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CHAPTER 1FREEZING OF MEAT1.1. THE PROCESS OF FREEZING1.1.1. Thermal aspectsMeat contains about 70 % by weight of water. The process of freezing converts most of the water into ice. Water in meat contains dissolved and colloidal substances, which depress the freezing point below 0C ; the extent of freezing point depression is proportional to the concentration of the solutes. A typical freezing point of meat is -1C to -2C. During freezing, water is gradually converted to ice and the concentration of dissolved organic and inorganic salts increases, depressing the freezing point continuously. Even at a temperature of -25C, only 90 to 95 % of the water is actually frozen. This does not include bound water (i.e. water chemically bound to specific sites such as carbonyl and amino groups of proteins, and hydrogen binding), as it is never available for freezing. Nevertheless, the largest part of of the water (about 75 to 80 %) is frozen between -1C and -5C. This temperature range is known as the critical or freezing zone.During the first stage of cooling the temperature falls fairly rapidly to just below 0C, the freezing point of water. As more heat requires to be extracted during the second stage, in order to turn the bulk of the water to ice, the temperature changes very little and this stage is known as the period of "thermal arrest". When about three quarters of the water is turned to ice, the temperature again begins to fall and during this third stage most of the remaining water freezes. A comparatively small amount of heat has to be removed during this third stage. (Fig. 1)

Fig.1 Temperature-time graph for meat during freezing9

1.1.2. Crystallization of water in muscle tissueThe crystallization of water exhibits two phenomena : the formation of crystal nuclei or nucleation, and the growth of the crystals originating from these nuclei. The local duration of crystallization (LDC), defined as the time taken for the temperature to fall from -1C to -7C, has a considerable influence on both these phenomena.The appearance of crystal nuclei requires a supercooling effect, which is greater in the case of a pure substance than for animal tissues. In the latter the model structure necessary for the formation of the nucleus is present already either in the form of impurities possessing the same crystal habit as ice or else in the form of water absorbed onto a substrate. The number of nuclei formed per unit of time and temperature increases with the magnitude of the supercooling effect.The concentration of extracellular liquid is lower than that of intracellular liquid. For this reason the supercooling effect is more pronounced on the extracellular space, and the crystal nuclei form preferentially in this environment. However, if the intensity of cooling is great enough, intracellular nucleation can take place because the development of the extracellular crystals is not fast enough to prevent the temperature dropping further in the interior of the cells.The growth of ice crystals occurs from the crystal nuclei. The temperature of these tends toward the equilibrium temperature of the ice-solution system although they are immersed in a supercooled liquid. This leads to crystallization in the form of dendrites or needles. The rate of growth and the final size of the crystals depend on the speed with which the heat is extracted.1.1.3. Freezing equipmentThere are three basic methods available for freezing meat :a)air blast freezing - where a continuous stream of cold air is passed over the product

b)plate or contact freezing - where the product is placed in direct contact with hollow, metal,freezer plates, through which a cold fluid is passed ; and

c)spray or immersion freezing - where the product is placed in direct contact with a fluidrefrigerant (limited applications).9

1.1.4. Effect of freezing rate on quality1.1.4.1. Microbiological qualityDuring freezing the cellular integrity of a portion of the microbial population is impaired. Those cells which suffer the least damage afterwards recover their initial metabolic properties, while the others die. "Slow" freezing favours this destructive action since the increase in ionic strength of the liquid phase, which is considerable between-2C and -7 C, is the cause of the denaturation reactions affecting the proteins of the cellular membranes and enzymes.The micro-organisms can be divided into three groups according to their sensitivity to the destructive action of freezing ; increasing order of sensitivity these are : (1) spores of the genera Clostridium and Bacillus as well as vegetative forms of Micrococcus, Staphylococcus and Streptococcus ; (2) a group composed chiefly of Gram-positive bacteria, such as Staphylococcus aureus, responsible for food poisoning ; (3) the most sensitive group, consisting of the majority of Gram-negative bacteria, such as Enterobacteriaceae and Pseudomonas organisms.The degree of inactivation is always very slight (about 0.5-1.0 log10 reductions).In industrial operations the rate of freezing is not susceptible to variations of sufficient magnitude to enable the process to be considered as exerting a "pasteurizing" effect.1.1.4.2. Nutritional qualityThe extent of probable denaturation of protein does not exhibit any repercussions on the nutritional quality of the meat. No significant difference in the degree of retention of B vitamins as a function of the rate of freezing has been demonstrated. Iron, for which meat supplies around 50 % of the nutritional requirement for humans, is also unaffected by the freezing process.1.1.4.3. Organoleptic qualityFlavour is unaffected by the rate of freezing.In a general fashion, the rate of freezing has no effect on the colour of the meat assessed after thawing.On the other hand, the appearance of the frozen product, which is of obvious commercial importance, is determined in part by the rate of local formation of ice crystals at the surface of the product. When this is high the small crystals which ensue exhibit an intense reflection of the incident light, which imparts a colour to the product ranging in shade from pale9

to bleached.It appears fairly definite that for pork, mutton, beef and poultry meat the tenderness is independent of the rate of freezing. Some authors, on the contrary, state that rapid freezing improves the tenderness of the product ; this can be explained, however, by the fact that the preparation of the meat before freezing has a determining effect on its tenderness.Freezing in advance of rigor mortis, that is while the muscles still contain ATP, gives rise, on thawing, especially for beef and mutton, to a very severe contraction called "thaw rigor", which brings about not merely an appreciable toughening but also a substantial loss of fluid. This phenomenon is especially marked on cooking straight from the frozen state. It can be limited or even avoided by encouraging the hydrolysis of ATP, either by an intermediate holding step of two to three weeks at -12C of by means of "slow" thawing.Another modification of tenderness, the so called "cold-shortening" effect can also be exhibited in the course of rapid freezing of muscles before the onset of rigor mortis. This toughening, once it has occurred, cannot be suppressed by any form of technological treatment. Hence, it is preferable to freeze only meat containing negligible amounts of ATP.1.1.4.4. Drip loss on thawingThe influence of the rate of freezing on the magnitude of the drip loss on thawing has been the subject of a considerable number of studies, the results of which are contradictory.1.2. STORAGE OF FROZEN PRODUCTSIn the course of their storage along the cold distribution chain, frozen products are liable to suffer certain modifying reactions : microbial development, physical changes, and biochemical reactions. These are the phenomena which mainly determine the life of frozen products, rather than the rate of freezing.Even though the severity of the changes depends both on the temperature and the activity of the water in the product, only the temperature is usually taken into account. This is explained by the fact that, for a given product, the water activity is fixed by the amount of water converted into ice, which is itself dependent on the temperature. (table 1)9

Table 1.Activity of water in meat as a function of temperatureTemp.(C)-1-5-10-15-20-25-30Aw0.9900.9530.9070.8640.8230.7840.746

1.2.1. Physical changes during storage1.2.1.1. Loss of waterIn the course of frozen storage the ice situated on the outer surface of the product sublimes, giving rise to an irreversible superficial dehydration effect which adversely affects the appearance of the product and is also conductive to oxidative reactions. This loss of water is assisted by forced air circulation. It becomes more important as the temperature is raised because the partial pressure of water vapour at the surface of the product increases with temperature. Packaging of meat in packing films with low permeability for water vapour and gas reduces those phenomena.The loss of water is aggravated by temperature fluctuations, when hoar frost deposits inside the wrapping. When the temperature of storage decreases, the ice sublimes from the surface of the product in the form of vapour, which deposits as hoar frost on the colder internal surfaces of the wrapping. During a rise in the temperature of storage, the opposite effect occurs, and the hoar frost is formed on the surface of the product. This water is not wholly reabsorbed by the product, and as the variation in temperature is repeated the mass of frost produced increases and can represent a significant loss. This phenomena is particularly marked in retail cold-storage cabinets.1.2.1.2. RecrystallizationThe modification of the number, size and shape of ice crystals is referred to as recrystallization.Several mechanisms have been put forward to explain the passage of water molecules from one crystal to another. All these take into account the fact that the vapour pressure at the surface of a crystal increases with the curvature of the surface. This gives rise to a diffusion of water vapour in the direction of the more lightly curved zones and an evolution of the crystal habit toward a practically spherical pattern. Likewise the small crystals tend to disappear in favour of the larger ones. These phenomena take place more rapidly as the temperature is raised.9

