The influence of the ionic environment on rheological properties of acidified micellar casein gels.

Membrane filtration allows for the physical separation of milk components and can be used as a single process or in multiple steps to recombine different component streams to create cultured milk products with unique functional properties. In this study, micellar casein (MC) powder was dispersed at 4% and 8% (wt/wt) protein in water, NaCl (10-100 mmol kg), or UF milk permeate (5.6%, wt/wt). Micellar casein is a dairy powder obtained by microfiltration that is mostly depleted of whey proteins and soluble salts. This study was conducted in several parts to investigate the effect of dispersant (solvent) on the pH of gelation and rheological properties of acid gels made from MC. In the initial phase, we prepared model acid gels by acidifying MC at cold temperatures (<4°C) and subsequent warming to 42°C to induce gelation. We also prepared MC gels by yogurt fermentation using commercial starter cultures incubated at 42°C. All MC samples were prepared in duplicate. Gel formation in MC samples occurred as high as pH 5.7 in the cold-acidified milk system. The pH of gelation was highly dependent on the protein level, dispersant used, and concentration of added salts. With increasing concentrations of added NaCl, a decrease in the pH of gelation was observed (from pH 5.7 to below 4.6), as well as a decrease in gel strength (from 175 to <1 Pa). Acid gelation was very dependent on the type of dispersant used to rehydrate the MC. At similar conductivities, MC dispersions with higher protein (8%) levels formed gels at higher pH values compared with lower protein (4%) dispersions. In microbially fermented yogurts, a maximum loss tangent value was observed during acidification in MC gels prepared with low-to-moderate ionic strength. This was unexpected, as the concentration of denatured whey proteins should be very low in these MC systems. No maximum loss tangent value was observed for MC samples dispersed in 100 mmol kg NaCl, as they did not form a gel until pH 4.5. Gel samples with large maximum loss tangent values also exhibited a more open, porous gel microstructure, which was indicative of weak gels that can be prone to syneresis. Results from this study could help dairy manufacturers to design specialty milk powders that have unique functionality when used for acid milk gels, such as yogurt.