Effects of guar galactomannan on wheat bread microstructure and on the in vitro and in vivo digestibility of starch in bread

Microscopy and in vitro digestibility methods were used to obtain information about the putative inhibitory effect of guar gum, an endospermic seed extract, on the rate of digestion of wheal bread starch. Guar seed endosperm was examined by bright field microscopy after staining with tolu;dine blue or ruthenium red, by epifluorescence microscopy after labelling with two fluorescein-labelled lectins specific for alpha-D-galactose and D-mannose residues, respectively, and by scanning electron microscopy (SEM). These methods showed that guar galactomannan, the main component of guar gum, was located both within the plant cell walls and as amorphous deposits within the cell vacuole. SEM examination showed that the latter persisted in the milled guar flour. Examination of wheat bread containing guar gum by SEM and by fluorescence microscopy using the labelled lectins revealed that the starch granules and surrounding bread matrix were coated with a layer of galactomannan mucilage. After guar bread was ingested by pigs, the dose association of galactomannan with the wheat starch persisted during the early post-prandial period (0-90 min), but this was greatly reduced at later post-prandial limes (greater than or equal to 180 min). The in vitro hydrolysis of starch in guar bread was found to be attenuated significantly compared with normal wheal bread. The results are consistent, therefore, with the concept that guar galactomannan acts as a physical 'barrier' to alpha-amylase-starch interactions and/or subsequent release of hydrolysed products (e.g. maltose). Such a process, in addition to the effect of guar gum on digesta viscosity, may be significant in reducing the rise in post-prandial glycaemia caused by guar gum in humans. (C) 1996 Academic Press Limited