The heterogeneity of the temperature within the product due to fluctuations in the ambient temperature is another cause of recrystallization. A temperature gradient induces a vapour pressure gradient in the same direction, and as a result a diffusion of water molecules from the warmer to the colder regions occurs.1.2.2. Hygienic and nutritional qualityMeat is sometimes contaminated with parasites which can be killed by an adequate period of storage at subzero temperatures. The cysticerci of Taenia saginata (beef) and Taenia solium (pork) are killed by three weeks' storage at -7 C. The regulations indicate that a joint of beef which is slightly infested is sanitized after ten days at -18C. The destruction of Trichinella spiralis larvae, however, requires temperatures below -18C ; 20 days at -18C is sufficient to avoid any risk to the consumer.The metabolic activity of the entire pathogenic bacterial flora is halted as soon as the temperature falls to 0C. Temperatures below -10C or -12C inhibit the growth of all bacteria and of the majority of yeasts and moulds, respectively. Storage at -18C can thus be regarded as an ideal method for stabilizing the microbial flora. Moreover, from the commencement of storage a certain proportion of this flora is destroyed.1.2.3. Organoleptic quality1.2.3.1. Texture and juicinessThe tenderness of meat after cooking decreases and dryness increases by progressive denaturation of myofibrillar proteins and by an increase in the insolubility of collagen in the course of the storage.1.2.3.2. ColourThe occurrence of freezer burn, which manifests itself in the form of brown patches of unappetizing appearance on the surface of red meat, is the evidence of surface dehydration, the mechanism of which has already been described. This problem may be counteracted by wrapping the product before freezing with an impermeable shrinkable film which acts as a barrier to the passage of water vapour.The haem pigment chiefly responsible for the colour of meat is myoglobin. Fig.2 indicates the colour assumed by the product as a function of the state of this pigment.9

Oxygenation

Reduced myoglobin Fe2+ (reddish-purple)

Oxymyoglobin Fe2+ (bright red)

ReductionOxidationOxidationReductionMetmyoglobin Fe3 + (brown)

Fig.2 The different oxidation/reduction states of myoglobin. After Renerre (1982)The oxidation of myoglobin to metmyoglobin should be avoided, as it imparts a brown colour to red meat which is a principal cause of consumer dissatisfaction. For certain fish such as haddock and whiting it gives rise to a grey discoloration. The temperature has a decisive influence on the rate of oxidation. It is at its maximum around -12C, then decreases as the temperature falls. In the region between -5C and -15C it is four to six times as great as at - 18C. Light triggers a photo-oxidation change in the myoglobin which reduces the storage life. This is over 90 days for beef held at -18C in the dark, but only 10 days under illumination at an intensity of 100 lux. There are wide differences in sensitivity, depending on the type of meat. In particular, the myoglobin of adult beef animals is more resistant to freezing than that of young bulls. For the latter, the rapid deterioration of colour constitutes the limiting factor for storage life.Lipolysis has a greying effect on the fat, and the oxidation of free fatty acids imparts a yellow colour. Mincing accentuates the latter reaction because the haem pigments of the lean meat which catalyse oxidative reactions are brought into intimate contact with the fat.1.2.3.3. FlavourDuring frozen storage, the activity of phosphatase is not totally inhibited. As a result one can observe a progressive fall in the acid-soluble nucleotides, in particular inosinic acid, the role of which as a flavour enhancer is well known.Lipids are the components chiefly responsible for the flavour of fresh meat. The breakdown of fatty material is an extremely important phenomenon, because it is this which9

limits the storage life of frozen meat in the majority of cases. Two successive reactions occur in the development of neutral tastes and odours which in extreme cases can lead to rancidity, namely lipolysis and oxidation of the free fatty acids.(a)Lipolysis

Lipolysis is an enzymatic reaction which hydrolyses the glycerides on the one hand and the phospholipids on the other, with the release of free fatty acid. This reaction can take place owing to the action of lipases of endogenous or microbial origin. If the latter have arisen before freezing, they are capable of acting during the period of storage. The kinetics of the lipolysis reaction, which can be inferred from determinations of the free fatty acids over a period, are governed by a logarithmic function of the time. The rate of reaction at -10C is from 15 to 25 times as great as at -20C, at which temperature it is still measurable.(a)Oxidation Oxidation of free fatty acids is an autocatalytic process.

- InitiationenergyRH R (free radical)Fe and Cu- Propagation R + O2RO (peroxiradicals)RO2 + RHRO2H (hydroperoxide) + R- TerminationFormation of inactive compoundsRadicals, peroxiradicals and hydroperoxides are neutral compounds which can be transformed into compounds of low molecular weight such as aldehydes and ketones causing rancidity.Oxidation is severely retarded by fall in temperature but is still detectable at9

-20C. The principal oxidative mechanism is direct attack by oxygen ; hence, owing to the fall in partial pressure of oxygen within the package, a vacuum-conditioning step linked with the freezing process can significantly retard the alteration of flavour.The deterioration of lipids also affects the unsaturated fatty acids. Both the content and composition of lipids are thus decisive. In order of increasing sensitivity to these reactions, we find the following sequence : beef, lamb, pork, poultry, fish. Table 2 indicates the effects of temperature, storage period, and type of product on the degree of flavour modification.Table 2. Storage period in days after which a taste panel reported a drop in flavour. Where no deterioration was observed the duration is quoted in parenthesesProductHolding -5Ctemperature -10C-20CRump steak (beef)20230(320)Minced beef51120(320)Calves liver, frozen in the fresh state60165(320)Calves liver, frozen after cold storage2452170Sides of pork27105180Minced pork4179230Pork sausage22110240

1.2.4. Storage lifeThe storage life depends in large measure on the factors designated PPP, or product, processing and packaging. Product : the species, the breed, the sex, age, and diet of the animals and the type of muscle selected all govern the composition of the product, for example its content of unsaturated fats, and by extension its keepability. Processing : refrigerated storage before freezing which is sometimes necessary (for example, for maturation of meat) or is used (for want of a better way) for fish, encourages the production of bacterial lipases and decreases the storage life. The incorporation of additives in the products affected has various repercussions : for example, ascorbate, which noticeably increases the storage life, and sodium chloride, which has the opposite effect in the presence of air4141and in the absence of antioxidants. Mincing reduces the storage life. Cooking has various effects depending on the product. Packaging : it should be remarked that vacuum packaging with the aid of a plastic film impermeable to water vapour is of considerable benefit in extending the storage life of those products which are susceptible to rancidity.- Calculation of the actual storage lifeStorage life is generally expressed in time at a constant temperature. In practice there are various phases between production and consumption during which temperature can vary : frozen, transport, distribution. This means that the storage life will be changed ( = actual storage life).The calculation proceeds stepwise as follows :a)Determination of storage life in function of the storage t (Fig.3).

Fig.3Storage life of chicken portions packed in polyethylene film in function offrozen storage temperaturea)Calculation of the storage life at fluctuating temperature during frozen storage, transportand distribution with the aid of the curve given in figure 3 (table 3).

