Study On Characteristics And Functional Properties Of Maillard Reaction Products From Whey Protein

Whey protein isolate(WPI) is a high quality protein at full price with the characteristics of broad source, cheap, easy digestion and absorption, high bioavailability and rich content of amino acid. It also contains the excellent functional properties such as good solubility and emulsifying activity and is widely used in the food processing. But WPI doesn’t have antioxidant activity, so it’s not capable to inhibit oil oxidation on the oil-water interfacial which limits its application in food containing oil-in-water emulsion. The issue selects whey protein isolate as the research object, prepare the Maillard reaction products(MRPs) with good antioxidant and emulsifying activity through glucosylation modification, then we studied the relationship between its functional properties and the molecular structure and the extent of reaction, its antioxidant mechanism in the oil-in-water emulsion. It can be applied to develop new, natural emulsifiers with strong antioxidant properties and provide technical support in food industry.We choose xylose, glucosan 20 k Da, glucosan 40 k Da and dextrin as the glycosyl donors to modify whey protein isolate through wet-heated Maillard reaction respectively, considering the emulsifying property, antioxidant activity, browning intensity, and the grafting degree, we select xylose as the glycosyl donor finally. Through single factor and orthogonal experiments, we deside the optimum conditions for Maillard reaction between whey protein isolate and xylose is protein concentration of 3%(w/v), protein and xylose mass ratio of 3: 2, reaction time 90 min, temperature is 95 °C and the initial p H 9.0. Glycosylation product under the optimum conditions has a deeper browning intensity and higher amount of intermediates, the degree of grafting increased rapidly in the initial stage of the reaction. Modified protein maintain the original emulsifying activity and emulsion stability, the transmittance of the emulsions formatted minimal changed and the stability of the system was optimal. Morever, the antioxidant capacity is great as the reducing power increased by 13.55 times more than the original protein, DPPH? radical scavenging rate increased 2.35 times, hydroxyl radical scavenging rate increased 20.82 times, divalent iron chelation rate increased by 1.64 and the fold increase in the rate of lipid oxidation inhibition 4.27 times. The amino acid composition analysis shows that the arginine, lysine, tyrosine and cysteine of WPI reduced significantly after reaction. In the first stage of the Maillard reaction mainly occurs carbonyl – ammonia reaction, protein and sugar graft done quickly, and no brown substance generation.To study the change of MRPs molecular structure under different degree of Maillard reaction and its mechanism of emulsifying and antioxidant activity, we use analysis techniques of infrared spectroscopy, circular dichroism, fluorescence spectroscopy, HPLC and SDS-PAGE. HPLC results show that the molecular weight distribution is increased from 2 segments to 3 segments. SDS-PAGE gel electrophoresis also confirmed that, with the progress of the Maillard reaction, the content of α- lactalbumin and β- lactoglobulin gradually reduced, resulting in a new macromolecules. FT-IR spectra shows that the change of the peaks of amide I and III bonds, enhance absorption peak of hydroxyl groups and carbon-oxygen bonds, confirmed the introduction of xylose molecules changed the secondary structure of WPI. The CD shows that with the deepening of the extent of reaction, the content of α- helix, β- sheet and β- turns reduced, but the random coil increased, protein structure tends to stretch. Fluorescence spectra showed that the maximum fluorescence intensity of the protein was significantly reduced, fluorescence spectral shift to a longer wavelength, the hydrophilic ability of the protein enhanced by Maillard modification.To investigate the emulsion stability and oxidation stability of the emulsion, we applied the products of the Maillard reaction in the oil-in-water emulsion, finding that the emulsion prepared by the MRPs with best emulsifying activity and emulsion stability has a smaller and uniform particles of grease, a relatively high negative charge and was difficult to gather stratified, therefore exhibited good emulsion stability over a wide p H range. After storageing 14 days, POV value of soybean oil and linleic acid emulsions prepared by the best MRPs represent 33.3% and 27.0% of which prepared by unmodified WPI, in terms of TBARS value, represent 35.8% and 24.4% respectively. The results show that the MRPs can effectively slow down the oxidation of fats, increase the oxidation stability of the system. Protein distributed at the interface retard oxidation of fat mainly by physical shielding and electrostatic repulsion, while the small molecules product in the aqueous phase mainly protect fat oxidation through its antioxidant capacity transfer and blocking free radicals by themselves.