The effect of salivary amylase on the viscosity behaviour of gelatinised starch suspensions and the mechanical properties of gelatinised starch granules

The digestion of gelatinised starch suspensions by salivary amylase was studied using a viscometric assay and also by measuring the mechanical properties of individual granules. Viscosity decay as a function of time was approximately exponential. A comparison with a reducing group assay showed that a 50% fall in viscosity corresponded to the scission of about 0.05% of the glycosidic bonds. The half‐time for the reduction of viscosity by the enzyme was found to be proportional to enzyme concentration but independent of starch concentration over a wide concentration range. This result suggests that the starch concentrations used were well below the saturating point for the enzyme, which was thus largely in the free state, and that the enzyme attacked the gelatinised granules throughout their structure rather than just at the surface. Measurements of the forces required to deform individual granules supported this hypothesis by showing that the gelatinised granules lost nearly all their mechanical strength during initial contact with salivary amylase, without any apparent change in diameter.