Experimental characterization of coalescence can be carried out using a variety of experimental techniques, many of which are similar to those used to monitor flocculation (Section 7.4.3). The most direct approach is to observe droplet coalescence using an optical microscope (Mikula 1992). An emulsion is placed on a microscope slide and the change in the droplet size distribution is measured as a function of time, by counting the individual droplets manually or by using a computer with image-processing software. It is possible to observe individual coalescence events using a sufficiently rapid camera, but these events are often so improbable in food emulsions that they are difficult to follow directly (Dickinson 1992, Walstra 1996a).
An alternative microscopic method involves the observation of the coalescence of single emulsion droplets at a planar oil-water interface (Dickinson et al. 1988). An oil droplet is released from a capillary tube into an aqueous phase and moves to the oil-water interface due to gravity (Figure 7.20). The time taken for the droplet to merge with the interface after it has arrived there is measured. The results from this type of experiment demonstrate that there are two stages to droplet coalescence: (1) a lag phase corresponding to film thinning, where the droplet remains at the interface but no coalescence occurs, and (2) a coalescence phase where the membrane spontaneously ruptures and the droplets merge with the bulk liquid. Droplets exhibit a spectrum of coalescence times because membrane rupture is a chance process. Consequently, there is an approximately exponential decrease in the number of uncoalesced droplets remaining at the interface with time after the lag phase. The major disadvantages of this technique are that only droplets above about 1 |im can be observed and coalescence often occurs so slowly that it is impractical to monitor it continuously using a microscope. To detect coalescence over a reasonably short period, it is necessary to have relatively low concentrations of emulsifier at the surfaces of the droplets, which is unrealistic because the droplets in food emulsions are nearly always saturated with emulsifier.
Droplet coalescence can also be monitored by measuring the time dependence of the droplet size distribution using an instrumental particle-sizing technique, such as light scattering, electrical pulse counting, or ultrasonics (Chapter 10). These techniques are fully automated and provide a measurement of the size of a large number of droplets in only a few minutes. Nevertheless, it is important to establish whether the increase in droplet size is due to coalescence, flocculation, or Ostwald ripening. Coalescence can be distinguished from flocculation by measuring the droplet size distribution in an emulsion, then changing the environmental conditions so that any flocs are broken down, and remeasuring the droplet size distribution (Section 7.4.3). If no flocs are present, the average droplet size remains constant, but if there are flocs present, it decreases. Coalescence is more difficult to distinguish from Ostwald ripening because they both involve an increase in the average size of the individual droplets with time.
As mentioned earlier, studies of coalescence can take a considerable time to complete because of the very slow rate of the coalescence process. Coalescence studies can be accelerated by applying a centrifugal force to an emulsion so that the droplets are forced together: the more resistant the membrane is to disruption, the higher the centrifugation force it can tolerate or the longer the time it will last at a particular speed before membrane disruption is observed (Sherman 1995). Alternatively, coalescence can be accelerated by subjecting the emulsions to high shear forces and measuring the shear rate at which coalescence is first observed or the length of time that the emulsion must be sheared at a constant shear rate before coalescence is observed (Dickinson and Williams 1994). Nevertheless, these accelerated coalescence tests may not always give a good indication of the long-term stability of an emulsion. For example, chemical or biochemical changes may occur in an emulsion which is stored for a long period that eventually lead to coalescence, but they may not be detected in an accelerated coalescence test. Alternatively, there may a critical force that is required to cause membrane rupture which is exceeded in a centrifuge or shearing device, but which would never be exceeded under normal storage conditions. As a consequence, these accelerated coalescence tests must be used with caution.
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