Changes in Composition and Amino Acid Profile during Dry Grind Ethanol Processing from Corn and Estimation of Yeast Contribution toward DDGS Proteins

Three sets of samples, consisting of ground corn, yeast, intermediate products, and DDGS, were provided by three commercial dry grind ethanol plants in Iowa and freeze dried before chemical analysis. On average, ground corn contained 70.23% starch, 7.65% protein, 3.26% oil, 1.29% ash, 87.79% total carbohydrate (CHO), and 17.57% total nonstarch CHO, dry matter basis. Results from Plant 1 samples showed that compared to ground corn, there was a slight but significant increase in the contents of protein, amino acids (AA), oil, and ash before fermentation, although starch/dextrin decreased sharply upon saccharification. After fermentation, starch content further decreased to about 6.0%, while protein, oil, and ash contents increased over 3-fold. AA increased 2.0−3.5-fold. Total CHO content decreased by 40%, and the content of total nonstarch CHO increased over 2.5-fold. Concentrations of these attributes fluctuated slightly in the remaining downstream products, but oil and ash were concentrated in thin stillage, while protein was concentrated in distiller grains upon centrifugation. When AA composition is expressed in relative % (protein basis), its changes did not follow that of protein concentration, but the influence of yeast AA profiles on those of downstream products became apparent. Accordingly, a multiple linear regression model for the AA profile of a downstream product as a function of AA profiles of ground corn and yeast was proposed. Regression results indicated that, with an r 2 = 0.95, yeast contributed about 20% toward DDGS proteins, and the rest came from corn. Data from Plants 2 and 3 confirmed those found with Plant 1 samples.