9.6.1.1 Yeast Metabolic Pathways

Glucose is converted into ethanol and CO2 via glycolysis, in the anaerobic pathway:

С6Н12Об ! 2C2H5OH + 2CO2 + Energy (Stored as ATP)

The overall reaction produces two moles of ethanol and CO2 for every mole of glucose consumed, with the reaction energy stored in 2 mol of ATP. Every gram of glucose converted will yield 0.511 g of ethanol, via this pathway. Secondary reactions consume a small portion of the glucose feed, however, to produce bio­mass and secondary products, Pasteur found that the actual yield of ethanol from fermentation by yeast is reduced to 95% of the theoretical maximum (Table 9.10). For maximum ethanol productivity, aerobic reaction should be avoided as in this

reaction, sugar is completely converted into CO2, cell mass and by-product with no ethanol formed.

9.6.1.2 Effect of Sugar Concentration

The primary reactant in the yeast metabolism is hexose sugar (glucose, fructose). The rate of ethanol production is related to the available sugar concentration by a Monod-type equation under fermentative conditions:

V = VmaxC/(Ks + Cs),

where

V = specific ethanol productivity (g ethanol/g cells/h)

Cs = Sugar substrate concentration (/g)

Ks = Saturation constant having a very low value (typically 0.2-9.4 g/l).

The yeast is starved at very low substrate concentrations (below 3 g/l) conse­quently, the productivity decreases [105]. At higher concentrations, a saturation limit is reached so that the rate of ethanol production per cell is essentially at its maximum up to 150 g/l sugar concentration. The catabolic (sugar) inhibition of enzymes in the fermentative pathway becomes important above 150 g/l, and the conversion rate is slowed down [72, 192].

An important secondary effect of sugar is catabolic repression of the oxidative pathways—Crabtree Effect. At above 3-30 g/l sugar concentration (depending on the yeast strain), the production of oxidative enzymes is inhibited [34, 127] thus, fermentative pathway is adopted. The Crabtree effect is not found in all the yeasts and is a desirable character in the industrial strains of yeast selected.