Colloids and Surfaces A: Physicochemical and Engineering Aspects
Aeration of emulsions by whipping
Introduction
Aerated food emulsions form an important class of food products. Examples are whipped cream, aerosol cream and ice cream. Macroscopically, aerated emulsions have a white opaque appearance and a relatively low density due to the high inclusion of gas bubbles in the system (expressed by the overrun, O, which is the relative increase of volume by the inclusion of gas). An important characteristic of these systems is their plastic behavior, due to the presence of a considerable yield stress. Ideally, the systems retain their yield stress during prolonged storage, and loss of overrun and serum leakage remain minimal. The integrity of these systems is mainly due to interactions between the main structural units, which are gas bubbles, emulsion droplets and in the case of ice cream, also ice crystals [1], [2], [3]. The emulsion droplets usually contain partially crystallized fat, and for this reason are called fat globules.
Many studies have demonstrated the importance of partial coalescence of fat globules for rendering stiffness to whipped emulsions [4], [5], [6]. In this process, the presence of fat crystals obstructs the complete merging of the fat globules into a single spherical shape. This leads to a clustering into stiffened structures in which the original fat globules remain recognizable. The sensitivity to partial coalescence of the fat globules depends on the solid fat content and the orientation of the crystals with respect to the droplet surface. These can be altered by adding certain surfactants and by subjecting the emulsion to a tempering treatment (partial melting of the crystal mass, followed by recrystallization) [7], [8].
Due to the optical opacity of aerated emulsions, it is difficult to determine the average size and shape of the gas bubbles. Electron microscopical studies have shown that during whipping spherical gas bubbles are formed, with number-averaged bubble diameters slowly decreasing to approximately 50 μm for normal whipping cream and 20 μm for homogenized whipping cream [9]. Air bubbles in ice cream usually range between 50 and 100 μm [10].
This work focuses on the high packing density of the structural components in aerated emulsions. In Table 1 a few examples of aerated emulsions are listed with estimates for the volume fractions of dispersed components, i.e. gas bubbles, ice crystals and fat globules. The sum of volume fractions of dispersed components exceeds the constraint of close packing for a monodisperse hard sphere dispersion, φ*=0.74. This has consequences for the physical behavior of the aerated emulsion. Even in the absence of an aggregated network of fat globules, the dense packing of structure elements will cause a significant increase of the viscosity and shear rate thinning behavior [11]. The packing constraints of the system also have large implications for the maximum attainable overrun, O(max). These aspects have been given little attention in the literature and their importance will be highlighted in this paper.
The organization of this paper is as follows. Firstly, an experimental study of the whipping process conducted on natural cream will be reported. It will be shown that the overrun of fully whipped cream strongly depends on the fat content of the cream. Special attention will be given to the position of the fat globules with respect to the bubble surface and to their degree of aggregation. Secondly, theories for the mechanisms underlying the whipping process will be derived in the theoretical section. Two important physical aspects will be highlighted. These are the packing constraints, and the dynamics of the formation, break-up and coalescence of the gas bubbles.
Thirdly, the experimental results will be discussed on the basis of the theoretical models for volume-constraints and the rheological processes during whipping.
Section snippets
Materials and methods
Creams were prepared by centrifugation from fresh milk at the pilot plant of NIZO food research, Ede, The Netherlands. Cream with a fat content of 30.7% (w/w) was sterilized (5 s, 140°C), stored one day at 4°C to initiate fat crystallization, and stored for 2 h at 10°C before whipping at this temperature. For the preparation of creams with varying fat content, fresh milk was separated into high-fat cream (fat content 42.4% (w/w)) and skimmed milk by centrifugation. The high-fat cream was
Packing constraints
The following assumptions were in the derivation of the model for the packing conditions in an aerated emulsion:
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The air bubbles are spherical and all of the same size, with radius Rg.
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The fat globules are spherical and all of the same size, with radius Re. The fat globules are assumed to be unaggregated in the continuous liquid, which is a reasonable assumption in the early stages of whipping, but certainly is not true at the endpoint of whipping.
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A layer of emulsion droplets is attached to the
Discussion
In this section, the whipping process will be discussed on the basis of the experimental observations and the theory of bubble break-up and coalescence. This will be used to explain the relation between the overrun and fat content of Fig. 1. To do so, firstly the mechanisms taking place during each of the three stages of whipping of cream with high fat content will be treated in detail. After that the deficient whipping properties at low and intermediate fat contents will be explained. Finally,
Conclusions
A model was developed that describes the physical constraints related to close packing of gas bubbles and emulsion droplets and a minimum concentration of free emulsion droplets needed to stabilize the gas bubbles during whipping. This model was used in combination with existing theories of bubble break-up and coalescence in laminar and turbulent flow to explain the process of whipping of natural cream and, in relation to this, the dependency between the overrun and fat content.
Three stages
Acknowledgements
Franklin Zoet, Jan de Wit and Angèle Jochems are acknowledged for carrying out the experimental work. Ton van Vliet and Pieter Walstra are acknowledged for critically reading the manuscript and making useful suggestions. This project was partially supported by The Netherlands Ministry of Agriculture, Nature Management and Fisheries.
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