Study On The Anti-oxidative Effects Of Conjugates Modified Upon The Maillard Reaction In Oil-in-water Emulsion System

In this study,Maillard reaction products(MRPs)were prepared by dry-heating whey protein isolate(WPI)with dextran derived from different WPI/dextran(WPI/D)ratio and different reaction time.Oil-in-water(O/W)emulsion were prepared by emulsifying MRPs with soy bean oil.The oxidative stability of emulsions and the anti-oxidant mechanism of MRPs in O/W emulsion system were investigated.On this basis,(-)-epigallocatechin gallate(EGCG)was conjugated with MRPs to improve the anti-oxidant property.The oxidation stability and antioxidant mechanism of the compound emulsions were investigated to provide technical and theoretical support in the food emulsion system to develop a natural and healthy emulsifier with strong anti-oxidant ability.Firstly,MRPs were prepared by dry-heating WPI and dextran derived from different WPI/D ratio and different reaction time under 60? and 79%RH.The reaction degree,emulsifying property and antioxidant property of MRPs and the oxidative stability soy bean oil emulsions stabilized by MRPs were investigated.With the increase of reaction time,the grafting degree was gradually increased and the molecular weight distribution became wider.The gradual grafting of the dextran chain promoted the exposure of hydrophobic groups in WPI molecules which increased the surface hydrophilicity of MRPs and reduced the hydrophobicity,thus increased the emulsifying ability.During the process of Maillard reaction,the antioxidative substances were gradually generated;the free radical scavenging ability and the metal ion chelating capacity were improved which led to the increase of antioxidant ability.MRPs were used as both good emulsifiers and anti-oxidant compounds to stabilize O/W emulsions and the oxidative stability was increased significantly.With the increase of reaction time,the oxidation stability was also increased.Compared with WPI emulsion,POV value of emulsions stabilized by MRPs incubated for 14 d were decreased by 78.6%,81.9% and 82.1% respectively when stored for 6 weeks.Emulsions stabilized by MRPs of higher dextran concentration showed more excellent oxidative stability.Secondly,the physical stability of emulsions were evaluated by measuring the particle size and interfacial protein concentration.Compared with WPI,the interfacial protein concentration was lower,but the particle size and zeta-potential of emulsions showed that the steric hindrance of dextran chain and excellent emulsifying ability efficiently increased the emulsion physical stability.However,during the storage period,the particle size did not change greatly,and it was suggested that the physical stability of the system had no significant effect on the oxidation stability in MRPs emulsion systems.After glycosylation,the interfacial protein adsorption was reduced.The highest absorption was 63.30% which can be indicated that at least about 40% of MRPs was remained in the aqueous phase.The radical scavenging ability of unabsorbed subnatants showed that MRPs absorbed on the interface took up stronger antioxidant ability.The oxidative stability of WPI emulsion whose subnatants were replaced by MRPs decreased but the increase of antioxidant ability of unabsorbed subnatants could increase the oxidative stability.The oxidative stability of MRPs emulsions whose subnatants were replaced by water showed that emulsions stabilized by MRPs from WPI/D 3:1 with thinner interfacial layer thickness had higher oxidation degree than MRPs from WPI/D 1:1 and 1:2 with thicker interfacial layer thickness.It can be demonstrated that the increase of interfacial layer thickness and the antioxidant ability of interface could both efficiently increase the oxidative stability of MRPs emulsions.Comparing MRPs emulsions with that whose subnatangts were replaced by water,a theoretical antioxidant mechanism of MRPs in O/W emulsion system were worked out.In the early stage of storage time,the interfacial layer thickness played an important role.Thicker interfacial layer could effectively prevent free radicals and delay the rate of oil oxidation.In the middle of storage time,the antioxidant capacity of the interface layer played a major role which was to say stronger antioxidant capacity could effectively prevent the formation of free radical chain reaction.In the later period,the stronger the antioxidant capacity of the unadsorbed water phase was,the stronger oxidative stability of the emulsion would be.Finally,the antioxidant capacity and emulsion oxidative stability were measured to choose the best method to conjugate with EGCG.The results showed that mixing MRPs with EGCG under pH 6.0 to let them non-covalently bonding was a desirable method.The fluorescence spectroscopy results showed that the higher grafting degree of glycosylation was,the less binding site of MRPs with EGCG would be but it did not affect the increase of antioxidant capacity.It was found that the conjugation of EGCG of small molecular weight will not affect the interfacial layer thickness of MRPs and it can be demonstrated that with the same interfacial layer thickness,the increase of antioxidant capacity of interface could significantly improve the oxidative stability of emulsions stabilized by MRPs from WPI/D 3:1 with thin interfacial layer thickness.