Preparation, characterization and properties of whey-soy proteins co-precipitates

The preparation and gelling characteristics of soybean protein-whey protein co-precipitates were investigated. Mixtures of defatted soy flour and whey powder (Mixed Powder, MP) and mixtures of soy protein extract and whey protein extract (Mixed Extract, ME) were used. The protein co-precipitates were prepared by combination of NaOH extraction and isoelectric-cooling precipitation (ICP) and isoelectric-heating–cooling precipitation (IHCP). The effect of extraction and co-precipitation on co-precipitate yield was investigated. Co-precipitate was obtained using the following conditions of extraction; extraction temperature, 40 °C; temperature of precipitation 98 °C, and pH of precipitation was 4.5. The yield of co-precipitates was affected by chelating agents and pH of precipitation and temperature of precipitation. SDS–PAGE and Native-PAGE slab gel electrophoresis studies indicated that polymerized products consist of proteins coming whey protein and soybean protein, in addition to a new band. Gels were prepared by heating 8%, 12% and 16% dispersions (pH 8.0) of protein co-precipitates at 95 °C for 90 min, and were characterized by measurement of gel strength, and water holding capacity (WHC). Sixteen percent gels from protein co-precipitate MPICP exhibited higher WHC and gel strength compared to WPICP, SPICP, and protein co-precipitates MEICP, MPIHCP and MEIHCP. Oscillation rheometric data indicated that protein concentration directly influenced gel rigidity and, thus, concentration at 16% resulted stronger gel strength and WHC.