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`C R C Critical Reviews in Food Technology
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`Microencapsulation in the food industry
`Leslie L. Balassa a , Gene O. Fanger b & Otto B. Wurzburg c
`a President, Balchem Corp., Slate Hill, N.Y.
`b Senior Research Chemist, Balchem Corp., Slate Hill, N.Y.
`c Vice President, Research, Starch Division, National Starch & Chemical Corp., Plainfield,
`N.J.
`
`Available online: 29 Sep 2009
`
`To cite this article: Leslie L. Balassa, Gene O. Fanger & Otto B. Wurzburg (1971): Microencapsulation in the food industry, C R
`C Critical Reviews in Food Technology, 2:2, 245-265
`
`To link to this article: http://dx.doi.org/10.1080/10408397109527123
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`
`MICROENCAPSULATION IN THE FOOD INDUSTRY
`
`Authors: Leslie L. Balassa
`Gene O. Fanger
`Balchem Corp.
`Slate Hill, N. Y.
`Otto B. Wurzburg
`National Starch and
`Chemical Corp.
`Plainfield, N. J.
`
`Referee:
`
`INTRODUCTION
`
`Anyone who has contemplated the mysteries of
`a bird egg, the durability of a bacterial spore, or
`the complexities of a living red blood cell, has,
`perhaps without realizing it, thought about micro-
`encapsulation.
`The variety and versatility of natural cellular
`membranes are well known and, indeed, the
`presence of
`such membrane
`structures was
`necessary before life could evolve and progress to
`its present complex forms.
`Many types of foods familiar to us utilize
`exterior cellular membranes, commonly called
`skins. Seeds, grains, fruits, and berries all have
`exterior skins which protect the interior oils or
`fruit juices from the harsh and unpredictable
`outside environment. These membranes control
`the loss of water, regulate the transfer of gases,
`provide isolation from airborne contamination,
`function as a temporary temperature insulation,
`and also often
`furnish protective supportive
`structures.
`Man-made microcapsules are similar to nature's
`in that they also consist of an outer film or skin
`
`and an inner "core" of liquid or solid material -
`the active material requiring protection.
`Microencapsulation
`technology has given
`man a means to duplicate some of the protective
`and selective properties of natural membrane
`films. Through a choice of different capsular film
`coatings, both gaseous and liquid transfer can be
`controlled. Oxygen transfer and resultant oxida-
`tion can be reduced. The transfer of salts or
`organic molecules can be regulated. By varying the
`wall thickness, or the physical properties, e.g.,
`toughness, flexibility, etc., of the capsule wall,
`strength and durability can be built into the
`capsule structure.
`A choice of capsule solubility and meltability is
`even possible. Thus, a capsule can be made to
`dissolve in hot water, but not cold; or to melt at a
`certain temperature, but not before. Even pH
`changes can be utilized for the controlled release
`of encapsulated materials.
`While there is some discussion concerning what
`exactly should be defined as a microcapsule, or a
`microencapsulation process, in order to include
`those processes most used in the food industry,
`this review uses the broadest interpretation of
`microencapsulation.
`
`July 1971
`
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`Schematic diagrams of microcapsules are shown
`in Figure 1, and 1 A.
`The applicability and the utility of micro-
`encapsulation in the food industry are becoming
`generally
`recognized. Besides
`the
`examples
`previously cited, utilization of sensitive or active
`problem materials is often enhanced by changing
`the apparent physical form. For instance, liquid
`flavor aromas and oils encapsulated in an edible
`starch or gelatin matrix result in dry pourable
`powders, which can be used directly in dry food
`
`show
`products. The encapsulated materials
`reduced reactivity with other food ingredients and
`have a lower susceptibility for oxygen-induced
`rancidity.
`
`Capsules can be produced in sizes varying from
`sub-micron to pea-sized particles.
`
`Where attractiveness is important, it is also
`possible to add food grade colors or pigments,
`either to the capsule wall or to the core liquid
`within.
