Three batches of milk were ultrafiltered to 60, 65, or 70% volume reduction before diafiltration. Starting diafiltration at 70% volume reduction took less time and water without affecting nutrient recovery. Whole milk was heated to 60, 72, and 82°C for 16 s. Milk representing each heat treatment was divided into three batches, one unacidified (pH 6.6), the others acidified to pH 6.2 and 5.8. The milk was ultrafiltered, diafiltered, and concentrated to 5x (80% volume reduction). Retentate was inoculated with .5% lactic culture and incubated at 28°C to pH 5.1. Each lot of fermented retentate was evaporated under 76 kPa vacuum until moisture was reduced to 35-38%, then made into pasteurized process cheese food by cooking to 82°C. The final product contained 43-44% moisture, 24-28% fat, 1.7% salt, and 2.5% sodium citrate. Fat and protein recovery were not affected by heat treatment or pH adjustment of the milk. Recovery of calcium, phosphorus, and riboflavin were significantly reduced following acidification of milk. Riboflavin recovery was higher when milk was preheated to 60°C as opposed to 72 or 82°C. Effect of cooking temperature on meltability of process cheese food was evaluated by repeating the above experiment at three cooking temperatures, 70, 76, or 81 °C. Cooking temperature significantly affected meltability. Cheese cooked to 70°C melted best for all treatments. At all cooking temperatures, cheese from unacidified milk (pH 6.6) had greater meltability than cheese from milk acidified to pH 5.8 or 6.2. Cooking temperature had a greater effect on meltability of process cheese food made from ultrafiltered retentate than calcium content. Preheating milk before ultrafiltration did not significantly affect meltability of pasteurized process cheese food. Meltability of pasteurized process cheese food was best when made from retentate heated (following ultrafiltration) to 61°C for 16 sand poorest when retentate was heated to 72 or 83°C. During ultrafiltration without diafiltration, amino acid analysis was on samples taken at 0, 20, 40, 60, and 80% volume reduction. There were no differences in amino acid composition (g/100 g protein) between milk and 5x retentate. Soluble nitrogen at pH 4.6 in pasteurized process cheese food was an approximate measure of undenatgred whey protein. As processing temperature increased from 66 to 82°C, undenatured whey protein decreased. Decrease in meltability due to increased processing temperature was related to denaturation of whey protein. Process cheese food made from blends of UF curd and Cheddar cheese had acceptable meltability with up to 66% UF curd when the final processing temperature was 68°C. Milk with high bacterial numbers (7.8 x 106 CFU/ml) was heated to 72°C for 16 s, acidified to pH 5.8 and ultrafiltered to a 5x concentration. Ultrafiltration proceeded normally and no processing difficulties were encountered.