Galactomannans occur in nature as the reserve polysaccharides in the endosperms of a wide range of legume seeds. These polysaccharides, in partially purified form, find widespread application as thickening and gelling (in the presence of other polysaccharides) agents in the food industry. Partially degraded guar galactomannan is used as a novel dietary fiber component.
Galactomannan is composed of a 1,4-β-linked D-mannan backbone to which single D-galactosyl units are attached to C-6 of some of the D-mannosyl residues. The major difference between galactomannans from different seed species is the ratio of D-galactose to D-mannose. However, some variation in the molecular weight of the galactomannans has also been reported. The D-galactose:D-mannose ratio of galactomannan from different varieties of a given seed species appears to be remarkably constant. In a detailed study of galactomannans, purified from approx. 50 samples of carob flour from diverse sources and from a range of carob varieties, it was shown that the D-galactose:D-mannose ratio is constant, i.e. 22 ± 1% (w/w) D-galactose and 78 ± 1% (w/w) D-mannose.1 In a parallel study on the galactomannan from guar seed varieties, a D-galactose content of 38 ± 1% (w/w) and D-mannose content of 62% ± 1% (w/w) was found. This observation would also appear to hold true for galactomannan from other seeds, e.g. fenugreek seed, but in these cases the studies have not been as comprehensive.
Galactomannans are quantitatively hydrolysed to D-galactose and manno-oligosaccharides by a mixture of α-galactosidase plus β-mannanase.
Thus, a procedure has been developed for the measurement of carob galactomannan and guar galactomannan (or, in fact, possibly any other galactomannan) based on the measurement of the D-galactose content.