Effect Of Thermal And Stretching Treatments On Physicochemical And Functional Properties Of Mozzarella Cheese

Thermo-stretching treatment during the production of Mozzarella cheese gives it unique functional properties when baked. Therefore Mozzarella cheese is widely used in pizza.In recent years, their consumption in China has grown fast. Different thermoplastic temperature and stretching degree have different effects on composition, protein structure and functional properties of Mozzarella cheese. In the present study various thermoplastic temperature(40℃, 60℃ and 80℃) and stretching degree(40, 80 and 120 times) were chosen to make Mozzarella cheese samples. Rheological experiments and Fourier transform infrared spectrometer were used to study the relationship between temperature and plastic transitional degree of the curd. Chemical analysis, confocal laser scanning microscope and texture analysis were used to test the composition, microstructure and texture properties of the cheese samples. Then some empirical test methods were used to determine the meltability and stretchability after heating of each cheese sample.The results indicated that, during the thermoplastic process, the G? and G? of curds decreased with increasing temperature, while Tan δ increased first and then decreased, with the gel point at 60-63℃. The amounts of β-sheets reduced while that of α-helices, β-turns and random coils rose with increasing temperature. The higher the thermoplastic temperature, the smaller the recovery of curd properties and structure was. There was some correlation between the rheological properties and secondary structure of the curd with the change of temperature. After different degrees of thermo-stretching treatment, the gross mass, moisture,fat, protein and calcium of cheese samples decreased, especially when stretched at 80℃. With the loss in moisture and fat greater than that of protein and calcium, the content of the latter both increased in cheese. Thermo-stretching at 40℃ or 60℃ did not change the relative content of γ1-3-casein, β-casein and αs1-casein, but stretching at 80℃ reduced the content ofαs1-Ⅰ-casein in cheese. When stretched at 40℃, there were some cracks in the microstructure and a recovery in the hardness and gumminess with increasing stretching degree, but this did not affect the relative content of secondary structures in casein. When stretched at 60℃, the protein formed into fibers and the hardness and gumminess increased. With the increasing stretching degree, the amounts of β-sheets and random coils rose but that of β-turns reduced.When stretched at 80℃, the protein would deforme and have a higher recovery in hardness and gumminess with increasing stretching degree. At the same time, the amounts of secondary structure in casein changed even more than that in 60℃.When baked, the cheeses which were stretched at 40℃ during manufacture had higher G? and G? and worse functional properties. The cheeses which were stretched at 80℃ had smaller change in Tan δ and decreasing stretchability with increasing stretching times. The cheese which was stretched 80 or 120 times at 60℃ both had better meltability and stretchability. Principal components analysis showed that there was a certain relationship among thermo-stretching treatments, physicochemical properties and functional properties of Mozzarella cheese. The gumminess and stretchability had better correlation.In summary, the changes of viscoelastic properties(G? and G?) and protein secondary structure of curd during heating made it plastic. When the temperature range was below or above the gel point, the curd would show different plasticity. The low thermoplastic temperature(40℃) below the gel point did not change the composition, texture and functional properties greatly. The medium thermoplastic temperature(60℃) was around the gel point and suitable for the formation of meltability and stretchability. The changes of cheese composition and structure when stretched 80 or 120 times at this temperature were more conducive to the formation of functional properties. The high thermoplastic temperature(80℃) above the gel point gave the cheese meltability when stretched 40 times. But the cheese components lost seriously with increasing stretching degree at this temperature which made the protein structure, texture properties and stretchability declined.