J . MICROENCAPSULATION, 1993, VOL. 10, NO.

4, 413-435

Microcapsules for food

R. ARSHADY
Department of Chemistry, Imperial College of Science,
Technology and Medicine, London SW7 2AY, UK
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(Received 24 March 1992; accepted 4 May 1992)

A systematic description of various processes employed for microencapsulation

of food additives is presented. Microencapsulation methods covered include
spray drying, fluidized bed coating, extrusion, solvent extraction, coacervation,
cocrystallization, liposome formation and molecular inclusion. Basic features of
microcapsular products, such as morphology, particle size, core/coat character-
istics, and release behaviour, are also highlighted.

Introduction
Microcapsules can be described as micron-size packages (figure l), composed of a
polymer wall (coat or shell), and an active ingredient referred to as core or nucleus.
T h e active ingredient may be a food additive, medicament, biocide, adhesive or
For personal use only.

another specialty material. As compared with conventional packages which facilitate

storage, transport, handling or presentation, microcapsules are generally employed
to enhance material performance or create new applications.
T h e origin of microcapsular products is generally ascribed to the development of
pressure-sensitive coatings for the manufacture of carbonless copying paper during
the 1950s (Green and Schleicher 1955). I n the food industry the history of
microencapsulation dates back to at least 1951, when Griffin patented a process for
the preparation of solid oil concentrates (Griffin 1951). I n this and related early work
the resulting products were similar to those currently obtained by cocrystallization
and extrusion (see later), and the processes were referred to as ‘locking’ rather than
microencapsulation (Mylne and Seamans 1954, Olsen and Seltzer 1945, Strashun
and Talbort 1954, Wellner 1953, Schultz et al. 1956). Today, several different
microencapsulation processes are widely employed for microencapsulation of food
additives, including flavours, preservatives, leavening agents, vitamins and
minerals. For literature on food additives see Smith (1991) and Furia (1972).
According to Versic (1988), there are currently several hundred microencapsulated
food ingredients used in the United States, the majority of which are natural and
artificial flavours and spices. An early review of microencapsulation in the food
industry has been presented by Balassa and Fanger (1971). More recently, various
aspects of flavour encapsulation have been discussed in a Symposium Proceeding
edited by Risch and Reineccius (1988). A feature article on microencapsulation with
reference to food ingredients has also been published by Dziezak (1988). These
reviews discuss details of specific microcapsules, rather than providing a systematic
coverage of the subject.
I n a series of recent articles I have presented a systematic description of
microencapsulation processes covering interfacial polycondensation (Arshady
1989 a), suspension crosslinking (Arshady 1989 b), coacervation (Arshady 1990 a),
0265-2048/93 $1000 1 1YY3 l’qlor & Francis Ltd
 414 R . Arshady
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Figure 1. Example of microcapsules containing a liquid (left) or solid (right).

and solvent extraction/solvent evaporation (Arshady 1990 b). These methods are
more usually employed in the chemical, agrochemical and pharmaceutical indus-
tries. T h e present article provides a systematic coverage of different methods
For personal use only.

employed for the microencapsulation of food ingredients (food additives). T h e main

processes covered include spray drying, fluidized bed coating (Wurster process) and
extrusion. Less frequently reported methods such as dehydration (solvent extrac-
tion), coacervation and cocrystallization are also briefly covered. T w o specialized
processes, namely molecular inclusion for the preparation of long-life flavours, and
liposome-entrapped proteinase for enhanced cheese maturation, are also high-
lighted. For general references on microencapsulation see Nixon (1976), Deasy
(1984), Kydonieus (1980), Kondo (1979) and Gutcho (1979).

