Changes in the decarboxylase activity of baker's yeast during the growth phase

Abstract

During the growth phase, baker's yeast is transferred stage by stage from anaerobic fermentation conditions to virogorously aerated conditions of cultivation. During the cultivation process its power to ferment glucose decreases to 4 5 of its original value. The decarboxylase activity, as determined by the decarboxylating power of pyruvic acid by the freeze-thawed cell preparations, was found to decrease to 1 6 of the original value by the time it reached the commercial yeast stage. The decrease was sharp during the growth period of the precommercial A 4 stage when cultivation proceeded under more aerobic conditions. The total thiamine content of baker's yeast decreases to about half during the aerobic phase of the cultivation. Even the addition of cocarboxylase—with or without the molybdate to inhibit phosphatases—to freeze-thawed cell preparations of the thiamine-poor yeast stages did not activate the decarboxylation of pyruvic acid. The decarboxylase activity of intact yeast cells with respect to pyruvic acid changes only slightly during the growth phase of baker's yeast. The cell membrane limits the penetration of pyruvic acid into the yeast cell, and hence its decarboxylation, to such extent that the decarboxylating power of the intact cell can amount to hardly one-twentieth of that expected on the basis of its carboxylase content. The permeability of the cell membrane increases somewhat, being twice the initial value on passing from the anaerobic stages to the aerobic ones. Hence, there is a twofold increase in the decarboxylating power of intact yeast cells when pasing from the anaerobic growth stages to the aerobic ones, although the decarboxylase activity contained in the cells falls to one-sixth of its initial value. During the cultivation period of a single yeast stage, the permeability of the cells is slightly greater during the budding phase than during the resting condition. The increase in permeability is at the most twofold, in the stage A 3. The speed of fermentation by commericial baker's yeast is limited by its carboxylase content. This is shown by the accumulation of pyruvic acid in a suspension of the aerobically cultivated commercial yeast stage during anaerovic glucose fermentation. Accumulation is slighter when the carboxylase-rich, fairly anaerovically grown A 2 stage is used.