Crystallization and Drying of Milk Powder in A

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Crystallization and Drying of Milk Powder in a Multiple

Stage Fluidized Bed DryerNima Yazdanpanah

a& Tim A. G. Langrish

a

aDrying and Process Technology Group, School of Chemical & Biomolecular Engineering, T

University of Sydney, New South Wales, Australia

Version of record first published: 27 Jun 2011.

To cite this article: Nima Yazdanpanah & Tim A. G. Langrish (2011): Crystallization and Drying of Milk Powder in a Multiple

Stage Fluidized Bed Dryer, Drying Technology: An International Journal, 29:9, 1046-1057

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Crystallization and Drying of Milk Powder in aMultiple-Stage Fluidized Bed Dryer

Nima Yazdanpanah and Tim A. G. LangrishDrying and Process Technology Group, School of Chemical & Biomolecular Engineering,

The University of Sydney, New South Wales, Australia

The rationale of this study has been to use fluidized beds tocrystallize amorphous spray-dried skim milk powders with multiplestages of processing at different temperatures and humidities withthe aim of rapidly making mostly crystalline powders. This paperdiscusses the performance of a multiple-stage fluidized bed dryer,and a combination of crystallization of lactose in spray drying at

high humidity (lactose nuclei formation) and subsequent fluidizedbed drying. Two different combinations of spray dryer andmulti-stage fluidized-bed dryer have been suggested to crystallizelactose in skim milk powder. The results show significant improve-ments in the crystallinity of the powders. Moisture sorption testand X-ray diffraction analysis were used to assess the crystallinityof the powders. The processed powders that were crystallized in ahumid-loop spray drying combined with a two-stage fluidized-beddryer/crystallizer showed 92% improvement in lower amorphicityby processing at different stages of 70C, 50% RH and 80C,50% RH for 15 minutes. The conventionally spray-dried powdersthat were crystallized in a three-stage fluidized-bed dryer/crystalli-zer showed 87% improvement in lower amorphicity (less moisturesorption) by processing at different stages of 60C, 50% RH;70C, 40% RH; and 80C, 40% RH for 20 minutes. Themultiple-stage fluidized bed system showed distinctive potential tocrystallize lactose significantly in skim milk powder using anindustrial-feasible process.

Keywords Crystallization; Fluidized-bed dryer; Lactose; Milkpowder; Spray drying

INTRODUCTION

An important application for studying the solid-phasecrystallization process in spray and fluidized-bed drying isthe production of milk powder in an industrial process con-figuration. Spray drying is a very significant part of milkpowder production where most of the powders specifica-tions and properties are determined to a large extent,[13]

while post processing in fluidized beds could modify them.Although the main morphological structure forms in thespray-drying chamber, fluidized bed drying has the poten-tial to improve powder crystallinity.[46]

More than 3.5 million tonnes of milk powders were pro-duced in the year 2009, and 2.8 million tonnes wereexported from main producer countries such as Australiaand New Zealand, which export 45% of the total worldmilk powder market. Processing, storage times, and hand-ling of these spray-dried powders are very sensitive to the

storage conditions, due to the presence of amorphouslactose in spray-dried milk powders that are produced byconventional drying facilities. The physical and thermo-dynamical states of milk powder, like the crystallinity ofthe powders, strongly depend on the process conditions,which can cause unstable powders. The unstable state ofpowders (amorphicity) causes some changes during sto-rage, such as stickiness, caking, degradation, and non-enzymatic browning.[7,8]

The transition from an amorphous solid-state to a vis-cous rubbery state occurs at the temperature known asthe glass-transition temperature (Tg). The presence of plas-ticizers, moisture in this case, reduces the glass-transition

temperature, and the crystallization of lactose occurs attemperatures that are above the glass-transition tempera-ture.[911] Among the different ingredients of milk powders,amorphous lactose is the most hygroscopic and unstablematerial. By sorbing moisture, the amorphous lactosebecomes sticky, and forms bridges with other particles thatleads to caking, forming a product that is non-free-flowingand difficult to handle.[12] Lactose is more stable in thecrystalline alpha monohydrate form; and crystalline lactoseor crystallized lactose-containing powders are free-flowingand have little tendency to agglomerate. With the presenceof sufficient moisture in powders, this transition can takeplace during spray drying and=or fluidized-bed drying

when the process temperature is much higher than theglass-transition temperature. Therefore, the powders canbe dried and crystallized simultaneously.

