Effect of processing and storage on the properties and microbiological quality of cowpeas (Vigna unguiculata)

Abstract

Cowpea (Vigna unguiculata) seed, flour and a protein-rich product (AP) were prepared by a dry process involving pin milling and air classification. A protein-rich product (DP) was also produced by slurry centrifugation and drum drying (wet process). The four products were used in storage studies at 64 or 79% equilibrium relative humidity (ERH) and 37°C for 6 months. Scanning electron and light microscopy showed protein particles and starch granules in the dry process products. Drum-dried DP appeared as structureless flakes. Dry processing concentrated the lipid and ash with the protein-rich fraction. The wet method reduced the quantity of free lipid in DP, (extractable with nonpolar diethyl ether). A polar solvent (chloroform/methanol, 2:1) removed substantial bound lipids from OP, the flour and AP. Linolenic and linoleic acids, determined by gas liquid chromatography (GLC), represented over 60% of the total fatty acids (FA) of cowpea lipids. Before storage, the amino acid composition of the cowpea products exhibited a deficiency in methionine plus cystine and high lysine, compared to the FAO/WHO (1973) reference protein. The available lysine determined by the fluorodinitrobenzene method ranged from 94.4% to 98.5% in the cowpea products. The functional properties evaluated in the cowpea products were nitrogen solubility, water hydration capacity (WHC), oil absorption, oil emulsification and foaming. Before storage the functional properties of the flour and AP compared favorably with those of soy products. The OP had the lowest nitrogen solubility, oil emulsification and foaming properties but was highest in WHC and oil absorption among the cowpea products. During the storage trials, the moisture content of the cowpea products and the ERH remained relatively stable at 64% ERH, both increased at 79% ERH. Storage at 64% ERH significantly decreased (P< 0.05) the aerobic plate counts (APC), yeast and mold counts and bacterial spore counts of the cowpea products. At 79% ERH the APC and yeast and mold counts of the seed significantly increased. Bacterial spores decreased. All microbial populations of the flour and AP initially decreased at 79% ERH, then increased beyond 2 to 4 months in storage. Microbial numbers decreased in DP throughout storage at 79% ERH but mold growth must have occurred since mycelium was present in the sample after 6 months of storage. The bacteria isolated from the stored products belonged to the genus Bacillus. Aspergillus ruber, A. amstelodami and A. chevalieri were the molds isolated from the products. All the stored products contained no aflatoxin. Lipid hydrolysis occurred in the seed, flour and AP stored at 64% and 79% ERH, as indicated by the increase in free fatty acids (FFA) and fat acidity. Losses later occurred in the polyunsaturated and saturated FFA of the flour and AP stored at 79% ERH. The decline in FFA corresponded with a decrease in fat acidity and an increase in the pH. The DP was remarkably stable to lipid hydrolysis at both storage conditions, as supported by the low levels of FFA and fat acidity. Losses in the polyenoic acids of the seed, flour and AP at the two storage conditions were attributed to oxidation. Decreases in the saturated FA of the products at the high ERH were partly attributed to mold metabolism. The high lipoxygenase activities in the dry process products supported the hypothesis of lipid oxidation. Increases in the total carbonyls determined by the thiobarbituric acid (TBA) assay was additional evidence of lipid oxidation of the seed, flour and AP during storage. Total carbonyls decreased during later stages of storage. The total FA of DP were stable to lipid oxidation. These results were consistent with the low TBA values and the lack of lipoxygenase in this product. Total lysine decreased in the cowpea products stored at 79% ERH for 6 months, particularly in DP. Available lysine was also lower in the stored products especially in DP. Methionine, cystine, arginine and glutamic acid decreased in some of the stored samples. The functional properties of the flour and AP either remained unchanged or were improved after 6 months of storage at 64% ERH. Storage at 79% ERH reduced functionality of the products. Both conditions of storage were detrimental to the functional properties of DP. The color of the dry process products remained relatively unaltered under storage at 64% ERH, but darkened at 79% ERH. Darkening was pronounced in DP at both conditions of storage. A taste panel detected significant differences (P< 0.05) between the flavor of the products stored at 64% ERH for 6 months and their control samples. Legume and bitter were the dominant flavors of the dry process products while toasted and nutty predominated in DP.