Effect of processing and storage on the properties and microbiological quality of cowpeas (Vigna unguiculata)
Date
1980-12
Authors
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Journal ISSN
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Publisher
ORCID
Type
Degree Level
Doctoral
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.
Description
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Citation
Degree
Doctor of Philosophy (Ph.D.)
Department
Food Science
Program
Food Science