41Table 3. Calculation of percentage value of acceptable loss for chicken portions packed in polyethylene filmPlaceHolding conditionsTimeTemp.Acceptable % per dayloss% period (Pj)Warehouse Transport Retail shop200 d 1 d60 d-24 C -15C -11C100/900=0.11100/400=0.25100/200=0.5Total22 0.253052.25

The loss percentage must be lower than 100 to consider the product as acceptable.1.3. THAWING1.3.1. Drip loss-exudationModification of the proteins and the cellular structure due to freezing causes a reduction in the capacity for retention of tissue fluid. During thawing, therefore, a part of the water resulting from melting of the ice crystals is not reabsorbed and gives rise to an exudate. This diminution of the strength of the bond between water and substrate also manifests itself on cooking, with a loss of juice greater by about 2 % than that which obtains in the case of fresh meat.In fact, it is impossible to state a priori what will be the loss of weight accompanying thawing, for it depends not only on exudation but also on various physical phenomena, linked to the method employed.1.3.2. Microbiological qualityThawing, of itself, brings about a certain destruction of the microbial flora during the rise in temperature between -7C and -1C. However, the surviving micro-organisms are in a position to resume their metabolic activity (multiplication and modification of the product) at the product surface as soon as it is thawed, and all the more rapidly as the temperature begins to rise. The abundant supply of nutrients (amino acids, peptides, mineral salts) in the exudate tends to promote this phenomenon.To avoid any development of mesophilic bacteria, some of which are pathogenic, it41is imperative to keep the temperature of the surface as low as possible.1.3.3. Industrial thawing processes1.3.3.1. Internal reheatingFor thawing by means of microwaves, the product is placed in a high-frequency alternating magnetic field, usually 2450 MHz ; the heat is then generated directly within the mass of the product as a result of molecular agitation induced by rapid changes in orientation of the electric dipoles. Thawing times are thus very brief. The penetration of the electromagnetic wave within the product is accompanied by an exponential decline in the strength of the signal in relation to the distance traversed. Thus the periphery of the product absorbs larger amounts of energy and heats up more rapidly than the centre. This heterogenicity is highly accentuated by the fact that local absorption of energy is increased as the proportion of water to ice increases, and is roughly ten times as great at -1C as at -10C.1.3.3.2. External reheatingIn order to thaw 1 kg of frozen meat from -20C, the same amount of calories is used as when meat is frozen to -20C. It is obvious that the thawing time is 2 to 3 times longer than the freezing time. This is due to the higher ability of heat transfer of frozen meat compared to that of fresh chilled meat. External reheating can be achieved with flowing water or in air.41CHAPTER 2COOKING OF MEAT2.1. TRANSFER OF HEAT2.1.1. Modes of heat transfer2.1.1.1. RadiationTransfer of heat by radiation takes place as electromagnetic radiation emitted by a hot body and absorbed by a cold body.2.1.1.2. ConvectionTransfer of heat via the flow of gas or fluid in the system.2.1.1.3. ConductionConduction tends toward an even temperature distribution within the medium, by transfer of energy from the zones where it is concentrated toward those where there is less. Meat can be considered as a solid in which only conduction takes part in the changes of temperature according to Fourier's law.2.1.2. Methods for cooking meatThe transfer of heat during cooking of meat has two distinct stages. First, energy is supplied to the surface, and then this energy penetrates into the meat. In practice it is rare of the energy to be supplied exclusively by a single method of heat transfer, though for each method of cooking there is a predominant mode of heating (table 4).As regards convection, the nature of the heating medium, for example water for boiled meats and the second stage of braising, steam for steam cooking, moist air for pot roasts, and molten fat or oil for frying. The rate of temperature rise depends on the mode of heating, the difference in the thermal capacity of the heating media, and especially the variable effect of the boundary layer.Radiation is used in grilling and spit roasting (the oven door is left open to avoid an accumulation of hot moist air around the meat), and conduction in the operation of sauteing and in the first phase of braising.41In certain special cases, heating is not effected by external heat transfer, but is achieved internally by the dissipation of energy within the body of the product to be heated. This is the case for heating by microwaves and direct ohmic heating.Table 4. Different modes of cooking meat, classified according to the principal mode of heat transfer and the nature of the heating mediumNature of heatingmediumConductionHeat transfer by ConvectionRadiationLiquid(aqueous medium)SteamMoist air Dry airMolten fatBraising (1st stage) Sautering, steakBoilingBraising (2nd stage) IncubationPot roastingFryingGrillingSpit roasting

2.2. LOSSES ON HEATINGWhenever meat is heated, a reduction in weight and volume due to loss of matter isobserved.Various factors are playing an important role.2.2.1. Temperature and duration of cookingNumerous authors have compared different cooking operations for different temperatures at the core of the sample, and have demonstrated an increase in the losses with temperature. For a given temperature at the core, however, the losses are influenced by the mode of heating and the temperature of the oven. The highest losses occur in moist heat rather than dry heat, this difference being due to crust formation with dry heat.Numerous authors consider it desirable to distinguish between loss of juice and losses due to evaporation. However, material balances for dry solids show that this is not feasible ; an appreciable part of the loss occurs as liquid, and the lost juice is immediately evaporated in the oven.With heating at constant temperature, the losses increase with the duration ofheating.412.2.2. Characteristics of meat2.2.2.1. Influence of zootechnical parametersThe absorptive capacity (which is different according to the species), species, sex, breeding methods, level of feeding and age have no influence on losses.2.2.2.2. Influence of the physical and chemical properties of the piece to be cookedThe only characteristic of meat for which the impact on losses on heating is certain is its pH. Variations in pH may be linked to conditions of slaughter, degree of maturation, or other treatment that the meat has undergone. The lower the pH the greater the losses will be due to the decreasing WBC.2.2.2.3. Technological treatmentsa)Slaughtering conditions

The conditions of slaughter can modify the quality of the meat ; in particular, how the carcasses are hung and refrigerated affects the length of the sarcomeres, depending on the muscles. Losses on heating increased with decreasing length of the sarcomeres, but the effect of this factor on heating losses still remains highly controversial. Electrical stimulation of the carcass causes hydrolysis of ATP and brings about a rapid fall in the pH of the tissues.a)Cooking process

Heat treatment under high pressure improves not only the tenderness as a result of the damage to the myofibrillar structure but also decreases the losses.c) Changes in pHChanges in pH can be brought about by immersion in different solutions ; the losses diminish as the pH moves away from the isoelectric point in either the acid or basic directions. The former case corresponds to a method of cooking now very rarely used, namely marinading. This method achieves not merely an increase in yield but also a marked increase in the tenderness of the cooked meat.41d) AdditivesThe use of various additives can likewise cause a reduction in losses on cooking. Those most often employed, after sodium chloride, are the polyphosphates. The action of polyphosphates is twofold ; they cause an increase in pH of the meat, this effect being accentuated by the presence of sodium chloride, which lowers the isoelectric pH of the meat by bonding of salt to the protein chains and by complexing the divalent ions present in the muscle they also allow expansion of the myofibrillar structure which can therefore retain more water.2.3. MODIFICATION OF CONSTITUENTS2.3.1. Modification of proteinsMuscle proteins can be divided into two groups according to their behaviour on heating : collagen, which is solubilized, and the myofibrillar and sarcoplasmic proteins, which become insoluble.2.3.1.1. Changes in collagenA native collagen fibre is practically inelastic. Heated in an aqueous medium it contracts sharply to about 75 % of its initial length, at a temperature between 55 and 70C, depending on the age of the animal.During prolonged heat treatment the solubility of the collagen rises both with the temperature and the duration of heating. The collagen denaturated by heat becomes more flexible, and it can distend by absorbing considerable amounts of water. Extended heating will result in the production of gelatin.2.3.1.2. Myofibrillar and sarcoplasmic proteinsThe solubility of myofibrillar proteins decreases abruptly between 55 and 60C ; the decrease for sarcoplasmic proteins is more gradual, beginning at 40C, and it is still incomplete at 100C.The SH content decreases as the temperature of heating increases and the meat becomes more reducing. There can even be, under drastic heating, some release of H2S, which may produce other effects such as corrosion of metal vessels.412.3.2. Physical and chemical properties2.3.2.1. pHDuring heating, the pH of the meat generally increases. A rise in pH is particularly noticeable up to 70C and the duration of heating is of minor importance. The initial pH of the meat is important : an increase in pH can be observed when that of the raw meat is below 6.4 and a fall when it is above this value. The basic phenomena exhibited in the rise in pH are poorly understood.2.3.2.2. Water holding capacityThe changes in the water holding capacity are chiefly due to tissue structure changes occurring in the course of the heating. Reduction in the water holding capacity becomes detectable above 40C and the most marked change occurs between 40C and 50C. The duration of heating has, at most, a 10 % influence on the loss of water holding capacity at 50- 60C ; at higher temperatures its effect becomes practically nil (Fig.4).