Many researchers have recommended that theamorphous-lactose fraction could be treated in a crystalli-zation facility after spray drying to crystallize lactose-containing powders, thus improving the physical propertiesof powders such as caking and agglomeration.[4,5,13]

Correspondence: Nima Yazdanpanah, Drying and ProcessTechnology Group, School of Chemical & Biomolecular Engin-eering, The University of Sydney, New South Wales, Australia;E-mail: [email protected]

Drying Technology, 29: 10461057, 2011

Copyright# 2011 Taylor & Francis Group, LLC

ISSN: 0737-3937 print=1532-2300 online

DOI: 10.1080/07373937.2011.561461

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Yazdanpanah and Langrish[4] have suggested continuedcrystallization in a fluidized bed dryer=crystallizer afterspray drying of skim milk powders; and Hynd[14] hassuggested a belt crystallizer after spray drying for wheypowder. Currently, fluidized-bed processing is widelyemployed after spray drying in dairy processing to agglom-erate, instantize, and dry powders where it may be possible

to perform further crystallization. Most of the researchthus far has been aimed at post-crystallizing whey pow-ders,[5,14] or pre-crystallizing them by high shear forces,[15]

but the recommended processes were time-consuming andmay not be suitable for industrial applications.

Fluidization of amorphous powders is very sensitive toprocess parameters like temperature, air velocity, andhumidity.[5,16] A significant problem with the fluidizationof milk powder and other lactose-containing powders usinghot humid gas is the stickiness of powders. A commonlyfound problem is the collapse of the bed being processedat high relative humidities, resulting in cake formation fromthe cohesion of all the particles in the bed and increasedstickiness of amorphous powder at elevated humid-ities.[17,18] Although the highest temperature and humiditygive the greatest crystallinity in the least time,[10,19] it isnot possible to fluidize hygroscopic=amorphous lactose-containing powders at these high temperatures and highhumidities. For instance, Ibach et al.[19] demonstrated thatwhey powder can crystallize at a relative humidity of 85%and a temperature of 100C in just 1 minute, while Nijdamet al.[5] showed that it was impossible to fluidize whey pow-der under those conditions. However, the upper limit ofprocessing at high temperature and humidity depends onthe amount of amorphous component in the powders.

Therefore crystalline powders or partially crystallized pow-ders from spray drying can be fluidized at higher tempera-tures and humidities that boost the crystallization rateand reduce the processing time.

In recent work, highly humidified hot air was used tocrystallize lactose and lactose in skim milk powders in avibrated fluidized bed;[4] however, the one-hour processingtime is not sufficiently short for industrial interest. Thepowders that were crystallized by that technique showeda high degree of developed crystallinity. Recently Islamet al.[20] have suggested a new technique called HumidLoop to spray-dry lactose, which uses highly humid airto obtain highly crystalline lactose from spray dryers.

Although the effects of other components, like proteins,on the crystallization rate, kinetics, crystal shapes, and dis-tributions have not been explained yet, this technique couldalso possibly apply to other lactose-containing powders.GEA=Niro is proposing a similar closed-cycle layout forsolvent recovery using nitrogen. Although the configur-ation is similar, the process conditions and aims are differ-ent, and the conditions are unlikely to lead to thecrystallization of amorphous solids.[21]

This work investigates the crystallization of milk powderin a multiple-stage fluidized-bed system and combinedmultiple-stage fluidized-bed system with spray drying ofmilk in hot humid air. The aim of this paper is to attemptto crystallize amorphous lactose in milk-powder drying,improving physical properties in a timely process with anindustrial approach.

MATERIALS AND METHODS

Commercial Skim Milk Powder (CSMP)

Skim milk powder (Coles, New Zealand, Batch No9199) with 4.1% w w1 moisture (dry basis) and an aver-age particle size of 120 mm was used as a reference material,being a typical commercial skim milk powder (CSMP).This material was compared with the products of thisexperiment.

Fresh Liquid Skim Milk, 35% (W W1) ConcentrateSkim Milk Slurry and Fresh Spray-Dried Milk Powder

Concentrated milk (3045% solid content) that is pre-pared by falling film evaporators is used in dairy plantsto feed industrial spray dryers.[22] To mimic the industrialconditions and to obtain larger particles, the concentratedfeed for spray drying was prepared by reconstitution offresh spray-dried milk powder to particular concentrations.Results of recent publications regarding the surface com-position of spray-dried milk powder[2] show that there-spray drying of reconstituted solutions maintained thecharacteristic physical properties of fresh milk.

Fresh skim milk (0.1 g fat.100 mL1 from local suppliers,Coles, Australia) was fed into a Buchi B-290 spray dryer(Buchi Labortechnik, Switzerland) with an inlet air tem-perature of 140C and an outlet air temperature of 67C.The powders were collected from the collection vessel atthe bottom of a cyclone and immediately reconstituted withdistilled water at 25C to a concentration of 35% w w1.This reconstituted slurry was fed again to a Buchi B-290spray dryer (Buchi Labortechnik, Switzerland) with an inletair temperature of 140C and an outlet air temperature of79C. The powders were collected from the collection vesselat the bottom of a cyclone and were immediately fed to thefluidized-bed dryers for the experiments.

