Spray drying is a commercial processes which is widely used in large-scale production of encapsulated flavours and volatiles (Deis, 1997). The merits of the process have ensured its dominance, these include availability of equipment, low process cost, wide choice of carrier solids, good retention of volatiles, good stability of the finished product, and large-scale production in continuous mode (Reineccius, 1989). According to Teixeira et al. (2004), this technique provides a high retention of aroma compounds during drying. Spray drying can be used for many heat-labile (low-boiling point) materials because of the lower temperatures that the core material reaches (Dziezak, 1988). Sharma & Tiwari (2001) and Re-MI (1998) presented a review on microencapsulation using spray drying. The process involves the dispersion of the substance to be encapsulated in a carrier material, followed by atomization and spraying of the mixture into a hot chamber (Dziezak, 1988; Watanabe et al., 2002). The resulting microcapsules are then transported to a cyclone separator for recovery. The formation of a stable emulsion, in which the wall material acts as a stabilizer for the flavour, is considerable.
Retention of volatile core material during encapsulation by spray drying is achieved by chemical and physical properties of the wall and core materials (Reineccuis & Coulter, 1969; Menting et al., 1970; Bomben et al., 1973; Leahy et al., 1983; Rosenberg, 1985; Reineccius, 1988; Rosenberg et al., 1990; Desobry et al., 1997), solid content of the dryer, processing temperature and also by the nature and the performance of the encapsulating support, i.e. emulsion-stabilizing capabilities, film-forming ability and low viscosity at a high concentration (Rosenberg et al., 1990; Goubet et al., 1998). The functionality profile of wall materials that are optimal for spray drying includes a high solubility in water, a low viscosity at high concentration, effective emulsification and film-forming characteristics and efficient drying properties (Reineccius, 1988; Re-MI, 1998). When core materials of limited water solubility are encapsulated by spray drying, the resulting capsules are of a matrix-type structure. In such, the core is organized into small droplets coated with wall materials that are embedded in the wall matrix. Microstructures of spray-dried capsules have been shown to be affected by wall composition and properties, flavour-to-wall ratio, atomization and drying parameters, uneven shrinkage at early stages of drying, the effect of a surface tension-driven viscous flow and storage conditions (Buma & Henstra, 1971a,b; Kalab, 1979; Greenwald, 1980; Greenwald & King, 1982; Keith, 1983; Mistry et al., 1992; Rosenberg & Young, 1993; Young et al., 1993a,b).
One disadvantage of spray drying is that some low-boiling point aromatics can be lost during spray drying and the core material may also be on the surface of the capsule, this would encourage oxidation and possible flavour changes of the encapsulated product (Dziezak, 1988; Desobry et al., 1997). Kerkhof (1994) has reviewed the difficulties in quantitatively understanding drying processes, identifying the non-linearity of the processes, the complex transfer processes and the tendency for the dominating phenomena to change during drying. The degree of aroma retention is strongly dependent on the moisture content of the final microcapsules and on the humidity of the exhaust air.
Other problems with spray drying for flavour or microencapsulation are that this technology produces a very fine powder, typically in the range 10–100 μm in diameter, which needs further processing, such as agglomeration, to make the dried material instantly or make it more soluble if it is a liquid application. The ability of agglomeration to influence the properties of spray-dried encapsulated flavourings is limited by the processing characteristics of the carrier materials (Risch, 1995; Buffo et al., 2002). Table 4 outlines the advantages and disadvantages of the spray-drying technique. To avoid these problems, spray-drying powders can be agglomerated by using the fluidized bed process.
Table 4. Advantages and disadvantages of the using of spray-drying
| Low operating cost|
| High quality of capsules in good yield|
| Rapid solubility of the capsules|
| Small size|
| High stability capsules|
| Produce no uniform microcapsules|
| Limitation in the choice of wall material (low viscosity at relatively high concentrations)|
| Produce very fine powder which needs further processing|
| Not good for heat-sensitive material|
Fluid bed spray coating is a three-step process. First, the particles to be coated are fluidized in the hot atmosphere of the coating chamber. Then, the coating material is sprayed through a nozzle onto the particles and film formation is initiated, there follows a succession of wetting and drying stages. The small droplets of the sprayed liquid spread onto the particle surface and coalesce. The solvent or the mixtures is then evaporated by the hot air and the coating material adheres on the particles (Fig. 5) (Jacquot & Pernetti, 2003). This technique relies upon a nozzle spraying the coating material into a fluidized bed of aroma particles in a hot environment (Fig. 6). The size of the product varies from 0.3 to 10 mm (Panda et al., 2001).
