Figure 2 Alcoholic fermentation and deviation from alcoholic to glycero-pyruvic fermentation. Black line: alcoholic pathway; red line: Glycero-pyruvic pathway; yellow box: intermediate metabolites involved in alcoholic fermentation block.
fermentation of fruit (cacao) and fruit juices (grape must and apple juice) is carried out by different microorganisms that act sequentially. The substrate fermentation is first achieved by apiculate yeast (Hanseniaspora), which is followed by elliptical yeast (Saccharomyces). Other fermenting yeast ascribed to the genera Candida, Kluyveromyces, Metschnikowia, Pichia, Saccharomycodes, Torulaspora, and Zygosaccharomyces are also sometimes found during natural alcoholic fermentation.
It is well established that the most important agent of alcoholic fermentation is S. cerevisiae, the yeast that is used widely in several fermentation industries (wine, beer, cider, and bread) as a microbial starter. S. cerevisiae becomes the dominant species during alcoholic fermentation of fruit and fruit juices because of the strongly selective environment due to the low pH and high sugar and ethanol concentrations, and the anaerobic conditions. The ecological distribution of S. cerevisiae and the role of different habitats in the evolution of this species are controversial. Numerous investigations have revealed the low diffusion of this yeast species in natural environments such as soil, fruit surfaces, and tree exudates. On the other hand, S. cerevisiae is found widely in wineries and other fermentation plants since it is used to carry out the fermentation processes.
In the winery, several environmental factors, including high ethanol and sugar concentrations, the presence of SO2, and others, can exert selective pressures on the S. cerevisiae population. Following these considerations, S. cerevisiae is defined as a domesticated species because of its selection through time in man-made environments.
Glycero-pyruvic fermentation is always concomitant to alcoholic fermentation, although it involves a very low percentage of sugar (5-8%). However, under particular fermentation conditions, some osmotolerant yeast species (e.g., Torulopsis magnoliae, Torulopsis bombicola, and Candida stellata) and other fermenting yeast (S. cerevisiae) can ferment sugar to produce glycerol and, for example, acetaldehyde, acetic acid, acetoin, 2,3-butanediol and succinic acid, all compounds that can be derived from pyruvate.
The change from alcoholic to glycero-pyruvic fermentation occurs mainly because of the need to regenerate NAD+ when the reduction of acetaldehyde to ethanol is not possible (Figure 2). This could be due to: (1) the nonavailability of acetaldehyde, if it is bound by sulfite; (2) the absence or low activity of pyruvate decarboxylase; and (3) the high activity of the aldehyde dehydrogenase enzyme (under alkaline pH) that catalyzes the reaction from acetaldehyde to acetate.
In the past, between the two world wars, glycero-pyruvic fermentation was exploited at an industrial level for the production of glycerol.
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