Hard Beans—A Problem for Growers, Processors, and Consumers

Abstract

Hard-to-cook and hard-shell are two textural defects associated with storage of legumes, as typified by the common bean. These defects can lead to failure to germinate, extended cooking times, reduced nutritional value, and economic loss throughout the food chain. Although these losses are predominate in tropical climates, beans stored in temperate areas also will harden eventually, depending on temperature and humidity. Hardened beans also often darken, causing further quality losses. Structurally, hard-shell is associated with the seedcoat and failure of water absorption, while hard-to-cook affects the cotyledons, rendering the cells unable to separate during cooking. Hardening of seedcoats during storage has been reported, and a mechanism based on oxidation and polymerization of phenolic compounds is suspected as being responsible, but few details of the hard-shell defect are known. The traditional theory used to explain the hard-to-cook defect is based on enzymatic hydrolysis of phytate, rendering it unable to chelate divalent cations that then migrate to the middle lamella and participate in crosslinking reactions with demethylated pectins. More recent evidence points to a multiple mechanism of bean hardening, with metabolism of phenolic compounds and membrane deterioration also involved. Control of bean hardening has been attempted at all levels of bean production, processing, and consumption. At present, control of storage conditions, manipulation of agronomic factors, and improved cooking techniques seem to be the best strategies to reduce bean hardening.