Moisture Sorption Tests

Moisture sorption behavior was studied with two repeatsamples of the CSMP, powders produced from conven-tional spray drying (lab scale) and processed powders fromthe proposed set-up. A mass of 11.5 grams of the powderwas placed on a 10-mm-diameter borosilicate glass Petridish with a nearly monolayer particle thickness, and themass change as a function of storage time was recordedonce per minute over a period of two to three days to reacha constant mass by using an analytical balance (0.0001g,

CRYSTALLIZATION OF MILK POWDER IN A FLUIDIZED BED DRYER 1047


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Mettler Toledo, AB 204-S, Switzerland). The sample andthe balance were placed in a sealed sorption box wherethe relative humidity (7075%RH) and the temperature(24.525C) were kept constant using a saturated salt sol-ution of sodium chloride and electric light bulbs, respect-ively. The levelling off of the curve indicates the end ofthe crystallization process.[23]

Oven Drying

The free moisture content (dry basis) was measured byweighing 3 g of a sample before and after drying in an oven(Labec, Australia) at 85C for two days. The skim milk wasdried at a temperature of 85C, as suggested by DIN 10321,instead of 102C as suggested by IDF standard 26A:1993,to prevent degradation and browning of the skim milkpowder.[24,25] The measured moisture content of the skimmilk were adjusted by adding 0.371% (w w1) to themoisture content determined by the decrease in mass, tocompare with moisture contents of material dried at

102

C, as recommended by De Knegt and Van DenBrink.[26] Each test was carried out in duplicate.

Scanning Electron Microscopy

A scanning electron microscope was used to observe thepowders in terms of the surface and bulk structures. Thesamples were prepared by placing a small amount of sam-ple on a carbon tape that was placed on an aluminium sam-ple disc. The sample was coated by a standard 30 nm goldlayer to produce the conductive surface (Emitech, K550X,Quorum Technologies, UK). The electron micrographswere produced using a Zeiss ULTRA plus (Carl ZeissSMT AG, Germany) scanning electron microscope

(SEM) in secondary electron mode with operating con-ditions of 5 keV. A range of 500 to 70,000 times magnifi-cation was used in the images.

X-Ray Diffraction (XRD)

X-ray Diffraction (XRD) was conducted using aSiemens D5000 diffractometer. The scanning range wasset to 550, the step size was 0.02 with a scanning rateof 1 step=s, and the operating conditions were 40 kV and30 mA. EVA evaluation program (DIFFRAC Plus, Brukeranalytical X-ray system, GmbH) was used for peak search-ing as part of quantitative crystallinity analysis.

PROCESS (DRYING/CRYSTALLIZATION) SET-UP

Actual Laboratory Fluidized Bed Set-Up (One Module)

The same fluidized bed dryer=crystallizer apparatus thatwas used for the previous study was employed here. [4] Avibrated fluidized bed (Fig. 1 has been used to fluidize thepowders with hot humid air. A variable speed centrifugalfan (0.75kW motor and 50 cm fan diameter, Western Elec-tric Australia, ABB speed controller), a counter-current

humidifier column (a packed-bed vessel with circulatinghot water (Thermoline, Australia), constructed in our lab-oratory), and a 1 kW electrical heater coil (constructed inour laboratory) have been used to generate the hot humidair. The supplied air was able to be conditioned to have atemperature range from 25 to 100C, a relative humidityfrom 10 to 95%, and a fluidization air velocity within thefluidization column from 0.1 m s1 up to 5 m s1 at ambi-ent conditions. The inside diameter of the fluidized bed col-umn is 100mm with a height of 50 mm. A stainless-steelmesh (45mm) plate with a diameter of 100 mm (Endecotts,UK) has been used as a bed support (air distributer). Asmall vibrated motor (IKA-werk, Germany) with a 2 cmunbalanced crankshaft was joined to the column and usedto agitate the bed at 500 Hz to break up the channels in thebed during processing and create smooth fluidization. Thefluidization column wall and bed support have been madefrom stainless steel. Heating insulation has been used on all

pipes and connections between the humidifier, the electri-cal heater, and the fluidized-bed chamber and fluidized-bedbody. Five RTD thermocouples (Pyrosales, Australia)have been connected to the system to measure the tempera-ture of the humidifier, the air heater, the inlet fluidizationair (dry-bulb), the wet-bulb temperature of the inlet air tothe fluidization chamber and the outlet air. All sensed tem-perature data were transferred to a computer by a datalogger (Datataker DT-505, Datataker Pty Ltd, Australia).The dry-bulb and wet-bulb temperature of the fluidizationair have been reported here. The wet-bulb temperatures ofthe inlet air were measured, and the dry-bulb temperatureof the outlet air was also measured. No significant

temperature difference was noticed between the inlet andoutlet dry-bulb temperatures in the well-insulated shortfluidized bed. No significant differences were foundbetween the inlet and outlet wet-bulb temperatures,because typically 1 g of moisture was adsorbed or desorbedin a 20-minute period, during which 2.3 kg of air (0.2 m=sthrough 10 cm diameter bed during 20 minutes) flowedthrough the bed. The whole set-up was run for two hoursto reach steady state (temperature and humidity) before

FIG. 1. A schematic diagram of the fluidized-bed system (one module)

taken from Yazdanpanah and Langrish.[2]

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feeding the powders, and all inlet process variables werenormally kept constant during each experiment. The aer-ation time for fluidizing the samples was up to one hour,and during this time material was well agitated withcontinuous vibration.

