Solvent toxicity proved to be the most severe limitation during the ABE fermentation process. To overcome this issue, more tolerant strains were designed (see Section 10.5) and integrated recovery techniques were applied (see Section 10.6).

While acetone and ethanol are only moderately toxic, butanol has disastrous effects on bacterial cells even at low concentrations. Already at a concentration of 1.1% (120 mM) butanol, the growth rate of C. acetobutylicum is decreased by 50% and at 1.5% (165 mM) butanol, growth is almost completely inhibited (Baer et al, 1987; Vollherbst-Schneck et al, 1984). This effect is caused by an increase in membrane fluidity (Baer et al, 1987, 1989; Ingram, 1976; Vollherbst-Schneck, 1984) and inhibition of membrane proteins such as transporters (Bowles and Ellefson, 1985; Moreira et al., 1981; Ounine et al., 1985) and ATPases (Terracciano and Kashket, 1986).