Chemical Components: Hemicelluloses
After cellulose, the next major polysaccharide resource is plant hemicelluloses. Unlike cellulose, hemicellulose is a collection of short branched polymers with lower DP values (i.e., typically 50–300), frequently have side chain groups and are essentially amorphous. The types and amoumt of hemicellulose are also species dependent. For example, typical softwood hemicelluloses are galactoglucomannan (Mannans), arabinoglucuronoxylan, arabinogalactan, and pectins; The dominant hemicellulose in softwood are glucomannan; Hardwood hemicelluloses are mainely glucuronxylan (xylans) and glucomannan. The dominant hemicellulose in hardwood is xylan. In spite of these, after hydrolysis, the monomer sugars from hemicellulose and cellulose are arabinose (C5),glucose (C6), mannose (C6), galactose (C6), xylose (C5) as well as some hexuronic acids: glucuronic acid, 4-methyl-glucuronic acid, and galacturonic acid.
After cellulose, the next major polysaccharide resource is plant hemicelluloses. Unlike cellulose, hemicellulose is a collection of short branched polymers with lower DP values (i.e., typically 50–300), frequently have side chain groups and are essentially amorphous. The types and amoumt of hemicellulose are also species dependent. For example, typical softwood hemicelluloses are galactoglucomannan (Mannans), arabinoglucuronoxylan, arabinogalactan, and pectins; The dominant hemicellulose in softwood are glucomannan; Hardwood hemicelluloses are mainely glucuronxylan (xylans) and glucomannan. The dominant hemicellulose in hardwood is xylan. In spite of these, after hydrolysis, the monomer sugars from hemicellulose and cellulose are arabinose (C5),glucose (C6), mannose (C6), galactose (C6), xylose (C5) as well as some hexuronic acids: glucuronic acid, 4-methyl-glucuronic acid, and galacturonic acid.
Fig. 1. Momomer sugars and hexuronic acids
Because the hemicellulose is Not crystalline in biomass, they are very accessible to chemicals and very reactive. As a result, they are easily extracted, hydrolyzed, and further degraded during the typical pretreatments of biomass. For example, during acid pretreatment, hemicellulose can be extracted by the cleavages of glycosidic linkages. Meanwhile, the ester and ether linkages are also cleaved to for acetic acid (from acetyl group) and methanol (from methyl group).
Fig. 2. Hemicellulose action under acidic conditions
One of the benefits for hemicellulose extraction is to open the cell wall pores to allow chemicals or enzymes to transport inside the cell wall more easily. On the other side, if not control very well, the monomer sugars produced will further degrade to form the by-products such as aldonic acids, furfural (or HMF), levulinic acid, and formic acid, etc. Both the furfural and weak acids are inhibitors of enzyme metabolism and should be minimized from the treatments.
Fig.3. Sugar degradation under acidic conditions
No comments:
Post a Comment