Basic features of food microcapsules

Food additives
A given food additive may impart texture or bulk, or it may play a more
functional role in terms of nutritional value, food preparation or preservation. Such
functional ingredients include processing aids (leavening agents and enzymes),
preservatives (acids and salts), fortifiers (vitamins and minerals), flavours (natural
and synthetic) and spices. Microencapsulation can be employed to ‘enhance’, ‘time’
or ‘tune’ the effect or function of these additives. Major benefits brought about by
microencapsulation of food ingredients are summarized in table 1.
Among the variety of ingredients named above, the use of microencapsulated
flavours is the most widely established. This follows from the fact that most aroma
substances (essential oils), are volatile compounds, and may evaporate long before
the food arrives on the table. Microencapsulation prevents (or minimizes) aroma
evaporation during processing, transport, storage and cooking. A different benefit of
microencapsulation is realized in the use of salt in meat processing. Salt is added to
processed meat products for controlling yeast growth, rancidity and water absorp-
tion. However, the added salt also causes tackiness and caking during processing,
and adversely affects food texture. Microencapsulation overcomes these problems
without impairing the beneficial functions of the salt (Dziazak 1988).
 Microcapsules for food 41 5

Table 1. Some of the main benefits of food microencapsulation.

Increasing shelf-life by preventing evaporation and oxidation (e.g. flavours).

Stabilization of sensitive components (e.g. vitamins).
Masking of undesirable taste (e.g. of vitamins apd minerals).
Improving processability and texture (e.g. meat products).
Preventing undesirable interactions (separation of incompatible ingredients).
Ease of handling (conversion of liquids to free-flowing powders).
Controlled release (sustained release or release at a given stage).
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Improved aesthetics, taste and colour.

Vitamins and mineral fortifiers are also common features of processed food and
animal feed. Microencapsulation increases the stability of vitamins during exposure
to heat, moisture and air (oxygen). In more delicate food products, microcapsules
can be designed to control the rate, or stage, at which the active ingredient is released
(i.e. to influence the timing of a given process or effect). An interesting example of
this is the use of microencapsulated proteinase for cheese ripening, to be highlighted
at the end of this review.

Wall polymers
For personal use only.

T h e most important criteria for selecting a given polymeric material for

microencapsulation of food ingredients are ‘edibility’ and ‘bland’ taste. Accordingly,
vegetable-based colloids and gums (e.g. starch and gum arabic; figure 2 ) are ideally
suitable for this purpose, because they are edible and many are constituents of
conventional human diet (Southgate et al. 1990, Glicksman 1983, Kester and
Fennema 1986). A number of chemically modified polysaccharides such as modified
starches (Wurzburg 1986, Turbiano and Lacourse 1988) are also used for food
microcapsules. Other naturally occurring polymers suitable for microencapsulation
of food ingredients include gelatin, vegetable and milk proteins, waxes and
(hydrogenated) fats (table 2 ) .
Other parameters in the choice of microcapsule wall are physicochemical criteria
such as molecular weights, solubility, glass/melting transition, crystallinity, film
formation and diffusibility. A practically more important criterion is often the cost of
the wall-forming material. This is particularly true in flavour microencapsulation
where the wall (or matrix) constitutes a large proportion (ca. 85-95 per cent) of the
finished product.

Morphology and performance

Basic features of microcapsular products include morphology, particle size,
particle size distribution and wall thickness (or core/coat ratio). From a manufactur-
ing point of view these criteria provide the means of ‘tuning’ microcapsule
performance and release behaviour. Some of the main considerations in the design of
microcapsules are listed in table 3. Four idealized microcapsular morphologies are
illustrated in figure 3. I n practice a wide variety of intermediate structures are
produced, depending on the nature of the starting materials and details of the
manufacturing process. More elaborate morphologies (e.g. double-walled or
microencapsulated microcapsules) are produced by sequential microencapsulation
processes.
 Microcapsules for food 43 1

J-1 Liporome-bound
proteinare

Cheele
vat
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0 0 0
Milk

I Curd formation

Cheddaring
intact liporomer

I Hooping
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Maturing cheese

Liberated
proteinare

Figure 18. Enhancement of cheese maturation by liposome-entrapped proteinase (from

Kirby and Law 1987).

Figure 19. Chemical structure of fl-cyclodextrin.

 Microcapsules f o r food 417

A 8
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C D
Figure 3. Idealized presentation of different microcapsular morphologies. A: Matrix
or monolithic type; B: reservoir type; C: polynuclear; D: microencapsulated
microcapsules.
For personal use only.

Table 3. Important criteria in microencapsulation of food additives.

The aim of microencapsulation should be clearly defined.

The active ingredient should not deteriorate during
microencapsulation.
The polymer coat should be edible, non-reactive, and tasteless.
Concentration of the active ingredient should be optimized with
respect to performance and cost.
Core release should be studied and optimized against application
parameters (dissolution, pH, temperature, pressure, etc.), as
may be necessary.
Cost of the polymer coat and microencapsulation process should be
justified in terms of improved performance.

