Manipulating the rheological properties and digestion kinetics of yogurts through protein composition and heat treatment

The functionality of gelled dairy products such as yogurt is dependent on the physico-chemical properties of caseins and whey proteins. This study investigates the effect of increasing the whey protein content and the application of different thermal treatment temperatures on the rheological properties and digestion kinetics of yogurt made using a standard dairy protein formulation (4:1 casein:whey protein ratio) versus a whey protein-enriched equivalent with a 1:1 casein:whey protein ratio. The yogurts contained 10 wt% protein, with the 1:1 casein:whey protein blend consisting of skim milk powder and whey protein isolate (WPI). The protein solutions underwent thermal treatments at 70°C, 75°C, or 80°C, followed by acidification using a yogurt bacterial culture. The firmness of the yogurt gels was analyzed using small amplitude oscillatory rheology, while microstructural changes were captured using confocal laser scanning microscopy. Yogurt digestion kinetics were studied using an in vitro digestion model based on the INFOGEST consensus protocol to simulate the human oral, gastric, and intestinal digestive phases. The degree of hydrolysis and molecular weight of the digested proteins were assessed using the TNBS method and SDS-PAGE, respectively. Results indicated that the thermal treatment temperature significantly influenced the rheological properties and digestion kinetics of the yogurts. The yogurts heat-treated at higher temperatures exhibited a higher degree of whey protein denaturation, leading to the formation of stronger gels and slower digestion. The 1:1 casein:whey protein test yogurt demonstrated distinct protein fragmentation during gastric and intestinal digestion compared to the control. This study successfully developed a yogurt variant with increased WPI content and explored how varying the protein composition and thermal treatment could modify its textural and digestive properties. These findings suggest a potential for designing dairy products with enhanced nutritional and functional qualities, possibly offering health-promoting effects beyond traditional yogurts.