Highlights
Both d-mannose and bioethanol were produced from coffee residue waste (CRW).
Ethanol pretreatment effectively enhanced hydrolysis.
Highest d-mannose retained after fermenting with S. cerevisiae KCTC 7906.
15.7 and 11.3 g (DW) of d-mannose and bioethanol, respectively, were recovered from 150 g ethanol-pretreated CRW.
Abstract
A novel, integrated process for economical high-yield production of d-mannose and ethanol from coffee residue waste (CRW), which is abundant and widely available, was reported. The process involves pretreatment, enzymatic hydrolysis, fermentation, color removal, and pervaporation, which can be performed using environmentally friendly technologies. The CRW was pretreated with ethanol at high temperature and then hydrolyzed with enzymes produced in-house to yield sugars. Key points of the process are: manipulations of the fermentation step that allowing bioethanol-producing yeasts to use almost glucose and galactose to produce ethanol, while retaining large amounts of d-mannose in the fermented broth; removal of colored compounds and other components from the fermented broth; and separation of ethanol and d-mannose through pervaporation. Under optimized conditions, approximately 15.7 g dry weight (DW) of d-mannose (approximately 46% of the mannose) and approximately 11.3 g DW of ethanol from 150 g DW of ethanol-pretreated CRW, were recovered.
Introduction
Due to increased demand for bioenergy to reduce heavy dependence on gasoline, the use of agro-industrial wastes in bioethanol production by hydrolysis and fermentation has received much attention because of its non-competitiveness with foodstuffs, abundance, and low cost (Hahn-Hägerdal et al., 2006, Lin et al., 2013, Mielenz, 2001, Sun and Cheng, 2002). Great effort has been made by scientists and engineers to expand the types of products generated by lignocellulosic biorefinery processing, including bioethanol and other valuable products like biomaterials, bioactive compounds, and carbohydrates derived from renewable materials (Devappa et al., 2015, Lau et al., 2012, Obruca et al., 2015, Sánchez and Cardona, 2008, Sarkar et al., 2012, Sokhansanj and Hess, 2009). Biosugar production is a promising new industry that has been developed by several companies for producing high-value sugars from lignocellulosic wastes (Bhuiyan et al., 1997, Fulger et al., 1985, Gómez et al., 2016, Hu et al., 2016, Zhang et al., 2009). Improvements in the development of lignocellulose-degrading enzymes have substantially enhanced sugar recovery via enzymatic hydrolysis. Unfortunately, biosugar production from lignocellulose frequently leads to a bottleneck in sugar separation and purification, which typically requires complex and high-cost separation technologies like membrane separation and simulated moving bed (SMB) chromatography. Thus, making lignocellulose-derived biosugar production feasible through improvements of fermentation and separation processes based on cost-effective, simple, eco-friendly, and high-efficiency processing methods, is a promising goal.
Coffee is one of the most consumed beverages worldwide. The direct discharge of coffee residue waste (CRW) into the environment without treatment could lead to serious environmental problems. Typically, CRW is generated from two main sources: cafeterias (coffee shops, restaurants, etc.) and instant coffee factories. The CRW contains a number of serious pollutants including organic material, caffeine, tannins, and polyphenols. It is estimated that these sources together produce approximately 6 Mt CRW/yr globally (Campos-Vega et al., 2015, Panusa et al., 2013). A mean of approximately 650 kg of CRW is generated from 1 t of green coffee beans during the standard production of instant coffee; however, many valuable products can be produced from CRW (Panusa et al., 2013). Fortunately, this residue is no longer considered “waste” based on its potential to produce bioenergy or value-added products from the substances present in CRW. Among the monosugars in CRW, mannose constitutes the largest portion (approximately 20–25%) of its total carbohydrate content (Hughes et al., 2014). This makes CRW an excellent source for mannose production. However, the mannose occurs mainly in the form of galactomannans, which are bound to arabinogalactans and cellulose to form polymers that are highly recalcitrant and hydrophilic; thus, the recovery of mannose from CRW by enzymatic hydrolysis is very difficult (Choi et al., 2012, Moreira et al., 2011, Nunes et al., 2005, Nunes et al., 2006). Despite wide applications in biological research and the pharmaceutical, food, and feed industries, the existing sources of d-mannose are limited. The largest supply is now extracted from plants and purified, probably because this is the largest source of raw sugar and currently the easiest process with the highest economic efficiency. Smaller portions of the total supply are provided by chemical and biological transformation of other sugars; however, despite many studies, this type of biological conversion remains far from practical reality (Gernhart et al., 2015, Sharma et al., 2014, Yokomizo, 2002, Zhang et al., 2009). Chemical extraction is a less desirable method due to the contamination by chemical extractants, whereas biological transformation has a very low efficiency. Based on the research of Zhang et al. (2009), the isolation and purification of d-mannose from palm kernels is currently considered the main and most effective method. Through this method, the total yield has reached approximately 48.4% of the weight of the palm kernels used. An unfortunate and important drawback of this method is the production of multiple harmful byproducts (e.g., furfurals) due to the use of the strong acids for hydrolysis, which consequently requires several filtration and purification steps. Thus, a simple and effective method to produce d-mannose from biomass wastes, particularly from a source as promising as CRW, is very attractive.
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