When fermenting on cassava (15–25%, w/v) with Clostridium acetobutylicum ATCC824, a severe delay
(18–40 h) was observed in the phase shift from acidogenesis to solventogenesis, compared to the cases
of fermenting on corn. By adding yeast extract (2.5 g/L-broth) into cassava meal medium when the delay
appeared, the phase shift was triggered and fermentation performances were consequently improved.
Total butanol concentrations/butanol productivities, compared to those with cassava substrate alone,
increased 15%/80% in traditional fermentation while 86%/79% in extractive fermentation using oleyl alcohol
as the extractant, and reached the equivalent levels of those using corn substrate. Analysis of genetic
transcriptional levels and measurements of free amino acids in the broth demonstrated that timely and
adequate addition of yeast extract could promote phase shift by increasing transcriptional level of ctfAB
to 16-fold, and indirectly enhance butanol synthesis through accelerating the accumulation of histidine
and aspartic acid families.