Spray Drying
Spray Drying is the most commonly used encapsulation method in the food industry. The process is economical and flexible uses equipment that is readily available, and produces particles of good quality (Heat, 1985;Heat and Reineccius, 1986; and Taylor, 1983). Spray drying is also one of the oldest encapsulation methods, having been used in the 1930s to prepare the first encapsulated flavours using gum acacia as the coating (Blenford, 1986).
Although most often considered a dehydration process, spray drying, can be used as an encapsulation process when it entraps 'active' material within a protective matrix formed from a polymer or melt.
The process involves three basic steps: preparation of a dispersion or emulsion to be processed, homogenisation of the dispersion; and finally, atomisation of the mass into the drying chamber.
The material to be atomised is prepared by dispersing an active material - usually a flavour or oil - into a solution of the coating with which it is immiscible. The coating is generally a food- grade hydrocolloid such as gelatine, vegetable gum, modified starch, dextrin, or nongelling protein (Balassa and Fanger, 1971). Following the addition of an emulsifier, the heated dispersion is homogenised to give an oil-in-water type of emulsion (Brenner, 1983), then atomised into a heated air stream supplied to the drying chamber (Sparks, 1981). As the atomised particles fall through the gaseous medium, they assume a spherical shape with the oil encased in the aqueous phase. This explains why most spray-dried particles are water- soluble. The rapid evaporation of water from the coating during its solidification keeps the core temperature below 100 ºC, in spite of the high temperatures used in the process (Brenner, 1983) The particles exposure to heat is in the range of a few seconds at most (Balassa and Fanger, 1971). This temperature increase required to remove a large volume of water (40%) from the solution may cause the loss of highly volatile or heat-sensitive flavour substances.
Spray dried ingredients typically have a very small particle size (generally less than 100 m) which makes them highly soluble but may present separation problems in dry blends. Separation can be prevented and fluidity improved by a separate agglomeration step, in which the encapsulated particles are treated with steam to induce their cohesion and form larger particles. Another problem associated with spray drying is that core material may adhere to the surface of the capsule, which presents potential for oxidation and changes in the flavour balance of the finished food product.
Water-soluble materials may also be encapsulated. However, instead of heaving a clearly defined core and coating as described above, the product consists of a homogeneously blended matrix of the polymer entrapping the core. These products are, sometimes, described as 'matrix particles' or 'entrapped ingredients'. They are also recommended to cover with a very fine film of coating.
A modification of the spray drying process that is suitable for encapsulating highly volatile or thermolabile substances is called 'cold dehydration process'. Here, an emulsion of core material in coating material solution is sprayed into a dehydrating liquid such as ethanol or polyglycols at room temperature or below. The microcapsules are recovered by filtration and vacuum dried at a low temperature.