Water-Extractable Nonstarch Polysaccharide Distribution in Pilot Milling Analysis of Soft Winter Wheat

Cereal Chem. 88(5):525–532 Commercial wheat (Triticum aestivum em. Thell) flour milling produces flour streams that differ in water absorption levels because of variability in protein concentration, starch damaged by milling, and nonstarch polysaccharides. This study characterized the distribution of water-extractable (WE) nonstarch polysaccharides (NSP) in long-flow pilot-milling streams of soft wheat to model flour quality and genetic differences among cultivars. Existing reports of millstream analysis focus on hard wheat, which breaks and reduces differently from soft wheat. Seven soft winter wheat genotypes were milled on a pilot-scale mill that yields three break flour streams, five reduction streams, and two resifted streams. Protein concentration increased linearly through the break streams. WENSP concentration was low and similar in the first two break streams, which are the largest break streams. Flour recovery decreased exponentially through the reduction streams; flour ash and water-extractable glucose and galactose polymers increased exponentially through the reduction streams. Protein concentration and WE xylan concentration increased linearly through the reduction streams. The ratio of arabinose to xylose in WE arabinoxylan (WEAX) decreased through the reduction streams, and response varied among the genotypes. Flour ash was not predictive of stream composition among genotypes, although within genotypes, ash and other flour components were correlated when measured across streams. The second reduction flour stream was the largest contributor to straight-grade flour WEAX because of both the size of the stream and the concentration of WEAX in the stream. Bakery performance of soft wheat flour across a wide range of products is defined by protein, damaged starch, and water-extractable arabinoxylan (WEAX) concentrations (Guttieri et al 2001; Gaines 2004). Yet soft wheat flour is traded primarily on the basis of flour ash and protein concentrations, and flour mill profitability is determined primarily by flour extraction rate. Increasing the aleurone and bran content in milled white flour typically increases water absorption and decreases soft wheat quality as flour extraction rates increase. The greatest mineral concentrations are in the aleurone and then the bran layers, the same layers that contribute to increased water absorption within soft wheat. This assumption underlies using flour ash as an indicator of quality in soft wheat (Souza et al 2002). Greater water absorption by flour decreases the suitability of flour for cookies, crackers, and other foods in which a finished baked or cooked product has a low moisture content. Starch damage resulting from milling is detrimental to the pastry quality of soft wheat flours (Gaines et al 1988; Barrera et al 2007). Starch damage is associated with increased water absorption. Flour milling alters the microstructure of the intact endosperm tissue. The compact, densely packed starch granules from the intact kernel are dispersed upon milling into aggregates of starch granules and protein matrix. In cryogenic scanning electron microscopy, some starch granules appear physically deformed as a consequence of milling (Rojas et al 2000). A subsequent study (Gangadharappa et al 2008) observed increased deformation of A-type starch granules with increasing severity of reduction milling. These studies, however, examined grain used for bread products rather than pastry products, milled using equipment optimized for production rather than for minimizing starch damage. Soft wheat has substantially less starch damage than hard wheat does (Ziegler and Greer 1971) when milled under comparable conditions. It is inherently misleading to generalize from hard wheat milling results to soft wheat milling. Hard wheat produces relatively little flour from the break streams compared with soft wheat (Ziegler and Greer 1971). Hard wheat kernels break to produce a more uniform distribution of particles than do soft wheat kernels. By contrast, soft wheat kernels break to give large proportions of small and large particles and few in the midsize range (Campbell et al 2007). The first break rolls are a critical control point in milling because particle-size distribution from the first break roll determines the stream flows for the remainder of the milling process (Fig. 1). For exam1 United States Department of Agriculture, Agricultural Research Service, Soft Wheat Quality Laboratory, Wooster, OH 44691. 2 Corresponding author. E-mail: [email protected] 3 Ohio State University, Ohio Agricultural Research and Development Center,