Carbon Dioxide Effects on Fuel Alcohol Fermentation

High levels of carbon dioxide are known to be inhibitory to yeast growth, at least at the low temperatures prevailing in the brewing industry, and have also been suggested to favor increased diversion of carbon to glycerol. Since it was not clear whether the inhibitory effects depend on the bulk concentration of CO₂ or on its partial pressure, it was not clear whether the same results would be obtained under the higher temperatures employed in fuel alcohol fermentation We first determined the conditions prevailing in an industrial corn-to-ethanol fermentation plant employing relatively small fermentors, then carried out laboratory fed-batch fermentations with glucose feed with CO₂ partial pressures of 0.5, 1.5, 2.5, and 3.5 atm absolute. Elevated carbon dioxide slowed the fermentation, particularly at the later stages, decreased the maximum number of viable cells obtained and increased cell death rates slightly. High carbon dioxide also decreased overall glycerol production. Low-level aeration also decreased glycerol productivity on a per-cell basis but stimulated cell growth to a compensating extent so that the final level was comparable to the control.