Actual Laboratory Spray Dryer

As the conventional spray-dryer method (lab scale), aBuchi B-290 mini-spray dryer (Buchi Labortechnik,Switzerland) with an inlet air temperature of 140C, anoutlet air temperature of 79C, a feed pump rate of 25%(8mL min1), and an aspirator rate of 80% (28 m3 h1),a rotameter setting of 55 mm (1 m3 h1) for the atomizingair flow, was used to dry a 35% concentrate solution ofmilk=s. This set-up was used to directly feed themultiple-stage fluidized-bed dryer.

To partially crystallize milk powder by spray dryingwith high humidity air, the same spray dryer was retrofittedwith a closed loop for the inlet=outlet air through a HumidLoop. This Humid Loop has been described in the work ofIslam et al.,[20] and it allows high humidities to be con-trolled during spray drying and crystallization.

The dryer was allowed to reach steady state for bothhumidity and temperature for at least 30 minutes withwater spraying before the concentrated milk was sprayed.The exhaust gas was re-circulated to the dryer inlet afterpassing through the fluidized-bed dryers=crystallizers andcondenser unit. The cooling water flow rate(% 90ml min1) through the copper helix coils within thecondenser unit was controlled at such a rate that the rela-tive humidity of the air into the spray dryer was %7580%.

Potential Industrial Combined Process Set-UpMany combined configurations of spray drying and flui-dized bed drying equipments are being used in milk pow-ders plants[22] and other drying facilities. These combinedconfigurations have been suggested to improve the qualityof products, to ease drying process, to reduce energy con-sumption, and to have better process control.[2730]

Figure 2 shows a possible configuration of the crystalliza-tion system for continuous drying and crystallization of35% concentrated milk solution with partial crystallizationin a Humid Loop spray drying and two-stage fluidized bed.The process included a common spray dryer that sucks theinlet air (drying air) from the condenser that controls the

humidity of the Humid Loop, and two modules of fluidizedbeds that crystallize lactose in milk powders bypost-processing of spray-dried powders. Different combi-nations of temperature and humidity were used in eachmodule so that the last stage was working at the highesttemperature and humidity. The concept of using the differ-ent stages of crystallization and the different settings oftemperatures and humidities will be explained in thefollowing section. The partial crystallization of lactose in

milk powder that had been done in the spray dryer cham-ber (with a Humid Loop) was continued with two stages ofsolid-phase crystallization in fluidized beds.

It is also possible to avoid using a Humid Loop, producetotally amorphous milk powder by spray drying, and thencrystallize lactose in milk powder by three modules offluidized-bed dryers=crystallizers with different sets of tem-perature and humidities for a continuous process(Figure 3). In this case, the partial crystallization of lactosein the spray dryer will not occur and raw amorphous milkpowder from the spray dryer will be crystallized in thethree-stage fluidized-bed crystallizer. Different numbersof processing stages with various processing conditionswere tested to achieve maximum crystallinity in milk pow-ders. It was found that the minimum number of effectiveprocessing stages to crystallize lactose in milk powder to

the most crystalline state in the shortest possible processingtime was three stages with the proposed temperatures andhumidities shown in Table 1.

The sizes of the fluidized beds depend on the outletpowder flow rate from the spray dryer and the residencetime needed for crystallization. At an industrial scale, these

FIG. 2. A schematic diagram of milk powder drying and crystallization

in a closed cycle with a two-stage fluidized-bed system.

FIG. 3. A schematic diagram of milk powder drying and crystallizationin a three-stage fluidized-bed system.



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dryers are not separate, and the stages are combined intoone body that is separated by internal baffles that allowzones with different operating conditions for step-wisecrystallization. The final fluidized bed cooler just decreasesthe powders temperature and has no significant effect onthe powder moisture content and=or crystallinity.

Processing Conditions

Fluidization Limits

Due to the hygroscopic nature of amorphous lactose,lactose-containing materials become sticky when exposedto humid environments. This sticky behavior causes pro-blems in fluidization with hot=humid air, and it is imposs-ible to achieve smooth fluidization for all temperatures andhumidities.[31] The upper limit of fluidization depends onthe percentage of crystallinity in the powders, therefore(for a given particle size) the worst fluidization correspondsto the highest amorphicity in the powders, while crystalline

milk powder has very smooth fluidization at high tempera-tures and humidities.[4] This problem is the main reason forsplitting the processing time into different modules to max-imize the crystallization rate by maintaining an adequatelyhigh differential temperature (TTg) in different stages cor-responding to different amounts of amorphicity in thepowders, which achieves a greater degree of crystallinityin less time.

