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Physical Microencapsulation Technologies

Spray Coating       Centrifugal Extrusion       Spinning Disk       Annular Jet      Spray Drying       Prilling     Extrusion

 

Spray Coating

Coating technologies have been used for many years to prepare pharmaceutical capsules in the traditional macro scale. The particles are tumbled in a pan or other device while the coating material is applied producing capsules we have all used at one time or another. Building on this technique Air-Suspension Coating/Fluidised Bed Coating (a more specific version is called Wurster Coating) of particles by solutions or melts gives better control and flexibility for microcapsule production. Particles of the active ingredient, spheres or granules are coated while suspended in an upward-moving air stream. They are supported by a perforated plate having different patterns of holes inside and outside a cylindrical insert. Just sufficient air is permitted to rise through the outer annular space to fluidize the settling particles. Most of the rising air (usually heated) flows inside the cylinder, causing the particles to rise rapidly. At the top, as the air stream diverges and slows, they settle back onto the outer bed and move downward to repeat the cycle. The particles pass through the inner cylinder many times in a few minutes.

In the food industry the Wurster process has been used to encapsulate/coat vitamins, minerals and functional food ingredients for example to mask an undesirable flavour or improve stability and shelf life. The coating possibilities are relatively unlimited for example it is possible to coat a hydrophilic active ingredient with a hydrophobic polymer, or a hydrophobic active ingredient with a hydrophilic wall material at various thicknesses for process optimisation. With irregular shaped crystals  it is likely that the coated particles would also be irregular in shape, although trials would have to be completed to be certain. Particles can be prepared with potential diameters of less than 50microns upwards.

 
 
Centrifugal Extrusion

The centrifugal extrusion process is a liquid co-extrusion process utilising nozzles consisting of concentric orifices located on the outer circumference of a rotating cylinder. A liquid core material is pumped through the inner orifice and a liquid wall material through the outer orifice forming a co-extruded rod of core material surrounded by the wall material. As the device rotates, the extruded rod breaks into droplets which form capsules. The rotational speed affects the capsule size which can be a little as 150microns in diameter and the active ingredients can be encapsulated to up to 80% per weight. Typical wall materials include gelatin, alginate, carageenan, starch, cellulose derivatives, gum arabic, fats and waxes or polyethylene glycol. Flavour oils for example are easily encapsulated using this methodology.
 
 
Spinning Disk

Similarly to centrifugal extrusion this spinning disk system uses rotational forces to create droplets. Typically the active ingredient is suspended in a wall material and dropped onto the rotating disk which throws the droplets out towards the circumference of the disk where the wall material solidifies through drying or chilling. As the spinning rate can be carefully controlled the disk process is able to yield narrow particle size distributions, and produce matrix particles of between 5 and 3000 microns.

 

 
 
Annular Jet

The technique involves two concentric jets, the inner containing the active ingredient and the outer generally molten wall material which solidifies when exiting the jet. The duel fluid stream naturally breaks into droplets which form the basis of the microcapsule, which depending on configuration can be matrix particles or core-shell in formation. Later adaptations have added a vibrational nozzle to help control the droplet size giving a more uniform product with lower microcapsule sizes down to sub-micron diameters. The liquid can consist of any liquids with limited viscosities e.g. solutions, emulsions, suspensions, melts etc. The soldification can be initiated by any common gelation system (e.g. sol-gel processing, melt) or by using an external additional binder system, (e.g. in a slurry).

 

 
 
Spray Drying

Traditionally the most common method of microencapsulation of food ingredients. To prepare materials for spray drying the wall material (e.g. maltodextrin, modified starch, gum or a combination of these is hydrated. The flavour or ingredient to be encapsulated is added to the carrier and homogenised or thoroughly mixed using a equivalent technique to create small droplets. Typically the ratio of carrier to flavour would be 4:1, however in some applications higher flavour loads can be used. The mixture is fed into the spray dryer where it is atomised through a nozzle or spinning wheel. Hot air flowing in either a co-current or counter current direction contacts the atomised droplets and evaporates the water producing a matrix particle containing small droplets of flavour. The dried particles are generally collected in a separate chamber through a cyclonic air stream.

 

There are many variants of the technique utilising a wide range of wall materials. Spray drying is carried out internally in virtually all major and medium size flavour companies where the encapsulated flavours, spices etc are either sold as spray dried individual active ingredients or as part of a formulated blend.

 
 
Prilling

Prilling sometimes called spray chilling, spray cooling or spray congealing is similar to spray drying in that the active ingredient is dispersed in a liquefied coating or wall material and atomised. However in these cases the mixture is atomised into cool or chilled air which solidifies the wall material around the active ingredient. In some cases two fluid nozzles can be used. Microcapsules or matrix particles can be produced generally from 10-400microns in diameter utilising a wide range of wall materials including waxes, hydrogenated vegetable oils and polymers. This technology has been applied to agrochemicals and drugs but can also be used for flavours and fragrances. As the melting point of the wall material can be carefully selected it is possible to develop control release formulations for a given active ingredient.

 

Prilling systems are capable of high throughput for major industrial application and contract manufacture capability is fairly widespread.

 
 
Extrusion

Encapsulation by extrusion involves dispersion of the active ingredient in a molten wall material often a carbohydrate such as starch or maltodextrins. The process can also be called melt-extrusion, glass encapsulation, melt-encapsulation or the Durarome process. The mixture is forced through a die producing a solid filament as the material cools in air or by falling into a dehydrating liquid such as isopropyl alcohol. The filaments are then milled and sieved to the desired particle size. Generally the product contains 8-20% flavour load, although the capsules size are in general fairly large (~500microns)

A typical commercially available product range utilises sugar as a wall material and with careful selection of the glass transition temperature can produce glassy matrix particles stable at room temperature which protect the flavours from oxidation. As the coating is sugar based they are particularly suited to sweet applications.

 
 
 
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