Preparation and characters of whey protein isolate-sodium tripolyphosphate aggregates by heating

Abstract: This study was aimed to prepare the whey protein isolate (WPI) - sodium tripolyphosphate (STPP) aggregates using heating at higher pH value and evaluate their characteristics. The results of single-factor experiment showed that the increase of viscosity of polymers was different from the increasing of WPI concentration, temperature, pH value, STPP content and aggregation time. The models were obtained by using a Box-Behnken optimization experiment design with the 4 factors (temperature, pH value, STPP content and aggregation time) based on the results of single-factor experiments. The results of Box-Behnken optimization experiment showed that the order of the effect of the 4 factors on viscosity was as follows: temperature > STPP content > pH value > aggregation time. The optimized condition determined was that 10% (w/w) WPI, 0.09% (w/w) STPP at 90°C for 42 min with pH value of 8.40, and the actual viscosity was 5083 mPa·s. The prepared WPI-STPP thermal aggregates were the thick sample with a semi flow state, and the regression model was fitted well. Determination of properties and structural analysis of WPI, WPI-STPP thermal aggregates and WPI aggregates showed the water holding capacity, surface hydrophobicity and rheological characteristics of WPI-STPP thermal aggregates were improved compared with WPI and WPI aggregates. For WPI aggregates, water holding capacity increased from 4.83 to 5.20 g per gram protein (P<0.05). However, the solubility of WPI-STPP thermal aggregates decreased from 88.5% to 34.50%, which was lower than that of WPI. Heat treatment and STPP significantly affected the surface hydrophobicity of the soluble aggregates. WPI-STPP thermal aggregates could form good cold-induced gels, which could widen its application in foods of gel type. When STPP was added, the average particle size of whey protein thermally polymerized increased from 31.39±1.81 μm for WPI to 292.09±2.17 μm for WPI-STPP thermal aggregates. The difference between strong and weak soluble gels could be assessed by the oscillatory dynamic experiments using parallel-plate geometries. Rotational rheometer showed that the rheological characteristics of WPI-STPP thermal aggregates were improved. The rheological characteristics were determined from storage and loss moduli as the functions of time and frequency. WPI-STPP thermal aggregates had higher storage modulus values. The results showed that the increasing of particles played a significant role in the water holding capacity and rheological properties of these dispersions. The microscopic structure analysis of WPI-STPP thermal aggregates showed that they denatured fully, and the larger irregular fractal aggregates of WPI-STPP thermal aggregates could be most useful to increase the viscosity. Transmission electron microscopy showed that heat-induced WPI-NaCl soluble gels had a dense structure and a higher number of cross-links. The utilization of WPI-STPP thermal aggregates is very attractive due to the low-complexity processing conditions needed, lower production cost and higher nutritive value. The production cost of yogurt is less than yogurt with pectin according to the optimal technological condition of the experiment. The application of this technology proposed in this paper will bring great economic benefits for the yogurt processing industry.