Effect Of Structure And Property Of Casein Or Casein-lactose System In Subcritical Water

This research adopted subcritical water as the modified technology, thenanalyzed the physicochemical properties, structure and activity in vitro digestion ofcasein after subcritical water treatment. At the same time, the casein-lactose system(CN-LA) consist the main monomer of milk was treated by subcritical water (SW) atdifferent times. The Maillard reaction (MR) process of CN-LA, phychemicalproperties and structure of the Maillard reaction products (MRPs) were determined.The results were as follows:(1)With the extension time of subcritical water treatment, the relative viscosityof casein solution decreased gradually, while its degree of browning increased. Whenthe temperature of subcritical water up to120℃, the solubility was reached to thehighest ater20min treatment, the strongest foaming capacity of the casein solutionwas obtained at40min and the maximum emulsifying activity was obtained at20min.There were8.73%,55.91%and23.59%improved than untreated samples,respectively. Adopted an appropriate subcritical water condition could be helpful tothe functional properties of casein. However, the solubility, foaming capacity andemulsifying activity of the casein woud be reduced under the prolonged treatment ofsubcritical water.(2)After subcritical water treatment, the content of free sulfhydryl group ofcasein was increased significantly, the content of α-helix was decreased rapidly, andthe content of unorder was increased slightly. All the results were showed thatsubcritical water treatment increased the flexibility of protein molecules.The particlesize of casein solution was increased rapidly after the first drop in subcritical water.However, the surface hydrophobicity of the molecule had an opposite trend. Thechange of infrared spectroscopy (FT-IR) and fluorescence intensity decreasing ofcasein solution, which indicated that subcritical water change the tertiary structure ofthe casein, significantly.Correlation analysis of physicochemical properties and structure of the casein insubcritical water showed that there were more or less correlations between the various physicochemical properties and structure of casein. It was an integratedresults of physicochemical properties and structure of casein.(3)In vitro digestion of casein, it found that the casein molecule was graduallydegradated to polypeptide with a continuous series of lower molecular weight.Subcritical water treatment can improve the activity in vitro digestion of casein.Compared to the native casein, both of the subcritical water and native casein, thereleased nitrogen were increased dramatically during the initial stage of pepsin andtrypsin digestion, and then showed no sigsificance, however, the released nitrogen ofSW casein were significantly higher than untreated casein during the whole digestion.It also illustrated that subcritical water could effectively improve the degestibilityand absorptal capacity.With the degree of hydrolysis of casein showed that the extent of digestion intryptic sharply increased after pepsin digestion. The reasons of this phenomenoncoud be that a large number of sites exposured for trypsin after digestion with pepsin,so that the degree of hydrolysis rapidly increased. Subcritical water treatment maychange the structure of casein, exposing more protein molecules originally locatedinside the binding sites of pepsin effective to accelerate the hydrolysis efficiency ofpepsin.(4)Adopted casein-lactose system as research object, The study of CN-LAsystem’s maillard reaction process, spectroscopic properties (eg. UV-Vis, FT-IR andfluorescence spectroscopy), antioxidant activities and antibacterial activity of MRPsunder subcritical water treatment were investigated. The results showed that, pH ofthe reaction system decrease and the content of lysine or arginine of caseinsignificantly reduced during the process of MR under subcritical water. Moreover, itcould promote the maillard reaction, made casein covalently crosslinked with lactoseto produce aggregates. For studying the change of antioxidant capacity of MRPsunder subcritical water, the reducing power, DPPH radical scavenging ability andtotal antioxidant capacity of MRPs was investigated. The results showed that:subcritical water treatment can effectively improve the antioxidant capacity ofCN-LA. Correlation analysis between the browning degree and antioxidant of MRPsshowed that the degree of browning and antioxidant activities of MRPs had good linear correlation. Studying antibacterial activity of the MRPs, it found that theMRPs produce by CN-LA under subcritical water had no significant antibacterial(such as E. coli, Salmonella and Staphylococcus aureus).