A Comparative Study of the Inter-RelationshipsBetween Mixograph Parameters and Bread-Making Qualities of Wheat Flours and Glutens

J Sci Food Agric 1996,72,71-85

A Comparative Study of the Inter-Relationships -

Between Mixograph Parameters and Bread- Y I

Making Qualities of Wheat Flours and Glutens Bhupendar S Khatkar, Alan E Bell and J David Schofield*

The University of Reading, Department of Food Science and Technology, Whiteknights, PO Box 226, Reading RG6 6AP, UK

(Received 25 August 1995; revised version received 11 December 1995; accepted 12 April 1996)

Abstract: The mixing characteristics and bread-making qualities of flours, reconstituted flours and glutens from a diverse range of wheat cultivars obtained from Canada, France and the UK, were investigated. Simple correlations were calculated among Mixograph parameters, loaf volume, Glu-l quality scores, protein content, SDS-sedimentation volume and baking absorption. The results indicate that the mixing properties of flours from cultivars in the medium-strong range are significantly influenced by their protein contents. On the other hand, the mixing properties of ‘extra strong’ or weak flours are relatively less affected by their protein contents, and it appears that the protein quality primarily controls their behaviour during mixing. Gluten samples, other than weak glutens from cvs Riband and Corin, required a longer time to mix to peak dough resistance (PDR) than their corresponding flour or reconstituted flour samples. However, the differences in mixing time were more pronounced between ‘extra strong’ glutens and their corresponding flours or reconstituted flours. The Mixograph parameter PDR showed highly significant (P < 0.001) correlations with loaf volume for flour, reconstituted flour and gluten samples. In each case, variation in PDR explained more than 65% of the variation in loaf volume, reaching about 83% for glutens. PDR was also significantly correlated with Glu-I quality scores. However, the mixing times for flours, reconstituted flours and glutens demonstrated no simple correlations with loaf volume. On the basis of the results, it appears that the 2 g Mixograph PDR value for flour or gluten may be used to assess flour or gluten bread-making quality. It may be used as a viable alternative to the baking test for the evaluation of the baking potential of flour or gluten, as well as an alternative to currently used ‘gel protein’-based procedures for differentiating between ‘strong’ and ‘extra strong’ wheats.

Key words: wheat cultivar, gluten, Mixograph, bread-making.

INTRODUCTION of the qualitative nature of the gluten proteins, which affects their rheological properties (Khatkar et al 1995).

The functional properties (mixing characteristics and Particular attention has been focused upon the glutenin bread-making quality) of a wheat cultivar are attributed fraction of gluten since it is qualitative variation in this largely to its gluten quality and quantity (for a review fraction that accounts for a substantial proportion of see Schofield 1994). Bread-making quality increases lin- the variation in bread-making quality that occurs early with increasing protein content within a cultivar, amongst wheat cultivars. (Schofield 1994; Weegels et a1 but, for a given protein content, bread-making quality 1996). differences among wheat cultivars are largely a function Quality testing is carried out routinely in order to

evaluate the bread-making potential of a cultivar. The * To whom correspondence should be addressed. methods used must be capable of differentiating

71 J Sci Food Agric 0022-5142/96/$09.00 0 1996 SCI. Printed in Great Britain

72 B S Khatkar, A E Bell, J D Schojeld

between wheat cultivars in terms of their protein (gluten) quality for bread-making, but they should also preferably lend themselves to rapid routine testing. Such tests might also be of benefit to the manufacturers and users of vital wheat gluten. Some form of baking test (for example, Finney 1984) is the most widely adopted procedure for assessment of the bread-making quality of either wheat flour or isolated gluten, but they require high levels of skill, are labour intensive and time consuming, and they often require relatively large sample sizes. Simpler, rapid indirect tests, such as the SDS- sedimentation test (Axford et al 1978), have been devised and widely adopted, but they do not always differentiate effectively between wheats of different quality, especially strong and ‘extra strong’ cultivars (Pritchard 1993; Pritchard and Brock 1994). To overcome the latter problem, measurements of gel protein or glutenin macropolymer (GMP) breakdown during dough mixing and of dynamic rheological properties of gel protein have been adopted in the UK for annual assessment of samples produced in Recommended List Trials (Pritchard 1993; Pritchard and Brock 1994). As with baking tests, however, these gel protein tests are intensive of highly skilled labour and time consuming, and they require the use of expensive equipment usually found only in specialised research laboratories.

The Mixograph technique (Finney and Shogren 1972) has been used extensively to try to predict the dough mixing properties of wheat cultivars. This technique has also been used for optimisation and/or to study the effect of various additives, such as oxidants, reductants, dough conditioners and enzymes, etc commonly used in baking (Tsen 1969; Lang et a1 1992). The recent development of a 2 g direct drive Mixograph interfaced to a personal computer for automated recording and interpretation of the results has transformed the Mixograph technique, and is especially valuable when sample size is limited (Rath et a1 1990).

In the present study, we have used the 2 g Mixog- raph, together with a small-scale baking test, to evaluate the bread-making qualities of flours and isolated glutens from wheat cultivars covering a wide range of bread-making performances. We have determined the correlations between the Mixograph parameters and loaf volume data in an attempt to select the Mixograph parameter that demonstrates the best correlation with bread-making performance.

EXPERIMENTAL

Wheat samples

Grain of 13 wheat cultivars used in this study were obtained from Canada (cvs Glenlea, Neepawa, Kate- pwa and Laura), France (cvs Aubaine, Florenc

Aurore, Courtot, Camp Remy, Capitole, Corin and Hardi) and the UK (cvs Hereward and Riband). These cultivars were selected mainly on the basis of their wide diversity of bread-making performances. Grains of individual cultivars were milled on a Buhler experimental mill (model MLU 202) into straight-grade flours after tempering for 24 h at 15.5% moisture level. An extra 0.5% moisture was added 30 min before milling. Flour extraction was calculated on a total product basis.

Analytical methods

Moisture, protein (N x 5.7) and ash contents were determined by standard AACC methods (1983). The sodium dodecyl sulphate (SDS) sedimentation volumes of flour samples were estimated according to the method of Axford et a1 (1978). Total lipid was extracted using chloroform by the batch extraction method (MacRitchie and Gras 1973), and was determined gravi- metrically by evaporating the filtrate under vacuum in a rotary evaporator at 30°C followed by oven drying to constant weight at 50°C. Water retention capacities (WRC) of flour samples were determined by the method of Sollars (1973) to estimate initial water absorption values.