`
`A.SINGLE
`
`CAPSULES
`
`Wai
`
`Internal
`
`Phase: Liquid
`Or Gas
`
`Irregular
`P a r t i c le
`
`Wai
`
`Internal Phase: S u s p e n d ed
`Solid in Liquid
`
`Dispersed
`Solid
`in Solid
`
`-Wall
`
`Internal Phase-- Dispersed
`Liquid in Liquid
`
`D i s p e r s ed
`Liquid
`in Solid
`
`Multiple Walh
`
`246
`
`CRC Critical Reviews in Food Technology
`
`FIGURE 1. Capsule Configuration.
`
`First
`
`Second
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`MICROENCAPSULATION IN
`THE FOOD INDUSTRY
`
`The food industry is constantly interested in
`improving the flavor, aroma, stability, nutritive
`value, and appearance of its products. Micro-
`encapsulation has much to offer to food producers
`
`in these areas. While some applications have been
`utilized for a considerable
`time, e.g., vitamin
`protection, many developments-are quite recent.
`Past studies have shown that encapsulation could
`furnish increased stability to vitamins, which are
`normally sensitive to UV radiation, light, oxygen,
`metals, and humidity. More recent work has
`
`CAPSULES
`B. MULTIPLE
`(AGGREGATE)
`
`Liquid
`Or Gas
`
`Capsule Wall
`
`nternal
`Phase
`
`Capsule Wall
`
`Irregular
`Particles
`
`Internal
`Phase
`
`Suspended
`Solid in Liquid
`
`Dispersed
`Solid in Solid
`
`Capsule Wall
`
`Internal
`Phase
`
`Dispersed
`Liquid in Liquid
`
`Dispersed
`Liquid in SoJid
`
`FIGURE 1A. Capsule Configuration.
`
`July 1971
`
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`shown that citrus oils and aromas can be success-
`fully protected from oxidative and even thermal
`degradation.
`Diet in the industrialized countries consists
`increasingly of processed food. As the length of
`time for storage and transit of foods from the
`place of manufacture to the market place in-
`creases, insuring a nutritive value equal to or
`surpassing that in the original food becomes both
`more important and more difficult.
`Essential or desirable components of foods such
`as fugitive aromas, flavors, vitamins, oxidizable
`constituents, and other ingredients sensitive to
`heat all tend to be affected to some degree by
`processing techniques.
`Microencapsulation can offer
`ducer:
`
`the food pro-
`
`1. a means to store sensitive food com-
`ponents
`for protection from other food
`in-
`gredients during storage,
`2. a means of adding nutritive materials to
`foods after processing and
`insuring that
`the
`original nutritive levels are not lost on extended
`storage under the expected storage conditions,
`3. a method of utilizing novel, fugitive, or
`otherwise sensitive components to prepare new
`and highly nutritional foodstuffs,
`4. unusual or
`time-saving
`anisms for foods and food products,
`5. additional attractiveness for the display
`and merchandising of food products.
`
`release mech-
`
`The present review covers the most generally
`used encapsulation techniques and systems in the
`food industry as well as the more recently intro
`duced or proposed techniques.
`
`SPRAY DRYING
`
`Spray drying is one of the oldest and certainly
`the most generally used method of encapsulation
`in the food industry. Spray drying is a form of gas
`solids contacting in which the solids phase usually
`contains a solvent or diluent, e.g., water, which is
`removed in the drying process.
`Spray drying achieves the same or similar
`results as most other encapsulation processes. That
`is, it covers an active material with a protective
`coating which is essentially inert to both the
`material being encapsulated and to the drying
`medium.
