materials for encapsulation of food ingredients

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Chapter 14Materials for Encapsulationof Food Ingredients:Understandingthe Propertiesto Find Practical

SolutionsC . McCrae,S. Debon, B. Guthrie, J . Heigs,G . Mondro,

and W. ShiehCargill R & D Centre Europe, Vilvoorde, Belgum

The food industry is continuously developing new materialsand formulations for the efficient encapsulation of foodingredients to make them easier to handle andproviding addedfunctionality. The material(s) that can be used includecarbohydrates, proteins, lipids, gums and cellulose, and thechoice depends on factors such as the nature of the corematerial, the expected requirements, for instance, oxidationand process stability and controlled flavour release, theprocess of encapsulation, labeling concerns and economics.Each type of material has its own particular strengths andcomplex structural designs may be required for optimalfunctionality. To help in the selection of the optimum system,this paper examnes the properties that make materials,particularly carbohydrates, applicable for encapsulation offood ingredients in terms of the basic underlying physico-chemical principles.

2009American Chemical Society 213

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214Introduction

Encapsulation is a technology for coating solids, liquids and gaseousmaterials into sealed capsules thatpreserve the substance in afinely dividedstateor as a whole ingredient and release their contents under certain conditions atspecific rates. In the case of microencapsulation, the capsules may vary in sizefrom several tenths of a micron to a few thousand micrometers. Nanocapsulesand capsules produced by macrocoating are generally less than 2000 andgreater than 5000 respectively (1). For each of these capsule classes, thetechnological challenges and type of coatings differ.

Numerous techniques have been evaluated for the encapsulation of foodingredients (including spray drying, extrusion, fluidized bed coating, spraycooling/ chilling, freeze drying, spinning disk and centrifugal coextrusion,coacervation, co-crystallisation, liposome entrappment and molecular inclusion(2-6). The selection of an appropriate technique depends on the type of capsulerequired, the physico/chemical properties of the core and coating material andeconomics. In the food industry, spray drying is the most widely used techniqueas it provides the most economical and flexible way to encapsulate hydrophobicingredients such as fats, oils, flavours and vitamins. It involves the formation ofan emulsion or suspension of core and coating material and nebulizing this feedsolution in hot air. Moisture evaporates quickly on contact with the hot airrendering free-flowing powder particles with the active core material entrappedinside a coating matrix. During spray drying, the temperatureof the core is muchlower (


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215the coating material. Knowledge on how different characteristics of coatingmaterial affect the stability of core material both during drying and storage canhelp the food developer in selecting the optimal system. Therefore, theproperties of coating material that control the coating's protective nature in themicroencapsulation of organic compounds have been investigated.

Propertiesof the carrierMolecular weght and viscosity

Hydrolysed starches are traditionally characterized by their dextroseequivalent (DE), which is ameasureof their reducing power as compared to thatof glucose (dextrose). An unhydrolysed starch has a DE value close to zero whileglucose has a DE of 100. If the DE is less than 20, starch hydrolysates are calledmaltodextrins. Corn syrup solids have a DE of 20 or greater. Qi and Xu (9)showed the molecular distribution of starch hydrolysates with different DEvalues and confirmed the data from Kenyon & Anderson (10) that the DE isroughly proportional to the reciprocal of the average degree of polymerization(Le. the average number of repeating glucose units in the carbohydratemolecules). However, by varying the type of hydrolyzing agent, it is possible toproduce products with the same DE value but different carbohydrate profiles( ) .

In agreement with earlier studies (12-14), Bangs & Reineccius (15) foundthat the DE of hydrolysed starch is inversely related to the retention of flavourcompounds during spray drying at constant infeed solids content. This relationheld truedown to DE 15 for all of 12 combined aroma compounds and was mostlikely related to an increase in the rateat which an amorphous film was formedaround droplets of the aroma compounds (16-18). Interestingly, Sheu &Rosenberg (19) found that this relation was of lesser importance in the presenceof an emulsifier. Furthermore, Bangs & Reineccius (15) found that flavorretention decreased with DE values at DE


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216that could be retained at 55% solids was ca. 20% (Table 1). Increasing the oilload to levels above 20% led to losses of oi l during drying.

