Breadmaking Quality of Selected Durum Wheat Genotypes and Its Relationship with High Molecular Weight Glutenin Subunits Allelic Variation and Gluten Protein Polymeric Composition Karim Ammar,1,2 Warren E. Kronstad,3 and Craig F. Morris4 ABSTRACT
Cereal Chem. 77(2):230–236
Twenty-seven durum wheat genotypes originating from different geographical areas, all expressing LMW-2 at Glu-B3, and five bread wheats were evaluated for flour mixing properties, dough physical characteristics, and baking performance. Gluten polymeric composition was studied using size-exclusion HPLC of unreduced flour protein extracts. As a group, durum wheats had poorer baking quality than bread wheats in spite of higher protein and total polymer concentrations. Durum wheats exhibited weaker gluten characteristics, which could generally be attributed to a reduced proportion of SDS-unextractable polymer, and produced less extensible doughs than did bread wheats. However, substantial variation in breadmaking quality attributes was observed among durum genotypes.
Better baking performance was generally associated with greater dough extensibility and protein content, but not with gluten strength related parameters. Extensibility did not correlate with gluten strength or SEHPLC parameters. Genotypes expressing high molecular weight glutenin subunits (HMW-GS) 6+8 exhibited better overall breadmaking quality compared with those expressing HMW-GS 7+8 or 20. Whereas differences between genotypes expressing HMW-GS 6+8 and those carrying HMW-GS 7+8 could only be attributed to variations in extensibility, the generally inferior baking performance of the HMW-GS 20 group relative to the HMW-GS 6+8 group could be attributed to both weaker and less extensible gluten characteristics.
Durum wheat (Triticum turgidum L. var. durum) represents only ≈8% of the wheat produced worldwide. In the Mediterranean basin, however, it accounts for 50–90% of the wheat produced by the different countries in the region (Bozzini 1988). Durum grain is used for the production of numerous types of pasta products, couscous, bulgur, and other foods. Approximately 24% of the durum wheat produced worldwide, and up to 70–90% in some MiddleEastern countries, is used in small breadbaking operations (Quaglia 1988). Generally, durum wheat is considered unsuitable for commercialscale breadmaking operations because it lacks the gluten strength found in most bread wheats (T. aestivum) (Boyaçioglu and D’Appolonia 1994). The absence of the D-genome in durum wheat, particularly chromosome 1D which carries major determinants of gluten strength and baking quality in hexaploid wheat (Welsh and Hehn 1964, Morris et al 1966, Schmidt et al 1966, Morris et al 1968), provides a genetic basis for this difference. However, improvement might be possible through breeding, as suggested by studies demonstrating the existence of a substantial variability for gluten strength and baking performance among durum wheats (Dexter et al 1981, Boggini et al 1988, Boggini and Pogna 1989, Peña et al 1994). The genetic progress achievable through breeding is largely dependent on the identification of genotypes with better quality attributes and of critical traits on which selection can be based. One such trait is obviously gluten strength. However, a more extensive evaluation of the physical properties characterizing the dough produced from durum wheat flour is needed to identify additional critical parameters. Because protein composition (qualitative as well as quantitative) is the major determinant of breadmaking quality in wheat, breeding efforts should focus on the manipulation of the various gluten protein components to yield a gluten complex with optimum properties. A relationship between breadmaking quality and
allelic composition at glutenin loci Glu-B1 and Glu-B3, coding for high and low molecular weight glutenin subunits (HMW-GS and LMW-GS), respectively, was reported in sets of Italian (Boggini et al 1988, Boggini and Pogna 1989) and Mexican (Peña et al 1994) durum genotypes. The relationship between gluten strength and LMW-GS alleles is robust and well established (Liu et al 1996). However, the relationship between HMW-GS allelic composition and breadmaking quality needs to be validated in a wider array of genotypes before HMW-GS can be considered reliable markers for selecting durum wheats with improved breadmaking quality. Differences in breadmaking quality among bread wheats have been related to the gluten polymeric composition (Singh et al 1990b, Gupta et al 1992) and particularly to the proportion of large-sized, SDS-unextractable polymers (Gupta et al 1993). Differences between durum and bread wheats, as well as among durum genotypes, with regard to the latter two attributes are not well characterized. The availability of such information could be helpful in developing a breeding strategy to improve the breadmaking quality of durum wheat. The present study was undertaken to evaluate the flour mixing properties, dough physical characteristics, and baking performance of 27 durum genotypes from different geographical areas, all expressing LMW-2 (allele conferring stronger gluten characteristics than the alternate allele LMW-1) at the Glu-B3 locus. These attributes were compared with those of five bread wheat checks included in the study. The relationship between breadmaking quality attributes and allelic composition at the Glu-B1 locus was investigated. Potential differences between durum and bread wheats and among durum wheats expressing different alleles at Glu-B1 with regard to the polymeric composition and estimated size-distribution of their gluten complex were studied using size-exclusion HPLC. Finally, an attempt to relate these differences to the variation in breadmaking quality attributes observed between the different groups of genotypes was made.
