Processing of Poultry || Influence of Processing on Product Quality and Yield

Influence of Processing on Product Quality and Yield

J. M. JONES'

18 Sywell Close, Norwich, UK

T. C. GREY

Agricultural and Food Research Council, Institute of Food Research, Bristol Laboratory, Langford, Bristol, UK

1. INTRODUCTION

The quality of poultry meat may be defined in terms of nutritive value, hygienic condition and sensory characteristics, such as colour, flavour, odour and texture. All of these aspects are important to the consumer in selecting the product at the point of sale and consuming it in the home, and may be influenced by production factors, such as breed of bird and husbandry, and by processing. This review will be largely concerned with the effects of processing on the sensory properties of the resultant meat.

Primary processing is defined as those stages involved in converting the live bird into an eviscerated carcass. Following harvesting and transportation, typical stages in primary processing are hanging birds on the processing line, stunning, bleeding, scalding, plucking, evisceration, spray-washing, chilling and packaging. Further processing implies portioning, deboning and preparation of 'value-added' products.

In addition to the eating quality of the meat, areas of concern to processors and consumers are control of the moisture and fat contents of the carcass, as well as product yield, and these topics will be discussed in some detail.

* Formerly of the Agricultural and Food Research Council, Food Research Institute, Norwich, UK.

127 G. C. Mead (ed.), Processing of Poultry© Chapman & Hall 1995

128 J. M. Jones and T. C. Grey

Many of the modern-day poultry industry's activities are significantly influenced by governmental regulations, and therefore the concluding part of this chapter will deal with various aspects of legislation and their effects on both processing and product quality.

2. POST·MORTEM CHANGES IN POULTRY MUSCLE

In the living bird, the breast and leg muscles are soft and freely extensible. Some time after slaughter, however, the muscles undergo rigor mortis, i.e. they stiffen and become inextensible. After a further period of time, rigor is resolved and the muscles relax.

The physical changes occurring during the transformation of muscle into meat are accompanied by a series of biochemical changes within the tissue. The relationships between physical and biochemical processes in skeletal muscle are very complicated, and a detailed discussion is outside the scope of this chapter. For a fuller treatment of the subject the reader is referred to the review by Greaser (1986).

Poultry muscle does not differ in structure from mammalian tissue, being made up of muscle fibres surrounded by connective tissue, principally composed of collagen. It must be stressed, however, that the white fibres which largely comprise the breast muscle of chicken will have biochemical properties differing from those of the red fibres of leg muscle. A fibre is itself composed of rod-like myofibrils, which run the length of the fibre and are the contractile units of the muscle. Generally, it is reactions within these units which determine the toughness or tenderness of poultry muscle, rather than the amount of connective tissue.

The myofibril is subdivided by thin partitions called Z-lines into sarcomeres approximately 2·0!lm long. These contain the proteins involved in muscle contraction, and are characterised by dark and light bands, when examined under the microscope. Such microscopic examination shows the dark A-band to occupy the middle of the sarcomere, while the light I-band fills the rest of the structure, forming a junction with the Z-line. The thick filaments of the A-band are composed principally of the protein myosin, while actin comprises the principal protein of the I-band. A cross-section through the myofibril shows that, where filaments overlap, six thin filaments are arranged hexagonally around each thick filament (Jones et ai., 1974).

Recent evidence indicates that the stoichiometric ratio of myosin to

Influence of Processing on Product Quality and Yield 129

actin in their natural complex of actomyosin differs between chicken breast and thigh muscles (Acton & Dick, 1986), a fact which may help to explain some of the differences in properties noted when these two meats are used in products (Jones, 1988a). Other proteins identified as being involved in the structure or function of the myofibrils of chicken and turkey muscle, and which may also be included in the salt-soluble, protein fraction of the tissue, are tropomyosin, troponins, a-actinin, C-protein and N-protein.

The most obvious physical change occurring in the post-mortem muscle is a shortening of the sarcomere and an accompanying increase in tension and toughness. The shortening results from the thin filaments sliding between the thick filaments as a consequence of interaction between specific sites on the myosin and actin molecules. After a further period of time, the muscle 'relaxes', becoming more extensible. At this stage, the Z-line has generally begun to disintegrate and there is a greater fragility of the myofibril, suggesting enzymic action by some of the various proteases present in the muscle. A component of the troponin fraction, and myosin, are among the proteins partially degraded (Hay et ai., 1973; Yamamoto et al., 1977). Jones et af. (1982) reported only a limited proteolysis during the first few days post mortem in the case of turkey breast meat.

The changes described above are probably a direct consequence of post-mortem metabolic changes in the muscle fibre or cell. On the death of the bird, the supply of oxygen and nutrients to the fibre ceases, and the concentration of the cell's principal energy source, adenosine triphosphate (ATP), is maintained only by the breakdown, firstly of creatine phosphate, and then of glycogen, the major storage carbohydrate of muscle. The breakdown of glycogen (glycolysis) is brought about mainly by the action of enzymes contained in the soluble sarcoplasm of the muscle, and results in the formation of lactic acid, hence largely explaining the well-known increased acidity of post-mortem muscle, with the pH declining from its initial value of ca 7·0, to an ultimate value of 5·6-5·8 in the large breast muscle and 6·1-6·4 in leg meat.

At first, the rate of muscle glycolysis is slow, with the A TP level being maintained by the breakdown of creatine phosphate but, as the latter energy source disappears, the rate of glycolysis increases, and the ATP concentration begins to decrease. When the concentration falls below between 20 and 30% of its initial value, the muscle loses extensibility and passes into rigor.

Poultry differs from most meat-producing species in that its muscle tends to enter rigor fairly quickly. For instance, based on evidence from biochemical studies, it has been claimed that broiler (chicken) breast muscle enters rigor within 60 min, while in leg muscle the time required is 30 min (Kijowski et aI., 1982). Grey & Jones (1977) found that, at about 1 h post mortem, the mean ATP concentration in the breast muscle of commercially processed broilers had declined from an initial value of ca 7·0,umoles/g tissue to 4·3,umoles, although the values for different birds within the experimental group varied from 1·1 to 6·5 ,umoles/ g, indicating a wide range of times for the onset of rigor. Similarly, Ma et al. (1971) found that rigor times in turkey breast muscle ranged from 25 to 391 min; in extreme cases, however, turkey breast muscles completed glycolysis within 5 min post mortem (Ma & Addis, 1973).

The rate of glycolysis, and hence rigor onset, is influenced by various processing procedures, and it is generally thought that the more rapid the rate of glycolysis, the tougher will be the resulting meat. However, Stewart et al. (1984) concluded that, while pH decline and muscle tenderness were parallel functions of post-mortem glycolysis and rigor processes, they were not necessarily involved in a cause-effect relationship.

Chicken breast muscle has been classified as pale, soft, exudative (PSE) or dry, firm, dark (DFD) on the basis of post-mortem decline in pH, the measurements usually being made some 15 min after slaughter (Kijowski & Niewiarowicz, 1978; Ristic, 1983). At this time, 'PSE' meat will have a pH value as low as 5·7, and 'DFD' meat a value of pH 6·4 or above. The breast muscle of some turkeys has been shown to exhibit the characteristics of the PSE condition (van Hoof, 1979).

Finally, as noted earlier, the mean pH value for chicken and turkey leg meat is generally about 6·0, compared with the 5·6-5·8 of breast meat. This difference will probably influence the properties of the two types of meat, as used in products, as well as being partly responsible for the poorer keeping quality of leg meat.

3. INFLUENCE OF PROCESSING ON SENSORY QUALITY OF POULTRY

3.1. Colour and Appearance Poultry differs from most of the species used for meat production in that it may be sold to the consumer with or without the skin attached.

Influence of Processing on Product Quality and Yield 13l

Thus, in the case of whole carcasses and some portions, consumer acceptance will firstly be influenced by the appearance of the skin, while the acceptance of other poultry products may depend upon the surface colour of the packaged meat.

Slaughter The appearance of poultry meat may be influenced by the effectiveness of stunning. Veerkamp (1987) reported the presence of more blood spots in the breast meat of broiler chickens stunned with a current of 100-200 rnA/bird, when compared with breasts from birds stunned with 50-60 rnA. This study showed that low-current stunning resulted in a more rapid breakdown of muscle glycogen, and hence a decrease in muscle pH value. This rapid decline in pH value, which may also occur when birds struggle during slaughter, has been shown to increase the darkness and redness of turkey breast muscle (Froning et al., 1978; Ngoka et al., 1982). On the other hand, Metz (1983) found no significant correlation between the pH value measured at IS min post mortem and the colour of turkey breast muscle. It was found that, in general, muscle pH values measured after 24 h were very variable, but the higher the pH value, the darker the meat. The situation is further confused by the fact that van Hoof (1979) found that the breast meat from turkeys slaughtered manually at the farm was significantly paler than that from birds similarly slaughtered after 4-h transport.

The differences noted by the various authors indicate that, in addition to peri-mortem struggling, other factors are probably involved in determining meat colour. For instance, it has been suggested that the cytochrome c content of breast muscle may be important, although the mechanism by which the protein exerts its effect on meat colour has not been established (Ngoka & Froning, 1982; Pikul et al., 1986).

None of the above studies established whether the observed colour differences would be detected by the consumer.

Scalding The scalding stage can have a profound effect on carcass appearance. It is here that poorly bled carcasses may show discoloration, while excessive periods or temperatures in the scald tank will result, at best, in a 'barked' appearance, or, at worst, produce a partially cooked flesh which results in carcass condemnation. For this latter reason, some processing plants in the United States of America have the facility to raise large turkey carcasses out of the scald tank in the event of a line

132 J. M. Jones and T. C Grey

breakdown. On the other hand, the seemingly damaging effects of high scald temperatures may also be used to the advantage of the processor. For instance, the yellow epidermis evident on maize-fed poultry may be removed by 'hard' scalding at 58-60°C to give white-skinned carcasses for those areas where this type of product demands a price premium.