Fig.4Water-holding capacity and duration of heating for minced beef heated byconduction at different temperatures2.3.2.3. Structure and dimensionsThe dimensional changes in response to heating are of myofibrillar origin.412.4. QUANTITATIVE CHANGES ASSOCIATED WITH LOSS OF JUICEThe dry matter content of the lost juice varied with pH and showed a minimum for pH values of 5.0-5.5, when the quantity is at a maximum, which results in loss of dry matter being practically constant for a given set of heating conditions. These losses, though very rarely taken into account, are not negligible, and they can amount to 10 % of the initial solids in the case of ground meat and as much as 20 % for meat in its original state. The materials lost comprise lipids, for which abundant data are available, as well as minerals, proteins, and various dissolved substances.2.4.1. Loss of lipids(i)The lipid content of the lean tissue increases, and the samples lose an appreciable amount of surface fat.

(ii)After cooking both the proportion of phospholipids in the meat and the proportion of phospholipids relative to the neutral lipids increase in samples from the raw to the cooked state.

(iii)Linoleic and arachidonic acids are probably substrates for oxidative reactions.In the juice it appears, that the ratio of the content of unsaturated to that of saturated fatty acids in the lipids in the juice is identical to that in the neutral lipids of the meat, but differs from that of the phospholipids. As regards the nutritional aspect, heating increases the unsaturated/saturated fatty acid ratio.2.4.2. Loss of mineralsPotassium, sodium, and calcium were particularly mobile, while on the contrary phosphorus and magnesium remained in the meat with their concentrations unaltered, and that practically all the iron was immobilized at temperatures above 70C. The loss of sulphur was negligible, while the addition of sodium chloride to the cooking water (for stews) had no significant effect on the migration of the other mineral constituents.412.4.3. Loss of nitrogenous constituentsLosses which can reach up to 20 % can occur.A large part of the nitrogen which is soluble initially becomes insoluble after heating, as already indicated, or after leaving the meat, in the juice.It must be stressed that this fall in solubility is partly masked by the solubilization of collagen in the course of prolonged thermal treatments. This solubilization becomes appreciable when the losses on heating are practically terminated ; the outward diffusion of solubilized collagen remains low, and therefore the content of soluble nitrogen increases.In stewing, the nitrogen originally present in the meat culminated in the cooking broth. The addition of salt to the broth influenced only the solubility of the lost protein ; of the 23 % of proteinaceous nitrogen present in the broth, 11 % remained soluble in the presence of salt and only 9 % without it. Non-proteinaceous nitrogen (defined as non-precipitable by 12.5 % trichoroacetic acid) which makes up three quarters of the total nitrogen in the broth, is composed of 35 % amino acids and dipeptides, 15 % polypeptides, with the remaining 50 % being largely made up of nucleotides.2.5. COOKING AND MEAT QUALITY2.5.1. Nutritional valueNutritional value is influenced by the loss or concentration of certain nutrients (essential amino acids, essential fatty acids, vitamins, etc.) and by changes in digestibility.The cooking of meat causes few changes in its amino acid composition.Meat is an important source of B group vitamins ; the vitamin content, however, is extremely variable, depending on the species and the mode of preparation of these vitamins ; thiamin exhibits the poorest heat stability.2.5.2. Microbiological qualitySee course "Food Microbiology and Food Preservation" (Prof.dr.ir.J. Debevere).412.5.3. Organoleptic quality2.5.3.1. ColourThe colour of meat is due to a pigment, myoglobin, a haemoprotein possessing certain compositional analogies with the haemoglobin of blood, with which it associated in the transport of oxygen. Myoglobin can be found in three different chemical states : reduced myoglobin (Fe2+), oxy-myoglobin (Fe2+), and metmyoglobin (Fe3+).The proteinaceous constituents of myoglobins are denaturated at temperatures between 80 and 85C, but the colour of the meat is modified from 40C.The action of heat on myoglobin leads to the formation of what is conveniently called ferrihaemochrome. This action, depending on the temperature, brings about a progressive destruction of the various parts of the molecule affecting first the globin, by denaturation, then the cleavage of the haem-globin linkage, and finally denaturation of the haem nucleus.During the preparation of most products, the action of nitrite on the myoglobin leads to the formation of nitrosomyoglobin which on application of heat is converted to nitrosomyochromogen by cleavage of the Fe-globin linkage, denaturation of the globin, and the binding of a second NO group to the Fe.reduction

NO2 NO-NO + myoglobinnitrosomyoglobin(purple-red)(bright pink)heatnitrosomyoglobinnitrosohaemochrome(bright-pink)For cooking temperatures greater than 90C, caramelization of the sugars together with Maillard reactions between the reducing sugars and the amino acids also occur, alongside the development of the pigment, in the formation of the brown colour of the cooked meat.412.5.3.2. FlavourRaw meat has a weak flavour ; its intensity develops on cooking, and different methods lead to different types of flavour.The development of flavour takes place from precursors present in the raw meat, those present in the lean being the cause of the meat taste, and those in the fat conferring a taste specific to the animal species.In the course of cooking, the reactions that govern the flavour of the meatare :(a)Autooxidation, hydrolysis, dehydration, and decarboxylation of the lipid constituents, fatty acids, lactones, and ketones.Reactions affecting the carbohydrates and which yield degradation products of importance as flavouring agents.The Maillard reaction between sugars and amino acids which leads to a wide range of compounds.An important series of inter- and intra-molecular reactions attributable to the reactivity of ammonia, hydrogen sulphide, mercaptans, etc.