`
`248
`
`CRC Critical Review's in Food Technology
`
`In the encapsulation of food additives, the
`"solids phase" may be either a liquid or even a
`volatile food flavor combined with a capsule wall
`material. As a rule, the wall-former is chosen from
`a group of food grade hydrocolloids such as
`gelatin, vegetable gum, modified starches, or
`dextrin. The solvent or diluent liquid is usually
`water in which the "solids phase" is dissolved,
`emulsified, or dispersed. This "solids phase" is
`essentially non-volatile under the conditions of the
`process.
`In the case of food flavors or oils, the hydro-
`colloid capsule wall formers are first dissolved in
`water. The oils or flavors, composed largely of
`essential oils, aldehydes, ketones, alcohols, esters,
`and organic acids, are then either emulsified (if
`water insoluble), or dissolved (if water soluble) in
`the hydrocolloid solution.
`Spray drying and other "spray" systems involve
`three basic steps:
`
`1. liquid atomization
`2. gas-droplet mixing
`3. drying from liquid droplets
`
`is accomplished by one of three
`Atomization
`atomizing devices:
`
`1. high-pressure
`nozzles)
`2. two-fluid nozzles
`3. high speed centrifugal disks
`
`nozzles
`
`(single-fluid
`
`With these atomizers low viscosity solutions of
`up to 2000 cP (Brookfield) may be broken up into
`droplets as small as 2ti. The largest drop sizes
`rarely exceed 500M(35 mesh). Because of the large
`total drying surface of the small droplets, the
`actual drying time in a spray dryer is in the range
`of fractions of seconds to a few seconds at most.
`The total residence time of a particle in a spray
`drying system is, on the average, less than 30 sec.
`Spray drying produces
`spherical
`shaped
`particles, the result of the free suspension of the
`liquid droplets in a gaseous medium in which the
`droplets acquire a spherical shape. These dried
`particles may be either solid or hollow depending
`on the characteristics of the material processed
`and on the drying conditions.
`A typical spray dryer is shown in Figure 2.
`Most spray dryers consist of a large, usually
`vertical, cylinder chamber (A), into which material
`
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`
`Drying
`Chamber
`
`Exhaust
`
`/=»_^-0
`
`Air
`
`^
`
`=^
`
`Solution
`
`////////////A
`
`Air
`
`heater
`
`FIGURE 2. Diagram of a typical spray dryer installation. (Reprinted with permission of Editors, Perrys Chemical Engineers Handbook,
`4th ed., 1963, 20. Courtesy of Nickols Engineering & Research Corp.)
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`
`is sprayed in the form of small droplets; (B) a large
`volume of heated gaseous fluid, usually air, is fed
`into the chamber; and (C) the heated gas must be
`introduced in sufficient volume and at a tempera-
`ture sufficiently high to completely evaporate the
`solvent from the droplets. The transfer of heat is
`accomplished through direct contact of the heated
`gas with the droplets dispersed and suspended
`therein. It is important that the drying is complete
`before the particles are separated from the gaseous
`medium. After drying
`the capsules must be
`separated. This separation may be accomplished
`partially in the drying chamber itself by gravity
`collection of the larger dried particles (D); the fine
`particles are separated from the gas in external
`cyclones, frequently with secondary bag collectors
`(E and F).
`Horizontal spray chambers are usually equipped
`with a longitudinal screw conveyor on the bottom
`of the drying chamber for the continuous removal
`of the settled coarse particles.
`Abundant published literature exists on dif-
`ferent theoretical approaches for the calculation of
`spray dryer requirements based on the properties
`of the specific liquid to be sprayed.
`Special designs of spray dryers may provide for
`cooling air to enter around the chamber or closed
`systems for recovery of solvents. Air sweepers and
`mechanical rakes are often added to remove dry
`product from
`the walls and bottom of
`the
`chamber. Some spray dry systems are combined
`with pneumatic conveyors in which the hot drying
`air is diluted with cool air for product cooling
`before separation.
`Spray dryers may operate with concurrent,
`mixed, or counter current flow of gas and sus-
`pended matter. The air may be heated by steam,
`or by direct or indirect fired air heaters. Inlet gas
`temperatures generally range from 149 to 816°C
`(300tol500°F).
