Influence of storage conditions on the development of hard-to-cook defect and the nutritional quality of selected common beans (Phaseolus vulgaris L.) grown in Kenya

Abstract

Common beans form one of the main sources of proteins in poor sub-Saharan Africa. However, common beans tend to develop the hard-to-cook (HTC) defect, which has been attributed to storage at temperatures higher than 25˚C and relative humidity (RH) of more than 70%. This results in increased cooking time, fuel consumption and water use thus reducing the rate of utilization of beans at domestic level. The main objective of this study was to understand the development of the hard to cook defect in Pinto and Red Haricot bean varieties during storage at varying temperatures and relative humidity (RH), and its effect on physical and nutritional quality. Samples were collected from Kenya Agricultural and Livestock Research Organization, (KALRO) Thika Station. The hard- (Pinto) and easy- (Red haricot) to-cook beans were stored at varying temperatures and RH (25˚C, 45˚C at 75% RH and 35˚C at 83% RH) levels using suitable incubators and salt solutions. The conditions were selected based on the temperature ranges that can be found in some regions across Kenya i.e. normal (25˚C), moderate (35˚C) and extremely high temperature (45˚C). the relative humidity were also selected on the same basis with 75% being the almost normal RH conditions while 85% being the extreme condition found in some regions in Kenya. Three (3) incubators were used for each set of temperature and RH, and sampling was carried out at 0, 2, 4 and 6 months. Pre-treatment with sodium carbonate, distilled water and calcium chloride was done followed by thermal treatment at 96.5˚C. The samples were assessed for physical (seed density, seed porosity, characteristic dimension of beans, hydration coefficient, and swelling coefficient), chemical (ash and proteins), nutritional (minerals and protein digestibility) and anti-nutritional changes (tannins and phytate). Beans stored at 45 ºC/75% RH were harder than the fresh beans (25˚C /75%) and the ones stored at 35 ˚C/83%. The hydration coefficient and swelling coefficient significantly decreased (P˂0.05) with increasing storage time. Soaking in distilled water and sodium carbonate for 16 hours reduced the cooking time from 3 hours to 1 hour for Pinto and 3 hours to 30 minutes for Red haricot while soaking in calcium chloride prolonged the cooking time for both to more than 5 hours. The increased relative humidity from 75% RH to 83% RH caused an increase in cooking time from 180 minutes to 270 minutes in Red haricot; and 210 minutes to 270 minutes in Pinto. Bean hardness, solutes and electrolytes leaching after 16 hours soaking significantly increased with increased storage temperature. The phytic acid content decreased significantly (P<0.05) with soaking and cooking. For Pinto beans stored at 35/83/6, the phytic acid content reduced by 33.2% after cooking without soaking, 39.3% after soaking in distilled water followed by cooking, and 65% after soaking in sodium carbonate followed by cooking. For Red haricot beans stored at 35/83/6, the phytic acid content reduced by 13.9% after cooking without soaking, 27.6% after soaking in distilled water followed by cooking, and 51.2% after soaking in sodium carbonate followed by cooking. The tannin content also decreased by 23.2% after cooking without soaking, 60.2% after soaking in distilled water followed by cooking, and 71.1% after soaking in sodium carbonate followed by cooking. For fresh Red haricot beans stored at 25/75/0, the tannin content reduced by 65.6% after cooking without soaking, 77.9% after soaking in distilled water followed by cooking, and 81.9% after soaking in sodium carbonate followed by cooking. There was no significant difference (P>0.05) in mineral content with increased storage. There was significant decrease (P<0.05) in protein digestibility with increased storage time. After storage for 6 months at 45/75, the digestible protein content reduced by 24.2% for Pinto and 28.2% for Red haricot, while at 35/83, the digestible protein content reduced by 28.2% for Pinto and 23.8% for Red haricot. It was concluded that the longer the beans are stored at higher temperatures and relative humidity, the more they develop the HTC defect. The high temperature and relative humidity caused a relative decrease in protein digestibility however, the protein and mineral content were not altered.