Effects of Different Cooking-Stretching Methods on the Physical Properties, Microstructure and Casein Molecular Characteristics of Rushan Cheese Curd

In order to investigate the effects of different cooking-stretching methods on the quality of Rushan cheese, using Rushan cheese curd as the object of study, the basic physicochemical properties, physical properties, microscopic and casein structures of Rushan cheese curd were determined after cooking-stretching using traditional manual and steam-screw methods. The results showed that after cooking-stretching, the protein mass fraction of Rushan cheese curd increased significantly, the fat mass fraction decreased significantly, the texture index and particle size increased significantly (P<0.05), and the viscoelasticity (G' and G'' values) was enhanced. Between the two cooking-stretching methods, the steam-screw method resulted in a curd with a higher protein content (29.23%±2.63%), and lower contents of fat and moisture (14.33%±0.29% and 51.05%±0.11%, respectively). The curd also exhibited higher elasticity, chewiness, hardness, adhesion and viscoelasticity, and more uniform particle size distribution. In its microstructure, the protein structure was dense and the fat was uniformly embedded in the protein matrix. The free sulfhydryl group in its casein was decreased by 22.21%, and the surface hydrophobicity was increased by 17.83%. The steam-screw cooking-stretching helped the formation of hydrophobic interactions and disulfide bonds of casein. Correlation analysis showed that fat content was negatively correlated with elasticity, hydrophobic interactions and hydrophobicity of casein (P<0.05), while protein content was positively correlated with elasticity (P<0.05). In conclusion, steam-screw cooking-stretching was conducive to the formation of dense, viscoelastic, and structurally stable Rushan cheese curd, but its texture was relatively hard, which was mainly related to the loss of fat and water. This study can provide some theoretical guidance for the industrial production of Rushan cheese.