Influence of calcium on beta-lactoglobulin denaturation kinetics: Implications in unfolding and aggregation mechanisms
Résumé
Much research dealing with the processing of milk by-products in heat exchangers has noted the key role of calcium in beta-lactoglobulin (beta-LG) fouling behavior. Nevertheless, the manner by which Ca affects beta-LG denaturation has rarely been quantified using reliable kinetic and thermodynamic data. To this end, the influence of Ca on beta-LG denaturation mechanisms in simulated lactoserum concentrates was studied on the laboratory-scale under 100 degrees C by HPLC analysis. The heat-treated solutions were composed of 53.3 g/L beta-LG and were enriched in Ca at various concentrations (0, 66, 132, and 264 mg/kg). The kinetic parameters (reaction order, activation energy, and frequency factor) associated with beta-LG denaturation, along with the unfolding and aggregation thermodynamic parameters were deduced from these experiments and discussed with respect to Ca content. We found that the multistage process characterizing beta-LG thermal denaturation is not greatly affected by Ca addition. In fact, the general model subdividing beta-LG denaturation mechanisms in 2 steps, namely, unfolding and aggregation, remained valid for all tested Ca concentrations. The change in the predominant mechanism from unfolding to aggregation was observed at 80 degrees C across the entire Ca concentration range. Moreover, the classical 1.5 reaction order value was unaffected by the presence of Ca. Interpretation of the acquired kinetic data showed that Ca addition led to a significant increase in kinetic rate, and more so in the aggregation temperature range. This indicates that Ca principally catalyzes beta-LG aggregation, by lowering the Coulombian repulsion between the negatively charged beta-LG reactive species, bridging beta-LG proteins, or via an ion-specific conformational change. To a lesser extent, Ca favors beta-LG unfolding, probably by disturbing the noncovalent binding network of native beta-LG. Simultaneously, Ca has a slight protective role on the native and unfolded beta-LG species, as shown by the increase in activation energy with Ca concentration. The calculation of thermodynamic parameters related to beta-LG denaturation confirmed this observation. A threshold effect in Ca influence was noted in this study: no further significant kinetic rate change was observed above 132 mg/kg of Ca; at this concentration, the studied solution was an almost equimolar mixture of beta-LG and Ca. Finally, we simulated the temporal evolution of beta-LG species concentrations at diverse Ca contents at 3 holding temperatures. The simulations were based on the acquired kinetic parameters. This permitted us to highlight the greater effect of Ca on beta-LG denaturation at high Ca content or for short-time heat treatments at temperatures near 100 degrees C, as in heat exchangers.