Cowpeas (Vigna unguiculata L. Walp) are legumes recognised as a good source of proteins in
developing countries. Cowpeas are mostly utilised as cooked whole seeds. This is often
achieved only after boiling for up to 2 hours, resulting in high energy consumption and a long
time for food preparation. Micronisation of pre-conditioned cowpeas (± 41 % moisture at 153
°C) reduces their cooking time. During micronisation, cowpea seeds are exposed to
electromagnetic radiation with a wavelength range of 1.8 to 3.4μm. For biological materials,
the penetration of infrared rays into the food material causes intermolecular vibration, this
result in a rapid increase in temperature and water vapour pressure within the seed.
Micronisation changes physico-chemical properties of cowpea seeds that may affect sensory
properties of cooked cowpeas. Micronisation may also affect cowpea bioactive components
such as phenolic compounds and hence their antioxidant properties and bioactive properties.
This study aimed at (1) determining the effects of micronisation of pre-conditioned cowpeas
on sensory properties of cooked cowpeas and (2) determining the effects of mironisation of
pre-conditioned cowpeas on the phenolic compounds, radical scavenging properties and their
protective effects against oxidative damage of biomolecules (i.e. low density lipoproteins
(LDL), deoxyribonucleic acid (DNA) and red blood cells (RBC).
© University of Pretoria
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Micronisation significantly reduced cowpea cooking time by 28 to 49 %, depending on
cowpea type. There were significant (P<0.05) increases in roasted aroma and flavour, mushy
texture and splitting in all micronised samples. Bechuana white, a light brown cowpea type,
was more mushy and split than others. There were significant decreases in firmness,
mealiness and coarseness after micronisation for all cowpea types. Micronised cowpeas were
darker (lower L* values) than unmicronised cooked cowpeas. Darkening was more evident in
light coloured than dark coloured cowpea types. Although micronisation reduces cowpea
cooking time, it also affects sensory properties of cowpeas. This might have an influence on
consumer acceptance of micronised cowpeas.
Twenty seven phenolic compounds were identified in the cowpea types studied: 6 phenolic
acids, 14 flavonols and 7 flavan-3-ols. Protocatechuic acid, p-coumaric acid, 4-
hydroxybenzoic acid and ferulic acid were the major phenolic acids in cowpeas. Catechin,
catechin-3-O-glucoside, myricetin, rutin, quercetin and its mono and diglycosides were
present in all cowpea types analysed. Dr Saunders (701.7−849.2 μg/g) (red in colour) and
Glenda (571.9−708.1 μg/g) (dark brown in colour) contained the highest total phenolic
contents, followed by Bechuana white (361.5−602.3 μg/g) (light brown in colour) and
Blackeye (152.0−224.5 μg/g) (cream in colour). More of the flavonols were identified in red
and dark brown compared to light brown and cream cowpea types. The red cowpea type
contained all the dimers and oligomeric flavan-3-ol species identified in this study.
In all cowpea types, extracts from unmicronised (uncooked) cowpeas inhibited copperinduced
LDL oxidation in a dose dependent manner. Extracts from all samples analysed
exhibited protective effects against AAPH (2, 2'-azobis (2-amidinopropane) hydrochloride)
induced RBC haemolysis and DNA damage. Extracts from more pigmented cowpeas, i.e. Dr
Saunders, Glenda and Bechuana white, had significantly (P<0.05) higher levels of total
phenolics, total flavonoids and radical scavenging properties than Blackeye (less pigmented).
Extracts from more pigmented cowpeas also offered higher protection against AAPH-induced
DNA and copper-induced LDL oxidation damage than extracts from less pigmented cowpeas.
These results indicate protection of biomolecules e.g. DNA, LDL and RBC) from oxidative
damage and have a potential to reduce oxidative stress implicated in the development of
chronic diseases. This is because cowpea phenolic compounds possess the ability to reduce
oxidative damage associated with development of these diseases.
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Pigmented cowpea types may be recommended for health applications as they show more
potential as source of antioxidants compared to the less pigmented cowpeas.
Extracts from micronised (uncooked and cooked) samples of Dr Saunders and Glenda
cowpeas had significantly higher concentrations of ferulic acid and p-coumaric acid
compared with unmicronised samples. Para-coumaric acid concentrations were higher in all
micronised samples of Blackeye cowpeas than in unmicronised samples. The micronisation
process could release cell wall bound ferulic acid and p-coumaric, increasing their
concentrations in micronised samples. On the contrary, extracts from all micronised samples
of Bechuana white and Glenda cowpeas had lower concentrations of catechin than
unmicronised samples. Results indicated that total extractable phenolics were lower in
micronised samples of cowpea types than unmicronised samples. Futhermore, extracts from
micronised samples of all cowpea types showed less protective effect against LDL oxidation
than extracts from unmicronised samples.
However, for most cowpea types there was no significant difference in total flavonoid
contents (TFC) and Trolox equivalent antioxidant capacity (TEAC) values of cooked samples
of both micronised and unmicronised. Micronisation did not affect the protective effects of
cowpeas against AAPH-induced RBC haemolysis and oxidative DNA damage.
Micronisation, followed by cooking, may have generated heat-induced antioxidants such as
Maillard reaction products contributing to radical scavenging properties in micronised
(cooked) cowpea samples. Though micronised samples had lower concentrations of some
phenolic compounds and total extractable phenolics than unmicronised samples, micronised
cowpea samples still exhibited radical scavenging properties and offered protective effects
against oxidative damage of LDL, DNA and RBC and therefore may offer potential health
benefits to consumers. / Thesis (PhD)--University of Pretoria, 2013. / gm2013 / Food Science / Unrestricted
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:up/oai:repository.up.ac.za:2263/32972 |
| Date | January 2013 |
| Creators | Kayitesi, Eugenie |
| Contributors | Minnaar, Amanda, De Kock, Henrietta Letitia, Duodu, Kwaku Gyebi |
| Publisher | University of Pretoria |
| Source Sets | South African National ETD Portal |
| Language | English |
| Detected Language | English |
| Type | Thesis |
| Rights | © 2013 University of Pretoria. All rights reserved. The copyright in this work vests in the University of Pretoria. No part of this work may be reproduced or transmitted in any form or by any means, without the prior written permission of the University of Pretoria |
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