Chilling and Freezing In the USA chickens are usually sold in the chilled state and turkeys as frozen carcasses, with nearly all poultry being chilled in water, whereas in the countries of the European Economic Community, chicken and turkey carcasses destined for the 'fresh' (chilled) market are usually chilled in air, while those prepared for the frozen sector also may be chilled in air, but more commonly are water (immersion) chilled. Apart from a temporary drying of the skin of air-chilled carcasses, there is little evidence to suggest that the chilling operation itself greatly influences the appearance of the unfrozen carcass. However, results of a recent study indicate that, in some localities, the water or ice used to maintain poultry carcasses in a cool condition might contain sufficient nitrates or nitrites to impart a pink coloration to the meat, when it is subsequently cooked (Nash et al., 1985). Although it is unlikely that difficulties will arise under normal circumstances, when poultry carcasses are held for a few hours, or overnight, in ice-water, discoloration problems may well be exacerbated if processing expediency requires carcasses to be stored in ice-water for extended periods, e.g. over a weekend.

As mentioned above, in some parts of the world both air- and immersion-chilling methods may be used in the production of frozen carcasses, but there is little information on how the appearance of the frozen carcass may be influenced by the method of chilling. A consumer acceptance trial carried out in the United Kingdom, using factory-processed broilers, showed that 70% of assessors preferred immersion-chilled carcasses, because they were 'fresher' and 'pale', whereas air-chilled carcasses were 'older' and 'darker' (Grey et al., 1982b). Since the poorer appearance of air-chilled birds results principally from carcass dehydration, it is probable that the problem could be overcome by using the evaporative chilling method developed by Veerkamp (1981, 1985), in which carcasses are periodically sprayed with water while passing through the air-tunnel.

In the study by Grey et af. (1982b), it was found that, regardless of

storage temperature, there was little change in the appearance of frozen chickens during the first six months in the freezer. Likewise, the carcasses remained stable for up to 12 months at -20°C, but the skin colour changed to grey-green, when carcasses were held at -12°C for more than 6 months.

The acceptable white colour of a frozen turkey carcass is obtained by rapid (crust) freezing of the outside after chilling: improper freezing leads to a visually inferior product (Jones, 1988b). Recent data are lacking for the effect of prolonged storage on the appearance of frozen carcasses, although it is well established that damage to the wrapping material may lead to 'freezer burn' and deterioration in appearance. A less common discoloration problem may arise if the calcium chloride 'brine', which is frequently used for crust-freezing turkeys, enters the pack and remains undetected. After a period of frozen storage, areas of green discoloration appear at the points of contact between brine and skin. Presumably, this latter type of problem would not be encountered in countries, such as Canada, where legislation demands the addition of a colouring agent to the immersion refrigerant, to facilitate detection of leaking bags (Agriculture Canada, 1986).

General Process Control The above examples are fairly obvious instances of inadequate quality control leading to product deterioration. In some cases, however, the causes of discoloration are less evident. A good example of a major problem involving ineffective monitoring of a process, together with poor plant design, was encountered some years ago in the UK, when a company manufacturing products from 'spent' laying hens found that the carcasses turned black or grey on cooking. Subsequent investigation (Grey et aI., 1978a) indicated that the problem arose from excessive use of chlorine, which produced a dilute acid in the process-water, the latter passing along copper pipes before coming into contact with the birds during chilling. The colourless copper salts from the water remained on the carcass skin, and probably were converted to a black sulphide during the cooking process.

Effects of Other Preservation Processes The decline in frozen poultry production has led to renewed efforts to develop methods for extending the limited shelf-life of chilled carcasses or portions. Since 'spoilage' of such products usually results

from the proliferation of microorganisms on the meat, an obvious and simple means of extending shelf-life is to retard bacterial growth by storing at low temperature, down to - 2°e. Although this is done in commercial practice, there seems to be little information on the effect of chill storage on meat colour. Ristic (1987) reported that chicken carcasses stored at O°C had a better appearance than those held at --1 or -4°C: at the latter temperature the meat had a significantly brighter colour. It should be emphasised, however, that, in this instance, the meat would have been below its freezing point, and this might have influenced the experimental results.

Another means of extending shelf-life is to modify the gaseous environment surrounding the refrigerated meat, and a number of studies have been concerned with possible effects on product colour.

There was no evidence of skin discoloration when chicken carcasses were held at 1°C in air containing 10 or 20% carbon dioxide or when packed in CO2 'snow' (Wabeck et al., 1968; Thomson & Risse, 1971). Although instrumental measurements showed a general decrease in the yellowness of the skin of chicken quarters stored in bulk-packs containing 80° CO2 in air, a sensory panel found the appearance of both skin and meat to be significantly better after 14 d than with samples held in air alone (Hotchkiss et al., 1985).

While the general appearance of chicken does not seem to be greatly affected by changes in the gaseous environment in which it is stored, the same is not necessarily true for other species of poultry. For instance, chilled turkeys packaged in oxygen-permeable film frequently show an acceptable blue-purple tinge, but when the carcasses are stored in evacuated, oxygen-impermeable barrier bags, undesirable colour changes may occur (Humphreys, 1985). Intact turkey drumsticks held under vacuum may lose moisture during storage, leading to the presence of unsightly 'drip' in the pack, while skinless turkey breast fillets (M. pectoralis profundus) held under vacuum, or in packs containing nitrogen and 20-30% CO2, rapidly lose their 'bloom' and may take on a bluish tinge. Although the colour of these portions is readily restored on exposure to air, attempts have been made to maintain an acceptable appearance of the packaged product by including oxygen in the packs, as frequently happens with red meats. Mead et al. (1983) found that the addition of 20% oxygen to the COz-containing packs enhanced the appearance of the raw breast meat by imparting a 'salmon-pink' colour. However, the enhanced colour intensity varied from muscle to muscle, and in some

instances, areas of pink coloration remained in the samples after cooking, thus conveying the impression of undercooking. An even more serious problem was the rapid development of unpleasant flavours in the 02-containing packs. Nonetheless, despite these apparent drawbacks, several turkey-meat products are held in a gas mixture containing CO2, Oz and Nz, and currently marketed in the UK.

The appearance of duck carcasses does not change significantly during storage under vacuum. However, a recent study showed that the skins of half-carcasses held in mixtures of COz and Nz rapidly took on a 'waxen' or 'milky' appearance which made the products commercially unacceptable (Mead et aI., 1986). Storage of half-ducks in air and COz did not produce this phenomenon.

Although ionising radiation is used in a number of countries as an effective means of destroying pathogens and extending the shelf-life of chilled poultry, it has been shown that such treatment imparts a pinkish or red colour to the meat (Coleby et al., 1960; Mead & Roberts, 1986).

3.2. Flavour Poultry-meat flavour is made up of several factors, including odour and taste, and its evaluation invariably depends upon a human assessor sampling the cooked meat. However, work with turkey has shown that panellists' personal preferences may vary widely (Webb et aI., 1972; Griffiths et al., 1984). Thus, in addition to applying the correct experimental design to flavour studies, it is essential to employ a suitably trained sensory panel.

Effect of Processing on Flavour There is little to suggest that the primary processing stages play an important part in determining meat flavour, the notable exceptions being the addition of salt in the case of birds prepared by the 'kosher' process (Mast & MacNeil, 1983) and the Injection of salt/polyphosphate solution into chicken breast meat prior to immersion chilling (Jones et al., 1980). However, 'dry' chilling of chickens by placing them in a blast freezer or in a tunnel where they can be exposed to liquid nitrogen is said to result in a flavour superior to that obtained with immersion (water) chilled carcasses (Hale et al., 1973; Arafa & Chen, 1978). On the other hand, Grey et al. (1982b) found the flavours of air- and water-chilled broilers prepared in a British processing plant to be similar. The same investigation showed that the

flavour of air-chilled chickens began to change significantly after 3 months at -12°C; at -20°C, 6 months elapsed before significant flavour changes were detected.

In order to improve the succulence of frozen turkey, some processors inject the chilled carcass either with an oil-based solution or with a broth. Recent sensory evidence indicates that the flavour of such 'self-basting' carcasses is significantly better than that of the unbasted product (Cornforth et al., 1982; Larmond & Moran, 1983).

Influence of Preservation Processes on Flavour of Chilled Poultry As mentioned above, storage at low temperatures or in an altered gaseous environment, are common means of extending the shelf-life of chilled (fresh) poultry. The effectiveness of low-temperature storage was shown by Barnes et al. (1978) who stored turkey carcasses at temperatures between +5 and -2°C. At +5°C, the average elapsed time before the detection of 'off' odours was 7·2 days, but at -2°C, this was extended to 38 days. Unfortunately, the investigation did not include evaluation of the flavour of the stored meat. Recently, Ristic (1987) reported that the flavour of chicken meat was significantly influenced by both temperature and time, when held at 0, -1, or -4°C for up to 22 days, although the sensory data obtained showed little difference between treatments without statistical analysis.

The flavour of cooked breast meat from ducks, which had been wrapped in low-density polythene and held in the raw state at - 1°C for up to 19 days, differed only 'very slightly' from frozen control meat, and even after 22 days was only ranked as 'slightly' different, while being judged to have more flavour (Barnes et al., 1979).

A simple means of altering the atmosphere surrounding the carcass or portion is to package the product in an oxygen-impermeable barrier film. During storage, there will be a decrease in O2 and a build-up of CO2 due to post-mortem respiration of muscle tissue. It is this build-up of CO2 , perhaps to only 3%, which extends shelf-life by retarding the growth of spoilage bacteria. However, as shown by the study of Igbinedion et al. (1981) on chickens, the beneficial effect of vacuum packing is more pronounced at 1° than at 5°C, thus indicating that reduction in temperature is essential for extension of shelf-life by this means.

The microbiological shelf-life of duck or turkey portions held in evacuated heat-shrunk 'barrier' bags was increased by some 50% over that of their polythene-wrapped counterparts (Barnes et al., 1979; Jones et al., 1982; Mead et aI., 1983). In the case of vacuum-packaged

ducks held at -1°C, a slight 'off' odour was apparent from 41 days (Barnes et al., 1979). However, a sensory panel found the cooked breast meat to be rancid, a result indicating that odour assessment may not be the best means of estimating shelf-life when growth of the usual aerobic spoilage bacteria is suppressed.

While the time to 'off'-odour development was extended from 16 to 25 days in the case of vacuum-packaged turkey breast fillets, and with drumsticks from 14 to 20 days, marked flavour changes preceded 'off' odour by several days (Jones et al., 1982). Unlike the rancid flavour noted above in the case of duck meat, turkey meat at the end of its shelf-life was variously described as 'gamey', 'fruity' or 'bitter'.

