Brewer’s spent grain as a source of renewable fuel through optimized dilute acid pretreatment
Introduction
Biomass represents the only renewable source capable of replacing fossil fuels for transportation. Advanced biofuels based on residual biomass constitute the most interesting alternative to conventional biofuels based on food feedstocks [1] and will play a crucial role in the transition from fossil fuels to sustainable resources. The use of lignocellulosic materials to produce biofuels and green chemicals is currently widely studied because lignocellulose is the most abundant renewable carbon source in nature. Among biomass resources, brewer’s spent grain (BSG) constitutes a prominent example of an agroindustrial, lignocellulosic residue with low-value applications (primarily animal feed) that has been proposed as a feedstock for fuel ethanol production [2].
To overcome the recalcitrant structure of lignocellulose, materials must be subjected to a pretreatment step that fractionates the biomass and facilitates the carbohydrate release which has been proved to be essential for producing fuels and chemicals from lignocellulosic materials [3]. Dilute acid pretreatment solubilizes hemicelluloses and opens the structure of the lignocellulosic biomass. Moreover, this technology has the advantage of not requiring special construction materials, thus improving industrial application possibilities [4]. After pretreatment, the conversion process also includes the hydrolysis of the pretreated materials by means of enzymes and fermentation of the resulting glucose to ethanol by selected microorganisms. Ethanol production can be increased by also transforming the sugars released during pretreatment; these sugars come from the hemicellulosic fraction of the lignocellulosic material that enters the liquid phase that is usually separated by filtration of the pretreated solids.
Recent studies have revealed several different bioproducts that can be obtained from BSG. For example, BSG is a source of antioxidant compounds that have been extracted using either conventional solvent extraction [5] or ultrasound or microwave assisted systems [6]. Ionic liquids have been used on BSG as a delignification method with the goal of obtaining sugars that may, in turn, be used for the production of other value-added products [7]. One of the main applications is the fractionation of protein from BSG; Rommi et al. [8] reported that the use of an enzyme (protease) was required for effective extraction of proteins from hydrothermally pretreated BSG. Qin et al. [9] compared several pretreatment options and concluded that hydrothermal pretreatment at 60 °C is an advantageous technology for protein extraction when both cost and environmental issues are taken into account. Dávila et al. [10] reported on the application of the biorefinery concept based on BSG; they concluded that the production of ethanol alone is not an economically feasible option, but heat integration could make the joint production of xylitol and polyhydroxybutyrate, along with ethanol, a viable alternative that would also benefit from the environmental point of view. The environmental impacts of a BSG-based biorefinery producing xylooligosaccharides and bioethanol are also discussed in a recent work by González-García et al. [11], who identified steam and enzyme production as environmental hotspots in the proposed biorefinery.
From the literature review, ethanol is still one of the products to be obtained from BSG given the high sugar content of this material. In this work, an experimental design considering the main operational variables was applied to optimize the pretreatment conditions of BSG to produce cellulose-enriched solids and liquids where hemicellulosic sugars and other compounds are included. This work addresses the integral conversion of all the sugars present in BSG, including those from the hemicellulosic and starch fractions and the glucose coming from enzyme hydrolysis of the cellulose contained in the pretreated solids. An ethanologenic Escherichia coli strain and a pentose-fermenting yeast, Scheffersomyces stipitis, were used to ferment starchy-glucose and hemicellulosic sugars, while Saccharomyces cerevisiae fermented the glucose derived from cellulose.
Section snippets
Raw material
Brewers’ spent grain (BSG), which was kindly donated by the Cruzcampo-Jaén brewery (Heineken España, S.A., Spain), was washed with distilled water in order to attain a neutral pH, dried at 50 °C and stored at 4 °C until use. The chemical composition of BSG is (dry weight): cellulose 15.2 ± 0.5, hemicellulose 25.1 ± 0.7 (xylan 16.9 ± 0.7, galactan 1.3 ± 0.0, arabinan 6.6 ± 0.3, mannan 0.3 ± 0.1), starch 5.3 ± 0.2, acid-soluble lignin 5.5 ± 0.3, protein 21.2 ± 0.2, ash 2.3 ± 0.1, acid-insoluble
Effect of dilute acid pretreatment on BSG
Dilute sulfuric acid pretreatment of BSG achieved the partial solubilization of the biomass; the solid recovery after pretreatment ranged from 36.9% (run 15, 130 °C, 3% H2SO4, 25 min) to 57.1% (run 6, 110 °C, 2% H2SO4, 10 min) depending on the pretreatment harshness (Table 2). This fact is related to the hydrolysis of hemicellulose and starch fractions and the solubilization of the extractives during pretreatment. Consequently, cellulose was enriched in the pretreated solids. Considering that
Conclusions
The acid pretreatment at optimized conditions (130 °C, 1% sulfuric acid, 26 min) efficiently fractionated brewer’s spent grain, with total recovery of starch and 94% of hemicellulosic sugars in the pretreatment liquor as well as 90% cellulose in the pretreated solid. The behaviour of two microorganisms able to ferment both pentoses and hexoses have been compared in this work. Overall, 100 kg of dried BSG yielded 18.1 kg ethanol (4.6 kg from cellulose by SSF at 25% solids and 13.5 kg by
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
Technical support from the Agrifood Campus of International Excellence (ceiA3) is gratefully acknowledged.
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2023, Current Opinion in Green and Sustainable ChemistryCitation Excerpt :This price reduced significantly from 3.36 USD/kg to 2 USD/kg when the BSG processing was raised from 100 and 2000 MT/day, when BDO titre was assumed to be 100g/L [23]. Complete valorisation of storage and structural carbohydrate fraction of BSG to ethanol was earlier attempted by Rojas-Chamarro et al. [24]. The authors developed an optimized dilute acid pretreatment strategy (12.5% solids; 1% H2SO4; 130°C; 26 min) that recovered 94% carbohydrates in the resultant solid residue.