Food aeration: Effect of the surface-active agent type on bubble deformation and break-up in a viscous Newtonian fluid: From single bubble to process-scale

Two continuous whipping devices, a rotor-stator (RS) and a narrow angular gap unit (NAGU), were used to produce aerated food with a 25% (v/v) gas fraction target. The liquid phase was a Newtonian model-solution containing 2% (w/w) of either whey proteins (WPC), sodium caseinate (SCN), or tween 20 (TW20). Strong differences emerged regarding gas incorporation and bubble size as a function of process parameters: namely, rotation speed and residence time. To improve understanding of the results obtained at pilot-scale, a second investigation consisting in the observation of the deformation and break-up of single gas bubbles has been undertaken using successively a Couette device and an impeller close to NAGU. For proteins, the observation of single bubble deformation and break-up showed that bubble break-up occurred by tip-streaming above a well-defined critical Capillary number Ca_c of 0.27 and 0.5 for SCN and WPC, respectively, whereas no break-up was observed with TW20 even though Ca reached 10. The poor foaming ability obtained with TW20 could be explained by a poor break-up mechanism, promoting coalescence and gas plugs at high shear instead of gas incorporation. Conversely, protein promote tip-streaming as the major break-up mechanism at low shear rate, explaining why rotation speed is not a key process parameter. Differences observed between SCN and WPC can be attributed to diffusion limitation for SCN when a much larger surface area is generated during aeration.

A review of bubble break-up

The coalescence and break-up of bubbles are important steps in many industrial processes. To date, most of the literature has been focussed on the coalescence process which has been studied using high speed cinematographic techniques. However, bubble break-up is equally important and requires further research. This review essentially details the break-up process and initially summarizes the different types of bubble deformation processes which lead to break-up. Break-up is considered in high and low turbulent (pseudo-static) conditions and the effect of fluctuations and shear forces on the break-up is reviewed. Different mechanisms of break-up are discussed including shearing-off, coalescence induced pitching and impact pinching following air entrapment. Also, the influence of bubble size, interfacial stability, and surfactant on break-up are reviewed and a summary of recent experimental techniques presented. Finally, the break-up process which occurs in micro-fluidics is summarized.