Effects Of Innovative Vacuum Cooling On The Quality And Safety Of Cooked Meat Product-based On The Perspective Of Microstructure

Cooked pork ham is regard as a typical low temperature-heated meat products.Some heat-resistant spores and spoilage bacterias are still surviving in low temperature-heated products due to its low sterilization temperature,which exhibited a potential risk for microbial safety.Vacuum cooling（VC）as a rapid cooling technology has attracted much attention in the low temperature-heated meat products based on the principle of latent heat of water evaporation under low pressure.However,in comparison with conventional cooling methods,vacuum cooling causes higher water loss and reduces the tenderness and juiciness of the product,which limits its widespread application in the meat industry.In order to optimise vacuum cooling for application in the cooked meat industry,some innovative vacuum cooling methods are still needed to develop.Despite significant efforts,some innovative method for VC,such as immersion vacuum cooling（IVC）,immersion vacuum cooling&agitation（IVCA）,immersion vacuum cooling&adjusting pressure reduction rate（IVC+APRR）,spray water vacuum cooling（SWVC）,pulsed vacuum cooling in water（PVCW）,vacuum cooling&immersion vacuum cooling（VC+IVC）,vacuum cooling&air blast cooling（VC+AB）had been developed in improving cooling rate,weight loss and product quality,it was still no enough due to some new problems.These new problems could be concluded as follows.Firstly,almost all of afore-mentioned innovative vacuum cooling had a lower cooling rate compared to vacuum cooling.Secondly,they were not able to exhibit a inhibition effect in the germination and outgrowth of C.perfringens spores during storage with temperature abuse.Finally,it was possible to result in a potential microbiological safety risk,in addition to augmenting the burden of the vacuum bump,as well as exhibit no significant inhibition effect on the growth of total viable count and lactic acid bacterial count during cold storage.Despite significant efforts,some innovative methods for vacuum cooling are still needed to exploit that improve these defects caused by afore-mentioned innovative vacuum cooling methods based on the Perspective of water characteristic and pore structure of samples,i.e.,immersion vacuum cooling with ultrasonic assistance（IVCUA）and vacuum cooling followed by ozone repressurization（Inh Vac）.1. Effect of Heating Treatment on Water Properties and Pore Structure of Pork Ham Injected with different Level of Brine SolutionNMR results showed that relaxation time for bound water(T₂₁,T₂₂)and free water(T₂₄,T₂₅),as well as relaxation peak area for free water(A₂₅)of samples with different injection level of brine solution,exhibited a increase as the increase of temperature from 32℃to 82℃.Inversely,relaxation time for immobile water(T₂₃)and relaxation peak area for immobile water(A₂₃)of samples with different injection level of brine solution exhibited a decrease with the increase of temperature from 32℃to 82℃.The MRI analysis revealed that pseudo-color image of samples with different injection level of brine solution was changed from yellow to blue with the increase of temperature from 32℃to 82℃.It was found that a new water population was developed in samples with different injection levels of brine solution as a result of heating treatment.In addition,the temperature for the appearance of new water population was higher with the increase of injection level of brine solution.It seemed to illustrate that the original immobile water either partly expelled from the sample matrix to become higher freedom of free water or alternatively it could be partly trapped in the inner of heat-induced gel to become lower freedom immobile water with the increase of temperature.In addition,samples injected with a higher level of brine solution could obtain a more dense gel structure,lower water loss and porosity,as well as higher T₂₃and lower T₂₅.The PCA results showed that the first and second principal components（PC）could explain 69.5%of the total variance.Rapid comparison between samples with different temperature and level of brine injection could be made by plotting the obtained PC scores based on the placement of each sample along with the components which were defined by the CL,NMR data and porosity through PC coefficients.2. Effect of Vacuum Cooling on the Water Properties and Pore Structure of Cooked Pork Ham Injected with