BIOTECHNOLOGY LETTERS Volume 17 No.3 (March 1995) pp.321-326 Received as revised 16th January INHIBITION EFFECTS OF ETHANOL CONCENTRATION HISTORY AND ETHANOL CONCENTRATION CHANGE RATE ON ZYMOMONAS MOBILIS Jinghong Li, P. James McLellan, and Andrew J. Daugulis ° Department of Chemical Engineering, Queen's University Kingston, Ontario, Canada K7L 3N6 SUMMARY The inhibition effects on Zymomonas mobilis of ethanol concentration history (time-integrated exposure to ethanol) and ethanol concentration change rate have been investigated. It was found that the ethanol concentration history had no significant effect on the fermentative capability of Z. mobilis, while the ethanol concentration change rate had a quite intense inhibitory effect. INTRODUCTION Ethanol is now well established as the primary end product responsible for inhibiting fermentations of both yeast and Z. mobilis (Ingrain and Buttke, 1984), but the mechanism of ethanol inhibition in fermentations remains in dispute. It has been well known that the yeast cell viability is severely reduced in "rapid fermentation" (Nagodawithana and Steinkraus, 1976). A related phenomenon has also been observed with Z. mobilis (Hobley and Pamment, 1994). Dasari et aL(1990) investigated the effect of ethanol concentration change rate on the fermentation kinetics of yeast. By feeding ethanol at various rates to low cell density cultures of Saccharomyces cerevisiae, it was shown that the sharp fall in viability when ethanol was produced during rapid fermentation of yeast is in part a direct consequence of the high rate of change of extracellular ethanol concentration. When compared at equal ethanol concentrations, the viabilities of yeast cells were significantly lower in the cultures m which the rate of change of ethanol concentration was highest, in spite of the shorter periods of exposure to ethanol. The effects of high rate of change of ethanol concentration persisted after ethanol addition had ceased: specific deactivation rates during the period of constant ethanol concentration, after ethanol addition was stopped, were 0.0031, 0.023, and 0.063 rnin t for the cultures to which ethanol had been added at 21, 41, and 124 g L t h -I, respectively. Recently, a similar experiment was performed on Z. mobilis (Hobley and Pamment, 1994). These authors found that increasing the rate of change of ethanol concentration from 21 to 83 g L-th~ did not lead to accelerated viability losses. In fact, during ethanol addition and when compared at the same 321