Partial coalescence

Partial Coalescence Partial coalescence occurs when two or more partially crystalline oil droplets come into contact and form an irregularly shaped aggregate. It is initiated when a solid fat crystal from one droplet penetrates the interface to the liquid phase of a second oil droplet (17, 28-31). Consequently,... 

Conclusion. Whereas O-W emulsions with small globules (say < 5 pm) that are covered with protein are very stable to coalescence, partial coalescence may readily occur if the oil in the globules becomes partly crystalline. The phenomenon is a good example of the complexity of stability problems that can be encountered in food systems. There are so many variables that it is generally not possible to predict quantitatively the rate of partial coalescence. Nevertheless, it is also a good example in that it shows how systematic research, making use of the fundamentals of colloid and surface science, can lead to the unraveling of such a complex problem. 

Partial coalescence. Partial coalescence occurs when two or more partially crystalline oil droplets come into contact and form an irregularly shaped aggregate [ 11 ]. It is initiated when a fat crystal from one partially crystalline droplet penetrates into the liquid portion of another partially crystallize droplet. Consequently, the lipid crystal is surrounded by lipid molecules instead of water molecules which is thermodynamically favored, that is the fat crystal is better wetted by liquid oil rather than water. Over time the droplets may continue to merge to further reduce the surface area of Hpid that is exposed to water. Nevertheless, the aggregates partly retain the shape of droplets from which they were formed due to the low mobility of molecules in fat crystal networks. 

Figure B2.4.1. Proton NMR spectra of the -dimethyl groups in 3-dimethylamino-7-methyl-l,2,4-benzotriazine, as a fiinction of temperature. Because of partial double-bond character, there is restricted rotation about the bond between the dunethylammo group and the ring. As the temperature is raised, the rate of rotation around the bond increases and the NMR signals of the two methyl groups broaden and coalesce.

The tendency is greatest, however, where pressures are close to atmospheric and "superheat" relative to atmosphere is least. Pipestill atmospheric towers and cat unit fractionators tend to fall in this category. Some operators consider that the likelihood is great that calculated condensation (dew) will coalesce to droplets which will gravitate (rain) when the partial pressure of condensibles at the dew point exceeds 1/3 atmosphere. With this factor and environmental protection in mind, some plants have diverted such releases into closed systems. Generally, however, this has not been of sufficient concern, and such releases have been treated as though they were all vapor. 

Separation of two liquid phases, immiscible or partially miscible liquids, is a common requirement in the process industries. For example, in the unit operation of liquid-liquid extraction the liquid contacting step must be followed by a separation stage (Chapter 11, Section 11.16). It is also frequently necessary to separate small quantities of entrained water from process streams. The simplest form of equipment used to separate liquid phases is the gravity settling tank, the decanter. Various proprietary equipment is also used to promote coalescence and improve separation in difficult systems, or where emulsions are likely to form. Centrifugal separators are also used. 

An attempt has been made by Tsouris and Tavlarides[5611 to improve previous models for breakup and coalescence of droplets in turbulent dispersions based on existing frameworks and recent advances. In both the breakup and coalescence models, two-step mecha-nisms were considered. A droplet breakup function was introduced as a product of droplet-eddy collision frequency and breakup efficiency that reflect the energetics of turbulent liquid-liquid dispersions. Similarly, a coalescencefunction was defined as a product of droplet-droplet collision frequency and coalescence efficiency. The existing coalescence efficiency model was modified to account for the effects of film drainage on droplets with partially mobile interfaces. A probability density function for secondary droplets was also proposed on the basis of the energy requirements for the formation of secondary droplets. These models eliminated several inconsistencies in previous studies, and are applicable to dense dispersions. 

Several variants of the CD model exist wherein only partial coalescence occurs. The final particle compositions are then only partially mixed, and thus will not be identical. 

A typical characteristic of many food products is that these are multi-phase products. The arrangement of the different phases leads to a microstructure that determines the properties of the product. Mayonnaise, for example, is an emulsion of about 80% oil in water, stabilized by egg yolk protein. The size of the oil droplets determines the rheology of the mayonnaise, and hence, the mouthfeel and the consumer liking. Ice cream is a product that consists of four phases. Figure 1 shows this structure schematically. Air bubbles are dispersed in a water matrix containing sugar molecules and ice crystals. The air bubbles are stabilized by partial coalesced fat droplets. The mouthfeel of ice cream is determined by a combination of the air bubble size, the fat droplet size and the ice crystal size. 

Altering the Ratio of Solvents Coalescence of an emulsion may also be achieved either by altering the ratio of the prevailing dispersed phase or even by partial evaporation of the solvent,... 

Partial Coalescence in Emulsions Comprising Partially Crystallized Droplets 167... 

Figure 5.21. Possible scenarios for partial coalescence, (a) Crystallization induced by contact between soUd particle and undercooled droplet, (b) Partial coalescence between two semiUquid droplets.

K. Boode and P. Walstra Partial Coalescence in Oil-in-Water Emulsions 1 Nature of the Aggregation. Colloids and Surfaces A Physicochem. Eng. Aspects 81, 121 (1993). 

H. D. Goff Instability and Partial Coalescence in Whippable Dairy Emulsions. J. Dairy Sci. 80, 2620 (1997). 

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