T h e thickness of the polymer coat (or more generally core/coat ratio) is an

important parameter in microcapsule performance. I n the case of microencapsulated
volatiles (e.g. flavours), the higher the corelcoat ratio, the lower the aroma retention
of the product.
T h e effect of particle size on microcapsule performance can be understood in
terms of two interrelated criteria, namely surface area and coat thickness. At constant
sample weight, smaller microcapsules have larger surface area, and hence faster
diffusion (lower retention) of volatiles. In addition, for a constant core/coat ratio, the
coat thickness decreases with decreasing particle size.
Another important parameter in microcapsule performance is the nature of the
polymer wail (or matrix), and especially its compatibility with the core substance. I n
general, amphiphilic residues on the polymer enhance core/coat miscibility and
facilitate microcapsule formation. For example, derivatized starches carrying
 41 8 R. Arshady

lipophilic groups (Turbiano and Lacourse 1988) or amino acid residues (Bangs and
Reineccius 1988) have been reported for flavour microencapsulation. It is also
noteworthy that the superiority of acacia (gum arabic) for microencapsulation may
be related to the presence of small quantities of proteinous substances in this
polymer (Anderson et al. 1985).

Nomenclature
Microcapsules are conventionally described by a variety of lengthy phrases such
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as starch-microencapsulated flavour, or flavour-containing starch microcapsules.

According to a recently proposed nomenclature (Arshady 1992), microcapsules can
be named by simple names, and with reference to their core, coat, or both core and
coat, as may be desired in the discussion. Thus, microcapsules illustrated in figure 4
(Zilberboim et al. 1986b) may be referred to by one of the following three names,
depending on the context:
(1) flavour microcapsules (named after their core);
(2) G E L microcapsules (named after their coat, G E L ABBREVIATED
notation for gelatin).
(3) Flavour/GEL microcapsules (named after both core and coat).
T h e distinction between core and coat is made by using the normal upper/lower case
letters for the name of the core, and upper case ABBREVIATED notations for the
For personal use only.

name of the polymer coat (see table 2).

Spraying processes
Spray drying
Microencapsulation by spray drying involves the dispersion of the active agent in
a polymer solution, followed by spraying of the mixture into a hot chamber. This
leads to evaporation of the polymer solvent, and hence the formation of matrix type
(or polynuclear) microcapsules. Figure 5 shows a schematic configuration of a
cocurrent spray dryer used for microencapsulation of flavours in the food industry.
The sprayer feed is prepared by mixing the core material with the polymer
solution in the presence of a surfactant (dispersing agent). Atomization and spraying
of the feed into the hot chamber produces a shower (i.e. microdroplets) of the

Figure 4. Microcapsules composed of ethyl caprylate core and gum arabic matrix (or coat)
(from Zilberboim et al. 1986 a).
 Microcapsules f o r food 419

Exhaust
air

*... .. *-.* .
*..
I

9.
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Microcapsules
Figure 5. Schematic presentation of cocurrent spray drying equipment.

polymer solution containing the dispersed active ingredient. In the chamber the
microdroplets lose their solvent to the hot air, which is blown concurrently with the
For personal use only.