2.5.3.3. JuicinessThe juiciness of meat has two components : the first is the impression of moistness during the first few mastications, due to rapid release of fluid by the meat ; the second is that of sustained juiciness largely due to the stimulatory action of the fat on salivation.For the same muscle, juiciness decreases significantly as the temperature of cookingincreases.2.5.3.4. TendernessTenderness is defined in terms of a set of mechanical sensations perceived in the bucal cavity during mastication. The mechanical behaviour of the heated meat is the sum total of that of each of its constituent. In matured raw meat, collagen is the principal cause of toughness, while the myofibrils contribute only a very slight resistance. The strength of these fibres, however, increases very rapidly under the action of heat ; collagen becomes progressively solubilized. Toughness is primarily a function of the length of the sarcomeres ; too rapid freezingof the carcass after slaughter, leading to a contraction in the cold, results in tough meat after heating, a toughness which cannot be reversed by any other cooking process.41CHAPTER 3DRYINGDrying of meat is based on the principle of reducing the water activity (aw).Pwater vapour pressure of the solutionaw =Powater vapour pressure of wateraw x 100 = E.R.H. (equilibrium relative humidity)The notion of aw is extensively treated in the course "Food Microbiology and Food Preservation" (Prof. dr. ir. J. Debevere).The value of aw is a decisive factor for the progress of biochemical and chemical reactions as well as for microbial growth. Its lower limit may be taken as 0.25 for lard. It may obtain values between 0.4 and 0.6 for dehydrated meats, 0.75 and 0.90 for dry sausage ; 0.8 and 0.9 for cured ham, and finally values approaching 1.0 in fresh muscle or adipose tissue. (Fig.5)

Fig.5Rate of deterioration of foods as a function of water activity. Along thex-axis are indicated the customary range of aw for : (1) muscle and fresh adipose tissues ; (2) dried ham, distinguishing between lean (2a) and fat covered (2b) ; dried sausage ; (4) dehydrated meat ; (5) lard413.1. TECHNOLOGY OF DRYING3.1.1. The dryerThe drying of cured ham and sausage takes place in an enclosure which is air-conditioned so that ventilation, relative humidity, and temperature can be controlled. The air circulation inside the dryer is ensured either by natural convection, where air masses migrate from a heat source to a cold sink, or, more usually, by forced draught from blowers situated in a separate enclosure, linked to the first via conduits. Several types of equipment work in this way.Temperature control at the centre of the dryer is ensured by a temperature sensor, usually of the thermoelectric type, which actuates heating batteries or cooling units. The heating batteries consist either of finned tubes, inside which steam or hot water is circulated, or electrical resistors. The cooling units are either of the evaporative type or heat exchangers supplied with a glycol-based coolant.Control of humidity is effected by a sensor such as a hair hygrometer or psychrometer. Dehumidification is performed by passing the air containing the water vapour from the products being dried over a cooling battery.3.1.2. Removal of waterThe proportions of moisture in the raw materials, that is, muscle and adipose tissues, that make up dehydrated meat products, usually amount to 70-75 % and 5-25 % respectively. In dehydrated meats, the moisture content is only 5-15 % while for cured ham it may be about 30 % for small-size products, and 3 0-45 % for larger ones. Because of the variable amounts of lipid in the comminuted meat products, it is customary to introduce a parameter which is practically independent of the extent of drying. This is the moisture content of the defatted product, abbreviated HPD :100 x total moisture contentHPD =100 - % of fatThe removal of moisture from the foodstuff results from two simultaneous transport processes, the transfer of heat, which occurs by convection of ambient air toward the product, and the transfer of moisture in the opposite direction.The kinetics of drying for meat are illustrated in figure 6.

41

Fig.6Drying curve of meatThe drying curve is rectilinear in function of the time (CD) with usually an inflection beyond point D in the case of high dehydration. In this case, there are two processes : (a) a rapid one concerning the free water, and (b) a slower one which concerns a fraction of the bound water.The rate of drying depends on the conditions in the chamber, and also on the intrinsic properties of the foodstuff.3.1.2.1. pH of the meatThe water holding capacity of the meat is strongly affected by changes in pH. (Fig.7)41

Fig.7Effect of pH on the water-holding capacity of 'homogenates' of beef (5 days postmortem) pH values adjusted with solutions of NaOH and HCl.In the pH range of meat, this capacity is particularly high at high pH values. At low pH values, obtained by the addition of acid to homogenates of meat, the capacity is again very high. In both cases, this situation results from the large difference between the pH and the isoelectric point of the meat proteins. The isoelectric point is the pH at which the protein carries the least electric charge. Addition of salt to meat does not alter its pH (or at least only slightly), but it does affect the position of the isoelectric point of the meat proteins, considered as a whole, which falls by about one pH unit for a salt concentration of 2 %. Thus the action of the salt enhances the water holding capacity of the meat proteins, only just noticeably at very low values of pH, very much more markedly at higher values. (Fig.8)

Fig.8Effect of the addition of sodium chloride (2 % on weight of meat) on the waterholding capacity of beef (5 days post mortem) at different pH values413.1.2.2. Fat content of meatThe fat acts as a brake on the escape of moisture. Increase in fat content causes an upper limit to dehydration to be rapidly attained.3.1.2.3. Degree of unsaturation of the fatBeyond a certain limit of insaturation, drying is strongly impaired, and the water cannot be removed from the product because of a clogging effect associated with the presence of fat of low melting point. This clogging action is initiated during the operation of grinding.3.1.2.4. Degree of comminution3.2. APPLICATIONS3.2.1. Dried sausage3.2.1.1. Raw materials and ingredientsDried sausage is an uncooked fine meat product, which is fermented and dried and is composed of roughly 2/3 lean meat and 1/3 fat. The lean meat may be derived from different animal species, although pork and beef are most frequently used. The fat, generally pork fat, must be firm and non-oily, and back fat is usually employed. To these two principal constituents are added sodium chloride, nitrates and/or nitrites, sugars, spices or aromatic compounds, and lastly polyphosphates, ascorbic and lactic acids, colouring matter, and ferments.The first stage in the manufacture of this product is grinding ; this is performed at a low temperature, slightly above 0C for muscle tissue and slightly below for fatty tissue. Under these conditions, clean cutting of fat is possible and the formation of a pure or melting of the fat can be prevented. Grinding is most often performed simultaneously on these two primary materials, sometimes after preliminary salting, either on the two constituents in a chopped state, or else on the lean meat alone. It facilitates protein solubilization. The mixture of the lean, the fat, and the other ingredients then undergoes a final process referred to as mingling. The mix, is then stuffed in either natural gut or artificial casing material at a temperature of -4 C. A draining stage can follow, before stoving for a period of 1-5 days at 8 5-90 % relative humidity and temperatures in the range 20-28C.413.2.1.2. The ripeningThe ripening is achieved at a certain adjustable temperature and relative humidity, all or not combined with a smoking step. During this process microbiological, biochemical, chemical and physical changes will take place. These changes will determine the quality of the finished product.The aim of the ripening is twofold. Firstly a pH decrease must be obtained and secondly a decrease of the water content (aw). The decrease in pH can be obtained on a bacteriological or chemical way. The drop in pH through microbial activities is the result of lactic acid production from added carbohydrates (glucose, sucrose, lactose). The production of lactic acid from sugars is achieved by lactic acid bacteria which are normally present in meat or which can be added as starter cultures.Starter cultures for dried sausages are commercially available under frozen or lyophilised form and are a mixture of lactic acid producing and nitrate reducing micro-organisms (table 5).Table 5. Micro-organisms used as starter cultures

Fam. LactobacillaceaeFam. MicrococcaceaeStreptomyces griseus Debaryomyces spp. Penicillium spp.- Lactobacillus spp. - Pediococcus spp. - Micrococcus spp.- Staphylococcus spp.The production of lactic acid will also improve the flavour of the dried sausage.Heterofermentative fermentation processes will result in the production of undesired flavour compounds. Hence only homofermentative species are used in commercial starter cultures.Commercial starter cultures used for manufacturing dry sausages are usually a mixture of two or three microorganisms, consisting of one or two strains of lactic acid bacteria (Lactobacillus spp., Pediococcus spp.) and one strain of micrococci (Micro-coccus spp., Staphylococcus spp.) or a yeast Debaryomyces hansenii).Some types of dry sausage are inoculated at the surface with moulds in order to obtain a specific flavour.Lactic acid bacteriaLactobacillus spp.Most of the lactobacilleae that are applied in commercial starter cultures for meat products are homofermenters ; heterofermenters may be present in raw flesh as contaminators. The most frequently used lactobacillae in starter cultures are Lactobacillus plantarum, Lactobacillus sakei and Lactobacillus curvatus.The most important properties are given in table 6.Table 6. Some important properties of lactobacillae used in starter culturesCharacteristicLactobacilus plantarumLactobacilus sakeiLactobacilus curvatusFormrodsrodsrodsCO2 from glucose---Fermentation of

