Physical properties of yogurt prepared from pressure-treated concentrated or fortified milks

Physical properties of yogurt, prepared from milk that had been membrane concentrated or fortified with skim milk powder (SMP) prior to high pressure processing, were evaluated. Pasteurised milk was concentrated by ultrafiltration or fortified with SMP and heat-treated at 85°C for 20 min or pressurized at 600 MPa for 15 min. Four types of yogurt were prepared: concentrated heat-treated milk (CH), fortified heat-treated milk (FH), concentrated pressure-treated milk (CP) and fortified pressure-treated milk (FP). Syneresis, penetration tests and compression tests were performed. Yogurts made from pressure-treated milk, either concentrated or fortified, showed no syneresis before centrifugation, whereas yogurts from heated milk lost about 10% of their weight due to whey drainage without centrifugation. Centrifugation at increasing speeds always resulted in more syneresis from heated milk yogurts than from the corresponding pressure-treated milk yogurt. Gel firmness, fracture stress and fracture strain values were higher in CP than in CH yogurts. Fortification with skim milk caused the opposite result, that is, textural parameters were lower in FP than in FH yogurts. This behaviour can be explained by the differences in whey protein denaturation and distribution and the changes in micelle structure due to fortification method and pressure treatment.