microdroplets. T h e resulting microcapsules are then transported to the cyclone

separator for recovery. Since the polymer solvent is water, recycling of the solvent
and treatment of the exhaust air (pollution control) are not needed.
T h e wall polymer used for flavour microencapsulation by spray drying is usually
a polysaccharide (e.g. modified starch or gum arabic), and inlet and outlet
temperatures of the hot chamber are about 180" and lOO"C, respectively. Natural
aroma substances are complex mixtures of relatively volatile organic compounds,
including alcohols, aldehydes, ketones, esters and ether. An intriguing question is
how such volatile compounds survive during the microencapsulation process at 100-
180°C. T h e answer lies mainly in the fact that the polar polysaccharide is relatively
impermeable to the non-polar aroma compounds. In other words, aroma com-
pounds are hydrophobic, and as such they have relatively low diffusibility through
the hydrophilic polysaccharide solution (or matrix).
It can also be reasoned that, initially, water from the outer surface of the sprayed
microdroplets is lost preferentially over the flavour compounds which are embedded
within the interior of the droplets. As the dehydration proceeds, the permeability of
the polysaccharide matrix remains substantially higher for the polar water, as
compared with those of the non-polar aroma compounds. As a result, water is
preferentially evaporated, while the oil (i.e. the non-polar flavour) is largely
preserved within the impermeable polysaccharide matrix. However, the degree of
aroma retention is strongly dependent on the moisture content of the final
microcapsules and on the humidity of the exhause air. Other parameters-such as
the type, molecular weight and concentration of the wall-forming polymer, degree of
flavour dispersion, and the size of the atomized microdroplets-also influence aroma
retention (Risch and Reineccius 1988, Reineccius et al. 1982, Anandaraman and
Reineccius 1986, Rosenberg et al. 1988,1990).
It is also noteworthy that microencapsulation by spray drying produces less
uniform microcapsules, as compared with other microencapsulation methods.
 420 R . Arshady
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A
- B

so , I
C

Figure 6. Typical orange oil/STA, microcapsules (from Chang et al. 1988).

Figure 6 (Chang et al. 1988) shows electron micrographs of typical orange oil
microcapsules produced by spray drying. T h e products are aggregates of a variety of
microcapsule forms and sizes and starch flocculates. These morphologies are
distinctly different from the uniform spherical particles obtained by, for example,
For personal use only.

extrusion and suspension methods.

Other spraying processes

T h e problems of flavour loss during the ‘drying’ stage of spray drying, and the
generally low flavour levels attainable by this process, have led to the investigation of
a number of alternative methods for dehydration of the atomized microdroplets.
These methods include spray freezing, spray chilling, spray cooling, spray on
moving oil, spray on desiccant powders and spray onto a dehydrating liquid. Flavour
encapsulation by the last-named method is outlined below. Patent literature on other
spraying methods has been reviewed by Balassa and Fanger (1971), and a
commentary on spray chilling has been contributed by Lamb (1987).
A typical procedure of spray dehydration is the one recently described by
Zilberboim et al. (1 986 a, b) for microencapsulation of a series of aroma compounds.
This process is based on the use of gum arabic (acacia) as the wall polymer, and
ethanol as the dehydrating medium. Thus, the core material is homogenized in a
20 per c q t acacia solution, followed by addition of a concentrated acacia solution to
produce a final polymer concentration of 40 per cent, and a core concentration of 10-
40 per cent in the feed mixture. This is then sprayed into ethanol at a rate of 25 g/min.
T h e resulting microcapsules are recovered by filtration, and dried at 50°C under
reduced pressure.
Flavour retentions of typical microcapsule preparations reported in this work are
indicated in figure 7. It is not surprising that aroma retention is also intrinsically
related to the nature of the flavour itself. T h e most volatile core substance indicated
in figure 7 (ethyl formate) is not only the least retained, but its retention is strongly
dependent of core/coat ratio. In contrast, the least volatile flavour (paprika oleoresin)
is the most efficiently retained, and its retention is only slightly affected by core/coat
ratio.
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I 1 r I 1 I I
0 10 20 30 40 50
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Flavor in feed (O/o)

Figure 7. Dependence of aroma retention o n flavour concentration in microencapsulation
feed. W , Paprika oleoresin; 0 , ethyl caprylate; A , ethyl butyrate; 0 ,ethyl acetate; 0 ,
ethyl formate (adapted from Zilberboim et al. 1986a).

Coating processes
Particle coating technology, including dip coating, pan coating and fluidized bed
coating (air suspension coating or Wurster process) are routinely employed in the
pharmaceutical industry. T h e Wurster process is also used for the microencapsul-
ation of food ingredients. A number of more specialized coating processes based on
the deposition of charged aerosols (Langer and Yamate 1966,1969) or sublimates
(vaporization) (Baer et al. 1961) have also been developed, but these are not usually
suitable for food products.
Air suspension coating was developed by D. E. Wurster in the 1950s; hence the
term ‘Wurster process’ (Wurster 1953,1966). T h e principle of the Wurster process
is similar to the principles of fluidized bed reactors in general. T h e main
coating/drying chamber of a conventional fluidized bed coater is shown schemati-
cally in figure 8.
T h e core particles are placed at the bottom of the central chamber (C), and blown
upwards through the central column by means of hot air blown in the same direction.
T h e coating polymer solution is also sprayed upwards through the central column
( C ) . In this way the core particles pass through a simultaneous coating/drying
process in the first flight upwards. After reaching the top of column (C) the partially
coated particles move downwards through the annular column (D) and undergo
further drying. At the end of column (D) they are redirected again upwards through
column (C), and the coating/drying operation is repeated until the desired coat
thickness is reached.
 422 R. Arshady
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Figure 8. Schematic presentation of fluidized bed coater/dryer. A: Air distribution plate;