A number of investigations into the extension of shelf-life have been concerned with the use of controlled or modified atmospheres, where the poultry was held in air or N2 containing various levels of CO2 and stored at chill temperatures. These studies have indicated that meat odour and flavour are influenced by bird species as well as by the composition of the gaseous environment.

Hotchkiss et al. (1985) studied chicken quarters held in bulk packs containing 80% CO2 for up to 28 days and found the flavour was not significantly different from that of fresh chicken; at 35 days, the flavour of the stored quarters was inferior. This investigation also showed that the odour of the raw meat held in CO2 was acceptable to a sensory panel for up to 35 days, in contrast to earlier studies which found that chicken carcasses held in air containing 10 or 20% CO2, or in solid CO2 (snow) developed 'sweet' or 'fruity' odours before controls held in air were 'off' (Wabeck et al., 1968; Thomson and Risse, 1971). The use of solid CO2 as a coolant also has been implicated as a cause of increased lipid oxidation, and hence rancidity, in frozen mechanically recovered chicken meat (Uebersax et al., 1977; Mast et al., 1979), thus presenting a potential problem for the processor using such material in further-processed products.

There is relatively little information on the influence of gaspackaging on the flavour of turkey and duck meat. Mead et al. (1983) found no deleterious flavour changes in turkey breast fillets held for 21 days at 1°C in 20 or 30% CO2 and N2 • However, if 20% O2 was added to the packs, in order to enhance product colour, marked deleterious flavour changes were noted within 4 days in the case of packs containing 30% CO2 and at 8 days when the CO2 concentration was 20%. The rapid onset of flavour change was unrelated to microbial growth.

A subsequent study with duck portions stored in CO2 and air or N2

showed there was no obvious relationship between odour and flavour changes for portions held in 20 or 80% CO2 in N2 (Mead et al., 1986). In the case of packs containing 20% CO2, 'off' odours were noted at 23 days and marked flavour differences at 30 days. With 80% CO2, clear flavour changes were obvious at 23 days, and there was no 'off' odour at 30 days. In contrast, both flavour and odour changes were obvious after 21 days of storage in the case of portions held in air cO_ltain:ng 20% CO2.

Raw chicken meat subjected to ionising radiation develops a transitory, but characteristic, 'irradiation odour', while flavour changes may be detected in the cooked meat (Coleby et al., 1960). However, the adverse sensory effects tend to be masked when the meat is cooked by roasting.

3.3. Tenderness Tenderness is a major criterion of poultry meat quality and as such has attracted a great deal of research effort as the processing industry has developed. It is assessed both by subjective (sensory panel) and objective (instrumental) methods, but there is not always a good correlation between the two techniques. Also, work on tenderness has suffered from the disadvantage that each research group has tended to set its own limits for determining whether a sample is 'tough' or 'tender'. For example, Lyon et al. (1985), using 2·54 cm x 7 cm strips of cooked chicken breast meat as experimental material, found a force to shear of 7·5 kg to be the point of demarcation between acceptably tender and unacceptably tough meat. On the other hand, Lee et al. (1979) considered a shear value of 82 kg/20 g ground, cooked breastmeat to be borderline.

Assessment of much of the published work is further complicated by (a) the fact that the exact source of the meat sample is not always clear, although it is well known that, for example, breast muscles can vary in texture along their length, and (b) a wide range of cooking conditions are applied in different countries.

Processing The stages involved in processing the carcass are known to have a profound effect on meat tenderness. In particular, the degree of bird-struggle occurring at, or during, slaughter has been shown to increase the toughness of turkey breast (Froning et al., 1978; van Hoof, 1979; Ngoka et aI., 1982). This was also borne out in an

investigation by Lee et al. (1979), which showed that broiler chickens allowed to struggle freely at slaughter without stunning, were significantly tougher, when cooked immediately after immersion-chilling or after 2 hours of 'ageing', than electrically stunned chickens. The difference between the two treatments was still evident after 24 h, with 44% of the 'no-stun' controls being judged as 'tough', compared with only 5% of the stunned birds. Concomitant biochemical studies indicated that, in the case of controls, the onset of rigor mortis occurred during scalding and plucking, two processing stages that have been implicated in toughness.

Ristic (1983) showed that the muscle pH value measured within 15 min of slaughter was lower in the case of broilers scalded at 60° than at 52°C, the effect being particularly marked with lower-grade carcasses. In the light of this, and of evidence that there was a cumulative effect from the replication of plucking units (Scholtyssek, 1980), it is perhaps surprising that the various scalding/plucking regimes in current use have not been extensively assessed. The observation that the voltage applied during stunning can influence the rate of decline in muscle pH (Ristic, 1983; Veerkamp, 1987) suggests that the effect of stunning on poultry-meat toughness also warrants further investigation.

Instrumental measurements made during the course of several studies showed younger or smaller turkeys to be tougher than older or larger birds, chilled under the same conditions, indicating that the rate of carcass chilling influenced meat toughness, although this was not always the case with breast meat (Scholtyssek & Klose, 1967; Welbourn et al., 1968; Ngoka et al., 1982).

Carcasses derived from female turkeys, with an average live-weight of 3·5 kg, were used in a comparison of the influence of two chilling systems, namely (i) a primary chill in a water-chilling system, followed by overnight storage in ice-water, and (ii) overnight chilling in a current of air at 1°C, on breast muscle (Dransfield et al., 1984). The slower air-chilling techniques produced a more tender meat, although the average shear forces appeared to be low: 3·0 kg force for immersion-chilled meat, 2·5 kg for air-chilled meat. This study supported other recent observations that the tenderness of breast muscle from turkeys prepared under conventional processing conditions may be very variable, and that prolonged 'ageing' or 'conditioning' of the carcass does not necessarily result in a tender product (Griffiths et al., 1984; Grey et aI., 1986; Seemann et at., 1986). In addition, the

observation by Griffiths et al. (1984) that uneviscerated turkeys cooled at ambient temperature (10-12°C) may be tough, indicated that rapid cooling was not the only causative factor in the problem.

Meat from commercially-processed chickens may also show a wide bird-to-bird variation in texture (Grey et al., 1982b; Janky et aI., 1982). In some instances, this variation may have arisen because immersion-chilled carcasses had been frozen shortly after slaughter, when they were in varying degrees of rigor mortis (Grey & Jones, 1977; Grey et al., 1982b) while, according to Janky & Salman (1986), in the USA the texture problem may be associated with 'chill-packed' chicken. Here, the carcass is crust-frozen at the processing plant and held at -2°C for shipping and storage, thus increasing product shelf-life, when compared with the more traditional, 'ice-packed' poultry.

Interestingly, chicken carcasses chilled by spraying with liquid nitrogen were found to be more tender than those chilled by water-immersion (Arafa & Chen, 1978). However, it was not clear whether the 8-min exposure of the warm carcass to the liquid nitrogen resulted in crust-freezing.

Although Grey et al. (1982b) found that the variation in texture could be reduced by holding immersion-chilled broilers in ice for a further 60 min or so before freezing, this procedure did not solve the problem completely. Furthermore, the delay involved would probably not be acceptable to many chicken processors, and hence other possible methods warranted investigation. For instance, it is well established that addition of sodium chloride (salt) to the chicken, either by injection (Jones et al., 1980), or during 'kosher' processing (Mast & MacNeil, 1983), will both tenderise and reduce texture variation, and recently there has been renewed interest in using salt as a tenderiser, especially in the USA, where its use as an aid in chilling raw poultry products is officially sanctioned.

Dukes & Janky (1985) found that cooling broiler carcasses in agitated brine (average content 50 g salt/kg solution) under commercial time and temperature conditions produced cooked breast meat that was significantly more tender than meat from water-chilled chickens. In the same study, 82% of members of a consumer panel preferred the brine-cooled product, with some 15% commenting that it was 'more tender'. In general, brine-cooling was more effective in tenderising 'tough' meat, i.e. that with a shear force of 8 kg/g meat, than 'tender' meat where the shear force was less than 4 kg/ g meat

(Janky et al., 1982). A similar situation prevailed in the case of quail breast meat (Legare et al., 1985). However, as discussed previously (Jones, 1986), problems due to enhanced sodium and water contents of the meat may be associated with this method of chilling. Also, in countries such as the USA and Canada, brine-chilled chicken would probably need to be specifically labelled (US Dept of Agriculture, 1985; Agriculture Canada, 1986).

Other Means of Tenderising Poultry Meat In some areas of the world, proteolytic enzymes, such as ficin, bromelin or papain, may be used to tenderise the muscle of older poultry, thus improving their consumer acceptability. That such a procedure was effective, was confirmed in a study by Cunningham & Tiede (1981), in which instrumental measurements and sensory analyses showed that the tenderness of drumsticks from yearling turkeys and of breast and thigh meat from 16-month old baking hens was significantly improved by injection into the chilled meat of a marinade containing 0·5 g papain/kg. Any adverse flavour effects imparted by the papain were apparently masked by the seasoning included in the marinade.

In addition to improving turkey-meat succulence and flavour, the self-basting process mentioned earlier also improves tenderness. Larmond & Moran (1983) injected the breast muscle of male turkeys of different grades with coconut oil, to a concentration of 30 g oil/kg, and subjected the cooked meat to sensory evaluation. The tenderness of breast muscle from basted A-grade carcasses was significantly improved by the treatment, but that from B-grade birds was largely unaffected. The use of broth as the self-basting medium was just as effective as oil-injection in improving the tenderness of turkey meat (Conforth et al., 1982).

There have been several investigations of the use of electrical stimulation as a means of tenderising poultry meat, and these are described in Section 6 of this chapter.

Influence of Portioning and Deboning on Product Tenderness A current feature of the poultry industry is the move away from whole-carcass production to preparation of portions or boneless cuts. In some countries, more than 50% of the chickens produced may be so treated. This development has resulted partly from the growth of

'fast-food' operations, and partly from the ease with which poultry meat may be converted into a 'value-added' food.

It has been known for some time that product tenderness is influenced by both the type of cut used in portioning the carcass and the time post-slaughter at which portioning is carried out (Klose et al., 1971, 1972; Lyon et al., 1973). Removal of the wing at its junction with the breast, or making a transverse cut through the breast before the onset of rigor increased toughness. It has been recommended that chicken carcasses should be held for several hours before portioning: in commercial practice this might be at least 4-6 h.