Table I. Influence of infeed solids content and oil load on the retention oforange oi l during spray drying.Carrier Orangeoilload (%)

Infeedsolids(%)

O il retention(%)Glucosesyrup DE52/

n-OSA starch (3:1,w/w)

20 30 92.8lucosesyrup DE52/n-OSA starch (3:1,

w/w)20 55 98.4

Glucosesyrup DE52/n-OSA starch (3:1,

w/w) 25 55 76.1

The importance of infeed solids content in retaining flavour during dryingwas related to an increase in therateof film formation (22). Interestingly, Bangs& Reineccius (15) showed that the advantageof higher infeed solids content athigher DE values did not offset the improved film forming ability of highermolecular weight dextrins for flavours with a boiling point >160C. Inagreement with later studies (21), infeed solids content influenced the morevolatilecomponents to agreater extent.The effectiveness of the matrix film in holding and protecting the corematerial is strongly depended on the physical stateof the matrix material. Thecontribution of the molecular weight of hydrolyzed starches to this aspect ofencapsulation has been further discussed below.

Amorphousstateof the carbohydrate matrixThe success of encapsulation dependsto a large extent on the formation of a

metastable amorphous structure, a glass, with a low permeability to organiccompounds but wil l let water diffuse through (Karel & Langer, 1988). Starchhydrolysis products are particularly useful for the formation of such a structurewith selected permeability. The faster the matrix material forms an amorphousfilm around the core material, the more organic compound wi l l be retained andthus, the higher for the flavour retention (23 & 24).Hydrophilic amorphous matrices are formed when a low water activity hasbeen reached upon rapid evaporation from droplet surfaces. The sudden waterreduction increases the glass transition temperature of the matrix material. Aslong as the storage temperature remains below the glass transition temperature,the resulting matrix is believed to be in the amorphous state, which ischaracterised by a very low mobility of the carrier molecules. Because of thislow mobility, permeability to organic compounds is strongly retarded and corematerial is released primarily by diffusion through the pores in the matrix (25 &


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21726). Consequently, release or retention of core material dependson factors suchas the chemical composition of the matrix, pore size, molecular size (or DE),wall thickness and contact surface area.

For the carbohydrate matrix to remain in the amorphous state, the glasstransition temperature may not drop below the room temperature. Thus,processes reducing the glass transition temperature need to be controlled. Forexample, increased hydrolyses results in starches with a higher DE andconsequently, a lower glass transition temperature (9 and 27). In addition,starches with a higher DE are usually more susceptible to hygroscopicity (10)and the increased uptake of moisture reduces the glass transition temperature.The relation between increased moisture uptake and glass transition temperaturehas been illustrated by the results in Figure 1 for a carrier comprising glucosesyrup (DE 40) and modified starch (3:1, w/w). Initially, the glass transitiontemperature was in the range 60 to 70 C. Increasing the relative humidity to60% increased the moisture content to 7% and reduced the glass transitiontemperature to well below room temperature. Thus, storage at low humidity(


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In Micro/Nanoencapsulation of Active Food Ingredients; Huang, Q., el al.;

ACS Symposium Series; American Chemical Society: Washington, DC, 2009.


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220Crystallisation of wall material ingredients

Hydrolysed starch products don't usually crystallise but loose flowabilityand eventually cake (10). This phenomenon can be attributed to the glasstransition properties of the hydrolysed starches (9). When the glass transitiontemperature drops below the storage temperature with increasing moisturecontent, a transition takes place from solid glass to a liquid-like rubbery state.Thehydrogen bonds responsible for the main structural forces in the amorphousmatrix are weakened and consequently, molecularmobility and hence diffusionincrease (25). When the moisture content continues to increase, the polymermatrix collapses resulting in caking (10). This caking, or collapse, is caused bythe inability of the carrier to support itself against gravity as viscosity decreases.Holes in the particlewall reduce, the surface matrix shrinks thereby forcing someor all of the core material to the surface and diffusion is yet again retarded,effectively by 're-encapsulating' the remaining organic compound (29, 31 & 32).This ability to release and reseal core material could be used to impart a desiredlevel of release depending upon the degree to which the collapse is allowed toprogress.