B: coating spray (delivered from a hydraulic or pneumatic nozzle); C: central coating/
drying chamber; D: outer drying chamber.

A
loo -
-

Time (h)
Figure 9. Core release from lactose/EDU microcapsules produced by fluidized bed coating
(EDU = Eudragit). Core content: (...), 10 per cent; (--), 20 per cent; (-.-), 30 per cent;
(-), 60 per cent (adapted from Fukumori et al. 1987).

T h e degree of aggregation and the overall quality of the microcapsules obtained

by fluidized bed coating are controlled by a number of process variables, including
the spraying rates of the coating solution and hot air, and temperatures and
humidities of the inlet and exhaust air. Release behaviour of the resulting
microcapsules can be controlled by the nature and proportion of the coating
polymer. Recently, Fukumori et al. (1987,1988 a, b), have reported a detailed study
of microencapsulation by air suspension coating. Figure 9 shows the dissolution
pattern of lactose/EUD microcapsules described by this group (1 987).
 Microcapsules for food 423

Extrusion
T h e technology of extrusion is employed in a variety of different modes for the
preparation of microcapsular products. In the simplest form an extrusion device
consists of a hypodermic syringe (droplet generator) and a droplet hardening bath.
T h e hardening bath may serve as a coolant for gelation or solidification of the
droplets, or it may contain a thermal or chemical agent for chelation or covalent
crosslinking. Alginate microcapsules containing whole cells (figure 10) (Lim and
Moss 1981) are routinely produced by extrusion into an aqueous solution of calcium
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chloride. T h e potential of this process for flavour microencapsulation has been

recently discussed by King (1988). For this and other extrusion processes to be
outlined below, a small percentage of a suitable drople_t stabilizer must usually be
added to the hardening bath to prevent the coagulation of the resulting
microcapsules.
T h e hardening of ‘liquid microdroplets’ and their conversion to ‘solid micro-
capsules’ can be accomplished by a variety of mechanisms, including melt
solidification, gelation, chelation, solvent extraction/evaporation, or covalent
crosslinking.
T h e use of basic extrusion technology in the food industry was patented by
Swisher (1957,1962), and by Beck (1972). T h e original method of Swisher has been
recently adopted by Miller and Mutka (1986), and is apparently the basis of a
commercial process for the production of microencapsulated flavours (Mutka and
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Nelson 1988). T h e latter process involves the use of a low-moisture carbohydrate

(e.g. sucrose, corn syrup or maltodextrin) as the matrix component, and isopropanol
as the hardening medium.
In a typical preparation, the carbohydrate is heated (cooked) to about 130°C to
reduce the moisture to less than 10 per cent. T h e flavour substance (15-35 per cent),

Figure 10. Hepatoma cells/ALG microcapsules produced by extrusion (from Lim and
Moss 1981).
 424 R . Arshady
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1
15 20 25 30 35
Oil in feed (9’0)
Figure 11. Dependence of aroma retention of flavour/SUCR microcapsules on cook
temperature and flavour concentration in the feed mixture (adapted from Mutka and
Nelson 1988).
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and a small quantity of an emulsifier, are then added with vigorous stirring to
produce a fine dispersion of the oil in the carbohydrate in a sealed reactor equipped
with a die. T h e reactor is subsequently pressurized and the mixture is extruded into
cold isopropanol with sufficient stirring to break up the produced filaments. Figure
11 shows the effect of cook temperature, emulsifier and flavour content on the
efficiency of microencapsulation (Mutka and Nelson 1988).