Different fluidization ability may occur for differentamounts of crystallinity in the powders, so in thefluidization=crystallization process, the humidity and tem-perature can be increased as a function of time while thecrystallinity of powders is developing. If the partial crystal-

lization of milk powders is carried out in the spray-dryingchamber, the produced powder is more easily fluidized atrelatively higher temperatures and humidities. Thereforethe initial stage of fluidized-bed processing could be per-formed at higher humidities or temperatures comparedwith raw powders from conventional spray dryers.

Many researchers have studied the sticky behavior offood powders.[16,32] Hennings et al.[16] found that, at acertain relative humidity, the temperature of skim milk

powder could be 25C more than the glass-transition tem-perature of lactose before the powder became sticky. Thesevalues are valid for mostly amorphous powder. Thepartially-crystallized powders have higher sticky-point tem-peratures.[33] The fluidized-bed crystallizer can be fed byhigher temperature= humidity air, and therefore one fluidi-

zation stage can be omitted.The process complication and number of required

fluidized-bed dryers depend on the fluidization ability andcrystallinity of the powders from the spray dryer. The pow-ders from conventional spray drying are more amorphous,and these powders need to be processed in three-stage flui-dized-bed dryers with a high residence time, while partial-lycrystallized powders from the Humid Loop spraydrying may be processed in two stages and with shortertimes. The process pathways and fluidization abilities ofdifferent powders are shown in Figure 4.

The bold continuous line in Figure 4A (left line) presentsthe upper limit of fluidization for the raw amorphous milk

TABLE 1Process conditions in the different sections of the conventional milk powder production=crystallization

with a three fluidized-bed crystallizer module

Processtemperature (C)

Air relativehumidity (%)

Residencetime (min)

Material inlet moisturecontent (%,w w1)

Material outlet moisturecontent (%,w w1)

Spray dryer Drying air 160 Inlet air 55%@25C 5.6 0.2Fluidized bed 1 60 3 50 1 10 5.6 0.2 7.9 0.2Fluidized bed 2 70 2 40 1 5 7.9 0.2 5.2 0.1Fluidized bed 3 80 2 40 1 5 5.2 0.1 4 0.1Cooler 25 55 5 4.0 0.1 3.7 0.2

FIG. 4. The upper limit on the relative humidity for which differenttypes (crystallinity) of skim milk powder can be fluidized and the process

pathways around different stage of crystallization=drying. A) Fluidizationof conventional spray-dried powders in three stages, B) Fluidization ofHumid Loop spray-dried powders in two stages.



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powder at the corresponding temperatures and humiditieswhile the bold dashed line in Figure 4A (top line) representsthe upper limit of fluidization of the mostly crystalline milkpowder. The two dashed lines between these two upper lim-its are representative limits for partially crystallized powderwith some percentage improved crystallinity during the

process. The safe process conditions of temperature andhumidity to keep the bed well fluidized and avoid cake for-mation should be below these upper limits. The processpath shows the initial conditions for processing and thechanges in different stages. Figure 4B shows the partiallycrystallized powder in Humid Loop spray drying has higherinitial upper limits of fluidization due to the lower amorph-ous contents. Murti[34] used a similar concept to assess thestickiness of milk powder by using a particle gun.

Process Set-Up

Tables 1 and 2 show some typical experiments per-formed for this study. In this experiment, a batch-wise flui-

dized bed was used, but the results are likely to be valid forcontinuous processes, especially at industrial scale, whereplug-flow fluidized-bed dryers are used, so that the batchtime here is equivalent to the residence time in the continu-ous process.

RESULTS

Surface Morphology

SEM micrographs show some morphological changesfor skim milk powders after processing in the fluidized bedsat different humidities and temperatures. Crystallized lac-tose appeared to be formed on the surface and inside of

the particles. Figures 5 and 6 show the different powdersbefore and after processing (crystallizing) by hot humidair in a fluidized-bed dryer. The surface of the raw milkpowders (produced by conventional spray drying) appearto be untextured and amorphous (Fig. 5A), while the pro-cessed milk powders have heavily textured appearances,suggesting that the particle is crystalline. Furthermore, nocrystalline lactose or lactose crystal nuclei appear on thesurfaces of the raw conventional spray-dried milk powders

(Fig. 5A), while the surfaces of the spray-dried powdersfrom a Humid Loop (Fig. 6A) contain some lactose crystalnuclei that show partial crystallization of lactose duringdrying with hot humid air. As Figure 5B shows, fine anduniform (approximately 200nm) lactose crystals haveappeared inside and on the surface of the processed pow-

ders from conventional spray drying after processing in athree-stage fluidized-bed dryer. The processed powdersfrom a three-stage fluidized bed contain very crystallinestructures throughout the particles that show deep moist-ure penetration into the particles and uniform crystalliza-tion of lactose in milk powder particles (Fig. 5B).

