docencia bioquimica de alimentos critical reviews in food science and nutrition 51 21491268

Critical Reviews in Food Science and Nutrition, 51:432–441 (2011)Copyright C©© Taylor and Francis Group, LLCISSN: 1040-8398 print / 1549-7852 onlineDOI: 10.1080/10408391003646270

Agglomeration of Food Powder andApplications

K. DHANALAKSHMI, S. GHOSAL and S. BHATTACHARYAFood Engineering Department, Central Food Technological Research Institute, (Council of Scientific and Industrial Research),Mysore 570020, India

Agglomeration has many applications in food processing and major applications include easy flow table salt, dispersiblemilk powder and soup mix, instant chocolate mix, beverage powder, compacted cubes for nutritional-intervention program,health bars using expanded/puffed cereals, etc. The main purpose of agglomeration is to improve certain physical propertiesof food powders such as bulk density, flowability, dispersability, and stability. Agglomerated products are easy to use by theconsumers and hence are preferred over the traditional non-agglomerated products that are usually non-flowable in nature.The properties of food agglomerates and the process of agglomeration like employing pressure, extrusion, rewetting, spray-bed drying, steam jet, heat/sintering, and binders have been reviewed. The physical and instant properties of agglomeratedfood products have also been discussed.

Keywords agglomeration, food powders, binder, functional properties

INTRODUCTION

Powdery food materials are frequently used for conveniencein applications during transportation, handling, processing, andfor product formulations. A variety of food powders havinga different source are used to serve specific purposes includingimproving sensory appeal and nutritional status of finished prod-ucts. Powders are characterized in terms of size, shape, and theirfunctionality, while there is a lack of knowledge about their be-havior under varying temperatures and moisture contents. Withthe increasing quantity of different powders like beverage pow-ders (coffee, tea, cocoa, milk, etc.), table salt, spice powders,cereal and pulses flour, additives, etc., are being produced infood industries, there is a need for detailed information abouttheir handling and processing characteristics, especially for foodpowders, and because of their complexity. Food powders areoccasionally used as such though their main use lies in furtherprocessing for developing different products (Table 1). Amongthe various frequently used processes the food powders undergo,the steps like agglomeration, compaction, instantization and en-capsulation are practiced to get products with specific purposesand for convenience.

Address correspondence to S. Bhattacharya, Food Engineering Department,Central Food Technological Research Institute, Council of Scientific and Indus-trial Research, Mysore 570020, India. Tel.: 0821-2513910, Fax: 0821-2517233.E-mail: [email protected]

AGGLOMERATION

Agglomeration, in general, can be defined as a process dur-ing which primary particles are joined together so that biggerporous secondary particles (conglomerates) are formed (Palzer,2005). According to this definition, even caking of hygroscopicraw materials during storage can be regarded as a kind of un-desired agglomeration. Agglomeration is basically a physicalphenomenon and can be described as the sticking of particulatesolids, which is caused by short-range physical or chemicalforces among the particles themselves as a result of physi-cal or chemical modifications of the surface of the solid. Thisphenomenon is triggered by specific processing conditions, orbinders and substances which adhere chemically or physicallyon the solid surfaces to form a bridge between particles (Pietsch,2003).

The process of agglomeration can be applied to many foodand non-food items. The non-food applications include the man-ufacture of ceramic objects formed by granular materials fromkaolin, feldspar, silica and silicon carbide, fish and mammalfeeds, household products (detergents for fabrics, dish, and hardsurface cleaning), microbiological products (enzyme, yeast, andbacterial granules), and a number of pharmaceutical productslike feed stock for tabletting, pellets, and encapsulates. The ag-glomeration of food powders is of recent interest in which thecontrol of porosity and density of material is the main interest.These have practical application like dispersability, wettability,

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AGGLOMERATION OF FOOD POWDER AND APPLICATIONS 433

Table 1 Different functional applications of food powder

Function Details of function Raw materials Specific applications Reference

Used directly Food additive, taste improver Salt, spice and sugar powder,milk powder, beveragepowder

Spray or drum dried milkpowder, baby foods,coffee, tea

Bergquist et al., 1992;Nasirpour et al., 2006;Hogekamp et al., 1996;Colarow, 2006; Sherwoodet al., 2008

Requires further processing Developing granulated andcompacted items

Cereal starch/flour, puffedcereals and pulses

Bouillon cubes, compressedbars/cubes, chocolate bars,granulated products andsoup mixes

Snow et al., 1999; Kimuraand Teraunchi, 1999;Haefliger et al., 2001;Pietsch, 2005

Coating material To add taste and flavor likesweet/salty, chocolate,antisticking agents and toimprove appearance, andadding nutrients

Cereal/pulse flour, sugar, salt,maltodextrin, spices,vitamins/minerals, artificialsweeteners, herb/plantextracts