`The spraying of the materials into the drying
`chamber is accomplished by one of the following
`means:
`
`1. Single-Fluid Pressure Nozzles
`These nozzles effect atomization by forcing
`the liquid, under high pressure and with a high
`degree of spin, through a small orifice. Pressures
`may range from 28 to 703 kg/cm2 '(400 to 10,000
`lb./sq in.), depending on the degree of atomization
`desired, the capacity to be reached or maintained,
`and the physical properties of the liquid to be
`
`250
`
`CRC Critical Reviews in Food Technology
`
`sprayed. Nozzle orifices may range from 0.0294 to
`0.381 cm (0.010 to 0.15 in.) diameter depending
`on the pressure desired for a given capacity and
`the degree of atomization required. When used at
`high pressures, particularly when solids are in
`suspension (as contrasted with solids in solution),
`the nozzle orifice is subjected to wear by erosion.
`Under such conditions, the orifice to be used is
`made of Stellite,
`tungsten carbide, or other
`abrasion-resistant material.
`
`2. Two-Fluid Nozzles
`The two-fluid nozzle spraying system does
`not operate as efficiently at high capacities as the
`single-fluid nozzle system and, consequently, is
`not used as widely on plant scale drying units. The
`The chief advantage of two-fluid nozzles is that
`they operate at relatively low pressures. The liquid
`being pumped at pressures 0.07 to 4.22 kg/cm2
`(14.2 to 60 psi) and sprayed or atomized at
`pressures under 0.70 to 7.03 kg/cm2 (10 to 100
`psi). The atomizing fluid may be air, nitrogen, or
`other inert gas, or even steam can be used if it does
`not degrade the materials being encapsulated.
`Recently, a
`two-fluid nozzle has been
`developed especially for spraying thick pastes
`not previously capable of being handled
`in
`ordinary atomizers.
`
`3. Centrifugal Disks
`These atomize liquids by extending them in
`thin sheets, which are discharged at high speed
`from
`the periphery of
`the
`rapidly
`rotating
`specially designed disk. The principal objective in
`disk design is to ensure that the liquid comes to
`disk speed and to obtain a uniform drop-size
`distribution in the atomized liquid. Disk diameters
`vary from 5 cm in small laboratory models to 30.5
`or 35.6 cm for plant size dryers. Disk speeds range
`from 3000 to 50,000 rpm. The high speeds are
`usually used only in small diameter dryers, while
`on plant size dryers speeds range from 4000 to
`20,000 rpm depending on disk diameter and the
`degree of atomization desired. The degree of
`atomization as a function of disk speed is affected
`by the physical properties of the product, by the
`disk diameter, and by the peripheral speed of the
`disk. The design of the disk and even its wett-
`ability by the liquid have an important bearing on
`the degree of atomization obtained.
`Centrifugal disk atomization is particularly use-
`ful with suspensions and pastes that often erode or
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`plug nozzles. Thick pastes can be handled if
`positive pressure pumps are used to feed them to
`the disk. Disks may be operated over a range of
`feed rates and disk speeds without producing
`excessive variations in the product.
`The centrifugal disks may be driven by any
`suitable means; however, high speed electric
`motors are preferred at present.
`Centrifugal disks make possible the spraying of
`uniform coarse particles in contrast to the pressure
`nozzles and the two-fluid nozzles which produce
`relatively small sized particles.
`
`Food Applications of Spray Drying
`The physical properties and even the compo-
`sition and the quality of the spray dried capsules
`are subject to considerable variations depending on
`the directions of flow of the inlet gas and its tem-
`perature, the degree and uniformity of
`ato-
`mization, the solids content of the feed, the
`temperature of
`the feed, and
`the degree of
`aeration of the feed.