1 Crop
and Soil Science Dept. Crop Science 225. Oregon State University. Corvallis, OR 97331. 2 Corresponding author. Phone: 541/737-5871. Fax: 541/737-0909. E-mail:
[email protected] 3 Crop and Soil Science Dept. Crop Science 231. Oregon State University. Corvallis, OR 97331. Phone: 541/737-3728. Fax: 541/737-0909. 4 USDA-ARS Western Wheat Quality Laboratory, E202 FSHN East, Washington State University, Pullman, WA 99164-6394. Phone: 509/335-4062. Fax: 509/335-8573. Publication no. C-2000-0212-05R. © 2000 American Association of Cereal Chemists, Inc.
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CEREAL CHEMISTRY
MATERIALS AND METHODS Plant Material and Experimental Design Twenty-seven spring durum wheat cultivars and breeding lines were selected from different geographical areas (Table I). Five bread wheat cultivars varying in breadmaking quality were included for comparison. All genotypes were grown in 5- × 1.5-m plots arranged in a randomized complete block design with two replicates during the spring of 1993 and 1994, near Pendleton, OR.
Milling, Flour Protein, and SDS-Sedimentation Test Before milling, samples (900 g) of clean grain from each plot were tempered for 24–36 hr to 17% moisture content for durum and 15.5% for bread wheat. Tempered wheat was milled in a Quadrumat Sr. mill according to the manufacturer’s instructions (Brabender OGH, Duisburg, Germany). The break and reduction flours were thoroughly mixed to yield a straight-grade flour on which subsequent analyses were performed. Percentage nitrogen was determined on 0.25 g of flour by the Dumas combustion method using a nitrogen analyzer (FP 428, Leco Corp., St. Joseph, MI) according to Approved Method 46-30 (AACC 1995) and reported as protein (N × 5.7) on a 14% moisture basis. Moisture content was determined using Approved Method 44-15A. SDS-sedimentation test was performed on 1 g of ground wheat according to Dick and Quick (1983). Sedimentation value (height of sediment in cm) was measured after 20 min in test tubes previously checked for i.d. uniformity.
the typically greater water absorption caused by high levels of starch damage occurring during milling of the much harder durum wheat grain (Dexter et al 1994, Peña et al 1994). Consequently, the alveograph test was performed for durum wheat samples with 55–59% water addition (15% moisture basis), rather than at 50% as in the original method. The resulting alveograms were used to determine the over-pressure (P, mm) as an indicator of dough tenacity or resistance to deformation, the abscissa (L, mm) at the point of bubble rupture which measures dough extensibility, and the deformation energy (W, 10–4 J) required to inflate the doughbubble until it ruptures. The configuration ratio P/L, an indicator of the rheological balance of the dough, was also computed. Baking Performance Evaluation Baking performance was evaluated by performing an optimized straight-dough bake test (Approved Method 10-10B, AACC 1995) using 100 g of flour (14% moisture basis). Optimum bake water absorption (%) and mixing time (min) were those resulting in dough with optimal handling characteristics as judged by three bakers. Loaf volume (cm3) was determined by rapeseed displacement on fresh loaves.
