Microstructure and Rheology of Process Cheese

Four batches of pasteurized process cheese were prepared from the same Cheddar cheese by cooking to 82C in the presence of sodium citrate, disodium phosphate, tetrasodium pyrophosphate or sodium aluminum phosphate. Each batch contained the same moisture (40.6%) and emulsifying salt concentration (2.5%). The process cheese was sampled for microstructural and rheological examination after 0, 5, 10, 20 and 40 min in the cooker at 82C. Even though each emulsifying salt affected the physical properties of the process cheese differently, the cheese generally became firmer, stiffer, more elastic and less meltable as cooking time increased from 0 to 40 min. These changes were accompanied by a decrease in the dimension of fat masses and an increase in the degree of emulsification as evidenced by scanning electron microscopy and transmission electron microscopy. The degree of emulsification (fineness of fat particles) seemed directly related to firmness, poor meltability, toughness, breaking force, apparent stiffness modulus, degree of elasticity , apparent ultimate stress and inversely related to hysteresis and apparent ultimate strain. Tetrasodium pyrophosphate produced the most rapid emulsification of the fat in the cheese and sodium aluminum phosphate the slowest. The effect of the other salts was intermediate. The softest most meltable cheese was poorly emulsified while the firmest most sliceable was well emulsified. Sodium citrate and tetrasodium pyrophosphate crystals remained undissolved in the cheese after 40 min in the cooker while sodium aluminum phosphate crystals were still undissolved after 10 min. There was a close statistical relationship among several of the rheological measurements viz. meltability and firmness, toughness and breaking force, and meltability and breaking force. Future rheological studies on process cheese should not require all of the above measurements. An increase in the fineness of the fat emulsion as determined by scanning electron microscopy was generally accompanied by increased firmness, poorer meltability and increased toughness.