EVALUATION OF ETHANOL AND BIOMASS PRODUCTION RATE BY DIFFERENT $SACCHAROMYCES~CEREVISIAE$ STRAINS DEPENDING ON EXTERNAL pH AND TEMPERATURE

Anahit H. Shirvanyan
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Abstract

Bioethanol production is in high demand due to its potential to replace transportation fuel and its ability to be renewable and long-lasting. Therefore, the bioconversion of fermentable sugars to ethanol is of central importance. The work aimed to explore yeast properties changes during fermentation at different pHs and temperature conditions. Biomass production, specific growth rate (SGR), fermentation products, and metabolite composition and quantity changes were studied at 25℃, 30℃, and 37℃ as well as pH 5 and pH 6.5 conditions. SGR data shows that at 25℃ and 30℃ pH 5 is optimal for yeast biomass production. At pH 5.0–6.5, the biomass production of the ATCC 9804 strain almost 2 times exceeds the same parameter for another strain. The highest biomass production was detected at 30℃ and pH 5. Ethanol production by yeast increases as the growth temperature decreases at pH 5 in contrast to pH 6.5, where the temperature changes within 30–37℃ range have no significant effect on it in both strains. At 30℃ ethanol concent­ra­tion in the extracellular medium reached ~117 mM in the case of ATCC 13007 strain, whereas for ATCC 9804 the same parameter was 1.4-fold lower. Both strains had the same substrate assimilation rate. Glycerol production reduces with increasing growth temperature and pH; the highest glycerol concentration (6.1 mM) was observed within 32 h growth of ATCC 9804 strain at 25℃, pH 5 and reaches 5.5 mM within 24 h growth of ATCC 13007 strain under the same conditions. At pH 5 and 6.5, the free energy for glycerol production was 2.3-fold and 4-fold higher compared to that of ethanol and acetate production processes in both strains. Gibbs free energy of ethanol production reaches the lowest value compared to the same parameter of acetate and glycerol production at pH 5, suggesting a preference for the alcoholic fermentation metabolic pathway under these conditions. The lowest acetate production was observed after 24 hours of growth of the ATCC 13007 strain at 37°C and pH 5, with ΔG = 173.9 kJ/mol. Obtained data highlights temperature stress mechanisms regulation of yeast and can be used for improving ethanol production processes and yeast genetic modification tools.
不同 SACCHAROMYCES~CEREVISIAE$ 菌株乙醇和生物质生产率受外部 pH 值和温度影响的评价
由于生物乙醇具有替代运输燃料的潜力,且可再生、使用寿命长,因此其生产需求量很大。因此,将可发酵糖类生物转化为乙醇至关重要。这项工作旨在探索酵母在不同 pH 值和温度条件下发酵过程中的特性变化。在 25℃、30℃ 和 37℃以及 pH 5 和 pH 6.5 条件下,研究了生物质产量、特定生长率(SGR)、发酵产物、代谢物组成和数量的变化。SGR 数据显示,在 25℃和 30℃条件下,pH 值为 5 最有利于酵母生物量的产生。在 pH 值为 5.0-6.5 时,ATCC 9804 菌株的生物量产量几乎是另一菌株相同参数的 2 倍。在 pH 值为 5 和 30℃时,酵母菌的生物量产量最高;在 pH 值为 5 时,酵母菌的乙醇产量随着生长温度的降低而增加;而在 pH 值为 6.5 时,温度在 30-37℃ 范围内的变化对两种菌株的乙醇产量均无显著影响。30℃ 时,ATCC 13007 菌株胞外培养基中的乙醇浓度达到约 117 mM,而 ATCC 9804 菌株的这一参数则低 1.4 倍。两株菌株的底物同化率相同。甘油产量随着生长温度和 pH 值的升高而减少;ATCC 9804 菌株在 25℃、pH 值为 5 的条件下生长 32 小时后,甘油浓度达到最高(6.1 mM),而 ATCC 13007 菌株在相同条件下生长 24 小时后,甘油浓度达到 5.5 mM。在 pH 值为 5 和 6.5 时,两株菌株生产甘油的自由能分别是生产乙醇和醋酸过程的 2.3 倍和 4 倍。在 pH 值为 5 时,乙醇生产的吉布斯自由能与乙酸盐和甘油生产的相同参数相比达到最低值,这表明在这些条件下酒精发酵代谢途径更受青睐。ATCC 13007 菌株在 37℃、pH 值为 5 的条件下生长 24 小时后,醋酸盐产量最低,ΔG = 173.9 kJ/mol。获得的数据强调了温度胁迫对酵母的调控机制,可用于改进乙醇生产工艺和酵母基因改造工具。
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