Study On Rheological Behavior And Its Mechanism Of Whey Protein Isolate/Lactose Gel Based On Glycation Reaction

Whey protein isolate(WPI)is the most abundant protein in mammalian whey,with high nutritional quality and diverse functional properties.The gelation of WPI is an important functional property for creating or modifying food texture.In the processing of dairy products,the gelation of WPI is inevitably affected by major coexisting components such as lactose.In recent years,researches has focused on the effects of covalent interaction(glycation reaction)between reducing carbohydrates(lactose)and WPI during processing on the functional properties of WPI,such as solubility,emulsifying,antioxidant activity and antigenicity.However,there are few studies about the thermal gelling properties of WPI-lactose conjugates prepared by glycosylation,and the effects of glycosylation on the gelling properties of whey protein and its intrinsic mechanism are not completely clear.Therefore,this paper carried out the following three parts of research: 1)Maillard reaction products with different incubation times were prepared by dry-heating treatment(60 ?C,79% RH).Sodium dodecyl sulfate polyacrylamide gel electrophoresis(SDS-PAGE),spectrophotometer and pH meter were used to evaluate the Maillard products and thereby obtain the sample with the maximum soluble glycation degree;2)The rheological properties of WPI-lactose conjugated gel were investigated by rheological tests including temperature sweep,amplitude strain sweep,frequency sweep and stable shear sweep,and compared with mixed gel.Fluorescence spectroscopy,spectrophotometer,scanning electron microscopy and temperature sweep experiments were uesd to study the changes of molecular structure,gel micostructure and molecular interaction of Maillard conjugated solution and mixed solution before and after heat gelation to explain the intrinsic mechanism of gel performance differences for providing theoretical support for expanding its application in food systems.The main conclusions of experiments were as follows:1.SDS-PAGE results showed that the molecular weight of glycated whey protein increased with the incubation time(0 to 2 days),while the molecular weight decreased and the band became shallower as time extended(more than 2 days).Therefore,whey protein and lactose were incubated for 2 days under dry heating to obtain the maximum degree of soluble glycosylation.Under this incubation condition,the pH of the conjugate solution was significantly reduced and its absorbance at wavelengths of 294 nm and 420 nm increased significantly,indicating the formation of a large number of Maillard intermediates and final products.In addition,its absorbance at 340 nm was greatly reduced,indicating that the amount of free amino groups decreased significantly after glycation.By calculation,the maximum degree of glycosylation reached 82%.2.Temperature sweep tests revealed that WPI solution and dry-heated WPI solution did not form a gel after heating at 90 °C for 30 min,but dry-heating conditions slightly weakened the rheological properties of WPI.The addition of lactose promoted WPI solution to form a good elastic gel after heat treatment.In addition,the conjugated gel had the lower damping factor than that of mixed gel,indicating that conjugated gel had better relative viscoelastic property.The results of amplitude strain,frequency,and stable shear sweeps indicated that the conjugated gel had the higher linear viscoelastic regions,lower frequency dependence and higher viscosity at high shear rates than mixed gel,which suggested that conjugated gel had a higher rupture modulus and contained more covalent links in its gel structure.3.Spectroscopic results indicated that Maillard reaction markedly changed the conformation of the protein molecule,embodied in the sharp decrease in fluorescence intensity,the apparent increase in surface hydrophobicity and reduced total free sulfhydryl content of conjugated solution.Compared with the mixed solution,the sulfhydryl content of the conjugated solution decreased significantly after heat gelation treatment,which indicated that the Maillard reaction promoted the formation of covalent bonds during thermal gelation of protein molecules.Temperature rise sweep results showed the mixed gel contained more non-covalent forces(hydrogen bonds,Van der Waals force)than conjugated gel,although both were depended on physical interaction forces.The results of scanning electron microscopy showed that the conjugated gel had a denser and smaller porous structure than that of hybrid gel.In conclusion,the glycation reaction could significantly change the molecular structure of whey protein,resulting in differences in molecular interaction and aggregation between the conjugate solution and the mixed solution during thermal gelation,thus forming gels with different microstructures and rheological properties.