Kinetics of disproportionation of air bubbles beneath a planar air-water interface stabilized by food proteins.

The rate of shrinkage of air bubbles of initial radii, r, from 50 to 150 microm injected beneath a planar air-water interface has been measured. Bubbles were stabilized by 0.05 wt% protein in approximately 0.1 mol dm(-3) ionic strength buffer at pH 7.0 and at room temperature. Four proteins were studied: commercial whey protein isolate (WPI), sodium caseinate, gelatin, and pure beta-lactoglobulin. Bubbles in all systems showed shrinkage due to diffusion of gas from the bubbles, which accelerated as the bubbles got smaller. Within approximately 1 h all bubbles had disappeared, having shrunk to below approximately 1 microm, so that in no cases was there evidence of stabilization via a surface rheological mechanism. The rates of shrinkage with the different proteins were not significantly different except in the case of gelatin, which at any given bubble size appeared to give a slightly higher rate, probably because the surface tension is higher for this system. A new theoretical analysis of the dissolution kinetics for the case of a bubble close to a planar interface has been developed. For caseinate and WPI a simple model incorporating a constant surface tension and a constant bubble-interface separation appears to account for the kinetics. Interestingly, the model predicts a linear dependence of r(n) versus time when n is closest to 3, in contrast to n = 2 expected from previous work. For gelatin and pure beta-lactoglobulin, the introduction of modest dilatational elasticities of approximately 2.3 and 7 mN m(-1), respectively, gives good agreement between theory and experiment. This is particularly the case for beta-lactoglobulin, where there is a noticeable slowing, but not cessation, of the shrinkage as the bubbles get smaller. In the light of these findings the practical significance of surface rheology with respect to stability to disproportionation is discussed. Finally, we present experimental evidence that a bubble stabilized by beta-lactoglobulin shrinks to a nonspherical protein particle consisting of the completely collapsed protein film.