Laccase-aided protein modification: Effects on the structural properties of acidified sodium caseinate gels

Trametes hirsuta laccase (EC 1.10.32) alone was not able to cross-link proteins effectively unless high dosages were used. Incorporation of ferulic acid enhanced the formation of intermolecular cross-links. Cross-linking was more effective when the reaction was carried out at 45 °C rather than at room temperature. Size exclusion chromatography showed that ferulic acid monomers disappeared from the solution in the presence of laccase. Force at rupture point of the caseinate gels treated with laccase without ferulic acid was not significantly different from the control, while stronger gels were achieved when ferulic acid was present. Rheological measurements showed that laccase treatment together with ferulic acid resulted in higher G′ values than the control. Gels were analyzed with confocal laser scanning microscope. A finer network was observed when the enzyme was used together with ferulic acid. Slight proteolytic activity resulted in a negative effect on the gel firmness and microstructure when laccase was used without ferulic acid.