Yeast Strains from Coconut Toddy in Sri Lanka Show High Tolerance to Inhibitors Derived from the Hydrolysis of Lignocellulosic Materials

We previously isolated 18 Saccharomyces cerevisiae strains from coconut toddy in Sri Lanka. All the 18 strains (SLY-1 to SLY-18) could grow aerobically up to 18 mmol L−1 vanillin in yeast extract–peptone–dextrose (YPD) medium, and the SLY-10 strain showed the highest vanillin tolerance (up to 21 mmol L−1). Among these 18 strains, even in the presence of 24 mmol L−1 vanillin, five strains (SLY-3, SLY-4, SLY-8, SLY-9 and SLY-10) produced alcohol at 72 h in the range of 11.4–33.1 g L−1 and the SLY-10 strain showed highest alcohol production. The five strains showed the conversion of vanillin to vanillyl alcohol. They also tolerated other strong inhibitors: 4-hydroxybenzoic acid (24 mmol L−1), furfural (30 mmol L−1), 5-hydroxymethyl-2-furaldehyde (5-HMF, 36 mmol L−1) and acetic acid (75 mmol L−1). All these five strains showed no growth and no alcohol production when cultured in the medium with inhibitor mixture of 2.7 mmol L−1 vanillin, 30 mmol L−1 furfural, 30 mmol L−1 5-HMF, 75 mmol L−1 acetic acid and 75 mmol L−1 formic acid but showed significant growth and alcohol production of 16.2–32.7 g L−1 with inhibitor mixture of 30% concentration (0.81 mmol L−1 vanillin, 9 mmol L−1 furfural, 9 mmol L−1 5-HMF, 22.5 mmol L−1 acetic acid and 22.5 mmol L−1 formic acid). The four strains (SLY-3, SLY-4, SLY-8 and SLY-9) were shown to tolerate the inhibitor mixture to a similar extent and better than SLY-10, while SLY-10 was the best to vanillin alone.