The Beer Brewer

The must (Juice) is transferred into open, stainless steel cylindrical fermentation vessels to begin the initial alcoholic fermentation, at temperatures around 25°C, to produce the white wine used as the Sherry base. Fermentation temperatures are slightly higher than those used for white table wines, consequently higher alcohols, such as isoamyl and phenethyl alcohol, are formed. Fermentation is initiated spontaneously by the yeasts that are part of the grape micro flora, or by the addition of specific inoculum of dry wine yeast. In spontaneously initiated fermentations, Kloeckera and Hanseniaspora yeasts have been identified as fermentation initiators, although the ubiquitous Saccharomyces cerevisiae ultimately dominates the fermentation, due to its comparative tolerance of alcohol. Kloeckera activity in the must leads to the production of glycerol, acetic acid and a variety of esters, before the species declines as S. cerevisiae predominates for the remainder of the fermentation.

S. cerevisiae metabolizes sugars contained in the must, primarily glucose and fructose, into ethanol and CO2 via the glycolytic pathway (shown in Figure 1), generating cellular ATP in the process.

Glycolysis provides the substrates that are utilized in respiration or fermentation. Preceding glycolysis, the hexose sugar is transported into the yeast cell via membrane bound permease transporters. During glycolysis, the hexose sugar undergoes a series of phosphorylation steps and cleavage to form triose phosphates, which subsequently form pyruvate. Respiration occurs only briefly in the must, as the yeast utilizes molecular oxygen to synthesize membrane material and to increase biomass. Respiration involves the generation of ATP from pyruvate via the TCA cycle (Figure 2). Under anaerobic conditions, which prevail in the must, ATP is generated predominantly via glycolysis (Figure 1). Fermentation (during glycolysis) commences with pyruvate, which is decarboxylated into acetaldehyde (and CO2), which in turn is reduced by the enzyme alcohol dehydrogenase to ethanol and rapidly exported from the cell and into the medium. The fermentation of pyruvate occurs as a redox balancing system to continue the fermentation process, with NADH re-oxidized to NAD+.

Continuing on the fermentation of sherry shortly …