Chemical composition of distillers grains, a review

Authors: Liu, Keshun

Submitted to: Journal of Agricultural and Food Chemistry
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/20/2010
Publication Date: 2/7/2011
Citation: Liu, K. 2011. Chemical composition of distillers grains, a review. Journal of Agricultural and Food Chemistry. 59:1508-1526.

Interpretive Summary: During the past decade, in the United States and elsewhere, the fuel ethanol industry has experienced a phenomenal surge in growth. In the U.S. alone, ethanol production increased from about 6.5 billion liters in 1999 to over 39 billion liters in 2009 (Renewable Fuels Association, 2010). This worldwide increase has been driven mostly by increasing demand for motor fuels as well as government mandates for alternate fuel oxygenates. At present the global ethanol supply is produced mainly from starchy or sugary grain feedstock, while a major process for making ethanol from starchy grains is the dry-grind method, which produces distillers dried grains with solubles (DDGS) as a major co-product. Like fuel ethanol, DDGS has quickly become a global commodity for trade. Distillers grain (DG) has a valuble nutrient profile, but its high compositional variation has been the main problem hindering DG use as a feed ingredient. Although DG has been in the market for a century, its surge in global supply in recent years has stimulated many new investigations into this important co-product of biofuel production. In particular, chemical composition of DG has been of great interest to researchers in animal science, ethanol producers, and traders in feed industry. Literature on the subject is abundant, but a large portion, particularly those published before 1990, dealt with DG from beverage ethanol production. There lacks a review with updated information about DG from fuel ethanol production. Thus, the objective of this article is to provide in-depth and up-to-date coverage of chemical composition of DG in term of concentrations (quantity) and composition (quality) of major and minor nutrients, changes during dry-grind processing, and the underlying causes for their higher variation in DG (as compared with other protein feeds, such as soy meal). In the final section, mycotoxins in DG are also discussed since there is an increasing concern with the issue. The ultimate goal is to develop strategies to control variation of DG quality, limit levels of mycotoxins, and thus solve the current major problems with use of DDGS as a feed ingredient.

Technical Abstract: In recent years, increasing demand for ethanol as a fuel additive and decreasing dependency on fossil fuels have resulted in a dramatic increase in the amount of grains used for ethanol production. Dry-grind is the major process, resulting in distillers dried grains with solubles (DDGS) as a major co-product. Like fuel ethanol, DDGS has quickly become a global commodity. However, high compositional variation has been the main problem hindering its use as a feed ingredient. This review provides updated information on chemical composition of distillers grains in term of nutrient levels, changes during dry-grind processing, and causes for large variation. The occurrence in grain feedstock and fate of mycotoxins during processing are also covered. During processing, starch is converted to glucose and then to ethanol and carbon dioxide. Most other components are relatively unchanged but concentrated in DDGS about 3-fold over the original feedstock. Mycotoxins, if present in the original feedstock, are also concentrated. Higher fold of increase in S, Na, and Ca, are mostly due to exogenous addition during processing, while unusual change in inorganic phosphorus (P) and phytate P indicates phytate hydrolysis by yeast phytase. Fermentation causes major changes, but other processing steps are also responsible. The causes for varying DDGS composition are multiple, including differences in feedstock species and composition, process methods and parameters, the amount of condensed solubles added to distiller wet grains, the effect of fermentation yeast, and analytical methodology. Most of them can be attributed to complexity of the dry-grind process itself. It is hoped that information provided in this review will improve the understanding of the dry-grind process and aid in the development of strategies to control the compositional variation in DDGS.