Submitted to: Inform
Publication Type: Other
Publication Acceptance Date: 3/15/2011
Publication Date: 4/20/2011
Citation: Han, J.C., Liu, K.S. 2011. Estimating the effect of fermentation yeast on distillers grains protein. INFORM, 22(4):244-247. Interpretive Summary: Increasing demand for ethanol as a gasoline additive and a desire to decrease dependency on fossil fuels have resulted in a dramatic increase in fuel ethanol production. According to the 2010 Ethanol Industry Outlook, ethanol bio-refineries in the United States converted 3.8 billion bushels of corn in 2009 into an estimated 10.6 billion gallons of ethanol and 30.5 million metric tons (MMT) of high-value livestock feed. The major method for making ethanol from corn is dry-grind processing, which includes basic steps of grinding (dry milling), slurrying, cooking, liquefaction, saccharification, fermentation, distillation, and co-product recovery. The major co-product is distillers dried grains with solubles (DDGS). It is common knowledge that a well-selected strain of yeast is essential for efficient fermentation during the production of ethanol fuel. Much less is known about the effect of fermentation yeast on the quality of ethanol co-products. This feature article discusses four methods for estimating the effect of fermentation yeast on quantity and quality of proteins in distillers grains products. Since the amino acid composition of yeast is nutritionally more balanced than that of corn, investigating yeast’s effect and accurately estimating its contribution toward DDGS will have a positive impact on feed and ethanol industries and at the same time increase our basic understanding of the processing system.
Technical Abstract: Distillers dried grains with solubles (DDGS) is the key co-product of bio-ethanol production from grains. Major factors affecting its quality and market values include protein quantity (concentration) and quality (amino acid composition). Yet, the effect of fermentation yeast on DDGS quality has not been well documented. This feature article discusses 4 methods that have been described to estimate yeast contribution in distillers grain products. Method 1 is by hamacytometer counting of yeast cells, which estimates 20% yeast by mass in distillers dried solubles. Method 2 is by assumption and calculation, which gives 3.9 % by mass and 5.3% by protein in DDGS. Method 3 is by averaging ratios in essential amino acid (AA) contents of yeast/DDGS, which indicates that yeast could provide up to 50% protein in DDGS. Method 4 is by a multiple linear regression of AA composition (% relative to total AA) of DDGS with those of corn and yeast as two independent variables, based on a recently published ARS study by the authors. It shows that fermentation yeast contributes about 20% DDGS protein, while corn accounts for the remaining 80%. Although the estimated results vary greatly with the methods chosen, a comparison seems to indicate that the multiple linear regression method is by far the most accurate estimation for the effect of yeast on the proteins of not only DDGS but also other downstream products.