On the surface of the spray-dried powders in a HumidLoop, after being processed with hot humid air in atwo-stage fluidized-bed dryer=crystallizer, some large lac-tose crystals were formed along with the smaller lactosecrystals. Large lactose crystals possibly were formed fromalready developed lactose nuclei or from highly saturatedregions of lactose due to the higher initial moisture con-

tents of the powders from the Humid Loop. The partiallycrystallized powders from a Humid Loop have highermoisture contents and lower glass-transition temperature;this could cause more crystals to grow in shorter times oreasier molecular movement in wet solid structures to formbigger crystals. Spray drying of powders in a Humid Loopneeds to be precisely controlled to give fine and uniform

TABLE 2Process conditions in the different sections of the combined crystalline milk powder production with

a two fluidized-bed crystallizer module (Humid Loop)

Processtemperature (C)

Air relativehumidity (%)

Residencetime (min)

Material inletmoisture content

(%,w w1)

Material outletmoisture content

(%,w w1)

Spray dryer Drying air 140 Inlet air 7580% @45C 8.8 0.2Fluidized bed 1 70 2 50 1 10 8.8 0.2 6.5 0.2Fluidized bed 2 80 2 50 1 5 6.5 0.2 4.1 0.1Cooler 25 55 5 4.1 0.1 3.8 0.1

FIG. 5. A) Surface of raw skim milk powder (amorphous) produced in aconventional spray-drying process at a high magnification (scale

bar 200 nm). B) Morphological structure of powder from conventionalspray-dried powder after processing in a three-stage fluidized bed dryer=crystallizer for 20 min (scale bar 1mm).



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lactose nuclei. This technique has already demonstrated theability to make large lactose crystals.[20] The crystalformation and crystallization control in such a highlyhumid drying media are under development and furtherapproaches to control crystal formation in spray drying

could help to produce uniform lactose crystals after proces-sing in fluidized beds system within the combined method.

Moisture Sorption Tests

It has been established[23,35,36] that the moisture adsorp-tion peak heights (changes in moisture content, %(100kg kg1 dry basis)) can be used to represent theextent of amorphicity in the samples.

The net percentage changes in mass (dry basis) as afunction of time for raw milk powder (from conventionalspray drying, curve 1), CSMP (commercial reference sam-ple, curve 2), partially crystallized powder in a Humid

Loop (curve 3), and processed powders in the three (curve4) and two-stage (curve 5) fluidized bed are displayed inFigure 7. The peak height (the height between the peakand the plateau of the crystallization process) representsthe amount of sorbed moisture by amorphous materials,which characterizes the degree of amorphicity for lactosein that material.

There were significant changes between differentsamples that were processed (crystallized) at differenthumidities and temperatures in the fluidized bed dryer=crystallizer. The first curve (highest peak) is the moisturesorption by raw skim milk powder from conventional lab-oratory spray drying, which was mostly amorphous. The

highest peak shows the greatest amorphicity comparedwith the other samples. The second curve (Fig. 7, curve2) shows lower moisture sorption by commercial skim milkpowder (CSMP), which could possibly be due to develop-ing the extent of crystallinity in some ingredients duringdrying or other deteriorative changes after drying or duringthe storage period (such as the Maillard reaction andaging). This situation could have occurred during large-scale industrial spray drying or during post processing (belt

dryer, agglomeration, or fluidized bed dryer=cooler) in adairy plant. The peak height differences between the firstand the third curves show the benefits of using a HumidLoop to develop the crystallinity of powders; the partiallycrystallized powders in a Humid Loop have less moisturesorption that represents less amorphous material in thepowders. The moisture sorption result of the powdersthat were processed in a three-stage fluidized bed dryer=

crystallizer for 20 minutes (from conventional spray-driedpowder, curve 1) is shown in curve 4. The significantchange between raw powder (curve 1) and the processedpowder from three-stage crystallization shows the signifi-cant improvement in less moisture sorption by materialafter crystallization due to the lower amorphous contentsof the processed materials. Curve 5 shows a significantreduction in moisture sorption by the processed powders(in a two-stage fluidized bed dryer=crystallizer for 15 min-utes) that had been produced (and partially crystallized) ina Humid Loop.

Table 3 describes the different types of powders thatwere used or produced in this research. Figure 7 shows a

gradual improvement in less moisture sorption that repre-sents the degree of amorphicity in the powders.

Bulk Crystallinity by X-Ray Diffraction

In this study, the XRD analysis has been used to evalu-ate the formation of lactose crystals and the improvementin the amount of crystallinity for different skim milk pow-ders before and after processing. The different types of

FIG. 7. Moisture sorption tests for different types of raw and processed

skim milk powders (sorption test environment was at a temperature of25C and 75% relative humidity).

FIG. 6. A) Surface of raw skim milk powder (partially crystallized) pro-duced in a Humid Loop (scale bar 1mm). B) Surface of processed pow-

der from partially crystallized particles in a Humid Loop after processingin a two-stage fluidized bed dryer=crystallizer for 15min (scalebar 1mm).