Mixing Properties and Dough Physical Characteristics Mixing properties were evaluated with a mixograph (National Mfg., Lincoln, NE) equipped with a 10-g bowl according to Approved Method 54-40A (AACC 1995). Enough distilled water was added to the flour (14% moisture basis) to result in a mixogram with optimum appearance, as judged by the amplitude and width of the trace. The mixing parameters measured were time-topeak (min), peak height (cm), and height of the curve after 7 min of mixing (cm), considered a measure of tolerance to over-mixing. The physical properties of the dough were analyzed on an alveograph (Chopin MA 82, Villeneuve-la-Garenne, France) according to Approved Method 54-30A. The procedure was modified for durum flours in that the volume of sodium chloride solution added to the flour was increased by 8–12% of that recommended in the original method. This modification was used to compensate for
Determination of Allelic Composition at Glutenin Loci Allelic composition at glutenin loci was determined by SDSPAGE (Laemmli 1970) fractionation of partially purified glutenin preparations (Singh et al 1991). The nomenclature used is that of Payne and Lawrence (1983) and Payne et al (1984) for the HMW glutenin alleles and the LMW glutenin alleles, respectively. Protein Extraction and SE-HPLC Analysis Total and SDS-unextractable protein were extracted following the procedures described by Gupta et al (1993). Total flour protein
TABLE I Meansa for Selected Quality Traits for Five Bread and 27 Durum Wheat Genotypes Grown Near Pendleton, OR, in 1993 and 1994 Genotype Klasic Florence-Aurore McKay Salambo Byrsa W.P.B. 881 Vic Renville ZY 8019 Mondur Valdur Monroe Quilafen OR 4910045 Valnova Capelli D86741 Altar 84 Brindur Lloyd Valgerardo D88450 Ambral Creso OR 918122 91EDUYT-12 Valgiorgio Carcomun ‘S’ OR 4910060 Razzak Valfiora Chen ‘S’ a b c
Origin CA-USA France ID-USA Tunisia Tunisia AZ-USA ND-USA ND-USA CIMMYTc France France ND-USA Chile OR-USA Italy Italy ND-USA Mexico France ND-USA Italy ND-USA France Italy OR-USA CIMMYT Italy CIMMYT OR-USA Tunisia Italy CIMMYT
HMW-GSb ... ... ... ... ... 6+8 6+8 6+8 7+8 6+8 6+8 6+8 6+8 7+8 7+8 20 6+8 7+8 20 6+8 7+8 6+8 20 6+8 7+8 7+8 20 20 20 7+8 20 7+8
% Flour Protein (14% mb)
Sedimentation Value (cm)
Alveograph W (10–4 J)
Loaf Volume (cm3)
12.7 13.0 11.3 11.9 11.8 14.4 14.6 14.8 14.7 12.3 12.8 13.9 12.8 13.7 13.8 14.1 12.4 12.4 13.5 12.9 12.3 13.0 12.4 11.8 12.4 13.1 13.1 12.0 11.8 11.8 13.2 12.0
8.2 7.4 7.3 6.7 5.1 4.9 4.0 3.8 3.7 4.5 4.3 3.4 4.2 4.2 4.2 3.9 3.1 4.0 3.0 3.7 4.1 3.6 3.4 4.2 2.5 3.7 2.6 2.8 3.2 3.8 2.6 4.0
300.2 371.1 231.5 247.9 227.2 305.3 165.4 175.6 305.4 160.4 193.3 169.2 218.6 281.3 306.8 267.2 117.7 188.8 117.8 172.9 267.7 138.2 196.4 202.0 96.0 224.9 151.6 169.6 150.4 179.9 186.6 211.0
1,081.3 993.8 956.3 938.8 861.3 816.3 793.8 781.3 781.3 743.8 738.8 728.8 712.5 692.5 682.5 677.5 645.0 628.8 627.5 621.3 608.8 606.3 598.8 576.3 567.5 562.5 535.0 497.5 497.5 473.8 463.8 442.5
Mean values represent average of four observations (two replicates, two years). High molecular weight glutenin subunits (Payne et al 1984). International Center for the Improvement of Maize and Wheat, Mexico. Vol. 77, No. 2, 2000
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was extracted by suspending 20 mg of flour in 1 mL of 0.05M sodium phosphate buffer (pH 6.95) containing 0.5% (w/v) SDS. The suspension was sonicated for 30 sec using a sonicator equipped with a 3-mm probe at maximum power (8–10W output). After clarification by centrifugation at 8,800 × g for 20 min, supernatants were filtered through a low protein-binding polyvinyledene difloride (PVDF) syringe filter with a pore size of 0.45 µm (Millipore, Bedford, MA). To quantify SDS-unextractable protein, 20 mg of flour was suspended in 1 mL of the same SDS-sodium phosphate buffer as for total protein, for 30 min with constant shaking but without sonication. After clarification by centrifugation, the residue was resuspended in 1 mL of SDS-sodium phosphate buffer with sonication (30 sec) to solubilize the SDS-unextractable fraction of the gluten polymer. After clarification, the resulting supernatant was filtered as described above. Size-exclusion HPLC analysis was performed according to the procedure described by Batey et al (1991) using a gold HPLC apparatus (Beckman, Fullerton, CA). The column was a stainlesssteel Waters Protein-Pak 300SW size-exclusion column (300 × 7.8 mm) packed with diol-coated silica beads (10 µm diameter) with an average pore size of 300 Å (Waters Corp., Milford, MA). The mobile phase was 50% (v/v) acetonitrile in HPLC-grade deionized water with 0.1% (v/v) trifluoroacetic acid, circulated at a constant flow rate of 0.5 mL/min, at room temperature (24–26°C). Eluting proteins were detected at a wavelength of 214 nm. Chromatograms were divided into three main areas: peaks I, II, and III corresponding to the polymeric, monomeric, and nongluten protein fractions, respectively (Singh et al 1990a,b; Batey et al 1991). The proportion of polymer in total protein was computed using [(area under peak I of total extract/total area of total extract) × 100]. The proportion of SDSunextractable polymer in total protein was computed using [(area under peak I of SDS-unextractable extract/total area of total extract) × 100]. The proportion of total and SDS-unextractable polymer present in the flour was determined by multiplying the proportion in total protein by the flour protein content. An additional parameter was considered: the proportion of SDS-unextractable polymer within the total polymeric fraction which was estimated using ([area under peak I of SDS-unextractable extract/area under peak I of total extract] × 100). The latter is considered an estimator of the size-distribution of the gluten polymer (Gupta et al 1993).
