Effect And Mechanism Of Transglutaminase Treatment On PSE Pork

PSE meat is a sort of pale,soft,exudative meat.It is commonly used to make re-composed meat products in many countries in the world because of its poor sensory properties,and it is believed as unfitted for sale as fresh meat.When PSE meat is processed to be re-composed products, gel strength is weaker and cooking loss is higher than that of RFN meat.How to promote its gel strength and how to reduce its cooking loss during heating are very important problems in the research field of PSE meat processing.Transglutaminase(TG,TGase) can catalyse the reaction of protein-protein cross-linking and gel strength would be strengthened,network of gel would be improved when TG was employed.The purpose of this research was to evaluate the effects of TG on PSE pork meat and investigate the mechanism of quality improvements of PSE meat by TG catalyzing,to provide theory evidence for enhancing qualities of products made from PSE meat and for promoting economic effects of PSE meat utilization.With longissimus dorsi muscle of pig as material,differences of characteristics between PSE and RFN meat were evaluated systematically, mechanism of quality improvements of PSE meat by TG catalyzing and utilization of TG in sausage were also investigated systematically,respectively,by using SDS-PAGE,DSC,SEM,rheological analysis,texture analysis,color difference analysis and other methods.The main results are as following:1.Differences of physicochemical properties between RFN and PSE pork meat(1) The results of color difference analysis showed that:The average value of L^* and b^* of PSE pork meat were 56.51±0.68 and 6.54±0.19,respectively,the average value of L^* and b^* of RFN pork meat were 56.51±0.68 and 6.54±0.19,respectively.The pH value of PSE pork meat was 0.5 much lower than RFN,when they were obtained after slaughter for 45min.The differences of color and pH between PSE and RFN are visible,and L^*,a^*,pH can be used as evaluation indexes for PSE pork meat discrimination.(2) Solubility of MP isolated from PSE pork meat was much lower than that of RFN pork meat under the same condition(pH,NaCl and TPP concentration are same).Turbidity of MP from PSE was much higher than that of RFN under the same condition,and thus,means the extent of aggregation of proteins in MP sol of PSE is higher than that of sol of RFN.The concentration of MP sol was 35mg/L,when sol was heated to 76.1℃,G' of PSE sample became to 19600Pa,while G' of RFN sample became to 30800Pa.It indicated that the capacity of gel forming of MP of PSE is poorer than that of RFN.2.Factors influencing TG treatment(1) Gel strength of MP from PSE or RFN would be evidently influenced by TG treatment.The optimal concentration of TG is about 0.3%.Gel strength of MP from PSE increased when 0.3%TG were added in,catching up with that of MP from RFN.It indicated that gel properties of PSE would be promoted when MP was treated with TG.(2) Within a certain range of pH(5.77 to 6.77),when pH become higher,gel strength and cooking loss of MP from PSE or RFN treated with TG would became lower.Because of it is some difficult to control the pH of meat batters for re-composed meat products and high pH value would decrease gel strength,so it is need not manually to control pH of meat batters.(3) Gel strength and cooking loss of MP from PSE or RFN treated with TG would be influenced evidently by ionic strength.Gel strength of MP from PSE treated with TG in a 0.6mol/L NaCl sol is 0.3594N,2.43 times more than that in a 0.3mol/L sol,while the cooking loss of the former was 27.13%,the later was 54.20%.To adjust the ionic strength is an important and useful method for promoting gel forming capacity of PSE MP.(4) Gel strength and cooking loss of MP from PSE or RFN treated with TG would be influenced evidently when TPP was added.When 0.15%TPP was added,gel strength of MP from PSE treated with TG is 0.4983N,1.39 times to that of samples without TPP,while the cooking loss of the former was just 34.80%,the later was 37.87%.To use TPP in sol is also an important and useful method for promoting gel forming capacity of PSE MP.(5) When the concentration of addition was lower than 4mmol/L,CaCl₂ would influence not evidently gel strength of MP from PSE or RFN treated with TG,but gel strength and cooking loss would evidently become higher when 8 to 10mmol/L CaCl₂ was added in.Gel strength and cooking loss would not be influenced evidently when 0 to 10mmol/L MgCl₂ or 0 to 2.5mmol/L FeCl₂ was added in.3.Effects of processing conditions and effectiveness of TG treatment(1) Gel strength and cooking loss of MP from PSE or RFN would increase with the increasing of temperature and time for heat-induced gel forming,but there were no evident differences of gel strength and cooking loss between samples treated at 4℃,10℃and 15℃for 2h,respectively.The same conclusion was obtained when incubated for different time(0h,1h,2h,3h and 4h,respectively) at 4℃.It is suggested that TG treatment should be performed at 4℃for 2h.