Antioxidant activity of milk suppresses laccase induced radicals and the subsequent modification of acidified milk protein gels

Laccase has been shown to crosslink milk proteins in model systems in literature. Owing to its acidic pH optimum, we postulated that it can improve the structure of milk gels in a post acidification step. ABTS radical intermediates induced by laccases in a model system were quenched in the presence of an antioxidant, namely ascorbic acid. This indicated that the antioxidant activity of different milk components can influence the reaction mechanism at a molecular level. Acidified microgel suspensions from skim milk with <5% and >99% denatured β-lactoglobulin and from sodium caseinate in distilled water or ultrafiltration permeate were treated with laccase at 20 °C for 24 h. This resulted in primarily lower molecular compounds, a decrease in gel firmness, apparent viscosity and particle size. Thus, substrate accessibility and milieu conditions influence the laccase induced crosslinking of proteins and its effect on gel structure.