EFFECTS OF PROTEIN COMPOSITION AND HEAT TREATMENT ON YOGURT GEL MICROSTRUCTURE, RHEOLOGY AND IN-VITRO DIGESTION

Candidate: Delnavaz Yazdansepas
Supervisor: Dr. Derick Rousseau

ABSTRACT

The first aim of this study was to formulate and investigate the structure and formation of a fat-free, high-protein model yogurt with a higher whey protein to casein ratio than found in cow’s milk. The second aim was to subject the reformulated yogurt to in vitro digestion in a model gastrointestinal tract to assess the effects of protein composition and heat treatment on yogurt breakdown. The yogurts were formulated with 10 wt% protein at 1:1 and 4:1 casein:whey protein ratios using micellar casein and whey protein isolate, the intent being the incorporation of a greater proportion of whey protein. Other than the protein, the yogurt formulations consisted of 75 wt% water, 14 wt% lactose, and 1 wt% bacterial culture. The 4:1 and 1:1 dairy solutions that formed the basis for yogurt-making were heat-treated at 70, 75, or 80°C to assess the effect of whey protein denaturation on the firmness and microstructure of the yogurts. The heat treatment of the dairy solution with a 1:1 casein:whey protein ratio at 75°C produced a yogurt with a similar appearance to the control with a 4:1 casein:whey protein ratio. The in vitro digestion of the yogurts showed that all proteins were fully digested by the start of the intestinal phase, with little difference based on protein composition. This study has shown that whey protein-rich yogurts can be used to formulate yogurts with a texture similar to yogurts with a standard 4:1 casein:whey protein ratio.