Ultrasonic flocculated emulsions

The attenuation coefficient in the flocculated emulsion is lower at low frequencies and higher at high frequencies than that of the nonflocculated emulsions. The decrease in attenuation at low frequencies on flocculation as a result of the thermal overlap effects mentioned earlier, whereas the increase at high frequencies results from increased scattering of ultrasound by the floes. The same ultrasonic spectroscopy technique has been used to study the disruption of floes in a shear field (38). As the emulsions are exposed to higher shear rates the floes become disrupted and their attenuation spectra become closer to that of nonflocculated droplets. 

In terms of measuring emulsion microstructure, ultrasonics is complementary to NMRI in that it is sensitive to droplet flocculation [54], which is the aggregation of droplets into clusters, or floes, without the occurrence of droplet fusion, or coalescence, as described earlier. Flocculation is an emulsion destabilization mechanism because it disrupts the uniform dispersion of discrete droplets. Furthermore, flocculation promotes creaming in the emulsion, as large clusters of droplets separate rapidly from the continuous phase, and also promotes coalescence, because droplets inside the clusters are in close contact for long periods of time. Ideally, a full characterization of an emulsion would include NMRI measurements of droplet size distributions, which only depend on the interior dimensions of the droplets and therefore are independent of flocculation, and also ultrasonic spectroscopy, which can characterize flocculation properties. 

D. J. McClements, N. Herrmann, Y. Hemar 1998, (Influence of flocculation on the ultrasonic properties of emulsions theory), J. Phys. D Appl. Phys. 31, 2950. 

Many dairy emulsions destabilize by flocculation and networking. Hib-berd et al, (1997a, b) obtained the ultrasonic response to flocculation showing that floe size increased during the experiment. The experiment was repeated with a higher level of hydroxyethyl cellulose (0.1%, v/v) with the result that flocculation occurred more rapidly with the formation of a densely connected network of particulate material. The residual root-mean-square error associated with fitting the ECAH model to the ultrasonic data at various points in the flocculation reaction increased rapidly at the onset of network formation and could, in principle, be used to detect such phenomena in a process context. 

Hibberd, D.J., Holmes, A.K., Garrood, M., Fillery-Travis, A.J., Robins, M.M., Challis, R.E. 1997a. Ultrasonic characterization of flocculation in oil-in-water emulsions. In Food Colloids—Proteins, Lipids and Polysaccharides (E. Dickinson, B. Bergenstahl, eds.), pp. 137-149, Royal Society of Chemistry, Cambridge. 

Ultrasonic techniques have also been used to study typical oll-ln-water emulsions as such and those undergoing either depletion flocculation or floe disruption. These studies are of industrial interest as flocculation of the droplets within an emulsion may often be the first stage in the deterioration of a product however, it can also be beneficial by Increasing the apparent viscosity of the product via the formation of a continuous network of droplets. 

The colloidal properties of emulsions are responsible for the quality of many foods. Ultrasound is sensitive to most of the properties of interest and can be used as both a research and a process-control tool by food scientists. As a research tool, ultrasonic measurements are particularly powerfid as they can be used to generate information not readily available by other methods - importantly, physical state, particle size, concentration, and flocculation in concentrated and optically opaque emulsions. In a process environment, ultrasonic measurements can be effected noninvasively in process lines and are therefore compatible with the stringent hygiene and cleaning requirements of food production. 

Below we describe the use of the ultrasonic monitor to detect creaming in a polydisperse concentrated emulsion, and to characterise flocculation from the creaming behaviour. The effects of added polymers on the flocculation and creaming processes are also described. 

In the previous section we demonstrated the use of ultrasonic velocity measurements to characterise creaming, and indirectly to characterise flocculation. However, there is more information to be obtained from an emulsion using ultrasonic spectroscopy. This involves measurement of phase velocity and attenuation of ultrasound as a function of frequency after propagation through the emulsion. There are a number of mechanisms by which ultrasound is attenuated by the emulsion, resulting in characteristic ultrasonic properties. Figure 4.15 shows the prineipal mechanisms of absorption. 

Additionally, this theory assumes that the scattering droplets are randomly positioned within the continuous phase and makes no allowance for droplet-droplet interactions. It is thus reasonable to expect deviations between theory and experiment for systems where the particles are flocculated. Recent work by McClements demonstrated that the attenuation and velocity of ultrasonic waves changed significantly when a sufficient concentration of surfactant micelles was added to an emulsion. Visual observations of the emulsion suggested that the ultrasonic changes were caused by the micelles acting as depletion flocculants. 

Ultrasonic Scattering from Emulsions during Depletion Flocculation... 

Figure 4.17 Emulsion of 5% v/v l-bromohexadecane with 0.5% HEC. (a) Final microscopic appearance of the emulsion, showing droplets flocculated in a network, (b) Measured ultrasonic spectra as a function of time...

In the presence of polymers the droplets are frequently flocculated, either by a depletion or bridging mechanism. The ultrasonic creaming data enable the degree of flocculation to be determined, and to distinguish between the behaviour of emulsions flocculated by the two mechanisms. 

Many ofthe technologies listed above can be used only with dilute suspensions. However, there are instances where particle characterization has to be performed in a concentrated phase in which dynamic processes such as aggregation, agglomeration, or flocculation may occur at a much faster rate. In other instances, such as in emulsion systems, dilution just is not feasible because the system may change due to the dilution process. The analysis of such concentrated samples is especially important in on-line processes where particles naturally exist in concentrated states. Since sound waves can travel through concentrated suspensions, ultrasonic analysis provides a mean to characterize particulate systems at concentrations up to 60% hy volume [34]. 

---