`Aqueous solution of the materials, such as the
`water soluble hydrocolloids or other polymers,
`used in the encapsulation of food materials form
`tough
`tenuous outer skins on drying which
`frequently
`result, in hollow spherical particles
`when spray dried. This is attributed
`to the
`formation of a case hardened outer surface on the
`particles which prevents liquid from reaching the
`surface of the particles from the interior.
`• The following properties of the spray dried
`capsules are the most important with respect to
`food applications:
`
`1. Quality of the encapsulated material
`a. retention without alteration of the
`original material in the capsule
`b. Production of a capsule containing the
`same ratio of volatile or encapsulated oil to wall
`forming material as was present in the emulsion
`prior to spray drying.
`2. Storage stability under the conditions of
`
`use
`
`3. Particle size range of the capsules
`4. Solubility characteristics
`5. Release properties
`a. solvent release
`b. temperature release
`c. pressure release.
`
`Spray dryers have been successfully employed
`
`for the drying of solutions, slurries (dispersions of
`dry material in liquids), .and pastes which:
`
`1. cannot be dewatered mechanically,
`2. cannot be exposed to high temperatures
`for prolonged periods of time,
`3. contain fine particles which will agglo-
`merate and/or fuse if not dried suspended in a
`gaseous drying fluid.
`
`While the gas temperatures in the dryer are
`usually high, the water evaporating from
`the
`liquid droplets offers some protection
`to the
`solids from
`the high gas temperatures. Drying
`in this system is carried out at essentially the
`drying air-wet bulb temperatures. However, under
`these conditions the evaporating water will azeo-
`tropically carry with it some of the volatile
`materials, and unless the dryer is closely controlled
`the particles may acquire a temperature in excess
`of the boiling points of some of the more volatile
`components, thereby volatilizing or degrading
`them.
`Liquids are usually encapsulated from their
`emulsions. Such emulsions are of the oil-in-water
`type and can be readily handled in spray dryers to
`give capsules which are essentially dried emulsion
`particles within which the encapsulated liquids,
`the active materials, are held in a micro dispersion.
`The composition and the preparation of the
`feed material control to an important degree the
`successful operation of the spray-dryer and the
`quality of the resulting capsules. The most im-
`portant factors affecting the preparation of food
`flavor emulsions for spray drying are as follows:
`
`1. proper choice of wall materials; as ex-
`plained in the following section, vegetable gum,
`dextrin, or modified starch are the preferred
`capsule wall materials
`2. high solids content of the wall material;
`the wall material is dissolved in water to give a
`solution of 30 to 40% solids
`3. critical control of the oil to wall ratio; the
`ratio of oil to dry wall material varies depending
`upon the particular system and equipment. In
`general, it usually runs about 20 to 80, though in
`some instances it may go as low as 15 to 85 or less
`and as high as 40 to 60. This, of course, depends
`upon the properties of the oil, use, and cost
`factors. Higher ratios of oil cause excessive losses
`in the process.
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`4. complete emulsification; the flavor oils
`containing
`some antioxidants are
`first pre-
`emulsified in the solution of the wall material.
`
`The pre-emulsification is usually accomplished
`with the aid of a propeller type mixer, and the
`emulsification is completed by passing the coarse
`emulsion
`through a pressure homogenizer or
`through a colloid mill.
`A simpler and more economical means of
`forming the emulsion is by the use of an efficient
`closed-turbine homogenizing mixer which, with
`chemically well-balanced compositions, gives e-
`mulsions comparable
`in particle size to
`that
`obtained through colloid milling and approaching,
`and at times equaling, the particle size obtained
`with
`the pressure homogenizers. Some homo-
`genizing mixers can be operated in a manner as to
`preclude the introduction of air into the emulsion
`during the process.
`The quality of the emulsion is dependent on
`the overall quality of the dispersion of the active
`material in the solution of the wall material and on
`the stability of the emulsions during processing.
`Assuring emulsion quality is one of the most
`important factors in a successful encapsulation by
`spray drying.
