PHYSICO-CHEMICAL PROPERTIES OF FLOURS AND CHARACTERIZATION OF PROTEIN AND MODIFIED PROTEIN ISOLATES FROM SOME UNDERUTILIZED LEGUME SEEDS

Abstract

This work involved the physicochemical properties and characterization of proteins of some underutilized legumes which may become industrially useful. Adenopus breviflorus benth seed, lima bean (Phaseolus lunatus), pigeon pea (Cajanus cajan) and two varieties of African yam bean (Sphenostylis sternocarpa I and II) were processed into flour, defatted using n- hexane and their proteins isolated using alkaline solubilization (sodium hydroxide) and acid precipitation (hydrochloric acid) at their various isoelectric pH. The protein isolates were modified using acetic anhydride and maleic anhydride. The seed flours, defatted flours, protein isolates and their modified protein isolates were analyzed for their proximate composition using standard methods. The mineral elements, antinutritional factors, amino acid composition, in vitro multienzyme digestibility and functional properties, such as water absorption capacity, oil absorption capacity, foaming capacity/stability, emulsion capacity/stability, least gelation concentration and protein solubility as a function of pH were also studied. The effects of ionic strength (NaCl, KCl, Na2SO4 and CH3COONa) on some functional properties of the seed flours were determined. The fatty acid composition of the extracted oils from the various seed flours was analyzed using standard method. The degree of modification was determined using standard method and infra red spectroscopic analysis was done to determine the effect of modification on the functional groups on the protein structure of the protein isolates. The evaluation of the protein sub-unit patterns in the protein isolates and modified protein isolates was also done using Gel filtration chromatography and sodium dodecyl sulphate polyacrylamide gel electrophoresis. The statistical analyses were carried out using SPSS 17 computer package. The triplicate results were analyzed and expressed as means ± standard deviation from mean with significant difference being within P ≤ 0.05. The results of the proximate composition show that the seeds are rich sources of protein and carbohydrate with crude protein ranging from 20.73% (S. sternocarpa I) – 30.44% (A. breviflorus) while A. breviflorus benth seed is very rich in crude fat (52.63% ) qualifying it as an oil seed. The seed flours and the protein isolates have high levels of phosphorus, sodium, potassium and magnesium while chromium, nickel, lead and cadmium were below detection limit. Na/K ratio of less than 1 was obtained in all the protein isolates. The amino acid composition of the seed flours and protein isolates revealed that the most abundant amino acid is glutamic acid followed by aspartic acid with values (141.60 -196.30)mg/g, (76.53-135.30)mg/g flour and (121.22-185.28)mg/g, (69.52-106.82)mg/g isolate respectively. The most concentrated essential amino acid is leucine which ranged from (71.37-89.13mg/g P) for the flours and (50.02-80.52mg/g P) for the isolates respectively. The limiting amino acids in both seed flours and protein isolates were methionine and cystine, threonine and valine. Linoleic acid was the predominant fatty acid in all the extracted oils followed by oleic acid. The seed flours were low in antinutritional factors, isolation significantly reduced the antinutrients. In vitro multienzyme digestibility values of the protein isolates were higher than those of the corresponding flours. Acetylation and maleylation of the protein isolates increased the water absorption capacity, foaming properties and emulsion properties. The seed flours and protein isolates are promising sources of proteins, mineral elements, and essential amino acids with excellent digestibility. The low antinutritional factors may also enhance their utilization in food systems. Considering the proximate composition and the functional properties of the protein isolates and modified protein isolates, they may serve as alternative protein source for supplementation in low protein foods.