The principal pathways used by yeast for the dissimilation of glucose are shown in Fig. 3.3. All yeasts predominantly utilise the Embden-Myerhoff glycolytic pathway for generation of ATP via substrate level phosphorylation. A proportion of the carbon flow devolving from glucose, or other sugars, is dissimilated via the hexose monophosphate shunt. This pathway is of importance for generation of NADPH for use in anabolic metabolism such as lipid synthesis. Bruinenberg et al. (1983) calculated that at least 2% of glucose metabolism had to be via the hexose monophosphate pathway where ammonium was the nitrogen source in order to satisfy anabolic requirements. A much greater proportion was required with certain carbon sources, such as pentoses. Since in the case of a brewery fermentation anabolic requirements are modest it seems likely that this pathway is of relatively small significance.
The product of glycolysis, pyruvate, occupies a major branch-point in metabolism. With respect to sugar catabolism, carbon flow may be directed towards acetyl-CoA and subsequent oxidation via the TCA cycle and oxidative phosphorylation, or into ethanol formation via acetaldehyde. Lagunas (1986) estimated that in Saccharomyces yeasts most carbon was dissimilated via the glycolytic pathway in order to generate ATP and thence to ethanol via the fermentative route. Thus, this author presented data indicating that the proportions of glucose, maltose and galactose utilised for ATP production were 73%, 69% and 53%, respectively. Of these, only 3%, 4% and 14%, respectively, were metabolised via the oxidative pathway. Most significantly, in the case of glucose and maltose, the fraction of carbon flux metabolised via fermentative or oxidative routes was independent of the presence of oxygen. Using the assumption that the yeast respiratory chain had two phosphorylation sites, Lagunas (1986) calculated that under aerobic conditions the yeast obtained 27%, 40% and 84% of the yield of ATP from respiration using glucose, maltose and galactose, respectively, the remainder deriving from substrate level phosphorylation.
It may be concluded from the foregoing discussion that the nature and concentrations of sugars in wort exert far-reaching effects on yeast metabolism. The effects on carbohydrate uptake of the spectrum of sugars present in wort has been discussed previously (Section 3.3.1). S. cerevisiae, in common with several other yeast genera, is usually described as being 'facultatively fermentative'. That is, it is capable, oo
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