Development of an advanced integrative process to create valuable biosugars including manno-oligosaccharides and mannose from spent coffee grounds

Highlights

• SCG-derived polysaccharides were produced through delignification and defatting.

•Enzymatic hydrolysis was significantly enhanced in SCG-derived polysaccharides.

•Manno-oligosaccharides, mannose, and bioethanol, were produced from SCG.

Abstract

Spent coffee grounds (SCG) or coffee residue wastes (CRW) provide excellent raw material for mannose and bioethanol production. In this study, SCG were used to produce valuable biosugars, including oligosaccharides (OSs), manno-oligosaccharides (MOSs), mannose, and bioethanol. SCG were subjected to delignification and defatting, producing SCG-derived polysaccharides. Two-stage enzymatic hydrolysis (short- and long-term) was performed to produce short-chain manno-oligosaccharides (MOSs) and monosaccharides (MSs), respectively. From 100 g dry weight (DW) amounts of SCG, approximately 77 g delignified SCG and 61 g SCG-derived polysaccharides, amounts of 15.9 g of first biosugars (mostly MOSs), 25.6 g of second biosugars (mostly MSs), and 3.1 g of bioethanol, were recovered. This technique may aid in the production of high-value mannose and OSs from SCG and other lignocellulosic biomasses that contain specific polysaccharides.

Introduction

In addition to the production of second-generation biofuels from lignocellulosic biomass, the preparation of high value-added products from wastes or residue materials via bio-refining processes could change human life and reduce global warming (Hu et al., 2016, Tuntiwiwattanapun et al., 2017). Among these processes, biosugar production from lignocellulose is a new and promising industry that could replace or complement present biofuel production methods, particularly for bioethanol. Biosugars, including oligosaccharides (OSs) and monosaccharides (MSs) can be produced from lignocellulosic biomasses. Their compositions vary, but they typically contain the following major components: glucan, lignin, xylan, mannan, arabinan, and galactan (Devappa et al., 2015, Hu et al., 2016, Mata et al., 2018, Vaidya et al., 2016). The components differ among different plants, rendering the biomasses good sources of specific polysaccharides, OSs, and MSs; e.g. mannan from softwood is an important source of mannose, second only to cellulose (Wolf et al., 2012). However, during bioethanol production, many components (saccharides, lignin, fats, and proteins) are present after hydrolysis by enzymatic or other physicochemical methods, and are then fermented to produce bioethanol. If the process is terminated after saccharification, complicated steps are required to remove all non-saccharide components, making the production of biosugars difficult. Furthermore, OSs production is even more problematic, requiring the regulation of saccharification (to produce principally OSs), and downstream separation and purification. Recently, enzymatic hydrolysis has been shown to afford many advantages in MS production under simpler operating conditions, in comparison with other physicochemical methods. Thus, it is important to develop methods for biosugar production featuring simple separation of OSs and MSs in a cost-effective and efficient manner.

In the context of the biorefinery concept, mannan biotechnology involves the collection, harvest, and production of mannose and MOSs, which are widely distributed in nature as constituents of plant hemicellulose and glycoproteins in yeast cell walls (FitzPatrick et al., 2010, Van Dyk and Pletschke, 2012). Many MOSs, including both α-MOS and β-MOS, which are produced from α-mannan and β-mannan polymers, respectively, are found in the cell walls of yeasts such as Saccharomyces cerevisiae, and in plants and/or seeds of coconut, coffee, locust, konjac, guar, and aloe vera, etc. (Blibech et al., 2011, Jian et al., 2013, Liyanage et al., 2015, Nguyen et al., 2017a). These form a relatively new class of OSs. Health-promoting MOSs have also attracted significant interest as prebiotics (selectively fermented ingredients with specific properties) (Roberfroid et al., 2010). During biofuel production, lignocellulosic biomass is commonly pretreated using various physiochemical methods prior to hydrolysis to MSs by enzymes and acids. However, during the production of OSs, or MOSs in particular, mannan polysaccharides are isolated before hydrolysis to ensure the purity of the final products, of which MOSs are the most abundant components. For example, to prepare α-mannan, S. cerevisiae cell walls were first extracted by autoclaving, followed by precipitation with Fehling’s reagent (Kocourek and Ballou, 1969), whereas β-mannan is commonly prepared from the plants mentioned above via extraction with hot water or an alkaline solution, followed by ethanol precipitation, which does not yield all of the mannan polysaccharides (Otieno and Ahring, 2012). Enzymatic and non-enzymatic (physiochemical) processes have both advantages and disadvantages, depending on the feedstock used, indicating that combinations of these approaches may aid MOSs production from different sources (Álvarez et al., 2017). Simple and efficient pretreatments are required to remove almost all non-polysaccharide components. Thus, the regulation of enzymatic or non-enzymatic hydrolysis is essential to obtain desired MOSs at high yields.

Coffee beans and spent coffee grounds (SCG) contain large proportions of mannans including galactomannans and arabinogalactomannans, which release indigestible MOSs after hydrolysis; these can be exploited as prebiotics. In vitro studies of MOSs digestibility and fermentation have reported that MOSs remained undigested until they reached the large intestine, and were then fermented by fecal bacteria into short-chain fatty acids, improving digestion (Salinardi et al., 2010, Tian et al., 2017). Small amounts of OSs are released from coffee beans during roasting and coffee making, and exhibit highly specific functions, e.g. acting as prebiotics, blocking pathogen attachment in the gut, and interacting directly with intestinal cells (Ballesteros et al., 2018, González-Delgado et al., 2017, Tian et al., 2017). Approximately 6 megatons of coffee beans are produced annually worldwide; 1 ton generates approximately 650 kg of SCG. Therefore, the production of biosugars, particularly MOSs and mannose, from SCG is of scientific and economic interest (Campos-Vega et al., 2015, Panusa et al., 2013). However, to produce MOSs and mannose, an alternative feedstock with lower non-polysaccharide content is required, which will simplify saccharification, separation, and purification. High-quality biosugars and their derivatives currently have more applications than bioethanol. Thus, a simple and effective method to simultaneously produce MOSs and mannose from SCG, would be of particular interest.

In this study, a novel integrated process to pretreat SCG via delignification and defatting to remove almost all non-polysaccharide components was developed. SCG-derived polysaccharides are then subjected to sequential short- and long-term saccharification, producing MOSs and mannose, respectively, as major components. The production of the final biosugar, in which mannose is the dominent component, features a modification that simplifies a previously described protocol (Nguyen et al., 2017a).

Read more: https://www.researchgate.net/publication/328131780_Development_of_an_advanced_integrative_process_to_create_valuable_biosugars_including_manno-oligosaccharides_and_mannose_from_spent_coffee_grounds