Statistical Analysis Statistical analyses were performed using SAS methods (SAS Institute, Cary, NC). Analysis of variance (ANOVA) for all traits was performed on data from both years combined. Genotypes were treated as fixed effects, while years were considered random effects. F-tests for significance of genotypic effects were calculated using the mean squares for year × genotype interaction as error term. Two sets of comparisons were made. First, durum wheats (27 genotypes, 108 samples) were compared to bread wheat checks (5 genotypes, 20 samples). Then, a three-way comparison was made between durum genotypes expressing HMW-GS 6+8 (11 genotypes, 44 samples), HMW-GS 7+8 (9 genotypes, 36 samples), and HMW-GS 20 (7 genotypes, 28 samples) at the high molecular weight glutenin locus Glu-B1. Statistical significance of the differences between group means was tested using pairwise t-tests (P < 0.05). Association between traits within the set of 27 durum genotypes was investigated by computing simple correlation coefficients (r) (n = 27) on genotypic means (average over replicates and years). RESULTS AND DISCUSSION ANOVA (Table II) indicated a significant year effect for all traits with the exception of tolerance to overmixing, estimated by mixogram height at 7 min. Genotypic effects were significant for all traits. Year × genotype interaction effects were also significant for all parameters measured, except for optimum dough development time (estimated by mixograph time to peak or bake mixing time) and bake water absorption. However, the mean squares for genotypes were substantially greater in magnitude than the mean squares for year × genotype interaction for all traits. Single year ANOVA (tables not shown) and mean comparisons indicate similar trends for both years. Therefore, means averaged over years are presented. As a group, durum wheats had significantly weaker gluten characteristics, produced more tenacious and less extensible doughs, and ultimately yielded smaller loaf volumes than did bread wheats (Table III). These observations could not be attributed to differences in protein quantity as durum wheats were characterized by a significantly higher flour protein concentration. Furthermore, differences
TABLE II Analysis of Variance (mean squares) for Breadmaking Quality and Size-Exclusion HPLC Parameters for Five Bread and 27 Durum Wheats Grown in a Randomized Complete Block Design Near Pendleton, OR, in 1993 and 1994 Source of Variation Year Flour protein (14% mb) Sedimentation value (cm) Mixograph time-to-peak (min) Mixograph peak height (cm) Mixograph height at 7 min (cm) Alveograph tenacity (P, mm) Alveograph extensibility (L, mm) Alveograph P/L ratio Alveograph deformation energy (W, 10–4 J) Bake absorption (%) Bake mixing time (min) Loaf volume (cm 3) % Polymer in total protein % Polymer in flour Polymer-to-monomer ratio % SDS unext. polymer in total polymer % SDS unext. polymer in protein % SDS unext. polymer in flour Degrees of freedom a b c
Rep.
CEREAL CHEMISTRY
Genotype
Y × Gb
Pooled Error
Coefficient of Variation (%)
27.57* c 1.45* 2.13* 0.45* 0.01† 3,376.37* 166.76* 1.56*
0.45†c 0.13† 0.26† 0.63* 0.26* 224.54 † 66.72 † 0.26*
3.60* 7.88* 1.13* 0.90* 0.50* 2,739.09* 1,031.53* 2.25*
1.26* 0.34* 0.16† 0.18* 0.10* 187.54* 70.97* 0.12*
0.35 0.06 0.11 0.06 0.04 74.35 31.15 0.06
4.62 5.97 12.69 4.84 4.78 8.99 8.28 15.24
9,173.35* 136.13* 2.15* 82,519.53* 103.73* 0.53* 0.0565*
943.29 † 14.63* 0.64† 3,553.91 † 1.20† 0.15† 0.0004 †
16,862.20* 19.74* 2.53* 102,645.34* 15.66* 0.75* 0.0138*
1,238.82* 2.21† 0.26† 3,143.72* 2.40* 0.19* 0.0018*
449.73 1.70 0.23 1,446.65 1.10 0.06 0.0009
10.13 1.94 13.32 5.55 2.77 5.06 0.59
39.17* 7.09* 0.16* 31
17.16 2.72 0.07 62
10.88 11.44 13.54
0.89* 172.14* 0.89* 1
4.67† 1.63† 0.05† 2
Replicates within years. Year by genotype interactions. *, † = significant and not significant, respectively (P ≤ 0.05).