Gluten isolation procedure

Flour (250 g) was defatted with chloroform (500 ml) by stirring vigorously under vacuum for 15 min on a mag- netic stirrer. The slurry was filtered through a sintered glass funnel with the aid of filter paper (Whatman no 4) under vacuum. The procedure was repeated twice on the residual flour. After extraction of lipid, the residual chloroform was evaporated by air drying at room temperature. Defatted flours were kneaded manually into dough balls using distilled water and rested in water at 15°C for 30 min. Glutens were hand-washed from doughs using 0.5 M NaCl (total volume 2.5 litres) at 15°C. The residual salt was removed from the gluten by three subsequent washes with distilled water. The washed glutens were freeze-dried and milled in a micro- hammer mill using a 1 mm screen and a 150 pm sieve. The dried gluten powder samples were analysed for protein (Kjeldahl N x 5.7) and moisture contents (130°C for 1 h).

Fractionation and reconstitution procedure

Cultivar Hereward flour (1.5 kg) was defatted, and gluten was isolated as described above. The washings, containing starch and water-solubles, were collected, freeze-dried separately and powdered using a mortar and pestle. The chloroform extracted lipid was added

Mixograph properties and bread-making qualities of flours and glutens 73

back to the combined freeze-dried starch and water- solubles powder in a Hobart mixer (model EB 10). The lipid-chloroform extract (450 ml) was slurried gently with the combined starch and water-solubles to ensure a uniform dispersion of the lipid. The chloroform was then allowed to evaporate at room temperature, and the reconstituted blend was stored at -20°C. Gluten (the same protein amount) from each cultivar was added whenever required to the above reconstituted blend prepared from cv Hereward flour in order to give a reconstituted flour at 12% protein content on a 14% moisture basis.

Mixograph measurements

Mixing tests were performed on flour (2 g), reconstituted flour (2 g) and isolated gluten (1.5 g) samples with a 2 g direct drive Mixograph (National Manufacturing Division, TMCO, Lincoln, NE, USA). Mixograms were also obtained for flour samples only using a conventional 10 g Mixograph for purposes of comparison. Samples were weighed at 14% moisture basis to an accuracy of 0-001 g, and all mixing experiments were carried out at a speed of 88 f 1 rpm and 25 k 1°C.

Initial water absorption values were estimated from the WRC values of flour samples. The optimum water absorption for each sample was then determined from several mixing and baking trials. Gluten samples were mixed at a final moisture content of 65% (w/w), the approximate moisture content observed for freshly washed-out gluten. The reconstituted flour samples were mixed at 60% absorption. To determine the reproducibility of the Mixograph parameters, six Mixograms were run on flour and gluten dough samples for cvs Hereward and Riband. All other mixing tests were performed at least in duplicate.

Mixograph parameters were determined using the computer software programme ‘Mixsmart’. For the flour-water dough Mixograms, the gain setting on the 2 g Mixograph was set to five turns clockwise as recommended in the ‘Mixsmart’ software handbook for the 2 g Mixograph. For the gluten-water dough Mixog- rams, the gain setting was set to fully anticlockwise in order to bring the maximum torque value within the range of the data handling system. All mixing parameters were measured from the centre line (mid-line) of the Mixogram, and all parameters except mixing time and work input are reported on a 100-unit scale, where 0 and 100 correspond to the minimum and maximum deflections, respectively, obtained with the 2 g Mixo- graph. Breakdowns (resistance and bandwidth breakdowns) were calculated as the change in the values 2 min after peak dough resistance (PDR). Mixing stability (MS) was assessed by measuring the bandwidth of the Mixogram 2 min after the PDR. Work input (WI) is calculated by integrating the area under the mid line of

the curve from time zero to the PDR, and is expressed in arbitrary units (YO torque. min).

Baking test

The bread-making performances of flours and reconstituted flours were determined using an optimised baking test following the procedure described by Finney (1984). The test baking formula was: flour (30 g, 14% moisture basis), compressed yeast (1.59 g), salt (0.45 g), sugar (1.8 g), fat (0.9 g), malted barley flour (0.075 g) and ascorbic acid (100 ppm, flour basis). Salt, sugar, ascorbic acid and yeast were added in solution form. Yeast was added as a suspension, which was mixed well each time before dispensing.

Doughs were mixed in a Simon Minorpin mixer. On the basis of preliminary range-finding experiments, it was found that a mixing time with the Minorpin mixer of 30 s longer than the Mixograph peak dough development time generally gave optimum bread-making performance. Thus, the mixing time in the baking test was optimised using the Mixograph mixing time as a guide, as practised by Finney and his co-workers (Finney 1984). Similarly, water absorption was also optimised. Doughs from ‘extra strong’ flours (eg cvs Glenlea, Aubaine and Florence Aurore) appeared ‘bucky’ when mixed according to the 2 g Mixograph peak times. These doughs were, therefore, also mixed for 2 and 3 min past the peak to assess the effect of extended mixing time on the dough handling properties and loaf volumes.

After mixing, doughs were placed in bowls, covered with a wet muslin cloth, and fermented for 90 rnin at 30°C and 98% RH. Doughs were punched after 52, 77 and 90 min in a machine moulder (Mono Universal) by passing through a set of rollers with a gap setting of 7 mm. After the final punch, the doughs were placed in lightly greased tins (internal dimensions : bottom, 35 x 70 mm; top, 50 x 77 mm and height, 40 mm) and proved for 36 rnin at 30°C and 98% RH.

Baking was carried out for 13 rnin at 232°C in a Simon rotary oven (Henry Simon Ltd, Stockport, UK). A water container was placed in the oven to provide adequate moisture conditions in the oven. Loaf volumes were measured by rapeseed displacement after cooling the loaves for 2 h on a wire mesh. Six loaves were baked for cvs Hereward and Riband and at least two loaves were baked for other samples. The standard deviations and coefficients of variation were below 5 ml and 2.5%, respectively.

Statistical analysis

All determinations were made at least in duplicate. Data were analysed using the statistical analysis system described by the SAS Institute (1985).

74 B S Khatkar, A E Bell, J D SchoJield

RESULTS AND DISCUSSION

Analytical properties

The analytical properties of the wheat flours used in this study are summarised in Table 1. The wheat cultivars represented a wide range for most of the quality attributes measured, except for lipid contents, which had a narrow range (1-10-1.36%). Three samples were from wheats with soft endosperm texture (cvs Riband, Corin and Capitole), the rest being hard. Flour protein contents, SDS-sedimentation volumes and WRC values ranged from 8.7 to 15.7%, 28 to 89.5 ml and 52.1 to 66.1 YO, respectively. The Canadian wheats had higher flour protein contents and SDS-sedimentation volumes than most of the French and the British wheats. The flour protein contents of the French cultivars were com- parable. SDS-sedimentation volumes for cvs Laura, Glenlea and Aubaine were high (> 84 ml), those for cvs Riband, Corin and Florence Aurore were low (< 54 ml), and the values for other cultivars were between 75 and 80 ml.

