Describe the concept of microencapsulation. What methods have been used to encapsulate labile bioactive food ingredients so that they are able to be absorbed in the gut?
Microencapsulation involves completely enveloping ingredients in a protective coating. Microcapsules are defined as being of a size between 0.2-5000 µm (Barbosa-Cánovas et al, 2005, p200). The ingredients within the coating are often labile; that is unstable and liable to change. These changes can be caused physically and chemically; for example by interactions with other chemicals in the food stuff in which they are contained, processes within the human body, when subjected to variations and extremes of the environment such as temperature, moisture, light and pH (e.g. in cooking or storage), mechanical stress, and in the presence of oxygen. The coating protects a foodstuffs labile bioactive materials, which allows the ingredients to be released at a desired time or point in the digestive system rather than being lost during a foods manufacture, preparation, storage, or when undesired in the mouth or stomach. For example, encapsulation of vitamins and minerals increases their stability to environmental conditions, prevents reactions with other ingredients and can reduce their off-flavours (Rahman, 2007, p548). Probiotic bacteria can be protected until they reach the gut where they can colonise and aid digestion. Other common materials that are encapsulated include flavours, essential oils, oleoresins, acids, alkalis, buffers, artificial sweeteners, preservatives, anti-oxidants, cross-linking agents, leavening agents, colourants and enzymes (Barbosa-Cánovas et al, 2005, p199). Flavours are the most commonly encapsulated ingredients.
This assignment will focus on the most common methods used to microencapsulate bioactive foodstuffs. Bioactive food ingredients include vitamins, minerals, enzymes, bacteria, essential oils and essential amino acids.
The process of encapsulating of ingredients has led to the development of materials that were not previously of any practical use in the food industry. As well as the food industry, microencapsulation is commonly used in medical applications to ensure drugs can be released in the correct part of the body at the correct time.
The protected materials with the capsule are called the core, internal phase, active material or fill (Gharsallaoui, 2009). The encapsulated core can be a solid, liquid or gas. The coating is called the wall, shell, carrier, encapsulant or membrane (Gharsallaoui, 2009). Microcapsules can take various forms such as multi-walled, spherical, irregular shaped and have multiple cores.
In the food industry the wall must be created from a food grade, non-toxic microencapsulating agent. Common materials used include Carbohydrates e.g. maltodextrin, modified maize starch, other starches and derivatives; Gums e.g. acacia gum, agar; Lipids e.g. fats, beeswax; Proteins e.g. gelatin, caesin; Celluloses, and Inorganic material e.g. clay, calcium sulphate; or any combination of these (Barbosa-Cánovas et al, 2005, p201). These materials readily form large polymer structures and are all relatively inert, which ensures they do not alter the core material. The wall must remain intact and continuous until the capsules reach the appropriate area or stage of digestion in the body. The material must not be completely indigestible as it must degrade and release the core. For example, if desired, it should breakdown and release the core material in the acidic conditions of the stomach. Degradation of the coating occurs by enzymic action, hydrolysis, slow disintegration, solvent action and other chemical reactions (Ref 7).
The chemical and physical properties of the wall material are important when considering the available manufacturing processes of microcapsules. The various methods of encapsulation described later require different properties from the coating material used. The density, crystallinity, orientation, solubility, (Barbosa-Cánovas et al, 2005), viscosity and melting-point are all important considerations when choosing a wall material.
The wall often has a neutral taste as to not affect the general taste of the food within. However, if coating a particularly unpleasant and volatile flavour the coating may be flavoured. The colour, texture and size of the capsule coating is also considered when looking at its intended final application (Kwak et al, 2003). Both beneficial and negative effects of these properties can affect the food product.
There are a large number of physical and chemical methods of encapsulation. These include: spray drying, spray cooling, spray chilling, emulsion forming, air suspension coating, extrusion, centrifugal extrusion, pan coating, freeze-drying, complex coacervation, rotational suspension separation, co-crystallisation, liposome entrapment and molecular inclusion. Spray drying and extrusion are the two major industrial processes (Madene et al, 2006). This assignment looks at methods used for encapsulating bioactive ingredients.
Extrusion
This is a continuous process in which the core material is not strictly encapsulated, but is trapped in a polymer with a continuous wall (Barbosa-Cánovas et al, 2005). An extruder is a mechanical mixer that consists of at least one screw in a barrel which can be tailored to the amount and type of shear needed (Ref 2). The core material is dissolved or suspended in a wall solution, forming an emulsion or dispersion. In all the methods described in this assignment where emulsions and dispersions are made, agents are often added to stabilise the emulsions and the solution homogenised to ensure an even distribution of particles/droplets. The resulting solution is heated to a molten state and added to the extruder. The rotation of the screw moves the material along the barrel. Individual sections of the barrel can be controlled to provide different temperatures. At the end of the barrel there is a very small diameter “die head” which shapes the final product into long filaments. As the solution leaves the extruder it passes into a dehydrating bath or temperature controlled area which hardens the wall material. Vibration can be used to create fine particles. The hardened capsules are sized and sorted.
