There is increasing demand in the European Union for vegetable protein used in
animal feed as well as for use in human foods such as preparation of culinary sauces,
mayonnaise and baking. Millions of tons of soy proteins are imported into the
European Union and the price is increasing annually. A market replacement share by a
similar vegetable protein by a few percent represents tens of millions of euros. Cowpea
(Vigna unguiculata L. Walp) is a well-established crop around the world and the
production of this grain has been increasing in Africa. Yet the exploitation of the
cowpea proteins as alternative to soy protein remains to be investigated.
As with other grain legumes, cowpea has high protein quality due to its high
levels of lysine which may be significant in balancing the deficiencies of this essential
amino acid in cereal-based diets. The primary limitation to the improvement of cowpea
proteins is the lack of information on the technology and characterisation of the cowpea
protein isolate, in comparison with well-established soy protein isolate. More
importantly, data on the evaluation of the functional properties of cowpea protein isolate
and modification of the protein structure for enhancing the functional behaviours are
lacking. Therefore, the major purpose of this study has been focused on the isolation
and characterisation of cowpea proteins, as well as evaluation of the functional
properties of the resultant products for appropriate food applications. The feasibility of
glycation and/or denaturation in order to improve the functional properties of the
proteins is also included.
The first phase of the study optimised the extraction conditions of cowpea protein
which resulted in 89% yield and 90% protein content reported here for the first time. Its
physicochemical and functional properties were compared to that of commercial soy
protein isolate (SPI) and whey protein concentrate (WPC 60). Compared to SPI, cowpea
protein isolate (CPI) had similar viscosity and solubility, but lower water holding and
fat absorption capacity, however the latter were comparable to that of WPC 60. The
gelation properties of CPI under different conditions are reported here for the first time.
The second phase of the study involved the thermal modification of cowpea
protein isolate (CPI) in solution by a) denaturation and b) simultaneous denaturation
and glycation with endogenous sugars and carbohydrates via the Maillard reaction.
Changes in physicochemical and functional properties were determined and compared
to that of SPI. Generally, glycated and denatured cowpea protein isolate (GCPI)
exhibited better functional properties than denatured CPI and native CPI. GCPI showed
improved solubility, emulsifying activity and stability, viscosity and foam stability
whereas denatured cowpea protein isolate (DCPI) exhibited better water holding
capacity, oil absorption capacity and gelation properties.
The third phase of the project studied the application of cowpea flour, CPI and
modified CPI in bread, mayonnaise and cakes. The fortification of bread with 5%
cowpea flour produced comparable textural and sensory properties to the control. The
protein isolate (CPI) could be incorporated in bread to 2% and GCPI up to 4% without
adversely affecting the bread physical properties such as crumb hardness and sensory
attributes. CPI could be incorporated in cakes to 20% while both DCPI and GCPI could
be incorporated to 40%. In mayonnaise, replacement of egg yolk with 20%GCPI
resulted in similar textural properties to the control, however sensory evaluation
reported a beany flavour. The effect of cowpea protein on pasting behaviour of rice
starch was investigated. The addition of GCPI had greater effect on pasting behaviour of
rice starch than CPI
