Investigation of effect of glycation and denaturation on functional properties of cowpea proteins

Abstract

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.