How does dry-heating affect the foaming properties of whey proteins and soy proteins in mixed systems?

Whey protein isolate (WPI) and soy protein isolate (SPI) are widely used by food industries due to their techno-functional properties. However, compared to SPI, WPI is often associated with high costs and a less sustainable life cycle. On the other hand, WPI displays some techno-functional advantages compared to SPI, as higher solubility for instance. Thus, mixing both ingredients has been pointed out as a good alternative to improve their properties. In this context, the objective of the present study was to evaluate the impact of dry heating treatments applied to mixtures of commercial WPI and SPI at different ratios on their foaming properties. WPI and SPI were mixed at total protein concentration of 2% w/w and ratios of 100:0; 75:25, 50:50, 25:75 and 0:100 (WPI : SPI). The different mixtures were lyophilized and a fraction of each obtained powder was submitted to dry heating at 80 oC for 48 h. Before dry heating, the solubility of mixtures was inversely related to the SPI content of the mixtures. After dry heating, the same trend was observed, nevertheless, for mixtures containing a high proportion of SPI the insoluble phase was less hydrated after treatment. Dry heating had a minor effect on the nature of proteins contained in the insoluble phase of the different mixtures. For mixture containing only SPI, before a heating, very small amount of beta-conglycinin and glycinin were soluble, however, after treatment, only A subunit of glycinin remained partially insoluble phase. Mixtures more concentrated in WPI were more prone to non-enzymatic browning during dry heating. Dry heating negatively affected the foaming capacity of all mixtures, which formed a smaller volume of foam when bubbled with 4 L of air (airflow: 4 L/min), compared to the non-treated samples. Before and after dry heating, SPI dictated the foam stability behavior of mixtures.