Grape marc as a source of carbohydrates for bioethanol: Chemical composition, pre-treatment and saccharification
Introduction
The carbohydrates in plant biomass can be used as a raw material for producing liquid biofuels and valuable biochemicals. However, plant material is heterogeneous and recalcitrant to degradation. Further, the carbohydrates may take a variety of chemical forms, including polysaccharides, oligosaccharides, monosaccharides, or form glyco-conjugates like glycoproteins and glycolipids. To overcome the structural complexity of plant biomass, research has focused on modifying plants to make the carbohydrates more available (Abramson et al., 2010). Alternatively, physical or chemical pre-treatments combined with enzymatic hydrolysis have been used to facilitate the liberation of fermentable sugars (Mosier et al., 2005). However, these additional processing steps are energetically and financially costly, and potentially create bottlenecks in large scale production (Klein-Marcuschamer et al., 2012). Thus the need to identify dedicated biomass sources that are structurally favorable in their native form, require negligible processing, and are socially and environmentally advantageous is essential to the progression and prosperity of the biofuels industry.
One way to circumvent the issues imposed by using classic lignocellulosic feedstocks is to identify waste materials that are produced in abundance, have been accumulated for other uses, can be sourced cheaply and have carbohydrates in a form amenable to fermenting microorganisms. One such potential source of raw material is grape marc. Grape marc (or pomace) is the waste that remains after the juice is collected from the pressing of grapes for wine production; it includes grape skin, pulp, seeds, stems and residual juice. The composition of grape marc is related to the grape variety, the method of processing, environmental conditions and the ratio of skin:seeds:stem.
The global production of grapes is 67.1 Mt/yr with China, the United States of America and Italy being the leading producers (FAO, 2015). It has been estimated that 18–20% of the grapes used for wine making remains as waste marc, which could generate up to ∼13.4 Mt (fresh weight) of waste biomass (Spanghero et al., 2009). Currently, the global accumulation of grape marc is lower at 4.8 Mt/yr (average of last 10 years; FAO, 2015) and is considered to have limited economic value. Grape marc is generally disposed of at a cost to the winery but may be utilized as filler in livestock feed, fermented to make the alcoholic beverage grappa, reapplied as fertilizer or used as a source of phenolic compounds.
In recent years, grape marc has been proposed as a raw material for bioenergy production (Toscano et al., 2013) and has been used to generate butanol and biogas (Cáceres et al., 2012, Law and Gutierrez, 2013). Grape marc may also be used to produce biofuel at the winery site and the evolution of local biofuel processing plants will obviate costs associated with transport of the biomass. However, further information about the composition of grape marc is required to allow estimates to be made of biofuel yields and hence facilitate cost-benefit analyses of different types of marc for biofuel production.
Here the composition of grape marc derived from two grape varieties, Cabernet Sauvignon and Sauvignon Blanc was investigated. Various pre-treatments were analyzed for their effect on depolymerisation of non-cellulosic polysaccharides, and the residual cellulosic biomass was subsequently hydrolyzed using crude cellulase preparations. Finally, the chemical composition of grape marc was compared with other known agro-industrial waste materials, and these data were extrapolated to estimate theoretical ethanol yields.
Section snippets
Collection and preparation of plant material
Material was collected after the pressing of Cabernet Sauvignon and Sauvignon Blanc grapes in both April 2012 and March 2013 at the University of Adelaide Waite Campus vineyards (Adelaide, Australia), n = 2 for each variety. Amounts of 30 g (three technical replicates) were separated into seed, skin and stem fractions. The weight of each fraction was recorded and the mass distribution calculated.
Whole grape marc was lyophilized (Labconco-Freezone, Kansas City, MO, USA) to determine moisture
Characterization of grape marc raw materials and soluble carbohydrates
After the grapes are crushed and processed for winemaking, approximately 20% of the starting material remains as a moisture-dense waste material (Spanghero et al., 2009). The composition of this heterogeneous material is affected by the percentage of seed, skin and stem in the total mass (Fig. 2). For both varieties, the majority of the weight (up to 80% in Sauvignon Blanc grape marc, Fig. 2) is attributable to grape skins, with seeds and stems present to a lesser degree. The moisture content
Conclusions
Grape marc is a waste material that is rich in carbohydrates. The majority of these carbohydrates are soluble monosaccharides (glucose and fructose) and structurally complex polysaccharides (pectins, heteroxylans, xyloglucan and cellulose). The soluble carbohydrates can be directly converted to ethanol through fermentation, yielding up to 270 L/tonne. Alternatively, ethanol yields may be increased by utilizing the whole material, which benefits from acid pre-treatment followed by enzymatic
Acknowledgements
This work was financially supported by the Australian Research Council, NSF EPSCoR Grant (S.D.) and USDA Hatch funds (J.S.). Thanks to Waite Analytical Services for assistance with elemental and nitrogen measurements, Professor Peter Hoffmann at the Adelaide Proteomics Centre for assistance with mass spectrometry, and Professor Vincent Bulone (University of Adelaide) for critical reading of the manuscript.
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