Glucose+++Saccharose++seldomLactose++seldomMaltose+-usuallyGluconic acid++-Starch---Mannitol+--D-ribose+++Lactic acid isomerDLDLDLAcetoin production+usuallyseldomNitrate reduction(a)+--Nitrite reduction(a)slight or -slight or --112O production (b)-usuallyusuallyBreakdown of arginine-usuallyusuallyGrowth 4Cslight or -++Growth 10C+++Growth 8 % NaCl+++Growth 10 % NaCl-slight or -slight(a) in media with little sugar and high p11 (b) in media with no added haemin.PediococciPediococci are all homofermenters. The most frequently used species are : Pediococcus pentosaceus, Pediococcus acidilactici. The specific properties are given in table 7.Table 7. Some important properties of pediococci used in starter cultures (a) in media with little sugar and high p11. (b) in media with no added haemin.CharacteristicP. pentosaceusP. acidilacticiFormcoccicocciCO2 from glucose--Fermentation of

Glucose++Saccharose++Lactose+-Maltose+-Gluconic acid--Starch--Mannitol--D-Ribose++Lactic acid isomerDLDLAcetoin production++Nitrate reduction (a)--Nitrite reduction (a)--112O production (b)--Breakdown of arginin++Growth 4C--Growth 10C+-Growth 8 % NaClslight or -+Growth 10 % NaCl-+41- MicrococciNext to lactic acid bacteria, Micrococcaceae are applied in fermentation of dry sausage, because of their catalase- and nitrate-reducing activity. Both the genus Staphylococcus and Micrococcus are used. The mutual differences between 2 genera are very small (table 8). The only difference lies in the fact that micrococci are aerobic and ferment glucose oxidatively, whereas Staphylococci are facultative anaerobic and form acid by anaerobic fermentation of glucose.The predominant representatives that are used in starter cultures are : Staphylococcus carnosus, Staphylococcus xylosus and Micrococcus varians.Table 8. Some properties of Micrococcaceae used in dry sausageCharacteristicS. carn osusS. xylosusM. variansAnaerobic growth with glucoseweak to +weakweakAerobic acid production from

Glucose+++Saccharose-+-Lactoseusually+weakMannitol++-Maltose-+-Nitrate reduction+++Acetoin production++-Gelatine splitting-weak-Growth at 15Cweak to +++Growth at 15 % NaCl+++41- YeastsIn certain specific cases, yeasts are applied. In starter cultures Debaryomyces hansenii is often added because of its high salt tolerance. Because they require oxygen in order to grow, yeasts only develop at the surface of the product.- MouldsFor the development of characteristic odours and flavours, yeasts can be inoculated on the surface of dry sausage. The strains that are available on the market are Penicillium nalgiovense and Penicillium chrysogenum.= FUNCTIONS- Lactic acid bacteriaLactic acid bacteria are mainly applied in fermentation of dried sausage, with different purposes.Lactic acid productionLactic acid bacteria in starter cultures have to be homofermenters, in other words, in accordance with the Embden-Meyerhof-Parnas pathway, glucose is converted into pyruvate, which again is converted into lactic acid and NAD by lactic acid hydrogenase and NADH2.Important is the rate of acid production and the value to which the pH can drop. The rate of acid production of a specific lactic acid bacterium is higher as a) there are more lactic acid bacteria initially present, b) the temperature is higher and c) the lactic acid bacteria can ferment the C-source faster. Monosaccharides are broken down more rapidly, whereas polysaccharides have to split up in their monomers first, which takes some time.An extra fermentable C-source has to be added to meat, which mostly contains low levels of carbohydrates due to the "post-mortem" phenomenon, by which glycogen is converted into glucose (glycolysis), and the latter is again broken down into lactic acid.41Lactic acid production is an important process in dry sausage fermentation. It causes a decrease in pH and as a result a) the undesired bacteria are inhibited in their growth ; b) the isoelectrical point of the meat proteins is achieved and the proteins thus become less soluble and denaturate (coagulation or gel formation) - this results in the drying out of the sausage (reduced aw) ; c) a solid product is obtained and d) nitrite is converted into NO, which contributes to colour formation and colour stability.Other antagonistic effectsOther ways in which lactic acid bacteria have an inhibitory effect on the present bacteria are : a) lactic acid has in its undissociated form a certain bacteriostatic effect ; b) acetic acid produced in small quantities has also a inhibitory effect ; c) lactic acid bacteria are demanding regarding their nutrients, and because of this lack of nutrients for other microorganisms may arise ; d) lactic acid bacteria produce hydrogen peroxide and this causes premature rancidity and colour deviations ; e) some lactic acid bacteria produce bacteriocins.- MicrococciMicrococci are applied both for fermentation of dried sausages as for cured meat products because they fulfil of a number of specific functions.Colour formationDuring the production of raw meat products, nitrite and/or nitrate are added to obtain an extended storage life but also for colour formation.Myoglobin is the most frequently found pigment in the muscles and is mainly responsible for the desired or undesired colour of meat. Myoglobin is a haemprotein that consists of a protein (globin) attached to a haematin nucleus with a central Fe2 + atom. The reactions of those haempigments determines the colour of fresh meat and meat products.The cause of colour formation in meat products is the reaction of nitric oxide with myoglobin which forms nitroso metmyoglobin, that is further reduced (in the presence of ascorbic acid and/or other reducing compounds) to nitrosomyoglobin. The latter provides a pink colour and protects the Fe2 + better from oxidation. Subsequently heat and/or acid environment are required for the formation of the more stable nitrosohaemochrome.Hydrogen peroxide produced by lactic acid bacteria has a negative effect on the colour formation. Hydrogen peroxide reacts with nitroso myoglobin and forms cholemyoglobin, which shows a grey-green colour.This proves also the value of catalase positive bacteria (micrococci, lactobacillae (haem-depending)).Nitrate is converted into nitrite by microbial reductases. This reduction occurs during the first 24 to 28 hrs. of fermentation, depending on chosen starter cultures, rate of pH reduction and initial nitrate concentration. Minimum pH for nitrate reduction is 5.2. Nitrate reduction in dry sausage is not optimal at low pH, since microbial reduction is an NADH-depending process. Synthesis of NADH is inhibited at low pH. This results in an insufficient production of nitrate if the pH rapidly decreases to below 5.4. The most favourable pH is 6.4.Breakdown of nitrite strongly depends on the pH. The lower the pH, the stronger the breakdown. To stimulate the reduction of nitrite to nitric oxide, ascorbic acid is used. In addition to this present oxygen will partly reconvert the nitric oxide to nitrate. It has to be said however, that if during the preparation of dried sausage, nitrite is added to the cutter, the oxymyoglobin formed due to the impact of air, is very rapidly oxidized to metmyoglobin, and by this a partial oxidation of nitrite to nitrate occurs.An extremely important reaction is the oxidation of myoglobin to metmyoglobin. During this process Fe2 + is oxidized to Fe3+, and the latter causes a brown colour. A low pH reduces the stability of the myoglobin compounds and the autooxidation.In the presence of nitrate and/or nitrite, a number of different processes may occur, and they may lead to the discolouration of raw meat products (Fig.9).AntioxidantSince micrococci protect the product from harmful influences of O2, they raise the chemical stability, in other words, through catalase (= antioxidant) they break down the rapidly oxidising peroxides, and this way they slow down the rancidity process.The nitrite, formed out of nitrate by micrococci also has an inhibitory effect on the breakdown of lipids. This effect is based on complexing prooxidative substances, like iron. The required amount of nitrite is still unknown.The formation of flavourBecause of their lipolytical activity, micrococci contribute to the formation offlavour.The nitrite formed by micrococci out of nitrate, reacts with the flesh components (alcohols, aldehydes, inosin and sulphur-containing molecules) and forms specific flavours. In order to obtain sufficient flavour, a quantity of 20 to 40 ppm is required.41