The apparent advantages (increased yield, saving of space and time) accruing from removal of meat from the bone soon after slaughter would seemingly benefit the processor producing fillets, rolls or roasts. However, there is a great deal of recent evidence to suggest that 'hot-boned' poultry meat will be tougher than tissue taken from chilled carcasses: such pre-rigor muscle is also said to be more 'springy' and 'chewy' than post-rigor meat (Lyon & Wilson, 1986).

Stewart et al. (1984) found that the toughness of breast muscle excised immediately after plucking did not differ significantly from that excised from the uneviscerated chicken carcass after 60 min at 25°C. However, at 2 hpost mortem, the muscle was significantly more tender, and after 4 h was as tender as that of control meat held on the carcass for 24 h. In contrast, when carcasses were held at 4°C for 4 h prior to muscle removal, the excised meat was significantly tougher, indicating that the holding temperature was an important factor.

In contrast to the uneviscerated carcasses used above, chicken halves derived from commercially eviscerated and chilled carcasses were used by Lyon et al. (1985) in their study of deboning. The resultant data indicated that de boning should be delayed for at least 6 h to ensure that 95% of breast portions would have shear forces below 7·5 kg, and hence be 'acceptably tender' to a sensory panel. Recently, Uijttenboogaart & Reimert (1987) confirmed that 'hot deboning' increases chicken breast-meat toughness, but found that the situation was complicated by the birds used in different trials being in varying degrees of rigor at the time of sampling.

Not all studies support the contention that pre-rigor chicken meat is tougher. Furumoto & Stadelman (1980) reported that rolls derived from 'hot' boned chickens were significantly more tender than those prepared from cubes of breast meat dissected from chilled birds. As discussed by Jones (1986), this result may have been a consequence of the chilling method employed.

The 'hot deboning' of turkey meat has not been so extensively examined: a somewhat surprising situation in view of apparent discrepancies in results from various studies. For instance, Furumoto & Stadelman (1980) found that rolls prepared from cubes of 'hot deboned' breast meat, after mixing for 30 min, were tougher than those from 'cold' boned meat, whereas an earlier study had suggested that rigor state was less important in determining product tenderness than the time at which the ingredients were mixed (Kardouche & Stadelman, 1978). In this latter investigation, it was also found that, while instrumental analysis showed 'hot' deboned meat to gradually ten de rise with 'ageing' time, a sensory panel was unable to find any effect of 'ageing'. This raises the question of whether consumers would be able to distinguish between 'hot' and 'cold' deboned meat in a turkey product.

Although the situation regarding deboned turkey meat might have been equivocal, this was not the case with quail meat, where experience with a further-processed product formulated from meat removed soon after carcass evisceration showed a major toughness problem (Legare et al., 1985). Investigations revealed that a holding period of 2 h did not maximise tenderness, and the authors recommended that breast fillets or meat deboned before 'ageing' should be 'aged' in brine in order to ensure a tender product. It would appear, however, that the need to hold the deboned meat before use might offset some of the advantages of employing the 'hot' deboning procedure.

4. FACTORS IN PROCESSING WHICH AFFECT YIELD AND THE EFFECTS OF EXCESSIVE FAT DEPOSITION

The maintenance of optimum yield at every stage in poultry meat production is important in the overall profitability of the enterprise. First, the grower is concerned with obtaining optimum live bodyweight to suit the market, at a minimum age and with good feed conversion. To achieve this, factors such as breed, strain, sex, diet and husbandry practice will all playa part.

Secondly, there are major losses to the processor in converting the live animal into a high quality meat product. As the birds progress from farm to factory, weight losses occur as a result of starvation, transport and holding-time prior to slaughter. During processing, the birds are stunned, bled, scalded and plucked, the feet and head are

removed, and the carcasses are eviscerated and chilled. Following insertion of the giblets, the carcasses are bagged and either stored at chill temperature or in the frozen state. Small variations in processing technique can have a significant effect on product yield.

In addition to preparing carcasses as described above, some poultry processors also produce cooked whole-birds or portions for the retail market. In these plants, the birds to be cooked are first injected with brine or marinated in a flavouring solution. Considerable variation in cooked yield is found due to factors which include oven cookingtemperature, air circulation, final product temperature, degree of browning required and the extent of uptake of the marinating solution.

In this section, the effects of husbandry on fat deposition and conversion of the live bird to a fresh or frozen product will be discussed.

4.1. Fat Deposition The growth of the broiler is always accompanied by an increase in total body fat, in which the abdominal, visceral and subcutaneous fat will be the main contributors (Summers & Leeson, 1979). While subcutaneous fat will possibly affect carcass 'finish' and eating quality, the fat in the visceral cavity is generally regarded as a waste product, both for the processor and ultimately the consumer.

Various methods are available to reduce fat content without significantly affecting the meat yield. Nutritionally, reducing the dietary energy to protein ratio has met with some success (Fraps, 1943; Donaldson et al., 1956; Griffiths et al., 1977; ten Have & Scheele, 1981). Low environmental temperatures have also been shown to reduce fat deposition without affecting protein content (Rinehart et al., 1975). In addition, it has been established that females deposit more abdominal fat than males (Edwards et al., 1973; Washburn et al., 1975), and that separate growing of the sexes would be advantageous. More recent approaches have involved genetic selection for reduced fat content and, more particularly, for low abdominal fat, without loss of bird weight. Leclercq {l985) has reviewed the genetic and physiological basis of fatness in poultry, indicating that direct selection can be made for abdominal fat-pad content, within a suitable nutritional regime. An added advantage of selection based on reduced abdominal fat is a reduction in the variability of the size of the fat pad between birds in anyone flock. A clear relationship between abdominal fat-pad size and total body lipid has been established (Whitehead & Griffin,

1984) and it can be concluded that, if a particular carcass 'finish' is required, e.g. for air-chilled broilers, then a compromise fat content has to be found which is acceptable to both grower and processor.

The situation with turkeys is very similar to that for broilers, except that development of the fat pad usually occurs after 8 weeks, particularly between 8 and 16 weeks of age. Bacon et al. (1986) obtained a correlation coefficient of 0·3-0·4 for body-weight and weight of abdominal fat-pad, but obtained a better correlation (0·6-0· 7) for the proportion of carcass fat and weight of abdominal fat-pad. The authors concluded that carcass fat content increased as a function of body-weight between 8 and 20 weeks, and that over this period females had a higher carcass fat content than males. Griffin & Whitehead (1985) have already established substantial differences in the fatness of male and female turkeys with high or low plasma levels of very low density lipoprotein. They also found a good relationship between the size of the abdominal fat-pad and total carcass fat. As with the broiler, live-weight increase is associated with a parallel increase in total carcass fat content.

The carcasses of market-age ducks contain, on average, ca 300 g/kg of fat, most of which is subcutaneous fat. Sheldon & Tarver (1987), in a survey involving seven brands of ducks commercially available in the USA, found considerable variation in the combined skin and subcutaneous fat contents, which ranged from 285 to 406 g/kg of the eviscerated weight. Elkin (1987) has recently reviewed the subject of duck nutrition and has shown that carcass composition and meat yield can be altered by breed, age, sex, weight, diet and growing temperature. Fris Jensen (1980), in a review of meat quality in ducks, stated that the Pekin breed is the duck that is mainly used for commercial production, but Muscovy and dwarf breeds are used when a lower fat content and higher meat yield are required. Reductions in duckling yield due to the presence of excessive fat are minimal during processing, when compared to losses on cooking which, with evaporative loss, can be as high as 390 g/kg of the ready-to-cook weight.

4.2. Effects of Catching and Transportation on Yield The effect of time of feed withdrawal prior to transportation of birds to the processing plant has been the subject of a number of studies. Times of 8-10 h (Carlson et al., 1975; Church & Binstead, 1986) are optimal for feed withdrawal in order to minimise yield loss. Chen et al. (1983) have studied the effect of temperature, and found that the rates

TABLE 1 THE EFFECT OF HOLDING TIME AND TEMPERATURE FOR LIVE BROILERS ON

EVISCERATED YIELD AS A PROPORTION OF POST-HOLD WEIGHT

Treatment

Control 8 h at 10°C 16 h at 10°C 8 hat 32·2°C 16 h at·32·2°C

Eviscerated yield / post -hold weight x 1000 (g / kg) MaLes FemaLes

668 677 685 678 679

671 663 681 670 688

Food withdrawn 12 h previously, but water available; overall mean live-weight before holding: males = 2348 g; females = 1971 g. After Chen et af. (1983).

of live body-weight loss were linear, and that losses were 2·19 and 5·13 g/kg for birds held in crates for up to 16 h at 10° and 32·2°C respectively (Table 1). In this latter study, food was withdrawn for 12 h, but the birds had access to water before the experiment began, from the time of crating. Veerkamp (1986) found that shrinkage and yield in broilers was also dependent upon age/weight and availability of water, as shown in Figs 1 and 2. NQ difference was found between the sexes, and the weight loss in birds receiving water was between 5 and 10 g/kg less than in those without water. Weight loss was between 2·0 and 2·4 g/kg/h, and this was comparable with data from an earlier study in which a value of 3·5 g/kg/h was obtained (Veerkamp, 1978). The difference between the two sets of results can be readily explained as a temperature effect since, in the more recent study, birds were kept in pens instead of crates prior to processing. Eviscerated carcass yields, based on post-holding rather than pre-holding weights, are shown in Table 1. These increased between 10 and 20 g/kg for holding times up to 16 h, and were dependent on holding temperature (Chen et aI., 1983). Yield increases of ca 10 g/kg were also found by Veerkamp (1986), after holding chickens for longer than 6 h and up to a total of 24 h, when comparison was made with eviscerated yields after holding between 0 and 24 h.

Weight losses occurring when Small White and Large White turkeys were held in crates, without access to feed and water, are shown in Tables 2 and 3 (Moran et at., 1970a, 1971). The latter authors have commented upon the inconsistency of the data with respect to weight,

.: . ~ 6 VI VI

Age x 36d

t. 43d

HOlding time (h) 20

FIG. 1. Weight loss as a percentage of pre-hold live-weight (with water). After Veerkamp (1986).

age and sex. It is obvious from the tables that holding losses are not directly related to a particular weight of turkey, and that other factors found with broilers (Chen et at., 1983), such as temperature and dietary energy, must also be important. Although there does seem to be a possible strain effect on holding losses, it has been shown clearly, within a particular strain and sex of turkey, that, as the age increases, the proportionate holding loss decreases.

