Abstract: In this work, the effect of acid-coagulation pH on the physicochemical properties, protein structure and microstructure of Rushan cheese curd was studied along with its effect on the stretch molding of Rushan cheese. The results showed that with the increase in acid-coagulation pH, κ-casein was degraded; the particle size of Rushan cheese curd rose initially and then declined, reaching an average of (26.74 ± 0.11) μm at pH 4.8, and so did all tested components, reaching the highest value at pH 4.8. The firmness of Rushan cheese curd increased significantly (P < 0.05). At an acid-coagulation pH of 4.8, the secondary structure of casein exhibited the smallest α-helix to β-sheet ratio and β-turn content, which were 0.398 and 20.55%, respectively. The flexibility and stability of the curd were excellent. The microstructure observation showed that the fat globules in the agglomerates were distributed in a uniform and orderly manner. It formed a dense milk gel with protein and water. In summary, the acid-coagulation pH affects the degradation and structural change of casein and consequently the physicochemical properties and structure of Rushan cheese curd. An acid-coagulation pH of 4.8 can impart a dense structure together with strong water-holding capacity and plasticity to Rushan cheese curd and make Rushan cheese easy to mold by stretching it. This study is expected to provide a scientific basis for the selection of processing parameters of Rushan cheese.

Key words: Rushan cheese; acid-coagulation pH; physicochemical properties; protein structure; microstructure; stretch molding