Soy protein gel systems with added phenolic acids : effect of gel matrix on protein digestibility and phenolics bioaccessibility : a thesis presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy in Food Technology at Massey University, Palmerston North, New Zealand

Abstract

This thesis studied how the physical properties of soy protein gels can affect proteolysis and the bioaccessibility of added phenolics. The types of gels developed were model systems for the commercial products firm and silken tofu, which differ in structure and composition. To begin with, a variety of gels (known as pressed and unpressed) with different physical characteristics were developed. Next, the properties of these pressed and unpressed gels were assessed when they were formed using two different mechanisms - acidification with glucono δ-lactone (GDL) or coagulation with magnesium sulphate (MgSO₄). Results showed that the gels formed with GDL (both pressed and unpressed) were firmer in texture and denser in microstructure than MgSO₄ gels, which resulted in a reduction in the extent of proteolysis. It was found that although the coagulation mechanism did not affect the total release of the phenolic acids on a percentage basis, GDL-induced pressed gels delivered larger masses of bioactives in the intestinal phase because they could retain more of the phenolics after pressing. It was hypothesised that at acidic pH the formation of complexes between phenolics and soy proteins is favoured which promotes their retention after pressing. Overall, it was found that the release of the bioactives was rapid in the unpressed gels and more gradual with a higher final concentration in the pressed gels, which can be attributed to the dense protein network of the pressed gels. In terms of the soy protein itself, this work showed that the extent of protein hydrolysis and the amino acid bioaccessibility were higher in the unpressed/soft gels than in pressed / firm gels. These results pave the way for the formulation of soy products that can modulate not only the bioaccessibility of phenolics (pressed gels), which was the main objective of this work but also gives insights into the best type of format or product to promote amino acid release during digestion (unpressed gels).