Techno-functional properties, dough rheology and bread quality of flours from infrared and microwave heat-treated bambara groundnuts grains

Abstract

This work was undertaken to investigate the effects of heat treatment (130 °C surface temperature) of pre-conditioned (20 or 53% moisture) Bambara groundnut (Vigna subterranea) seeds using infrared, microwave energy alone and in combination on the techno-functional properties of their flours, and the effects of these flours on dough rheology. Heat treatment caused starch pre-gelatinisation and denaturation of protein in resulting flours, which significantly increased with moisture level and treatment time (0,5 and 10 minutes). Endothermic peak by DSC decreased with increased treatment time for all heat treatments in flours from 53% moisture level seeds. Pasting temperature of Bambara groundnut flours significantly (P<0.05) increased with heat-treatment times (0,5 and 10 minutes), but final paste viscosities decreased. The low viscosity can be explained by unavailability of starch to form a viscous paste due to surrounding protein matrix around the starch granules. Proteins have been proposed as the major biomolecules responsible to coat starch and make it unavailable to paste, thus, a low paste viscosity. In the second research chapter, proteins were removed by trypsin and the flours were analysed. CLSM did not show a protein signal after the trypsin treatment. The removal of Bambara groundnut proteins significantly (P<0.05) increased the flour paste viscosities. During infrared, microwave and combination heat treatment of Bambara groundnut seed, there was disulphide bonding and polymerisation of Bambara groundnut protein indicated by an increase in higher molecular weight band intensity from SDS-PAGE. FTIR showed a reduction in the α-helix and an increase in β-sheet for all the heat treatment methods indicating changes in the Bambara groundnut protein secondary structures. Molecular changes of Bambara groundnut protein caused by heat treatment play a major role in low paste viscosities of resulting flours from heat-treated Bambara groundnut seeds. And lastly, flours from heat treated Bambara groundnut seeds were composited to wheat (15 and 30% Bambara groundnut flour addition) and their effects on dough rheology and bread making properties were investigated. Mixolab, Alveograph and the creep and recovery test were used to investigate dough rheological properties of wheat-Bambara groundnut composite dough. There was an increase in flour water absorption from the mixolab (P>0.001) with addition of Bambara groundnut flours from heat treated seeds when compared with the addition of flours from untreated Bambara groundnut seeds. Bambara groundnut flour negatively affected viscoelastic behaviour and increase the energy required to inflate the dough (W) while dough elastic compliance (J1) was reduced. The addition of Bambara groundnut flour to wheat flour results had negative effects on dough viscoelastic properties which subsequently affected bread properties. Addition of heat-treated flour provided bread with softer crumb and high specific volume when compared with untreated flours. Compositing wheat flour with resulting flours from heat treated Bambara groundnut seeds may be useful in the production of composite bread with softer crumb structure. In conclusion, infrared and microwave heat treated Bambara groundnut grain changed the molecular structure of the protein by increase in β-sheet and disulphide bond formation; and pregelatinized the starch. The protein thus became hydrophobic and coated the starch to reduce pasting viscosity. This change also increased dough water absorption and produce softer bread when compositing with wheat flour.