SOME EFFECTS OF LACTIC-ACID BACTERIA ON LABORATORY-SCALE YEAST FERMENTATIONS

A laboratory-scale system, involving the mashing of distiller's malt and the fermentation of the resulting wort by distiller's yeast, was used to simulate commercial-scale malt whisky fermentations. Deliberate addition of pure cultures of lactic acid bacteria, previously isolated from a commercial malt whisky distillery, permitted the effects of bacterial contamination to be studied under controlled conditions. Bacterial contamination led to reduced yields of ethanol, lower yeast crops, reduced carbohydrate utilization, greatly increased acidity due mainly to production of lactic acid, and lower final pH. Contaminated fermentations showed greatly increased foam production. Certain bacterial strains caused premature flocculation of yeast cells. Reduced ethanol yields were not due simply to competition for carbohydrate: other likely mechanisms for the effect are discussed.