Potential Mechanisms To Investigate Effects Of Superchilled Storage And Modified Atmosphere Packaging On Quality Characteristics Of Beef Muscle

China is a large country of beef production and consumption.In recent years,with the improvement of living standard,consumers require more fresh and healthy meat products.Low temperature storage,including chilled,frozen and superchilled storage,has been widely used for beef preservation.Traditional chilled storage cannot prevent beef from deteriorative process over a long time.Frozen storage is commonly used for long-term preservation of beef muscle,however,meat quality is generally reduced because of severe water loss after thawing.Superchilled storage as a new refrigeration technology has been emerged and effectively implemented for aquatic products.Now there is an increasing interest in the application for meat preservation.Packaging is another factor affecting meat quality.High-oxygen modified atmosphere packaging is usually applied to maintain an attractive red color of beef muscle during retail display.Therefore,the objectives of this study were to investigate the effect of superchilled storage and modified atmosphere packaging on quality characteristics of beef muscle,and to explore the underlying mechanisms of beef tenderness and water holding capacity during superchilled storage.This work was divided into two parts.In the first part,the effect of superchilled storage on shelf life and quality characteristics of beef muscle was determined compared with chilled and frozen storage.From the perspectives of protein structure,calpain activity,microstructure and water distribution,the underlying mechanisms of beef tenderness and water holding capacity during chilled,superchilled and frozen storage were also explored.In the second part,the effect of modified atmosphere packaging combined with superchilled storage on beef quality was investigated.Additionally,oxygen concentration in modified atmosphere packaging was optimized.The main results of this research are as follows:（1）Based on the total volatile basic nitrogen（TVB-N）value,the shelf life of chilled and superchilled beef muscle was 5 weeks and 12 weeks,respectively.Frozen storage can extend the shelf life of beef muscle to at least 24 weeks.As the storage time increased,samples from all treatments showed an increase of the total aerobic count（TAC）,pH value and thiobarbituric acid reactive substances（TBARS）value.L^?value initially increased then decreased,a^?showed a downward trend and b^*values remained increasing.The lower the storage temperature,the slower the rate of change.Warner-Bratzler shear force（WBSF）of samples from all treatments tended to decrease with storage time,and chilled storage was the best condition for beef tenderness.Superchilled steaks possessed higher water loss and cooking loss compared to chilled and frozen samples,indicating poor water holding capacity.（2）A rapid decline of WBSF value was observed during chilled storage,mainly due to the significant proteolysis of troponin T and desmin caused by calpains.Superchilled steaks exhibited much slower decrease rate of calpain activity than chilled steaks,and superchilled storage did not induce great changes in degradation of troponin T and desmin.A slight decrease of WBSF value of superchilled samples was owing to structural changes with a weakening and breakage of myofibers caused by the combined effects of ice crystal and calpain activity.For frozen beef steaks,no major changes were observed with respect to protein degradation and microstructure,resulting in higher WBSF values during storage.（3）Severe degradation of myofibrillar proteins was observed in chilled beef steaks,leading to partial bound water and immobilized water transferred into free water.Superchilled storage did not induce serious proteolysis of myofibrillar proteins.The level of protein denaturation of superchilled samples was higher at later storage.Additionally,myofibers were severely damaged,improving the fluidity of immobilized water located in the myofibrillar protein network.Partial immobilized water was transferred into free water,leading to lower water holding capacity.For frozen beef samples,no major changes were observed in regard to the degradation of myofibrillar protein and microstructure,resulting in lower water loss and cooking loss.（4）Compared with chilled storage,superchilled storage combined with modified atmosphere packaging inhibited the increase of TAC and pH value of beef muscle,restrained color deterioration and lipid oxidation to some extent.However,the water loss of superchilled beef steaks was significantly higher than that of chilled samples.（5）Surface color of both normal-pH and high-pH beef steaks was improved by 60%O₂-MAP and 80%O₂-MAP.The appearance of high-pH sample was enhanced to similar levels as the initial lightness,redness and yellowness of normal-pH steaks after 4 days storage.Steaks in 60%O₂-MAP and 80%O₂-MAP had lower metmyoglobin reducing activity and higher TBARS values,but deeper oxygen penetration depth that may have masked discoloration resulting from metmyoglobin formation under the surface.While both 60%O₂-MAP and 80%O₂-MAP improved the appearance of beef muslce,60%O₂-MAP may be preferred owing to the lower level of lipid oxidation.