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lactose crystals, which were formed in milk powders byprocessing at hot and high humid conditions, were ident-ified by the location of peaks in the reference data of pre-vious studies.[3740] The most noted representative peaks

were located at 12.5,16.4, 20.1 for a-lactose monohy-drate; 10.5, 21 for anhydrous b-lactose; 19.1, 21.1 forthe mixture of anhydrous a=b with a molar ratio of 5:3;and 19.5, 21.2 for mixture of anhydrous a=b with a molarratio of 4:1.[40,41] The progress of lactose crystallization wasobserved from the increasing intensities of the peaks in theXRD patterns at different angles, in the range of 1921,specifically at 19.1 and 20.

The mostly crystalline skim milk powder[4] that wasstored at a condition of 25C and 75% RH for two weekswas used as a reference point for nominally 100% lactosecrystallinity in milk powder when the quantitative analysis

of crystallization was done. Raw commercial skim milkpowder was used for XRD scanning by assuming minimum(almost zero) lactose crystallinity. Figure 8 shows individ-ual different XRD patterns for: a) mostly crystalline skimmilk powder; b) powders from the humid-loop after pro-cessing in two-stage fluidized bed crystallization (powderno. 5); c) CSMP that was processed in three-stage fluidizedbed crystallization (powder no. 4); and d) powders that

TABLE 3Different types of the raw and crystallized powders that were used and produced in this research

Powders DescriptionAverage particle size

(micron, D50)Moisture content

(%, w w1)

1 Raw powder From conventional spray drying 30 5.60.22 CSMP Commercial type 180 4.13 Partially crystallized From Humid Loop spray drying 90 8.80.24 Processed in three-stage Processed in three-stage fluidized

bed from powder 1390 3.70.2

5 Processed in two-stage Processed in two-stage fluidizedbed from powder 3

470 3.80.1

Corresponding numbers to the curves in Fig. 7.

FIG. 8. X-ray diffraction patterns for: A) mostly crystalline skim milk powder; B) processed in two stages from Humid-Loop powder (powder no. 5);C) processed in three stages (powder no. 4); D) partially crystallized in Humid Loop spray drying (powder no. 3). Peaks in the X-ray diffraction patternsfor different lactose crystals and crystalline polymorphs are shown as vertical lines.



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were partially crystallized in humid loop spray drying(powder no. 3). The noisy and relatively flat XRD patternfor raw SMP is not shown here. The diffraction pattern formostly crystalline SMP shows clear sharp peaks associatedwith a-lactose monohydrate at high intensity. There is nodistinguishable peak for b-lactose at 10.5, and the peakintensities for a=b mixtures with different molar ration at

19.1 and 19.5 are very low. The vertical line at the highestpeak point (20.0) was considered to indicate the maximumamount of lactose crystallinity in milk powder. The pat-terns for processed powders in multi-stage fluidized-bedcrystallizers (Fig. 8B, C) include clear representative peaksfor the different lactose crystals polymorphs, while the a-lactose monohydrate peaks have the highest intensities.The peaks at 10.5, 21 show the formation of anhydrousb-lactose, and the peaks at 19.1, 19.5, 21.2, and 21.1

show the formation of anhydrous a=b lactose polymorphs.The intensities of anhydrous a=b lactose polymorphs peaksare relatively low. The b-lactose crystals or the b-lactosecontaining crystals (polymorphs) were formed due tohigh-temperature crystallization or low-humidity con-ditions;[37,40] the crystallized powder from a three-stagefluidized-bed crystallizer (Fig. 8C, powder No. 4) thatwas processed at lower humidity (initial moisture contentof powder, lower humidity fluidization air) containsslightly higher peaks from anhydrous b-lactose crystals.The partially crystallized powders from Humid Loop spraydrying (Fig. 8C, powder No. 3) show some improvement incrystallinity but at a low intensity, while most of the lactosecrystals (and nuclei) are in the a-lactose monohydrateform.

DISCUSSIONYazdanpanah and Langrish[4] demonstrated the crystal-lization of lactose and milk powder in a fluidized-beddryer=crystallizer. The crystallinity of crystallized powderwas assessed by different techniques. That process was rela-tively time-consuming, while the multiple-stage fluidizedbed systems in this research are capable of improving thecrystallinity of powders in minutes. By increasing the pro-cess temperature and=or increasing the water activity bysorbing more moisture at a higher relative humidity, thetemperature between the material temperature (T) and itsglass-transition temperature (Tg) will be increased and thecrystallization time will be decreased.[9,10,42] Therefore, as

the results show for crystallized powders, by increasingthe material moisture content during processing(high-humidity process air at different stages and higherinitial moisture contents), the sorption peak height dra-matically decreased for the processed powders. This lowpeak height suggests that less amorphous lactose (the mosthygroscopic component) remained in the milk powder. Theshorter process time and the higher degrees of crystallinitysupport the hypothesis of high crystallization rates

occurring at high humidity and=or hot-processing con-ditions.[10,43] The crystallization boost in a fluidized bed isgoverned by the fluidization upper limits of temperatureand humidity that depend on the degree of amorphicityof powders. Partially crystallized powders, fromhumid-loop drying, may have better fluidization abilitywith highly humid air, causing acceleration of the crystalli-

zation rate in very moist powders. This very lowglass-transition temperature, and relatively low requiredprocess temperature and short residence time, avoid degra-dation and denaturation in the powders.

