The supramolecular organisation of β-casein: effect on interfacial properties

Milk caseins are natural emulsifiers widely used in food processing applications. Four different caseins exist in milk. β-Casein is one of the most soluble, although not completely so. This protein can exist in solution in a molecular or aggregated state depending on concentration, temperature and calcium content. Many laboratory studies use β-casein assuming it is in the molecular state, without verifying its exact state of aggregation. The aim of this work was to characterise the conditions required to obtain different β-casein aggregation levels and to study their effect on interfacial properties. Different aggregation states have been obtained by varying β-casein concentration, temperature and calcium content and characterised with dynamic light scattering, confocal microscopy and fluorescence spectroscopy. Four aggregation states of β-casein (1 g/L) have been studied: a molecular state (7-8 nm), a micellar state (20-25 nm), a polymeric state (20-25 nm) and an aggregated state (>1 μm). Their interfacial properties at the air-water interface have been followed in dynamic conditions through isotherms, using a Langmuir film balance. We showed that ionic bridging had little impact on β-casein interfacial properties. In contrast, the hydrophobic interactions were more significant in defining the adsorbed film structure. Thermal aggregation in solution was found to be totally reversible, whereas the changes seen in the surface film were not reversible. The interfacial film structure present at 35 °C was independent of whether the film had been spread at 35 °C from a preheated solution or spread at 15 °C and then heated to 35 °C. This was not the case for the structure present at 15 °C.