The Effect Of Sarcoplasmic Protein On Myofibrillar Protein And Water Holding Capacity Of Myofibrils Under PSE Conditions

PSE pork（pale,soft and exudative）is a typical quality-deteriorated meat occurred in the pork production industry such as pig breeding,slaughtering and meat processing lines.Its appearance is characterized by white surface,soft texture and high-water permeability.In addition,compared with normal meat,PSE meat often has abnormally high cooking loss and poor taste,which makes PSE meat less preferred by consumers.Since PSE meat is more prone to be spoiled by microorganisms and always has lost quite a lot of proteins due to high purge,PSE meat has caused huge economic loss to the whole meat industry.The mechanism explaining the loss of water-holding capacity of PSE meat has not yet been fully discovered,but it is in consensus that the cause can be that the accelerated glycolysis within carcass post-mortem under high temperature（>35°C）leads to the abnormally-fast decrease of carcass p H,resulting in a large amount of denaturation of sarcoplasmic protein and myofibrillar protein in muscle.Therefore,it is very important to study the changes of physicochemical properties of sarcoplasmic proteins and myofibrillar proteins under PSE conditions,regarding their interaction and impacts on the water-holding capacity of meat matrix.The main results presented in the current thesis are as follows:1.Optimization of extraction solution composition for sarcoplasmic protein extraction.The longissimus dorsi muscle of pigs before and after rigor accomplished were used.The extraction rate of sarcoplasmic protein,polyacrylamide gel electrophoresis SDS-PAGE,particle size and Zeta potential were used as main parameters to compare4 extraction solutions and 3 solid-liquid ratios.The results showed that compared with pure water,extraction solutions with potassium ions had a significant improvement in the extraction rate of sarcoplasmic protein.The results of SDS-PAGE of sarcoplasmic protein extracts showed that the sarcoplasmic protein composition and quantities that could be extracted from meat were significantly improved when the p H was kept neutral by adding phosphates.Considering that the concentration of potassium ions in muscle cells is close to 100 mmol/L,composition of K₂HPO₄（20 mmol/L）:K₂EGTA（100 mmol/L）:KPropionate（1 mol/L）=92:2:6（v/v）was finally chosen to obtain potassium with concentration of 100.8mmol/L in the final optimized extraction solution.2.Changes of physicochemical properties of sarcoplasmic proteins（SP）and sarcoplasmic proteins+myofibrillar proteins（SP+MF）were analyzed under simulated PSE conditions in vitro.In this section,9 treatment conditions designed with PSE conditions（40°C,p H 5.5）as the center were selected to study the denaturation of SP and SP+MF under.Protein sedimentation rate,SDS-PAGE were used to analyze the denaturation of sarcoplasmic proteins.It was found that although the total amount of denatured sarcoplasmic proteins at low p H and high temperature was close to heat-denatured ones,the denatured protein components were different.Furthermore,the changes of physicochemical properties of SP and SP+MF during denaturation were investigated by surface hydrophobicity,particle size,Zeta potential and Fourier transform infrared spectroscopy.The results showed that there was no linear relationship between particle size and sedimentation rate.The sedimentation of sarcoplasmic protein was closely related to its surface charge.When p H was close to isoelectric point,the net charge of sarcoplasmic protein molecules was close to zero,resulting in a large number of aggregation and sedimentation of sarcoplasmic protein.However,when the temperature increased and the p H values were 7.0 and 8.0,there was no significant change in the surface charge of sarcoplasmic proteins.At this time,the increase of sedimentation rate was caused by the expansion of the tertiary and tertiary structures of proteins and the exposure of the internal hydrophobic groups under the action of high temperature.The above results showed that the denaturation of sarcoplasmic proteins under PSE conditions was the result of the combined factors of high temperature and low p H,and the stability of some protein components of sarcoplasmic proteins in rigor-accomplished pork became worse,which affected the extraction results of sarcoplasmic proteins.3.Under PSE conditions,the interaction between sarcoplasmic protein and myofibrillar protein during denaturation affected water holding capacity.Based on the results of Chapter 2,three representative conditions were selected to study the effect of denatured sarcoplasmic proteins on the water holding capacity of myofibrillar proteins.Two models of myofibrillar protein and myofibrillar protein were designed to simulate the effect of sarcoplasmic protein on water-holding capacity under different conditions.Centrifugation method was used to detect the water-holding capacity of myofibrils.In order to simulate the structure of undamaged muscle fibers,non-destructive low-field nuclear magnetic resonance（LF-NMR）was selected to detect the water distribution.The results showed that in the myofibrillar protein model,the water holding capacity of myofibrillar protein treated with sarcoplasmic protein under PSE condition was significantly improved compared with that of the sole treatment,while there was no significant difference in water holding capacity between the two treatments under physiological conditions and high temperature conditions.When the experimental model was replaced by muscle fiber,whether it was co-treated with sarcoplasmic protein did not have a significant impact on the relaxation time(T₂₁),which reflects the bulk water content within muscle fiber.The reason for this phenomenon is that although muscle fiber has been treated as permeable muscle fiber,external ions can affect muscle fiber through the muscle membrane,but the sarcoplasmic protein that can enter the internal interaction with myofibrillar protein through the muscle membrane is still far less than that of the myofibrillar protein model.At this time,the original structure of myofibril was still maintained,and the contact space between sarcoplasmic protein and myofibrillar protein was much smaller than that of sarcoplasmic protein and myofibrillar protein co-homogenized in the myofibrillar protein model.Therefore,sarcoplasmic protein failed to have a significant impact on the water holding capacity of muscle fibers.The above results showed that under PSE conditions,due to the combined effect of temperature and p H,sarcoplasmic protein and myofibrillar proteins would interact to form gel,which had a positive effect on the overall water-holding capacity.