Abstract
Propagation conditions have been shown to be of considerable importance for the fermentation ability of Saccharomyces cerevisiae. The limited tolerance of yeast to inhibitors present in lignocellulosic hydrolysates is a major challenge in second-generation bioethanol production. We have investigated the hypothesis that the addition of nutrients during propagation leads to yeast cultures with improved ability to subsequently ferment lignocellulosic materials. This hypothesis was tested with and without short-term adaptation to wheat straw or corn stover hydrolysates during propagation of the yeast. The study was performed using the industrial xylose-fermenting S. cerevisiae strain CR01. Adding a mixture of pyridoxine, thiamine, and biotin to unadapted propagation cultures improved cell growth and ethanol yields during fermentation in wheat straw hydrolysate from 0.04 g g−1 to 0.19 g g−1 and in corn stover hydrolysate from 0.02 g g−1 to 0.08 g g−1. The combination of short–term adaptation and supplementation with the vitamin mixture during propagation led to ethanol yields of 0.43 g g−1 in wheat straw hydrolysate fermentation and 0.41 g g−1 in corn stover hydrolysate fermentation. These ethanol yields were improved compared to ethanol yields from cultures that were solely short-term adapted (0.37 and 0.33 g g−1). Supplementing the propagation medium with nutrients in combination with short-term adaptation was thus demonstrated to be a promising strategy to improve the efficiency of industrial lignocellulosic fermentation.
Highlights
The limited tolerance of Saccharomyces cerevisiae to inhibitors present in lignocellulosic hydrolysates is a major challenge in second-generation bioethanol production (Palmqvist and Hahn-Hägerdal 2000; Hemansi et al 2019)
Results showed that the addition of a mixture of pyridoxine, thiamine and biotin, during propagation led to an increase in the ethanol yield on total sugars during wheat straw hydrolysate (WSH) fermentation (0.43 g g−1), compared to non-supplemented, unadapted cultures (0.07 g g−1, Fig. 1a)
The role of nutrient additions during propagation has not been extensively investigated for lignocellulosic hydrolysate fermentations, and here we demonstrated the potential to improve the process by focusing on the propagation step
Summary
The limited tolerance of Saccharomyces cerevisiae to inhibitors present in lignocellulosic hydrolysates is a major challenge in second-generation bioethanol production (Palmqvist and Hahn-Hägerdal 2000; Hemansi et al 2019). The present study takes another approach by investigating the effects of nutrient addition to the propagation on lignocellulose fermentation performance of S. cerevisiae. It investigates whether the efficiency could be improved further by combining it with short-term adaptation. Supplementation of nutrients such as biotin, magnesium, or zinc to the fermentation medium have been reported to improve ethanol tolerance, and ethanol productivity in media without lignocellulose hydrolysates (Dombek and Ingram 1986; Winter et al 1989; Alfenore et al 2002; Zhao et al 2009). Information is lacking on the effects of addition of nutrients to the propagation media and how such additions change the cells capacity to ferment lignocellulose hydrolysates
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