Study On Stability And Oxidation Of Milk Protein During Heat Treatment And Storage For Liquid Milk

Ultra-high temperature(UHT)processed milk has long shelf life at room temperature.The consumption of UHT milk is very large in developing countries.The quality and flavor of UHT milk often deteriorate during storage,which poses enormous challenge to the dairy industry.In this paper,the effects of heat treatment and storage on milk quality and sensory characteristics,heat stability of milk protein,degradation and oxidation of milk protein were studied with the aim of providing theoretical basis for scientific production and storage of liquid milk.Firstly,the properties were studied including changes in the physical and chemical properties and sensory quality of milk during heat treatment and storage.Pasteurization(PAST)and UHT had no significant effect on the quality and sensory quality of milk,but result to loss of vitamin C(7.9%-12.8%)and dissociation of small amount of colloidal calcium(0.7%)into free calcium.PAST milk maintained good physical and chemical stability after 7 days cold storage(2-4°C).Loss rate of vitamin C increased to 19.7% and free calcium content increased by 12.3%.PAST milk had a little rancid and plastic taste.After UHT milk was stored at room temperature for 6 months,free amino acids content increased,the loss rate of vitamin C and pantothenic acid were 27.0% and 35.0%,and free calcium content increased by 6.5%.Stability of milk colloid decreased,and UHT milk had undesirable flavors such as plastic flavor,oxidation flavor,metal flavor and stale flavor.The stability of casein micelles is the essential for colloid stability.After PAST and UHT heat treatment,the degradation rate of milk protein was 2.2% and 6.1%,respectively.PAST and UHT both induced dissociation of casein from casein micelle,and both heat treatments maintained good colloidal stability.After 7 days of cold storage,27.2% caseins were degraded,the content of non protein nitrogen increased to 27.3%,and whey protein(40.0% ?-Lg and ?-La,25.0%BSA)were associated with casein micelle.After 6 months storage,the degradation rate of milk protein was 29.5%,and non-protein nitrogen content increased to 82.4%.With increase of storage time,the proportion of casein(?s1-CN,?s2-CN and ?-CN)decreased and particle size of casein micelle increased in micelle phase,resulting in significant decrease of milk colloid stability.Heat-induced redox reaction of milk protein continued during storage,which changed the composition and structure of milk protein.The mainly heat-induced oxidation of milk protein was amino acid side chain oxidation,amino acid N-terminal oxidation,protein cleavage,lactosylation and Maillard reaction.The oxidation degree of UHT milk protein was significantly higher than that of PAST milk.The main oxidation products of UHT milk were methionine oxide(M+O,M+2O),pyroglutamic acid,lysine lactosylation and protein N-terminal lactosylation.The oxidation products of PAST milk were methionine sulfoxide and pyroglutamic acid.The oxidation degree(amino acid side chain oxidation,heat damage and lactosylation)of UHT milk protein was gradually increased with the storage time.Amino acid side chain oxidation was the main oxidation reaction of PAST milk protein and other oxidation modification were relatively minor.The degrees of oxidation,deamidation and thermal damage of whey protein were higher than those of casein.PAST did not induce Maillard reaction and protein cross-linking.UHT induced higher Maillard reaction in casein than in whey protein.Maillard reaction continued during storage led to cross-linking of milk protein.Retention of lactoferrin is required by the quality standards for the evaluation of the quality of dairy products.A kinetic model for the heat stability of lactoferrin was established.Higher iron saturation led to better in heat stability.When heat treatment was over 72°C/15 s,lactoferrin began to denature and degrade.The degradation products were 35-60 k Da and 20-25 k Da protein fragments.Increase in heat treatment intensity led to increase in protein fragment content,which decreased with increase of iron saturation.The antibacterial activity of natural iron free lactoferrin was higher than that of natural iron-binding lactoferrin.Heat treatment had no significant effect on antibacterial activity of lactoferrin with different iron saturation.After 85°C/15 s heat treatment,antibacterial activity of iron semisaturated lactoferrin and iron saturated lactoferrin increased.Pasteurization of milk and refrigerated storage(7 days)of PAST milk inhibited E.coli growth.Native lactoferrin was not detected in UHT milk,but fresh UHT milk and 1 month storage UHT milk were observed of the inhibibiton for E.coli growth.It is very important to understand the thermal processes utilized in milk processing,as this provides insights on the changes in composition and structure of milk proteins during heat treatment and storage as well as facilitate nutrition,sensory and shelf-life quality assurance of milk products.