Study On The Trend Of Advanced Glycation End Products In Pork During Storage

Fresh meat is rich in various nutrients needed by human body,especially animal protein,and meat food has an important position in people’s dietary life.Pork is a common source of animal protein intake in our daily life,its muscle contains up to 20%protein,myogenic fibrous protein accounts for 60%～70%of the total protein in muscle structure,and sarcoplasmic protein accounts for 20%of the total protein.During storage and processing,physical or chemical changes in protein are closely related to changes in texture and flavor of meat.Meat products usually have a shelf life of about seven days at 4°C.During storage and transportation of the products,proteins are highly susceptible to denaturation,oxidation,degradation and other adverse reactions.For this reason,the choice of different preservation techniques or the selection of suitable additives to inhibit the degree of protein degradation of raw meat during storage has also received more attention.Advanced glycation end-products are a group of chemically stable substances produced by complex reactions between the reactive carbonyl groups of reducing sugars(glucose,fructose,etc.)and their derivatives,and the amino groups of peptides,proteins,amino acids and nucleic acids under non-enzymatic conditions,and are widely found in living organisms and food.In this study,two representative AGEs(N~ε-carboxymethyllysine,CML;N~ε-carboxyethyllysine,CEL)in pork loin were analyzed by HPLC-MS/MS and HPLC during storage and subsequent heat treatment,and oxidation and degradation of fats and proteins.The correlation between the oxidative degradation of fat and protein and the formation of AGEs was clarified,and the mechanism of AGEs formation during storage and subsequent heat treatment was investigated to provide a theoretical basis for optimizing the freezing conditions and processing methods of pork to reduce the formation of AGEs.The main findings are as follows.(1)SDS-PAGE,TCA-soluble peptides,Schiff bases,glyoxal(GO),acetone aldehyde(MGO)and two typical advanced glycation end-products(AGEs)(N~ε-carboxymethyllysine,CML;N~ε-carboxyethyllysine,CEL)were analyzed in refrigerated(4°C±0.5°C,7 d)and frozen(-20°C±2°C,7d)-refrigerated(4°C±0.5°C,7 d)storage and subsequent changes in the levels of heating(100°C/30 min).Compared to frozen pork,raw meat samples that were frozen and then refrigerated showed elevated levels of TCA-soluble peptides,Schiff bases,CML and CEL,and decreased,but not significantly,levels of GO and MGO.During frozen-then-chilled storage of pork,there was a significant correlation between CML/CEL and levels of Schiff bases(r=0.942-0.966),GO(r=-0.794-0.934)and MGO(r=-0.538-0.867),with the exception of CML and MGO(r=-0.384).In addition,the correlation between TCA-soluble peptides and other indicators was higher in frozen-then-chilled storage than in refrigerated storage,suggesting that freezing treatment prior to refrigerated storage could accelerate the formation of AGEs by promoting protein degradation.(2)The changes in TCA-soluble peptides,Schiff bases,GO,MGO,CML,and CEL of pork with added sucrose(4%)were investigated under nine freeze-thaw cycles and during subsequent heating.It was found that an increase in the number of freeze-thaws promoted protein degradation and oxidation reactions,while the addition of sucrose further promoted the reactions,but not significantly,ultimately leading to higher levels of TCA-soluble peptides,Schiff bases,GO,MGO,CML,and CEL in the pork patties with the addition of sucrose than in the blank group by 4%,9%,214%,180%,3%,and56%,respectively.In the blank group,TCA-soluble peptides and Schiff bases showed significant positive correlation with AGEs(P<0.05),while GO and MGO showed significant negative correlation with AGEs(P<0.05),while no good correlation was found between the number of freeze-thaws and CML and CEL(r=0.181 and 0.232,P>0.05).In the pork group with added sucrose,however,the correlation between GO,MGO and AGEs became less significant.(3)The mechanisms of the effects of two typical commercial cryoprotectants(4%sorbitol,0.15%polyphosphate)on the oxidative degradation of proteins,lipid oxidation,and the generation of advanced glycation end-products were investigated under nine freeze-thaw cycles and during subsequent heating.After freeze-thaw cycles,TCA-soluble peptides in raw meat increased by 22%(blank group),18%(sorbitol group)and7%(polyphosphate group),respectively,while the average levels of TCA-soluble peptides in the sorbitol and polyphosphate groups were 2%and 4%higher than those in the blank group,respectively,suggesting that freeze-thaw treatment further promotes protein degradation,while the addition of two commercial antifreeze agents may slow down its degradation.In addition,the levels of GO,MGO,fluorescent AGEs,CML and CEL in raw meat also increased with the number of freeze-thaws,with the mean levels of GO and MGO in the sorbitol group being significantly higher than those in the blank group by 36%and 45%.