Phosphorylation of egg white proteins by dry-heating in the presence of phosphate

Food proteins were phosphorylated by heating in a dry state in the presence of phosphate. When casein, whey protein isolate (WPI), and egg white proteins (EWP), which were lyophilized from their solutions in a phosphate buffer, were dry-heated at various temperatures and pH levels for 1-5 days, EWP was more highly phosphorylated than casein and WPI. Phosphorylation of EWP was promoted with a decrease of pH from 7.0 to 3.0 when the incubation temperature was raised from 55 to 100 degrees C. The phosphorus content of EWP increased from 0.08 to 0.64% by dry-heating at pH 3.0 and 85 degrees C for 5 days in the presence of phosphate. The electrophoretic mobility of EWP increased with an increase in the phosphorylation level. The heat-induced polymerization of EWP by dry-heating was not affected by the presence of phosphate. Although the solubility of EWP decreased by dry-heating at pH 3.0-5.5, the phosphorylation depressed the insolubilization at low pH. The phosphate bonds in phosphorylated EWP (P-EWP) were stable at pH 2.0-10.0 and were more acid-labile and base-stable than phosphoesters of egg riboflavin-binding protein (RfBP). (31)P NMR spectral data suggested that besides phosphoesters, phosphodiester and polyphosphate bonds were introduced in P-EWP. Heat stability of EWP was improved, and calcium phosphate-solubilizing ability of EWP was enhanced by phosphorylation.