Bioethanol production from cellulose-rich corncob residue using thermotolerant yeast

Abstract

Thermotolerant yeasts are usually employed for bioethanol production from lignocellulosic biomass due to their numerous advantages. Therefore, this study aimed to isolate, select, and screen the promising thermotolerant yeast as a platform for second generation bioethanol production. The most suitable yeast was selected based on the thermotolerant and ethanol-tolerant abilitics. Afterwards, the cthanol production from cellulose-rich comncob (CRC) hydrolysate by using the selected yeast strain was studied. Furthermore, the effect of temperature on bioethanol production by separate hydrolysis and fermentation (SHF), simultancous saccharification and fermentation (SSF), and prehydrolysis-simultaneous saccharification and fermentation (pre-SSF) processes using the selected thermotolerant yeast strains was subsequently examined. Eventually, the feasibility of selected thermotolerant yeast on biocthanol production in the up-scale production was also considered. Forty-two thermotolerant yeast isolates were isolated and screened from five different Loog-Paeng (traditional alcoholic beverage starter culture) sources. Four isolates, including isolates SB1, SC10, G3, and TC-5 were further selected based on their ability to produce ethanol and cthanol-tolerant at celevated temperature. Based on their 26S rDNA sequences, isolates SB1, SC10, and G3 were identified as Candida glabrata with the GenBank accession number of MN784460, MN784462, and KY618710, respectively. Meanwhile, isolate TC-5 was identified as Saccharomyces cerevisiae with the GenBank accession number of KY681804. Thereafter, the selected thermotolerant yeasts were preliminary studied for their ability to produce bioethanol from CRC hydrolysate (54.04 g/L of glucose, 14.18 g/L of xylose, and 0.65 g/L of arabinose) at 42°C. Among these, C. glabrata KY618710 and S. cerevisiae KY681804 produced the highest ethanol concentration of 19.05±0.41 and 18.38±0.32 g/L, respectively. The effect of temperature on bioethanol production via SHF, SSF, and pre-SSF processes by strains KY618710 and KY681804 were investigated at 35-42°C and compared with commercial S. cerevisiae. The results from SHF, SSF, and pre-SSF process revealed that temperature provided a negative effect on bioethanol production by commercial S. cerevisiae. The results from SHF process showed that the highest cthanol concentration in a range of 20.33-21.64 g/L were obtained from strain KY618710 and KY681804, when the cultivation temperature was set at 37 and 40°C. Unfortunately, the ethanol concentration, ethanol yield, and theoretical ethanol yield of strain KY618710 and KY681804 decreased when the temperature was elevated to 42°C. Bioethanol production via SSF process at 40°C revealed that strain KY681804 could produce the highest ethanol concentration of 20.92±0.34 g/L. Besides these, strain KY618710 also showed the comparable highest bioethanol concentration of 20.88±0.76 g/L at the same temperature. With regards to the pre-SSF process, strain KY618710 and KY681804 showed the comparable ethanol concentration of 20.46±0.16 and 20.89±0.44 g/L. In addition, the produced bioethanol via SHF, SSF, and pre-SSF processes were not significantly difference. However, the SSF process was preferable than other processes due to their short overall process time and the simple operation procedure. Nevertheless, strain KY618710 showed a comparable bioethanol concentration and ethanol productivity to that of strain KY681804, which is one of opportunistic human pathogenic yeast. Therefore, the thermotolerant S. cerevisiae KY681804 and the SSF process were chosen for next study. The effect of CRC solid loading on bioethanol production via SSF process employing S. cerevisiae KY681804 was carried out at 40*C. The 7.5, 10, 12.5, and 15% (w/v) of CRC solid loading were investigated. The highest ethanol concentration of 35.91±0.30 g/L, with the theoretical ethanol yield of 87.86±0.73% were obtained from 12.5% CRC solid loading. To reduce the substrate inhibition, the bioethanol production via fed-batch SSF process was also examined under the same experimental condition with SSF. The ethanol concentration of 38.23±0.19 g/L with the theoretical ethanol yield of 93.51±0.47% was attained when 12.5% (w/v) of CRC solid loading was used. Surprisingly, cthanol production from the same concentration of CRC via fed-batch SSF process was significantly higher than batch SSF process. Therefore, bioethanol production via fed-batch SSF process with 12.5% of CRC solid loading by strain KY681804 was then performed in a 5-L stirred tank bioreactor with 3-L reaction volume. The maximum ethanol concentration, theoretical ethanol yield, and ethanol productivity were 31.96±0.78 g/L, 78.20±0.19%, and 0.222±0.001 g/L/h, respectively. The results from this study revealed that the newly isolated thermotolerant S. cerevisiae KY681804 is an alternative yeast strain that suitable for bioethanol production under clevated temperature. In addition, strain KY681804 might be a promising ethanol producing-thermotolerant yeast which could be applied in SSF process using various types of lignocellulosic biomass.