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Production of high-concentration bioethanol from cassava stem by repeated hydrolysis and intermittent yeast inoculation

Tanaka, Kazumasa, Koyama, Mitsuhiko, Pham, Phan Thi, Rollon, Analiza Palenzuela, Habaki, Hiroaki, Egashira, Ryuichi, Nakasaki, Kiyohiko
International biodeterioration & biodegradation 2019 v.138 pp. 1-7
Saccharomyces cerevisiae, acid hydrolysis, autoclaving, bioethanol, biomass, cassava, ethanol, ethanol fermentation, ethanol production, furfural, glucose, hydrolysates, temperature, xylose, yeasts
Methods for obtaining high-concentration (ca. 40 g/L) ethanol from cassava stem (CS) were explored by investigating the effects of acid hydrolysis conditions, hydrolysate concentration, and intermittent inoculation of yeast on ethanol fermentation. Two-step acid hydrolysis demonstrated that a reduction in autoclaving temperature from 121 °C to 111 °C remarkably lowered the furfural concentration in the hydrolysate, with only ±10% differences in the glucose and xylose concentrations. For the concentration of sugars in the CS hydrolysate, the acid hydrolysate prepared with CS concentration of 200 g/L was used instead of distilled water for the second step of acid hydrolysis using new CS biomass (i.e., two-times acid hydrolysis). This produced a concentrated hydrolysate containing 95 g/L of glucose and 0.95 g/L furfural. Saccharomyces cerevisiae strain IAM 4178 was inoculated into the concentrated CS hydrolysate after verifying its furfural tolerance. Furfural was rapidly degraded and glucose was converted to ethanol during the startup period, whereas cell density decreased to approximately 10% of the initial value and ethanol production ceased by 72 h. IAM 4178 was then re-inoculated intermittently to maintain the cell density at 108 cells/mL, which eventually resulted in the complete uptake of glucose and the production of high concentrations of ethanol, up to 37.5 g/L.