232
(Year)a
371.27* 52.30* 0.97* 31
in breadmaking quality attributes could not be explained by differences in SE-HPLC parameters estimating the proportion of total gluten polymer. In fact, the proportion of polymers in the total protein as well as in flour was, on average, greater for durum than for the bread wheat checks. The average polymer to monomer ratio in durum wheat protein was significantly greater than in the bread wheat. Therefore, not considering the size-distribution of the gluten polymer, the present results suggest that the glutenin components of durum genotypes expressing LMW-2 at Glu-B3 are not deficient in the ability to form a polymeric structure. Differences in average dough development time between the two groups, a strength related attribute estimated by the mixograph time to peak or bake mixing time, are consistent with the differences observed for gluten strength. However, it should be noted that the average bake mixing time was not significantly different between the two classes, and none of the mixograph parameters, aside from time to peak, significantly differentiated between the durum and bread wheat groups. Thus, differences in bake mixing properties between the two groups were not as marked as differences in gluten strength, dough extensibility, or loaf volume. Breadmaking quality, particularly loaf volume, improves with increased gluten strength in bread wheat (Orth et al 1972, Moonen et al 1982, Campbell et al 1987, Cressey et al 1987, Branlard et al 1991, Hamer et al 1992) and durum wheat (Dexter et al 1981, Quick and Crawford 1983, Holm 1985, Boggini and Pogna 1989, Peña et al 1994). In the present study, differences in loaf volume between the durum and bread wheats can partly be explained by the weaker gluten characteristics exhibited by durum flours, evidenced by substantially lower alveograph deformation energies W and sedimentation values (Table III). Whereas mean sedimentation values (Table I) and loaf volumes separated durum wheats from bread wheats into essentially discrete classes, there was a substantial overlap between the two groups for mean deformation energies W. This suggests that part of the deformation energy of some durum genotypes with high W values did not contribute to improving the baking performance. The weaker gluten characteristics of durum wheats can partly be attributed to the reduced ability of glutenin subunits to form large-sized polymers, possibly due to a reduced intermolecular disulfide bonds formation capacity, as indicated by the substantially lower proportion of SDS-unextractable polymers (in total polymer, protein, and flour). As the size of gluten polymers is inversely related to extractability in SDS buffers and a minimum
molecular size is required for a polymer to effectively contribute to chain entanglements and tensile strength (Bersted and Anderson 1990), a lower proportion of SDS-unextractable glutenin polymers would be expected to result in a weaker gluten complex. Some of the durum genotypes were characterized by a greater proportion of SDS-unextractable polymers than some of the bread wheat checks, as indicated by the overlapping group ranges for these parameters. Therefore, additional factors that are not directly related to gluten strength, polymer size distribution, or capacity for intermolecular disulfide bonds formation are preventing the durum gluten complex from producing loaf volumes equivalent to those of bread wheats. The lack of adequate dough extensibility, as indicated by the lower alveograph L value and greater average configuration P/L observed for durum wheats, appeared to be one such factor. The continuity of the gluten complex depends not only on covalent cross-links but also on noncovalent, secondary links (hydrogen bonds and hydrophobic interactions) between adjacent polymers (Bloksma 1990). Consequently, it is conceivable that glutenin subunits or gliadin components in bread wheats have molecular features promoting better secondary interactions between adjacent gluten polymers than the analogous components in durum wheats. A considerable variability for most breadmaking quality attributes was observed within the set of 27 durum genotypes tested in the present study (Tables III and IV). This was particularly the case for loaf volume as individual sample values were 390–935 cm3. The range in genotypic means was also substantial (442.5–861.3 cm3). The identification of attributes that were associated with baking performance within the present set of genotypes was attempted using correlation analysis (Table V). The association between