| Protein-phenolic interactions are common in foods with reported effects on nutritional and functional properties of foods. The objectives of this research were to investigate the casein-phenolic acid interactions in a model system and in chocolate as a processed food product. Casein-phenolic acid interactions were induced by heat incubation of casein with protocatechuic acid or p-coumaric acid at 55°C (pH 7, 2 h); caseins were isolated from milk chocolate and white chocolate by precipitating caseins at its isoelectric point (pH 4.6). Casein-phenolic complexes were identified using polyacrylamide gel electrophoresis (Native- and SDS-PAGE) and reversed-phase high performance liquid chromatography (RP-HPLC). Degree of hydrolysis of casein-phenolic complexes was investigated using tryptic hydrolysis; sodium dodecyl sulfate gel electrophoresis (SDS-PAGE) and reversed-phase high performance liquid chromatography (RP-HPLC) were used to identify the hydrolysates of casein-phenolic complexes.;Total phenolic contents of original and defatted milk chocolate and casein isolated from milk chocolate (1.905, 1.644 and 1.018 mg/g respectively) were higher than those of original and defatted white chocolate and casein isolated from white chocolate (1.678, 0.723 and 0.000 mg/g respectively) respectively. Native- and SDS-PAGE results revealed that casein-phenolic interactions were induced by heat incubation and occurred during the processing of milk chocolate; minor changes in the migration of casein fractions and aggregation of casein subunits were observed after heat incubation of casein with protocatechuic acid and in casein isolated from milk chocolate; there is no observed change with Native- and SDS-PAGE electropherograms after casein incubated with p-coumaric acid and in casein isolated from white chocolate. In vitro hydrolysis of casein control (C), casein-protocatechuic acid complex (CPA), casein-p-coumaric acid complex (CCA), casein isolated from milk chocolate (CMC) and casein isolated from white chocolate (CWC) by trypsin showed degree of hydrolysis of 19.3%, 18.6%, 17.7%, 10.4% and 17.8% respectively; SDS-PAGE revealed that the three major casein fractions alpha-, beta- and kappa-caseins in casein control, casein-protocatechuic acid complex, casein-p-coumaric acid complex and casein isolated from milk chocolate and white chocolate were hydrolyzed. Both protocatechuic acid and p-coumaric acid affected the peptide profiles of casein hydrolysates; the peptide profile of casein isolated from milk chocolate was affected by phenolic compounds. |
