Effect Of Heat Treatment On The Structure And Digestive Properties Of Goat Milk Proteins

Goat milk is an emerging force in the recent development of the dairy industry because of its rich nutritional profile,along with better digestibility and lower allergenicity.Goat milk protein is an important nutrient in goat milk and is the main carrier to achieve nutritional functions,especially for infants and special dietary groups goat milk protein has a high nutritional value.Heat treatment is a process that cannot be neglected in commercial dairy products,although it can ensure microbiological safety and extend the shelf life but can change the structure,digestive and absorption properties and biological functions of dairy proteins.Currently,there are relatively more studies on the effects of different heat treatments on the structure and digestive properties of bovine milk proteins,while few studies have been reported on the thermal stability and digestive properties of goat milk proteins.The differences in composition,structure and function between bovine milk proteins and goat milk proteins cannot be directly migrated for use.Therefore,studying the effects of common commercial heat treatments on the structural,functional and digestive properties of goat dairy products produced during processing is an important reference for the utilization of goat milk resources under existing production conditions.The results of this study also have an important reference value for the development of new functional goat dairy products.The aim of this study was to investigate the effects of five common commercial heat treatments,including low temperature long time pasteurization(LTLT),high temperature short time pasteurization(HTST),ultra-high temperature instantaneous sterilization(UHT),spray drying(SP)and retention sterilization(RS),on the structural and digestive properties of goat milk proteins.Firstly,the effects of different heat treatments on the structure of goat milk proteins were investigated in terms of their secondary and tertiary structure,degree of oxidation and interactions.Secondly,the effects of different heat treatments on the digestive properties of goat milk proteins were investigated using a simulated infant in vitro digestion model.Finally,peptidomics techniques were used to identify the peptide types and contents of digested goat milk proteins,and to analyze the effects of heat treatment on the function and degree of glycosylation of digested goat milk peptides.The main findings were as follows:(1)Effect of commercial heat treatment on stability,structure,degree of oxidation and content of melad reaction products of goat milk proteins: the degree of damage to goat milk proteins by different heat treatments was LTLT,HTST < UHT /SP < RS.The changes of LTLT and HTST treatments on the accumulation of goat milk protein particle size,potential and melad reaction products compared to fresh goat milk were not significant,whey protein secondary The RS treatment had the most significant effect,with a 418% increase in carbonyl content and a 65% decrease in sulfhydryl content compared to fresh goat milk,and a 95%denaturation of whey proteins.reaction product accumulation than UHT;(2)Effect of commercial heat treatment on in vitro digestion of goat milk protein simulated infants: The characteristics of gastrointestinal digestion of infants with different heat treatments of goat milk protein were investigated by in vitro infant digestion models.Heat treatment during gastric digestion led to a decrease in the hydrolysis of goat milk proteins to the extent of RS>UHT/SP>HTST/LTLT,while heat treatment during gastric digestion resulted in denser chyme and smaller particles of curd,with the densest chyme morphology in RS and the smallest particles of curd in UHT,where the compact protein structure affected the effect of gastric digestive enzymes.Milk protein digests with a relative molecular mass of 15-20 k Da during digestion were the main component responsible for the differences in hydrolysis.Also the content of free amino acids became more different between fresh goat milk and different heat-treated goat milk as the gastric digestion process progressed.During intestinal digestion,the main proteins of both fresh and different heat-treated goat milk were digested and the free amino acid content was not significantly different in the heat-treated groups compared to fresh goat milk,except for RS.However,the degree of protein hydrolysis decreased after heat treatment,and the degree of decrease was RS/UHT/SP>HTST/LTLT;(3)Effect of commercial heat treatment on peptide composition and distribution after digestion of goat milk proteins: 792,754,750,783,776 and 836 peptides were identified from fresh goat milk,LTLT,HTST,UHT,SP and RS digests(I60),respectively.With deeper heat treatment,the differences in the different heat-treated goat milk peptide species were greater compared to fresh goats.In terms of peptide length,the proportion of digested peptides >10 peptides decreased for seed pasteurized goat milk and increased for higher heat treatments UHT,SP and RS >10 peptides.Also heat treatment caused a decrease in functional peptide content after goat milk protein digestion,with SP causing the greatest loss of functional peptides.In addition,heat treatment caused an increase in glycosylated peptide content after digestion of goat milk protein,where UHT,SP and RS had 14,12 and 9significantly increased glycosylated peptide content compared to fresh goat milk,and LTLT and HTST had relatively less effect on glycosylated peptide content.