The health benefits of legumes have been known for many years. They are considered to be a food staple in the diet of humans. Dry beans have been implicated in the prevention and management of type II diabetes mellitus and in the reduction of risks associated with colon cancer. Dry beans are good sources of complex carbohydrates, protein, dietary fiber, vitamins and minerals. However, use of the starch fraction of dry beans is limited due to the lack of molecular structure-functionality relationships. The specific objectives of this study were: (1) to determine and compare the physicochemical properties of starch isolated from six varieties of Phaseolus vulgaris, namely black bean, dark red kidney bean, light red kidney bean, navy bean, pinto bean and small red bean, (2) to determine and compare the digestibility, estimated glycemic index, and molecular weight distributions of isolated raw, canned and stovetop-cooked bean starches, (3) to investigate the effects of canning and a traditional cooking method (boiling) on the digestibility and molecular weight distributions of starch isolated from dried bean. The physicochemical characteristics of starches isolated from six varieties of Phaseolus vulgaris grown in Michigan were determined. Starch yield varied from 22.8% to 34.1% on a whole seed basis (~ 10% moisture content). The raw isolated bean starches exhibited high resistant starch contents ranging from 41.9% (light red kidney bean) to 55.7% (pinto bean), whilst total amylose content varied from 28.0% (pinto bean) to 29.8% (dark red kidney bean). Significant differences were observed for the gelatinization transition temperatures, pasting parameters, and resistant starch contents of the isolated dark red kidney and light red kidney bean starches when compared to isolated starch from black, navy, pinto and, small red beans. All isolated bean starches displayed the characteristic C-type X-ray diffraction pattern of legumes. The crystallinity and B-type starch polymorph contents ranged from 36.1% to 24.9% and 19.6% to 15.6% respectively. This study demonstrated that the chemical compositions of beans are different for different varieties, and thus the varieties exhibit different physicochemical properties. The weight average molecular weight distribution (Mw) of starches isolated from native, canned, and stovetop-cooked beans were analyzed using high-performance size exclusion chromatography with multi-angle laser light scattering and refractive index detectors (HPSEC-MALLS-RI). Results revealed that amylose of isolated native dark red kidney bean starch had the smallest Mw (1.0 x 106 g/mol), whereas isolated native pinto bean starch had the largest value of Mw (1.8 x 106 g/mol). The Mw values of amylopectin for isolated native bean starches ranged from 2.4 x 107 g/mol to 3.9 x 107 g/mol. Isolated canned and stovetop-cooked bean starches displayed a mono-modal Mw distribution, with a reduction in high molecular weight fractions, whereas isolated native bean starch Mw distribution was bi-modal. Results of in vitro á-amylase starch hydrolysis showed ranges of rapidly digestible starch (RDS) (1.95-2.71%), slowly digestible starch (SDS) (14.36-17.39%), and resistant starch (RS) (78.95-83.7%) among the tested isolated native Phaseolus vulgaris starches. However, RDS, SDS, and RS fractions in isolated canned bean starches ranged from 7.58-13.21%, 20.75-24.90%, and 63.17- 68.54%, respectively. Isolated stovetop-cooked bean starches yielded similar results: RDS, 7.37-10.61%; SDS, 22.55-26.84%; and RS, 62.55-68.59%). The hydrolysis indices (HI) and glycemic indices (GI) were marginally greater for isolated canned bean starches than for stovetop-cooked starches.
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