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. ScopeAeration, or the incorporation of air bubbles, is one of the fastest growing processing operations in the food industry [1].A wide variety of aerated food products are available; a few examples include whipped cream, ice cream, mousse, meringue, souffl, bread, cake, carbonated soft drinks, beer, champagne and cappuccino foam.In this thesis we will concentrate on one class of aerated food system: the aerated emulsion.In aerated emulsions such as whipped cream, ice cream and milkshakes, a three-phase partially crystallised fat-in-water emulsion is transformed into a four-phase system as air bubbles are incorporated.This thesis is focussed on the role of the interaction between emulsion (fat) droplets and air bubbles during the aeration of emulsions.We pay special attention to the role of emulsion droplet/air interaction in the development of structure in whipped cream.However, in a wider frame of reference, emulsion droplet/air interaction may be an important parameter controlling emulsion stability during any processing operation where emulsion droplets come into contact with air such as stirring, pouring or mastication.In the present chapter, we begin by describing the main processes that take place during the aeration of cream by whipping (section 1.2).Since the interaction between emulsion droplets and the air bubble surface is one of the main steps in the development of whipped cream structure, we go on to describe the phenomenon of droplet entering and spreading at the air/water interface (section 1.3).Finally, the aim (section 1.4) and outline (section 1.5) of this thesis are presented. Aerating cream by whippingIn whipped cream, air bubbles are incorporated into the cream by mechanically beating air into the system.This can be achieved, for example, by using a hand mixer.The processes that take place during the whipping of cream have been extensively studied, and a number of reviews can be found in the literature [2][3][4][5].During whipping, foam containing large air bubbles stabilised by adsorbed proteins is initially formed [2,6].As whipping continues, the air bubbles become smaller and fat globules (emulsion droplets) adsorb to the air bubble surface [7][8][9][10].The adsorbed fat globules coexist with adsorbed protein at the air bubble surface [6,8,10].Further,