Different Levels of Brine SolutionThe NMR results showed that relaxation peak area for bound water(A₂₁,A₂₂),immobile water(A₂₃)and free water(A₂₄,A₂₅)exhibited a decrease during vacuum cooling process.It was noted that relaxation time for immobile water(A₂₃)didn’t change during the whole vacuum cooling process.In addition,the pore structure results revealed that samples subjected to vacuum cooling treatment exhibited a larger range of pore size distribution,porosity and pore diameter than those treated by conventional cooling methods.The Bi-plot results showed that the average temperature reduction rates exhibited a significant correlation with some NMR parameters and pore structure parameters.The VIP results revealed that T₂₃and A₂₃exhibited a more significant effect on the average temperature reduction rates compared to T₂₅and A₂₅.In addition,pore structure parameters such as APDV,TIV,Permeability and Tortuosity could play a more important role in evaluating cooling rate compare to the parameter of porosity（P-M）.Samples injected with 30%brine solution had a lower cooling rate compared to those injected with 40%brine solution during vacuum cooling process,despite of their porosity were almost the same.The results might be explained that samples with 30%brine injection level had a larger proportion on the count of small pore,higher tortuosity as well as lower permeability than those injected with 40%brine solution.PLS model used to establish the relationship between pore structure characteristics,water characteristic and average temperature reduction rate during vacuum cooling cooked pork ham process,which not only improved cooling mechanism,but also provided the theoretical foundation for the feasibility of innovative vacuum cooling technologies,such as vacuum cooling followed by ozone repressurization or immersion vacuum cooling with ultrasonic assistance.3. Effect of Inh Vac Treatment on the Germination and Outgrowth of C.perfringens Spores in Cooked Pork HamThe effects of ozone concentration（X₁,5-15 g/m3）,treatment time（X₂,10-30 min）as well as repressurization pressure（X₃,0.5-1.5 atm）during Inh Vac of cooked pork ham were evaluated and optimised by response surface methodology（RSM）for the first time.The germination and outgrowth of C.perfringens spores in cooked pork ham were estimated.Results showed that the best combination of the operational parameters was that ozone concentration 15 g/m³,treatment time 20 min,repressurization pressure 1 atm.By treatment with the combined operational parameters,the color value,TBA value,as well as total viable count（TVC）and lactic acid bacteria（LAB）count were determined under the storage of（4±1）℃to evaluate the quality of samples.IC and VC treatment were used as control.The color results showed that samples treated by Inh Vac had a higher（P<0.05）b*value and lower（P<0.05）a*value compared to those treated by IC and VC.The TBA results revealed that Inh Vac treatment exhibited a slightly higher（P>0.05）TBA value compared to IC and VC treatment.The TVC and LAB evolution results demonstrated that samples subjected to Inh Vac treatment exhibited a lower（P<0.05）microbial count than those treated by IC and VC treatment.The germination and outgrowth of C.perfringens spores in cooked pork ham treated by Inh Vac,were significantly inhibited during storage with temperature abuse.So,Inh Vac treatment can be regard as a feasible cooling method used to improve the microbial safety of samples duing storage and transportation.4. Modeling C.perfringens Growth Parameters in Cooked Pork Ham Cooled by Gaseous Ozone Repressurization during Vacuum CoolingWe evaluated the effect of vacuum cooling followed by ozone repressurization（Inh Vac）on C.perfringens germination and outgrowth in cooked pork ham after exponential chilling（from 54.4 to 7.2°C in 12,15,18,or 21 h）and isothermal storage（20,25,30,36,or 45°C）.We compared these effects to ice cooling（IC）and vacuum cooling（VC）.In samples treated by Inh Vac,bacterial growth was<1 log after 21 h of cooling from 54.4 to 7.2°C.However,bacterial growth in samples subjected to IC and VC were>1 log after 15 and 18 h of cooling,respectively.Compared to VC and IC,Inh Vac treatment increased the lag phase（λ）,decreased growth rates(μ_max),and extended the sample shelf life at all storage temperatures.Additionally,all statistical indexes showed that a primary modified Gompertz model and a secondary Square Root model best fit the data.In addition,a computer simulation