Double-capillary extrusion
For the formation of reservoir-type microcapsules a variety of double-capillary
extrusion devices have been described. In one design (Matsumoto et al. 1986,
Sudekum 1976), the core substance and the wall polymer are fed through,
respectively, the inner and outer opening of a coaxial double capillary (figure 12).
T h e core substance is usually a liquid and the polymer may be applied as a solution or
melt (the core and coat fluids must be immiscible). T h e two fluids form a unified jet
flow at the tip of the coaxial nozzle, which breaks up to form the corresponding
microdroplets. These are then led into an appropriate medium for hardening of the
wall polymer by cooling, solvent extraction, chelation or crosslinking.

Multi-orifice centrifugal extrusion

An alternative extrusion design for the preparation of reservoir microcapsules is
based on a multi-orifice cylinder with a central rotating disc (figure 13). T h e actual
mechanism of microcapsule formation is very similar to that of coaxial capillary
design discussed above. T h e wall-forming polymer solution (or melt) is fed through
the grooves above and below the central rotating disc; hence the formation of a fluid
film across each orifice. T h e rotating disc pushes the core substance (a liquid or
slurry) into the wall fluid at the centre of the orifice. This causes the wall fluid to
distend, break up, engulf the core, and produce a shower of consecutive droplets.
Hardening of the polymer coat is then accomplished in a hardening medium as
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Figure 12. Schematic presentation of a coaxial double-capillary extrusion device for the
generation of reservoir type microcapsules.

I
A ,

,I :: ....>!a:.:. ....
.e.;\::_

T
Figure 13. Schematic presentation of multi-orifice extrusion device. A: Coating fluid; B:
core material; C: orifice chamfer; D: orifice;E: rotating disc (carrying the core material);
F: rotor.
 426 R . Arshady

described above. T h e multi-orifice extrusion design is highly efficient and can

accommodate a variety of accessory facilities for large-scale production. For details
of this and related microencapsulation methods see Somerville (1962) and Goodwin
and Somerville (1974).

Recycling centrifugal extrusion

In a recently described process, the technology of rotating disc extrusion is
combined with a facility for recycling of the excess coating fluid (Sparks 1987, Sparks
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et al. 1986). According to this method, the active ingredient (a particulate solid) is
dispersed in the coating fluid in the usual way. T h e 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.

Other microencapsulation methods

A broad spectrum of well-established processes employed for microencapsul-
ation of food ingredients were discussed in the preceding sections. A number of less
frequently reported techniques are also briefly covered below for the sake of
completeness. It should also be noted that microencapsulation technology is, to a
large degree, an art, and as such it is wide open to innovation, novelty and
specialization. Two interesting examples, namely liposome entrapment and mole-
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cular inclusion, are highlighted at the end of this review to illustrate the potential of
‘specialized’ microencapsulation processes in food technology.

Suspension processes
T h e term ‘suspension’ in the present context refers to processes involving
microcapsule formation by two-phase suspension systems. For matrix-type (or
polynuclear) microcapsules, for example, the active ingredient is dissolved or
dispersed in a solution of the wall-forming polymer. This is then stirred in an
immiscible liquid (suspension medium) to form a ‘microdroplet’ suspension
(figure 14). T h e microdroplets obtained in this way are subsequently hardened, and
hence converted to the corresponding ‘microcapsules’. T h e resulting microcapsular
product is then recovered from the suspension medium (and droplet stabilizer) by
decantation, filtration or centrifugation, depending on particle size.
A typical example of microencapsulation by a suspension process according to
figure 14 is the preparation of coffee aroma/GEL microcapsules by solvent
extraction (dehydration) (Balassa and Brody 1968). In this work the liquefied coffee
aroma is well dispersed in a gelatin solution in the presence of an emulsifier (e.g. a
Tween). The dispersion is then added into mineral oil with continued stirring to
form the desired water in oil (w/o) suspension. T h e mixture is subsequently added to
anhydrous alcohol; hence dehydration and conversion of the droplets to the
corresponding microcapsules. T h e product is finally recovered and freed from
mineral oil and traces of water by centrifugation (or filtration), washing and drying.

