ABSTRACT
Crystallization in emulsions is an increasingly important area both technically and scientifically. Agrochemicals, pharmaceuticals [1], ceramic manufacture, food, cosmetics [2], speciality chemicals, photographic emulsions are a few examples of industries in which emulsion crystallization is widely employed or in develop ment. Ceramics manufacture may seem an unlikely candidate for the use of emul sions, but emulsion crystallization offers more uniform stochiochemistry, smaller ceramic particle size, and a superior fired ceramic [3,4]. Crystallization of ceram ics is but one example of a new approach to materials processing based on emul sions [1,5-11]. In foods, emulsion crystallization was discovered accidentally, first as part of the butter-churning process [12] and then in margarine manufac ture. Margarine and fatty spreads are oil-in-water emulsions in which a crystal network stabilizes and structures what is basically a liquid, imparting upon it solid properties. Margarine manufacture is a little more complicated than simple emulsion crystallization [13]. While nucleation is initiated in the dispersed oil phase of an oil-in-water emulsion, the emulsion is inverted under shear during the crystallization process so that crystallization completes when the oil forms the continuous phase. The result is a kinetically stable water-in-oil emulsion, which would otherwise be a thermodynamically stable oil-in-water emulsion. This has the interesting property of inverting back to a water continuous emulsion when the crystal network melts in the mouth. In ice cream, the role of crystalliza tion in stabilizing and structuring the product is even more complex [14,15]. In pharmaceuticals [1] and agrochemicals, emulsions permit the delivery of expen sive and possibly hazardous chemicals in a water continuous system. This makes it much easier to control dosage; it also reduces hazards through concentration
reduction and the presence of a water barrier. Normally the drug or chemical is dissolved in the dispersed oil phase. Crystallization may be used to structure the emulsion, imparting upon it the properties of a soft solid so that it adheres to surfaces. Fat crystals play an important role in structuring emulsions [14-18].
