The Physicochemical Properties, Encapsulating And Release Performance Of Casein Maillard Reaction Products

Casein micelle as a natural nano-capsular vehicle for nutraceuticals, has the properties oflow solubility and emulsion ability at pH near the isoelectric point. The casein-dextranconjugates via Maillard reaction is expected to overcome these problems. Casein-dextranconjugates, amphiphilic self-assembling copolymers that form micellar nanostructures, couldbe used as a carrier system for hydrophobic nutrients. In this thesis, casein-dextran conjugateswere purified from Maillard reaction products containing un-reacted protein and dextran usingultrafiltration. On the basis of studing relationship beween physicochemical properties ofcasein-dextran conjugates and Maillard reaction time, we explored the environmental pHresponse of conjugates and analysed control release performance of conjugates micellesloaded lutein in simulated gastrointestinal liquid digestion model.The degree of the Maillard reaction is greatly affected by reaction time. We used acombination of circular dichroism and fluorescence technique s to study the sequencestructural changes associated with casein-dextran conjugates at different reaction time. After24hours of Maillar d reaction, glycosylation had a stabilizing and increasing effect on thesecondary structure of casein, the content of α-helices of casein reduced from35%to15.3%,in contrast random coil structure content increased from13%to44.9%. The results ofintrinsic fluorescence with the far-UV CD confirmed that part of α-helices and β-sheettransformed to random coil, which was conducive to the formation of hydrogen bonds and theexposure of hydrophobic amino acid residues. The structural changes of casein-dextranconjugates associated with structural changes of casein heat treatment process, but dextranprevented excessive changes of the casein, which indicated the enhancement of thermalstability by glycosylation. The exposure of surface hydrophobicity leaded to better surfaceactivity, after24hours of Maillard reaction, the emulsifying activity and emulsion stability ofthe conjugates were increased by1.15times and1.66times, respectively. In addition, thesolubility of conjugates improved significantly.The aggregation behavior and pH dependence of casein-dextran conjugates separatedthrough ultrafiltration were explored by using fluorescence spectroscopy and dynamic lightscattering. The results showed that casein-dextran conjugates began to self-assemble when theconcentration of casein exceeded0.4mg/mL. The conjugates micelles had compact structureor relative loose structure at different pH conditions. The structures of micelles at pH nearisoelectric point were the most compact, and presented good stability which was due to thesteric hindrance. Meanwhile, emulsion properties revealed that the emulsifying activity andemulsion stability of the conjugates micelles were increased by3.17times and4.33times atpH near the isoelectric point, respectively. Therefore, the pH dependence of casein-dextranconjugates micelles might provide more choice for the use in the field of nutraceuticalnanocarriers.Casein-dextran conjugates micelles were effectively used to encapsulate lutein with theencapsulation efficiency77.5%~95.1%. The average diameter and polydispersity index ofmicelles were110~150nm and0.2~0.4, respectively. The shielding capability of conjugates was able to protect the lutein and improve its stability. The retention rates of lutein inconjugates were86.3%after4weeks storage at25°C and92.8%after4weeks storage at4°C. The conjugates also could serve as a shield against light, relative humidity and oxygen.The retention rates of lutein were93.5%after4weeks exposure to30%relative humidity,89.2%after4weeks exposure to light and87.2%after4weeks exposure to oxygen,respectively. The protective effect of conjugates attributed to theirs oxidation resistance andphysical shielding effect at the interface.The controlled release of lipophilic nutrients in the human gastrointestinal fluid is acrucial factor influencing their absorption. In this study, casein-dextran conjugates throughMaillard reaction were used as the matrix, and lutein was encapsulated in the conjugatesmicelles by self-assembly technique. The pH and ionic strength sensitivity were investigatedby dynamic light scattering, and the digestion behavior was evaluated through in vitrosimulated digestion experiments. It was found that the pH and ionic strength stability ofconjugates micelles was significantly superior to casein micelles, and enhanced with theincrease of the molecular weight of dextran moieties attached to the casein molecules. Thecumulative release rate showed that lutein could be bound with protein in conjugates anduniformly dispersed in simulated gastric fluid. Whereas micelles rapidly disintegrated andreleased the embedding lutein within0.5h in simulated intestinal fluid, which led to theexistence of lutein in the mixed micelles and the increase of the bioaccessibility. Therefore,casein-dextran conjugates could be used as the carrier of lutein, and effectively control itsrelease behavior in the simulated gastrointestinal fluid.