Flavours play an important role in the food industry. Their preservation in food is usually of high priority to the food manufacturers. They are expensive to preserve and incorporate into food substances. This is due to the fact that they are volatile and delicate. Different types of methods have been used in the past to preserve them in foods cost effectively.
One of the methods which have achieved the most success is the process of encapsulation. There are different methods used in encapsulation to preserve flavours. The stability of flavours is of high importance and high degree of interest. This is because it is directly related to quality of and acceptability of foods. The processes involved in the manufacture, storage and packaging of food often leads to changes in its overall flavour by reducing the intensity of the aroma compound or producing off-flavour components (Lubbers at al., 1998)
Encapsulation involves the use of technique in which a substance is coated with or entrapped in another material. There are a number of encapsulation methods in use today that is well accepted and developed in the food, pharmaceutical, and cosmetic industries (Heinzen, 2002).
This report critically examines the conventional encapsulation methods used in the preservation of flavours and then compares it to the more recent development where biotechnology has been employed in the encapsulation through the use of microbial cells in forming the encapsulated wall.
Microcapsule system
Encapsulation involves the use of microcapsule system in which the flavour is protected and isolated from the external medium. These microcapsules may consist of a core that is surrounded by a wall of either a uniform or non-uniform thickness.
A few number of encapsulating methods exist (versic, 1998), but a large range of different materials can be used as encapsulating material. Examples of materials that can be used are carbohydrates, lipids, proteins cellulose and gums (Brazel, 1999). These materials used in the wall have their individual advantage and disadvantage. With recent advances in biotechnology, microorganism cells are also now being used for encapsulation. Their use has a peculiar advantage and benefit as they are more resistant to many environmental processes and their constituent organic base which can be digested by enzymes.
CONVENTIONAL METHODS OF ENCAPSULATION
There are basically two main broad processes involved in encapsulation. These are chemical and mechanical processes.
Mechanical process
Under this process, the various methods that can be used are:
Spray-drying
Freeze drying extrusion Fluidized bed
Chemical process
Coacervation
Co-crystallization
Molecular inclusion
Interfacial polymerization
Spray-drying method
This is the most common and cheapest technique used in flavour encapsulation (Deis,1997). The cost of production is relatively low compared to the other conventional methods.
Compared to the freeze drying, its cost is about 30-50 times cheaper (Desobry, Netto and Labuza,1997). This technique gives a high retention of aroma compounds during drying (Teixeira et al.,2004)
It is suitable for many low-boiling point materials because of the lower temperatures that the core materials in them reach (Dziezak, 1988).
There are three steps involved in this encapsulation technique (Dziezak,1988)
a) preparation of the dispersion or emulsion to be processed
b) homogenization of the dispersion and
c) atomization of the mass into a drying chamber
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After this, the resulting microcapsules are then transported to a cyclone separator for recovery. The ability of the encapsulated material to retain flavour and volatile material during encapsulation is due to the physical and chemical properties of the wall and core materials (Bomben et al., 1973).
For wall materials to function optimally in the spray- drying technique, it should be highly
