S25: The Control and Elimination of Undesirable Microbes to Improve Yields in Ethanol Fermentation for Fuel

Ethanol is gaining wide acceptance as a renewable fuel.  Fuel ethanol production in the U.S. has increased from 1.7 billion gallons in 2000 to over 10 billion gallons in 2009.  Because of this increase, the effect of bacterial contamination on ethanol production has come under increased scrutiny.  Bacteria that enter the plant with incoming feedstocks can reduce fermentation efficiency by competing with yeasts for available nutrients.  Since the majority of these contaminants are lactic acid bacteria (LAB), they also produce metabolites such as lactic and acetic acid that further inhibit the growth of yeasts.  Most plants employ a combination of methods to try to minimize the effects of bacterial contamination including a rigorous CIP regimen, pH adjustment, and a wide variety of antimicrobials.  Despite these attempts, most facilities continue to experience periodic upsets.  Upsets can lead to reduced ethanol production coupled with high levels of residual sugars, thus making the fermentation process far less economical and contributing to additional maintenance costs.  We have surveyed a wide number of commercial ethanol plants to understand where bacterial contamination arises, what effect various unit operations (such as mixing, liquefaction, cooking, and cooling) have on populations of contaminant bacteria, and what strategies can be used to minimize the risk.  This presentation will review the basics of bacterial control in fuel ethanol by drawing on our experiences as well as the scientific literature.  The benefits, as well as drawbacks, of a range of strategies will be discussed as part of case studies.