Storage-induced changes in protein interactions and protein solubility in common beans

The common bean is a traditional food in the human diet, being a good source of proteins, minerals and vitamins. Storage under conditions of high temperature and humidity causes a texture defect known as ‘hard-to-cook’, which reduces the acceptability of the beans, increasing the cooking time required to soften the cotyledons. This phenomenon may result, amongst other factors, from protein denaturation. Thus, the study of the proteinprotein interactions occurring in ‘hard-to-cook’ beans could shed light on the mechanisms involved in the emergence of such a defect. In this context, the present study investigated the interactions causing protein denaturation in beans stored under high temperature and high relative humidity. The aging of two common bean batches of each of two varieties was accelerated in an oven at 40oC and 76% RH for 15, 30, 45, 60 and 75 days. For both lots of each variety, fresh beans stored at 5oC were considered as the control. The total protein content, soluble protein in buffers with different denaturing agents and the contents of sulphydryl and disulphide groups were analyzed. The results for the solubility data were used to determine covalent and non-covalent interactions in the proteins. The percentage of soluble protein reduced with increase in storage time, which could be related to protein denaturation, leading to the aggregation of molecules. Covalent interactions made little contribution to reducing protein solubility, as confirmed by the small variation in disulphide bonds during storage.