algorithm was developed to dynamically estimate and predict the growth of C.perfringens in cooked pork ham.The computational algorithm was based on the implicit form of the modified Gompertz model,the growth kinetics of C.perfringens in cooked ground beef（Square Root model）,and the fourth-order Runge-Kutta numerical model.This algorithm was validated using a cocktail of three strain of C.perfringens spores grown under isothermal,square-waved,linear cooling,and exponential cooling temperature profiles.In general,the results of computer simulation matched closely with the experimental data with the absolute error less than 1 ln CFU/g.The newly-eatablished computer simulation algorithm can achieve the prediction of C.perfringens in samples treated by Inh Vac under temperature fluctuation,which provides a theoretical foundation for its application on actual cooling chain.5. Feasibility Analysis for the Application of Vacuum Cooling with Ozone Repressurization on Inbibiting of Germination,Outgrowth of C.perfringens Spores of Cooked Pork Ham.Inh Vac treatment has been proved as an effective cooling method,which can be used to inhibit the germination,outgrowth of C.perfringens spores during storage with temperature abuse.However,the inhibition mechanism is still unclear.The results of SEM observation showed that Inh Vac treatment had a more significant inhibitory effect on the morphology of C.perfringens vegetable cells compared to IC and VC treatment.Conversely,it had a insignificant inhibitory effect on the morphology of C.perfringens spores cells which was similar to IC and VC treatment.In addition,the fluorescence assay revealed that Inh Vac treatment could lead to more significant damage to the integrity of C.perfringens spores cell membrane compared to IC and VC treatment.Although it was verified that Inh Vac treatment could damage the integrity of cell membrane of C.perfringens spores,but not result in the inactivation of C.perfringens spores.The pore structure analysis showed samples treated by Inh Vac could obtain a more obvious increase in the porosity and enlargement in the pore size distribution than those treated by IC and VC.Thus,it was reasonable to infer that Inh Vac treatment could lead to better inhibitory effects on C.perfringens germination and outgrowth in cooked pork ham due to a higher concentration of ozone permeability into the samples.The oxidation stability tests showed that samples treated by Inh Vac didn’t cause a shorter induction period（IP value）compared to those treated by IC and VC.It seems to illustrate that ozone permeated into the inner of samples has a preferential oxidation reaction with the cell membrane of C.perfringens rather than fat and oil of the sample itself.These above-mentioned results are enough to prove that Inh Vac treatment is a feasible cooling method applied to cooked meat products.6. An Innovative Method of Immersion Vacuum Cooling for Cooked Pork Ham:Immersion Vacuum Cooling with Ultrasonic AssistanceImmersion vacuum cooling with ultrasonic assistance（IVCUA）,is compared with immersion vacuum cooling alone（IVC）and vacuum cooling（VC）for cooling time,mass loss,colour,texture profile,water mobility and compartmentalization,total microbial count（TBA）and lactic acid bacterial（LAB）evolution for cooked pork ham.The results revealed that IVCUA clearly enhanced the boiling intensity of impregnation liquid compared to IVC.The total cooling time using IVC（126.56 min）for samples from 72 to 4°C was significantly longer（P<0.05）than that of IVCUA（96.89 min）and VC（80.48 min）.The cooling time of samples from 10 to 4°C by IVC（46.26 min）was higher（P<0.05）than that by IVCUA（28.55min）.There were no significant differences（P>0.05）in mass loss,colour,and texture profile among all samples.However,IVCUA had a higher transverse relation time of bulk water(T₂₄)and MRI proton densities（P<0.05）,as well as a more uniform water distribution compared to IVC.In addition,it was concluded that samples treated by IVCUA had a lower TBA and LAB count compared to those subjected to VC and IVC during cold storage,indicating a longer shelf-life.Cooling profiles of samples were tested using USDA cooling growth model for C.perfringens in beef broth.According to the model,none of the analyzed profiles would support significant growth of the bacteria.The results showed that IVCUA not only can offset the water loss but not comprise cooling rate,as well as improve quality and microbial safety compared to IVC.