4.3. Losses Due to Killing and Bleeding Blood loss in broilers has been studied in some detail, mainly because of the problems associated with killing efficiency, i.e. ensuring that birds are dead before entering the scalder. It was also thought that residual and pooled blood in the tissues may affect the appearance of the carcass, particularly after scalding and plucking (Abram &

Weight loss ('k)

J. M. Jones and T. C. Grey

Age: x=V, d +=.t1 d ('=.~() d

Holding time (h)

FIG. 2. Weight loss as a percentage of pre-hold live weight (without water). After Veerkamp (1986).

TABLE 2 MEAN HOLDING LOSSES IN SMALL WHITE BROILER·TYPE TURKEY AITER 12 h

IN CRATES

Age (weeks)

10 11 12 13 14 15 16 17

Males Live weight Holding loss

(g) (g/kg)

3943 39 4194 43 4535 42 5029 38 5632 32 5963 37

After Moran et al. (1971).

Females Live weight Holding loss

(g) (g/kg)

2250 50 2448 55 2928 42 3106 45 3361 42 3680 34

TABLE 3 MEAN HOLDING LOSSES IN LARGE WHITE TURKEYS AFTER 12 h IN eRA TES

--"----

Age Males Females (weeks) Live weight Holding losses Live weight Holding losses

(g) (g/kg) (g) (g /kg) -----------

14 4389 61 15 16 5360 57 17 7721 61 5865 62 18 6156 61 19 9053 58 6658 58 20 21 10462 42 7425 34 22 11277 45 23 12183 33 24 8533 26 25 13 445 29 26 27 14388 24

After Moran et at. (1970a).

Goodwin, 1977; Harris & Carter, 1977; Kuenzel et al., 1978). Griffiths et al. (1985) have studied the effects of various methods of slaughter, i.e. stunning at 55, 80 or 105 V, birds electrocuted at 240 V or anaesthetised and then exsanguinated on the right side of the neck, such that all blood vessels were cut on that side. It was concluded that the method of slaughter made no difference to the amount of blood lost after 1, 2, 3 or 4 min bleed-time. Most of the blood was lost within 2 min of cutting the blood vessels, and appearance was unaffected by the method of slaughter. No relationship was apparent between the quantity of blood lost and the concentration of haemoglobin in the breast muscle or the whole carcass, and the overall conclusion was that the individual response to exsanguination rather than slaughter technique was the major determinant of the amount of blood lost. Heath (1984) has reviewed some of the contradictory findings on the slaughter of broilers. The amount of blood in the live bird has been estimated at 80 g/kg (Kotula & Helbacka, 1966) and the proportion of blood loss in a broiler strain was found to be affected by both age and sex (Grey et al., 1982b). As the age of the broiler increased from 21 to 364 days, the amount of blood loss significantly decreased, although

TABLE 4 VARIATION IN AMOUNTS OF INTESTINE. BLOOD AND FEATHERS FOR

BROILERS OF VARIABLE LIVE-WEIGHT

Live-weight (kg) Min Max

1-63-1·72 1·73-1-81 1·82-1·90 1·91-1·99 2·00-2·09

Intestine Min Max

a56·5-75·1 48·1-68·8 44·8-61·8 45·0-68·4 41·0-60·0

Blood Min Max

30·8-44·3 35·0-39·1 29·4-42·6 33·3-56·7 35·6-49·4

a g/kg live-weight. After Benoff & Wing (1985).

Feathers Min Max

44·7-61·6 44·0-56·5 48·0-68·0 44·0-65·0 39·6-60·3

the differences were relatively small, and male broilers lost more blood than females. Moran & Orr (1969) reported combined dressing losses i.e. weight losses after bleeding and plucking, for broilers between 6 and 10 weeks of age, to be 117 g/kg and 125 g/kg for males and females respectively. Similar figures were obtained for a variety of breeds (Moran et al., 1970b).

Benoff & Wing (1985) have studied losses during processing of Cobb broilers, and their general conclusion was that, within a live bodyweight range of 1·6-2·1 kg, the variation in blood loss and degree of feathering, both between and within strains, was considerable. Data obtained from their study are shown in Table 4.

Blood and feather losses from turkeys have been reported by Clayton et al. (1978), who showed that the loss after plucking declined as bird age increased. In the case of males, losses ranged from 91 g/kg at 12 weeks to 68 g/kg at 24 weeks; for females the values were 91 and 72 g/kg at weeks 12 and 18 respectively. Moran et al. (1970a, 1971) demonstrated a similar weight/age effect with male and female turkeys. The highest loss after bleeding and plucking (100 g/kg) was found in 12-week males weighing 4-0 kg, and this decreased to a minimum value (77 g/kg) obtained with 27-week males weighing 13·3 kg. Salmon (1979) found that 18-week male and female turkeys, weighing 8·76 and 5·91 kg, lost 28·4 and 26·5 g/kg of blood respectively. Feather loss averaged 49·0 g/kg for both sexes. Earlier data from Hammond (1944) showed a blood loss of 31 g/kg and a feather loss of 69·3 g/kg from small female turkeys weighing 4·1 kg. In the same study, male turkeys of 10·82 kg mean live-weight lost 41·6 g/kg as blood and 45·8 g/kg as feathers.

Table 5 shows blood and feather losses and eviscerated yields from a number of crosses and strains of duckling. Blood and feather losses are less variable than in turkey. However, since most of the feathers are normally dried after removal and sold for use in the domestic bedding industry, the removal of feathers during processing does not normally represent a financial loss to the processor.

4.4. Eviscerated Yield In current commercial practice, eviscerated yield is normally based upon live-weight at the time of processing i.e. starved weight. Holding losses have previously been discussed; however, Chen et al. (1983) found that most of the losses occurred in the viscera, particularly after holding for 8 h at 10°C (Table 6). Once processing conditions have been standardised, only small differences in eviscerated yield are found within a particular breed or strain. Merkley et al. (1980) compared five strains of broilers at 56 days and failed to demonstrate any significant difference in eviscerated yield, but found small, though significant differences, in fat pad, liver and heart weights, when expressed as a proportion of 'plant' weight (Table 7). Although the question of fat-pad size has been discussed earlier, it is worth noting the important observation of Merkley et at. (1980) that the fat pad varied with the time of year at which the birds were processed, ranging for males from 27·6 g/kg in December to 34·0 g/kg the following September and for females from 35·1 g/kg to 42·3 g/kg over the same period. Veerkamp (1980) has shown that differences between plants in processing conditions can result in considerable variation in eviscerated yield. For seven plants, taking a single day's production, the mean eviscerated dry yield varied from 610 to 680 g/kg. The differences in eviscerated yield were unexplained but must be due

TABLE 6 RELATIVE WEIGHT OF VISCERA TO EVISCERATED CARCASS WEIGHT AFTER

HOLDING IN CRATES FOR UP TO 16 h AT 10°C

Treatment

Control 8h at 10°C 16 hat lOoe

Viscera + head + Jeet:wt/eviscerated wt x 1000 (g/kg) Males Females Mean

360 354 327

359 379 338

360 366 332

After Chen et al. (1983).

TABLE 7 MEAN COMMERCIAL YIELDS OF BROILERS AS A PROPORTION OF PLANT" WEIGHT

Bird strain Ab B C D E

Eviscerated yield" 653-6 654·0 654·7 656·0 653·2 NS (g/kg live-weight)

Fat pad 37·2ak 34·5bc1m 35·1bk1m 33·4cm 35·6bkl P<O·01 (g/kg eviscerated weight)

Liver 17·6ak 17·3abkl 17·0bc1m 16·7cm 16·9c1m P<O·OI Gizzard 13·3 13·5 13·1 1304 13·5 NS Heart 05·5ak 05·3abkl 05·1 bc1m 04·9cm 05·0cm P<O·OI Neck 61·3 61·5 61·8 61·1 62·2 NS Total 788·5 786·1 786·8 785·5 786·4 Mean plant wt (g) 2020 1950 1974 1978 1961 P<O·01

(both sexes) -----~~---

" Plant weight is live weight immediately before processing. b 132 birds of each sex. abed: mean values within a row not showing the same superscript differ significantly (P < 0·05); kIm: mean values within a row not showing the same superscript differ significantly (P < 0·01); NS: not significant. After Merkley et al. (1980).

partly to machine adjustment, pre-holding conditions, neck-skin trimming and partial removal of kidney. Eviscerated yields have been shown to increase with age (Moran & Orr, 1969; Grey et al., 1982a), the increase being mainly due to a relative decrease in the size of the organs removed, when expressed as a proportion of live body-weight (Crawley et al., 1980).

Clayton et al. (1978) found no significant differences between sexes or breeds of turkey in the yield of oven-ready carcasses; there was, however, an age effect. Oven-ready yield increased from 790 to 820 g/kg between 12 and 18 weeks. Commercially processed, ovenready yield differs from the eviscerated yield only by the increase in weight caused by the incorporation of giblets (neck, heart, liver and gizzard). Clayton et al. (1978) found that giblet weight decreased with age from 65 g/kg at 12 weeks to 57 g/kg at 18 weeks of age. In a recent study, using BUT 8 male turkeys, Grey (unpublished) found no significant change between 18 and 22 weeks of age in the unwashed eviscerated weight as a proportion of live body-weight. Although there was an increase in live body-weight of ca 3 kg over the period in

TABLE 8 CHANGE IN EVISCERATED WEIGHT AS A PROPORTION OF LIVE WEIGHT IN BUT 8

MALE TURKEYS AGED BETWEEN 10 AND 22 WEEKS

Age (weeks): lOa 12 14 16 18 20 22

Mean live-weight (kg) 5-91 7·70 8·64 10·62 13·74 14-62 16·46 Eviscerated weight (kg) 4·30 5·57 6·48 8·19 10·84 11·50 13·02 Eviscerated weight/live-weight

x 1000 (g/kg) 726·4 725·3 749·8 770·6 789·1 786·1 790·7

an = 5 for each age group. Grey (unpublished data).

question, the proportionate eviscerated yield remained constant, thus indicating that, in this age range, the weight of viscera as a proportion of live-weight is increasing, and would have commercial significance with respect to killing age/weights. The data are shown in Table 8.

