Improvement of functional properties of Soy protein
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The objective of this thesis was to study the effect of heat treatment and glycation on five industrially important functional properties of soy protein, namely solubility, emulsifying ability, water holding ability, acid gelation ability in soy yogurts and the heat stability of soy protein emulsions. These objectives were achieved completing three tasks: 1) The physicochemical properties were studied of Soycomil K, a commercially available, insoluble soy protein concentrate. Differential scanning calorimetry (DSC) analysis showed that it was 28% denatured compared to native, laboratory SPC. Further analysis showed that the aggregated structure is more hydrophobic and there are more disulfide bonds than found in commercially available soy protein isolates (SPI). Heat treatment at alkaline pH and low protein concentration increased its solubility. Heat treatment at 100 °C increased SoyComil’s solubility significantly compared to heat treatment at 70 °C. Glycation of SoyComil K with glucose at 70 °C increased solubility compared to the control, whereas glycation at 100 °C decreased solubility. Heat treatment of Soycomil increased its solubility more than glycation. 2) A soy yogurt with smooth texture and high water holding capacity (WHC) was developed with SPI heated in the presence of pectin and glucose, followed by homogenization with the oil ingredient of the yogurt recipe. The texture matched that of commercially available yoghurt manufactured with soymilk. A yogurt made with SPI only showed low WHC. These results provide evidence that combined heat treatment and glycation of SPI improved its functional properties. A study of the structure of the yogurt showed that the majority of bonds were hydrophobic bonds, whilst electrostatic and disulfide bonds played a small part in maintaining the yogurt structure. 3) A study of the rate of aggregation of SPI stabilized emulsions at pH4.5 showed that the heat stability of SPI emulsions was strongly dependent on protein concentration and temperature. A second study showed that the presence of polysaccharides either improved or had little effect on heat stability depending on the concentration of polysaccharide added. This was attributed to interactions between soy proteins and specific polysaccharides. A study of the heat stability of mixed whey protein concentrate (WPC) and SPI emulsions showed that WPC dominated the oil droplet destabilization behaviour, and that low proportions of WPC were able to slow down the heat-induced breakdown of SPI/WPCemulsions containing a high proportion of SPI.