Visualisation of foam microstructure when subject to pressure change

Abstract

We report the development of a methodology to study the static and dynamic effects of pressure on the stability of air bubbles to coalescence in process and formulation regimes that are relevant to much of the food industry. In this initial work a model food foam was studied using a combination of a Linkam pressure cell and direct in situ imaging of the foam by confocal scanning laser microscopy (CSLM). Observations were made during pressure changes from 11 bar absolute to 1 bar, from 11 to 6 bar, and from 6 to 1 bar and the resulting changes in the foam microstructure were captured. Analysis of the digital images obtained was then used to quantify the effects of pressure on bubble size distribution. The results obtained show that the foam under study was relatively stable under quiescent conditions but could de-stabilise when subject to a pressure change similar to that of a typical industrial process. The larger the pressure change that was applied to the foam, the more coalescence resulted. However, most of the coalescence events were noted to take place over a small pressure window. The resulting change in the air bubble size distribution through pressure changes must be directly related to the nature of the air/water surface and how this responds to a change in surface area. It is also likely to relate to the solubility of the gas in the continuous phase as a function of dynamic change.