HARD-TO-COOK PHENOMENON IN COMMON BEANS - A REVIEW

Legumes are one of the world's most important sources of food supply, especially in developing countries, in terms of food energy as well as nutrients. Common beans are a good source of proteins, vitamins (thia mine, riboflavin, niacin, vitamin B6) and certain minerals (Ca, Fe, Cu , Zn, P, K, and Mg). They are an excellent source of complex carbohydr ates and polyunsaturated free fatty acids (linoleic, linolenic). Howev er, common beans have several undesirable attributes, such as long coo king times, being enzyme inhibitors, phytates, flatus factors, and phe nolic compounds, having a ''beany'' flavor, and being lectins and alle rgens, which should be removed or eliminated for effective utilization . Grain quality of common beans is determined by factors such as accep tability by the consumer, soaking characteristics, cooking quality, an d nutritive value. Acceptability characteristics include a wide variet y of attributes, such as grain size, shape, color, appearance, stabili ty under storage conditions, cooking properties, quality of the produc t obtained, and flavor. Storage of common beans under adverse conditio ns of high temperature and high humidity renders them susceptible to a hardening phenomenon, also known as the hard-to-cook (HTC) defect. Be ans with this defect are characterized by extended cooking times for c otyledon softening, are less acceptable to the consumer, and are of lo wer nutritive value. Mechanisms involved in the HTC defect have not be en elucidated satisfactorily. Attempts to provide a definitive explana tion of this phenomenon have not been successful. The most important h ypotheses that have been proposed to explain the cause of bean hardeni ng are (1) lipid oxidation and/or polymerization, (2) formation of ins oluble pectates, (3) lignification of middle lamella, and (4) multiple mechanisms. Most researchers have reported that the defect develops i n the cotyledons. Recently, some authors have suggested that the seed coat plays a significant role in the process of common bean hardening. A better knowledge of cotyledon and seed coat microstructure may lead to a better understanding of the causes of seed hardness. In order to prevent the development of the HTC defect several procedures have bee n proposed: (1) appropriate storage, (2) controlled atmospheres, and ( 3) pretreatments. Probably, the most workable solution to the hardenin g phenomenon may be the development of materials less prone to HTC phe nomenon. Decreasing cooking time, increasing nutritive value, and impr oving sensory properties of seeds with HTC defect would have great nut ritional and economical impact. Furthermore, an understanding of the m echanisms leading to reversibility of this phenomenon would provide in sight into the development of the defect itself and would aid in the s earch for appropriate methods to prevent it. Efforts to develop techno logical processes are needed in order to transform the HTC beans into edible and useful products. Several economic alternatives to utilize H TC common beans have been proposed: (1) dehulling, (2) extrusion, (3) solid state fermentation, (4) quick-cooking beans, and (5) production of protein concentrates and isolates and starch fractions.