In extractives, a fraction of chemical components are those containing sugars, referred to as either water-soluble-carbohydrate, or non-structural carbohydrate, or free sugars, which is mostly composed of the monosaccharide of D-fructose, the disaccharide of sucrose, the trisaccharide of raffinose, and tetrasaccharide of stachyose as well as the group of polysaccharides known as fructans. In addition, arabinogalactan can be classified as among the extractives.
These non-structural sugars tend to be intermediates and primary products of photosynthesis and serve functions in storage, translocation, and metabolic utilization of carbon as well as protection against abiotic stresses. They also play an important role in the dynamics of piles of stored biomass.
The non-structural sugars tend to be present in much higher concentrations in grassy than woody feedstocks. As reported (Chen et at, 2007 and 2010),carbohydrates (primarily sucrose, glucose, and fructose) were found to be the predominant water-soluble components of corn stover and switchgrass, accounting for 18-27% of the dry weight of switchgrass extractives and 30−46% of the dry weight of corn stover extractives (4−12% of the dry weight of feedstocks). The actual level of free sugars in harvestable biomass depends on CO2 assimilation and demand for these assimilates, mainly for respiration and the production of protein and structural carbohydrates (White, 1973). Given that such needs vary greatly with growth cycle, nutritional state, tissue, season, and management regimes, the non-structural sugar concentrations tend to be much more dynamic than those of the structural polysaccharides (Wulfes et al., 1999).
Whether or not these sugars add to sugar yields in fractionation procedures will depend on whether or not they are retarded by process conditions.
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