Microwave-assisted Proteolysis Of Whey Proteins

Bovine whey protein has a high nutritional value and is the important ingredient in infantformula. With respect to human milk, bovine milk contains more β-lactoglobulin as a majorprotein allergen for infants (2%～3%). Therefore it is important to produce hypoallergenicinfant formula to decrease anaphylaxis of β-lactoglobulin. Proteolysis is a commonly usedmethod to decrease the anaphylaxis of β-lactoglobulin, and microwave-assisted proteolysis ofwhey protein could decrease the reaction time and the allergy reaction of infants. Howeverfew studies have been conducted on the effect of microwave on β-lactoglobulin and theproteolysis process.This paper investigated the effect of microwave on7proteases commonly used in foodindustry, and papain was the most stable enzyme among the proteases under microwave andselected for further analysis. The structure and anaphylaxis of β-lactoglobulin undermicrowave changed. The microwave-assisted hydrolysis of whey proteins with papain wasinvestigated. The main results in this study were as follows:(1) After microwave treatment, enzyme activity reduced gradually with the order ofpapain, alcalase, bromelain, pepsin, trypsin, flavourzyme and neutrase. Papain was the moststable protease in microwave system among the seven proteases.(2) Circular dichroism spectroscopy showed that the secondary structure of papain in themicrowave system did not change significantly. The α-helix, random coil contents of thetrypsin decreased, and the β-sheet and β-turn contents increased with increasing microwavepower and microwave time. Fluorescence intensity of proteases changed significantly afterthe microwave treatment. In general, the fluorescence intensity of papain decreased but with aincrease of trypsin and the fluorescence had red shift for the two enzymes. In addition, thedegree of intensity change and the red shift increased gradually with the increase ofmicrowave power and time. The microenvironment of the tryptophan of papain and trypsin inthe microwave system were altered markedly.(3) The conformation and properties of β-lactoglobulin changed after microwavetreatment, with the fluorescence intensity increased gradually and the microenvironment ofprotein changed. The binding of β-lactoglobulin with ANS increased, and the binding ofβ-lactoglobulin with CPA decreased. The polarity and the secondary structure ofβ-lactoglobulin changed after microwave treatment. Under microwave treatment, near-UVcircular dichroism signal on the chart changed significantly and turned into the moltenglobular state when the temperature reached65℃. β-Lactoglobulin had the maximum contentof free sulfydryl groups and no polymerization. The anaphylaxis of β-lactoglobulin decreasedgradually with the increasing of processing time of microwave.(4) The optimum reaction conditions for microwave-assisted proteolysis ofβ-lactoglobulin with papain were as follows: the addition of enzyme amount at4mg perprotein, the temperature at65℃, and the pH at6.5. There were correlations between thehydrolysis degree of papain hydrolyzed whey protein and the allergenicity of whey protein.Specifically, the higher degree of hydrolysis was, the lower the allergenicity was. When the temperature was higher than70℃or the pH was at6.5, the allergenicity of hydrolysatedecreased significantly under the lower degree of hydrolysis.(5) There were differences of the molecular weight distributions of peptides between themicrowave and heat treated samples. Compared with the heat treated samples, themicrowave-treated samples contained more fractions whose molecular weight was less than2000Da. Further analysis of the fractions indicated that the molecular weight distribution andcontents of these fractions were different for the microwave-treated and heat-treated samples.New peptides were found after the microwave treatment.In summary, the conformation of β-lactoglobulin changed after microwave treatment,which may cause the change of microwave-assisted hydrolysis on whey protein. Thehydrolysis degree of papain hydrolyzed whey protein and the allergenicity of whey proteindecreased after microwave-assisted proteolysis on whey proteins. It has great prospects formicrowave-assisted proteolysis on food proteins to be used in the food industry.