`
`Capsule Wall Materials Used in Spray Drying and
`Other "Spray" Systems
`The wall material is chosen so as to meet as
`closely as possible the following properties:
`
`1. low viscosity at high solids,
`2. ability to disperse or emulsify the active
`material and stabilize the emulsion produced,
`3. good spraying at relatively high solids
`content,
`4. non-reactivity with the material to be
`encapsulated both during processing and on pro-
`longed storage,
`5. complete release of the solvent under
`drying conditions,
`6. ability to hold and to seal the active
`material within its structure during processing and
`on storage,
`7. ability to provide maximum protection to
`the active material against environmental condi-
`tions (heat, humidity, etc.),
`8. food grade materials,
`9. solubility in solvents accepted in the food
`industry, e.g., water, ethanol, etc.
`
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`
`CRC Critical Reviews in Food Technology
`
`10. be chemically non-reactive with the active
`material,
`solubility
`specified or desired
`1 ]. meet
`properties of the capsules, and release properties
`of the active material from the dried capsules.
`
`Since no wall material will have all the proper-
`ties listed, in practice, a single wall former is
`seldom used. Rather, they are employed either in
`combination with other wall formers and/or modi-
`fiers such as plasticizers, oxygen scavengers, anti-
`oxidants, chelating agents, and surfactants.
`The most generally used wall materials for the
`encapsulation of food additives by spray drying
`are selected from the following types:
`
`1. vegetable gums, e.g., gum acacia, gum
`traganth, etc,
`2. starches, modified starches,
`3. dextrins,
`4. proteins, e.g., gelatin, soy protein,
`5. sugars, e.g., sucrose, dextrose, etc.,
`6. cellulose esters and ethers.
`
`Advantages
`in-
`1. Ready availability of equipment,
`the
`cluding that used for
`the preparation of
`encapsulating compositions, e.g., for dissolving the
`wall materials and for
`the dispersion of
`the
`emulsions.
`2. Low processing cost, based on the use of
`heated air as the drying medium.
`3. Production of high quality capsules in
`good yield, provided that active materials of low
`volatility and high resistance to oxidation are used.
`4. Rapid solubility of the capsules because
`of their small size, (less than 100 n and their large
`surface area).
`5. Stability of most food grade wall materi-
`als during the drying process.
`
`Disadvantages
`1. A limitation in the choice of wall materi-
`als to those which have low viscosities at relatively
`high concentrations.
`2. The necessity of utilizing only wall ma-
`terial solutions and dispersions readily sprayable to
`form sufficiently small droplets which dry within
`the short residence time in the spray chamber. The
`small droplets result in small capsules providing
`relatively low protection to some active materials.
`3. Volatilization and/or degradation by heat
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`and oxidation of the active material within the
`fluid droplets during drying and immediately
`thereafter due to excessive heat and oxidation.
`4. The formation of azeotropes of the active
`material with the solvent, usually water, being
`evaporated from the sprayed, suspended droplets,
`tending to remove the lower boiling fractions of
`the active material.
`5. Formation of a surface film of some of
`the less volatile fractions of the active material,
`which remains on the surface of the capsules and is
`oxidized by the oxygen in the air. Thus, some
`citrus oil emulsions in a gum acacia solution on
`spray drying can form capsules which soon devel-
`op distinctly oxidized (rancid) odor.
`6. Formation of mostly hollow capsules
`with a thin shell, thereby exposing larger surfaces
`to oxidation than would solid capsules of equal
`weight.
`7. The degraded or oxidized film of active
`material may be washed from the surface of the
`capsules with suitable solvents. However, this is
`usually considered uneconomical. An even more
`important consideration is that unless the solvent
`is carefully chosen and completely removed, it will
`leave a residual odor, on the capsules, which may
`be even more objectionable than the oxidized
`material on the surface. Spray drying in an inert
`atmosphere eliminates
`the oxidation during
`processing; however, all of
`the other effects
`resulting from dry small droplets at relatively high
`temperatures in a hot gaseous medium will remain.