Vitamin C is often encapsulated in this way (Barbosa-Cánovas, 2005).
Centrifugal extrusion
In this method a rotating extrusion head of concentric nozzles sprays a high pressure jet of liquid core material surrounded by a sheath of liquid wall material. The formation of droplets can be finely controlled by vibrating or pulsating the nozzle in a technique called Prilling. Forming electrostatic fields can also create droplets. Rayleigh instability causes droplets of core to be formed as the liquid jet moves through the air and these are covered by the coating material (Ref 7). Multi-nozzle systems, rotating disc atomizers and jet cutting (a rotating device cuts through the jet with thin wires to separate droplets) are also used in the mass production of microcapsules by this method (Kailasapathy, 2002). The wall hardens either by evaporation of a solvent from the wall material or by falling into a hardening solution.
Spray drying
This method is widely used for microencapsulation in the food industry because spray drying technology and machinery is abundant and is commonly used to produce low water activity products to reduce microbiological risk, reduce chemical interactions and reduce storage and transport costs (Gharsallaoui, 2009). This method is therefore economical and easily scaled up. The process can be continuous and can be fine tuned.
In this method the core material is dissolved or suspended in a polymer wall solution forming an emulsion or dispersion. The solution is finely sprayed or atomised via nozzles into a drying chamber where the solvent evaporates and the core is trapped in the dried particle (Ref 7). The nozzles can be specially-designed to assist particle formation by separating drops by mechanical vibration or ultrasonic frequencies (Ref 4).
The relatively short time in the drier allows labile bioactive ingredients to be handled, however very heat sensitive materials such as bacteria may not be suitable for the spray drying method (Kalasapathy, 2002).
The solvent used is often water which does not harm the core material and is evaporated easily from the polymer wall thus trapping the core almost instantaneously (Gharsallaoui, 2009).
The wall material in this process is made of polymers which exhibit high solubility, film forming, emulsification and drying properties (Young et al, 1993). Gums, carbohydrates, waxes and proteins have all been used as wall materials using this method and the material used depends greatly on the material being coated. For example a common spray drying material is maltodextrin but it cannot retain oils like modified starch or Gum Arabic can. The polymer solutions must have a low viscosity at a high concentration to allow easier spraying and drying (Young et al, 1993). The higher the concentration of the polymer the quicker the drying time because less solvent is contained within the solution (Barbosa-Cánovas et al, 2005 p206).
Spray cooling, chilling or congealing
This method of preparation is the same as for spray drying, but rather than the final material being dried it is rapidly cooled by spraying into a chilled chamber, thus hardening the wall material and trapping the core.
The most common wall material for this method are vegetable oils with low melting points; 32-42°C for spray chilling, 45-122°C for spray cooling (Barbosa-Cánovas et al, 1995). Because low temperatures are used heat sensitive core materials like bacteria can be used with the process. Once consumed in a food the core is released when the wall melts. This method is suitable for introducing bioactive materials to foods like ice cream.
Emulsion technique
Water soluble core material is dispersed in an aqueous phase and the wall material is dispersed in an organic phase e.g. oil. When mixed together this forms an oil-in-water emulsion, which is then homogenised. The core droplets in the solution are hardened by the addition of a gelling agent, a cross-linking agent or by cooling. After hardening, the capsules which have formed are washed with water and the oil is removed (Kailasapathy, 2002). Small amounts of residual oil can be left in the capsule, which may not make this method suitable for low-fat formulations (Kailasapathy, 2002).
If the core material is hydrophobic then it is added to an oil phase. This is then added to an aqueous polymer phase to form an oil-in-water emulsion. This is turn is added to another hydrophobic oil phase containing the polymer wall material to form an oil in water in oil emulsion. The process then continues as above (Ref 3).
Complex coacervation
This is considered as the original method of encapsulation having been used by Green & Scheider to produce carbonless copying paper in 1955 (Madene et al, 2006).
In this process the core material is dispersed within an aqueous solution of two polymer substances. The pH or temperature is adjusted so the positive charge on one of the polymers is neutralised by the negative charge on the other, which causes phase separation of a polymer layer containing both polymer wall substances (Yeo et al, 2005). This polymer phase is called coacervate (Madene et al, 2006). This phase completely surrounds the core material. The wall material is then stabilised often by heating, forming cross-links between the polymers, or by desolvation techniques (Ref 6). The microcapsules formed are collected by filtration or centrifugation, washed and dried, often by spray drying.
A Gelatin-Gum Arabic polymer system has been shown to work effectively using this method (Yeo et al, 2005).
This is a relatively high cost method but is suitable for the microencapsulating all kinds of labile bioactive ingredients (Ref 6).
