Can ethanol partially or fully replace sulfuric acid in the acid wash step of bioethanol production to fight contamination by Lactobacillus fermentum

In the bioethanol industry, the acid washing applied to yeast cell mass after fermentation cycles is not always effective in removing the unwanted bacteria. In fact, it poses safety hazards due to the handling of large volumes of sulfuric acid. We explored whether sulfuric acid could be replaced, partially or completely, in the acid washing step to control bacterial growth. To achieve this, we evaluated: (1) the effect of ethanol and pH on growth of Lactobacillus fermentum in MRS medium; (2) ethanol as a substitute to or adjuvant of sulfuric acid in the cell treatment against L. fermentum; (3) acid treatments of cells with or without 5% ethanol in cell-recycled batch fermentation with an industrial strain of Saccharomyces cerevisiae and contaminated with L. fermentum. There was a total loss of L. fermentum viability in the following cell treatments: pH 2.0 with 5% ethanol, pH 3.0 with 20% ethanol, only 22% ethanol and pH 2.0 with 3 mg L–1 Kamoran®. Switching the treatment from pH 2.0 with 5% ethanol to pH 2.0 during the fermentative cycles was effective in non-sterile must samples that were not contaminated with L. fermentum, because the native bacteria did not survive the acid treatment (pH 2.0).