Development of the hard-to-cook defect in dried beans (Phaseolus vulgaris) and cowpeas (Vigna unguiculata)

Dawn Lynnette Hentges, Purdue University

Abstract

Two cultivars of cowpeas (Vigna unguiculata) and five cultivars of dry beans (Phaseolus vulgaris) were monitored for development of the hard-to-cook defect during storage in the following conditions: 5$\sp\circ$C, 30% RH; 29$\sp\circ$C, 30% RH; 5$\sp\circ$C, 65% RH; and 29$\sp\circ$C, 65% RH. Cookability, moisture content, soakability, leachability, phytate content, phytase activity, starch content, pectin content, and protein content of the legume seeds were measured after 0, 1 1/2, 3, 6, 9, 12, 18, and 24 months of storage. It was attempted to relate these physical and chemical properties to cooking time of the legume seeds. High temperature and high humidity appeared to be necessary during storage to promote increased cooking times of legume seeds. The seeds stored at 29$\sp\circ$C, 65% RH had much longer cooking times than seeds stored in the other environmental conditions. The seeds stored in the other conditions maintained relatively constant cooking times throughout the storage period. The hard-to-cook defect was reversed when beans having long cooking times were stored at 6.5$\sp\circ$C, 71% RH. This reversibility appears to depend on storage time. For hard-to-cook beans which characteristically have a high percentage of uncooked beans, the 50% cooking time provided a more accurate description of the cooking time than the calculated geometric mean cooking time. Several physical and chemical components seem to contribute to the development of the hard-to-cook defect in legume seeds. Cowpeas and dried beans requiring long cooking times had decreased phytate contents, decreased water-soluble (high methoxyl) pectin, and increased EDTA-soluble (low methoxyl) pectin contents. This evidence supports the proposed theory of the hard-to-cook defect involving phytate-mineral and mineral-pectin interactions. Phytase activity decreased during storage of beans. Membrane deterioration allowing migration of minerals was indicated by increased electrolyte leakage of soaked legume seeds. Starch and protein may also play a role in the development of the defect since decreased starch and protein solubility was observed in hard-to-cook legume seeds. Since seeds having the defect were shown to have decreased cooking times after additional storage at 6.5$\sp\circ$C, 71% RH, the mechanism(s) leading to the development of the hard-to-cook defect must be easily reversible.

Degree

Ph.D.

Advisors

Weaver, Purdue University.

Subject Area

Food science

COinS