On the basis of the SDS-sedimentation test, the flour samples from all the Canadian wheats plus cv Aubaine could be classified as being of very good bread-making quality and cvs Riband, Corin and Florence Aurore could be classified as being of poor bread-making quality, with the rest of the cultivars being classified as being of good bread-making quality. As will be dis- cussed below, despite its very low SDS-sedimentation value (54 ml), cv Florence Aurore exhibited very strong Mixograph characteristics (both for flour-water and gluten-water doughs, Table 2), and its gluten produced good bread loaf volumes (see Table 6 below) in the reconstitution baking test (where the protein in the reconstituted flour was from cv Florence Aurore and

the other constituents, except protein, were from cv Hereward). Bread baked from unfractionated cv Flor- ence Aurore flour was poor, however. On the contrary, cv Aubaine had a very high SDS-sedimentation volume (84.5 ml) but poor bread loaf volume both for unfractionated flour and reconstituted flour. These observations clearly suggest that the SDS-sedimentation test alone is insufficient to assess the true bread-making potential of a wheat cultivar.

Comparison of the 2 g direct drive Mixograph and the conventional 10 g Mixograph

The 2 g direct drive Mixograph was compared with the 10 g Mixograph by running the flour-water dough Mixograms on both the machines. The 10 g Mixograph had been standardised previously according to the conditions and procedures outlined by Finney and Shogren (1972). The traces from the two machines were compar- able, and the results showed very highly significant correlations for mixing times (r = 0.96, P < 0.001 and n = 32) and peak dough resistance (r = 0.90, P c 0.001 and n = 32). The 2 g Mixograph results also showed a close relationship with the 35 g standard Mixograph results in a study reported by Rath et a1 (1990). The small differences observed in the present study between the mixing times and peak dough resistances from the two Mixographs may be due mainly to the manual interpretation of the results from the conventional 10 g Mixograph and partly due to some differences in the mixing actions of the two instruments.

Reproducibility of the 2 g Mixograph parameters

The reproducibility of the 2 g Mixograph results (Table 3) was established by running six Mixograms on flour

TABLE 1 Analytical properties and milling yields of flours"

Cultivar Country of Moisture Proteinb Ash SDSsed vol WRC' Lipid Milling yield Endosperm origin (%, w/w) (YO, db) (%, db) (ml) (YO) (%, db; n = 2) (%; n = I ) texture

Glenlea Canada 12.8 (0.07) 15.7 (0.18) 0.54 (0.05) 86.0 (1.6) 62.0 (0.35) 1.29 73.3 Hard Neepawa Canada 13.2 (0.03) 14.1 (0.11) 0.48 (0.05) 80.0 (1.0) 66.1 (0.12) 1.11 70.5 Hard Katepwa Canada 13.7 (044) 14.0 (0.09) 0.47 (0.03) 79.5 (2.2) 66.0 (0.33) 1.25 70.1 Hard Laura Canada 14.0 (0.06) 13.7 (0.13) 0.48 (0.04) 89.5 (1.0) 63.5 (0.31) 1.24 71.0 Hard Hereward UK 134 (0.05) 11.7 (0.11) 0.48 (0.03) 76.0 (1.3) 60.0 (0-40) 1.10 70.5 Hard Aubaine France 12.7 (0.07) 11.7 (0.08) 0.42 (0.03) 84.5 (1.3) 64-8 (0.30) 1.14 72.6 Hard Florence Aurore France 13.9 (0.04) 11.4 (0.09) 0.46 (0.01) 54.0 (1.2) 63.4 (0.10) 1.18 70.1 Hard Courtot France 14.2 (0-03) 10.9 (0-14) 0.44 (0.04) 78.0 (0.5) 60.8 (0.33) 1.21 70.4 Hard Hardi France 12.9 (0.05) 10.1 (0.13) 0.39 (0.04) 75.5 (1.2) 59.4 (0.43) 1.18 73.1 Hard Camp Remy France 13.4 (0.07) 10.0 (0.13) 0.41 (0.04) 75.5 (1.0) 56.7 (0.30) 1.26 73.2 Hard Capitole France 12.6 (0.08) 10.0 (0.07) 0.37 (0.03) 64.0 (1.0) 52.5 (0.35) 1.30 70.6 Soft Corin France 13.7 (0.05) 9.9 (0.08) 0.40 (0.01) 54.0 (0.8) 53.4 (0.41) 1.36 68.3 Soft Riband UK 13.4 (0.09) 8-7 (0.11) 0.38 (0.03) 28.0 (1.2) 52.1 (0.50) 1.29 68.5 Soft

The standard deviations (un- are given in parentheses; n = 5. Kjeldahl protein (N x 5.7). Water retention capacity.

Mixograph properties and bread-making qualities of flours and glutens

12.4 17.2 151’ 5.4 19-9 136 5.2 18.9 134 5.7 14.5 119 7.3 16.7 119 3.8 17.3 114 4-8 15.5 109 4.6 16.0 lO5i

75

9.0 19.9 169’ 9.0 24.3 168

20.7 14.5 136 12.8 20.1 125 15.5 17.3 114 11.3 18.0 114 10.7 19.0 112 10.7 20.6 1021

TABLE 2 Mixing characteristics“ of flour, reconstituted flour and gluten samples

’

Cultivar M T PDR B W P R RBD BWBD MS wi Comments (rnin) (MU)b ( M U ) ( M U ) ( M U ) ( M U ) (AU)’

4.4 16.9 204’ 4.8 20.2 176 8.6 13.6 163 6.6 17.4 152 6.9 15-7 152 6.7 14.4 150 4.7 15.1 149 5.6 14.0 114,

Flour sample Aubaine Glenlea Florence Aurore Laura Neepawa Katepwa Hardi Courtot Capit ole Hereward Camp Remy Corin Riband LSDd (P < 0.05)

Reconstituted flour Aubaine Florence Aurore Glenlea Capitole Camp Remy Laura Hardi Courtot Neepawa Katepwa Hereward Corin Riband LSDd (P < 0.05)

Gluten sample Aubaine Florence Aurore Glenlea Capitole Camp Remy Laura Hardi Courtot Neepawa Katepwa Hereward Corin Riband LSDd (P < 0.05)

’

6.33 43.3 20.3 4.65 70.2 37.6 5.17 50.1 20.6 3.30 66.9 29.6 3.22 56.9 25.3 3.37 53.6 24.1 3.55 45.5 20.2 3.12 53.5 24.0 3.90 37.9 21.1 3.40 44.9 20.3 3.72 38.9 20.6 2.78 38.9 18-9 2.23 33.6 14.0 0.35 4.8 2.0

