Coacervation

Coacervation is a colloidal chemical phenomenon first used as a microencapsulation technique in the early 1940s by Green of the National Cash Register Corp. It is considered by many to be the first true microencapsulation process.

Simple coacervation involves dispersing a colloid, eg, gelatin, in an appropriate solvent, eg, water. A core material, eg, a hydrophobic citrus oil, is dispersed in the mixture with agitation. By one or more method, such as the addition of sodium sulfate and lowering the temperature, the solubility of gelatin in water is reduced, creating a two-phase system. The colloid-rich phase appears as an amorphous cloud in the colloid-poor, more aqueous phase. If left to stand, the minute droplets would coalesce forming a separate liquid layer. However, under proper conditions (and most literature refers to coacervation as an art as well as a science), the coalescence of the polymeric colloid occurs around the suspended core particles of citrus oils, creating small, still unstable microcapsules. Final steps in the process include adding a suitable cross-linking, or hardening, agent such as glutaraldehyde; adjusting the pH; and subsequent collecting, washing, and drying of the now-stable citrus oil encapsulate.

Coacervation as described is also referred to as aqueous-phase separation, or oil-in-water encapsulation. Complex coacervation is possible only at pH values below the isoelectric point of gelatin. It is at these pH values that gelatin becomes positively charged, but gum arabic continues to be negatively charged. A typical complex coacervation process begins with the suspension, or emulsification, of core material in either gelatin or gum arabic solution. Then the gelatin or gum arabic solution (whichever was not used to suspend the core material) is added to the system while mixing continuously. The pH is adjusted to 3.84.3, the system is cooled to 5°C (41°F) and the gelled coacervate capsule walls are insolubilized with glutaraldehyde, or another hardening agent. Microcapsules are collected, washed, and dried.

It is possible to microencapsulate hydrophilic core material in oil-soluble coatings. A polar core is dispersed in an organic, nonpolar solvent at an elevated temperature. The coating material is then dissolved in the solvent. Encapsulation is achieved by lowering the temperature and allowing the polymeric coating material to emerge as a separate coacervate phase microencapsulating the core particles. The coating gradually solidifies and remains insoluble in cold solvent. This process is called either water-in-oil microencapsulation, or organic-phase separation. It is used typically in the pharmaceutical industry for encapsulation with ethylcellulose. However, ethylcellulose does not have general approval for use in the food industry.

Coacervation is an efficient, but expensive, microencapsulation technique. When small particle sizes are required it is probably the only process that can produce submicron-size particles. With typical payloads in the range of 8595%, it might be expected that the process would be economical to allow for its application in the food industry for myriad ingredients, such is not the case. Aside from a few specialized flavor applications there are no current uses for encapsulated food ingredients by the coacervation process; costs are much too high (2-4).

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