`8. Oxygen-free closed systems are required
`when employing spray drying to remove flam-
`mable organic solvents from the capsules to reduce
`explosion hazards. Since the solvent vapors repre-
`sent only a small fraction of the drying gas, the
`recovery of the solvents is rarely economical;
`consequently,
`their loss or the cost of their
`recovery is a burden on the economics of the
`process.
`of employing non-
`9. The difficulty
`flammable solvent systems, such as chlorinated
`solvents, due to their inherent toxicity and some
`of their extremely toxic decomposition products
`at elevated temperatures. These products, espe-
`cially in the presence of oxygen, produce a serious
`pollution problem if not removed from the emis-
`sion gases.
`
`In order to avoid some of the disadvantages of
`spray drying, e.g., the exposure of the active
`
`material to oxygen or even to inert gases at high
`temperatures, several spray systems and techniques
`have been developed. The most important of these
`spray systems are the following.
`
`SPRAYING ON A HYDROPHOBIC
`FLUIDIZED POWDER
`
`A schematic diagram of this process is shown in
`Figure 3. A liquid composition, consisting of an
`active material dispersed in a solution of a wall
`material in which the solution of the wall material
`is the external phase, is sprayed onto a fluidized
`bed of a hydrophobic powder or onto a moving
`belt covered by a thin layer of a hydrophobic
`powder.
`The spraying equipment used in this case may
`be the same or similar to that employed
`in
`conventional spray drying. Single fluid pressure
`nozzles are most frequently used, though two-fluid
`nozzles and spinning disk atomizers may also be
`utilized to good advantage.
`Where a hydrophobic powder is used, it may be
`
`E M U L S I ON
`
`r -( DRY
`
`F L O)
`
`' • J 0 g &*
`
`FIGURE 3. Spraying emulsion on a fluidized powder.
`U.S. Patent 2,756,177; Belgium Patent 669,166. Klaui, H.
`M. et al.. Fat-soluble vitamins, in International Encyclo-
`paedia of Food and Nutrition, Vol. 9, Morton, R.A., Ed.,
`Pergamon Press, Oxford. 1970, 135. With permission.
`
`July 1971
`
`253
`
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`
`a modified starch, e.g., Dry Flo (National Starch
`& Chem. Co.) calcium stearate, magnesium
`stearate, or high specific gravity inorganic materi-
`als, e.g., barium sulfate coated with a hydrophobic
`material. In this system the wall material is
`preferably gelatin and the dust is kept at a
`temperature below the gel point of the gelatin
`composition. The powder, by adhering to the
`surface of the capsules, keeps them from agglom-
`erating while the excess hydrophobic powder acts
`as a moisture sink, accelerating the removal of
`excess moisture from the capsules.
`
`Advantages
`1. This system is particularly useful in the
`encapsulation of vitamin A in gelatin since it has
`the advantages of solvent dehydration "at a lower
`cost.
`
`2. Thermal degradation of the active ma-
`terial in this system is limited to exposure to the
`50 to 55°C temperature of the gelatin solution
`during the preparation of the dispersion and
`during spraying.
`3. The temperature is or may be lowered to
`below the gel temperature as soon as the sprayed
`droplets leave the nozzle. The time and conditions
`of exposure to higher than ambient temperature
`are shorter and therefore less severe than in spray
`drying.
`4. Oxygen may be excluded without great
`expense.
`
`Disadvantages
`1. The most serious difficulty of this system
`would appear to be the separation of the capsules
`from the excess dust, especially of non-gelling wall
`compositions.
`2. It is most difficult to lower the moisture
`content of the capsules below 5% if this should be
`required. (It has been reported that vitamin A
`gelatin capsules show inferior long-term stability if
`their moisture content is above 3%).
