| Controlled freezing point (CFP) storage refers to the storage of fresh food within the temperature range below0℃but above the freezing-point, which can maintain physiological activity of fresh food to the lowest degree, also maintain its normal metabolism, inhibit the growth of microorganisms and cause no cold damage, thus achieve the effect of maintaining long-time freshness. In recent years, CFP technology has been widely applied in the US, Japan and South Korea, etc, and domestic studies on CFP have also gradually increased. Certain achievements have been made on the storage of blueberry, pears, spinach, Penaeus vannamei and chicken at CFP. But present CFP storage studies are mainly related to fruits, vegetables and aquatic products; CFP storage studies on the chilled meat, particularly the beef, are relatively fewer. This study mainly deals with the beef stored at CFP, combined with vacuum conditions, preservatives, modified atmosphere packaging separately as the storage methods for retaining freshness, the physicochemical properties and structural characteristics of the beef during storage were measured, electronic nose technology was used for the beef storage shelf life prediction, which proved good results. The main conclusions are as follows:(1) Beef of different parts and from different regions was selected for testing at freezing point to determine the relationship between the freezing point and moisture content&the soluble solids. The results showed that beef of different carcass parts and from different regions varied in terms of freezing point, moisture content and soluble solids, the moisture content and soluble solids of the beef from the adjacent parts on the carcass were relatively similar; different parts of the carcass had larger effect on the three parameters of freezing point, moisture content and soluble solids than that of different regions; the correlation analysis indicated that moisture content was spuriously correlated with the temperature value of freezing point, therefore the moisture content alone can not be used for prediction of freezing temperature value; there was a strong negative correlation between soluble solids content and freezing temperature value; the regression analysis on the relationship between the freezing temperature and soluble solids resulted in a unary linear equation Y=-0.098X-0.208, which was verified to be capable of giving reasonable prediction of temperature values of freezing point for the beef of different carcass parts and from different regions.(2) The storage methods of CFP+vacuum (Cv),0℃+vacuum (Zv),4℃+vacuum (Fv),0℃(Z) were combined with vacuum-packing respectively to investigate the change of beef quality indicators during storage. Given the total plate count and total volatile basic nitrogen during storage, the shelf life of beef under four conditions was32days,24days,20days and8days. The change rate of total plate count, TVB-N, and pH at the same temperature was Zv<Z, therefore, the vacuum packaging can better maintain the beef quality under the same condition. Under the storage conditions of Cv, Zv and Fv combined with vacuum packaging, the change rule of several properties was basically Cv<Zv<Fv. The higher the temperature was, the faster the change in physicochemical properties of beef, Cv condition extended the shelf life of beef, but vacuum packaging had greater rate of water loss than non-vacuum packaging, while the food color was adversely affected in the vacuum packaging due to the anoxic condition. The correlation research shows the TVB-N, total plate count, cooking loss, sensory color and sensory odor can be a reflection for the quality change of the chilled fresh beef in cold storage. The pH can only indicate the beef quality under non-vacuum storage condition.(3) A systematic study in relation to the macroscopic to microscopic structure change of the chilled beef under the three storage conditions Cv, Zv, Fv was conducted., the other indicators (TPA parameters, tenderness, muscle fiber diameter, protein solubility, myofibrillar fragmentation index, sulfhydryl content, secondary structure content), except the sarcomere length, had significant or highly significant correlation with the storage time (p<0.05or p <0.01), signifying these indicators were of a certain indication to the storage shelf life. The structural indicators show the change of indicators is such that the higher the temperature is, the faster the indicator changes, so the temperature has a large impact on the storage shelf life. For the above three kinds of storage conditions, based on the indicators observed, the structure stability under Cv condition is best maintained, followed by Zv condition and Fv condition comes last.(4) The measured TPA parameters (hardness, cohesiveness, elasticity, colloidality, chewiness) and tenderness of the beef showed that with the extension of storage time, the cohesiveness and tenderness first increased and then decreased, for elasticity, there was a certain degree of fluctuation, while hardness, colloidality and chewiness were declining. The correlation analysis revealed there was a significant correlation between tenderness and TPA parameters. The beef muscle sarcomere length and radius measured with confocal microscopy showed the sarcomere length first decreased and then increased, while the muscle fiber radius beccame smaller due to dehydration. Because of action of endogenous and exogenous microbial enzymes, the protein was degraded constantly, resulting in the increase in MFI during storage. MFI can reflect the degradation degree of the tissue structure, and the correlation analysis indicates that it has a highly significant correlation (p<0.01) with tenderness indicator that can also reflect the degree of degradation, as a result, MFI can be used to measure the MFI beef tenderness.(5) The solubility of the protein during storage was declining caused by the denaturation of the protein. The change of thiol group reflects the degree of oxidation of the protein; the thiol was continuously oxidized during storage to become disulfide bond, with the content decreasing. Protein solubility is significantly correlated with thiol levels (p<0.01). Fourier transform infrared spectroscopy was applied for further analysis of secondary structure content change of the soluble protein extracted, and the result showed that a-helix content of protein secondary structure decreased at the end of storage, while the content of β-pleated sheet, β-turn and random coil increased at a certain degree, was a certain, which was probably due to the protein oxidation attacked by free radical and free radical capturing the hydrogen ion, leading to breakage of hydrogen bonds, and a-helix structure was damaged and changed toward the random coil.(6) The preservative chitosan, polyphenols and Nisin combined with CFP for storage of beef. It was found that the preservation effect of these three methods was chitosan> TP> Nisin, and there existed significant interaction between these three kinds of preservatives. Under CFP condition, the optimum compound conditions for compound preservative are: chitosan1.70%, Nisin0.03%, polyphenols0.24%. A regression model was build on Mintab15software, resulting in the regression equation of Y=12.55-1.08A-0.70B-1.08C+0.99A2+0.73B2+0.52C2+0.38AB+0.25AC+0.26BC, with model validation error of2.28%, which can better reflect the preservation effect of the compound preservative combined with CFP storage on beef. Compared with pure CFP storage, the combination of CFP storage with compound preservative combination can effectively extend the shelf life of chilled beef.(7) The study on beef CFP storage combined with modified atmosphere packaging found that compared with the CFP storage alone, the combination of CFP storage with modified atmosphere packaging can extend the shelf life of beef for over4days, the preservation effect of50%02+50%CO2group and30%02+70%CO2group was better than that of80%02+20%CO2group. The beef TVB-N under CFP storage combined with modified atmosphere packaging was far less than the control group of CFP storage, no significant difference was found between several groups of modified atmosphere packaging. High oxygen modified atmosphere packaging will increase the rate of water loss of beef. High oxygen content gained better meat color, yet causing brown stain on the beef surface in later period of the storage, and the higher the carbon dioxide will also adversely affect the color. CFP storage combined with50%O2+50%CO2can better maintain the meat freshness.(8) The electronic nose was used for freshness prediction of the beef stored at CFP and under normal storage. Electronic nose can well distinguish the beef of different storage times. The electronic nose builds models based on parameters of total plate count, TVB-N and sensory evaluation value, which can be used for indicator forecast of the beef not involved in modeling. The regression analysis of the predicted and measured values show the model can well predict total plate count, TVB-N and sensory evaluation value of the beef under the corresponding storage conditions. The establishment of an electronic nose model provides an effective and efficient method for determining the quality of chilled beef under different storage conditions, which is of great significance for quality control of chilled fresh beef during storage. |
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