A Novel Technique that Enables Efficient Conduct of Simultaneous Isomerization and Fermentation (SIF) of Xylose

Abstract

Of the sugars recovered from lignocellulose, D-glucose can be readily converted into ethanol by baker's or brewer's yeast (Saccharomyces cerevisiae). However, xylose that is obtained by the hydrolysis of the hemicellulosic portion is not fermentable by the same species of yeasts. Xylose fermentation by native yeasts can be achieved via isomerization of xylose to its ketose isomer, xylulose. Isomerization with exogenous xylose isomerase (XI) occurs optimally at a pH of 7-8, whereas subsequent fermentation of xylulose to ethanol occurs at a pH of 4-5. We present a novel scheme for efficient isomerization of xylose to xylulose at conditions suitable for the fermentation by using an immobilized enzyme system capable of sustaining two different pH microenvironments in a single vessel. The proof-of-concept of the two-enzyme pellet is presented, showing conversion of xylose to xylulose even when the immobilized enzyme pellets are suspended in a bulk solution whose pH is sub-optimal for XI activity. The co-immobilized enzyme pellets may prove extremely valuable in effectively conducting "simultaneous isomerization and fermentation" (SIF) of xylose. To help further shift the equilibrium in favor of xylulose formation, sodium tetraborate (borax) was added to the isomerization solution. Binding of tetrahydroxyborate ions to xylulose effectively reduces the concentration of xylulose and leads to increased xylose isomerization. The formation of tetrahydroxyborate ions and the enhancement in xylulose production resulting from the complexation was studied at two different bulk pH values. The addition of 0.05 M borax to the isomerization solution containing our co-immobilized enzyme pellets resulted in xylose to xylulose conversion as high as 86% under pH conditions that are suboptimal for XI activity. These initial findings, which can be optimized for industrial conditions, have significant potential for increasing the yield of ethanol from xylose in an SIF approach.

Novel two‐stage fermentation process for bioethanol production using Saccharomyces pastorianus

“…The drawbacks of these non‐ Saccharomyces species are slower ethanol production and higher nutrient requirements compared to S. cerevisiae . Recently, extracellular conversion of xylose to xylulose by xylose isomerase has been reinvestigated using S. cerevisiae , where S. cerevisiae consumes xylulose via the pentose phosphate pathway . Specifically, Rao et al improved the immobilization process for xylose isomerase by coupling the enzyme with urea to increase the xylose to xylulose reaction rate due to more optimal reaction pH.…”

High Gravity Fermentation of Sugarcane Bagasse Hydrolysate by Saccharomyces pastorianus to Produce Economically Distillable Ethanol Concentrations: Necessity of Medium Components Examined

“…First, the yeast biomass production was accounted for in the conversion of glucose to ethanol. Conversely, most reported ethanol fermentations use yeast inoculums on the order of 10 to 50 g dcw/L (or 1.75 to 3.5 × 10 7 cell/mL) [18,30,32,[34][35][36]58,59]. In these high inoculum cases, it is not apparent that the yield from sugar to ethanol accounts for the sugar used to generate the biomass, which for the 10 g/L case would be at least 20 g/L of glucose.…”

d‐Xylose consumption by nonrecombinant Saccharomyces cerevisiae: A review

“…Although xylulose is a rare sugar in nature (Granström, Takata, Tokuda, & Izumori, 2004), the ability of S. cerevisiae to ferment this pentose has attracted considerable attention because the xylose present in lignocellulosic biomass can be isomerized into xylulose by the addition of commercial xylose (glucose) isomerase to the medium (a bacterial enzyme used in the industrial production of high-fructose syrup), allowing the fermentation of this abundant carbon source (de Bari, Cuna, Di Matteo, & Liuzzi, 2014;Gong, Chen, Flickinger, Chiang, & Tsao, 1981;Hahn-Hagerdal, Berner, & Skoog, 1986;Linden & Hahn-Hagerdal, 1989;Milessi et al, 2018;Rao, Chelikani, Relue, & Varanasi, 2008;Yuan, Rao, Relue, & Varanasi, 2011;Yuan, Rao, Varanasi, & Relue, 2012). However, the rates of cell growth, xylulose consumption, and ethanol production and yield from this carbon source are significantly lower than the results obtained from the fermentation of other sugars by S. cerevisiae, like glucose or galactose (Jeppsson et al, 1996;Mittelman & Barkai, 2017;Tamari, Rosin, Voichek, & Barkai, 2014;Yu et al, 1995).…”