7.07 52.3 27.1 6.78 53.5 29.1 5.97 66.7 35.3 4.75 51.4 28.9 4-23 58.0 33.3 3.25 66.1 35.2 3.33 57.1 32.9 2.94 57.7 32.8 2.96 55.2 29.3 2.89 54.0 29.7 2.49 55.5 31.3 2.17 48.0 26.3 1.42 44.5 23.6 0.32 3.5 1.7

sample

10.70 48.7 18.6 11.91 52.5 21.6 9.03 64.5 22.7 6.89 51.3 21.3 4.75 57.5 25.0 3.74 65.3 22.2 4.19 58.0 24.0 3.43 62.7 22.6 4.23 54.2 21.1 4.56 52.3 19.8 3.11 49.5 19.6 2.49 46.9 18.6 2.10 40.2 14.9 0.25 2.2 1.1

1.5 7.4 5.4 9.7 5.8 5.8 3-6 6.0 2.3 4.5 1.8 3.2 5.5 1.5

4.7 3.3 9.1 5.4 2.9 9.9 6.5 6.9 6.1 4.1 4.7 3.8 7.2 0.5

2.2 2.6 5.5 2.6 3.0 8.1 4.3 6.1 5.4 4.2 4.5 3.4 4.3 0.3

‘Extra strong’

Strong

Weak

‘Extra strong’

Strong

Weak

‘Extra strong’

Strong

Weak

“ MT, mixing time; PDR, peak dough resistance; BWPR, bandwidth at peak dough resistance; RBD, resistance breakdown; BWBD, bandwidth breakdown; MS, mixing stability and WI, work input. All Mixograph parameters are average of at least two determinations.

MU, Mixograph units. Arbitrary units (% Torque. min).

* LSD, least significant difference.

76 B S Khatkar, A E Bell, J D Schofeld

TABLE 3 Reproducibility of 2 g Mixograph parameters for flours and glutens from cv Hereward

(good bread-making wheat) and cv Riband (poor bread-making wheat)”

Sample Mixograph parameter?

MT P D R B W P R RBD B W B D M S wi (min) (MUY ( M U ) ( M U ) ( M U ) ( M U ) (AU)d

Flour sample cv Hereward

Mean 3.40 SD 0.1 1 CV(%) 3.2

cv Riband Mean 2.23 SD 0.06 CV (%) 2.7

Gluten sample cv Hereward

Mean 3.1 1 SD 0.07 CV(%) 2.3

Mean 2.10 SD 0.05 CV (W) 2.4

cv Riband

44.9 1.67 3.7

33.6 1.20 3.6

49.5 0.73 1.5

40.2 0.40 1 .o

20.3 0.92 4.5

14.0 0.64 4.6

19.6 0.63 3.2

14.9 0.35 2.3

4.5 4.8 0.30 0.32 6.7 6.7

5 .5 4.6 0.35 0.28 6.4 6.1

4.5 5.6 0.20 0.21 4.4 3.8

4.3 6.1 0.21 0.25 4.9 4.1

15.5 0.44 2.8

9.4 0.35 3.7

14.0 0.24 1.7

8.8 0.1 1 1.3

109 2.99 2.7

59 1.72 2.9

114 1.71 1.5

68 1.09 1.6

n = 6; SD = standard deviations (0”- of mean; CV = coefficients of variation. * MT, mixing time; PDR, peak dough resistance; BWPR, bandwidth at peak dough resistance; RBD, resistance breakdown; BWBD, bandwidth breakdown; MS, mixing stability; and WI, work input. ‘ MU, Mixograph units.

AU, arbitrary units (% torque. min).

and gluten sample from cvs Hereward and Riband. The reproducibility of the Mixograph parameters was generally good for both the flour and gluten samples from both the cultivars. The coefficients of variation for the Mixograph parameters from the flour samples were from 3 to 5% except for the resistance breakdowns (RBD) and bandwidth breakdowns (BWBD) (about 7%). Reproducibility of the Mixograph parameters for gluten samples was better than that for the flour samples. The strong mixing characteristics of cv Here- ward flour and gluten and the weak mixing characteristics of cv Riband flour and gluten are consistent with our previous observations regarding the fundamental rheological properties of glutens from these two cultivars as measured by small deformation controlled stress rheometry (Khatkar et a1 1995).

Flour-water dough Mixograph parameters

The Mixograms for some unfractionated flours and reconstituted flours are presented in Fig 1 to illustrate the nature of the Mixograms obtained, and all the Mix- ograph results are summarised in Table 2. The ‘strength’

of each cultivar could be judged from the Mixograms on the basis of the mixing time (MT), PDR and work input (WI) requirement, which is a function of MT and PDR, as well as the general shape of the Mixograph curve. Flour-water doughs from cvs Aubaine, Glenlea and Florence Aurore exhibited the Mixograph characteristics of ‘extra strong’ wheats with longer mixing times (4.65-6-33 min), medium to high peak dough resistances (43.3-70-2 MU) and higher work input requirements (180-220 AU). Flour-water doughs from cvs Riband and Corin, in contrast, were weak and developed quickly with little work input ( G 8 2 AU) requirement. Other cultivars had mixing properties typical of strong doughs. Some of the cultivars charac- terised as ‘strong’ covered a wide range as far as their PDR and WI values are concerned, but, based on their similar mixing requirements (MT) and the general shapes of the mixing curves, and since there was no clear cut-off point within the ranges, they are included in a single category.

To segregate the effect of protein quality from that of protein quantity, Mixograms were run on reconstituted flours, in which the glutens (same protein amount) from


B C ‘Extra’ strong A

2

Strong s C P a

.-

a 3 a

B

3

k 0

P)

.s 4

Weak

5

6

Miring time (min)

Fig 1. Mixing curves of (A) unfractionated flours, (B) reconstituted flours, and (C) glutens. Culltivars are indicated by numbers from 1 to 6: 1, Glenlea; 2, Aubaine; 3, Courtot; 4, Hereward; 5, Corin; 6, Riband.

individual cultivars were reconstituted with combined starch plus water-solubles, and lipid isolated from cv Hereward flour. In a control experiment, cv Hereward doughs from unfractionated flour and the flour that was fractionated and reconstituted in the original proportions of gluten, starch, water-solubles and lipid, produced identical Mixograms (Fig 2), indicating that the fractionation and reconstitution procedures had not affected the functionality of the components in terms of dough-mixing properties.

