Impacts Of Arabinoxylans And Their Derivatives On Oxidation And Stability Of Milk Protein Emulsion

Dairy products or milk drinks with fat soluble functional components are prone to oxidize and deteriorate during processing and storage.It has been reported in the literature that several polysaccharides revealed certain antioxidant capacity in oil-in-water（O/W）emulsion systems,but most of them reduced the physical stability of the emulsion at neutral p H.In this study,arabinoxylan（AX）,with both antioxidant capacity and interfacial activity,was added to milk protein emulsions to investigate the effects of different structures of AX on the physical and oxidative stability of milk protein emulsion.The main contents and results are as follows:（1）Characterization of structure and properties of AX isolated from 4 different sources,and their interaction with milk proteins in solution.AXwheat,AXcorn,AXrice and AXrye were extracted by alkali method from four types of grain bran（wheat bran,corn bran,rice bran and rye bran）,respectively.The molecular structure and antioxidant activities of the four AX were analyzed.The zeta potential,average particle size and rheological properties of AX,WPI,SC,AX/WPI mixed solution and AX/SC mixed solution were measured.Results showed that there were differences in the molecular structures of the four AX.The order of protein content was AXrye<AXcorn<AXwheat<AXrice.The order of average molecular weight was AXrice<AXcorn=AXrye<AXwheat.AXcorn and AXrice had higher branching degree than AXwheat and AXrye.The Fe²⁺chelating capacity of AXwheat was much higher than that of the other three AX,and the Fe²⁺chelating capacity of AXrice was the lowest.AXcorn had the strongest free radical scavenging ability and reducing power.The order of thickening effect of the four AX was as follows:AXwheat>AXcorn>AXrice>AXrye.Four AX and two milk proteins might form soluble complexes in solution through electrostatic force and hydrogen bond.（2）The antioxidant capacity of the 4 sources of AX in WPI emulsion and SC emulsion,and their impacts on the stability of emulsions.The impacts of AX addition on the physical stability of emulsions were examined by analyzing the apparent viscosity,zeta potential,microstructure,average particle size and creaming stability of different emulsions before and after storage.Peroxide value（POV）and thiobarbituric acid reactant（TBARS）concentration with time were investigated to evaluate the antioxidant capacity of AX in milk protein emulsion.Results showed that all the four AX can be adsorbed on the surface of oil droplets.When the level of AXcorn or AXrice reached0.5%（w/v）,it would cause flocculation of WPI emulsion,while AXwheat and AXcorn were prone to promote the flocculation of SC emulsion.All WPI emulsions showed slight coalescence during storage,but SC emulsions exhibited high coalescence stability.The emulsions had high creaming stability during storage when the content of AX was less than0.5%（w/v）.All the four AX samples improved the oxidation stability of emulsions,with the effect of AXrice was the highest.Through comprehensive comparison,it was found that the influence of AX on the physical and chemical stability of the two milk protein emulsions was smiliar.It is preliminarily determined that AXrice was the most suitable antioxidant,and the appropriate addition amount was 0.3%（w/v）.（3）Antioxidant capacity of Maillard modified AX in SC emulsion,and their impacts on the stability of emulsion.AX-SC covalent conjugates were prepared by dry heat treatment at the weight ratio of AX to SC of 3:1,under 60°C and 79%relative humidity.SDS-PAGE and HPSEC were employed to confirm the formation of AX-SC covalent conjugates under heat treatment.AX-SC was added into the preparation of emulsion.Results showed that the addition of AX-SC higher than 0.3%would reduce the flocculation stability of the emulsion during storage.After 21 days of storage,the particle size of all emulsions was increased significantly.The emulsions containing 0.3%AXrice-SC or 0.3%AXrye-SC and the emulsions containing 0.5%AX-SC showed slight phase separation after storage.AX-SC could inhibit the formation of lipid oxidation products in emulsion by increasing viscosity of emulsions and interfacial layer thickness of oil droplets.The contents of POV and TBARS in AXrice-SC emulsion were the lowest.（4）AXrice was hydrolyzed with xylanase to improve the stability of emulsion.Arabinoxylan hydrolysates（AXH）with different molecular weights were prepared by xylanase hydrolysis of AXrice,and their molecular structure and antioxidant properties were characterized.O/W emulsions were prepared with 0.3%（w/v）AXH,and their physical and oxidative stability were evaluated.Results showed that at p H 7.0,the addition of 0.3%AXH0（Non hydrolyzed AXrice）and AXH30（AXrice hydrolyzed for 30 min）would cause flocculation and/or slight creaming of the emulsion,while AXH60（AXrice hydrolyzed for60 min）would not affect the physical stability of the emulsion.At p H 5.0,except that AXH0emulsion showed a small amount of flocculation,other emulsions had high physical stability.At p H 3.0,the physical stability of all emulsions was very poor due to the reduction of electrostatic repulsion force between oil droplets and the complexation between AXH and SC.Under all p H conditions,AXH significantly improved the oxidation stability of the emulsion.In conclusion,the influence of AX on the stability of WPI emulsion and SC emulsion was similar.Maillard modified AX could not improve their stabilizing effect on SC emulsion.AXrice exhibited the highest antioxidant capacity in milk protein emulsion among the four AX,and the addition amount of 0.3%（w/v）was enough to significantly improve the oxidation stability of emulsions.AXH60 could be used as an antioxidant to prepare milk protein emulsion with high physical stability and oxidation stability under neutral and slight acid conditions.This study provides a theoretical basis for the improvement of quality and efficiency of natural extract AX in the processing of functional dairy products.