Salt-spicy snacks,confectionery, sugar coatedproducts

Teunou and Poncelot, 2002;Jinapong et al., 2008;Kowalska et al., 2005

Intermediate raw material Suitable as a food componentonly after significantprocessing

Corn flour/starch,rice/wheat/pulse powder,proteinconcentrates/isolates,modified starches, casein,egg powder

Soup mix, traditional productlike pan cake, dosa/idli,high protein foods likemeat analogues

Cunningham, 2007; Takeitiet al., 2008; Pietsch, 2005

Minor ingredient Increasing consistency andfor gelling

Starches, crude gelatin, gums,pectin

Snacks, jams, jellies, desserts,soups, sauces

Zhao and Bertrand, 2007;Caspers et al., 2001

Microbial source Microbial culture in driedform

Yeast media, bacterialculture, enzymes

Formulations containinglactic acid bacteria, yeastpowder

Harkonen et al., 1993

Flavoring Natural and synthetic flavors Cheese, spices Cheese flavored extrudedsnacks, mixed spice flavor

Fuchs et al., 2006; Buffoet al., 2002; Stahl, 2005

Fruit and vegetable powders Adds specific fruit andvegetable taste

Dehydrated fruits andvegetables

Onion/garlic/potato/tapioca/mango powders,fruit-based beveragepowder

Pietsch, 2005; Cremer, et al.,2008

Aid for drying Provides a non-sticky surface Cereal starches/flour Bread crumb, rolling/sheetingmachines

Takeiti et al., 2008|

sinkability, and solubility. Agglomeration is also referred to asintantizing, because rehydration and reconstitution are impor-tant properties of foods that decide its convenience at domesticas well as industrial sectors. Agglomerates have both coarse andopen structures varying from 0.1 to 3 mm. Agglomeration im-proves the dispersability of the formed products that are wetteduniformly when put in either cold or hot water.

The present article focuses on different processes of agglom-eration and applications of agglomerated products in food andallied industries. The associated mechanism of formation andstructure of the agglomerated products is also discussed.

APPLICATIONS

The major applications of agglomeration in food includeeasy flow table salt, dispersible milk powder and soup mix,instant chocolate mix and beverage powder, compacted cubesfor nutritional intervention programme, health bars using ex-panded/puffed cereals, etc. (Table 1). Compacted cubes maybe defined as the cuboids having high density that are madeout of powdery food ingredients by the application of pressure.

The most common application of punch-and-die presses in thefood industry is for the production of bouillon cubes with di-mensions typically in the range 13–15 mm (Pietsch, 2005). Inthe context of instantizing, the technique of tumble/growth ag-glomeration is frequently used in the food industries to improvethe reconstitutability of a number of products including flour,cocoa powder, instant coffee, dried milk powder, sweeteners,fruit beverage powder, instant soup, and spice. Extrusion is ex-tensively used for shaping the product during the manufactureof a variety of ready-to-eat breakfast cereals (Barbosa-Canovaset al., 2005).

A major application of the agglomeration process is in theproduction of instant products in which primary particles areagglomerated to give a granule-shaped product with improvedwettability, dispersability, and dissolution characteristics com-pared to the original primary particles. Food particulates areencapsulated to increase shelf life, masking taste or odor, andimproving appearance. Coated agglomerated granules are pro-duced for controlled release of constituents within the particleby providing a coating which dissolves at a given rate in aparticular temperature or pH, or to protect unstable ingredientsfrom degradation by heat, moisture, or light (Dewettinck andHuyghebaert, 1998; Teunou and Poncelet, 2002). The materials

434 K. DHANALAKSHMI ET AL.

used for coating in food industries are mainly water-solublebiopolymers, lipids, milk proteins or corn proteins, gums suchas locust bean gum, carboxymethylcellulose, sodium alginateand kappa-carrageenan, protein concentrates (sodium caseinate,lysozyme), blood plasma concentrate (Dewettinck et al., 1998),gelatin and starch hydrolysates (Dewettinck et al., 1999), andlipids (Eichler, 1996).

A process for the production of granular cocoa (Kimura andTeranchi, 1999), agglomerated powdered beverage (Camp andFischbach, 1990), a gelling aid consisting of particulate sugar(Caspers et al., 2001) and granulation of protein-rich fraction offlour (Berizzi, 2004) have been patented in which agglomera-tion is a critical processing step. Table 1 lists the products andthe materials that are used to prepare agglomerated products(Knight, 2001). The patented processes include the productionof instant powder (Strommen et al., 2001), infusing flavors intocereal grains and the agglomeration of grains into a unitary foodproduct (Capodieci, 2002), methods for binding agglomeratedproteins (Grossman et al., 2007), and agglomerated starch prod-uct which has improved flow properties and disintegrates at sub-stantially the same rate in media of varying pH (Cunningham,2007). Zhao and Bertrand (2007) have developed a method forproducing instantly dispersible pregelatinized starches for dif-ferent food products. The list of products includes fish and dairyproducts of commercial importance; microbial products and en-zyme granules also find the applications in different food andpharmaceutical industries. As an example, the flow chart forthe production of instant agglomerated soymilk powder is pre-sented in Fig. 1 which shows the use of maltodextrin as a binder(Jinapong et al., 2008).