Comparison of the Mixograph results for the flour- water doughs and their corresponding reconstituted flour-water doughs (Fig 1 and Table 2) demonstrated

that the overall strengths of cvs Neepawa and Kate- pwa doughs were reduced, whilst the overall strength of cvs Capitole and Camp Remy were increased for the reconstituted flour-water doughs. Similar results were obtained for gluten-water doughs (Fig 1 and Table 2). The Mixograph trends for other cultivars remained unchanged between flour-water doughs and their corresponding reconstituted flour-water doughs. These shifts for those cultivars are due primarily to the significant increases in the PDR values for reconstituted flours of cvs Capitole and Camp Remy. The PDR and other Mixograph parameters (except mixing time) for the reconstituted flour-water doughs (12% protein, 14%

78 B S Khatkar, A E Bell, J D Schofield

I 1

Miring time (mid

Fig 2. Mixograms from cv Hereward (A) unfractionated flour and (B) reconstituted flour.

moisture basis) had values greater than those for the corresponding flour-water doughs for all cultivars, except for Canadian wheats (Table 2). This is due presumably to the fact that all the unfractionated flours, other than Canadian wheat flours, had protein contents less than the protein contents of the reconstituted flours, that is, 12% (on 14% moisture basis). The direct relationship of protein content with the PDR has also been demonstrated in previous studies (Finney and Shogren 1972; Lang et al1992).

The reconstituted flours from the ‘extra strong’ cvs Aubaine, Glenlea and Florence Aurore and from strong cvs Capitole and Camp Remy had longer mixing requirements than the corresponding unfractionated flours, whereas the reconstituted flours for other cultivars developed more quickly than the corresponding flour-water doughs (Table 2). This observation indicates that gluten protein composition is important for the mixing requirement. Finney and Shogren (1972) have shown an inverse relationship of protein content with mixing time up to 12% protein content, but further increasing the protein content did not influence the mixing time. Our observations are consistent with this explanation for glutens from moderately strong and weaker wheats, but the results for the ‘extra strong’ wheat flours were not consistent with the previous findings.

Gluten-water dough Mixograph parameters

The gluten Mixograms (Fig 1) differentiated the cultivars under study into ‘extra strong’, strong and weak

based on their overall work input requirements (Table 2). This classification is consistent with the reconstitution mixing experiments (Table 2). The Farinograph has been used in previous studies (Bushuk 1963; Doguchi and Hlynka 1967) to study the mixing properties of commercial and laboratory prepared glutens. Using the Farinograph, Doguchi and Hlynka (1967) reported that the mixing curve for the gluten of a hard red spring wheat displayed stronger mixing characteristics than that for a soft winter wheat. However, the large sample size required, the dificulty experienced in the manual recording of parameters, and the optimisation required to centre the curve on the 500 Brabender Units (BU) are considered to be the main drawbacks of the Fari- nograph method, which seriously limit its application to studying gluten (Neufeld and Walker 1990).

In this investigation, we observed that the gluten- water doughs exhibited higher work input requirements than those of the corresponding flour-water doughs or reconstituted flour-water doughs. It is interesting that most gluten samples, except weak glutens from cvs Riband and Corin, also required a longer time to mix to peak than the corresponding flour and reconstituted flour samples (Table 2). However, the differences in the mixing time were more pronounced between ‘extra strong’ glutens and their corresponding unfractionated or reconstituted flours. Isolated gluten is a concentrated protein mass (about 75% protein) that appears to require an initial induction period for hydration before the gluten-matrix starts to develop.

The high M , glutenin subunit compositions of the cultivars examined here are given in Table 4. Consider- ing the subunit composition of these cultivars, our Mix- ograph results broadly agree with the reports (Campbell

TABLE 4 High M , glutenin subunit compositions and Glu-1 quality

scores of wheat cultivars

Cultivar High M, glutenin subunits Glu-1 scores“

Glu-A1 Glu-BI Glu-DI

Glenlea 2* 7 + 8 5 + 1 0 10 Laura 1 7 + 8 5 + 1 0 10 Katepwa 2* 7 + 9 5 + 1 0 9 Neepawa 2* 7 + 9 5 + 1 0 9 Aubaine 2* 7 + 9 5 + 1 0 9 Florence Aurore 2* 7 + 9 5 + 10 9 Courtot 2* 7 + 8 2 + 1 2 8 Camp Remy 2* 7 3 + 1 2 6 Hardi 2* 7 2 + 1 2 6

Hereward Null 7 + 9 3 + 12 5 Corin Null 6 + 8 3 + 12 4 Riband Null 6 + 8 2 + 12 4

Capitole Null 7 + 9 2 + 12 5

According to Payne et a1 (1987)


et a1 1987; Cressey et al 1987; Ng and Bushuk 1988; Dong et al 1992; Gupta and MacRitchie 1994) that subunits 5 + 10, 17 + 18, 7 + 8, 1 and 2* have positive effects on mixing time and dough 'strength'. On the other hand, subunits 6 + 8, 2 + 12, 3 + 12 and 20 were positively related with short mixing time and dough weakness. Both quantitative and qualitative effects may contribute to quality differences associated with specific high M I glutenin subunit alleles. Gupta and MacRitchie (1994) reported that the high M I glutenin subunits 5 + 10 and 2 + 12, 17 + 18 and 20, also 1 and 2* are produced in similar quantities. They concluded that the qualitative differences in these subunits are responsible for their different effects on Mixograph peak dough development time and Extensograph maximum resistance (Rmax) values. In another study (Lundh and Mac- Ritchie 1989), differences in the dough development time between an excellent bread-making quality wheat (Mexico 8 156) and a moderately good bread-making

quality wheat (Halberd) were attributed to differences in their proportions of glutenin. These cultivars were reported to possess a similar high M I glutenin subunit composition.

Inter-relationships of Mixograph parameters for unfractionated flours, reconstituted flours and glutens

Reconstituted flour Mixograph parameters displayed better correlations with the gluten Mixograph parameters than the correlations of the unfractionated flour Mixograph parameters with the reconstituted flour or gluten parameters (Table 5). This is presumably because of the elimination of the protein content effects. Another noteworthy observation was that the PDR consistently showed the highest correlations with most of the Mix- ograph parameters (except with MT and WI, which are

TABLE 5 Relationships of Mixograph parameters" of unfractionated flours, reconstituted flours and glutens

~

M T PDR B W P R RBD B W B D M S W I LV

Unfractionated flours Reconstituted flours

M T 0.95*** PDR 0.19 BWPR 0.09 RBD -0.19 BWBD -0.19 MS 0.38 WI 0.92 * * * LV -0.01

Unfractionated flours Glutens

M T 0.93 * * * PDR 0.1 1 BWPR 0.18 RBD - 0.44* BWBD -0.54** MS 0.52** WI 0.92***

Reconstituted flours Glutens

M T 0.97*** PDR 0.23 BWPR 0.3 1 RBD -0.41* BWBD -0.41* MS 0.58** WI 0*99***

0.21 0-81*** 0.68*** 0.62*** 0.80*** 0.31 0.28 0.82***

0.20 0.77*** 0*40* 0.71*** 0*61*** 0.02 0.34

0.19 0*92*** 0.70*** 0*63*** 0*57** 0.35 0.36

0.31 -0.21 0-83*** 0-52** 0.71*** 0*41* 0.58** 0.72*** 0.76*** 0.81*** 0.22 -0.65*** 0-40* -0.17 0*78*** 0.62***

