Ethanol production by thermotolerant yeasts have been extensively studied,
because thermotolerant yeasts are capable of growth and fermentation during the
summer months in non-tropical countries as well as under tropical climates. The aim
of this study was to isolate and characterize thermotolerant yeast to produce ethanol.
Thermotolerant yeast strain S1 was isolated from silage sample in Suranee University
of Technology farm. According to the morphological and biochemical
characterization, morphology of the thermotolerant yeast strain S1 cells are ovoidal to
elongate, 2.7-4.2 x 5.6-10.1 µm, single or in pairs, budding cell are present and
pseudomycelium are developed. The features of the appearance of cultures when
cells grown in liquid medium after 3 days at 40 °
C, heavy, dry climbing pellicles are
formed on the surface of liquid medium and the growth is butyrous and light cream
colored on agar medium. It can grow and ferment when glucose was used as a carbon
source. This yeast was not producing killer toxin against with Saccharomyces
cerevisiae EC 1118, Escherichia coli ATCC 25922 and Bacillus subtilis ATCC 6633.
Genetic analysis was determined on the basis of 18S rDNA sequence, the results
showed that the S1 strain had 99% similarity to Issatchenkia sp. The optimization of
nitrogen and carbon (glucose) sources, and pH for ethanol production at 40 °
C was
observed using M9 minimal medium. In shake flask experiment, the thermotolerant
yeast strain S1 gave the highest ethanol concentration (74.58% theoretical yield) at pH 4.0 when ammonium sulphate (1%, w/v) and glucose (5%, w/v) were used as nitrogen
and carbon sources, respectively. The optimum condition for ethanol production in
shake flask experiment was also conducted in 2 L fermenter. The yeast gave the
highest cell dry weight and ethanol yield within 24 hours and the ethanol production
was decreased within 72 hours. In contrast to aeration for 24 hours, ethanol
production was stable about 76.06% of theoretical yield. However, the ethanol
production was lower than that of shake flask experiment. Additionally, the
production of some organic acids using HPLC technique was investigated. High
amount of an unknown compound produced during the yeast growth and its ethanol
production was also detected at the retention time of 13.5 minutes. Therefore, ethanol
production by thermotolerant yeast strain S1 should be considered because this yeast
produces an unknown compound (amount 40x of acetic acid), this might involve in
some metabolisms of the yeast during fermentation.