loaf volume and gluten strength was either weak, if sedimentation value was used as indicator of gluten strength, or nonexistent, if deformation energy W was used. The association between loaf volume and SEHPLC parameters was also weak to nonsignificant depending on which SE-HPLC parameter was considered. In fact, total protein content correlated better with loaf volume than any of the strengthrelated attributes or SE-HPLC parameters. The attribute that correlated most highly with loaf volume was dough extensibility (r = 0.80, P ≤ 0.0001) as measured by the alveograph L value. The importance of dough extensibility in relation to its effect on baking performance was emphasized in a number of studies reporting strong associations between loaf volume and dough extensibility in bread wheats (Shogren et al 1962, Bettge et al 1989, Addo et al
TABLE III Comparison Between Group Meansa and Rangesb of Breadmaking Quality and Size-Exclusion HPLC Parameters for Five Bread and 27 Durum Wheats Grown Near Pendleton, OR, in 1993 and 1994 Bread Wheats (5 genotypes, 20 samples) Mean Flour protein (14% mb) Sedimentation value (cm) Mixograph time to peak (min) Mixograph peak height (cm) Mixograph height at 7 min (cm) Alveograph tenacity (P, mm) Alveograph extensibility ( L, mm) Alveograph P/L ratio Alveograph deformation energy ( W, 10–4 J) Bake absorption (%) Bake mixing time (min) Loaf volume (cm 3) % Polymer in total protein % Polymer in flour Polymer-to-monomer ratio % SDS unext. polymer in total polymer % SDS unext. polymer in protein % SDS unext. polymer in flour a b c
12.2a c 7.0b 3.2b 5.2a 4.1a 82.4a 91.3b 0.94a 275.6b 64.7a 4.0a 966.3b 35.8a 4.5a 0.64a 52.7b 19.1b 2.4b
Range 10.6–13.4 4.4–8.5 1.6–4.0 4.5–5.9 3.2–4.5 60.9–99.3 57.1–108.6 0.57–1.63 195.1–398.2 60.3–70.0 2.8–6.0 800–1,110 27.9–41.3 3.3–5.2 0.44–0.79 34.5–65.2 10.5–24.6 1.3–3.1
Durum Wheats (27 genotypes, 108 samples) Mean 13.0b 3.7a 2.6a 5.0a 4.2a 98.4b 63.0a 1.70b 197.0a 67.9b 3.6a 633.4a 38.3b 5.1b 0.71b 35.4a 13.5a 1.8a
Range 10.9–16.8 2.2–5.4 1.5–4.5 3.6–6.4 3.1–5.1 43.6–160.6 32.9–91.7 0.65–3.98 63.6–365.5 62.9–73.2 2.2–8.6 390–935 33.8–43.2 4.2–6.2 0.59–0.86 17.9–55.3 6.6–22.0 0.9–3.3
Each genotype within a group is represented by four samples (two field replicates for each crop year). Ranges of individual sample values within each group (not ranges of genotypic means). Values followed by the same letter in the same row are not significantly different (P < 0.05). Vol. 77, No. 2, 2000
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TABLE IV Comparison Between Group Meansa and Rangesb of Breadmaking Quality and Size-Exclusion HPLC Parameters for 27 Durum Wheats Grouped by HMW-GSc Composition, Grown Near Pendleton, OR, in 1993 and 1994 HMW-GS 6+8 (11 genotypes, 44 samples) Mean Flour protein (14% mb) Sedimentation value (cm) Mixograph time to peak (min) Mixograph peak height (cm) Mixograph height at 7 min (cm) Alveograph tenacity (P, mm) Alveograph extensibility ( L, mm) Alveograph P/L ratio Alveograph deformation energy ( W, 10–4 J) Bake absorption (%) Bake mixing time (min) Loaf volume (cm 3) % Polymer in total protein % Polymer in flour Polymer-to-monomer ratio % SDS unext. polymer in total polymer % SDS unext. polymer in protein % SDS unext. polymer in flour
13.2a d 4.0b 2.6b 5.0a 4.2a 83.3a 70.8b 1.2a 183.5a 67.7a 3.4b 705.8b 38.2b 5.2a 0.71a 37.4b 14.3b 1.9b
Range
HMW-GS 7+8 (9 genotypes, 36 samples) Mean
10.9–16.8 2.7–5.4 1.5–4.1 4.0–6.0 3.1–5.0 55.2–147.4 42.9–91.7 0.6–2.9 99.8–365.5 63.4–71.8 2.4–5.4 535–935 34.8–40.7 4.2–6.2 0.60–0.79 24.0–55.3 9.4–22.0 1.2–3.3
12.9a 3.8b 2.9a 4.9a 4.3a 114.8b 56.9a 2.1b 229.1b 68.2a 4.1c 604.4a 37.8a 5.0a 0.70a 39.4b 14.9b 2.0b
Range 11.0-15.6 2.2-4.7 1.9-4.5 3.6-6.4 3.1-5.1 43.6-159.5 32.9-76.2 0.7-4.0 63.6-344.8 63.1-73.2 2.2-8.6 390-810 33.8-41.7 4.2-5.8 0.59-0.82 25.7-54.6 8.8-21.3 1.1-2.7
HMW-GS 20 (7 genotypes, 28 samples) Mean 12.9a 3.1a 2.1c 5.3b 4.2a 101.1b 58.5a 1.9b 177.1a 67.9a 3.1a 556.8a 38.9b 5.1a 0.73b 27.1a 10.6a 1.4a
Range 10.9–15.0 2.2–4.2 1.5–2.7 4.6–6.2 3.8–5.0 58.4–160.6 33.8–90.5 0.7–3.5 108.2–313.6 62.9–72.8 2.3–3.9 405–730 36.5–43.2 4.5–5.8 0.66–0.86 17.9–41.7 6.6–16.1 0.9–2.2
a
Each genotype within a group is represented by four samples (two field replicates for each crop year). Ranges of individual sample values within each group (not ranges of genotypic means). c High molecular weight glutenin subunit. d Values followed by the same letter in the same row are not significantly different (P < 0.05). b
