ADSA Dairy Foods Graduate Student Paper Competition

Our objective was to determine the effect of heating on the structure of nonfat Mozzarella cheese, and then to relate changes in structure to changes in cheese opacity. Cheese was made according to a direct-acid, stirred-curd procedure. Cheese samples, at 4◦C, were taken on d 1 and placed into glass bottles, which were sealed and heated. Once the cheese reached 10◦C or 50◦C, the bottles were placed on a scanner and color values measured. Samples of cheese at 10◦C and 50◦C were also taken on d 1 for structural analysis. Scanning and transmission electron micrographs of cheese were scanned and digitally analyzed. Applying heat increased cheese opacity, as measured by L-values (91.5 versus 78.0; P < 0.001), and induced changes in cheese structure. Larger protein aggregates (7.0 nm2 versus 1.9 nm2; P = 0.074) and increased protein concentration in the protein matrix were observed in cheese at 50◦C (i.e., increased aggregate density, 157 versus 91; P = 0.008). Applied heat would favor hydrophobic interactions, and possibly, re-association of β-casein and calcium with the protein matrix, promoting protein-toprotein interactions. Thus, the protein matrix contracted, occupying less cheese matrix area (69 versus 99%; P = 0.022) and microphase separation occurred, causing serum pockets to grow in size (from a maximum diameter of 1.0 μm to 7.0 μm), and microstructural heterogeneity increased. It is proposed that the increased size of aggregates and heterogeneity of the cheese at 50◦C promote light reflection, thus increasing cheese opacity. We concluded that applying heat alters protein interactions in the cheese matrix, which is manifest as changes in cheese structure. Such changes in structure help provide an understanding of changes in cheese opacity.