Comparison of Pork Quality in Different Frozen Storage Time and Its Change Mechanism

In order to understand the possible reasons and mechanisms of pork quality changes in frozen storage, the quality indicators such as water holding capacity, texture and color were tested to pork for 4 months and 8 months of frozen storage. The type and distribution of moisture in pork samples were detected by low-field nuclear magnetic resonance (LF-NMR), and the correlation between different water distribution and water holding capacity, texture was explored. The changes in pork flavor components and the types of characteristic substances were discussed by the electronic nose and lipidomics analysis based on the time-of-flight mass spectrometer. The results showed that the quality of frozen pork for 8 months significantly deteriorated, and the morphology of the muscle fibers was looser, the water relaxation time was delayed, the peak area was reduced, compared with pork frozen for 4 months. The quality indicators such as water holding capacity and texture of pork were positively correlated with frozen storage time, and negatively correlated with moisture relaxation time and peak area, which also indicated that the main reason for the deterioration of pork texture after a long period of freezing storage might be including the migration and loss of bound water and non-flowing water in pork. The partial least square regression analysis (PLS-DA) of pork electronic nose data found that the flavor components of pork also changed significantly after long-term frozen storage. The changed flavor components were mainly alkanes, aromatic components, weakly polar compounds, methane, and alcohol compounds. The identification of five different compounds related to oxidation by high-resolution mass spectrometry indicated that the significant changes in pork flavor might be caused by the oxidation of lipids in muscle tissue after a long period of freezing storage.