Effect of galactose content on the solution and interaction properties of guar and carob galactomannans

Abstract Guar galactomannan has been modified by treatment with an α- d -galactosidase A preparation from lucerne seeds. This enzyme was purified by affinity chromatography on N -ϵ-aminocaproyl-α- d -galactopyranosylamine linked to Sepharose 4B, had a high activity towards galactomannans, and was completely devoid of β- d -mannanase. On incubation for 2 h, this enzyme removed ⪢ 75% of the galactose from guar galactomannan with no concurrent decrease in viscosity. Eventual decrease in viscosity was associated with the formation of insoluble, mannan-type precipitates. This phenomenon, although directly related to the galactose content of the galactomannan, was also time-dependent. The limiting viscosity numbers calculated for the “mannan backbones” of α- D -galactosidase-treated, guar galactomannan having galactose-mannose ratios of 38:62 to 15:85 were the same. Modified, guar galactomannan (at 0.4% w/v) having a galactose-mannose ratio of 20:80, or less, forms a gel on storage at 4° over several weeks. Also, gel particles form when solutions of these galactomannans are passed through a freeze-thaw cycle. Samples containing ⪡ 10% of galactose rapidly precipitate from solution even at 30°. The interaction of guar galactomannan with xanthan is greatly increased by removal of galactose residues. Samples having galactose-mannose ratios of ~19:81 interact with xanthan to essentially the same degree as carob galactomannan (Gal/Man = 23:77).