0.25 -0.15 0*78*** 0*50* 0.47* 0.04 0*57** 0.85*** 0.52** 0.79 * * * 0.15 - 0.45* 0.41* -0.09

0.05 -0.17 0.92*** 0.51** 0*83*** 0.05 0.56** 0.81*** 0.56** 0*75*** 0.48* - 0.43* 0.25 - 0.06

0.01 0.73*** 0.62*** 0,74* * * 0*90***

-0.51** 0.10 0.65***

0.50** 0.68 * * * 0.58** 0-26 0.40* 0.13 0*56** 0.68***

-0.03 0.45* 0.70*** 0*63*** 0.31 0.48* 0*74*** 0.24 0*76*** 0.14

-0.16 0 4 * 0.10 0.59**

-0.17 0.29 0.76*** 0.04 0.33 0*53** 0.89*** -0.58** 0.84*** -0.50**

- 0.20 0.85*** -0.01 0.31

0*85*** 0.47* 0.32 0.13 0.18 0.13 0.85*** 0.34

0.83*** 0.40* 0.26

- 0.03 -0.19

0.38 0.89***

0.93*** 0.34 0*41*

-0.33 -0.34

0.63*** 0*98***

-0.01 0.78 ** * 0.73*** 0*60** 0.74***

0.08 0.82***

-0.16

-0.11 0.77*** 0.52** 0.72*** 0.64*** 0.13 0.08

0.09 0*90*** 0.70*** 0.66*** 0*67*** 0.29 0.26

MT, mixing time; PDR, peak dough resistance; BWPR, bandwidth at peak dough resistance; RBD, resistance breakdown; BWBD, bandwidth breakdown; MS, mixing stability; WI, work input; and LV, loaf volume. *, **, *** indicate that the parameters are significantly, highly significantly and very highly significantly correlated at 5, 1 and 0.1% probability, respectively.

80 B S Khatkar, A E Bell, J D Schojield

strongly inter-related, and which had no relationships with loaf volume either) for unfractionated flours, reconstituted flours and glutens.

Inter-cultivar variation in bread-making quality

An optimised baking test was employed with unfractionated flours and reconstituted flours to determine directly the bread-making performances, in terms of loaf volume, of the wheat cultivars under study. The reconstitution baking tests were carried out in order to investigate the effect on bread-baking quality of inherent protein quality variation amongst cultivars. Doughs from most flours (except cvs Glenlea and Aubaine) were developed well in the Simon Minorpin mixer (at mixing times 30 s past their peak time previously determined using the 2 g MixographX and produced satisfactory loaves. However, dough from cvs Glenlea and Aubaine appeared ‘bucky’ and underdeveloped, and they produced loaves of inferior volume. Doughs from these cultivars and cv Florence Aurore dough were, therefore, mixed for 2 and 3 min extra in order to assess the response to extended mixing time of dough-handling properties and loaf volumes.

Extended mixing times led to poorer baking quality (ie poorer crumb grain and decreased loaf volume) for cvs Aubaine and Florence Aurore (Fig 3). Loaf volumes of these cultivars were considered optimum without extended dough mixing times. Cultivar Florence Aurore dough became sticky when mixed for either an extra 2 or 3 min. The dough handling properties of cv Aubaine remained satisfactory, however. Importantly, extended mixing time substantially improved the dough development and baking quality (improved crumb grain and increased loaf volume) of cv Glenlea (Fig 3). Cultivar Glenlea dough mixed for 3 min extra produced the

. a.7 0 1 2 3

Extended mixing time (min)

Fig 3. Effect of extended mixing time on loaf volume response of unfractionated flours from cvs m, Glenlea; 0 , Aubaine;

and A, Florence Aurore.

greatest loaf volume. Beyond 3 min of extra mixing, the dough was difficult to handle. The loaf volume for cv Glenlea dough mixed for an extra 3 min was, therefore, taken as the loaf volume in the correlation studies. The positive response of cv Glenlea dough to extended mixing time suggests that some flours, such as Glenlea flour, need higher work inputs to achieve their full baking potential. This concept has been demonstrated previously by Kilborn and Tipples (1972). Recent research has shown that ‘extra strong’ cultivars, such as Glenlea, can be differentiated on the basis of the properties of their very high molecular weight SDS- unextractable glutenin polymer fractions [gel protein or glutenin macropolymer-GMP-fractions (Graveland et a1 1980; Weegels et a1 1994)l. GMP from ‘extra strong’ cultivars depolymerises at a much lower rate during dough mixing than that from weaker cultivars (Pritchard 1993; Pritchard and Brock 1994; Weegels et a1 1995). The elastic modulus of GMP fractions from ‘extra strong’ cultivars are also considerably higher than those from weaker cultivars (Pritchard 1993). Such parameters are presently used in annual official cultivar trials in the UK since they differentiate ‘extra strong’ and other cultivars more effectively than parameters, such as SDS-sedimentation volume (Pritchard 1993).

The poor baking performance of cv. Aubaine (Table 6 ) for unfractionated flour and reconstituted flour may be attributable to the ‘extra strong’ nature of its gluten proteins that prevents dough expansion during fermentation and the initial baking stage. It may also be pos-

TABLE 6 Bread-making qualities (loaf volumes) of unfractionated flour

and reconstituted flours’ ~

Unfractionated flour Reconstituted flour

Cultivar Loaf Cultivar Loaf volume volume

(ml) (ml)

Laura 200 Glenlea 208 Glenlea 196 Courtot 208 Neepawa 190 Laura 205

Katepwa 180 Katepwa 192 Capitole 176 Florence Aurore 192 Camp Remy 174 Hardi 192 Hardi 170 Camp Remy 190 Hereward 168 Capitole 187 Corin 158 Hereward 181 Aubaine 157 Corin 174 Florence Aurore 157 Aubaine 173 Riband 153 Riband 1 70

Courtot 180 Neepawa 200

Loaf volumes are the means of at least duplicate determinations and the standard deviations of the means and coefficients of variation were below 5 ml and 2.5%, respectively.


sible that the 'extra strong' flour of cv Aubaine requires mixing at higher speed, at least higher than that produced in the Simon Minorpin mixer, for effective dough development. On the other hand, cv Florence Aurore gluten produced good bread loaf volume in the reconstitution baking test, even though its unfractionated flour had poor loaf volume (Table 6). This suggests that cv Florence Aurore flour may be deficient in some inherent non-gluten component that is necessary for good baking quality. In a study reported by Lukow et al (1989), the bread-baking qualities of flours from the Canadian wheat cvs Lennox and Vernon were judged as being poor. These cultivars have high Glu-I scores of 9 similar to that of cv Florence Aurore examined here. These results suggest that, besides protein quantity and quality, the non-gluten constituents may also play an important role in determining the baking quality of the flours from some wheat cultivars. Weegels et al (1995) have shown that low M , non-gluten proteins have significant effects on dough rheological properties, which may be related to their ability to retard glutenin re- polymerisation during dough resting. Furthermore, surface active low M, non-gluten proteins which have a foam stabilising effect, are known to be present in wheat flour (Blochet et al 1991; Wilde et a1 1993), and these may play a role in gas retention in dough (Gan et a1 1995). Wheat flour also contains sulphydryl peptides, in particular glutathione, which can interact with gluten proteins (Chen and Schofield 1995; Schofield and Chen 1995) and which can have significant effects on dough rheology (Grosch and Sarwin 1994). Whether deficiency in one of these factors can explain the observations for cv Florence Aurore unfractionated flour, on the one hand and the reconstituted flour and gluten on the other is not known.