Applications of agglomeration also include production of ar-tificial sweeteners (Fotos and Bishay, 2001), a granular flavoringfor chewing gum that facilitates for enhanced flavor retention(Hyodo et al., 2003), and a free-flowing granular dried soup mix

Soybean

Soymilk

Soaked, heated to 70-80oC for 10-15 min, ground with water and extracted

Concentrated at 40oC using ultrafiltration unit

Concentrated soymilk

Spray dried

Spray dried soymilk

Agglomeration in a top spray fluidized bed granulator

Instant agglomerated soymilk powder

Maltodextrin

Figure 1 Flowchart for the production of instant soymilk powders.

with relatively narrow particle size distribution to aid dispersion(Haefliger et al., 2001). Besides the mentioned agglomeratedand granulated products, there exists ample scope to developmany more such convenience products. Table 2 gives the list ofthe various applications of agglomeration processes in food andallied industries.

AGGLOMERATION PROCESSES

The process of agglomeration may be conducted in severalways in conjunction with other unit operations such as spraying,steaming, and drying. The selection of a specific agglomerationprocess depends on several factors including physical and chem-ical properties, average particle sizes of initial raw material(s)and product, thermal sensitivity, and on the requirement of spe-cial properties like instant solubility, easy flowability, etc. Thetechnologies applied vary widely in their process conditions andadhesion principles to bind the primary particles together. Thecommonly used agglomeration processes can be divided intothree groups (Schuchmann, 1995; Pietsch, 2002; Hogekampet al., 1996) like (a) pressure agglomeration (e.g., tableting), (b)growth agglomeration (e.g., granulation, pelleting) such as wetagglomeration and dry agglomeration and (c) agglomeration bydrying (e.g., spray drying).

Depending on whether or not a binder liquid is involved in theprocess, it can be subdivided into “wet” and “dry” agglomera-tion methods. Each method exploits certain binding mechanisms(Ennis et al., 1991) for granulation. For wet growth agglomera-tion processes, the term “granulation” is often used. The subse-quent sections describe the different processes of agglomerationthat is frequently used in agri-food processing industries.

Pressure Agglomeration

Compaction is a process whereby small particles are boundtogether to form larger cohesive masses in which the originalparticles can still be identified (Snow et al., 1999). Examples ofdry granulation methods are roll compaction and uniaxial diecompaction. Dry granulation is of particular interest in industryas the final product requires no liquid binder and drying process,and therefore costs less to operate as it requires simpler equip-ment (Augsburger and Vuppala, 1997). It has special advantagefor handling of moisture sensitive material.

In pressure agglomeration, compressive force acts on a con-fined mass of particulate solids, which are then shaped anddensified. Pressure agglomeration is normally carried out intwo stages; the first stage comprises a forced rearrangement ofparticles due to applied pressure, and the second step consistsof a steep pressure rise during which brittle particles break andmalleable particles deform plastically.

The success of compression or compaction agglomerationprocess depends on the effective utilization and transmissionof the applied external force, and on the ability of the material


Table 2 Application of agglomeration processes in food and allied industries

Technology Method Equipment Application Reference

Agitation Methods Tumbling agglomeration Rotary drums, pans, bowland plate granulators

Fruit and vegetablepowders

Pietsch, 2005; Cremeret al., 2008

Mixer agglomeration Horizontal pan, pin mixer Cereal powder/starch Cunningham, 2007|Powder clustering Powder blenders like

conical, vertical shaft,ribbon mixers,agglomerators, flow jetmixing systems

Beverage powder, cerealpowder/starch

Kowalska and Lenart2005

Pressure methods Piston/rotary typecompaction

Compacting/tablettingpresses

Bouillon cubes,nutritional bars

Snow et al., 1999;Pietsch, 2005

Roll pressing Compacting rolls Health bars Snow et al., 1999Extrusion Piston/screw extrusion

systemsCereal powder/starch Pietsch, 2005

Thermal methods Steam jet agglomeration Vertical jet agglomeration Cereal powder/starch Hogekamp et al., 1996Fluidised bed Fluidised bed dryers Milk powder, baby food Buffo et al., 2002

Spray and dispersion methods Spray drying Spray dryers Milk powder, coffeegranules, ice creammixes, flavors, lipids,caroteniods

Vega et al., 2005;Gharsallaoui et al.,2007

Spray onto dispersedpowder

Fluidized/sprouted beds Milk powder, baby food,cereal flour

Thomas et al., 2004;Nasirpour et al., 2006

Agglomeration in liquidmedia

Ribbon mixers Cereal powder/starch Buffo et al., 2002

to form and maintain inter-particle bonds during pressure com-paction (or consolidation). This aspect is controlled in turn bythe geometry of the confined space, the nature of the appliedloads, and the physical properties of the particulate materialand of the confining walls (Snow et al., 1999). The differentadvantages of compaction agglomeration include an increasein product density and requirement of a small amount of liq-uid binder. Compaction agglomeration is carried out by usingthe equipment like the piston and moulding presses, tablettingpresses, and roll presses. Tablet presses require highly flowablefeed material to uniformly fill the press chambers prior to com-paction. Uniformity of particle size, particle shape, roughnessof surface, cohesiveness due to the chemical nature of the com-pound, and moisture content contribute to the flow and filling ofthe tablet presses (Snow et al., 1999).

