Functional Properties And Application Of Xylo-oligosaccharide Modified Casein

This research was supported by the"13th Five-Year Plan"key research and development project(2017YFD0400603).Casein has a simple production process,low cost,rich amino acid composition and high nutritional value.It is the most commonly used protein component in special medical food.However,due to its instability at isoelectric point,high temperature and high salt concentration,it is necessary to enhance the stability and functional properties of casein through some modification methods.Xylo-oligosaccharides have a sweet taste similar to sucrose,low viscosity and decrease with increasing temperature,stable at high temperature and a wide p H range,and can promote the proliferation of intestinal probiotics.Glycosylation reaction under controlled conditions can be done without any obstacles,not need to add chemical reagents,also does not produce harmful secondary compounds,can optimize the functional characteristics of protein,to produce a new protein,sugar copolymer,widely used in many fields,and have a higher p H after glycosylation properties,such as stability,thermal stability and storage stability.Therefore,in this study,xylo-oligosaccharide was used to modify casein glycosylation,to explore the mechanism of glycosylation reaction and its effect on casein functional properties,and to study its effect on the stability of whole nutrition special medical food emulsions.The main research contents include the following three aspects:(1)Casein and xylo-oligosaccharide were used as substrates for Maillard reaction.Grafting degree,Browning degree,chroma,particle size and potential of the products were measured and characterized under different substrate mixing ratio and reaction time.The results showed that the degree of grafting and Browning increased with the increase of reaction time and sugar ratio.When casein:xylo-oligosaccharide(W/W)was 1:3,the maximum grafting rate was 64.25±2.23%.The experimental results of chromaticity showed that the brightness and yellowness of the glycation product became dark and increased,which was positively correlated with the formation of Amadori product.The particle size decreased with the increase of the two compounds,and the solution of glycosylated products was more stable,but the potential change was not significant.The structural changes of casein during glycosylation were analyzed by SDS-PAGE,Fourier transform infrared(FTIR),ultraviolet(UV),endogenous fluorescence spectrum and circular dichroism.SDS-page showed that xylo-oligosaccharide was successfully grafted to casein and macromolecular polymer was formed.Infrared spectrum,fluorescence spectrum and ultraviolet spectrum and circular dichroism and displacement showed that the characteristic peak changes in casein with low poly xylose in covalent bonding,and protein glycosylation reaction of chromophore exposure,protein conformation changed,alpha helix and beta folded content reduced,random coil increased,from order to disorder.(2)The functional properties of casein,casein-xylo-oligosaccharide mixture and glycosylated products,such as solubility,water retention,oil retention,emulsifying ability and foaming ability,were measured.The results showed that the solubility of glycosylated products was significantly higher than that of pure casein and unreacted mixture,indicating that casein grafted with xylo-oligosaccharide produced compounds with higher water solubility.Because most of the mixture is dissolved in the solution rather than swelling,it cannot combine with more free water,so the water holding capacity decreases.However,after glycosylation modification,the particles are smaller,the specific surface area is larger,and the oil holding capacity is stronger.The emulsification and foaming properties of the glycosylated products were also improved.The emulsification of the glycosylated products reached the maximum value of 50.48±0.55 m~2/g at 8 h,and the emulsification stability was 78.91±2.61%,which was 59.34%higher than that of the untreated samples.Scanning electron microscope(SEM)was used to observe the changes of the microstructure of the product.The results showed that the apparent morphology of the grafted sample tended to be dispersed and uneven,which promoted the interaction between the conjugate and water,and proved the improvement of solubility.The effect of glycosylation on denaturation temperature of casein was measured by differential scanning calorimeter.The results showed that glycosylation increased denaturation temperature and enthalpy,and enhanced thermal stability of casein.The changes of volatile flavor components before and after glycosylation were determined by gas chromatography-mass spectrometry.The results showed that the modified protein had positive effects on the content and types of flavor substances.(3)The whole nutritional special medical food emulsions were prepared using casein,casein-xylo-oligosaccharide mixture and casein-xylo-oligosaccharide copolymer as protein components and emulsifiers respectively.Soybean oil was used as oil component.The micromorphology of the emulsion was observed by laser confocal microscope.The p H stability,centrifugal stability,thermal stability,dilution stability and storage stability of the emulsion were evaluated by measuring particle size,potential,rheological properties,turbidity and other indicators.The results showed that the spatial grid structure of sugar chain in copolymer reduced the diffusion of droplets in concentrated solution,stabilized the contact between droplets,and improved the stability of emulsion.Glycosylation could reduce the isoelectric point of casein and enhance the p H stability of emulsion.If the dilution ratio is too large,the protein concentration in the solution is too low,and the strong interfacial tension between water phase and oil phase cannot be produced,which leads to the aggregation of oil droplets and the instability of the emulsion system.With the increase of storage time and temperature,casein and casein-xylo-oligosaccharide mixture emulsions coagulated into frozen shape,particle size increased to more than2000 nm,absolute potential decreased,viscosity and turbidity increased,while glycosylated product emulsions had good stability,all indexes had no significant change.