Composition, gel properties and microstructure of quarg as affected by processing parameters and milk quality

Quarg cheese was manufactured on a small scale from batches of milk which had received different heat treatments (72°C for 15 s or 90°C for 10 min), different levels of added KIO₃ (0 or 0.1 mM) and different levels of plasmin proteolysis premanufacture (none or 6 mg L^-1 plasmin added and incubated at 37°C for 6 h). Plasmin digestion premanufacture affected the microstructure (changed appearance in electron micrographs) and rheology (lower G' values) of quarg. Plasmin digestion also increased cheese moisture (P < 0.05) and decreased cheese protein and moisture adjusted yield (P < 0.05). High heat treatment of milk gave a higher moisture quarg with more open microstructure and less firm texture. Both β-lactoglobulin and α-lactalbumin levels in whey were decreased by this treatment (P < 0.001) indicating their denaturation and incorporation into the curd. Addition of KIO₃ before high-heat treatment reduced α-lactalbumin denaturation, but gave a cheese of similar composition to that made from high heated milk controlled. Rheological properties of the KIO₃-treated samples were poor, and microstructures appeared different. Milk quality (plasmin content) and processing conditions used (extent and nature of whey protein denaturation) appear to significantly affect the composition and structure of quarg.