Research Of The Digestion And Absorption Mechanism Of Milk Proteins During The Heat Treatment And Storage

Pasteurization(PAST)and ultra-high temperature processing(UHT)are the most common heat treatments in dairy industry.Changes in the structures and properties of milk proteins during heat treatment and storage could affect the quality and nutritional value of milk.The aim of this study was to investigate the effects of heat treatment and storage on the physicochemical properties of milk and the compositions and structures of milk proteins in PAST milk and UHT milk.The in vitro dynamic digestion model,Caco-2 cell monolayer and Sprague Dawley(SD)rat model were used to investigate the milk protein digestibility and absorption,the compositions and contents of protein hydrolysates and the changes in peptide mapping of hydrolyzed peptides.Such study could provide theoretical directions for liquid milk during processing and storage.Firstly,the physicochemical properties,the compositions and structures of milk proteins in PAST milk(stored at 4 ^oC for 7 days)and UHT milk(stored at room temperature for 6 months)were investigated.During the storage,the size of casein micelle increased and the zeta potential of the micelle decreased,which led to the increase of milk viscosity.Because of the hydrolysis action of plasmin,while the contents of milk proteins were decreased,the contents of free peptides and amino acids were increased during storage.The surface hydrophobicity of protein was increased and large casein aggregates begun to appear during storage.?-CN and?-Lg as the form of heat-induced aggregates in serum phase were recombined to the surface of casein micelle.?_s1-CN and?-CN in micelle phase entered into serum phase,especially?-CN dissociated from casein micelle when PAST milk stored at low temperature,resulting in the loose micelle structure.The abundances of lactulose lysine and N?-carboxymethyl lysine increased during storage,indicating Maillard reaction was undergoing.Compared to PAST milk,the Maillard reaction in UHT milk was more significant during storage,which reduced the whiteness of UHT milk.An in vitro dynamic gastric digestion model was established to investigate the composition and structure of the curd from milk during the gastric digestion.The hydrolysis rates and the degrees of hydrolysis of milk proteins,the hydrolyzed peptides and free amino acids in the gastric juice were also investigated.RM and PAST-D0 in the stomach formed a dense curd which had low water holding capacity.The curd structure was loose from UHT milk and stored PAST milk,which could induce a fast hydrolysis of milk proteins in the curd.Native?-Lg was not hydrolyzed in the stomach.Heat treatment had little effect on the hydrolysis degree of casein.However,29%?35%of?-Lg was hydrolyzed during gastric digestion.Storage resulted in an increase in the hydrolysis degrees of casein and?-Lg to 73%?80%and32%?44%.PAST had little effect on the compositions and abundances of hydrolyzed peptides,which were mainly composed of 10?20 peptides.UHT and storage increased the numbers and abundances of peptides,in which the proportion of<10 peptides was relatively higher(54%-68%).Because of heat treatment and storage,the main hydrolysis regions of?-Lg begun to hydrolyze.The numbers and abundances of peptides from the main hydrolysis regions of?-CN were the highest compared to other proteins.The high heat treatment and prolonged storage time induced the high amount of free amino acids.Heat treatment and storage resulted in that the amino acid balance was altered and the proportion of essential amino acids to total amino acids was decreased.However,the proportion of branched chain amino acids to total amino acids did not change significantly during the heat treatment and storage.In vitro intestinal digestion model and Caco-2 cell monolayer model were used to study the digestion and absorption characteristics of milk protein and its hydrolyzed peptides during intestinal digestion.Heat treatment and storage induced a fast hydrolysis of milk protein during the intestinal digestion.Moreover,the higher heat treatment intensity and longer storage time resulted in a faster hydrolysis rate of milk protein.The complete hydrolysis time of PAST and UHT milk were 5 and 2 min,respectively.Qualitative and quantitative results obtained using peptide mapping exhibited little significant differences between RM and PAST milk(PAST-D0 and PAST-D7)in the hydrolyzed regions,the compositions and abundances of hydrolyzed peptides.The hydrolyzed peptides were mainly composed of 5?10 peptides.The longer storage time of UHT milk resulted in higher numbers and lower contents of peptides,in which the proportion of>10 peptides were higher.?-CN and?-Lg for PAST and UHT milk were completely hydrolyzed during intestinal digestion.The degree of hydrolysis of?-CN for PAST was higher than that for UHT milk,while the degree of hydrolysis of?_s1-CN for UHT was higher than that for PAST milk.Caco-2cell monolayer experiments showed that the peptide absorption rate for RM and PAST-D0 were 98%and the absorption peptides were mainly derived from?_s1-CN,?-CN and?-CN.The peptide absorption rate for PAST-D7 was 96%,and the absorption peptides were mainly derived from?-CN,?-CN and?-Lg.The peptide absorption rate for UHT-D0(96%),UHT-3m(91%)and UHT-6m(90%)were lower than that for PAST milk and the absorption peptides were mainly derived from?-CN and?-Lg,in which the proportion of>10 peptides was relatively higher.The numbers and abundances of bioactive peptides from UHT milk were significantly lower than those from PAST milk,and the absorbed bioactive peptides were mainly from?-CN.90%of angiotensin converting enzyme(ACE)inhibitory peptide NMAINPSK was absorbed intact through Caco-2 cell monolayers via paracellular pathway and 1.9%of the peptide was hydrolyzed to the metabolite(NMAINP)by the small intestinal brush border peptidase.The higher heat treatment intensity and longer storage time resulted in the higher amounts of free amino acids.However,processing and storage had little effect on the amino acid balance.SD rat model was used to study the digestibility of milk protein in gastrointestinal tract.The hydrolysis of caseins for RM were slow in rat stomach.?-Lg was resistance to pepsin hydrolysis in rat stomach,which was emptied into the small intestine as intact?-Lg.Heat treatment and storage accelerated the hydrolysis rate of casein and?-Lg during gastrointestinal digestion.There were little differences in the numbers and abundances of hydrolyzed peptides between RM and PAST milk(PAST-D0 and PAST-D7).The longer storage time of UHT milk could result in the higher numbers and abundances of peptides in rat stomach and intestine,the higher proportion of>10 peptides in intestinal residual peptides and residual bioactive peptides,which could influence the bioavailability and functionality of milk protein.The higher heat treatment and longer storage time could induce a faster amino acid absorption rate.The apparent digestibility of PAST and UHT milk was 89.6%?90.4%and 87.2%?88.3%,respectively.The postprandial blood nitrogen reached the maximum value at 120 min for SD rat.Heat treatment and storage had effects on the composition,structure and bioavailability of milk protein in liquid milk,especially in the long life-shelf of UHT milk.Reasonable improvement of processing and storage time would be beneficial to improve the quality and nutritional value of liquid milk.