1990, Branlard et al 1991, Janssen et al 1996). In the present study, dough extensibility was essentially unrelated to gluten strength as evidenced by the lack of correlation with either sedimentation value or deformation energy. In addition, it was not directly correlated with any of the parameters estimating the proportion of SDSunextractable polymers, indicating that, unlike gluten strength, it was not directly related to the size-distribution of the durum gluten polymer or to the glutenin subunits ability to form intermolecular disulfide bonds. When the durum genotypes were grouped according to allelic composition at the HMW-GS locus Glu-B1 (Tables I and IV), genotypes expressing HMW-GS 6+8 were characterized by higher overall breadmaking quality than those expressing either alternate alleles at Glu-B1. Genotypes from this group were characterized by a significantly greater dough extensibility and a mean configuration ratio P/L that was closest to that of the bread wheat checks. They produced bread loaves that were, on average, larger than those produced by genotypes carrying HMW-GS 7+8 or HMWGS 20. Although, as previously mentioned, loaf volume was associated with flour protein concentration when all durum genotypes are considered (Table V), differences in flour protein concentration between HMW-GS groups were not significant (Table IV). In addition, differences between HMW-GS groups in terms of the proportion of total polymer (in protein and in flour) and polymer-to-monomer ratio were not consistent with those observed for loaf volume. Therefore, the relatively better baking performance observed for genotypes expressing HMW-GS 6+8 could not be attributed to either total protein or total polymer concentrations. Differences in gluten strength between the three groups were not as pronounced as between durum and bread wheats, and different measurements of strength yielded different results. The deformation energy W and dough development time parameters were, on average, greater for genotypes expressing HMW-GS 7+8 than for those carrying HMW-GS 6+8. However, this apparent indication of a greater gluten strength did not translate into better baking performance of the HMW-GS 7+8 group compared with the HMW-GS 6+8 group. Furthermore, there were no significant differences between these two groups in average sedimentation value or SE-HPLC parameters related to the SDS-unextractable polymeric fraction of the gluten complex. Whereas no significant difference 234
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was observed between genotypes expressing HMW-GS 20 and HMW-GS 6+8 in terms of the mean deformation energy W, the latter group was characterized by a significantly greater dough development time, sedimentation value, and SDS-unextractable polymer concentration than the former. These observations suggest that, in the present set of genotypes, the deformation energy W provided an inflated estimation of strength, particularly for genotypes expressing HMWGS 7+8 and HMW-GS 20. This apparent strength was not confirmed by the “chemical” indicators of gluten strength such as sedimentation values and SDS-unextractable polymer concentration. Higher deformation energies observed for these two groups of genotypes were generally the result of an excessive tenacity (P) with no coincident increase in extensibility (L). This is evident even when all genotypes are considered together (Table V) as the deformation energy was highly correlated with tenacity (r = 0.84, P ≤ 0.0001) but was not at all associated with extensibility (r = –0.14, P ≤ 0.46). The difference in baking performance between durum genotypes expressing HMW-GS 6+8 and those carrying HMW-GS 7+8 could not be attributed to differences in gluten strength or gluten polymer size. These differences were related to differences in dough extensibility, which suggests that subunits 6+8 might possess some molecular features that promote better secondary interactions between adjacent gluten polymers than those resulting from the presence of subunits 7+8. On the other hand, the superior baking performance of genotypes expressing HMW-GS 6+8 relatively to those expressing HMW-GS 20 seems to be attributable to a combination of greater gluten strength and dough extensibility of the former group. A relationship between breadmaking quality and allelic variation at Glu-B1 was reported in sets of Italian (Boggini et al 1988, Boggini and Pogna 1989) and Mexican (Peña et al 1994) durum genotypes. However, both of these studies associated the presence of HMW-GS 7+8 with better baking performance than that of HMW-GS 6+8, which is clearly in disagreement with the results of the present evaluation. This discrepancy originates from the fact that different attributes were most critical to better baking performance in the different sets of genotypes tested. Whereas gluten strength was the major factor affecting baking performance in the studies mentioned above, dough extensibility (which was greatest for the HMW-GS 6+8 group) was the most important attribute for