Protein quality and quantity play important roles in the dough for efficient gas retention during fermentation and the early stages of baking. Some low protein flours with good quality gluten, such as cv Courtot, may perform as well as some high-protein flours, eg flours from cvs Katepwa and Neepawa. On the other hand, flours having poor protein quality, such as cvs Riband and Corin, yield doughs of inferior dough handling properties and poor bread-baking results (Table 6). According to the baking test on reconstituted flours, the cultivars could be classified according to their gluten qualities into very good (cvs Glenlea, Courtot, Laura and Neepawa), good (cvs Katepwa, Florence Aurore, Hardi, Camp Remy, Capitole and Hereward) and poor (cvs Corin, Aubaine and Riband) bread-making wheats.

Inter-relationships among Mixograph parameters and other quality parameters

Tables 7a and 7b show the correlation coefficients among Mixograph parameters and other quality parameters (loaf volume, flour protein content, SDS- sedimentation volume and baking absorption) obtained for unfractionated flour, reconstituted flour and gluten samples. As might be expected, flour protein content exhibited very strong relationships ( P < 0-01) with all flour quality attributes except mixing time (Table 7a), suggesting a broad influence of flour protein content on the flour quality attributes.

SDS-sedimentation volume, a measure of good quality protein, was closely related ( P < 0.01) to most quality attributes under investigation (Table 7a), but showed no significant associations with RBD and

TABLE 7a Correlation coefficients among Mixograph parameters and other quality attributes' for 13 wheat flour samples

~ ~

M T P D R B W P R RBD B W B D M S W I SDS F P B A Glu-1 score LV

MT PDR BWPR RBD BWBD MS WI SDS FP BA Gfu-I score LV

1 .o 0.16 0.20

-0.29 -0.10

0.46* 0.86*** 0.36 0.26 0.57'. 0*51**

-0.14

1 .o 0.77*** 0.27 0.16 0.19 0.60** 0.34 0.54.' 0.60**

MT, mixing time; PDR, peak dough resistance; BWPR, bandwidth at peak dough resistance; RBD, resistance breakdown; BWBD, bandwidth breakdown; MS, mixing stability; WI, work input; SDS, SDS-sedimentation volume; FP, flour protein; BA, baking absorption; and LV, loaf volume (unfractionated flour). *, **, *** indicate that the parameters are significantly, highly significantly, and very highly significantly correlated at 5, 1 and 0.1% probability, respectively.

82 B S Khatkar, A E Bell, J D Schofield

TABLE 7b Correlation coefficients among Mixograph parameters” and loaf volumes for reconstituted flour and gluten samples

~~ ~

MT PDR BWPR RBD B W B D M S WI Glu-1 score LV

Reconstituted flour sample MT 1 *o PDR 0.27 BWPR 0.18 RBD -0.14 BWBD -0.11 MS 0.37 WI 0.98*** Glu-1 score 0*53** LV 0.14

Gluten sample MT PDR BWPR RBD BWBD MS WI Glu-1 score LV

1 a 0 0.11 0.18

-0.51** -0*45*

0.47; 0.97*** 0.52** 0.09

1.0 0.94*** 0.55** 0.79*** 0.02 040* 0.67** 0.81***

1 .o 0.82*** 0-6 1 ** * 0.60** 0.42* 0.27 0.63*** 0.91***

1 .o 0.41; 0.71*** 0.19 0.30 0.49* 0.78***

1 .o 0.16 0.29 040* * * 0.35 0.35 0.72***

1 .o 0438*** 1 .o

-0.74*** -0.55;; 1 .o -0.10 - 0.03 0.40. 1 .o

0.35 0.48* - 0.09 0.56** 1.0 0.51** 0.72*** -0.07 0.22 0.68*** 1 .o

1 .o 0.89*** 1 .o

- 0.40* -0.34 1 .o -0.38 -0.35 0.56;. 1.0

0.39 0.2 1 0.21 0.63*** 1 .o 0.63*** 0.67*** 0.29 0.26 0.68*** 1 .o

MT, mixing time; PDR, peak dough resistance; BWPR bandwidth at peak dough resistance; RBD, resistance breakdown; BWBD, bandwidth breakdown; MS, mixing stability; WI, work input and LV, loaf volume (reconstituted flour). *, **, *** indicate that the parameters are significantly, highly significantly and very highly significantly correlated at 5, 1 and 0.1% probability, respectively.

BWBD. RBD and BWBD both relate to mixing tolerance, indicating that mixing tolerance is not affected by the factors associated with protein quality, but which has been reported (Hoseney 1985) to be directly related to compounds found in the water-solubles fraction. RBD and BWBD were not significantly related to work input requirement (a measure of dough ‘strength’) for unfractionated flour, reconstituted flour or gluten samples. In addition, the MT for unfractionated Bour and reconstituted flour samples exhibited no significant association with RBD or BWBD. However, for gluten samples, from which water-solubles are absent, an inverse relationship (P .c 0.05) was observed between mixing time and both the RBD and BWBD (Table 7b). These findings again indicate that RBD and BWBD Mixograph parameters for flour are related to components in the water-solubles fraction.

Examination of the correlation matrix of Mixograph parameters (Tables 7a and 7b) for flour, reconstituted flour and gluten samples showed that RBD and BWBD had a very strong positive association (P < 0.01) with PDR and bandwidth at peak dough resistance (BWPR). This implies that cultivars with high PDR and BWPR values (eg cvs Laura and Glenlea) will also have high RBD and BWBD values, implying poor mixing tolerance. High PDR and BWPR values are related to good dough ‘strength’ and good bread-rnaking quality, as observed in this study for cvs Laura, Glenlea and

Courtot (see Tables 6, 7a and 7b). On the other hand, poor mixing tolerance is thought to be related to poor dough ‘strength’ and poor bread-making quality. These observations led to the conclusion that, in cases where PDR is high (a55 MU), high values of RBD and BWBD, implying poor mixing tolerance, should be viewed with caution as far as bread-making quality is concerned.