Coacervation
Coacervation is the method initially employed by Green and Schleicher in the
1950s to produce pressure-sensitive dye microcapusles for the manufacturing of
carbonless copying paper (Green and Schleicher 1955). T h e term ‘coacervation’ had
been introduced some 20 years earlier by Bungenberg de Jong and Kruyt to describe
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Droplet formation
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Droplet hardening

1
Figure 14. Schematic presentation of microencapsulationby suspension processes. Droplet
hardening may involve crosslinking, solvent extraction or melt solidification.

macromolecular aggregation (phase separation) by partial desolvation of fully

solvated macromolecules (Bungenberg de Jong 1949). Coacervation in the presence
of a liquid or solid core material (i.e. microcapsule formation by coacervation) is
complicated because different polymers have varying film-forming behaviours on
different surfaces and in different media. I t is, however, generally recognized that for
successful microencapsulation by coacervation the core material must be compatible
with the excipient polymer, and insoluble (or scarcely soluble) in the coacervation
medium.
For the purpose of the present discussion, two alternative (or complementary)
mechanisms of microencapsulation by coacervation may be considered, as depicted
in figure 15 (Arshady 1990a). According to one mechanism the core droplets/
particles are gradually coated by the newly formed coacervate nuclei. Alternatively,
relatively large coacervate drops (or even macroscopic phase separation) -?re
formed, followed by bulk encapsulation of the core droplets/particles.
 428 R . Arshady

\
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Figure 15. Microencapsulation by coacervation: gradual coat formation from coacervate

For personal use only.

droplets, or phase separation followed by bulk microencapsulation. I / / / , Polymer

molecules; 8 , core particles or droplets; m , polymer coacervates.

Cocrystallization
Cocrystallization (or coprecipitation, as may be the case) can be regarded as a
microencapsulation process in a broad sense, rather than the formation of a strict
microcapsular morphology. As the name implies, this method involves the
simultaneous crystallization/precipitation of the active ingredient with a matrix
material (e.g. a carbohydrate). T h e process is relatively simple, and especially useful
when high core/coat ratios are not required.
An interesting example of food microencapsulation by cocrystallization is the
recently described preparation of peanut butter microcapsules (Chen et al. 1988). In
this work, a supersaturated sucrose syrup is maintained at a temperature high enough
to prevent crystallization. T h e desired proportion of the active ingredient is then
added with vigorous mixing. This leads to the cooling of the mixture, and hence
crystallization of the sugar and entrapment of the ingredient. T h e bulk product is
subsequently dried (if necessary), screened for particle size classification, and
packed. Figure 16 shows a photomicrograph of a cocrystallized aggregate, and an
electron micrograph of the porous structure formed in this process (Chen et al.
1988).

Liposome entrapment
Liposomes (lipid vesicles) are produced routinely by sonication, extrusion, and a
number of related techniques (Gregoriadis 1984). In either case the resulting
vesicles may be composed of one, a few, or a multitude of lipid layers (uni- or multi-
lamellar liposomes). Active ingredients can be entrapped within the aqueous phase
of the vesicle (i.e. liposome interior), or incorporated within the lipid structure. A
schematic illustration of a multi-lamellar vesicle (MLV), with active ingredients in
both aqueous and lipid regions is shown in figure 17 (Gregoriadis 1987).
 Microcapsules f o r food 429
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For personal use only.

Figure 16. Photo- and electron micrographs of aggregate particles produced by crystalliz-
ation of sucrose syrup in the presence of flavour (i.e. cocrystallization) (from Chen
et al. 1988).

For most food uses, liposomes are not very attractive, because of their ‘liquid’
nature and poor retention, as compared with microcapsules. However, Kirby and
Law (1978) have recently reported the use of liposome-entrapped proteinase in
cheese-making. It is noteworthy that maturation of cheese is an enzymic process
involving the formation of small ‘flavouring’ molecules from fats, carbohydrates and
proteins (Law 1982). This is a slow process. For example, Cheddar requires a period
of about 1 year for full maturation (ripening). Evidently, the economics of such a
lengthy ‘flavouring process’ is rather unfavourable. T h e inclusion of additional
 430 R. Arshady
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Figure 17. Schematic presentation of a multi-lamellar liposome with active ingredient in

both aqueous and lipid regions (from Gregoriadis 1987).
For personal use only.

enzyme(s) in the milk is, therefore, an obvious way of enhancing the rate of the
maturation process. T h e method developed by Kirby and Law involves the addition
of liposome-entrapped enzyme to the milk shortly before renneting (figure 18)
(Kirby and Law 1987). More recently, Karel and Langer (1988) have reported
the use of microencapsulated liposomes (microencapsulated microcapsules, see
figure 3), and have outlined potential uses of microencapsulated enzymes in cheese
making and other food processes.