Nitrosomyoglobin (bright pink) Fe2+Heat/smokingNitrosohaemochrome (stable pink) Fe 2+Cholemoglobin (green)HexoseLactic acid bacteriaLactic acidNitrosometmyoglobin (brown) Fe3+Myoglobin (purple-red) Fe2+Metmyoglobin (brown) Fe3+NitrateNitriteNitric oxideFigure 9. This diagram shows the convertions of haem pigments during fermentation of raw meat products. The different enzymes are : 1) nitrate reductase, 2) nitrite reductase, 3) oxydase, 4) enzymes of the EMP pathway41Shelf lifeNitrite is generally known as an inhibitor of numerous micro-organisms. The growth of various food pathogens such as Clostridium botulinum, Salmonella spp., and Staphylococcus aureus is inhibited by 80 to 150 ppm nitrite.Since nitrite inhibits various types of bacteria, there is a possibility that added and/or formed nitrite inhibits the starter cultures, and this results in a less imposed or unsuccessful fermentation.- YeastsBy consuming oxygen and breaking down peroxides, Debaryomyces protects the product from breakdown of fat, and thus improves the quality of the fermented product.- MoldsMoulds have an antioxidative effect. By consuming oxygen they provide a reduction of the oxygen pressure against the surface. In addition to this, they break down peroxides. They only grow at the surface and this way they protect the product from the harmful influences of light. This will generally improve colour stability and slow down the rancidity process. Moreover, moulds create a favourable "micro-climate" at the surface, which prevents dehydration of the surface of the sausage or loss of fat during maturing process. Finally, moulds have a lipolytic and proteolytic effect, and they also break down lactic acid. This contributes to the development of a typical flavour.3.2.2. Cured hamCured ham is an uncooked pork butchery product which is first dried and then left to mature. Retention of the original anatomical structure distinguishes it from the dry sausage already described. However, it is similar to this last product, belonging to the second category of 'foodstuffs of intermediate equilibrium moisture content' at the close of drying, and by the importance in manufacture of curing agents. The raw material is pork ham joints taken preferably from carcasses chilled quickly after slaughter and kept refrigerated after cutting, thus limiting microbial growth.Curing is preceded by a preparation stage, in the form of successive rubbing, either in the fresh state or after freezing, in which case penetration of salt and loss of41water are accelerated. The first rubbing is done with a complex mixture consisting principally of salt, nitrates, and spices, together with sugars and water in some cases. Subsequent rubbing is usually confined to the application of salt alone, to maintain the maximum level of salt concentration. The duration of contact with the salt varies with the weight of the ham, but for commercial production about 20 days is usual. This operation takes place in the cold, usually at temperatures between +4C and +1C. During this stage the product loses around 2 to 4 % of its weight in the form of water and halophilic proteins. The ensuing stage, termed 'desalting', is intended, among other things, to remove excess salt. It consists of dipping in cold running water, followed by draining for a period of 12-24h. In industry, this phase consists of initial brushing, followed by spraying with lukewarm water, and drying at 2-5C for 3-6 weeks. The loss of weight, at the close of this treatment, amounts to 10-12 %. Next comes stoving, an operation which, though often neglected in the traditional manufacturing process, seems to be of prime importance for flavour, and is practised systematically on a commercial scale. A temperature of 20-30C is maintained for a week, while the relative humidity fluctuates between 65 and 85 %. During this phase of treatment, degradation of proteins and fats is initiated, while the loss of weight reaches an overall figure of 16-18 % of the initial weight.The final phase is drying, which lasts from 2 to 6 months and on a commercial scale is carried out at a temperature of 13-16C and a relative humidity of 75-85 % ; intermittent ventilation reduces the risk of crust formation. In industry there are distinct stages, the last of which is called 'finishing'. Its duration directly determines the flavour and texture characteristics of the final product. To limit total loss of weight to 25-28 % it is preceded by an operation in which the exposed muscle parts are coated with pork leaf fat or lard.41CHAPTER 4SALTING AND CURING4.1. SALTSalt is the additive with the longest history of use, and it has a great number of roles.4.1.1. Functions of salt4.1.1.1. Bacteriostatic roleProperly speaking, salt is not an antiseptic agent, because it does not kill bacteria, or only very few ; it retards or inhibits the growth of most of them, however, if present at a high enough concentration. It is generally accepted that at 10 % concentration the growth of a large number of micro-organisms is inhibited while at 5 % only the anaerobes are affected. In the past, meat was preserved for fairly long periods at salt concentrations of 7-8 %. Today, however, changed consumer preferences have made it necessary to reduce the amount to below 3 %, which means that additional treatment, such as refrigeration, is necessary to complete the bacteriostatic action of the salt.The action of salt is linked to its concentration in the aqueous phase, which is why for products which undergo dehydration (cured hams, dried sausage) for example, it may be necessary to refrigerate at the start of the process, when moisture content is still fairly high, while it is no longer necessary at the end of the treatment.4.1.1.2. Flavouring roleSalt imparts a salty taste which is chiefly due to the Cl- ion, the cation Na+ also having the ability to stimulate the taste buds. It is important to remember that because of the formation of a complex with proteins, which is stable in the cold but breaks down on heating, only the remaining salt which remains free can produce a salty taste. This is why, for the same salt content, a raw product tastes less salty than when it is cooked.The fat always tastes only slightly salty, since low moisture content means that very little salt can penetrate.414.1.1.3. Influence on the water holding capacity of meatThe addition of salt to raw meat, at the levels traditionally employed, lowers the pHi of the proteins. For this reason, under the conditions of manufacture of meat products (pH 5.5- 6.0), the gap between the pHi of the proteins and pH of the medium is increased, causing an increase in the water holding capacity.4.1.1.4. Action on proteinsBy increasing ionic strength, salt increases the solubility of muscle proteins thereby improving their emulsifying and binding capacity etc.4.1.1.5. Action of fatSalt promotes oxidative changes and rancidity development in fats ; it is thus detrimental to fat preservation.4.1.2. Penetration of salt into meatThe penetration of salt into the meat entails an equilibrium between the concentrations of salt inside and at the surface of the piece. All other things being equal, the rate of penetration diminishes as this equilibrium is approached. In addition, a variety of other factors influence the rate at which equilibrium is reached. There are both extraneous and internal factors.4.1.2.1. Extraneous factorsA rise in temperature favours penetration of salt. Though hardly noticeable in the lower range, the effect becomes more marked at temperatures above 15C. There is a linear relationship between the concentration of salt in the brine and the rate of penetration. Finally, equilibrium is attained more rapidly as the ratio of brine to meat is increased. It is not always desirable, however, to increase this ratio as a means of accelerating the process, since the amount of meat constituents lost by diffusion into the brine is also increased.414.1.2.2. Internal factorsThese principally concern the pH. The higher the pH, the less easily the salt penetrates. As a result of these observations the distinction between meats with a closed structure (poor generation of salt) and open structure (good penetration) can be made. Additionally, the presence of fat in the inter- and intra-muscular connective tissue impedes salt penetration. The 'history' of the meat before brining also has a marked effect. In particular, a freeze/thaw cycle has a noticeable effect on salt penetration.4.2. NITRITE AND NITRATE4.2.1. Role of nitrite in cured products4.2.1.1. Nitrite and colourSee chapter 3, figure 9.4.2.1.2. Nitrite and flavourThe flavour of meat products will be improved by adding nitrite. The products responsible for the characteristic flavour therefore appear only after reaction between nitrite and the meat constituents. Relatively low quantities of added nitrite (25 ppm) were sufficient to give the characteristic flavour, while with excessive amounts (300 ppm) a deterioration of flavour was reported, possibly as a result of oxidative changes.4.2.1.3. Nitrite and microbiological qualitySee course "Food Microbiology and Preservation" (Prof.dr.ir. J. Debevere).4.2.1.4. Oxidizing and antioxidant action of nitriteThe oxidizing action of nitrite and its affinity for haemoglobin are responsible for its direct toxicity.In meat however, nitrite is considered as exhibiting an antioxidant effect. The mechanism is still not fully understood.414.2.2. Nitrite and meat constituentsThe different roles of nitrite already discussed stem from its reactions with the various constituents of the meat.4.2.2.1. Reactions with proteinsIn addition to myoglobin, various proteins offer a certain number of potential reactions sites for nitrite, and the muscle contains a wide range of proteins of varied composition and properties. A quantitative reaction takes place between nitrite and the SH group of cysteine with the formation of nitrosothiols in meat (5 to 15 % of added nitrite). A large fraction of nitrite is bound to proteins independently of the SH groups. A significant proportion of the "loss" occurred in the sarcoplasmic fraction and part of the nitrite is bound to the myofibrils.4.2.2.2. Reactions with adipose tissueThe adipose tissue is composed of lipids connective tissue and water. NO produced from NO can react with unsaturated fatty acids. Nitrosopyrollidine can be obtained from collagen incubated in the presence of nitrite. In adipose tissue most of the added nitrite (80-90 %) remains free, but 2-5 % is connected with the connective tissue, and another smaller fraction with the lipids depending upon the degree of unsaturation.4.2.2.3. Reactions with carbohydratesGiven the very low level of carbohydrates in meat post mortem this type of reaction is quantitatively very limited. However, it cannot be completely ignored, since various sugars are added in certain manufacturing processes, and because amino sugars are present, associated with the connective tissue, which can be converted by nitrite into aldehydes.4.2.2.4. Formation of nitrate from nitriteThe formation of nitrate in products treated solely with nitrite has often been reported (20-3 0 % of the added nitrite can be converted into nitrate). In systems containing ascorbic acid and metmyoglobin, a large fraction of the nitrite was converted into nitrate ; in samples of salted meat nitrate is formed in greater quantities in the presence of ascorbate.44424.2.2.5. Gas formationThe van Slyke reaction, in which nitrite in contact with an amino acid produces nitrogen, can take place in processed meats. It is favoured by low pH and high temperatures. Both NO and N2O are formed by incubation of nitrite with ground muscle preparations, together with N2, though more slowly.4.3. POLYPHOSPHATES4.3.1. Phosphates used in curingThe phosphates used for processed meat production are alkali metal salts of some of the acids. They are mainly :(i)disodium polyphosphate Na2H2P2O7tetrasodium pyrophosphate Na4P2O7pentasodium tripolyphosphate Na5P3O10sodium pentapolyphosphate Na7P5O16 (v) hexametaphosphate sodium salt or Graham's salt (NaPO3)n where n is well above 6, despite its name, with the corresponding potassium salts.

