Continuous ethanol production in a nonconventional five-stage system operating with yeast cell recycling at elevated temperatures
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Three ranges of increasing temperatures (35–43, 37–45, 39–47°C) were sequentially applied to a five-stage system continuously operated with cell recycling so that differences of 2°C (between one reactor to the next) and 8°C (between the first reactor at the highest temperature and the fifth at the lowest temperature) were kept among the reactors for each temperature range. The entire system was fed through the first reactor. The lowest values of biomass and viability were obtained for reactor R3 located in the middle of the system. The highest yield of biomass was obtained in the effluent when the system was operated at 35–43°C. This nonconventional system was set up to simulate the local fluctuations in temperature and nutrient concentrations that occur in different regions of the medium in an industrial bioreactor for fuel ethanol production mainly in tropical climates. Minimized cell death and continuous sugar utilization were observed at temperatures normally considered too high for Saccharomyces cerevisiae fermentations.