Construction of a Thermotolerant Saccharomyces cerevisiae Strain for Bioethanol Production with Reduced Fermentation Time and Saccharifying Enzyme Dose

A thermotolerant Saccharomyces cerevisiae mutant strain, TT6, was constructed after multi-parental hybridization of five mutant strains obtained by UV or NTG treatment of the original strain, S. cerevisiae KV1. When incubated at $40^{\circ}C$ in YPD broth, TT6 began to grow exponentially in 10 h, but KV1 did not show any noticeable growth even after 22 h. The thermotolerant growth of TT6 was confirmed by serial dilution assay at $42^{\circ}C$; TT6 grew at a cell concentration ($10^{-5}$) 10,000 times lower than that of KV1 ($10^{-1}$). Whereas ethanol production from YP containing 23% (w/v) glucose by KV1 decreased with increasing temperature from $30^{\circ}C$ to $36^{\circ}C$, ethanol production by TT6 did not decrease at temperatures up to $37^{\circ}C$. When TT6 was tested for ethanol production at $36^{\circ}C$ by simultaneous saccharification and fermentation (SSF) from 23% corn, 24 h of fermentation time or 50% of the glucoamylase dose was saved when compared with KV1 at $30^{\circ}C$. The ethanol yield from corn by SSF with TT6 at $36^{\circ}C$ was 91.7% of the theoretical yield, whereas that of KV1 at $30^{\circ}C$ was 90.6%.