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 (thiamine, riboflavin, niacin, vitamin B6) and certain minerals (Ca, Fe, Cu, Zn, P, K, and Mg). They are an excellent source of complex carbohydrates and polyunsaturated free fatty acids (linoleic, linolenic). However, common beans have several undesirable attributes, such as long cooking times, being enzyme inhibitors, phytates, flatus factors, and phenolic compounds, having a "beany" flavor, and being lectins and allergens, which should be removed or eliminated for effective utilization. Grain quality of common beans is determined by factors such as acceptability by the consumer, soaking characteristics, cooking quality, and nutritive value. Acceptability characteristics include a wide variety of attributes, such as grain size, shape, color, appearance, stability under storage conditions, cooking properties, quality of the product obtained, and flavor. Storage of common beans under adverse conditions of high temperature and high humidity renders them susceptible to a hardening phenomenon, also known as the hard-to-cook (HTC) defect. Beans with this defect are characterized by extended cooking times for cotyledon softening, are less acceptable to the consumer, and are of lower nutritive value. Mechanisms involved in the HTC defect have not been elucidated satisfactorily. Attempts to provide a definitive explanation of this phenomenon have not been successful. The most important hypotheses that have been proposed to explain the cause of bean hardening are (1) lipid oxidation and/or polymerization, (2) formation of insoluble pectates, (3) lignification of middle lamella, and (4) multiple mechanisms. Most researchers have reported that the defect develops in 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 been proposed: (1) appropriate storage, (2) controlled atmospheres, and (3) pretreatments. Probably, the most workable solution to the hardening phenomenon may be the development of materials less prone to HTC phenomenon. Decreasing cooking time, increasing nutritive value, and improving sensory properties of seeds with HTC defect would have great nutritional and economical impact.(ABSTRACT TRUNCATED AT 400 WORDS)

Hard‐to‐cook phenomenon in common beans — A review

Grain quality of common beans

Legumes provide an important part of the world's protein requirements, as well as other nutrients, but they are underutilized as food. A major factor limiting expanded consumption is storage induced textural defects that prolong cooking time and demand correspondingly higher energy requirements for preparation. These defects, including the hard-to-cook phenomenon and hardshell, are initiated by structural and compositional factors but can be at least partially controlled by storage and processing conditions. Structural components implicated include seed coat, cell walls, middle lamella, starch granules and membranes while, compositionally, proteins, carbohydrates, phytate, polyphenols and lignin may also be important. Methods of texture measurement of legumes, including mechanical, physical and sensory properties are reviewed. The most commonly held hypothesis for the mechanism of the hard-to-cook defect states that beans fail to soften after exposure to high temperature and humidity storage because the increased water activity potentiates phytase which hydrolyzes phytate, rendering it no longer capable of chelating the Ca++ and Mg++ ions of the middle lamella and, hence, undissoluble. More recent evidence, however, provides potential roles for lignification, starch gelatinization and perhaps other mechanisms depending upon conditions. The possibility of a nonenzymatic as well as an enzymatic route is raised and thoughts presented on a course of action for future research.

A review of textural defects in cooked reconstituted legumes —the influence of structure and composition

Black beans stored for 7–14 days at 41°C, 100% RH, required more cooking time than fresh beans and thus paralleled cooking time for beans stored forr more than 1 yr at room temperature. With corrections for solids lost during soaking, water absorption did not differ between fresh and stored beans. The rate of electrolyte leakage was greater from stored beans than from fresh samples, indicating that the cotyledon deteriorated during aging. Bean moisture content was found to be related to cooking time. In general, the higher the moisture content after soaking, the shorter the cooking time. However differences in cooking times between fresh and aged samples persisted regardless of bean moisture content.

Hard‐to‐Cook Phenomenon in Beans: Effects of Accelerated Storage on Water Absorption and Cooking Time

This review integrates current knowledge on the hard‐to‐cook (HTC) defect in legume seeds, with emphasis on the cellular and biological changes during storage and soaking, and the physicochemical changes during heating. Several postulated mechanisms, including the pectin‐cation‐phytate model, cell lignification, pectin β‐eliminative degradation, and protein denaturation in relation to starch gelatinization, are discussed in the context of current evidence. Subsequently, a developmental model of legume hardening is presented. It is held that the HTC defect develops during aging and soaking and is exhibited through cooking. During the process, there are many events involved. Free radical formation, lipid peroxidation, acid formation, membrane deterioration, protein denaturation, and leakage are events associated with aging and soaking, whereas pectin decomposition and solubilization, protein coagulation, and starch gelatinization are events that occur during cooking. Cooked HTC seeds are characteri...

Cellular, biological, and physicochemical basis for the hard‐to‐cook defect in legume seeds

Storage under unfavorable conditions is known to adversely affect the cooking quality of legumes by leading to the “hard-to-cook” defect. This phenomenon was studied by examining the texture and microstructure of Ghanainan cowpeas, variet “Adua ayera,” following storage at 0°C 80% RH; 21°C 35% RH and 29°C, 85% RH for up to 12 months. Scanning electron microscopy revealed no changes in raw beans after 12 months storage but soaking in water produced a loss of some protein bodies in seeds stored at 29°C. Texture measurement indicated that the rate of cooking of the beans decreased with increasing storage temperature and that storage at 29°C introduced the formation of the “hard-to-cook” defect. The micro structure of the defective beans showed an incomplete break down of the middle lamella which may partially explain this defect.

Peter W. Voisey

Papers

EFFECT OF STORAGE TIME AND CONDITIONS ON THE HARD-TO-COOK DEFECT IN COWPEAS (Vigna unguiculata)