It is noteworthy that loaf volume had the highest correlation with PDR among the Mixograph parameters for flour, reconstituted flour or gluten samples (Tables 7a and 7b). Importantly, the correlation coefficient for the relationship between PDR and loaf volume was maximal for gluten samples, accounting for about 83% of the variation in loaf volume. Although PDR was very closely related to loaf volume for both flour and gluten samples in this study, the importance of this parameter for prediction of the baking potential of flour or gluten has not highlighted in the past.

Surprisingly, mixing time and work input showed no correlation with loaf volume, but exhibited a very strong inter-relationship between themselves for flour (r = 0.86***), reconstituted flour (r = 0.98***) and gluten samples (r = 0.7***) (see Tables 7a and 7b). Similar observations were reported elsewhere for the relationship between mixing time and loaf volume (Dong et al 1992; Preston et al 1992). Mixing time showed no correlation with flour protein content, but


work input exhibited a close relationship with flour protein content (Table 7a). This explains why the correlation coefficient between work input and mixing time was comparatively low for flour samples compared with those for gluten or reconstituted flour samples.

Association of high M , glutenin subunits with Mixograph parameters and bread-making quality

Glu-1 quality scores, a measure of bread-making quality, showed the highest correlation with PDR for flour, reconstituted flour and gluten samples. Glu-I scores explained 60% and 68%, respectively. of the variation in loaf volume for unfractionated flour and reconstituted flour samples. Lukow et a1 (1989) reported a similar correlation of Glu-I scores with loaf volume for a set of 67 cultivars. Correlation studies (Campbell et a1 1987; Cressey et a1 1987; Payne et al 1987; Ng and Bushuk 1988; Lukow et a1 1989; MacRitchie et a1 1990; Singh et al 1990; Dong et al 1992; Preston et al 1992; Gupta and MacRitchie 1994) have indicated that dough ‘strength’ and bread-making potential of flours are positively related with subunits 5 + 10, 7 + 8, 17 + 8, 1 and 2* and negatively associated with subunits 6 + 8, 2 + 12 and 20.

In this study, cultivars having subunits 6 + 8 at Glu-B1 (eg cvs Riband and Corin) had weak doughs and low bread loaf volumes. However, some cultivars, such as Courtot, Hardi and Capitole, although having subunits 2 + 12 (Table 4), produced strong doughs and good loaf volumes (Table 6). Additionally, cultivars containing subunit pair 7 + 8 at the Glu-B1 locus in com- bination with either subunit 2* or 1 or 5 + 10 had greater PDRs and loaf volumes. The PDR had been found to have the best correlation with loaf volume in this study (Tables 7a and 7b). It was also observed that, where subunits 7 + 8 and 2* occurred together, eg in cv Courtot, the presence of subunit pair 2 + 12 at the Glu-D1 locus did not have a negative effect on the bread making quality. Similar results were obtained by Singh et al (1990) for Australian wheat cultivars Cook, Gabo, Oxley and Timgalen.

GENERAL DISCUSSION

The Mixograph studies with flour and gluten samples (gluten studies with both isolated gluten and with gluten in reconstituted flours) indicate that the differences in the mixing characteristics among weak and ‘extra strong’ wheat cultivars are due mainly to differences in gluten protein quality with protein content playing only a minor role. Amongst those cultivars classed here as

‘strong’, the mixing properties were significantly influenced by protein content. This was confirmed by the fact that the Mixograph measurements ranked ‘extra strong’ and weak cultivars in the same order for flours, reconstituted flours and glutens. However, the ranking orders of the cultivars with strong mixing characteristics were different for flours, reconstituted flours and glutens, presumably because, for glutens and reconstituted flours, the variation in protein content was removed. The PDR showed a very highly significant correlation (P < 0.001) with Glu-1 quality score and loaf volume. The highest coefficients were observed between the PDR values for glutens and loaf volume in the reconstitution baking test, accounting for about 83% variation in loaf volume. On the other hand, the mixing time showed no significant correlation with loaf volume and most other quality factors in this study or reports published elsewhere (Dong et al 1992; Preston et al 1992).

The 2 g Mixograph proved to be a simple, rapid and effective instrument for evaluating flour and gluten protein quality. Its small sample size requirement pro- vides an opportunity to investigate the direct effects of individual protein fractions on gluten functional properties, and may allow plant breeders to assess protein quality more effectively at an early stage in the breeding process.

At present in the UK, part of the annual quality assessment procedure used to evaluate wheats grown in the UK in Recommended List Trials involves measure- ment of the breakdown rate during mixing and the fundamental rheological characteristics (the dynamic elastic or storage modulus, G‘, in particular) of the gel protein or glutenin macropolymer (GMP) fraction of gluten (Pritchard 1993; Pritchard and Brock 1994). This is done especially to provide information about ‘extra strong’ characteristics of wheats due to cultivar, season and site. The practical use of such tests is limited, however, because of high cost; they are time consuming, labour intensive, require a high level of technical skill to carry out the tests and interpret the results, and require the use of expensive equipment found only in few research laboratories. Such procedures do not lend themselves readily to routine testing, such as testing of breeding lines or intake testing at flour mills.

The 2 g Mixograph procedure appears, on the basis of the results presented here, to be equally capable of differentiating amongst wheats differing in strength characteristics, including ‘extra strong’ types, from wheats of more usual dough strength for bread-making purposes. In contrast to the gel protein tests, however, the Mixograph procedure is relatively rapid (PDR values for flour are obtained in well under 10 min, for example), and suffers from none of the disadvantages of the gel protein breakdown and rheology procedure.

It is tempting to speculate, therefore, that, in addition to its potential use as a tool for predicting the bread-

84 B S Khatkar, A E Bell, J D Schojield

making quality of wheat flour or gluten, the 2 g Mixog- raph procedure might provide a viable, much lower cost, alternative to the presently used gel protein tests for differentiating the strength characteristics of wheats. Before such an option is considered, however, much more detailed information is required on factors such as season-to-season and site-to-site variation within cultivar, and contemporary cultivar collections relevant to particular national contexts need to be evaluated.

ACKNOWLEDGEMENTS

One of the authors (BSK) is grateful for the award of a scholarship from the Commonwealth Scholarship Com- mission (UK), which enabled this work to be carried out. Thanks to Drs F Townley-Smith, Agriculture and Agri-Food Canada, Winnipeg Research Station, Canada; Dr B C Morgan, Canadian Grain Commis- sion, Grain Research Laboratory, Winnipeg, Canada and Dr G Branlard, INRA-Station d’Amblioration des Plantes, Montpellier, France for their help in providing the wheat samples used in this study.

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Documents

A Comparative Study of the Inter-RelationshipsBetween Mixograph Parameters and Bread-Making Qualities of Wheat Flours and Glutens