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Relations of Grain Proximate Composition and Physical Properties to Wet-milling Characteristics of Maize

S.R. Fox2, L.A. Johnson2,3, C.R. Hurburgh, Jr.2, and C. Dorsey-Redding2.

The relations of proximate composition and physical properties of 27 maize hybrids to laboratory wet-milling characteristics were determined. No single trait accounted for more than 40% of the variation in starch yield or 60% of the variation in protein content of the recovered starch. Merely having greater starch content in the kernel did not increase starch yields (r = 0.20). Hybrids with smaller protein contents (r = -0.63) and greater thousand grain weights (r = 0.43) yielded more starch, whereas hybrids with greater protein contents (r = 0.77) and with harder endosperms (r = 0.58) gave greater residual protein contents of recovered starch. The best models for predicting starch yields included: 1) grain protein content; and 2) one of the following, test weight, absolute density, kernel hardness (Stenvert sample height), and water absorptivity (index or initial rate). The preferred model for starch yield (percentage of starch yield = 58.2 - 3.6 (percentage grain protein) + 0.5 (test weight)) accounted for 61% of the variation. Starch protein-content was a function of the grains' protein and oil contents. The preferred model for protein content of recovered starch (percentage protein in starch = -1.28 + 0.23 (percentage grain protein) + 0.13 (percentage grain oil)) accounted for 66% of the observed variation.

The growing importance of wet-milling and other maize-processing industries has heightened interest in the relations of grain quality factors to end-use value. Current grading standards do not relate well to estimated end-use value of maize processed by the wet-milling industry (Watson 1987a). Based on grain proximate composition, the values of maize hybrids to wet mills have been estimated to range by as much as 15% (Hurburgh 1990). Maize with greater-than-normal starch contents are believed to have greater end-use values. Factors other than starch content of the grain, however, probably also relate to yields of starch and other high-value products from wet-milled maize. Understanding these relations may lead to a means of evaluating wet-milling potential at the time of first sale and/or purchase by wet-milling companies. It may also lead to improved hybrids yielding greater proportions of high-valued products, or ones that are more amenable to processing with reduced costs.

Watson and Hirata (1954) reported on a method for determining millability of steeped grain by visually estimating the amount of starch released when thinly sectioned slices of steeped maize kernels were brushed. No correlation was found between the visual millability score and the yield of starch from wet milling. Freeman and Watson (1969) reported that gluten/starch separation in graduated cylinders was indicative of millability. Maize of high millability yielded sharp delineations between the milky, white starch layer at the bottom of the cylinder and the less dense, but highly pigmented, gluten layer. These investigators, however, did not establish quantitative relations among the relative proportions, sharpness of delineation between deposits, and starch recovery in wet milling.

Freeman (1973) concluded that test weight, kernel size, stress cracks, and moisture content of maize gave little indication of product yields upon wet milling. Likewise, Vojnovich et al (1975) observed no significant correlation between starch recovery and test weight, although they did find a high negative correlation (r = -0.93) between starch yield and drying temperature. Brown et al (1979) reported quantitative relations between steeping index (starch yield) and test weight (r = 0.57), stress cracks (r = 0.47), and viability (r = 0.54). They concluded that none of the measures good predictors of wet-milling performance.

Watson (1987b) contended that, based on experience kernel hardness, density, and breakage susceptibility are important in determining yields of wet-milled products, but presented no statistical correlations of these properties to product yields. Weller (1987) and Weller et al (1988) related starch yield to starch content (r = -0.65), test weight (r = 0.24), and ethanol-soluble protein (r = 0.45) in the grain. They examined only four hybrids for differences in wet-milling characteristics, and thus, their correlations were not statistically significant. They also observed that starch yield decreased as harvest moisture and drying temperature increased.

Laboratory wet milling is time-consuming and labor-intensive. It is impractical to use such procedures to predict the wet-milling performance of the grain at the time of sale. Rapid methods for determining quality factors indicative of wet-milled product yields would be very useful to the wet-milling industry and to breeders (Biss and Cogan 1988). The objective of this research was to determine the relations (correlations) of various grain composition factors and physical properties to yields and compositions of the products recovered from laboratory wet milling.