Eviscerated yield (ready-to-cook, excluding giblets), as a proportion of live body-weight in female Pekin ducks aged between 7 and 9 weeks, was 620 g/kg and that of males 600 g/kg (Luhmann & Vogt, 1975). Small variations in eviscerated yield have been found between breeds; the data from a number of studies are shown in Table 5.

Immersion-chilled carcasses absorb a variable amount of water during chilling, the effects of this treatment on yield will be discussed in Section 5.

4.5. Yields of Portions and Muscle The yield of cut-up portions or edible meat is not markedly influenced by processing other than by the method of dissection or portioning. Veerkamp (1983) has discussed this aspect of processing and commented that a standardised dissection procedure and uniform means of presenting the data were urgently required. Steps in this direction have now been taken by the World's Poultry Science Association (WPSA), European Federation Working Group on Poultry Meat Quality, which has adopted a description of parts and standardised method of dissection evolved by Uijttenboogaart & Gerrits (1982). (The booklet now in its second edition can be obtained from the Danish Branch of the WPSA, Copenhagen.)

Other factors, such as age, sex and breed playa more important role in influencing meat yield, although in terms of processing, chilling has also a significant effect and will be discussed later.

5. ABSORPTION OF EXTRANEOUS WATER; FACTORS AFFECTING UPTAKE AND RETENTION

The commercial processing of poultry requires the use of water at most stages of the operation and it has been estimated that a minimum of 10 litres/kg of carcass is used in transforming the live bird into a finished product (Mast & Veerkamp, 1981). Water is used mainly to scald, pluck, clean and chill birds during processing at line speeds of up to 6000 birds/h, and is also used to clean the processing plant and equipment. Consequently, it is inevitable that some of the water used is absorbed and retained in the carcass, in varying quantities, depending upon the scalding, spray-washing and chilling procedures used in the factory.

Extraneous water from frozen and thawed, immersion-chilled poultry has, in the past, resulted in the formation of excessive drip, and thus it is important for both the consumer and processor that the amount of extraneous water in these carcasses should be controlled. Hence this section will be concerned with the method of entry of water, the factors which influence its absorption and retention, and the regulations governing the use of water and its control during commercial processing.

5.1. Method of Entry, Accumulation and Retention of Water Heath et al. (1968), using a static system rather than any applied agitation, dipped birds in tritiated water at specified temperatures (0·5, 6·1 and 11· 7°C), so that only the breast was in contact with the water. Birds were removed after 2, 4, 6 and 8 h, and the degree of water penetration measured by comparing the specific activity of the chill-water and the amount absorbed in the skin or at specific depths of breast muscle. The greatest water penetration was found in the skin (0·21 ml/g tissue), which decreased as the depth of penetration increased viz. through the M. pectoralis superficialis and M. pectoralis profundus. Both time of immersion and increase in water temperature increased the absorption of water. Sanders (1969) used fluorescent-dye tracing of water entry and retention in broilers, which were subjected to varying degrees of agitation during a 20 min period of chilling. Very little penetration occurred through the skin, the primary route of entry into the areas under the skin being through body-cavity openings. It was also found that penetration of muscle tissue was progressive, beginning at the tip of the keel bone and over the clavicle. The

amount of dye bound by the tissue did not correlate with the total amount of water absorbed during chilling. However, the degree of agitation used influenced penetration of the dye into the tissues.

Veerkamp et al. (1973) investigated the absorption of water by the skin under commercial and simulated commercial conditions, using a 5 min draining period. With uneviscerated broilers in which the vent and neck were sealed, mean water absorption by 1 kg carcasses was found to be 20 g at 20°C and 10 g at 10°C, after a 30 min period in agitated chill-water. Carcass weights were measured immediately before chilling and after the drainage period. Thomas & McMeekin (1982), using scanning electron microscopy, found that water immersion of skin from poultry carcasses caused the skin to swell, and exposed deep channels and crevices as a result of water absorption. The amount of water held as a surface film on the skin was measured by desorbing the water onto weighed filter pads of unit area and, on average, was shown to be 0·008 g/cm2 . Assuming the total skin-surface area of a 1 kg broiler is ca 1200 cm2 (Thomas, 1978), a value of ca 10 g of water can be calculated for such a carcass. This weight will be a minimum amount of water absorbed on the skin surface and, of course, will depend upon the time and temperature of immersion. Thus, the figures obtained by Veerkamp et al. (1973) are in reasonable agreement with this calculated value. It is clear that the main site of entry of water into the carcass can only be through tears in the skin or cut surfaces. Klose et al. (1960) studied the uptake of water after spray-washing and draining, by subjecting broilers which had a minimum vent cut to various chilling procedures. The highest absorption and retention of water after a 10-15 min drainage period was after 30 min in ice-slush with tumbling. The amount absorbed was equivalent to 11·7% of the eviscerated weight prior to chilling and of this, 4·9% was found to be retained between the skin and muscle. Analysis of skin and M. pectoralis superficialis for water content showed that only small changes occurred in comparison with the total water absorption; this was particularly evident with skin, when the water content was calculated on a fat-free basis. The amount of water absorbed is therefore influenced by the degree of agitation and the drainage period, since this work confirmed the conclusion of Lentz & Rooke (1958) that approximately half of the moisture absorbed during chilling was lost during subsequent draining. Most of the water would seem to be retained between the skin and body of the carcass, but amounts can vary considerably and may result in the formation of

TABLE 9 EFFECf OF QUALITY OF EVISCERATION ON WATER RETENTION BY BROILERS

No. of carcasses

Description Range of water Mean retention

(%) -------

24 24 25 23

Minimal cuts, complete evisceration Minimal cuts, incomplete evisceration Extensive cuts, complete evisceration Extensive cuts, incomplete evisceration

Grey & Jones (unpublished results).

4·1-12·6 2·0-13·0 1·0-15·9 4·4-22·7

6·2 6·2 9·7

large water pockets, particularly in the area under the breast skin, close to the wing (Jonas, 1981). Entry of water has also been shown as a consequence of the method of opening the visceral cavity (Kotula et al., 1960); cuts which opened the inguinal area of either one or both thighs resulted in increased absorption. The effects of good and bad standards of evisceration on water retention are shown in Table 9 (Grey & Jones, unpublished results). Thus poor evisceration technique can influence both the uptake and retention of water from immersion chilling.

S.2. Effects of Processing on Water Uptake and Retention Most studies on the effects of immersion chilling on water uptake have been carried out on commercially plucked and eviscerated carcasses. During the latter processes, water is initially absorbed in variable amounts. Bolder (1975) has shown that 3-6% can be absorbed during pre-chilling procedures with hard-scalded broilers (61-62°C), and Veerkamp et al. (1973), examining the effect of spray cooling, obtained a mean value of 5 ·9% with broilers having an average weight of 750 g. Klose et al. (1960) found an increase in weight of ca 3% after spray-washing the outside and inside of the carcass. Woltersdorf (1971) used the fat-free dry matter of chickens x 3·5, defined as physiological water, and deducted it from the chemically determined total water content to calculate the amount of extraneous water. It was found that after scalding, the mean extraneous water content was 2·5%, after plucking 2·0%, after evisceration 2·0% and after a spray-wash 3·1 %. From the data quoted above, it is obvious that considerable variation in water uptake can occur during the pre-chill

processing of broilers, and the uptake will be influenced by the number of times the birds are washed during evisceration.

Legislation within the European Economic Community (EEC) has laid down minimum requirements for water usage during the spraywashing and immersion-chilling of poultry carcasses. In the United Kingdom, EEC Regulations have been introduced in the form of the Poultry Meat (Hygiene)(Amendment) Regulations 1979, which stipulate that all poultry intended for immersion chilling must be thoroughly washed by spraying with water, and subsequently immersed in water without delay. The minimum amounts of water to be used for spraying and chilling in relation to carcass weight are shown in Table 10. If more than one tank is used for immersion chilling, then the required amount of water can be divided between the tanks, although there is a minimum requirement for water usage in the final tank. Water must flow through the tanks in the opposite direction to that of the birds and the water temperature must not exceed 16°C, where carcasses enter the first tank, or exceed 4°C at the point that they finally leave the second tank. Birds must not remain longer than half an hour in the first tank or longer than is necessary in the remainder of the system.

The entry of water into the carcass has already been discussed, and Brant (1963) and Thomson et al. (1974) have reviewed factors which influence water absorption and retention. Most of the data quoted in these reviews were obtained with the use of through-flow chillers or simulated laboratory chillers rather than commercial counter-flow systems; nevertheless, many of the conclusions drawn are applicable to the counter-flow method. Factors such as time in chill, agitation of the carcasses and the extent of the evisceration cuts have all been shown

TABLE 10 WATER USAGE REQUIREMENTS FOR SPRAY-WASHING AND WATER CHILLING IN

THE EEC

Carcass weight Spray-wash Complete chilling Final tank (kg) (litres) system (litres)

(litres)

<2·5 1·5 2·5 1·0 2-5-5 2·5 4·0 1·5 >5 3·5 6-0 2·0

... ---~~

TABLE 11 EFFECf OF EVISCERATED WEIGHT ON THE AMOUNTS OF

WATER ABSORBED BY CARCASSES

No. of carcasses Weight range (kg)

Water absorption (%)

108 112 63 40 16

1·0-1·1 H-1·2 1·2-1·3 1·3-1·4 1·4-1·5 1·5-1·6 1·6-1·7

Grey & Jones (unpublished results).

9·1 9·6 8-4 8·0 7·6 7·6 6·8

to influence the amount of extraneous water absorbed and retained by the carcasses. Grey & Jones (unpublished results), in studies carried out with through-flow chillers, prior to the introduction of EEC regulations, have further shown that the amount of water retained by carcasses, expressed as a percentage of eviscerated wieght, is greater in smaller birds, as shown in Table 11, although the mean weight of water retained actually increased slightly with increasing carcass weight. Thus, the amount of water absorbed would seem to be decreasing with increase in eviscerated weight, the difference in water absorption between the heaviest and lightest birds being 2·3% (in favour of the heavier birds). It has also been found that the time of immersion in the first tank (the washer) significantly increased the amount of extraneous water taken up and retained (Grey & Jones unpublished results: Table 12). Pavlus & Szentkuti (1970) reported that water uptake in the washer increased with increasing temperature. Klein & Takla (1975) found that doubling the time of immersion in the washer and chiller resulted in an increase in water absorption of 2%. However, they also noted that increasing the drainage (drip) time considerably reduced the amount of water retained. Thus, rather than increase the drip-time some commercial processors have now installed a slowly rotating, serrated drum, down which the carcasses slide after coming out of the chiller. This efficiently reduces the amount of water retained and helps the processor to meet the extraneous water-content limit (which will be discussed overleaf).