The fluid bed process is already widely employed in the phamaceutical and cosmetic industry, both of which have a greater budget for processing than the food industry, but its use in the food industry to encapsulate flavours has also been studied (Dezarn, 1998; Lee & Krochta, 2002). It is the most suitable method for encapsulating spray-dried flavours because the wall materials used in flavour systems are readily dissolved and form strong interparticle bridges on re-drying (Buffo et al., 2002). This technology allows specific particle size distribution and low porosities to be designed into the product (Uhlemann & Mörl, 2000). Other advantages of fluidized bed (Mujumdar & Devahastin, 2000) are:
high drying rates because of good gas-particle contact, leading to optimal heat and mass transfer rates;
smaller flow area;
high thermal efficiency; lower capital and maintenance costs;
ease of control.
According Chua & Chou (2003), the fluidized bed dryer is a low-cost drying technology for developing countries compared with some state-of-the-art drying equipment, such as spray dryers.
The freeze-drying technique, which is lyophilization, is one of the most useful processes for drying thermosensitive substances that are unstable in aqueous solutions. In this process, upon water crystallization, the non-frozen solution is viscous and the diffusion of flavours is retarded. Upon starting freeze drying, the surface of the solution becomes an amorphous solid in which selective diffusion is possible (Karel & Langer, 1988). Buffo & Reineccius (2001) compared spray drying, tray drying, drum drying and freeze drying to encapsulate cold-pressed orange oil Valencia with gum acacia and modified food starch. They concluded that freeze drying is the process that gives the most desirable properties to spray-dried powder. Minemoto et al. (1997) compared oxidation of menthyl linoleate when encapsulated with either gum arabic by hot air drying and freeze drying. These authors showed that freeze drying was better than hot air drying. Indeed, the menthyl linoleate encapsulated by freeze drying was more slowly oxidized at any relative humidity and this did not change during storage. Heinzelmann & Franke (1999) showed that the production of dried microencapsulated fish oil by freezing and subsequent freeze drying offered an opportunity to achieve a product with good resistance to oxidation. It was shown that the freeze drying process maintained the shape of the microcapsules because of fixation by freezing (Nagata, 1996).
Extrusion was first patented in 1957 and further developed by the group that originally patented the technique (Swisher, 1957). Encapsulation of flavours via extrusion has been used for volatile and unstable flavours in glassy carbohydrate matrices (Reineccius, 1991; Blake, 1994; Benczedi & Blake, 1999; Gunning et al., 1999; Qi & Xu, 1999; Saleeb, 1999; Benczedi & Bouquerand, 2001). The principal advantage of the extrusion method is the stability of flavours against oxidation. Carbohydrate matrices in the glassy state have very good barrier properties and extrusion is a convenient process enabling the encapsulation of flavours in such matrices (Gouin, 2004). However, process parameters and diffusion of flavour from extruded carbohydrates is enhanced by structural defects such as crakes, thin wall, or pores formed during or after processing (Wampler, 1992; Villota & Hawkes, 1994). Extrusion of polymer solutions through nozzles to produce either beads or capsules is mainly used on a laboratory scale (Heinzen, 2002).
Recycling centrifugal extrusion
The technology of rotating disc extrusion is combined with a facility for recycling of the excess coating fluid. The core material is dispersed in the carrier material. The suspension is extruded through the rotating disc in such a way that the excess coating fluid is atomized and separated from the coated particles. Excess coating fluid is then recycled, while the resulting microcapsules are hardened by cooling or solvent extraction. Optimizing the cook temperature, pressure, emulsifier level, residence time and extrusion vessel pasteurization permits the production of encapsulated flavouring with a high flavour load. But, as stated previously, the diffusion of flavours out of extruded carbohydrates is enhanced by structural defects such as cracks, thin walls, or pores formed during or after processing (Miller & Mutka, 1986; Wampler, 1992). However, the major problem still facing this process is related to emulsion stability, this is difficult to obtain in extremely viscous carbohydrate melts (Risch, 1988).
