Product Inhibition in Bioethanol Fermentation: An Overview

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Many organisms are used for alcohol production in an industry that includes Saccharomyces cerevisiae, Zymomonas mobilis, and Clostridium spare better microbes with respect to ethanol production and ethanol tolerance, Zymomonas mobilis can use glucose, sucrose, and fructose through Entner-Deodoroff pathway. In bioprocessing product inhibition is undesirable that limits the product's final titer and volumetric productivity more precisely known as product toxicity those utilizing the whole cell as biocatalyst. During the ethanol fermentation the yield of cell mass decreases gradually as the ethanol concentration increases progressively indicating product inhibition. The decrease in cell mass concentration is the accumulation of ethanol in the fermentation broth beyond the limits. This is because the increase in alcohol concentration during fermentation destroys the microorganism lipid bilayer membrane and denatures the enzymatic protein thereby creating instability conditions. The product inhibition is very well studied only in the batch reactor. Conventionally maximum ethanol concentration of 7-8% (v/v) is achieved in the time frame of 50-70 hr with the operating temperature of the 32-34oC and stirring rate of 180rpm during fermentation. To overcome this problem the continuous product removal solves the product inhibition through maintaining the ethanol concentration below the inhibitory limit.