Energy stress has a profound effect on yeast metabolism and can promote aerobic formation of ethanol with high specific productivity. Oxidoreductive metabolism induced by elevated concentrations of glucose cuts in sooner when the cells are further stressed by depressed levels of potassium ion. Although this opens new approaches for ethanol with aerobic instead of anaerobic conditions, progress is slowed by inhibition as ethanol accumulates. It cannot yet be demonstrated that both high ethanol productivity and high concentration can be achieved. This research uses auxostat mode of continuous culture with self-adjustment by the organisms of their feed rate. State-of-the-art computer control stabilizes the cultures. This work includes, 1. testing of various strains of yeast; 2. investigate effects of energy stress on Zymomonas, a bacterium used commercially to produce ethanol; and 3. operate with flocculating yeast to establish high reactor cell densities. Ethanol produced by microorganisms already competes with ethanol made from petroleum and promises to assume an expanding role in automotive fuels. Greater productivity and a rugged aerobic process would lower costs. Preliminary work demonstrates outstanding productivity per cell but cannot maintain it as ethanol accumulates to inhibit the process. This is an important, real effect, and the challenge is to extend it to commercially relevant ranges of ethanol concentration. This research tests other yeasts and a bacterium, Zymomonas, to exploit new insights into a region of physiology that has had little attention. The novel computerized continuous culture apparatus feeds the cells at a rate determined by their own metabolism. This is unique in displaying growth rate responses to changes in conditions and leads to efficient screening of key experimental factors.