TABLE V Simple Correlation Coefficients (r)a Between Breadmaking Quality and Size-Exclusion HPLC Parameters and Flour Protein, Sedimentation Value, Alveograph Values, and Loaf Volume for 27 Durum Wheat Genotypes Grown Near Pendleton, OR, in 1993 and 1994 Flour Protein Flour protein (14% mb) Sedimentation value (cm) Mixograph time to peak (min) Mixograph peak height (cm) Mixograph height at 7 min (cm) Alveograph tenacity (P, mm) Alveograph extensibility ( L, mm) Alveograph P/L ratio Alveograph deformation energy ( W, 10–4 J) Bake absorption (%) Bake mixing time (min) Loaf volume (cm 3) % Polymer in total protein % Polymer in flour Polymer-to-monomer ratio % SDS unext. polymer in total polymer % SDS unext. polymer in protein % SDS unext. polymer in flour a b
Sedimentation Value 0.19
0.19 –0.01 0.70** 0.54** 0.01 0.56** –0.27 0.40* 0.64** –0.18 0.72** –0.32 0.87** –0.16 0.27 0.20 0.49**
0.54** 0.02 0.52** 0.35 0.18 0.14 0.62** 0.45* 0.53** 0.56** 0.24 0.33 0.23 0.71** 0.74** 0.72**
Alveograph W 0.40* b 0.62** 0.41* 0.44* 0.85** 0.84** –0.14 0.53** 0.78** 0.49** 0.34 0.02 0.43* –0.00 0.63** 0.62** 0.68**
Alveograph L 0.56** 0.18 –0.21 0.37 0.12 –0.61** –0.86** –0.14 0.09 –0.50** 0.80** –0.26 0.44* –0.27 0.02 –0.03 0.14
Loaf Volume 0.72** 0.56** 0.11 0.54** 0.54** –0.16 0.80** –0.51** 0.34 0.47* –0.16 –0.29 0.60** –0.31 0.42* 0.36 0.55**
Correlation analysis performed on genotypic means (average of four observations; two replicates for two years). *, ** = significant at P = 0.05 and P = 0.01, respectively.
breadmaking quality in the present set of genotypes. In any event, these conflicting conclusions suggest that the allelic composition at Glu-B1 per se cannot be considered a reliable indicator of the breadmaking quality potential of a durum genotype. In summary, the results of the present study indicate that, for durum wheats to achieve baking performances comparable to those of bread wheats, substantial improvements in gluten strength and, most importantly, gluten extensibility are required. SE-HPLC analysis revealed that a proportion of large-sized polymers (SDSunextractable) equivalent to that found in bread wheats is not sufficient, as it contributes to improving the durum breadmaking quality only up to a certain level (through an increase in gluten strength). Components with molecular properties that increase gluten extensibility, presumably by enhancing secondary interactions between gluten polymers, appear to be required to produce bread loaves comparable to those produced from bread wheat flours. SEHPLC analysis failed to provide parameters that could be used as reliable predictors of dough extensibility or baking performance. Therefore, based on our results, its usefulness as a tool to aid in selecting for improved breadmaking quality potential in durum wheat appears to be limited. Durum genotypes expressing HMWGS 6+8 were characterized by a greater dough extensibility and better overall baking performance than those expressing HMW-GS 7+8 or HMW-GS 20. However, conflicting conclusions reported in other studies as to which allele is associated with better baking performance suggest that the relationship between allelic composition at Glu-B1 and breadmaking quality is dependent on the set of genotypes evaluated. Therefore the use of Glu-B1 encoded HMWGS alleles as markers to select for improved breadmaking quality in durum wheat might not be justified. ACKNOWLEDGMENTS We thank Doug A. Engels, Herb C. Jeffers, Mary L. Baldridge, and Art D. Bettge from the USDA-ARS Western Wheat Quality Laboratory at Pullman, WA, for their expert help during the baking performance evaluation. We thank Robert B. Drynan and Mark Kruk from the Wheat Marketing Center in Portland, OR, for providing us access to their alveograph. LITERATURE CITED Addo, K., Coahran, D. R., and Pomeranz, Y. 1990. A new parameter related to loaf volume based on the first derivative of the alveograph curve. Cereal Chem. 67:64-69. American Association of Cereal Chemists. 1995. Approved Methods of the AACC, 9th ed. Method 10-10B, approved November 1995; Method
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[Received March 10, 1999. Accepted December 7, 1999.]
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