Super critical fluid technology
There are two different types of this technology (i) Rapid expansion of Supercritical solutions (RESS) and (ii) Supercritical fluid anti-solvent (SAS) or Gas Anti solvent (GAS) (Ref 5).
Carbon dioxide is the most commonly used supercritical fluid because of its low critical temperature (which makes it suitable for use with heat sensitive materials) and pressure (Ref 5). It is also recognised as safe and non-toxic and is commonly used within the food industry.
In RESS processing a supercritical solvent (usually carbon dioxide) containing the core and wall materials is passed through a small nozzle under pressure (Ref 5). A sharp drop in pressure causes the wall material to drop out of the solvent and coat the core material. Microcapsule formation is almost instantaneous.
The SAS and GAS methods are similar to RESS but the wall and core materials are precipitated from a liquid solvent. SAS cannot be used for water soluble compounds because of the low water solubility in CO2.
These techniques can create very small particles of <1µm (Ref 5).
The mild conditions involved in this method make it suitable for labile materials such as vitamins. No harsh treatments like pH, temperature or organic solvents are needed.
These methods offer fine tuning of particle size, shape, purity and form.
Rotational Suspension Separation
This method is also known as Centrifugal Suspension Separation and is a continuous and quick production process.
Core materials are suspended in a solution of wall material. The core material should have larger particles than the coating solution to make this method the most efficient (Barbosa-Cánovas et al, 2005, p213). This solution is passed over a rotating disk atomiser which creates two types of particles (i) core material with coating (ii) coating only. The particles are thrown to the edge of the disk and fall into a cooling or drying chamber. A filter or sieve sorts the smaller coating only particles and returns them for recycling.
Air suspension coating
This method is also known as spray coating or fluidized bed processing and is used for encapsulating solid core materials (Barbosa-Cánovas et al, 2005 p215).
Solid core material particles are lifted in an upwards moving air stream to the top of a chamber. At the top of the chamber the slower air flow causes the particles to settle to the sides. A fine mist of coating material is sprayed onto the particles, which hardens through cooling or evaporation. Because the particles are moving about randomly a uniform layer of coating is built up on them. The process continues until all the particles are coated, at which point they are removed from the chamber.
If a multi-walled capsule is required the process can be repeated with another coating material. Microcapsules already made by another process can have another wall added; this allows the manufacturer to fine tune the capsule release properties.
Co-crystallisation
This method of encapsulation is not widespread but has been shown to be effective in encapsulating essential oils (Madene et al, 2006).
In this method a supersaturated, high Brix (95-97 ° Brix) sucrose syrup is heated to temperatures above 120°C where upon it spontaneously crystallises (Madene et al, 2006). Core materials are added at the point of crystallisation, and the solution is rapidly mixed which leads to nucleation around the core material which is then trapped inside the resulting sucrose crystals (Astolfi-Filho et al, 2005).
Liposome entrapment
Liposomes are spherical, lipid (usually composed of phospholipids) membranes (Ref 8). When an appropriate lipid solution is mixed with an aqueous solution of core material a lipid bilayer is formed around the core material with the lipids hydrophilic heads attracted to the aqueous core and hydrophobic tails facing away. This bilayer acts as a closed shell and traps an aqueous solution inside (Barrow et al, 2007). An attractive feature of this encapsulation method is that liposomes can contain both water soluble ingredients in the interior of the lipid membrane and hydrophobic ingredients such as oils can dissolved into the lipid membrane itself (Barrow et al, 2007).
Liposomes have been used to encapsulate enzymes (Fox, 1999), proteins, hormones and fat soluble vitamins (Barrow et al, 2007).
This is considered a gentle method of encapsulation because no solvents, surfactants or emulsifiers are needed (Barrow et al, 2007).
Molecular inclusion/Cyclodextrins
Microencapsulation using Cyclodextrins has been successful with lipophilic food ingredients such as the fat-soluble vitamins (Szente, Szejtli, 2004).
Cyclodextrins are cyclic oligosaccharides created by enzyme degradation of starch and contain 6, 7 or 8 glucose monomers (Ref 9). Their structure is cone-like and smaller “guest” hydrophobic core material molecules are able to be included in the hydrophobic cavity (Ref 10, Madene et al, 2005). The core material can be incorporated into the cyclodextrins by mixing an aqueous core material and cyclodextrins solution and filtering the precipitated complex or by blending cyclodextrins with the core material (Madene et al, 2006).
Pan coating
In this rather old fashioned and simplistic method the core material is tumbled in a device like a drageé pan used for confectionery and slowly sprayed with the coating material as a liquid (Ref 7). This method is not entirely suitable for microencapsulation because the result products are generally quite large.
As described above, there are many possible methods for microencapsulating bioactive food materials which will certainly lead to foods of improved nutritional character in future years.