Abstract
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).
Highlights
Brazilian distilleries use non-sterile sugarcane musts for bioethanol production, which may result in lower ethanol yields due to bacterial contamination
We explored whether ethanol can partially or fully replace sulfuric acid in the cell treatment step, aiming for complete loss of viability of L. fermentum
Compared to the control medium with pH 6.5, the medium with pH 5.0 caused a decrease in the maximum specific growth rate of the bacterium from 0.284 to 0.037 h–1
Summary
Brazilian distilleries use non-sterile sugarcane musts for bioethanol production, which may result in lower ethanol yields due to bacterial contamination. Bacteria consume sugars and produce various byproducts other than ethanol, causing inhibitory effects on yeast viability. They may induce yeast cell flocculation and impair the fermentation process (Beckner et al 2011; Lopes et al 2016; Ceccato-Antonini 2018). Large volumes of sulfuric acid are required in the acid washing treatment; centrifugation operations are hindered due to yeast flocculation. They result in a decrease in the ethanol production by 10,000–30,000 L
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