Effects of single and combined cell treatments based on low pH and high concentrations of ethanol on the growth and fermentation of Dekkera bruxellensis and Saccharomyces cerevisiae

Abstract

The alcoholic fermentation in Brazil displays some peculiarities because the yeast used is recycled in a non-aseptic process. After centrifugation, the cells are treated with acid to control the bacterial growth. However, it is difficult to manage the indigenous yeasts without affecting the main culture of Saccharomyces cerevisiae. This work evaluated how the cell treatment could be modified to combat contaminant yeasts based on the differential sensitivities to low pH and high concentrations of ethanol displayed by an industrial strain of S. cerevisiae and three strains of Dekkera bruxellensis, which are common contaminant yeasts in Brazilian fermentation processes. The tests were initially performed in rich medium with a low pH or a high concentration of ethanol to analyse the yeast growth profile. Then, the single and combined effects of low pH and ethanol concentration on the yeast cell viability were evaluated under non-proliferative conditions. The effects on the fermentation parameters were also verified. S. cerevisiae grew best when not subjected to the stresses, but this yeast and D. bruxellensis had similar growth kinetics when exposed to a low pH or increased ethanol concentrations. However, the combined treatments of low pH (2.0) and ethanol (11 or 13 %) resulted in a decrease of D. bruxellensis cell viability almost three times higher than of S. cerevisiae, which was only slightly affected by all cell treatments. The initial viability of the treated cells was restored within 8 h of growth in sugar cane juice, with the exception of the combined treatment for D. bruxellensis. The ethanol-based cell treatment, in despite of slowing the fermentation, could decrease and maintain D. bruxellensis population under control while S. cerevisiae was taking over the fermentation along six fermentative cycles. These results indicate that it may be possible to control the growth of D. bruxellensis without major effects on S. cerevisiae. The cells could be treated between the fermentation cycles by the parcelled addition of 13 % ethanol to the tanks in which the yeast cream is treated with sulphuric acid at pH 2.0.

Potassium metabisulphite as a potential biocide against Dekkera bruxellensis in fuel ethanol fermentations

“…Comparing the experiments by Bassi et al . (), in which a decrease of 14% in alcohol production and 10‐fold in the number of D. bruxellensis were verified when 13% ethanol was added to the acidic solution, with those of the present study carried out in the same conditions, it is better to add ethanol to the acidic solution than PMB.…”

“…In summary, the addition of PMB to the acidic solution in which the yeast cells are commonly treated in the fermentation industry specifically overloaded S. cerevisiae, impairing alcohol production and not affecting the contaminant D. bruxellensis yeast, despite the sensitivity displayed by the latter to sulphite in pure cultures in both growing and fermenting conditions. Comparing the experiments by Bassi et al (2013), in which a decrease of 14% in alcohol production and 10-fold in the number of D. bruxellensis were verified when 13% ethanol was added to the acidic solution, with those of the present study carried out in the same conditions, it is better to add ethanol to the acidic solution than PMB.…”

“…Would it be better to control D. bruxellensis growth using PMB, even at the expense of lower alcohol yield? An ethanol-based cell treatment, as proposed by Bassi et al (2013), despite slowing the fermentation, decreased and controlled the D. bruxellensis population while S. cerevisiae took over the fermentation during the growth cycles. Both PMB added to the fermentation medium in each cycle (this study) and ethanol added to the acidic solution helped to control D. bruxellensis growth but with a lower alcohol yield.…”

Ethanol production from Dekkera bruxellensis in synthetic media with pentose

“…In comparison to xylose, it presented slightly lower levels of ethanol. pH values of around 2 are reported in the literature as being able to limit Dekkera growth in YEPD medium (Bassi et al, 2013). According to Freer et al (2003), D. bruxellensis is able to produce high amounts of acetic acid from glucose in a wellmixed medium, even when presenting a slow metabolism.…”

Bioethanol strains of Saccharomyces cerevisiae characterised by microsatellite and stress resistance

“…A study conducted by Bassi et al 26 demonstrated that PE-2 was only minimally affected by cell treatments based on low pH and or ethanol addition.…”