| Freezing is an important way to ensure the quality and nutritional value and prolong the consumption cycle of meat.However,the traditional freezing method is easy to form uneven ice crystals between muscle cells.The mechanical damage in the freezing process will lead to cell atrophy,distortion and protein denaturation.After thawing,it will cause problems such as nutrition and juice loss,sensory and texture deterioration,which restrict the development of meat industry and cause huge economic losses.Ultrasound-assisted freezing(UIF)is a new green freezing technology which couples ultrasound and food freezing technology.In this paper,the influence of UIF with different power densities on the changes of tissue structure,water state and quality of porcine M.longissimus lumborum were studied,the mechanism of UIF on the protein structure and thermal stability of myofibrillar protein(MP)was analyzed,the mechanism of the effect of UIF on the emulsification and gel properties of MP was clarified,providing a theoretical basis for the processing and application of UIF in high quality frozen meat.The main conclusions are as follows:(1)Effects of UIF on water state,fiber structure and quality of muscle.The cooling process and quality changes of air freezing(AF),immersion freezing(IF)and UIF with different power densities were evaluated.The freezing time-temperature curve showed that the phase transition time and total freezing time of UIF samples were much less than those of others;The microstructure of frozen muscle showed that compared with AF,the ice crystal size of 30 W/L UIF(UIF-30)sample decreased by 66.31%,the fiber structure was complete and compact,the tenderness increased by 15.57%,the total water loss decreased by 5.05%and the content of freezable water decreased significantly,indicating that UIF reduced the mobility and loss of combined water and free water;The freezing rate had a significant positive correlation with ice crystal size(r = 0.982),and had a significant correlation with thawing loss or other qualities,indicating that the pork quick freezing technology combined with ultrasound can improve the fiber structure and storage quality of frozen meat by regulating the freezing process and inhibiting the growth of ice crystal.(2)Effects of UIF on the structure and stability of pork MP.The effects of AF,IF,UIF on the structural changes of pork MP were studied,the solubility and thermal stability were also analyzed.The results of circular dichroism,endogenous fluorescence spectrum and gel electrophoresis showed that the freezing process caused the cross-linking and degradation of MP,resulting in the exposure of thiol,hydrophobic groups and the change of MP spatial conformation.In contrast,UIF-30 had less damage to MP,with high contentof ?-helix,and little change of the fluorescence intensity and surface hydrophobicity.Appropriate ultrasound(30 W/L)significantly improved the solubility of total protein(116.6 mg/g)and myosin thermal denaturation temperature(57.07?),effectively inhibited the deterioration of MP structure and thermal stability during freezing,and better maintained protein quality.(3)Effects of UIF on the emulsification and gel function of pork MP.Extraction of MP from different frozen processed pork was used to study the influence mechanism of freezing method on emulsification and gel properties of MP.The results showed that UIF significantly improved the emulsifying activity and stability of MP,obtained a small and uniform emulsion droplet,and the emulsion showed better viscoelasticity in rheological scanning.The heat induced gel of UIF contained high ?-sheet content.The dense morphology of the UIF gel was observed,indicating that UIF weakened the disordered aggregation of protein gelation process,resulting in a 12.30% decrease in cooking loss and a higher gel strength.The improvement of emulsification and gelation indicated that UIF was more conducive to maintaining the processing quality of frozen meat. |
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