Abstract Liquid whey protein concentrates were prepared from various types of cheese wheys by ultrafiltration. After pH adjustment, the whey ultrafiltration retentates (UFRs) were mixed with skimmilk to study the heat stability of skimmilk blends with a range of casein/whey protein ratios. Upon heating at 93°C, skimmilk blends containing cottage cheese (acid) whey UFR coagulated when the casein/whey protein ratio was less than 1:1. No heat-induced coagulation at 93°C was observed for skimmilk blends containing low calcium whey retentates, including rennet whey UFR, ultracentrifugal whey UFR or decalcified acid whey UFR. Electron micrographs of heated skimmilk blends with variable casein-whey protein contents demonstrated major structural changes affected by the individual type of whey UFR used. Heating of blends containing decalcified acid whey UFR resulted in the formation of large, loosely structured protein aggregates. Skimmilk blends with higher whey protein/casein ratios exhibited differences in heat stability related to the use of whey protein concentrates from high calcium (acid) or low calcium (rennet) type whey. The heat instability appeared to be due to calcium-modulated coprecipitation of whey proteins and casein micelles.