`
`SPRAYING ON
`A MOVING OIL SURFACE
`
`This spraying system is similar to that described
`under Spray Drying. The fluidized bed of powder,
`described under Spraying on a Hydrophobic Fluid-
`ized Powder, is replaced by a moving surface of a
`hydrophobic oil or by another hydrophobic organ-
`ic liquid. The moving surface may be the surface
`
`254
`
`CRC Critical Reviews in Food Technology
`
`of a body of oil subjected to agitation or an oil
`coated moving belt (Figures 4 and 5).
`In practice, the oil may be an organic liquid,
`mineral oil, castor oil, mineral spirits, silicone oil,
`
`E M U L S I ON
`
`FALLI NG
`FILM OF
`CASTOR OIL
`
`COOLING, THEN DEHYDRATING WITH ALCOHOL
`
`FIGURE 4. Spraying emulsion onto a moving oil
`surface. U.S. Patent 3,143,475. Klaui, H.M. et al.,
`Fat-soluble vitamins, in International Encyclopaedia of
`Food and Nutrition, Vol. 9, Morton, R.A., Ed., Pergamon
`Press, Oxford, 1970, 137. With permission.
`
`EM U L SI ON
`
`/
`
`/ /
`
`\ \
`
`_^
`
`C O N V E Y OR
`
`B E LT
`
`( OR O T H ER
`
`S U R F A CE
`
`M O V I NG
`
`IN R E L A T I ON TO S P R A Y I NG
`
`D E V I C E)
`
`S U R F A CE
`
`R E P E L L A NT
`
`TO
`
`D R O P L E TS
`
`FIGURE 5. Spraying emulsion onto a moving oil
`surface. Belgium Patent 589,766. Klaui, H. M. et al.,
`Fat-soluble vitamins, in International Encyclopaedia of
`Food and Nutrition, Vol. 9, Morton, R.A., Ed., Pergamon
`Press, Oxford, 1970, 135. With permission.
`
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`

`
`or a liquid fluorohydrocarbon, etc. It is used to
`keep the liquid capsule particles separated until
`they are either hardened: 1) by gelation, 2)
`chemically by the use of crosslinking agents, e.g.,
`aldehydes, or 3) through dehydration, e.g., by an
`alcohol.
`
`Advantages
`
`(1). This system makes the preparation of
`completely dehydrated capsules with less than 3%
`moisture content. In the case of gelatin capsules,
`the exposure to thermal degradation is limited to
`the operation prior to and during spraying.
`(2). Dehydration capsules with cold water
`soluble wall materials, e.g., gum acacia, dextrin,
`etc., can be prepared by the application of this
`system.
`(3). Since this system can be operated at
`temperatures as low as 5 to 10°C and in an inert
`atmosphere, heat or oxidative degradation can be
`reduced or eliminated in the process.
`
`Disadvantages
`
`(1). In the case of water soluble wall materials
`and organic soluble active materials, e.g., flavor
`oils, alcohol dehydration can result in severe losses
`of alcohol-soluble active material from the capsule.
`(2). Washing of the capsules to remove the
`residual alcohol may also be required.
`
`SPRAYING INTO DEHYDRATING
`LIQUID OR DEHYDRATING
`VAPORS
`
`This system utilizes the same emulsification of
`dispersion techniques and similar spraying equip-
`ment as described under Spray Drying.
`
`Advantages
`
`(1). The technique lends itself to low tem-
`perature operation with the exclusion of air.
`(2). With active materials which are insoluble
`in the dehydrating liquids or with capsule wall
`compositions which resist penetration by the
`dehydrating liquids, this system offers technical
`possibilities not matched by other systems.
`(3). Thus by careful selection of the wall
`materials, water soluble materials may be encap-
`
`sulated in water soluble capsule wall materials.
`(4). A dehydrating
`liquid
`is used as the
`contact medium rather than heated air or a
`hydrophobic