Molecular inclusion
Various preparations of microencapsulated flavours described in the preceding
sections afford products with shelf-lives suitable for most customary food uses.
However, in all of these products the flavour is physically entrapped, and as such it is
subject to escape by evaporation and deterioration by oxidation. One method which
provides an essentially 'chemical entrapment' is molecular inclusion by cyclodex-
trins (figure 19). Technicaily, molecular inclusion (molecular encapsulation) of
aroma compounds is relatively simple, and is reminiscent of precipitation and
crystallization processes (Szejtli 1982, Pagington 1985, Reineccius and Risch 1986,
Szente and Szejtli 1986). In a typical preparation, B-cyclodextrin is dissolved in a
water-ethanol mixture (1 : 2) at a temperature of about SS'C, followed by gradual
addition of an ethanolic solution of the flavour ingredient while stirring. T h e
solution is brought gradually to room temperature to effect crystallization. T h e
mixture is then cooled to O'C, and the resulting cyclodextrin-flavour adduct is
recovered as a crystalline powder.
In molecular inclusion the flavour molecules become individually complexed
within the cavity of cyclodextrin molecules. T h e complexes formed in this way are
highly stable (even at temperatures of up to 2OO0C),but release the flavour upon
hydration (i.e. in the mouth). T h e data presented by Szente and Szejtli (1988) show
 Microcapsules for food 43 1

J-1 Liporome-bound
proteinare

Cheele
vat
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0 0 0
Milk

I Curd formation

Cheddaring
intact liporomer

I Hooping
For personal use only.

Maturing cheese

Liberated
proteinare

Figure 18. Enhancement of cheese maturation by liposome-entrapped proteinase (from

Kirby and Law 1987).

Figure 19. Chemical structure of fl-cyclodextrin.

 432 R. A r s h a d y

Table 4. Reported shelf-life for inclusion complexes of various flavours with /I-cyclodextrin
(adapted from Szente and Szejtli 1988).

Flavour content (%)

Encapsulated flavour In 1977 In 1987

~ ~~

Garlic oil 10.2-1 0 4 10.0-1 0.3

Onion oil 10+10.6 10'2-10.4
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Caraway oil 10.5 9.9-1 0.2

Thyme oil 94-9.8 9.0-9.2
Lemon oil 8.9-9.1 8.6-8.8
Carrot oil 88-9.0 7.9-8.3
Anise oil 9'0-9.2 9.0-9.3
Peppermint oil 94-9.7 9.0-9.2
Marjoram oil 8.8-9.0 8.0-8.2
Orange oil 9.0-9.5 60-7.0
Tarragon 10.0-10.3 8.8-9'0
Mustard oil 10.8-1 1.o 11.0-1 1.2

that flavour contents of molecular inclusion products remain virtually unchanged

For personal use only.

during a 10-year period (table 4). I t is noteworthy that cyclodextrins are relatively
expensive, and such long-life flavours are not needed for everyday food uses.
However, novel materials and technologies provide us with new possibilities;
economics and markets follow the course of novelty in time.

Concluding remarks
Many food ingredients (flavours, leavening agents, vitamins and enzymes) are
microencapsulated by a polymer coat or matrix to enhance food quality. Several
different microencapsulation processes, and a variety of edible polymeric materials
(e.g. polysaccharides, gelatin, fats and waxes), are used to obtain microcapsular
products with different morphologies, particle size and other characteristics. Highly
specialized microcapsules can be designed, for example, to produce long-life
flavours, o r to speed up the process of cheese maturation. However, despite
widespread use of microencapsulated ingredients in the manufacture of food
products, details of the coating processes involved are not fully understood. In
particular, the fine 'tuning' of microcapsules for optimum performance requires a
thorough understanding of the 'polymer processes' involved in microencapsulation.
Multidisciplinary studies of these problems involving microencapsulation experts,
as well as food and polymer scientists, are suggested to mark the future progress of
the subject.

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