Functional and physicochemical properties of gamma-irradiated cowpea flours and pastes

Abstract

Cowpea (Vigna unguiculata L. Walp) is an important potential source of protein in many parts of Africa. However, cowpea use is confined primarily to boiled whole seeds or traditional food preparations such as akara and moi-moi prepared from cowpea flours or pastes. By creating variety in the foods made from cowpeas, its utilization may be enhanced. The functional properties of cowpeas are mainly responsible for the quality of cowpea foods such as akara and moi-moi and are derived mostly from the physicochemical properties of their proteins and starches. Irradiation can change the physicochemical properties of cowpea proteins and starches and possibly lead to modification of functional properties of cowpea flours and pastes. Such potential modification may enable a wider application of cowpea flours and pastes in food systems. Protein and starch-related functional properties of cowpea flours and pastes irradiated at 2, 10 and 50 kGy (and proteins and starches isolated thereof) were determined. The physicochemical properties of isolated proteins and starches were studied using differential scanning calorimetry (DSC), Fourier transform infrared (FTIR) spectroscopy, size exclusion high performance liquid chromatography (SE-HPLC) and electrophoresis (SDS-PAGE). These physicochemical properties were determined to help in explaining the fundamental reasons for the potential irradiation-induced modification of functional properties of cowpea flours and pastes. Irradiation mainly at 10 and 50 kGy resulted in modification of most protein-related functional properties of cowpea flours and pastes as well as proteins isolated thereof. Nitrogen solubility index decreased markedly whilst oil absorption capacity increased in both flours and pastes. The extent of modification of functional properties such as decreased nitrogen solubility index was more in flour proteins than in paste proteins owing to higher protein denaturation in the former in a mostly dose-dependent manner. It appears that irradiation modified protein-related functional properties of cowpea flours and pastes mainly through unfolding of polypeptide chains. SE-HPLC data revealed increases in molecular weights of proteins isolated from both irradiated cowpea flours and pastes suggesting possible protein cross-linking. However, SDS-PAGE seemed to exclude disulphide bonds formation with irradiation. The most probable mechanism of protein cross-linking therefore, may be via bityrosine cross-links. More protein cross-linking occured in paste proteins than in flour proteins possibly because of the secondary radiolytic effects of added water in the pastes. However, the extents of modification of functional properties were more in flours than pastes. Thus, it may be assumed that protein cross-linking contributed to functional modifications to a lesser extent than protein denaturation although the interplay of both physicochemical properties is likely. Irradiation at all the doses employed caused significant decreases in all starch-related functional properties (i.e. RVA pasting properties, swelling index and gel strength) due possibly to starch degradation. The extent of modification was more in starches from flour than in starches from pastes possibly because of hydoxy radical (OH*) mediated starch cross-linking occurring and counteracting starch degradation somewhat in the pastes. The possibility of starch degradation occurring with irradiation was indicated by decreased starch crystallinity (indicated by the increases in DSC peak gelatinisation temperatures). Although starch degradation may have occured, FTIR revealed that starch granule surface order was not affected by irradiation under both dry (flour) and wet (paste) conditions. Gamma irradiation, depending on dose, has the potential of modifying protein and starch-related functional properties of cowpea flours and pastes as well as proteins and starches isolated thereof, by altering the physicochemical properties of cowpea proteins and starches. The modified functional properties may be exploited in various food systems and this may potentially broaden application of cowpea flours and pastes in addition to their present use in traditional African foods such as akaraand moi-moi.