Fermentation of oat and soybean hull hydrolysates into ethanol and xylitol by recombinant industrial strains of Saccharomyces cerevisiae under diverse oxygen environments

Authors: CORTIVO, PAULO ROBERTO D - Federal University Of Rio Grande Do Sul; HICKERT, LILIAN RAQUEL - Federal University Of Rio Grande Do Sul; Hector, Ronald - Ron; AYUB, MARCO ANTONIO Z - Federal University Of Rio Grande Do Sul

Submitted to: Industrial Crops and Products
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/5/2018
Publication Date: 3/1/2018
Citation: Cortivo, P.R.D., Hickert, L.R.H, Hector, R., Ayub, M.A.Z. 2018. Fermentation of oat and soybean hull hydrolysates into ethanol and xylitol by recombinant industrial strains of Saccharomyces cerevisiae under diverse oxygen environments. Industrial Crops and Products. 113:10-18. doi: 10.1016/j.indcrop.2018.01.010.
DOI: https://doi.org/10.1016/j.indcrop.2018.01.010

Interpretive Summary: Oat and soybean hulls are agriculture byproducts with low value. In this study, two yeast strains were evaluated for their ability to ferment sugars derived from oat and soybean hulls into higher-value products. The two yeast strains were first modified to metabolize the five-carbon sugar xylose. This first step is required because xylose is the second most abundant sugar in nature and its use is essential for processes that use agricultural materials. The yeast strains were tested using differing levels of sugars derived from either oat or soybean hulls. One of the strains, YRH396, outperformed the other and was studied further. This strain was able to consume up to 73% of the available xylose. When oxygen was limited, an unwanted byproduct of was formed and the sugar was not completely converted to ethanol. These results suggest that additional modifications are necessary for an industrial application of these types of genetically modified yeast strains. This study will be of interest to producers using biomass-derived sugars as a feedstock for manufacturing bio-renewable fuels and chemicals.

Technical Abstract: In this study, we evaluated the capacity of recombinant industrial Saccharomyces cerevisiae YRH 396 and YRH 400 strains to ferment sugars from oat hull and soybean hull hydrolysates into ethanol and xylitol. The strains were genetically modified by chromosomal integration of Pichia stipitis XYLI/XYL2 genes and the overexpression of S. cerevisiae XKS1 genes, in order to have the ability to metabolize xylose, one of the main sugars in lignocellulosic biomass. The strains YRH 396 and YRH 400 were tested by fermenting acid and enzymatic hydrolysates of oat and soybean hull, with different concentrations of sugars, in orbital shaker under conditions of anaerobiosis and oxygen limitation. The YRH 396 strain showed the best kinetic parameters for the production of ethanol and xylitol, thus its metabolism was further studied in bioreactor cultivations. Under anaerobiosis, the maximum consumption of xylose was approximately 35% when using hydrolysates containing similar concentrations of glucose and xylose, whereas when hydrolysates mainly composed of xylose were used, this strain showed a consumption of 73% of the xylose, reaching yields of ethanol of 0.33'g'g-1. In bioreactor cultivations under oxygen limitation, xylose consumption reached approximately 65%, and the main product was xylitol, reaching a final concentration of 8.17'g'L-1. These results suggest that, in addition to an adaptive evolution process, molecular modifications are necessary for an industrial application of these type of genetically modified strains.