TABLE 12 EFFECf OF PASSING BIRDS TWICE THROUGH THE FIRST CHILLER UNIT ON WATER

UPTAKE AND RETENTION

Once through first chiller Twice through first chillera

Initial Weight Chill Initial Weight Chillb

weight gain time weight gain time (g) (%) (min) (g) (%) (min)

1359 4·6 29 1521 11·9 40 1502 7·1 32 1390 7·1 43 1788 5·0 45 1670 10·9 36 1695 5·1 29 1700 6·8 40 1467 6·0 32 1649 13·8 43 1294 9·6 38 1555 9·9 39 1492 5·2 39 1672 8·8 41 1349 9·6 39 1437 9·1 41 1510 5·1 35 1781 8·3 34 1514 7·1 34 1507 6·7 38

Means 1497 6·4 35 Means 1588 9·3 40

a All carcasses proceeded through the complete chilling and drainage system, except that in this case they were removed from the exit of the first chiller and returned to the same tank at the entry point. b Total time through chiller units. Grey & Jones (unpublished results.)

5.3. Control of Extraneous Water in the EEC Council Regulation No. 2967/76 (a) governs the control of water uptake and retention by the carcass during chilling and immediately prior to freezing, and (b) defines methods for estimating extraneous water in the fro~en product, using a thaw test and/or chemical methods. At present, the regulation, and amendments, are only applicable to frozen or deep-frozen chickens, hens or cocks; they involve the following:

The In-plant Method (Annexe I) Enforcement of EEC regulations in the UK is provided by the Poultry Meat (Water Content) Regulations (Anon., 1984a). Annexe I of the EEC Regulation requires producers of frozen chicken, hens and cocks to carry out a test at least once during each working period of 4 h. The test involves a random selection of 25 carcasses, at a point immediately before the final spray-washer, where the birds are tagged, weighed and returned to the line to proceed through the washing and chilling

procedures. The birds are finally removed at the end of the drip-line and the total weight of the first 20 carcasses out of the chiller is used to determine the moisture uptake by subtracting their total weight at the first weighing point. The increase should not exceed 5% of the initial weight of the sample, or any other figure allowing compliance with the total permitted extraneous water content of 7-4%. The main problem associated with this type of test has been that uptake of water by carcasses prior to chilling can be as high as 6%, and no account of this is taken in the test. Uptake can also be varied by the number of spray-washers used during different stages of evisceration (Bolder, 1975).

Once the carcass has been bagged, frozen and transported to any other EEC country, the regulatory authorities there can remove birds for the determination of extraneous water. The first method that can be used for this purpose is the thaw test.

The Thaw Test (Annexe II) This is the Rapid Detection Method which involves thawing carcasses plus offal in a polythene bag for a specific period of time, according to weight, at 42 ± 2°e. The average moisture loss on thawing is determined for 20 carcasses and expressed as a percentage by weight of the frozen or deep-frozen carcass including offal. If the figure obtained is greater than 5·2%, it is highly likely that the amount of water absorbed during processing has exceeded the limit value. Should this be the case, then a chemical analysis would be applicable, in accordance with the methods described in Annexes III or IV of the regulations. However, the chances of samples failing the Annexe II test are minimal, even after allowing for the presence of offal (Jonas, 1981). Daelman & van Hoof (1975) compared the different methods for determining water uptake during chilling and obtained a linear correlation coefficient of 0·82 between uptake and drip-volume; they further found that an uptake of 8% was equivalent to a drip-volume of 6% and, when pre-chilling uptake was considered, the total extraneous water content was of the order of 10-11 %. No allowance was made for the effect of the presence of offal, which has a higher physiological water content and may therefore contribute significantly to the drip-volume. Erdtsieck (1975) has discussed the factors which influence drip; of these, rate of freezing seems to be the most important, and this has been confirmed by B0gh-Sorensen et al. (1983), using broilers selected for a specific water uptake of 5 %. It was concluded that slow freezing results in an increase in drip, although differences

between the freezing procedures used were small. The injection of polyphosphate into the musculature was found by van Hoof & Daelman (1975) to affect the drip-loss from frozen and thawed carcasses and, in this case, the amount of extraneous water could not be reliably assessed by use of the drip method. This was confirmed for polyphosphate by Grey et al. (1978b) and for polyphosphate and salt injections by Jones et al. (1980). The effects of various injection-doses of polyphosphate on water absorption and thaw-loss are shown in Table 13. It was suggested by van Hoof & Daelman (1975) that the detection of polyphosphate could be carried out on the drip-water, using the total inorganic phosphate content. Grey et al. (1977, 1979) evolved a specific and reliable method for detecting the presence of both polyphosphate and sodium chloride in breast muscle.

Enforcement authorities in some EEC countries have decided not to use the thaw test but to directly apply the Annexe III method which will be discussed below. The thaw test was introduced mainly to give a rapid indication of the extraneous water content of the carcass, but the relationship between thaw-loss and total extraneous water is highly variable. However, if the result of the thaw test gives a loss greater than 5·2% of the frozen carcass weight, including offal, it is likely that an excessive amount of water has been absorbed and a chemical method of determination needs to be carried out on the same batch of frozen birds.

Chemical Method (Annexe III) This is based on a method developed by Jul et al. (1974), which utilises the relationship between protein and water in the carcass. An evaluation of the method within the Community (EEC, 1976) and subsequent studies by Scholtyssek et al. (1979), Ehinger & Thomson (1980) and Jonas (1981) have established that the method is practicable, although it has limitations in that the chances of failing the test can be affected by both carcass weight and the inclusion of offal. In this method, seven frozen carcasses, including offal, are analysed either individually or together by thoroughly mincing until an homogeneous sample is obtained. Protein and moisture determinations are carried out, and the total weight of protein (g) is then inserted in a limit regression equation. The result is compared with the determined weight of total moisture in the carcass or carcasses, and if the determined moisture exceeds the figure obtained previously, then the batch of birds has failed the test. The regression equation allows for a

-------------

---------

limit of 7·4% extraneous water in the whole carcass, and it should be noted that this figure also includes water absorbed prior to chilling. If the limit value is exceeded, the holder of the poultry can request that a counter analysis be carried out, using the same method. In the event that the carcasses fail the test, then the packaged birds must carry a label stating 'Water content exceeds the EEC limit'.

Chemical Method (Annexe IV) The method was developed by Woltersdorf (1971), who found that with dry-processed birds, the ratio of water content to fat-free dry matter (FFDM) in the edible parts of the carcass was constant. However, the constant used resulted in an overestimate of water content, and subsequently a regression equation was introduced for determination of the physiological water content. The method requires the carcass to be thawed and the thaw-loss recorded; the carcass is then de boned and the edible parts i.e. carcass meat, including meat from the neck, without bones or cartilage, with or without giblets, are then combined and frozen. The edible parts are then cut into 3 mm discs and minced three times to obtain an homogeneous mixture, from which samples are taken for water and fat analyses. Total water and fat values are then inserted in regression equations given in the Annexe. The physiological water content is used to calculate the amount of extraneous water which, with this method, should not exceed 6%. Annexe IV is quite laborious and time-consuming, and tends to give results which are more variable than those from the Annexe III method, particularly between different laboratories. This may be due to the difficulty of obtaining reproducible dissection procedures between laboratories. In addition, regression equations are also given in both annexes to cover birds which have been labelled as 'dry chilled poultry'. Neither method is ideal, but most laboratories opt for the Annexe III method because of the relative ease with which the determination can be carried out and the more consistent results obtained.

Even if washing and chilling procedures were defined, as in the USA, the result obtained with the in-plant method (Annexe I) may not necessarily be confirmed by the other tests because of variables such as spray-water pressure and degree of agitation of carcasses in the chilling system. Nevertheless, if the regulations are to succeed,

pre-chill water absorption and retention should be controlled just as much as water retention during the chilling stages which are covered by the Annexe I test.

6. ELECTRICAL STIMULATION

Considerable research has been carried out on the theory and application of electrical stimulation to red meat, and this has recently been reviewed by Chrystall & Devine (1986). The purpose of electrical stimulation is to speed up the development of rigor mortis, thus increasing the rate of tenderisation during subsequent 'ageing' at chill temperatures, and to avoid the phenomenon of 'cold shortening' encountered in some types of meat. With poultry, the bird-to-bird variation in muscle texture may be due to the response of the birds to both ante- and peri-slaughter treatments. Therefore, it would seem logical to expect electrical stimulation to result in a more uniform and acceptable product.

Jenson et al. (1979) applied electrical stimulation to broilers, using equipment that was capable of applying a variable pulse rate (1·25-50 pulses/s), at any voltage up to 800 V. Stimulation was applied for 1 min after bleeding (3 min post mortem), or after scalding and plucking (6 min post mortem). The overall conclusions were that electrical stimulation, although lowering the muscle pH value, did not have any clear effect on the ultimate texture of the cooked meat. In addition, there seemed to be no consistent trend in the results obtained. Lockyer & Dransfield (1986) applied low voltage (94 V) electrical stimulation to broilers immediately after bleeding, in a commercial processing plant. The broilers were subsequently airchilled and frozen, either immediately after chilling or after overnight 'ageing' at chill temperature. The electrically stimulated birds were generally tougher than the non-stimulated ones, even after overnight 'ageing'. The same study examined the effects of low-voltage electrical stimulation on commercially processed turkeys of ca 3·5 kg liveweight. Stimulation slightly reduced toughness in immersion-chilled turkeys but increased the variation in tenderness of air-chilled birds, when compared with non-stimulated, air-chilled controls. Air-chilled birds were held in air overnight and spin-chilled birds in ice-slush for a similar period, before the breast muscles were removed and frozen.

