Yeasts acid tolerance

Acid-tolerant lactic acid bacteria would solve this latter problem and have indeed been obtained from screening [44] and via whole genome shuffling [45]. Alternatively, a lactate dehydrogenase gene could be inserted in a yeast, such as S. cerevisiae. These grow well at low pH and efficiently channel carbon into the glycolysis pathway under anaerobic conditions [46, 47]. Such constructs were indeed found to produce up to 55 g L-1 of lactic acid at pH 3.6 [46]. 

De-novo fermentation of ASA in an acid-tolerant yeast, combined with in-pro-cess product removal, has the potential of eliminating salt production altogether. Unfortunately, there is no microbe that naturally produces ASA and the reported yields are minute until now [176]. 

Rnstler, S. and Stolz, A. 2007. Isolation and characterization of a nitrile hydrolysing acid tolerant black yeast—Exophiala oligosperma R1. Applied Microbiology and Biotechnology, 75 899-908. 

Yeast (qv) metabolize maltose and glucose sugars via the Embden-Meyerhof pathway to pymvate, and via acetaldehyde to ethanol. AH distiUers yeast strains can be expected to produce 6% (v/v) ethanol from a mash containing 11% (w/v) starch. Ethanol concentration up to 18% can be tolerated by some yeasts. Secondary products (congeners) arise during fermentation and are retained in the distiUation of whiskey. These include aldehydes, esters, and higher alcohols (fusel oHs). NaturaHy occurring lactic acid bacteria may simultaneously ferment within the mash and contribute to the whiskey flavor profile. 

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