High-pressure agglomeration is characterized by a large de-gree of densification resulting in low-product porosity. Typi-cally, the products from high-pressure agglomeration featurehigh strength immediately after discharge from the equipment.Low and medium pressure agglomeration yield relatively uni-form agglomerates of elongated spaghetti-like or cylindricalshape, whereas high-pressure agglomeration produces pillow oralmond-like shapes (Barbosa Canovas et al., 2005). Mukherjeeand Bhattacharya (2006) and Ghosal et al., (2010) reported theinter-relationship between rheology of powder and texture ofcompacted mass using model food powder system in presenceof binders.

One of the most common binding mechanisms in these pro-cesses is caused by the short-range molecular attraction forces,that is, electrostatic and van der Waals forces, rather than solidbridging forces. Because these forces are reduced in liquids by

a factor of around 10, the particles bonded by them can disperseeasily in liquid, exhibiting expected instant properties (Pietsch,1999).

Extrusion Agglomeration

In pressure agglomeration, applying external forces to partic-ulate solids in more or less closed dies forms new and enlargedentities. Pressure or press agglomeration using an extruder ispossible for commercial applications of size enlargement by ag-glomeration. In the extrusion agglomeration process, a powdermixture is blended with binder liquid, additives, or dispersantsand then extruded at low pressure followed by drying, cooling,and crumbling to get the final instant product (Pietsch, 1999).Many food products such as snack bars and confectioneries areprocessed and/or finished by pressure agglomeration, mainlyextrusion (Pietsch, 2005). Extrusion has also been used to en-capsulate flavors, vitamin C, and colors (Dziezak, 1988). Themajor advantage of extrusion is its outstanding protection offlavors against oxidation (Barbosa-Canovas et al., 2005) whilebeing a commercially-viable continuous-processing technique.

Tumbling of Powders (Rewetting Agglomeration)

The tumbling agglomeration involves both disintegration ofweaker bonds and re-agglomeration by abrasion transfer and co-alescence of larger units. Coalescence occurs at contact pointsand additional growth of the agglomerate may proceed by fur-ther coalescence, or by layering, or both. The particles to be


agglomerated are larger; however, the particle-to-particle adhe-sion needs to be increased by the addition of binders, such aswater or other more viscous liquids, depending on the propertiesof the particles being agglomerated and the required strength ofthe agglomerate structure (Barbosa-Canovas et al., 2005).

In food industries, most units use static and vibrating flu-idized beds to mix the powder, promote inter-particle collisionsand dry the granules (Coucoulas, 1992). Granules are formed byshear processing in planetary mixers, ribbon blenders, Z-bladeunits, and high-speed intensive mixers. For example, granulatedenzyme products can be manufactured by mixing enzyme solu-tion with a suitable filler to form dough, which is then pressedinto fine granules. The granules are then sprayed with a suit-able binder and further dried in a fluidized bed dryer (Harkonenet al., 1993). Sometimes, a large amount of fine products arereprocessed, causing economical burden to this technology.

Straight through Agglomeration

A liquid concentrate is used in this process. When powdersare produced by spray drying, the agglomeration process canbe accomplished in a fluidized bed connected directly to thespray dryer, where the operating conditions can be controlled sothat the partially dried particles formed in the upper part of thedryer are still sticky. Fine particles, either from recycle or thedrying chamber, are fed into an external fluidized bed to undergocluster formation. Sometimes, steam or atomized water can beinjected into the fluidized bed to assist in the agglomerationprocess. Final drying and cooling are also accomplished in thebed, and the agglomerated product is removed for storage orpackaging. This procedure is more adequate for coffee and babyfoods (Masters and Stoltze, 1973), skim and whole milk, non-caking whey, milk replacer, and ice cream mix. In instant wholemilk production, the straightthrough process is followed by theaddition of lecithin that improves wettability and dispersability.