(2) Under the certain condition(protein concentration is 35mg/L,pH 6.07),with TG,NaCl and TPP concentration as variable factors,a Box-Behnken design was employed to predict the effects of factors on gel strength.The calculation formula is as follows:gel strength=0.88-1.459X₁-1.359X₂ +3.39X₃+1.5X₁X₂+4.5X₁X₃+1.5X₂X₃+0.25X₁X₁+0.75X₂X₂-20X₃X₃ Under the optimal condition (TG concentration X₁=0.33%,NaCl concentration X₂=0.5mol/L,TPP concentration X₃=0.14%),gel strength of MP from PSE was 0.58N,higher than that of RFN without TG treatment.(3) It was presented in the experiment that gel strength and cooking loss of MP from PSE or RFN treated with TG were influenced by different heating pattern.Gel strength and cooking loss would be much higher when heated in a water bath from 20℃to 75℃(temperature at the center of gel),contrast to heated in a 75℃water bath till the temperature at the gel center increased to 75℃.It is believed helpful to enhance the working effect of TG when sol was heated in certain procedure.4.Mechanism of qualities improving of PSE pork product by TG treatment(1) It was proved that MP from PSE or RFN can be linked together by TG treatment in SDS-PAGE experiments and this kind reaction is a reversible reaction.Because of the difference of solubility under different conditions(pH,NaCl or TPP concentration),color of remaining band of myosin and actin in the profile of TG treated samples were difference,means the concentration of reaction products were not the same under different conditions.(2) The aggregation and cohesion of MP from PSE or RFN would be enhanced when MP were treated with TG.After TG treatment,turbidity of MP(from PSE or RFN) sol is higher than that of no TG treatment during heating,and the transformation temperature become lower than that of no TG treatment.Turbidity of MP sol treated with TG changed under different pH,different NaCl or TPP concentration,because the solubility and cross-linking reaction induced by TG affected the aggregation and cohesion of MP sol.(3) It was proved by DSC experiment that chemical properties of MP from PSE or RFN changed when treated with TG.The corresponding temperature to peak value of heat flow in DSC profile changed when TG were used.The more TG was added,the more temperature decreased within 0-0.5%TG addition.(4) Results of rheological properties showed that,G' and phase angle(δ) of MP from PSE or RFN changed evidently when treated with 0.3%TG.The transformation temperature would become lower when 0.3%TG was used.The conclusion can be drawn from DSC and rheology test that TG catalyzed the reaction of protein-protein cross-linking and the denaturalization temperature of products is lower than MP.The second G' peak value of heated sol of MP from PSE is 1200Pa when treated with 0.3%TG,more than that of RFN samples without TG(1160Pa),indicating that gel properties of MP from PSE treated with TG was better than that of MP from RFN without TG.(5) Results of chemical force test showed that,disulfide bonds play a most important role for gel strength in heat-induced gel made from PSE or RFN without TG treatment,while hydrogen bonds,electrostatic interaction,hydrophobic interaction were subordinate force.Non-disulfide covalent bonds and disulfide bonds were the main chemical force to maintain the three-dimensional structure of gel treated with TG.5.Effects of TG treatment in PSE meat sausage processing(1) Single-factor experiment showed that,TG treatment can promote gel strength of sausages made from PSE or RFN pork meat and the optimal addition amount of TG is 0.3%.The optimal temperature and time of TG treatment was at 4℃for 2h.Two heating procedures were tried for gel forming:heat in a water bath from 20℃to 75℃(till the temperature in sausage center rose to 75℃) and heat in a 75℃water bath(till the temperature in sausage center rose to 75℃).Gel properties of sausage formed by the former procedure were better than the last one.(2) Gel strength and cooking loss of sausages treated with TG would be evidently affected by salt and TPP concentration,but color was not affected evidently.Single-factor experiment showed that,gel strength was the highest and the cooking loss was the lowest when salt concentration was 3.0%and TPP concentration is 0.15%,respectively.(3) Under certain conditions,when more than 0.1%TG was added,xanthan can promote gel strength and can decrease cooking loss of MP from PSE or RFN,but it is not known whether or not TG can catalyze cross-linking between MP and xanthan.Gel strength and cooking loss of sausage made from PSE or RFN pork meat treated with TG decreased evidently when xanthan were added. Gel strength of sausages made from PSE or RFN decreased sharply when more than 0.1%xanthan were added in.(4) Addition of carrageenan can evidently decrease cooking loss of sausage and the gel strength would not be affected.A very good gel can be made when 0.8%carrageenan was employed.(5) With TG,NaCl and TPP concentration as variable factors,a Box-Behnken design was employed to predict the effects of factors on gel strength.The calculation formula is as follows: gel strength=35.775-85.5X₁+9X₂-188X₃+25X₁X₂+350X₁X₃+70X₂X₃-72.5X₁X₁-2.9X₂X₂-490X₃X₃ Under the optimal condition(TG concentration X₁=0.50%,NaCl concentration X₂=3.25%,TPP concentration X₃=0.22%),gel strength of MP from PSE was 37.63N,close to that of RFN without TG(38.56N).