Saturation of Respiratory Capacity

Fermentation of sugars to ethanol is an anaerobic process. However, ethanol has been found in aerated processes when the concentration of sugar is high. This is the "Crabtree effect" and was for many years considered catabolite repression. In other words, it was postulated that excess glucose repressed its use in the pathways that lead to ethanol. Fairly recently it has been shown that this hypothesis is false, and the effect seems to be over saturation of the respiratory pathways. The yeast cannot pass the excess sugar through the main path and simply shunt it through an alternate route to ethanol.

The observations in continuous culture with aeration and ethanol are that dilution rates that are far from the growth rate maximum result in quite low sugar concentrations and no overload of respiratory capacity. As we have seen, the sugar concentration rises abruptly and steeply when approaching the maximum specific growth rate, and this results in the formation of ethanol. This is seen in the following figure drawn by computer simulation:

It is practically impossible to operate a chemostat near the maximum specific growth rate because this is a high-gain region for control. However, an auxostat works just fine in this region. There is potential for a rugged aerobic process that produces ethanol at high rates in an auxostat. Unfortunately, our group found that accumulation of ethanol diminishes the effect of over saturation. Perhaps genetic engineering can overcome this restriction.

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