Introduction

What is Microencapsulation ?

Application of Microencapsulation

Hot melt coating

Cyclodextrins

Extrusion

Emulsion Stabilisation

Jet break-up Processes

Super Critical Fluid technology

Complex Coacervation

Introduction

Preparation of protein- and polysaccharide-based microcapsules and microspheres using the “emulsion crosslinking” or “emulsion stabilisation” technology is very frequent. This method can be used for the encapsulation of soluble or insoluble liquids, or solid agents to be released by diffusion, erosion or dissolution.

Technology

Emulsion stabilisation is a technique based on single or double emulsions. Since many biopolymers are generally water-soluble, two systems can be distinguished when biopolymers are used as wall material :

  Hydrophilic active ingredients :

Single water-in-oil (W/O)-emulsions are employed if the active ingredient is water-soluble. In this case an aqueous biopolymer solution containing the active ingredient is emulsified in a hydrophobic phase like vegetable oil or organic solvent (figure 1). When the desired droplet size is obtained, the matrix material is stabilized by crosslinking. Then, the oil phase is removed by washing with solvents like hexane and the particles are isolated. The particles can either be dried to obtain a powder, or used as a slurry.

Figure 1

  Hydrophobic active ingredients :

Double oil-water-oil (O/W/O)-emulsions can be used if the active ingredient is hydrophobic. The active ingredient is first added to an oil phase. This oil phase is then emulsified in the aqueous biopolymer phase to form an O/W-emulsion. Then the O/W-emulsion is added to a hydrophobic phase to form the double O/W/O-emulsion. Common challenge with emulsion stabilisation for hydrophobic ingredients is retention of the core ; often losses occur during the encapsulation process (figure-2).

Figure 2

The main advantages of this technology are the flexibility in controlling the degree of stabilisation and the small particle size that can be obtained. Disadvantages include the costs and effort related to removal of oil phase and the loss of encapsulant during processing.

Moreover, Spray Drying of emulsions is another technology for production of microcapsules with emulsions as a starting point. This technology will be described in a separate technologic chapter (spray drying).

Polymers most commonly used in this technology

Different biopolymers can be used for the preparation of microparticles by emulsion stabilisation. Polysaccharides such as dextran, derivatives of starch and cellulose, chitosan, and different carbohydrate-based hydrocolloids are frequently used. They have the advantage of being abundant, relatively inexpensive, and biodegradable. They possess some functionality for derivatization. An emulsifier is required to emulsify the aqueous phase in the oil phase. Proteins are also abundant, relatively inexpensive, and biodegradable, but moreover they have extremely good emulsifying properties and a lot of functional groups available for modification. In general, proteins are suitable for microsphere preparation if they are (partly) soluble in water. Besides the soluble proteins (e.g. gelatin, albumin casein, and whey protein), also less water-soluble proteins have shown to be suitable for microsphere preparation under certain conditions, including soluble wheat gluten, zein, soy protein, pea protein, and potato protein. Albumin is by far the most investigated natural polymer in microparticle preparation by emulsion stabilisation. The pH of the aqueous solution has a major influence on particle formation when proteins are applied. This might be related to the differences in protein solubility at various pH-values, as proteins are least soluble around the pI.

Application

  Drug delivery systems
  Encapsulation of fragrances for laundry applications
  Encapsulation of ingredients for food applications
  Encapsulation of pesticides