Bacterial contamination and its effects on ethanol fermentation

Abstract

Samples were collected from a commercial ethanol production plant to enumerate the bacterial contamination in each step of a starch based ethanol production process. Though the slurry of raw material used in the process carried bacteria with various colony morphology in the order of 10(4) per ml, only the colonies of white and circular form survived and propagated through the processes to the order of 10(8) per mi at the end of fermentation. Almost all of the bacterial isolates from the fermentation broth were lactic acid bacteria. Heterofermentative Lactobacillus fermentum and L. salivarius, and a facultatively heterofermentative L. casei were major bacteria of an ethanol fermentation. In a batch fermentation L. fermentum was more detrimental than L. casei to ethanol fermentation. In a cell-recycled fermentation, ethanol productivity of 5.72 g l(-1) h(-1) was obtained when the culture was contaminated by L. fermentum, whilst that of the pure culture was 9.00 g l(-1) h(-1) Similar effects were observed in a cell-recycled ethanol fermentation inoculated by fermentation broth collected from an industrial plant, which showed a bacterial contamination at the level of 10(8) cells per ml.