Controlled flavour release
Controlled release may be defined as a method by which one or more active agents or ingredients are made available at a desired site and time and at a specific rate (Pothakamury & Barbosa-Canovas, 1995). Many researchers have sought a better understanding of the effects that govern the flavour release from complex matrices as this represents an important target in many fields, including the food industries (Guichard, 2000). An overview of physical chemistry relevant to flavour release has been presented previously (Taylor, 1998). For matrix systems encapsulating volatile compounds, release depends on several mutually dependent processes such as diffusion of the volatile compound through the matrix, type and geometry of the particle, transfer from the matrix to the environment, and degradation/dissolution of the matrix material (Pothakamury & Barbosa-Canovas, 1995).
De Roos (2000) showed that two factors control the rate of flavour release from products, the comparative volatility of the aroma compounds in the food matrix and air phases under equilibrium conditions (thermodynamic factor) and the resistance to mass transport from product to air (kinetic factor). The mechanism of release for the capsule may be based on solvent effects, such as melting, diffusion, degradation, or particle fracture (Table 5).
Table 5. The mechanistic of flavour-controlled release (Richard & Benoît, 2000)
|Encapsulation technique||Controlled release mechanistic|
|Simple coacervation||Prolonged release|
|Complex coacervation||Prolonged release (diffusion) and started release (pH, dehydration, effect mechanical, dissolution or enzymatic effect)|
|Spray drying||Prolonged release and started release|
|Fluid bed drying||Started release (pH or heat treatment)|
Flavour retention in the matrix is greatly dependent on the type of food ingredient and the physico-chemical properties of the flavour compound. Retention will clearly induce a noticeable decrease in flavour perception. Generally, flavour release decreases with increasing lipid level in the food matrix, with the exception of hydrophilic compounds possessing log P values [P, volatile permeability (mol m−1 s−1 per bar)] near or below zero (Guichard, 2002). The presence of salts increases the volatility of an aroma compound; this is in contrast with the effects of salts on other small molecules, such as caffeine or naringin, where they induce a solubilization effect (Druaux & Voilley, 1997). Boland et al. (2004) investigated the release of eleven flavour compounds from gelatine, starch and pectin gels. These authors showed that flavour release was significantly affected by the texture of the gels. Thus, the gelatin gel showed large increases in flavour release in the presence of saliva, while the starch and pectin gels showed a reduction in flavour release under these conditions. Interactions between proteins and aromas have been the subject of numerous studies (Lubbers et al., 1998), showing that covalent binding, hydrogen binding and hydrophobic interactions are all detectable.
The advantages of controlled release are: the active ingredients are released at controlled rates over prolonged periods of time; loss of ingredients during processing and cooking can be avoided or reduced; reactive or incompatible components can be separated (Dziezak, 1988; Brannon-Peppas, 1993).
Release of flavour by diffusion
Diffusion is controlled by the solubility of a compound in the matrix (this establishes a concentration in the matrix which drives division) and the permeability of the compound through the matrix. Diffusion is important in food because it is the dominant mechanism in controlled release from encapsulation matrices (Crank, 1975; Cussler, 1997).
The vapour pressure of a volatile substance on each side of the matrix is the major driving force influencing diffusion (Gibbs et al., 1999a). The principal steps in the release of a flavour compound from matrix system are: diffusion of the active agent to the surface of the matrix; partition of the volatile component between the matrix and the surrounding food and transport away from the matrix surface (Fan & Singh, 1989). It should be obvious that if the food component is not soluble in the matrix, then it will not enter the matrix and so diffusion will not take place irrespective of the pore size of the matrix (Reineccius, 1995).
Two distinct mechanisms of diffusion may apply. One mechanism is molecular or static diffusion, which is caused by the random movement of the molecules in the stagnant fluid. The rate of molecular diffusion varies only slightly with flavour type. The second mechanism is eddy or convective diffusion, which transports elements of the fluid from one location to another, carrying with them the dissolved solute. The rate of eddy diffusion is usually much higher than the rate of molecular diffusion and is independent of the flavour type (Roos, 2003).