Hard-to-Cook Defect in Black Beans: The Effect of Soaking in Various Aqueous Salt Solutions

Abstract

In order to study the effect of various salts on the hard-to-cook phenomenon, black beans (Phaseolus vulgaris) were stored under tropical conditions to induce the defect and soaked in various aqueous sodium salt solutions prior to cooking. Statistical analyses of hardness data showed that storage conditions and pH of soaking solutions did not produce significant effects but anion type was important in inducing softness as follows: CO3−2>EDTA−2=NO3−>SO4−2=Cl−. Softening was also promoted by increasing the ionic strength of the soaking solution. Scanning electron microscopy of treated beans indicated that storage conditions contributed more to the degree of cell separation after cooking than salt treatment, which had little effect. These results bring a mechanism based on chelation and ion exchange into question. Differential scanning calorimetry data can be interpreted as meaning that soaking beans in salt solution caused a major reduction (10–15°C) in the temperature of protein denaturation. This suggests that aqueous salts influence storage proteins, perhaps by solubilization and rendering them more thermally labile.