In practice, the polyphosphates marketed for use in meat processing are mixtures of the above compounds in various proportions determined by regulatory controls and by the purpose to which they are put. These mixtures are characterized essentially by their phosphoric anhydride P2O5 titre (between 59.5 and 70 % in the calcined dry residue), by the pH of a 1 % aqueous solution (between 3.6 and 9.0), and by their cyclic phosphate content (less than 8 %).All these polyphosphates, during the processes of manufacture of the finished product, undergo hydrolysis, assisted by a pH in the acid range, reverting to orthophosphate. This means that in the finished product only orthophosphate (with perhaps the most stable pyrophosphates) can be detected.4.3.2. Action of polyphosphates during curingThe basic role of polyphosphates is to promote the binding of water to the proteins of muscle. This action affects the yield of finished product, the quality of emulsions, and the organoleptic characteristics of the product. The mode of action is the combined result of several different phenomena.444.3.2.1. pHThe pH of meat used for the manufacture of fine meat products normally lies between 5.4 and 5.8. The isoelectric zone of the proteins of the meat is 5.10-5.20, which pH the power for water binding capacity is at a minimum. The mixtures of polyphosphates used commercially have a pH (in a 1 % aqueous solution) below but quite close to 9. The polyphosphates contained in these mixtures have pH values in 1 % aqueous solution between 8.3 and 10.4, with the exception of hexametaphosphate (pH 6.4) which is used in small amounts, and acid pyrophosphate (pH 4.2), which is used only in mixtures intended for dried sausage.The addition of polyphosphates to the meat thus raises its pH by 0.2-0.5 units, allowing for the doses employed and the buffering power of the meat itself, and so increases the water holding capacity by shifting the pH further away from the isoelectric point.However, the increase in the water holding capacity obtained by the use of polyphosphates is greater than that resulting from a corresponding increase in pH obtained by other means. The pH rise is thus not the only factor responsible.4.3.2.2. Formation of complexes with Ca++ and Mg++Polyphosphates are able to form complexes with the alkaline earth cations Ca++ and Mg++. These cations are present at low but not insignificant levels (9 and 20 mg respectively per 100 g wet wt). It is known that a portion of these cations is bound to the myofibrillar proteins and that the bound fraction increases during the onset of rigor mortis. This cation fixation may form bridges between the negative charges carried by the polypeptide chains of the proteins. The Ca++ and Mg ++ bridges act as rigid links within the protein network and hence reduce its water binding capacity. The formation of complexes between polyphosphates and these cations thus cleaves a certain number of these bridges, resulting in some slackening of the network, and thereby an increase in its water holding capacity.4.3.2.3. Dissociation of actomyosinPolyphosphates have a dissociating action on the actomyosin complex : myosin and actin will be formed resulting in a swelling of the structure. This supports the idea of the bonds between the myosin filaments and actin, in conjunction with enhanced hydration of the myofibrillar network.444.4. ASCORBIC ACID (VITAMIN C) AND SODIUMASCORBATEAscorbic acid (better known as vitamin C) or its more stable sodium salt, is used in the preparation of processed meats for its reducing properties. As it is insoluble in lipids it cannot exert any antioxidant action on the adipose tissues, but in the meat it reinforces the reducing action of the muscle tissue and protects the myoglobin from oxidation in non-matured uncooked products. In the presence of nitrite, it favours the formation of nitric oxide and the production of nitroso pigments. In addition, its action enables the level of residual nitrite in processed meat products to be reduced and thus reduces the risk of formation of nitrosamines.4.5. SUGARSThe sugars used in salted and cured meat products include sucrose, lactose, glucose and starch derivatives in various degrees of hydrolysis (maltodextrins).In the field of salted products the reducing sugars lactose and glucose are used together with sucrose, which although itself not a reducing sugar, is rapidly hydrolysed to fructose and glucose. Their role is to reinforce the reducing power of the medium and especially to act as carbon sources for the bacteria capable of reducing nitrate to nitrite. Their presence is thus of particular benefit for salted products containing nitrates (or a mixture of nitrite and nitrate) which demand a developed reducing microflora, but which are used less and less frequently.The sugars are also able to play a different role, that of supplementing the dry matter content in order to avoid an excessive moisture content in the product. To prevent such abuses, however, the regulations specify an upper limit for the permissible content of dissolved sugars, which is dependent on the type of product concerned.