Abstract
The qualitative and quantitative composition of volatile compounds in fermented distillery mash determines the quality of the obtained distillate of agricultural origin (i.e., raw spirit) and the effectiveness of further purification steps. Propan-2-ol (syn. isopropyl alcohol), due to its low boiling point, is difficult to remove by rectification. Therefore, its synthesis needs to be limited during fermentation by Saccharomyces cerevisiae yeast, while at the same time controlling the levels of acetaldehyde and acetic acid, which are likewise known to determine the quality of raw spirit. Lactic acid bacteria (LAB) are a common but undesirable contaminant in distillery mashes. They are responsible for the production of undesirable compounds, which can affect synthesis of propan-2-ol. Some bacteria strains are able to synthesize isopropyl alcohol. This study therefore set out to investigate whether LAB with S. cerevisiae yeast are responsible for conversion of acetone to propan-2-ol, as well as the effects of the amount of LAB inoculum and fermentation parameters (pH and temperature) on the content of isopropyl alcohol, acetaldehyde, lactic acid and acetic acid in fermented mashes. The results of NMR and comprehensive two-dimensional gas chromatography coupled with time of flight mass spectrometry (GC × GC-TOF MS) analysis confirmed the ability of the yeast and LAB strains to metabolize acetone via its reduction to isopropyl alcohol. Efficient fermentation of distillery mashes was observed in all tested mashes with an initial LAB count of 3.34–6.34 log cfu/mL, which had no significant effect on the ethanol content. However, changes were observed in the contents of by-products. Lowering the initial pH of the mashes to 4.5, without and with LAB (3.34–4.34 log cfu/mL), resulted in a decrease in propan-2-ol and a concomitant increase in acetaldehyde content, while a higher pH (5.0 and 5.5) increased the content of propan-2-ol and decreased acetaldehyde content. Higher temperature (35 °C) promoted propan-2-ol synthesis and also resulted in increased acetic acid content in the fermented mashes compared to the controls. Moreover, the acetic acid content rose with increases in the initial pH and the initial LAB count.
Highlights
The basic raw material for the production of pure vodka is ethyl alcohol of agricultural origin, which is obtained from a distillate of agricultural origin via the rectification process
The main aims of the present work were, : (i) to confirm whether the acetone present in sweet mash is metabolized to propan-2-ol by Saccharomyces cerevisiae yeast and five strains of lactic acid bacteria; (ii) to examined the effect of the three variables, i.e., initial pH of sweet rye distillery mashes, the fermentation temperature and the initial LAB count, on the propan-2-ol, acetaldehyde and weak acids content in mashes fermented by S. cerevisiae yeast
There is nothing in the literature that proves whether acetone, which occurs in sweet mashes as a product of the thermal-pressure treatment of cereal grains, can be a source of isopropyl alcohol synthesis during ethanol fermentation in the presence of yeast and/or bacteria
Summary
The basic raw material for the production of pure vodka is ethyl alcohol of agricultural origin (i.e., rectified spirit), which is obtained from a distillate of agricultural origin via the rectification process. Only the levels of aldehydes (calculated as acetaldehyde) and acids (calculated as acetic acid) are limited [2], while in rectified spirit, the content of higher alcohols, methanol and esters is restricted [1]. The concentration of these compounds significantly decreases during the rectification process. Vodkas produced in EU countries which are exported to other non-EU countries must fulfil the additional requirements of the foreign recipients, as has been detailed in a previous work [3] One of these additional restrictions is the level of propanol-2-ol in pure vodkas and ethyl alcohol of agricultural origin [3]. Due to the difficulty of removing this compound in the rectification process, the synthesis of isopropyl alcohol needs to be limited during the process of fermenting distillery mashes
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