Crystallization, like structural collapse, is promoted when the glasstransition temperature drops below the storage temperature with increasingmoisture content. It's a two-step process with the phaseof initial nucleation andsubsequent crystal growth. At a temperature above that of glass transition,particles have sufficient mobility to associate and form crystalline nucleus. L ikecollapse, crystallisation also leads to a reduction in pore size. As a result, corematerial is forced out from the crystallized matrix to the surface (27 & 31).Water may also be released from the crystallized areas into the amorphousregions, further plasticizing the wall material and decreasing the stability of thesystem (25).Trehalose is an interesting molecule because it possesses a high glasstransition temperature and, unlike hydrolysed starch, crystallises. In addition, itcrystallises mainly as trehalose dihydrate thus immobilizing water and keepingthe water activity at a low level. These interesting properties of trehalose havelead to a study on the use of trehalose as wall material in the encapsulation oforange oil. Table 2 lists the results obtained shortly after spray drying for themoisture content and the glass transition and melting characteristics of twopowders, one encapsulated with glucose syrup and n-OSA starch and the otherwith trehalose and n-OSA starch. Differences between the powders in moisturecontent were insignificant. In the presence of glucose syrup (DE 38), the wallmaterial was fully amorphous with a glass transition range in the region 78-95C. When trehalose replaced glucose syrup, the glass transition temperature wasfound to be slightly lower (70-82 C) and a melting transition was observed.The extent of crystallinity was, derived fromthe melting transition, only ca 8%.Thus, both powders were fully or almost fully amorphous.


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221

TableII. Moisturecontent andgasstransition and melting characteristicsof two spray-dried powders comprising orange oil (20%) and wall material

of different compositions immediately after spray drying.Wall material

Parameter Glucose syrup D E38/ n-OSA starch(3:1, w/w)Trehalose/n-OSAlucose syrup D E38/ n-OSA starch(3:1, w/w) starch (3:1, w/w)Moisture(%)a 2.67 2.79Glass transitiontemperature ( C) bonset 78.70.4 70.70.9peak 87.41.2 74.21.1end 95.0 1.3 81.81.1M elting temperature ( C ) bOnset N o melting 90.30.7Peak N o melting 98.1 0.7Crystallinity (%)c Fully amorphous Ca. 8

aInfra-red moisture balance(130C, 20mn);bDSC program:Equilibratedat20C andramped from20to 130C (scan rate5C/min). Averageof 3 replicates;CalculatedwithA H m (trehalose dehydrate)=86.2J/g()

TableIII. Influence of low molecular weight carbohydrates on the retentionof orange oil during spray drying.

Carrier Orange oilload (%)Infeedsolids

(%)O i l retention

(%)GlucosesyrupD E 42/n-OSA starch(3:1, w/w) 20 55 96.6

Trehalose/ n-OSAstarch (3:1, w/w) 20 55 95.2Trehalose/ n-OSAstarch (3:1, w/w) 20 55 98.6Erythritol/n-OSAstarch(3:1, w/w) 20 55 35.4

Isomalt/n-OSA starch(3:1, w/w) 20 55 94.5


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222The extent of crystallinity in the orange oil powder encapsulated withtrehalose was too small to affect the retention of orange oil during drying (Table3). Similar results were obtained with isomalt (Table 3). However, erythritolresulted in a crystalline powder (extent of crystallinity was ca. 82%) with poorflavour oil retention (Table 3). Since no orange oil was lost with glucose syrup,trehalose or isomalt, these results suggest that for good flavour retention amatrix, whichdoesnot crystallise during processing, is required.In the amorphous state, Drusch,Serfert, van den Heuvel andSchwarz(33)found that trehalose was a better ingredient for encapsulation of fish oil thanglucose syrup (DE 38) due to its ability to provide better protection againstoxidation. These results were obtained at a very low relative humidity. Figure 3demonstrates the moisture sorption properties of similar carriers but for theencapsulation of orange oil under conditions of increasing relative humidity. Thepowder encapsulated in the presence of glucose syrup exhibited a continuousincrease in the uptake of moisture as the relative humidity increased. In contrast,aplateau was observed for the powder encapsulated in the presence of trehaloseat relative humidities in the region of 53.3 to 85.0%. This plateau correspondedto complete crystallisation of trehalose (Table 4). At complete crystallisation,there was no significant increase in the uptake of moisture (Figure 3) andconsequently, the glass transition temperature did not significantly decrease(Table4).Collapse/caking was observed when the powder encapsulated with glucosesyrup increased in relative humidity to 43% and at a relative humidity of 70%,this powder liquefied. Crystallisation of trehalose led to a hardening of thepowder, limiting the range of applications to capsules requiringstorage at lowhumidity. Drush,Serfert, van den Heuvel and Schwarz. (33) came to the sameconclusion based on the observation thatrapid oxidationfollowed crystallisation