Thompson et al. (1987) found that low-voltage stimulation (45 V) for nine or 18 s significantly reduced the shear values of hot-deboned broiler breast fillets, after overnight 'ageing' in crushed ice, freezing and ultimately thawing for 24 h at 7°C. Despite this treatment, all the fillets were still tough (mean shear value 9·1 kg force/g sample), compared with non-stimulated hot-deboned controls (shear value 11·9 kg force/g). Increasing the voltage had no significant effect on the tenderness of hot-boned or 'aged'-and-boned fillets. However, highvoltage stimulation (820 V) after bleeding significantly decreased the shear values of fillets removed after chilling for 45 min under simulated commercial conditions and then stored in crushed ice for 48 h, drained and frozen. The mean shear value for this treatment was 6·5 kg force/g, compared with the control mean of 9·4 kg force/g. It was concluded that the 820 V stimulation was more closely related to myofibrillar disruption and increased sarcomere length than to any change in pH value or ATP disappearance. It should be noted that the mean shear value of fillets from the 820 V-treatment was still considerably higher than that obtained from non-stimulated carcasses, from which fillets were removed after 'ageing' for 48 h in crushed ice, viz. 6·5 kg force/g, compared with 3·6 kg force/g. Within the de boning times studied, electrical stimulation had no significant effect on water uptake, drip-loss, cooking loss or moisture content of hot-boned, chill-boned or 'aged' -and-boned fillets.

Maki & Froning (1987) used high-voltage stimulation on 15 turkeys, with a Cervin FS stunner set at the highest setting, 800 V, 340 rnA. Stimulation after bleeding was applied for 4 s duration, followed by 5 s rest, until a total of 36 s stimulation had been given. After evisceration, all the birds were chilled overnight and frozen at -29°C. Fifteen control turkeys were treated similarly except that they were not stimulated. A statistically significant reduction in Warner-Bratzler mean shear value was found for the breast meat of electrically stimulated birds, i.e. 1·93 kg/2 cm core, compared with the control value of 2·74 kg/2cm core. A small but significant increase in the mean Hunterlab aL tristimulus value was obtained for the stimulated group, indicating an increase in redness of the breast muscle.

A recent study at the Institute of Food Research, Bristol (Grey, unpublished results), using low-voltage (94 V) stimulation, has also shown that hot-boning of broilers and subsequent 'ageing' of the deboned breast meat in an air-chiller at + 1°C overnight failed to yield more tender meat, shear values being considerably higher than those

Peak force (kg)

o hour 1 hour 2 hours 4 hours 24 hours

Chill time

~ Control

_ Stimulated

FIG. 3. Variation in shear force with 'ageing' time after muscle removal 2·5 h post mortem. Birds electrically stimulated for 3 min; into chill at 20 min for ca

2·5 h. Breast removed and stored at 1·5°C for varying lengths of time.

obtained from muscles aged on the bird for a similar period i.e. 6·65 kg force compared with 1·73 kg force. If, however, a delay was introduced before deboning, the breast fillets continued to tenderise during subsequent storage at + 1°C in air. For example, following normal processing procedures, during which electrical stimulation was applied after bleeding, carcasses were held at chill temperature until 2·5 h post mortem and then deboned. Samples were also taken at intervals after this time, as shown in Fig. 3. Ten stimulated and ten non-stimulated broilers were compared at each time interval, all the birds being from the same flock and ultimately cooked from chill temperature to an internal muscle temperature of 90°C. Although at 24 h post mortem both groups of muscles had tenderised, the electrically stimulated muscles were significantly more tender than the control. Also, the variation between birds was markedly reduced in the electrically stimulated muscles (Fig. 4). Hence, the delay in deboning had led to a fall in meat temperature and at the same time allowed the muscles to go into rigor whilst still on the carcass, thus reducing the shortening effect obtained as a result of hot-deboning. Electrical stimulation had hastened both the onset of rigor and the initial rate of tenderisation, which subsequently slowed during overnight 'ageing' at chill temperature. To be a viable proposition for the poultry industry, the ideal time to debone broilers would be after the

g 3 :;; " .:; .. 'tI

; 2 'tI c:

" ~ '0 III c: " .. l:

J. M. Jones and T. C. Grey

o Electrically stimulated

• Control

HOlding time Ch) 24

FIG. 4. Mean values for standard deviation of texture measurement (kg force).

normal chilling period at ca 1·5 h post mortem, rather than having to 'age' the carcasses for 6-24 h.

Further research will no doubt ascertain whether electrical stimulation and deboning immediately after chilling will produce a uniformly tender product that will continue to 'age' during chill-storage, packaging and transportation to retail outlets.

7. IMPACT OF LEGISLATION ON PROCESS CONTROL

In view of the current concern with animal welfare and product quality, it is not surprising to find a number of countries with a great deal of legislation relating both to the treatment of poultry at the time of slaughter and the control of operations involved in carcass production. Much of this legislation can influence the quality attributes discussed in this and other chapters.

For example, directives of the EEC are directly reflected in the UK poultry hygiene regulations (Anon., 1976, 1979) which govern both carcass preparation and inspection. In the USA, extensive legislation relating to the inspection of poultry is described in the Code of Federal Regulations (US Dept of Agriculture, 1985).

In the UK, the Slaughter of Poultry (Humane Conditions) Regulations 1984 (Anon., 1984b) specify the maximum times for which domestic fowl and turkeys may be suspended from shackles prior to stunning (3 and 6 min respectively), as well as the minimum time which must elapse between neck cutting and scalding or plucking (90 s for domestic fowl; 2 min for turkeys). Although the regulations apply to other species such as ducks, geese and guinea fowl, no times for hanging or bleeding are specified for those birds.

Legislation in the USA demands that poultry shall be slaughtered in accordance with good commercial practice, which results in thorough bleeding of the carcass and ensures that breathing has stopped prior to scalding. As in the UK, any bird not bled properly before being scalded will be condemned as unfit for human consumption. The same applies to over-scalded carcasses, where the flesh appears to be cooked.

Both UK and US legislation demand that each plucked poultry carcass shall be opened in such a way that the cavity and viscera may be readily inspected. However, in the USA, the type of opening cut is considered to be related to the chilling procedure, and hence any modification of the cut requires a new procedure to be established to the satisfaction of the official inspectorate. Federal regulations also state that opening cuts shall be made in a manner which ensures that the skin between rib cage and thigh is not cut or torn during evisceration. In particular, the use of the 'bar cut' requires the permission of the plant inspector.

UK regulations (Anon., 1979) specify the volumes of water to be used for spray-washing eviscerated carcasses, the type of system (counter-flow) to be used for immersion-chilling, the volume of water to be used per carcass during chilling (based on weight), inlet and outlet temperatures, as well as the maximum length of time a carcass may reside in the first stage of the chilling system. Although the carcasses should be brought to a temperature of 4°C as quickly as is practicable, the hygiene regulations do not state that this has to be achieved in the immersion-chilling system (see also Sub-section 5.2).

In some cases, eviscerated poultry may be portioned without prior chilling, thus, perhaps, allowing for the use of the 'hot' deboning method mentioned earlier. American legislation is generally the same as that in Europe in that, after evisceration, the carcass must be chilled immediately. In this case, the internal temperature of the bird must reach 4·4°C or below in a time commensurate with carcass weight. For

TABLE 14 RELATIONSHIP OF CARCASS SIZE TO MINIMUM WATER USAGE IN

CONTINUOUS CHILLING SYSTEMS

United Kingdom a Canada b USA c

Carcass wt Water Carcass wt Water Species Water (kg) (lUres) (kg) (litres) (litres)

<2·5 2·5 <2·5 2·0 per fryerd 2·27 2·5-5 4·0 2·5-6·5 2·75 >5·0 6·0 >6·5 3·5 per turkey 4·54

Maximum permitted temperature at warmest point in system eC) 16 18 18·3

a Anon. (1979). b Agriculture Canada (1986). C US Dept. of Agriculture (1985). d A young chicken generally under 13 weeks of age.

instance, with carcasses weighing under 1·82 kg, the time is 4 h: at 3·63 kg or more, the elapsed time must not be greater than 8 h. There is no requirement for the counter-flow type of immersion chilling but, where continuous chillers are used, the water usage in the chillers is related to the type of bird, and the agitation in the chiller must cease when the equipment is stopped. As shown in Table 14, Canadian regulations also specify water usage requirements in the case of continuous chillers. In addition, the regulations stipulate a carcass temperature of 4°C or lower, although they do allow for carcasses to be packed before the 4°C target has been achieved, providing that the poultry is frozen immediately after packing (Agriculture Canada, 1986).

The situation in the EEC Member States regarding the control and estimation of water taken up by chicken carcasses destined for the frozen market, has been discussed in Section 5.

Briefly, in the case of water-chilled birds, the total permitted amount of extraneous water is 7·4%, of which approximately 5·5% may be taken up during immersion chilling while, with the so-called 'dry' chilling method, the total uptake limit is 2·5%. There are no regulations in the EEC relating to the moisture content of frozen turkeys and ducks nor to that of unfrozen poultry.

In both the USA and Canada, the situation regarding the moisture content of poultry is very different to that pertaining in Western

Europe. First, the water content is estimated solely by an 'in-plant' test, generally using 10 carcasses per test, and secondly, the maximum amount of water which may be retained after chilling is governed by species, carcass weight, type of marketing presentation ('ice-pack', chilled and bagged, or frozen) and whether the carcass is to be cut up. For example, while chicken carcasses in bags may retain 80 g water/kg after chilling, similar birds destined for 'ice-packs' may retain 120 g/kg. However, the loss of moisture from the 'ice-packed' carcasses during holding and transportation must be such that, on arrival at their first destination, the carcasses comply with the relevant lower moisture-retention limit.

An even greater contrast between European and North American practices relating to moisture control, is the fact that in both the USA and Canada the government inspector at the processing plant will retain those carcasses in which moisture uptake exceeds the specified limits. Such retention remains in force until further 'in-plant' tests indicate that compliance has been achieved, either by allowing extra drainage time or by removal of trapped water.

8. CONCLUSIONS

The quality of poultry meat can be affected markedly by the processing techniques applied in converting the live bird into the finished product. This is particularly apparent with regard to meat tenderness and in the case of water-uptake control.

There is a need for further work on the effects of various techniques for extending the shelf-life of chilled poultry and on sensory properties, especially flavour. Also, there is a need to establish an accurate means of determining product shelf-life.

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