Spray-bed Dryer Agglomeration

A fluidized bed is integrated into a spray dryer chamber, com-bining spray drying with fluidized-bed agglomeration. Particlesformed in the spray drying-zone enter the integrated fluidizedbed at the bottom of the dryer with high moisture content, andbecome agglomerated in the bed where they are vigorously ag-itated by high fluidization velocity (Quek et al., 2007; Gonget al., 2008). An external fluidized bed is connected to the inte-grated fluidizer for final product drying and cooling. This typeof dryer is most suitable for small to medium sized plants andcan produce agglomerated powder with excellent properties. Asan example, the flow chart for the production of granular encap-sulated flavor powder has been cited in Fig. 2 where maltodex-trin/gum acacia/modified starch have been used as a carrier offlavor. The raw materials are subjected to agglomeration usinga fluidized bed granulator (Buffo et al., 2002).

Atomizer Wheel

This method is used when powder cannot withstand a forcefulagglomeration process or where small-sized agglomerates aredesired. The layout closely resembles the agglomerating tubemethod. The only difference is that a rotary atomizer replacesthe agglomerating tube. The powder falls around the rotatingatomizing wheel and is sprayed with water or binder solution.This system is used for certain baby foods, beverage whiteners,and cocoa/sugar mixtures (Masters and Stoltze, 1973; Jinaponget al., 2008).

Steam Jet Agglomeration

Steam jet agglomeration is a continuous process, which hasbeen used in the food industries for several years to produceagglomerates with favorable instant properties from fine pow-ders. Free falling particles are wetted by turbulent free jets ofsteam. The colliding wetted particles form agglomerates pro-vided that their relative kinetic energy can be dissipated bythe viscous liquid layers on the particles surfaces (Schuchmannet al., 1993). The material to be agglomerated should preferablybe water-soluble; insoluble or water-repellent substances can beprocessed if mixed with a sufficient amount of water-solublematerial such as sucrose or a monosaccharide. Furthermore,the wettability of hydrophobic substances can be improved byadding a surface-active agent, for example, in the case of co-coa powder, lecithin. A typical application is instant beveragesintended for reconstitution with water or milk. The steam jet-agglomeration process combines a low number of particles per

Carriers (maltodextrin/gum acacia/modified starch)

Dissolved in water at 82oC for 45 min

Dispersion containing carrier substances

Allowed to stand overnight at room temperature

Hydrated carrier + flavour

Emulsification for 5min using a high speed mixer

Carrier-flavour emulsion (1:4)

Spray dried encapsulated flavour powder

Spray drying

Agglomerated encapsulated flavour powder

Agglomeration using fluidized bed

Figure 2 Flowchart for flavor encapsulation.


unit volume with short average residence time and narrow res-idence time distribution (Schuchmann et al., 1993) to produceproducts with uniform size and properties.

If the feed contains dry agglomerates, for example, finesbound together by van der Waals forces, these agglomeratesalso take up water, further stabilizing these agglomerates. Afteran average residence time ranging from 1 to 2 s, the particlesleave the agglomeration zone and enter the drying zone. Dryingof the agglomerates causes crystallization of soluble substancesin the liquid bridges joining the primary particles, finally turningthe liquid bridges into solid ones. On the other hand, dependingon the drying rate and temperature, these bridges may consist ofan amorphous structure. In any case, the agglomerates formedby collision in the wetting zone as well as any agglomerateshaving entered with the feed are stabilized.

Binders

Binders are adhesives that provide the cohesiveness essentialfor the bonding of solid particles during the process of ag-glomeration. In wet granulation process, binders promote sizeenlargement and thereby improve flowability of the blend dur-ing the manufacturing process. During wet massing, the bindermay be dissolved in the granulating solvent, which is then addedto the powder, or mixed dry with the powder and the granulat-ing solvent (generally water) (Barbosa-Canovas et al., 2005).Binders are classified as natural polymers, synthetic polymers,or sugars. Some commonly used binders in wet granulationare starch, gelatin, acacia gum, sodium alginate, alginic acid,methylcellulose, Na-carboxymethylcellulose, glucose, sucrose,and sorbitol. In the food system, binders are used in aqueoussolution of lactose or dextrose, gelatine, or a food gum. In prod-uct, the nature of the binder is a major factor that determines thegranule strength, attrition resistance, and granule dustiness. Itaffects the dispersion and dissolution properties, ingredient re-lease rate, and the chemical stability of the ingredients (Knight,2001).

The factors influencing the binder efficiency are concen-tration, viscosity, mechanical properties of the binder, inter-particulate interactions between the binder and the substrate,and binder distribution. Binders differ in their bonding effi-ciency. For example, gelatin or acacia provide high hardnessand slow disintegration to the agglomerate. Methylcellulose pro-duces granulations that compress easily. Glucose or sucrose canbe applied as syrups in concentrations above 50% in wet gran-ulation processes exhibiting good bonding properties, althoughsucrose produces hard and brittle bridges (Barbosa-Canovas etal, 2005). In the agglomeration process, the binder providescapillary and viscous forces that give the wet granules mechan-ical strength and particularly with high shear mixer granulation.The viscosity of the binder is a very important variable affectingthe rate of size enlargement, morphology of the granule, and thesize distribution obtained (Mills et al., 2000; Keningley et al.,1997). A multidimensional model enables the study of criticalparameters in binder granulation such as the reaction rate (so-

lidification of binder) and the size of the added binder droplets,which demonstrates its promising potential (Braumann et al.,2007).

