Ethanol Production from Hydrolyzed Kraft Pulp by Mono- and Co-Cultures of Yeasts: The Challenge of C6 and C5 Sugars Consumption

Abstract

Second-generation bioethanol production's main bottleneck is the need for a costly and technically difficult pretreatment due to the recalcitrance of lignocellulosic biomass (LCB). Chemical pulping can be considered as a LCB pretreatment since it removes lignin and targets hemicelluloses to some extent. Chemical pulps could be used to produce ethanol. The present study aimed to investigate the batch ethanol production from unbleached Kraft pulp of Eucalyptus globulus by separate hydrolysis and fermentation (SHF). Enzymatic hydrolysis of the pulp resulted in a glucose yield of 96.1 ± 3.6% and a xylose yield of 94.0 ± 7.1%. In an Erlenmeyer flask, fermentation of the hydrolysate using Saccharomyces cerevisiae showed better results than Scheffersomyces stipitis. At both the Erlenmeyer flask and bioreactor scale, co-cultures of S. cerevisiae and S. stipitis did not show significant improvements in the fermentation performance. The best result was provided by S. cerevisiae alone in a bioreactor, which fermented the Kraft pulp hydrolysate with an ethanol yield of 0.433 g·g −1 and a volumetric ethanol productivity of 0.733 g·L −1 ·h −1 , and a maximum ethanol concentration of 19.24 g·L −1 was attained. Bioethanol production using the SHF of unbleached Kraft pulp of E. globulus provides a high yield and productivity.Energies 2020, 13, 744 2 of 15 sustainability, has a low and stable price, and practically does not demand extra land [6,7]. There are some facilities producing 2G bioethanol on a commercial scale. However, large-scale production still faces some technological barriers that must be overcome in order to achieve a cost-competitive production [8]. Due to the recalcitrance of LCB, a costly pretreatment step is required, which is the main technological bottleneck of 2G bioethanol production. The release of enzymatic and fermentation inhibitors during pretreatment is another limitation [9].Pulp and paper mills have the infrastructures and logistics to handle LCB, and chemical mills employ technology required for LCB fractionation and conversion [10]. Bioethanol has been produced from different feedstocks such as Kraft pulp, spent sulfite liquor, and pulp and paper sludge [11]. Chemical pulping processes can be considered as a LCB pretreatment since they promote delignification and target hemicelluloses to some degree [12]. Chemical pulping represents about 77% of the virgin pulps produced globally, and more than 95% of these chemical pulps are Kraft pulps [13]. These pulps are produced by Kraft pulping involving the reaction of white liquor, i.e., an alkaline aqueous solution of sodium hydroxide and sodium sulfide with a pH of 14, with lignin at high temperature (150-170 • C). This reaction promotes lignin breakdown and degradation with the release of phenolic fragments, removing almost 90% of the lignin from the wood. Kraft pulping also leads to hemicelluloses and some cellulose loss and decreases the degree of polymerization of cellulose [14]. The utilization of Kraft pulping as a pretreatment method for LCB has ...

Sustainable Second-Generation Ethanol Production from Switchgrass Biomass via Co-fermentation of Pentoses and Hexoses Using Novel Wild Yeasts

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Subcritical Water as Pretreatment Technique for Bioethanol Production from Brewer’s Spent Grain within a Biorefinery Concept

“…A slight decrease in ethanol concentration was observed after reaching its maximum, together with an increase in biomass concentration. Branco et al [ 34 ] already observed a slight increment in biomass concentration when ethanol started to be reassimilated due to the ability of S. cerevisiae to consume ethanol when fermentable sugars dropped. In this regard, biomass concentration still increased after glucose exhaustion also due to consumption of xylose and probably other metabolites present in the hydrolysate.…”

On the Optimization of Fermentation Conditions for Enhanced Bioethanol Yields from Starchy Biowaste via Yeast Co-Cultures

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