of trehalose.

EmulsificationcapacityThe retention of oil during spray drying has been found to improve onaddition of an emulsifier. For example, Trubiano & Lacourse (34) showed thateven though maltodextrins matched the viscosity of gum A rabic, they didn'tencapsulate as much oil possibly due to lack of emulsifying properties (Table 5).GumArabic is a complex mixture of macromolecules comprising predominantlycarbohydrate and a small proportion (2%) of protein. Concentrations of 12% orhigher are required to stabilize 20% (w/w) orange oil-in-water emulsions. Undertheseconditions, Randall, Phillipsand Williams(35) showed that only 1-2% ofgum Arabic bound effectively at the oil droplet surface and this adsorbedmaterial contained a high proportion of protein. It has been proposed that the


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225protein component of gum Arabic embeds in the oil while the carbohydratecomponent extends into the water phase (36). The emulsifying propertiesinherent to gumArabic are believed to be partially responsible for its ability toimprove oi l retention during drying. A stable emulsion of fine droplets of corematerial as infeed solution iscritical to microencapsulation.

Risch & Reineccius (37) demonstrated the importance of particle size in theencapsulation of organic compounds. A smaller particle size led to a betterretention of citrus oil during drying and yielded spray-dried powders, whichcontained less extractable surface oil. These findings are in agreement with theresults obtained from various studies and listed in Table 5. Surface oil may becritical to powder stability because oil droplets on the surface of powderparticles are not protected against atmospheric oxygen. However, a reduction inextractable surface oil with smaller particle size did not result in better shelfstability or resistance to oxidation. The fact that surface oi l is not critical to shelfstability has been confirmed by various studies (32, 34 & 38), which suggeststhat other factors such as matrix porosity may be more significant in determiningthe rateof oxidationof encapsulated core materials.

A s an alternative to gumA rabic, a chemically modified starch has been usedsuccessfully in the microencapsulation of organic compounds. The starchderivative is prepared by a standard esterification reaction using n-octenylsuccinic anhydride (n-OSA). The substitution of a hydrophobic octenyl sidechain is at a level of about 1 per 50-60 anhydrous glucose units, whichcorresponds to 3% n-OSA in weight, the maximumlevel approved for food use.To lower viscosity, n-OSA starches are hydrolyzed either by acid thinning,pyrodextrinization or enzyme hydrolyses. Enzymes that can be used are the a-and -amylase or a combination of both. n-OSA Starches offer excellentemulsifying properties leading to good quality infeed solutions with average oildroplet sizes of less than one micron (Table 5: Cargill and 32). Compared togum A rabic, n-OSA starch provides wall materials that have higher levels ofretained core material and less extractable surface oi l (Table 5:32 & 34). Theseimproved properties result in a direct economic benefit to the manufacturer anduser of the product. They did not, however, result in better oxidation stability.Both Trubiano & Lacourse (34) and Partanen , Y oshii, K all io, Y ang & Forssell(32) found that the oxidation resistance of wall material comprising n-OSAstarch was similar, but not worse, to thatof gumA rabic.

In our studies, excellent results were obtained at an oil load of 20% and aninfeed solids concentration of 55%. Retention of oil during spray drying washigh (Table 1 and 5) and due to the high DE of the coating material, the spray-dried powder displayed excellent stability against oxidation (Figure 2). Inaddition, Figure 4 illustrates that the spray-dried capsules had little surfaceindentations (Figure 4), which likely contributed to the excellent stability duringstorage.


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materials for encapsulation of food ingredients

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