Optimization of the interaction of whey protein concentrate-cress seed gum using response surface methodology (RSM) and investigating the foaming properties of the optimal sample

Proteins and polysaccharides are natural biopolymers that consider among the most widely used hydrocolloids in the food industry (Gaonkar and McPherson 2006) Which are often used simultaneously to improve functional properties. In electrostatic interaction, positively charged proteins with polysaccharides containing negative groups below the protein isoelectric point (PI) (LEE, MORR, and HA 1992), Leads to the formation of macromolecular particles by creating molecular aggregates rich in biopolymers suspended in the aqueous phase, called complex coacervate, which cause the coagulation and spontaneous separation of the phase. The ratio of protein-polysaccharide biopolymers and the pH of the mixture are important factors influencing the adsorption and compatibility of protein- polysaccharides as well as the characteristics of their aggregates (Shu et al. 1996). For this purpose, the optical density (turbidity) of protein-polysaccharide mixtures and the yield of dried sediments in this study as a basis for optimizing the complex coacervate process as in previous studies (Wang, Adhikari, and Barrow 2014; Huang et al. 2012) were used. In the present study, the optimization of complex coacervate formation conditions of whey protein concentrate-cress seed gum as well as the foaming properties of complex coacervate were studied for the first time. Whey proteins, as amphiphilic macromolecules, can adsorb at the interface (Scheer, Kruppke, and Heib 2001; Tamm et al. 2012; Wierenga and Gruppen, 2010) and form a viscoelastic adsorbent layer (Wilde 2000). Also, whey protein acts as a foaming agent with the ability to be adsorbed at the joint water-oil and water-air interface through hydrophobic interactions or disulfide bridges, and intermolecular bonds (Nicorescu et al. 2008; Nicorescu et al., 2008; Dickinson, 1992; Forschen, 2017). Cress seed gum has rheological, emulsifying, favorable foaming properties and is stable in a wide range of heat, cold, salt, and pH with synergistic effects in the presence of sugars (sucrose, lactose). These properties are important in stabilizing emulsions and foams. The general purpose of this study was to achieve the optimal point of electrostatic interaction of different ratios of whey protein concentrate with cress seed gum in the pH range of 2 to 7 using response surface methodology and to investigate the foaming properties of the coacervate complex at the optimal point in comparison with Pure control protein in the same ratio.

The raw materials of this study included cress seed, whey protein concentrate with 80% purity (from Milli com., Germany), and hydrochloric acid with 37% purity (from Merck Com.). Different protein-polysaccharide mixtures were prepared to optimize the electrostatic interaction with the ratio of biopolymers (1: 5 to 5: 1 w / w) and pH of interaction (2 to 7). Mixtures, in the ratio determined by the software and the total concentration of biopolymer (0.3%), were prepared, and after 15 minutes of stirring and equilibration with the environment, were adjusted by hydrochloric acid of 0.1, 0.5, and 1 (n) to the desired pH, and stirred for 5 minutes at each pH with a magnetic stirrer at 400 rpm. Optical Density (OD) of protein: polysaccharide mixtures was evaluated by visible-ultraviolet optical spectrophotometry (Unico, Model S-2150, USA). To optimize the electrostatic complex coacervation process, the coacervation yield was determined. The result of the interaction of whey protein concentrate and cress seed gum as a function of pH and protein to gum ratio was investigated by measuring the sediment phase. For investigation of the foaming properties, the dispersions were homogenized using a homogenizer (Ultra Turrax T25 Digital) for 5 minutes at a speed of 10,000 rpm. After recording the sample volume in the dimension and before homogeneity, the foaming capacity was calculated. To determine the stability of the foam, changes in the volume of the samples were recorded and calculated after 30 minutes of foaming. In this study, the effect of two independent variables including the ratio of biopolymers in the range of 0.2-5 (w/w) and the pH within the range of 2 to 7 were analyzed using Design-Expert software based on two responses including turbidity and yield by the combined central design (CCD) with 5 replications to optimize the electrostatic interaction of whey protein concentrate with cress seed gum.

The results obtained by using response surface methodology showed that the independent variables (ratio of whey protein to cress seed gum and pH) in this study affected the studied responses (turbidity and sediment percentage), although pH had the greatest effect. Optimal conditions for the formation of the maximum protein-polysaccharide coacervate complex were obtained in the ratio of protein to polysaccharide 5: 1 (w / w) and pH 4.24. The results of foaming properties at the optimal point showed an increase in foaming capacity and foam stability compared to pure protein (control sample). This increase in foaming properties is probably due to the presence of cress seed gum and its effect on the formation of a thick viscoelastic film, improved protein adsorption at the interface, and increased bulk phase viscosity.