Formation And Rheological Properties Of Acid-induced Gels Made From Soy Protein-stabilized Emulsions

Soy protein-stabilized emulsions prepared were used as stock emulsions for the development of acid-induced emulsion gels. The stability, interfacial and rheological properties of emulsions were very important for the following investigation on the acid-induced emulsion gels. Rheometer is the main research tool for this so called“soft condensed matter”. The experimental data of rheology measurements was fitted with the predicted values under the frame of theoretical model. All these works will provide reliable theoretical knowledge for the understanding and development of the acid-induced soy protein-stabilized emulsion gels in the area of food, pharmaceutical and cosmetics. Therefore, the interfacial and rheological properties of emulsion, gelation behaviour of emulsions, the effect of filler particle (oil droplets) on the rheological properties of emulsion gels and the effect of gel matrix (unadsorbed soy protein aggregates, USPA) on the large deformation and fracture properties of emulsion gels were studied.The effects of pre-/post- heat treatment and soy protein concentration on the interfacial and rheological properties of emulsions were investigated. Four types of emulsions made with unheated/native and heated soy protein solutions (NSPE and HSPE) as well as emulsions with post heat treatment after homogenization (post-NSPE and post-HSPE) were studied. For the NSPE and HSPE, the increasing soy protein concentration decreased the average droplet size (d32) of emulsions and the protein adsorption at the interface. Within the range of protein concentration (0.5 - 4.0%) studied, the interfacial protein concentration of oil droplets decreased and then leveled off at ~ 2.0 mg/m2, suggesting a monolayer adsorption. Heating on soy protein solutions at 100°C for 30 min resulted in a smaller average oil droplet size, a higher protein adsorption and a lower tendency for oil droplet flocculation, while the post heat treatment on the NSPE and HSPE induced only slight changes in interfacial and rheological properties (p < 0.05). The rheological studies have shown that increasing soy protein concentration (referring to the protein concentration in the soy protein solutions before homogenization) resulted in pseudoplastic emulsions with significantly higher low-shear viscosities due to the presence of the increasing unadsorbed protein in the aqueous phase. All the experimental data fitted the Ostwald-de Waele model quite well. The viscosity at 1 1/s of shear rate was inversely proportional to the d32 value, indicating a positive relation to the total interfacial area per unit volume (m2/mL). This linear dependence is also established at different low shear rates for four types of emulsions. The gelation behaviour and rheological properties of gels was investigated for the NSPEand HSPE. Previous heat treatment on soy protein solutions led to a higher storage modulus (G′) and a shorter gelation time of acid-induced emulsion gels. A maximum in tanδwas observed in the NSPE gels but no maximum was appeared in the HSPE gels. Increasing the acidification temperature decreased the G’ and gelation time (tgel). The dependence of the storage modulus (G’) on the protein concentration (c) can be scaled with a power law: G’ ~ cA. The simple time-scaling model (G’/G’max ~ t/tgel) did not fit for the gelation profiles of NSPE but fitted for the gelation profiles of HSPE well, although there was a more pronounced deviation from the set of curves at the lowest protein concentration, 0.5%. The fractal scaling model and the simple time-scaling model above fitted the experimental data very well for the acid-induced soy protein-stabilized HSPE gels with varying oil volume fraction.The effect of filler particle (oil droplets) on the rheological properties of HSPE gels having the different strength of gel matrix was investigated at small deformation. Series of emulsions were prepared by filling two stock emulsion droplets (Emulsion ?, d32 = 0.491μm; Emulsion ??, d32 = 0.650μm) into the solutions of 6.0% (w/v) soluble soy protein aggregates. The storage modulus (G’) of emulsion gel after 20 h of GDL acidification, taken as gel strength, increased with the volume fraction of oil droplets and the final soy protein aggregates (FSPA) concentrations in the aqueous phase. The smaller droplet sizes the higher gel strength for the emulsion gels with the same oil volume fractions. Experimental results revealed that the effect of the G’m of matrix on the G’ of emulsion gels was in agreement with the van der Poel’s theoretical predictions, especially in the 20 - 35% of oil volume fraction range. The van der Poel’s theory fitted the experimental data of emulsion gels containing smaller droplets better, in the range of droplet sizes we tested.The effect of gel matrix (unadsorbed soy protein aggregates, USPA) on the large deformation and fracture properties of HSPE gels was investigated with a rheometer. In this work, N-ethylmaleimide (NEM) was added to prevent the formation of disulfide-linked structures. The storage modulus (G′) and fracture properties of the non-treated and NEM-treated emulsion gels were investigated in the absence and presence of USPA. In the absence of USPA, a decrease in the G′of emulsion gels was observed after NEM treatment. However, in the presence of USPA, the addition of NEM only slightly decreased the G′(p < 0.05). For both non-treated and NEM-treated emulsions, a ~ 63-folds higher G′value was obtained after 1.5% (w/v) USPA addition. These results revealed the presence of sulphydryl group - disulfide bond interchange reactions at ambient temperature and under acidic conditions. In the absence of USPA, the sulphydryl group - disulfide bond interchange reactions was the main interactions responsible for the higher G′values of non-treated emulsion gels, but for the emulsions with USPA presented, the large quantity of non-covalent interactions (e.g. hydrophobic group & hydrogen bonds) is the main interactions responsible for the aggregation and gelation of emulsion droplets. In the absence of USPA, NEM treatment decreased the fracture stress but increased the fracture strain of emulsion gels. However, a completely contrary phenomenon was observed in the presence of USPA. In the presence of USPA, the sulphydryl group - disulfide bond interchange reactions formed in the non-treated emulsion gels did not increase the final G′but increased the stability of network. A power law relation was observed between the USPA concentration and the final G′, as well as between the oil volume fraction and the fracture stress/strain.