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Why can bacterial action spoil a fermented mash before distillation?

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distilled spirit The fermentation and distillation process for producing whiskey. The production of whiskey begins with grinding grain into a meal, which is cooked. Malt is introduced to the meal, which results in mash that is cooled and pumped into a fermenter, where yeast is added. The fermented mixture is heated in a still, where the heat vaporizes the alcohol. The alcohol vapours are caught, cooled, condensed, and drawn off as clean, new whiskey. This liquid is stored in a cistern room, and water is added to lower the proof (absolute alcohol content) before the whiskey is placed in new charred oak barrels for aging and later bottling.

Bacterial action can spoil a fermented mash because, if fermentation is allowed to continue beyond the optimal period, bacteria can metabolize the mixture, leading to a reduction in the ethyl alcohol content. This process taints the flavor and aroma of the final product.1

While yeast is essential for converting sugars into alcohol, the environment of the mash—including its acidity, available carbohydrates, and temperature—determines which microorganisms thrive. If conditions are not strictly controlled, lactic-acid bacteria, which produce lactic acid as a primary byproduct of fermentation, can proliferate. Because the growth of various microorganisms is highly sensitive to these environmental factors, any deviation from the ideal conditions can allow undesirable bacterial activity to interfere with the intended alcoholic fermentation.213