Agglomeration by Heat/Sintering

Sintering is a process in which the particles in a powder masscan be bonded in solid state at elevated temperatures below themelting or softening temperature of the materials. The drivingforce for sintering is diminution of the surface area of the as-sembly of original particles. The accessible internal surface ofthe particles or surface of the pores between the particles fea-tures a specific surface energy. This specific energy is due to thefact that surface atoms have no neighbors. The reduction of thefree surface leads to the reduction in surface energy. Therefore,sintering occurs with a reduction in the total surface energy andaccordingly, the total free energy of the powder decrease withsintering (Pietsch, 2002). The viscosity of material can be de-creased by increasing temperature or moisture level to enablethe material to build viscous bridges between the particles byviscous flow and the process is called sintering (Palzer, 2005).A simple method for the production of chocolate beverage gran-ules has been reported by Omobuwajo et al. (2000) wherein al-kalized cocoa powder (91–94%), malt extract (2–3%), skimmedmilk powder (3–5%), and vitamins and minerals (0.5–0.8%) aremixed with granular sugar to obtain the chocolate drink powderblend; later, it is heated over a metal plate with constant agitationto produce instant chocolate beverage granules.

At a certain elevated temperature, atoms and molecules be-gin to migrate across the interface where particles touch eachother in solid state. Depending on the temperature, time, and theintensity of contact, the diffusion of matter forms bridge-likestructures between the surfaces, which solidify upon cooling.This may result in a densification of the compact, which is dueto an elimination of pores and associated shrinkage.

It is desirable to understand the nature of the bonds betweenparticles in agglomerates that are formed during the processof agglomeration. The bonding mechanism involved is inter-molecular forces (Van der Waals force and hydrogen bond),electrostatic force, liquid bonds, solid bridges, and mechanicalinterlocking. The intermolecular attractive force between par-ticles is inversely proportional to the seventh power of theirseparation distance. As the roughness of surface increases theeffective separation distance, Van der Waals forces are of lowimportance in granulating systems; electrostatic force is alsoof the same order of magnitude as the Van der Waals forces.Absorbed liquid binder layers on the particle surface have theeffect of smoothing out surface roughness and thus particle sep-aration distances tend to decrease leading to an increase in Vander Waals forces. The mobile liquid bonds have an importantrole during the granulation process. An increase in the quantityof liquid thus changes the nature of the bonds and influencesthe overall granule strength. Particles containing liquid bondsat the contact point between individual particles are stated to bein the pendular state; an increase in the liquid content gives rise


to the funicular state, and finally to the capillary state in whichthe inter-particle space is saturated with liquid.

Though theory on the binding of particles by liquid is avail-able, the same is not true on solid bridges between particles.The strength of such bridges depends on the amount of mate-rial present and its structure. A finer crystal structure results instronger bonds and there exists some correlation between bondstrength and higher drying temperatures. Granule strength is afunction of the structure and physical properties of the binderused.

In agglomeration processes, granules grow by the successiveaddition of primary particles to an agglomerate that is alreadyformed; this happens when two particles or two granules arebrought into contact with sufficient liquid binder. In tumblingbeds of powder, two agglomerates colliding are kneaded to-gether by the tumbling action of the mixer to form near spherical-shaped granules. Subsequent growth occurs by a crushing andlayering mechanism in which the smallest and weakest granulesare crushed by larger ones and the material becomes redis-tributed around the surface of the large granule in a uniformlayer (Smith, 2003).

CHARACTERIZATION OF AGGLOMERATEDPRODUCTS

The knowledge and characterization of raw materials andproducts are essential to select appropriate method and ma-chine, optimize processes, functionality, product formulation,and reduce the cost of the product. The bulk properties of foodpowders are a function of physical and chemical properties of thematerial, the geometry, size, and surface characteristics of theindividual particles, as well as the whole system. Parameters thatdetermine the properties of agglomerates include those relatedto primary particles and agglomerates. Thus, the measurementof powder property is important because these properties in-trinsically affect powder behavior during storage, handling, andprocessing.

Physical Properties

Bulk Density and Porosity

The density of the sample is conventionally referred to asmass per unit volume. It is considered quite relevant for deter-mining other particle properties such as bulk powder structureand particle size requiring a careful definition. Density of apowder system decides the container volume, requirement ofpackaging materials, and selection of machinery for processing.Depending on how the total volume is measured, different def-initions of particle density can be given; these are true particledensity, the apparent particle density, or the effective (or aero-dynamic) particle density. Most food particles have densitiesconsiderably lower of about 1000–1500 kgm−3.

