Changes in composition and amino acid profile during dry grind ethanol processing from corn and estimation of yeast contribution toward DDGS proteins.

Authors: HAN, JIANCHUN - Northeast Agricultural University; Liu, Keshun

Submitted to: Journal of Agricultural and Food Chemistry
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/1/2010
Publication Date: 5/3/2010
Citation: Han, J., Liu, K. 2010. Changes in composition and amino acid profile during dry grind ethanol processing from corn and estimation of yeast contribution toward DDGS proteins. Journal of Agricultural and Food Chemistry. 58:3430-3437.

Interpretive Summary: A major process for making ethanol from corn is the dry-grind method, which produces distillers dried grains with solubles (DDGS) as a major co-product. Production of DDGS has increased significantly in recent years, as the number of dry grind ethanol production facilities increases. DDGS has a valuble nutrient profile and currently, most DDGS is being used for animal feed, including fish feed. Factors that affect quality and marketability can have marked effects on value of the co-product. One major factor is variation in chemical composition. One cause for varying DDGS composition could be differences in processing methods. Another cause could be varying contribution of yeast biomass, particularly with respect to protein contribution from yeast. Yet, the proportion of yeast protein to corn protein in DDGS is not well documented in the literature. This significance of this study lies in three aspects. First, it was the first time to monitor changes in contents of protein, oil, ahs, starch, total carbohydrate and total non-starch carbohydrate throughout the whole dry grind ethanol process from corn. It was also the first time to document changes in amino acid profile during the dry grind process. Second, the study was the first time to propose a multiple linear regression model, linking AA profile of a downstream product as a function of AA profiles of ground corn and yeast. By regression of this multiple linear model, a good estimate of yeast contribution toward DDGS models could be obtained. Regression results indicated that yeast contributed about 20% toward DDGS proteins, and the rest came from corn. Third, the observations obtained from the sample set collected from one ethanol plant were all confirmed from sample data of two additional ethanol plants. Information gained in this study can help us better understand causes for nutrient variation in DDGS, and develop strategies to modify processing steps for maximum balance of nutrients in DDGS.

Technical Abstract: Three sets of ground corn, yeast, intermediate masses and DDGS were collected from three commercial ethanol plants in Iowa. Samples were freeze-dried before chemical analysis. Compared to ground corn, there was slight increase in contents of protein, oil, ash, and amino acids (AA) before fermentation although starch/dextrin decreased sharply upon saccarification. Upon fermentation, starch content further decreased to about 6.0%, while protein, oil, ash contents increased over 3 fold. Total carbohydrate content decreased by 40%, and the content of total non-starch carbohydrate increased over 2.5 fold. Concentrations of these attributes fluctuated slightly in the remaining downstream products, although oil and ash were concentrated in thin stillage while protein in distillers grains upon centrifugation. Data from the three plants gave similar observations. When AA is expressed as relative % (protein based), the change in AA profile did not follow that of protein content. Upon fermentation, some AA increased, others decreased, and still others unchanged. Furthermore, the influence of yeast AA profile on those of downstream products became apparent. Accordingly, a multiple linear regression model for AA profile of a downstream product as a function of AA profiles of ground corn and yeast was proposed. Regression results indicated that yeast contributed about 20% toward DDGS proteins, and the rest came from corn.