The application of high-potential thermotolerant yeasts is a key factor for successful ethanol production at high temperatures. Two hundred and thirty-four yeast isolates from Greater Mekong Subregion (GMS) countries, i.e., Thailand, The Lao People's Democratic Republic (Lao PDR) and Vietnam were obtained. Five thermotolerant yeasts, designated Saccharomyces cerevisiae KKU-VN8, KKU-VN20, and KKU-VN27, Pichia kudriavzevii KKU-TH33 and P. kudriavzevii KKU-TH43, demonstrated high temperature and ethanol tolerance levels up to 45 °C and 13% (v/v), respectively. All five strains produced higher ethanol concentrations and exhibited greater productivities and yields than the industrial strain S. cerevisiae TISTR5606 during high-temperature fermentation at 40 °C and 43 °C. S. cerevisiae KKU-VN8 demonstrated the best performance for ethanol production from glucose at 37 °C with an ethanol concentration of 72.69 g/L, a productivity of 1.59 g/L/h and a theoretical ethanol yield of 86.27%. The optimal conditions for ethanol production of S. cerevisiae KKU-VN8 from sweet sorghum juice (SSJ) at 40 °C were achieved using the Box–Behnken experimental design (BBD). The maximal ethanol concentration obtained during fermentation was 89.32 g/L, with a productivity of 2.48 g/L/h and a theoretical ethanol yield of 96.32%. Thus, the newly isolated thermotolerant S. cerevisiae KKU-VN8 exhibits a great potential for commercial-scale ethanol production in the future.
Gamma-aminobutyric acid (GABA) is an inhibitory neurotransmitter that can be found in many plants, especially beans. Beans are normally used for producing vegetarian foods, such as bean milks, bean sprouts, and tofu. Thus, the aims of this study were to determine the GABA content in various germinated beans (yellow beans, black beans, green beans, and red beans) as well in tofu products made from different types of germinated beans. The results showed that soaking and germination significantly contributed to an increase in GABA production. The GABA content increased to a maximum value of 0.89, 3.09, 3.93 and 4.78 mg/g in yellow beans, red beans, green beans, and black beans, respectively. Moreover, due to the bean characteristics, green beans, red beans, and black beans were collected at 6 h after germination while yellow beans were collected at 0 h after germination. As a result, only yellow bean sprouts could be used for tofu production since they are composed of a high amount of proteins and a low amount of carbohydrates. The GABA content in tofu was 0.55 mg/g, which was lower than that in soybean milk (0.65 mg/g), likely due to the filtration and pressing processes of tofu production.
The highly expressed genes encoding heat-shock proteins, HSP82 and SSA4, potentially play an important role in helping S. cerevisiae KKU-VN8 cope with various stresses that occur during high-temperature fermentation, leading to higher ethanol production efficiency.
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