Characterization of cellobiose fermentations to ethanol by yeasts

Abstract

Twenty‐two different yeasts were screened for their ability to ferment both glucose and cellobiose. The fermentation characteristics of Candida lusitaniae (NRRL Y‐5394) and C. wickerhamii (NRRL Y‐2563) were selected for further study because their initial rate of ethanol production from cellobiose was faster than the other test cultures. C. lusitaniae produced 44 g/L ethanol from 90 g/L cellobiose after 5–7 days. When higher carbohydrate concentrations were employed, fermentation ceased when the ethanol concentration reached 45–60 g/L. C. lusitaniae exhibited barely detectable levels of β‐glucosidase, even though the culture actively fermented cellobiose. C. wickerhamii produced ethanol from cellobiose at a rate equivalent to C. lusitaniae; however, once the ethanol concentration reached 20 g/L, fermentation ceased. Using p‐nitrophenyl‐β‐D‐glucopyranoside (pNPG) as substrate, β‐glucosidase (3–5 U/mL) was detected when C. wickerhamii was grown anaerobically on glucose or cellobiose. About 35% of the β‐glucosidase activity was excreted into the medium. The cell‐associated activity was highest against pNPG and salicin. Approximately 100‐fold less activity was detected with cellobiose as substrate. When empolying these organisms in a simultaneous saccharification‐fermentation of avicel, using Trichoderma reesei cellulase as the saccharifying agent, 10–30% more ethanol was produced by the two yeasts capable of fermenting cellobiose than by the control, Saccharomyces cerevisiae.