Salted and unsalted nonfat mozzarella cheese was made by direct acidification and stored at 4°C over 60 d. Changes in cheese opacity were measured by using reflectance L* values while the cheese was heated from 10 to 90°C, then cooled to 10°C, and reheated to 90°C. A characteristic opacity transition temperature (TOP) was obtained for each cheese. Both salt content and storage time influenced TOP. Opacity during heating, cooling, and reheating formed a hysteresis. At d 1, the unsalted cheese became opaque when heated to 20°C, but the salted cheese required heating to 40°C. As the salted cheese was aged, its TOP increased so that by 60 d the cheese did not become opaque until it was heated to 70°C.