Hard‐to‐cook phenomenon in legumes

Abstract

Hard‐to‐cook legumes (HTCL) have been observed for centuries. Different types of HTCL are recognized in freshly harvested crops or are developed during storage. Two types of hard shell, which is due to seed coat impermeability to water, are identified: (1) the reversible hard shell that occurs in freshly harvested seeds and disappear during storage; (2) the other type is the irreversible hard shell that develops during storage. Sclerema is due to unhydratable cotyledons (despite removal of seed coat) during soaking, and it may develop during storage. Some cases of reported hard shell may be due to sclerema, since no testing was done to prove seed coat impermeability. Hard texture and sandy texture (i.e., grainy or granular texture) have been noticed in freshly harvested and in stored legumes. In this paper, variations in the physical properties and the chemical composition of the seeds of different legumes and of different varieties are summarized in several tables. The following topics are discussed with regard to different types of HTCL: the environmental factors affecting their development; the structural and the chemical changes associated with their presence; the treatments which are used or suggested to control their development or to improve the texture of HTCL; and the possible relations between these subjects. Several propositions concerning the mechanisms involved in hard‐texture seeds are discussed. Soft texture is associated with ease of cell separation of the cotyledons in cooked legumes, indicating the significance of the middle lamella and the cell wall. At present, it seems that the main mechanisms include phytate, phenolic compounds, and divalent cations (Ca, Mg) and their interaction with pectic substances and proteins. Starch, lipids, tannins, and lignin may have a secondary effect. More knowledge is needed about cell wall components, structure, and interactions, and about the effect of environmental conditions on them.