Noncatalytic hydrolysis of guar gum under hydrothermal conditions Original Research Article

Guar gum, a naturally occurring heteropolysaccharide made of mannose and galactose, was hydrolytically degraded without a catalyst in a batch reactor to produce water-soluble (WS) saccharides including mono- and oligosaccharides. The degradation was carried out under hydrothermal conditions over ranges of temperature from 180 to 240 °C and of reaction time from 3 to 60 min. Guar gum was readily dissolved and hydrolyzed, and the major products identified in the WS components were oligosaccharides with degrees of polymerization up to about 20, monosaccharides containing mannose and galactose, and 5-hydroxymethyl-2-furaldehyde (5-HMF). At 200 °C, the oligosaccharide yield, obtained from the difference between the yields of the total WS saccharides and monosaccharides, showed the highest value of 94.4% at 7 min among all conditions studied, on the basis of the saccharide content in the initial sample. The oligosaccharide yield decreased with reaction time, and the yield of monosaccharides correspondingly increased, and reached the highest value of 34.5% (mannose 22.8%, galactose 11.7%) at 60 min. The monosaccharides produced were further decomposed to secondary products such as 5-HMF. The maximum yield of 5-HMF obtained was 26.3% at 220 °C and 30 min. The production and the decomposition of galactose somewhat preceded those of mannose.