Study On The Change Of Goat Milk Protein During Heated Processing Based On Proteomics And It Is Effect On The Quality Of Yogurt

Milk protein is the main index to determine the nutritional quality of milk,while thermal processing will affect the stability of proteins.Therefore,it is imperative to study the effect of heat treatment on milk protein,and proteomics is a relatively advanced technology in protein research so far.In this study,changes of the goat milk protein were investigated during homogenization and heated processing by label-free quantification.The thermal denaturation degree of milk protein under different heat treatments was analyzed by SDS-PAGE and HPLC.Finally,yoghurt was produced by fermentation of different heat-treated samples.The best heat-treatment conditions for yoghurt were preliminarily deduced through determing pH,water holding capacity,viscosity and elastic modulus.Total of 310 and 315 proteins were quantified in control group(CG)and homogenized group(HG),respectively,and there were 16 significantly different proteins.For HG,the goat milk protein particle sizes were smaller and more evenly distributed and exhibited an increase in the regular arrangement of the secondary structures.Proteomics analysis verified that xanthine dehydrogenase and asparaginase-like 1 expression in CG were higher than in HG,whereas the opposite was observed for fructose-bisphosphate aldolase,?-casein,and ?-casein.Significant changes were found in the homogenization-treated goat milk proteome that were related to goat milk glycolysis/gluconeogenesis metabolism.Milk proteins have shown to be very sensitive to heated processing.This study investigated the heat-dependent changes of goat milk proteins using label-free quantification(LFQ).A total of 843 proteins were identified in all the samples,of which 625 proteins were quantified.There were 527,543,537,533 and 539 proteins quantified in the CG and heated groups(HGs: HG1,HG2,HG3 and HG4),respectively,and 438 proteins were common to all groups.The effects of high temperature/short time(HTST)treatment on proteins were similar to ultra-pasteurization(UP)and ultra-high temperature(UHT),but the low temperature/long time(LTLT)was different.Proteomics analysis demonstrated that heated processing increases the digestibility of proteins and is beneficial for anti-atherosclerosis therapy.The thermal denaturation degree of the milk protein between the control group(CG)and the heating group(HG1,HG2,HG2',HG3,HG3' and HG4)were compared by SDS-PAGE and HPLC.The value of pH,viscosity,storage modulus G'(elastic modulus),loss modulus G''(viscosity modulus),and water holding capacity were determined to evaluate the quality of the yogurt.SDS-PAGE results showed that the condition of 120±5 °C/10 s had the greater effect on milk protein.The changes of ?-lactalbumin and ?-lactoglobulin,and ?-Casein and ?-Casein in the five heat treated samples were completely consistent,but ?-Casein was slightly different.The HPLC results showed that the degree of denaturation of goat milk protein increased significantly with the increase of heating temperature and time(P<0.05).The pH of yoghurt for all the samples showed a decreasing trend during the 7 h fermentation at 43 °C.In addition,the results of storage modulus G'(elastic modulus)and loss modulus G''(viscosity modulus)indicated that G' and G'' of all samples showed an increasing trend,and G' of 7 groups of samples were slightly higher than G''.That is,the test sample had good gel state,and HG2 and HG3 show good viscoelasticity.In this study,the viscosity of the 7 groups of yoghurts showed a nonlinear decrease,that is,the samples displayed shear thinning.And the viscosity of HG2 and HG3 were better than other groups.For HG1 and HG3,there were significant differences compared with the other five groups(P < 0.05).It was concluded that the sterilization conditions of 85±5 °C /15 s and 120±5 °C /4 s were suitable for yoghurt fermentation.