Effect of sugars, galactose content and chainlength on freeze-thaw gelation of galactomannans

Cryogels of locust bean gum (LBG) were prepared by freezing and thawing 1.0 wt% solutions incorporating sucrose, glucose, fructose or sorbitol at concentrations of 40. 45, 50, 55 and 60 wt%, and were characterised by compression testing. Gel strength showed an initial increase and subsequent decrease with increasing concentration of sugar. Maximum strength was attained at 45 wt% fructose, 50 wt% sucrose or sorbitol, and 55 wt% glucose. but increased in the same order: fructose < sucrose = sorbitol < glucose. The initial increase in gel strength is attributed to the reduction in water content with increasing concentration of sugar; the subsequent decrease is tentatively ascribed to inhibition of polymer-polymer association by binding of sugar molecules to the polymer chains, with differences in gel strength arising from differences in strength of binding. Galactomannan samples with mannose: galactose (M/G) ratios spanning that of LBG were prepared by treatment of guar gum with alpha-galactosidase, and cryogels (1.0 wt%) were prepared at a fixed concentration (50 wt%) of sucrose. A sharp increase in gel strength was observed at M/G ratios around. and above, that of LBG, and is attributed to increasing content of unsubstituted sequences of mannan backbone long enough to form stable associations as junctions in the cryogel network. The samples of partially debranched guar gum had substantially lower molecular weights than LBG (attributed to some slight beta-mannanase activity in the alpha-galactosidase used in their production), but the mechanical properties of the LBG cryogels fitted in reasonably well with the M/G-dependence observed for the other samples indicating that within the range studied (similar to 10 to similar to 2000 kD.). chainlength has little effect. (c) 2006 Elsevier Ltd. All rights reserved.