Appropriate freezing temperature to maintain beef protein stability and inhibit myowater state changes

Abstract: Meat in the freezing process will produce a series of complex physical and chemical and biochemical changes, and these changes have important influence on the quality, especially on the thawing loss, seriously affecting the product quality and enterprise benefit. At present, the freezing temperatures of frozen meat production, -18, -23 and -38℃, are usually applied in China's meat industry, but the effects of different freezing temperatures on the protein stability and moisture state changes in freezing process are not very clear. The effects of freezing temperature on protein denaturation and muscle water distribution of beef were studied in this research. Differential scanning calorimeter (DSC), Fourier transform infrared spectroscopy (FTIR), low field nuclear magnetic resonance (LF-NMR), magnetic resonance imaging (MRI) and other methods were used in the research. In the process of freezing-thawing, the freezing-thawing curve, protein thermal stability, protein secondary structure, protein surface hydrophobicity, moisture state change and thawing loss were analyzed. The freezing point temperature of beef sample is -1.1℃, and below this temperature, the ice crystals begin to form. With the decrease of temperature, the majority of water forms ice. In this temperature range (zone of maximum ice crystal formation), the protein is prone to degeneration. So freezing temperature and freezing speed are important factors to affect the protein denaturation of beef. The lower the freezing temperature of beef, the shorter the time used in the zone of maximum ice crystal formation, the more favorable for controlling the shape, size and distribution of ice crystals in beef, and the longer the time required for thawing to 4℃. The anti degeneration ability and thermal stability of the muscle myosin, myogen and actin were higher under -38℃ than that under the -18℃ freezing condition. Freezing would lead the protein alpha helix to shift to random coil, that was, the ordered structure became disordered structure. But the red shift or blue shift of the peak wave number of beef in the freezing-thawing process indicated that beef protein may have structural degeneration and renaturation and aggregation. FTIR results showed that freezing at -38℃ was the most favorable for maintaining the stability of beef protein secondary structures. Freezing at -18 and -23℃ could lead to the increase of surface hydrophobicity of beef protein, while the surface hydrophobicity of the protein was significantly decreased after thawing (p<0.05). When thawed, MRI results showed that juice loss in beef occurred, especially the parts close to the edge of beef. After thawing of frozen samples from -38℃ to 4℃, we could know that the trend of the immobile water shifting to free water state was lower than that from -23 and -18℃ (p<0.05), and the thawing loss was also the least (p<0.05) based on the LF-NMR results. Therefore different freezing temperatures in beef freezing-thawing process could impact the myofibrillar protein thermal stability, protein secondary structure characteristics, protein surface hydrophobicity, moisture state changes of beef. Overall, the lower the freezing temperature, the more conducive to maintain the water in frozen beef, thermal stability and protein space structure, and what was more, after thawing, the juice loss was less. Protein denaturation, moisture state changes, thawing loss and other indicators mutually support each other. These experimental results provide a reference for the process technology formulation of the frozen meat production and preservation.