Study On Effects Of Freezing And Thawing On Water Holding Capacity And Structure Of Pork

Frozen meat is the main raw material of manufacturing meat products. A basicunderstanding of the significance effects of raw material on processed meat qualityattributes (e.g. water-holding capacity, thawing loss, cooking loss) is important for the meatprocessing industry to control and optimize the production in an economically favourableway. Freezing procedure and frozen storage influence meat quality attributes such asthawing loss, colour and tenderness. The most serious problem in our country presently isthe water holding capacity of some frozen meat is very poor. Starting with the investigationon the water holding capacity and structure of frozen storage pork under conventionalproducing process, we found out the quality varied according to the frozen storage time;and then studied on the optimum frozen time postmortem and coating pork with SodiumAlginate in order to improve the freezing procedure and the quality of frozen pork.NMR was applied as a means to inspect the quality of raw material; Thawing processalso was researched to ensure better thawed meat quality. The aim is to provide a theorybasis and technique for the inspection of quality during freezing process and frozen storage.The contents and results are as follows.1. Effects of Conventional Freezing Processing and Frozen Storage onPork Water Holding Capacity and StructureEffects of conventional freezing processing and frozen storage at -18℃on porkwater-holding capacity and structure were investigated. Thaw loss, cook loss, pressing loss,protein solubility, pH and structure of the meat were measured. The results show:increasing the frozen storage time, thaw loss, cook loss and pressing loss increased, andprotein solubility (including total protein solubility and myofibrillar protein solubility)decreased, and the size of crystals in the samples grew up gradually, this resulted in thewater holding capacity of the frozen meat decreased gradually. After 1 month storage theindexes varied a little, and indexes varied significantly after 2 months frozen storage, after5 months frozen storage there were exceeding differences among indexes compare to the former. Kinetics equations of different indexes were obtained using linear regression.2. Definition the Optimum Freezing Time after Slaughter forManufacturing PorkPaired experimental design was adopted to investigate the quality difference betweenpork freezing at different time postmortem. Freezing loss, thawing loss, cooking loss,pressing loss, protein solubility, pH and scanning electron microscopy of the meat weremeasured. Freezing meat 45 min postmortem significantly reduced total loss, thawing loss,cooking loss and press loss when compared to meat frozen at 4 h and 24 h postmortem.The sooner after slaughter the meat was frozen, the better the water holding capacity was.Both sarcoplasmic protein solubility and pH of pork were significantly higher in meatfrozen at 45 min than meat frozen at 4 h and 24 h postmortem, while the total proteinsolubility and myofibrillar protein solubility were lower in meat frozen at 45 minpostmortem. Increasing the time postmortem before freezing resulted in larger interspacesbetween fibres of fresh muscle. This leads to differences in size and distribution of icecrystals in pork frozen at different time postmortem, and thus changes the water holdingcapacity of the frozen and thawed meat. No differences were seen in meat samples frozen atdifferent times postmortem and thawed under the same conditions.3. Using NMR to Forecast the Aptness for Freezing of PorkNMR was used to investigate the chemical shift graphs of fresh pork at differenttime postmortem, and at the same time the ice crystals of meat freezing at differenttime postmortem were researched with SEM. The results showed there weresignificant (P＜0.05) correlation between distribution and size of ice crytals freezingat different time postmortem and the chemical shift graphs of fresh pork at differenttime postmortem different. Using chemical shift graphs of meat could forecast whetherthe meat was suitable for frozen processing, and also could forecast the water holdingcapacity of it. 4. Coating with Sodium Alginate and Its Effects on the FunctionalProperties and Structure of Frozen PorkThe main objective of this study was to explore an edible coating with SodiumAlginate for improving the frozen pork quality. The results show that coating porkwith sodium alginate can decrease thawing loss of meat; Coating with sodium alginatewas very useful to maintain the functional properties of frozen pork; Coating withsodium alginate could affect total protein solubility, probably. Both different sodiumalginate and CaCl₂ concentration had significant influences on active sulphydryl;CaCl₂ had a significant influence on shear force and pH, Ca²⁺ can tender the meatdirectly or indirectly, and affect the pH of meats. The optimal coating conditions,which were found in the experiments, were sodium alginate concentration: 3%,concentration of CaCl₂:7%, and the time of sodium alginate solution on the meatsurface reacts with CaCl₂ solution: 5～7min. During frozen storage, comparison to thecontrol, the treatment using the optimal coating condition can decrease thawing loss,shear force and thiobarbituric acid reactive substances (TBARS), and can increasetotal protien solubility and sarcoplasmic protein solubility of the frozen pork after 7months storage. This indicates that coating pork with sodium alginate can maintain thequality of frozen pork during frozen storage. There were no significant differences ofstructures between the control and the treatment using the optimal coating condition.5. Effect of Different Thawing Rate on Thawed Meat QualityUniform design was adopted, and three factors, i.e., freezing rate with two levels,thawing rate with six levels and individual (three pork M. Longissimus dorsi musclesfrom three pigs), were considered. After thawed thawing loss, cooking loss, pH, waterholding capacity under pressure (pressing loss) and protein solubility were measuredto study the different effects of the three factors considered. Scanning electronmicroscopy (SEM) and transmission electron microscopy (TEM) were also conductedto research the structure. The results showed: All the three factors considered hadsignificant effects on thaw loss and cook loss, and the effects of thawing rate andfreezing rate on thaw loss and cook loss were nonlinear, there were interactionsbetween freezing rate and thawing rate. Water holding capacity and pH were correlated to different individuals, and were independent of freezing rate and thawing rate.Thawing rate had some effect on protein solubility, especially on total proteinsolubility and myofibrillar protein solubility, and other factors considered had noeffects on it. Freezing at different conditions affected the size and distribution of icecrystals. Different freezing rate and thawing rate had some bad effect on theultrastructure of thawed meat, the quicker the thawing rate, the worse the effect.