Relationship between α-amylase degradation and the structure and physicochemical properties of legume starches

Abstract

Starch from cultivars of black bean, pinto bean, smooth pea, lentil and wrinkled pea starches were isolated and their composition, physicochemical properties and susceptibility towards porcine pancreatic α-amylase determined. The yield of starch ranged from 16.4 to 34.1% on a whole seed basis. The shape of the granules varied from round to oval to irregular. Bound and total lipids ranged from 0.26 to 0.80% and 0.35 to 0.84%, respectively. The total amylose content ranged from 30.5 to 78.4%, of which 10.3 to 12.2% was complexed by native starch lipids. The X-ray diffraction pattern was of the ‘B’ type in wrinkled pea starch and of the ‘C’ type in other starches. The relative crystallinity and the ‘B’ polymorphic content ranged from 17.7 to 33.4% and 27.1 to 92.2%, respectively. The starches differed in the degree of swelling, extent of amylose leaching, gelatinization temperatures and gelatinization enthalpy. The above differences were more pronounced between cultivars of black bean and lentil. The results showed that starch chain interactions within the amorphous domains were more pronounced in wrinkled pea starch. All starches exhibited a biphasic hydrolysis pattern, a relatively rapid rate at the initial stage followed by a progressively decreased rate thereafter. Wrinkled pea starch exhibited a much higher initial velocity than the other starches. Cultivars of black bean and lentil showed significant differences in their initial velocities. However, differences in initial velocity between cultivars of smooth pea and pinto bean were not significant. Black bean, lentil and wrinkled pea starches showed a plateau at 93, 85 and 65% hydrolysis, respectively. The time taken for the appearance of the plateau was identical for black bean cultivars, but was different for cultivars of lentil. Pinto bean and smooth pea cultivars showed no plateau. At the end of the assay period (120 h), cultivars of each legume species were hydrolyzed to the same extent, and the extent of hydrolysis among the legume species followed the order: black bean>lentil>smooth pea>pinto bean>wrinkled pea. The X-ray pattern and the ‘B’ polymorphic content of all starches remained unchanged on hydrolysis. However, the relative crystallinity increased in wrinkled pea, but remained unchanged in the other starches. On hydrolysis, the apparent amylose content decreased in all starches. The extent of this decrease was most pronounced in wrinkled pea. In all starches, the enthalpy of gelatinization decreased, and the gelatinization transition temperatures increased slightly on hydrolysis. This study showed that the rate and extent of hydrolysis was influenced by structural organization of the starch chains within the native granule, and by the extent of interaction between hydrolyzed amylose chains.