The Fermentation Characteristics of Newly Selected Thermotolerant Yeasts at High Temperature

In order to develop a method of economical production and to reduce energy-consumption in fuel alcohol production, we investigated the fermentation characters of two newly selected thermotolerant yeasts. The RA-74-2 showed stable and superior fermentability between 30 and $40^{\circ}C$ in 20% glucose media in comparison to the industrial strains. The optimum concentration of glucose for economical fermentation at $40^{\circ}C$ was 15-18%, and organic nitrogen was necessary for a satisfactory fermentation. The optimum pH was 4.0 and aeration was adversed for high temperature fermentation. Agitation was an important factor at $40^{\circ}C$ and the addition of magnesium ion 0.2% was required in this experiment. When the inoculum was increased, ethanol productivity as well as the speed of fermentation increased. On the other hand RA-912, which can grow at $48^{\circ}C$, showed similar fermentability between 30-$45^{\circ}C$ in 20% glucose media As the concentration of substrate decreased, fermentation ratio increased at $45^{\circ}C$ (45%, 65%, 95% fermentation ratio in 20%, 15%, 10% glucose media, respectively). Also, requirement of organic nitrogen and magnesium ion in RA-912 was similar in RA-74-2. The optimum pH for fermentation was 5.0, and the effects of agitation were enhanced at $37^{\circ}C$ than at $45^{\circ}C$. As the inoculum was increased, fermentation speed became more enhanced but the ethanol productivity was less affected. RA-912 showed fermentability with various substrates. Among the substrates used, inulin was the most promising substrate for the high-temperature fermentation. When 14.5% inulin was used as the substrate, 93% and 55% fermentation ratios were shown at $37^{\circ}C$ and $45^{\circ}C$, respectively.