The objective of this study was to investigate the role of casein as a percentage of true protein and total protein content on the physical and sensory properties of skim milk beverages. Pasteurized fluid skim milk was subjected to ceramic microfiltration and diafiltration to produce 95% serum protein reduced fresh liquid micellar casein concentrate (MCC) as retentate with about 8.4% protein. Microfiltration permeate from skim milk was ultrafiltered and diafiltered to produce liquid serum protein isolate (SPI) at about 24% protein. MCC, SPI, lactose monohydrate, cream and deionized water were formulated into 20 skim milk beverages (0.2% fat) with 5 casein:true protein ratios (5, 25, 50, 75, and 80%) and 4 protein levels (3.00, 3.67, 4.34, and 5.00%), with constant lactose (4.65% anhydrous lactose). The experiment was replicated twice. Hunter color and relative viscosity were measured at 4°C, 20°C, and 50°C. A trained panel evaluated flavor, appearance and texture attributes. As true protein levels increased, the milks became more white (higher L value), less green (lower negative a value) and more yellow (higher b value) (P < 0.05). As casein as a percentage of true protein increased, the milks were more white and green (P < 0.05). Milks were more white at 50°C compared with 4°C (P < 0.05). Following pasteurization, milks were generally more white, less green and more yellow (P < 0.05). Relative viscosity increased with increasing protein levels and casein as a percentage of true protein and decreasing temperature (P < 0.05). Pasteurization increased sensory opacity and whiteness (P < 0.05) as did casein as a percentage of true protein and protein content. Cooked/sulfur and cardboard flavors, viscosity and throat cling increased with protein content (P < 0.05) while increased casein as a percentage of true protein decreased aroma intensity, cardboard flavor and astringency (P < 0.05) and increased cooked/milky, cooked/sulfur and throat cling (P < 0.05). These results demonstrate that membrane fractionation can be applied to optimize physical and sensory properties of milk beverages.