Air suspension coating

Air suspension coating, also termed fluidised bed processing or spray coating, is accomplished by suspending solid particles of core material in an upward-moving stream of air, which may be heated or cooled. The coating, which may be in a molten state or dissolved in an evaporable solvent, may be selected from among cellulose derivatives, dextrins, emulsifiers, lipids, protein derivatives, and starch derivatives. The coating is atomised through nozzles into the chamber and deposits as a thin layer on the surface of suspended particles. The turbulence of the column of air is sufficient to maintain suspension of the coated particles, allowing them to tumble and thereby become uniformly coated. Upon reaching the top of the air steam, the particles move into the outer, downward-moving column of air, which returns them to the fluidised bed with their coating nearly, dried and hardened. The particles pass through the coating cycle many times per minute (Sparks, 1981). With each successive pass, the random orientation of the particles further ensures their uniform coating. According to Percel (1988) the process typically takes from 2 to 12 hours to complete and achieves exceptionally good coverage, leaving only about 0.2-1.5% of the particles uncoated. In addition, air suspension coating can be used with core particles ranging from 50 to 500 m.

Extrusion, as a low temperature encapsulation method, involves forcing a core material dispersed in a molten carbohydrate mass through a series of dies into a bath of the hydrating liquid. Upon contacting the liquid, the coating material, which forms the encapsulating matrix, hardens to entrap the core material. The extruded filaments are separated from the fluid bath, dried to mitigate hygroscopicity (an anticraking agent such as calcium triphosphate can facilitate this), and sized.

The work that originally led to the extrusion/encapsulation process was done by Schultz et al. (1956) of the United States Dept. of Agriculture Albany (California) Laboratory and involved 'locking' orange oil in an amorphous carbohydrate mass.

Most of the research developments made to date concern the composition of the material that forms the encapsulating matrix. For example, the dextrose equivalent (D.E.) of corn syrup solids was found to be important in reducing the product's hygroscopicity (Crocker and Pritchett, 1978). Other parameters important in developing an 'improved' encapsulated flavour - i.e., one that contains 12-16% flavour compared to the current commercial levels of 8-8.5% - include the emulsifier content, the flavour oil content, and the emulsification pressure.

The extrusion process is particularly useful for heat-labile substances and has been used to encapsulate flavours, vitamin C, and colours. Encapsulated flavour are soluble in cold or hot water, making them suitable for use in a variety of dry food applications such as drink mixes, cake mixes, gelatine dessert mixes, and cocktail mixes. The water phase of natural essence is also encapsulated by this process and is commercially marketed. About 100 different flavours have been encapsulated by extrusion. Furthermore, because the alcohol removes all traces of core material from the product's surface, the encapsulated flavour exhibits good shelf live, often exceeding two years.

Spray cooling and Spray chilling are two encapsulation processes that are similar to spray drying in that both involve dispersing the core material into a liquefied coating and spraying through heated nozzles into a controlled environment (Bakan and Anderson, 1978). The principal differences between this processes and spray drying, however, lie firstly, in the temperature of the air used in the drying chamber and secondly, in the type of coating applied. Spray drying uses heated air to volatilise the solvent from a coating dispersion; in contrast, spray cooling and spray chilling use air cooled to ambient or refrigerated temperatures considerably below the solidification point of a molten fat or wax coating.

In spray cooling, the coating substance is typically some form or derivative of vegetable oil, but a wide variety of other encapsulating materials may be used. These include, e.g., fat and stearine with melting points of 45 to 122 ºC as well as hard mono- and diglycerides with melting points of 45 to 67 ºC. Taylor (1983) indicates that the mono- and diglycerides facilitate dispersion of the encapsulate in the finished, reconstituted food product and may also be considered part of the overall emulsification system.

In spray chilling, the encapsulating material is selected from a range of fractionated vegetable oils or hydrogenated vegetable oils with comparatively lower melting points, 32 to 42 ºC. Coatings with even lower melting points may be used; however, their end products may require specialised handling and storage conditions (Taylor, 1983).

Spray chilling is used primarily for the encapsulation of solid food additives such as ferrous sulphate, acidulants, vitamins, and solid flavours, as well as for sensitive materials or those that are not soluble in 'normal' solvents (Taylor, 1983). Liquids may also be encapsulated following their conversion to a solid form, perhaps by freezing. The end products of the process, resembling fine beadlets of a large particle size, are water insoluble but release their contents around the melting point of the wall material. Because of the controlled release properties of the particles, the process is suitable for protecting many water-soluble materials such as spray dried flavours which may otherwise be volatilised from a product during thermal processing. Spray chilled products have applications in bakery products, dry soup mixes, and foods containing high level of fat (Blendford, 1986).