The density is of primary importance in applications involv-ing bulk flow of air around the particles like in fluidization, ofliquid as sedimentation, or flow-through packed beds. The mea-surement of bulk density is of fundamental use by the industryto adjust storage, processing, packaging, and distribution con-ditions. Bulk density is the mass of the particles that occupiesa unit volume of a bed, whereas porosity is defined as the vol-ume of the voids within the bed divided by the total volume ofthe bed. There are several types of bulk density based on themethod of volume determination. The bulk density (ρb) of pow-ders is the mass of the particles that occupies the unit volumeof a bed. It is determined by particle density, which in turn isdetermined by solid density, particle internal porosity, and alsoby special arrangement of the particles in the container. Bulkdensity includes the volume of the solid and liquid materials,and all pores. Compact or compacted density is determined af-ter compressing the powder’s bulk mass by mechanical pressureand impact(s). Tap or tapped density results after a volume ofpowder has been tapped or vibrated under specific conditions.Loose bulk density is measured after a powder is freely pouredinto a container. Aerated bulk density is used for testing underfluidized conditions or during pneumatic conveying. The vol-ume fraction of air over the total bed volume is called porosity(Barbosa and Juliano, 2005).

Flowability

Powder flow is defined as the relative movement of a bulkof particles among the neighboring particles or along the con-tainer wall surface (Peleg, 1978). The forces involved in powderflow are gravity, friction, cohesion (inter-particle attraction), andadhesion (particle-wall attraction). Particle surface properties,particle shape, size distribution, and the geometry of the systemare factors that affect the flowability. It is, therefore, quite dif-ficult to have general theory applicable to the flow of all foodpowders in all possible conditions that might be developed inpractice (Peleg, 1978).

The practical objective of powder flowability determinationis to provide both qualitative and quantitative knowledge of pow-der behavior which can be used in the designing of equipment. Inorder to flow, the powder must fail and its strength must be lessthan the load put on it. The basic properties describing the failurecondition are: (i) the angle of wall friction (ii) the effective angleof internal friction (iii) the failure function (iv) the cohesion and(v) the ultimate tensile strength (Barbosa-Canovas et al., 2005).The cohesion is a function of inter-particle attraction and is dueto the effect of internal forces within the bulk, which tend toprevent planar sliding of one internal surface of particles uponanother. The ultimate tensile strength of a compact powder is themost fundamental strength mechanism, representing the mini-mum force required to cause separation of the bulk structurewithout major complication on particle distribution within theplane of failure. There are many empirical tests used to assessflowability, including measurement of angles of repose (Teu-nou et al., 1995), Johanson indicizer (Johanson, 1992), Hausner


ratio, and Carr indices, flow-through opening and compressiontesting (Peleg, 1978), Jenike shear cell (Jenike, 1964), and by theupward and downward movement of a rotating blade employ-ing a texture measuring system (Mukherjee and Bhattacharya,2006).

Structural Features

Particle Size and Shape

Particle size is one of the most important physical character-istics of particles. In the case of simple shapes such as the sphereor cylinder, the size is explicitly determined by one or severaldimensions. However, particles are of irregular shape so that alarge number of dimensions would be required to describe thesize and shape. The term size of a powder or a particulate mate-rial is relative. To determine the particle size, in principle, anymeasurable physical property which correlates with characteris-tic geometric dimensions or equivalent dimensions can be used(Schubert, 1987). The bulk density, compressibility, and flowa-bility of food powders are highly dependent on particle size andits distribution. The common convention considers that for aparticulate material to be considered a powder, its approximatemedian size (50% of the material is smaller than median sizeand 50% larger) should be less than 1 mm. It is also a com-mon practice to talk about “fine” and “coarse” powders; severalattempts have been made to standardize particle nomenclaturein certain fields. A significant number of food powders maybe considered to be in the fine range. Methods for measuringparticle size are sieving, microscope counting techniques, sed-imentation, and stream scanning. The median particle sizes incommon food commodities have been indicated by Barbosa-Canovas et al. (2005).

Particle shape influences factors such as flowability of pow-der, packing, interaction with fluids, and coating of powderssuch as pigments (Barbosa-Canovas et al., 2005). Generallythe particle shapes are defined as acicular (needle shape), an-gular (roughly polyhedral shape), crystalline (freely developedgeometric shape in a fluid medium), dentritic (branched crys-talline shape), fibrous (regular or irregular thread-like), flaky(plate-like), granular (approximately equidimensional irregularshape), irregular (lacking any symmetry), modular (rounded ir-regular shape), and spherical (global shape), etc.

