Emulsion droplet spreading at air/water interfaces: mechanisms and relevance to the whipping of cream

In this thesis, the interaction between emulsion droplets and expanding air/water interfaces was investigated.The objective was to deepen our knowledge concerning the physical processes that take place at the expanding air surfaces that form during aeration of emulsions.Emulsions can become aerated as a result of various processing operations, for example, stirring or pouring.Moreover, emulsions may be aerated with the intention of producing an aerated food product such as whipped cream or ice cream.Emulsion droplet/air interaction can have important consequences for emulsion stability.For example, emulsion droplet spreading at the air/water interface can initiate a collective oil spreading mechanism, resulting in the spreading of many oil droplets.This may lead to coalescence of the emulsion droplets.The tendency for an oil droplet to spread at an expanding air/water interface depends on the values of the dynamic interfacial tensions at the air/water, oil/water and oil/air interfaces.This can be expressed in terms of a dynamic spreading coefficient; when the spreading coefficient is positive, oil spreads out of the droplets.Experimental results confirmed that oil indeed only spreads out of emulsion droplets if the dynamic spreading coefficient is positive.The tendency for an emulsion droplet to spread at the air/water interface could be controlled by manipulating the surface expansion rate, the protein type and concentration, and type and concentration of emulsifier in the emulsion.The presence of crystalline fat, although relevant to the stability of emulsions exposed to shear, was not found to influence the spreading behaviour of emulsion droplets at the air/water interface.The results of the emulsion droplet spreading experiments lead to the development of a model that describes the whipping time of cream in terms of the proportion of the air bubble surface for which the spreading coefficient is positive.Experimental results for the whipping of model creams could be well explained by this model.