Changes in Physicochemical Properties of Dry Beans (Phaseolus vulgaris L.) during Long-Term Storage Marı ´a A. Martı ´n-Cabrejas,* ,† Rosa M. Esteban, † Pedro Perez, † Gladys Maina, ‡ and Keith W. Waldron § Departamento Quı ´mica Agrı ´cola, Facultad de Ciencias, Universidad Auto ´noma de Madrid, 28049 Madrid, Spain, Kenyan Agricultural Research Institute (KARI), Nairobi, Kenya, and Institute of Food Research, Norwich Research Park, Norwich NR4 7UA, United Kingdom Five different bean varieties (Phaseolus vulgaris L.) from Kenya, which were obtained either freshly collected or after having been stored for 5 years in tropical conditions (30-40 °C, >75% RH), were compared for their cooking characteristics. Beans under storage were susceptible to the hard-to- cook (HTC) defect, and the bean cooking time was up to 12 times that of the fresh beans, limiting their acceptability. A study was undertaken to investigate changes in nutritionally relevant physicochemical properties of beans during storage. Chemical analysis showed that storage resulted in a decrease in pH and an increase in titratable acidity in all varieties. Total polyphenol, non- tannin polyphenol, tannin, and lignin contents were determined; storage-induced HTC beans contained lower levels of total polyphenols (especially, non-tannin polyphenols) than the fresh beans but higher levels of tannins in all cultivars. Significant increases in lignin and lignified protein were also detected and accompanied by increases in cooking time. A reduction in phytic acid was also observed. These results are discussed in relation to the mechanisms which may underlie the development of the HTC defect. Keywords: Beans; hard-to-cook; phenolic compounds; storage INTRODUCTION Dry beans of Phaseolus vulgaris are important food sources, especially in developing countries where they are a major source of dietary protein. Phaseolus beans are a good source of proteins, vitamins, and certain minerals (Ca, Fe, Cu, Zn, P, K, and Mg). They have beneficial effects on human health, being an excellent source of complex carbohydrates and polyunsaturated fatty acids. However, in tropical countries, storage of common beans under adverse conditions of high tem- perature and high humidity renders them susceptible to a hardening phenomenon, also known as the hard- to-cook (HTC) defect. Beans with this defect are char- acterized by extended cooking times (Jones and Boulter, 1983; Vindiola et al., 1986), are less acceptable to the consumer (Burr et al., 1968), and are of lower nutritive value (Sievwright and Shipe, 1986). In addition, in the raw state, common beans contain antinutritional and toxic substances such as polyphenols, phytates, enzy- matic inhibitors, and lectins (Gupta, 1987; Liener, 1989) which limit their acceptability and should be removed or eliminated for effective utilization (Pusztai and Palmer, 1977). At present, most studies carried out in extended storage at high temperature and high humidity have been directed to highlight the postharvest physiological changes in several physical and chemical components in relation to cooking times of legume seeds (Stanley and Aguilera, 1985; Plhak et al., 1989; Hentges et al., 1991). However, little work has been done to determine the changes in antinutrient compounds in beans in relation to the development of the HTC phenomenon. It has been reported (Martin-Cabrejas et al., 1995) that fresh and HTC beans contained nutritionally significant amounts of lectins and trypsin and R-amylase inhibitors; HTC samples had higher levels of lectin and lower levels R-amylase inhibitor, while the amounts of trypsin and chymotrypsin inhibitors were the same. In recent years, concern over nutritionally harmful effects of certain biological compounds such as phytate and phenolics has increased (Bjorck and Nyman, 1987). These compounds occur naturally in the seeds of cereals and legumes and, if present in appreciable quantities, can lower the nutritional value and biological avail- ability of dietary proteins and minerals. Phytic acid reduces the availability of minerals by forming com- plexes (Hallberg and So ¨ lvell, 1967; Na ¨ vert et al., 1985). Polyphenols are known to inhibit the activity of pro- teolytic enzymes, hence reducing protein digestibility; recently attention has been paid to the role of these antinutrients in affecting the availability of carbohy- drates for enzymic digestion (Bjo ¨ rck and Nyman, 1987). However, there is little information regarding the effect of long-term storage on the changes in the different types of phenolics (total polyphenols, non-tannin polyphe- nols, tannins, and lignin). The main objective of the present work was to determine the effects of the HTC phenomenon on several physicochemical parameters and, especially, phenolic constituents and phytic acid of seed bean varieties typically grown and stored in Kenya, under ambient environmental conditions. MATERIALS AND METHODS Bean Samples. Dry beans (P. vulgaris) of five different cultivars, namely, Mwitemania, Canadian Wonder, Mwezi Moja, Rose Coco, and Red Haricot, were supplied by the * To whom correspondence should be addressed (tel, 34-1-3973831; fax, 34-1-3973830; e-mail, maria.martin@ uam.es). † Universidad Auto ´noma de Madrid. ‡ KARI. § Institute of Food Research. 3223 J. Agric. Food Chem. 1997, 45, 3223-3227 S0021-8561(97)00069-1 CCC: $14.00 © 1997 American Chemical Society