Solid and Liquid Bridges

In general, interaction between particles is regulated by therelationship between the strength of the attractive (or repul-sive) forces and gravitational forces. For all particles in theamorphous rubbery state (or above glass transition tempera-ture), forces causing primary particles to stick together are inter-particle attraction forces (Van der Waals or molecular forces andelectrostatics forces), liquid bridges, and solid bridges (Hartleyet al., 1985; Seville et al., 2000). Inter-particle forces are in-versely related to the particle size (Rennie et al., 1999; Adhikari

et al., 2001). Van der Waals and electrostatic attraction are notas high as the inter-particle connecting force coming from liq-uid bridges (Schubert, 1987). Van der Waals forces arise fromelectron motion among dipoles and act over very short distanceswithin the material structure, becoming prevalent when the par-ticle size is less than 1 µm (Hartley et al., 1985). Electrostaticforces are longer ranging forces that arise through surface dif-ferences on particles and are present when the material does notdissipate electrostatic charges.

Solid bridge is formed as a result of sintering, solid diffusion,condensation, or chemical reaction and arises from the materialdeposited between the agglomerated particles. Solid bridges canalso be built up by chemical reaction, crystallization of dissolvedsubstances, hardening of binders, and solidification of meltedcomponents (Loncin and Merson, 1979). Liquid bridges are re-lated to chemical interactions between particle components andresult from the presence of bulk liquid (generally unbound wa-ter or melted lipids) between the individual particles. In liquidbridges, the force of particle adhesion arises either from sur-face tension of the liquid/air system (as in the case of a liquiddroplet) or from capillary pressure. Composition of the liquidin the bridge varies in different food materials. The “bridgingpotential” or “stickiness” is related to factors such as powdermoisture, fat or low-molecular-weight sugar content, and theshape of the particles (Barbosa-Canovas et al., 2005).

Instant Properties

The instantaneous properties of agglomerates are the mostdesirable properties of agglomeration processes and they can bemeasured by the following four dissolution properties when ag-glomerates are spread on the surface of liquid (Schubert, 1987).These are wettability (liquid penetration into a porous agglom-erate system due to capillary action), sinkability (the sinkingof agglomerates below the liquid surface), dispersibility (thedispersion of agglomerates with little stirring), and solubility(dissolving of soluble agglomerates in the liquid).

The ability of the bulk powder to imbibe a liquid under theinfluence of capillary forces is called wettability (Freudig et al.,1999) and it is the ability of the powder particles to overcomethe surface tension between themselves and the liquid (Fanget al., 2008). In technical processes, the wetting of the particlesis often the rate-controlling step and depends largely on particlesize. The importance of wettability lies in the development andquality control of several food powders like milk powder, instanttea and coffee, and instant mixes for which claims have beenmade that they have instant properties like solubility. The con-ditions favoring good wettability are large particles with largepores in between, high porosity as long as a critical porosity isnot exceeded, and small contact angle (Freudig et al., 1999). Thepresence of free fat in the surface reduces wettability. The selec-tive use of surface-active agents, such as lecithin, can sometimesimprove wettability in dried products containing fat.

Sinkability is defined as the falling of powder particles be-low the surface of an aqueous phase or liquid (Thomas et al.,


2004). It depends mainly on the particle size and density, sincelarger and denser particles usually sink faster than the finer andlighter ones. A higher particle density combined with a lowerquantity of air content enclosed within powder particle resultsin a faster sinking rate (Caric and Milanovich, 2002). Swellingcan strongly inhibit sinking (Freudig et al., 1999).

Dispersability describes the ease with which the powder maybe distributed as single particles over the surface and throughoutthe bulk of the reconstituting water. In dispersability measuringtest, it is essential to assume that following a short period ofdispersing, soluble particles are completely dissolved and sus-pended particles are regarded as residual material. There areseveral techniques for measuring the dispersibility, includingthe measurement of dispersion kinetics using an optical fibersensor to collect the light backscattered by the particles in sus-pension (Galet et al., 2004; Vu et al., 2003).

Solubility refers to the rate and extent to which the com-ponents of the powder particles dissolve in water. Solubilitydepends mainly on the chemical composition of the powder andits physical state. It is also the final step of powder dissolutionand is possibly the key determinant of the overall reconstitu-tion quality. It is used to represent the complete phenomenonof milk powder recombination, comprising soluble componentssuch as lactose, undenatured whey protein, and salts, as well asdispersible components like casein (Thomas et al., 2004). Solu-bility is also used to quantify the rate of dissolution to describethe various combinations of milk powder reconstitution proper-ties. Generally, the dissolution rate is flavored by the presence ofsmall hydrophilic molecules on the surface (Lillford and Fryer,1998).

CONCLUSIONS

Properties of food powder and their conversion to agglomer-ated products are important areas that directly affect the selec-tion of process equipment, storage ability, and product formu-lation. The area of understanding the behavior of food powderis still at an infant stage though there is considerable progressin other areas like ceramic, mining/metallurgy, and pharmaceu-tical technology. Scope exists to study the properties of foodpowder with an emphasis on application in developing differentfood formulations. The specific areas that require attention arecharacterization and understanding of food powders along withfunctional behavior. It is also desirable that raw materials fromdifferent sources are also considered for research investigationsthat would improve our existing knowledge on food powders.

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