Physicochemical characteristics of yam bean (pachyrhizus spp) seed flour
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Yam bean (Pachyrhizus spp) is a robust legume crop native to South and Central America. The yam bean is mainly grown for its root tubers which are used as food. The yam bean seeds contain high amount of proteins and oil but are not consumed because they contain the toxic compound, rotenone. The aim of this study was to determine the physicochemical characteristics of yam bean seed flour as the first step towards identifying potential uses of the seeds. Seeds from the three cultivated species of yam bean plants grown on-station at National Crops Resources Research Institute (NaCRRI), Namulonge- Uganda were collected from CIP- Uganda. The seeds were collected from ten selected accessions (2 accessions of P. erosus, 4 accession of P. ahipa and 4 accessions of P. tuberosus). The seeds were milled and a portion of the flour was defatted using hexane. Part of the defatted flour was used to prepare a protein isolate using the isoelectric precipitation method. The proximate composition of the whole seed (from the three species) and defatted yam bean seed flour from P. erosus was determined. The defatted flours and their protein isolate were also analyzed for water and oil absorption capacity, bulk density, protein solubility, least gelation concentration, emulsifying capacity, emulsion stability, foaming capacity and foam stability as well as pasting properties (for flour only). The in vitro protein digestibility of P. erosus seed protein was determined using the pepsin–pancreatin enzyme system. The protein was fractionated based on solubility in different solvents. The electrophoretic pattern (using SDS-PAGE under reducing and non-reducing conditions) of the defatted seed flours and their protein isolates from P. erosus was also determined. The yam bean seeds were found to contain about 29.2-32.1% proteins, 31.3-33.0 % carbohydrates, 24.1-25.6 % crude fat, 7.5- 8.1% crude fiber and 3.4- 4.1% ash. The defatted P. erosus seed flour contained about 45.6- 48.8 % protein; 32.6-36.5 % total carbohydrate, 6.7 -7.1 % crude fiber and 6.0- 6.4 % ash and 5.2%crude fat. Regarding functional properties, the protein solubility, least gelation concentration, water absorption capacity and oil absorption capacity of the defatted yam bean seed flours were found to be 68.0-70.4%, 14%, 2.8-2.9% and 1.5% respectively. The defatted flour had about 35.7-36.0% emulsifying capacity, 33.2-33.5% emulsion stability, 42% foaming capacities and 25.1-25.8% foam stability. The defatted yam bean seed protein isolate had least gelation concentration of 8%, water absorption capacity 2.9-3 g/g, oil absorption capacity 0.8 g/g, protein solubility 80.4-81.6%, foaming capacity 37.0-37.2%, foam stability 73.4-74.1%, emulsion activity 12.9-14.7% and emulsion stability of 9.2%. The defatted yam bean seed flours had pasting temperatures of 80.0-81.3oC, peak viscosity of 145-146 RVU, trough viscosity of 95.11-102.03 RVU, breakdown value of 43-51 RVU, setback value of 252 -258 RVU and final viscosity of 348-360 RVU. Results of protein fractionation showed that albumins were the most dominant protein fraction (52.7-53.8%) followed by globulins (17.5-19.8%), glutelins (8.0-9.6%) and prolamines (2.6-2.7%). In vitro protein digestibilities of P. erosus for both raw and cooked samples were about 87.5 and 84.3%, respectively. The electrophoretic pattern of yam bean seed protein showed major bands corresponding to molecular weights of 35, 55 and above 85 kDa. These bands mainly represented albumins and globulins. Electrophoresis results indicated that there were no observable differences in the electrophoretic patterns of the two accessions of P. erosus. The results of this study indicated that yam bean seed flour and its protein isolate have potential for use in food and industrial applications like stabilization of foams and gel formation.