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

Zhou, Yang (2003) Relationship between α-amylase degradation and the structure and physicochemical properties of legume starches. Masters thesis, Memorial University of Newfoundland.

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Abstract

Starches from different cultivars of black beans, pinto beans, smooth peas, lentils and wrinkled peas were isolated and their composition, physicochemical properties and susceptibility towards porcine pancreatic a-amylase were determined. -- The yield of starch ranged from 16.4 to 34.1% on a whole seed basis. The shape of the granules in black bean, pinto bean, smooth pea and lentil varied from round to oval to irregular, while compounds in rounded rosette were observed for wrinkled pea starch. Bound and total lipids ranged from 0.26 to 0.80% and 0.35 to 0.84%, respectively. The total amylose content of black bean, pinto bean, smooth pea and lentil were in the range of 30.5-39.3%, whereas that of wrinkled pea was much higher (78.4%). The percentage of lipid-complexed amylose in native starches ranged from 10.3 to 12.2%. The X-ray diffraction pattern was of the 'B' type in wrinkled pea starch and of the 'C' type in the other starches. The relative crystallinity and the 'B' polymorphic content ranged from 17.7 to 33.4% and 27.1 to 92.2%, respectively. Wrinkled pea starch exhibited the lowest relative crystallinity (17.7%) and the highest 'B' polymorphic content (92.2%). -- The swelling factor (SF) and the extent of amylose leaching (AML) of native starches were in the range of 3.4-17.7 and 11.0-17.8%, respectively. The gelatinization temperatures (To , Tp, Tc) and enthalpy (ΔH) of native starches (with the exception of wrinkled pea starch) were in the range of 60.0-65.7°C, 66.0-76.5°C, 76.4-88.8°C, and 14.6-20.1mJ/mg, respectively. Differences in SF, AML, To , Tp, Tc, and ΔH between cultivars of the same species were more pronounced in black bean and lentil starches. Wrinkled pea starch did not show an endothermic peak indicating that starch chain interactions within the amorphous domains were more extensive in wrinkled pea starch. -- All starches exhibited a biphasic hydrolysis pattern, i.e. a relatively rapid rate initially followed by a progressively decreasing rate thereafter. Wrinkled pea starch exhibited a much higher initial hydrolysis velocity than did 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 the black bean cultivars, but was different for the lentil cultivars. Pinto bean and smooth pea cultivars showed no plateau. At the end of the assay period (120h), 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. Scanning electron micrographs showed that starches were slightly eroded during the initial hydrolysis stage (<20% hydrolysis), but the integrity of most of the granules was well maintained. However, roughened surfaces and disc like depressions were obvious for all starches, except for lentil starch. No morphological differences were observed between cultivars from the same species for both native and hydrolyzed starches. The X-ray diffraction pattern and the 'B' polymorphic content of all starches remained unchanged upon 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 demonstrated that differences in the composition and physicochemical properties of starch between cultivars from the same species were marginal. The rate and extent of hydrolysis were influenced mainly by structural organization and interactions of the starch chains within the native granule, as well as by the extent of association between hydrolyzed amylose chains.

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