Layering of Sodium Caseinate Submicelles in Thin Liquid Films— A New Stability Mechanism for Food Dispersions

Abstract

Drainage and stability mechanisms of thin foam and emulsion films containing aqueous sodium caseinate solutions were investigated. We found that the films thin stepwise by stratification. The heights of the film step-transitions were in the same range as the effective size of the casein-aggregates, the so-called submicelles (about 20 nm). This showed that microlayering of submicelles takes place in the stratifying film and, at a step-transition, a layer leaves the film. The number of steps increased with increasing caseinate concentration. Increase of temperature accelerated the step-transitions. At a lower temperature the drainage was not only slower but the last transition was inhibited and a final film containing a layer of submicelles formed. The stepwise film thinning process could also be stopped at high film thicknesses by decreasing the film size, i.e., by forming smaller drops or bubbles. The stepwise thickness transition results could be explained by the theory of “vacancy” mechanism. It was found that the film and the emulsion are very stable if the drained emulsion film contains layer(s) of submicelles. This new mechanism of microlayering in foam and emulsion films can be important in the stability of food systems because the concentration (0.1–4 wt%) and temperature (25–80°C) ranges where the phenomenon was observed are similar to those found in practical applications.