Augmenting Pentose Utilization and Ethanol Production of Native Saccharomyces cerevisiae LN Using Medium Engineering and Response Surface Methodology

Abstract

Economics of ethanol production from lignocellulosic biomass depends on complete utilization of constituent carbohydrates and efficient fermentation of mixed sugars present in biomass hydrolysates. Saccharomyces cerevisiae, the commercial strain for ethanol production uses only glucose while pentoses remain unused. Recombinant strains capable of utilizing pentoses have been engineered but with limited success. Recently, presence of endogenous pentose assimilation pathway in S. cerevisiae was reported. On the contrary, evolutionary engineering of native xylose assimilating strains is promising approach. In this study, a native strain S. cerevisiae LN, isolated from fruit juice, was found to be capable of xylose assimilation and mixed sugar fermentation. Upon supplementation with yeast extract and peptone, glucose (10%) fermentation efficiency was 78% with ~90% sugar consumption. Medium engineering augmented mixed sugars (5% glucose + 5% xylose) fermentation efficiency to ~50 and 1.6% ethanol yield was obtained with concomitant sugar consumption ~60%. Statistical optimization of input variables Glucose (5.36%), Xylose (3.30%), YE (0.36%), and peptone (0.25%) with Response surface methodology led to improved sugar consumption (74.33%) and 2.36% ethanol within 84 h. Specific activities of Xylose Reductase and Xylitol Dehydrogenase exhibited by S. cerevisiae LN were relatively low. Their ratio indicated metabolism diverted toward ethanol than xylitol and other byproducts. Strain was tolerant to concentrations of HMF, furfural and acetic acid commonly encountered in biomass hydrolysates. Thus, genetic setup for xylose assimilation in S. cerevisiae LN is not merely artifact of xylose metabolizing pathway and can be augmented by adaptive evolution. This strain showed potential for commercial exploitation.

Strain Improvement of Native Saccharomyces cerevisiae LN ITCC 8246 Strain Through Protoplast Fusion To Enhance Its Xylose Uptake

“…After 48 h, cultures were centrifuged at 8,000 rpm for 10 min and supernatants were discarded. Pellet was processed for determining XR (xylose reductase), XDH (xylitol dehydrogenase) [20] and XKS activities in intracellular milieu.…”

d‐Xylose consumption by nonrecombinant Saccharomyces cerevisiae: A review

“…Recently, other publications have reported S. cerevisiae yeast isolates from fruit juice capable of consuming some xylose aerobically (Nasir, Rahman, Hossain, & Choudhury, ; Sharma et al, ). In a screen of six industrial strains for development of recombinant yeasts able to ferment xylose, one strain (YB2625, isolated from bagasse) after transformation with a xylose utilization pathway showed faster growth on xylose and higher ethanol yields, when compared with the other recombinant industrial strains (Hector, Dien, Cotta, & Qureshi, ).…”

“…As pointed out early, media composition can have a significant impact on xylose consumption by nonrecombinant yeast strains. Using response surface methodology and statistical optimization of input variables, Sharma and co‐workers () were able to improve the utilization of xylose in glucose/xylose co‐fermentations by a natural LN ITCC 8246 yeast strain, which in a previous work showed good growth on paddly straw hydrolysates, producing ethanol (Arora, Priya, Sharma, Sharma, & Nain, ). This strain was shown to consume 3.7 g/L xylose (from an initial concentration of 20 g/L) in 72 hr, producing 1.1 g/L of ethanol (a Y e/x of 0.30 g ethanol/g xylose, fermentation efficiency of 58%).…”

“…Recently, other publications have reported S. cerevisiae yeast isolates from fruit juice capable of consuming some xylose aerobically (Nasir, Rahman, Hossain, & Choudhury, 2017;Sharma et al, 2018).…”

Natural Saccharomyces cerevisiae Strain Reveals Peculiar Genomic Traits for Starch-to-Bioethanol Production: the Design of an Amylolytic Consolidated Bioprocessing Yeast

“…The ethanologenic yeast Saccharomyces cerevisiae represents the ultimate candidate for bioethanol production due to the ease of cultivation and the generally recognized as safe (GRAS) and qualified presumption of safety (QPS) status (Sharma et al, 2018;Favaro et al, 2019b). Moreover, robust industrial strains have been adapted to stressful conditions and present favorable traits such as high fermentation rate, general robustness, tolerance to low pH and osmotic stress.…”