Mechanism Underlying Changes in the Water Retention of Tan Sheep Meat during Cold Storage

In order to reveal the mechanisms underlying the change in the water-holding capacity of Tan sheep meat during postmortem chilled storage, the pH, cooking loss and centrifugal loss of Longissimus dorsi muscle from six-month-old Tan sheep were measured at 0, 4 and 8 days of chilled storage. Using proteomics based on isobaric tags for relative and absolute quantification (iTRAQ), the level of protein expression in Tan sheep meat was investigated. The results showed that the pH decreased significantly and then increased slightly with chilling time. The cooking loss and centrifugal loss increased continuously. Proteomic analysis showed that tropomyosin 2 (TPM2), myosin light chain 1 (MYL1), troponin I2 (TNNI2), troponin C2 (TNNC2), troponin T3 (TNNT3), myosin light chain 2 (MYL2), actinin α3 (ACTN3), actin (cytoplasmic 1) (ACTB), actin α1 (ACTA1), and actinin α2 (ACTN2) were identified as core differential abundant proteins. TPM2, MYL1, TNNI2, TNNC2, TNNT3, MYL2 and ACTN2 were up-regulated at 0 versus 4 days and down-regulated at 4 versus 8 days; ACTB and ACTA1 were down-regulated at 0 versus 4 days and up-regulated at 4 versus 8 days. ACTN3 was continuously up-regulated from day 0 to day 8. These proteins, located in the sarcomere, the muscle fibers and the cell membrane, could participate in the regulation of muscle contraction and ATPase activity by binding to cytoskeletal proteins or ions during postmortem aging, thereby leading to muscle contraction and changes in muscle structure, finally affecting the water retention of Tan sheep meat. Muscle pH had significant effects on ACTB, ACTA1, MYL1, TNNC2, TNNT3, ACTN2, TPM2 and TNNI2. Muscle pH as well as MYL1, ACTA1 and ACTN3 had significant effects on the cooking loss. MYL2 and ACTN3 had important effects on the centrifugal loss.