Brewers’ yeast has a very strong tendency toward fermentation and will respire only when the concentration of fermentable sugars is very low and oxygen is available. In beer making, yeast will ferment rather than respire, regardless of the oxygen concentration, because the wort usually supplies an overwhelming abundance of fermentable sugar.
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All fermentable sugars, including fructose, maltose, and sucrose (and galactose to a limited extent), induce the Crabtree effect, but glucose exhibits the strongest effect. Brewers’ yeast is often said to prefer fermentation only when glucose levels are high, but “high” is a relative term; glucose in excess of about 0.4% (w/v) will bring on the Crabtree effect; most worts (both all-malt and adjunct) contain an excess of 1% glucose, which is more than enough glucose to induce the Crabtree effect. The other fermentable sugars present in wort induce the Crabtree effect as well (for example, a wort with an O.G. of 1.040 [10 °P] is 10% sugar). This high concentration of sugar makes it virtually impossible for brewers’ yeast to respire in wort.