Soft Wheat Quality as Related to Protein Composition

Cereal Chern. 76(5):650-655 Soft red and white winter wheats from the eastern United States, used primarily to produce cookies, cakes, and biscuits, have quality requirements very different from those of bread wheats. In general, soft wheats have been bred to have low protein content, and conventional wisdom has been that protein composition of soft wheat is relatively unimportant. To test this hypothesis, relationships between soft wheat protein composition and end-use functional quality characteristics were examined. Quantitative protein compositions of eight cultivars of soft wheats grown in a wide area of the eastern United States during seven years (53 samples total) were analyzed by size-exclusion HPLC. Results were statistically correlated with numerous chemical and physical characteristics and quality factors of these wheats, their flours, and of cookies baked from the flours. Since HPLC became available, many studies have related composition, amount, and size of proteins of hard red winter and spring wheats to baking quality (Huebner and Bietz 1994). In one early study, reversed-phase HPLC also related a durum wheat gliadin fraction to pasta-making quality (Burnouf and Bietz 1984). Methods using size-exclusion HPLC have become especially valuable for indicating quality (Graybosch et al 1994, Huebner and Bietz 1994). One recent study showed amounts of hard red winter wheat y-gliadins correlated almost perfectly with loaf volume (Huebner et al 1997). Such analyses are quick, accurate, and can be done on small flour samples. Because the composition and amount of proteins within individual cultivars also vary with environment, however, it is nearly impossible to predict breadmaking quality solely on the basis of cultivar. HPLC may identify potential problems during breadmaking by indicating flours unsuitable for specific uses. Soft wheats have quality factors and end uses very different from those of hard wheats. Soft wheats typically have been bred to have low protein content. This has contributed to the general belief that, in soft wheats, protein composition is of little importance. There is much quality variation among soft wheats, however; some soft wheat cultivars can be used to produce breads as well as typical cookie and cake products. This suggests that proteins may, in fact, also be major contributors to soft wheat functionality and product quality. In contrast to research on proteins of hard wheats, there have been relatively few studies of the relationship between soft wheat proteins and products. Methods developed for and applicable to hard wheats have provided some new information but have been of limited usefulness for indicating suitability of soft wheats for products as diverse as cookies, cakes, gravies, and breadings. We therefore developed and applied a procedure for detailed quan1 Presented in part at the AACC 80th Annual Meeting, San Antonio, TX, November 1995. 2 National Center for Agricultural Utilization Research, Biomaterials Processing Research, USDA, Agricultural Research Service, 1815 N. University Street, Peoria, II. 61604. Names are necessary to report faetual1y on available data; however, the USDA neither guarantees nor warrants the standard of the product, and the use of the name by USDA implies no approval of the product to the exclusion of others that may also be suitable. 3 Corresponding author. E-mail: [email protected] 4 Midwest Area, USDA-ARS, 1815 N. University St., Peoria, II. 61604. 5 USDA-ARS, Soft Wheat Quality Laboratory, Wooster, OH 44691. Publication no. C-1999-0727-03R. This article is in the public domain and not copyrightable. It may be freely reprinted with customary crediting of the source. American Association of Cereal Chemists, Inc., 1999. 650 CEREAL CHEMISTRY For the entire sample set, wheats containing high molecular weight glutenin subunits 2+12 showed significantly different properties and cookie characteristics from those with subunits 5+10, but amounts of most individual fractions correlated poorly with quality descriptors. For individual soft wheat cultivars, however, amounts of many individual gliadin and glutenin subfractions correlated significantly with quality descriptors such as SDS sedimentation, mixograph absorption, peak mixing time, mixograph number, cookie diameter, and top grain. Protein contents as a function of genotype and environment also differed greatly among cultivars, as did ratios of gliadin to glutenin. These results clearly revealed that suitability of soft wheat cultivars for specific products can be rapidly determined by quantitative and qualitative analyses of protein composition. titative analysis of soft wheat flour proteins using HPLC. Protein compositions were statistically related to wheat and flour functional properties and to product quality. Results show that rapid quantitative protein compositional analysis by HPLC can identify and select soft wheats with optimal properties for specific end uses. MATERIALS AND METHODS Samples Flour samples were from the Soft Wheat Quality Laboratory (SWQL), Wooster, OH. Cultivars used were six soft red winter wheats (Argee, Becker, Caldwell, Cardinal, Pioneer 2555, and Tyler) and two soft white winter wheats (Augusta and Frankenmuth) grown during a seven-year period over a wide area of the eastern United States. These cultivars were chosen because of their known variation in biochemical, milling, and baking properties, as well as variation in protein content and other physical characteristics. Fifty-nine wheats and their flours had been subjected to 27 biochemical, milling, compositional, and cookie baking tests at the SWQL (Finney and Bains, ill press). In the present study, 53 of these wheat samples were used. Protein Extraction Flour samples (75 mg) were first extracted at 3-5°C with 1 mL of 0.05M NaCI for 20 min with vortexing to remove most albumins and globulins, including proteases. After centrifugation, this extract was discarded. Gliadins were then extracted at room temperature with 0.9 mL of 70% ethanol in lO-mL polypropylene tubes for 30 min with vortex mixing. After centrifugation at 15,000 x g for 10 min, the residue was reextracted with 0.6 mL of ethanol as above. After centrifugation, extracts were combined for HPLC analyses (Huebner and Bietz 1986). Glutenins were then extracted from the residue with 1 mL of 5M urea in 0.05M sodium phosphate buffer, pH 7.7, containing 0.1% dithiothreitol. After 2 hr of vortex mixing at 33°C, 5 JlL of 50% 4-vinylpyridine in 60% propanol were added to alkylate cysteine residues. After brief vortex mixing, samples were centrifuged at 17,000 x g for 10 min, and clear supernatants were transferred to autosampler vials. Approximately 1 hr after alkylation, sample pH was lowered to 3.1 ± 0.2 by addition of 4-5 JlL of 60% trifluoroacetic acid in concentrated acetic acid. SE·HPLC Gliadin and reduced-alkylated glutenin fractions were analyzed by SE-HPLC with an SP8700 solvent delivery system and an SP8780XR autosampler (Thermo Separations Products, San Jose, CA) using a 1x 30-cm Superose-12 column (pharmacia LKB BiG-