Particle size distribution of the resultant processed pow-ders has shown some particle agglomeration for the pro-cessed powders compared with the raw skim milk powderbefore processing in a fluidized-bed system (Table 3). Theagglomeration is not avoidable in this technique due toprocessing by high-humidity air and at temperatures abovethe glass-transition temperatures of the powders. Figure 5Bshows that the porous configuration of the crystallized milkpowders agglomerates then so that these porous agglomer-ated particles easily sink and disperse in solvent (water)during rehydration process.[44]

The comparative peak heights (improvements) for rawand crystallized milk powders have been reported inTable 4. Moisture sorption tests have shown that thereare significant differences between the powders that wereprocessed under different conditions and the amorphousmilk powder. Amorphous skim milk powders from conven-tional spray drying that were processed in a three-stage flui-dized-bed dryer=crystallizer have 87% less moisturesorption compared with the initial state. This loweramount of moisture sorption shows the lower fraction of

amorphous lactose (most hygroscopic component) in theprocessed powder. The 20-minute processing time at hightemperature and humidity conditions was therefore suf-ficient to mostly crystallize the SMP from conventionalspray drying. The partially crystallized powders that wereproduced from a Humid Loop spray drying show 36%lower moisture sorption, supporting the suggestion thatthe Humid Loop technique reduces the degree of amorphi-city of materials by drying them under humid conditions.The moisture sorption behavior of the partially crystallizedpowder was considered to be the base state when the moist-ure sorption of other process powders from this materialwas calculated. Therefore the processed powder (crystal-

lized from the Humid Loop spray-dried powder) in atwo-stage fluidized bed dryer=crystallizer showed 92% lessmoisture sorption compared with the moisture sorptionlevel of the raw material (conventionally dried). Thus the15-minute processing time of the partially crystallized pow-der in a two-stage fluidized bed was adequate to completethe crystallization of lactose in the milk powder fromHumid Loop spray drying. The significant reduction(8792%) in the moisture sorption and amorphicity of the



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powders in the order of minutes has emphasized the poten-tial of this multiple-stage processes for industrial applica-tions. Although the sequential batch processes that wereperformed in this research have some dead-time betweendifferent stages, a horizontal-vibrated fluidized-bed dryerwith different zones of temperature and humidity couldsave even more process time by eliminating thesedead-times and reach the same level of crystallinity in ashorter time.

Table 4 summarizes the statistical analysis for themoisture sorption peak heights of all the milk powders(Table 3) and reduction in the degree of amorphicity

with respect to the sorption peak heights of raw powdersbefore crystallization in a fluidized-bed system (basecase).

To confirm the results of the moisture-sorption tests interms of the amount for crystallinity of different powders,an XRD analysis has been done on the various powders.The XRD patterns of processed powders show clear sharppeaks for lactose crystals in the processed milk powderswith different intensities. The ratios of a- and b-lactosemay change as a result of changes in temperature and wateractivity during the crystallization process. The differentintensities of peak heights that indicate the amount of crys-tallinity in the powders have been used as a scale for differ-

ences in the powder crystallinities. For this comparison, thehighest peak intensity of mostly crystalline SMP (Fig. 8A)at 20.0 was assumed to be 100% crystalline and the noisyXRD pattern of raw SMP with no significant peak wastaken as 0% crystallinity. The three other intensities havebeen rescaled accordingly. Figure 9 shows the five XRDpatterns for different powders on this scale. Like themoisture-sorption test curves, the step-wise improvementsin the crystallinity of the powders could be quantified by

XRD analysis. The comparative peak points in thepartially crystallized powder and the processed powdersin fluidized-bed crystallizers have supported the improve-ments in crystallinity that were assessed by moisture sorp-tion tests. The peak intensities (crystallinity) from the XRDanalysis show higher amounts of crystallinity in compari-son with the moisture sorption tests. For instance, the pro-cessed powders from the two-stage fluidized bed and theHumid Loop (powder No. 5, Figs. 7, 8B) show 94% crys-tallinity from the XRD pattern and 92% crystallinity (lessmoisture sorption) in the moisture sorption test. Overall,the XRD analysis shows 24% more crystallinity compared

with the moisture sorption tests, which could be due to

FIG. 9. Rescaled X-ray diffraction patterns for raw, processed, andmostly crystalline skim milk powders. The peak heights from 20.0 to

20.2 have been used as references (Fig. 8). The mostly crystalline skimmilk powder was assumed to be 100% crystalline (top peak) while theraw SMP was taken as being nearly zero crystallinity.

TABLE 4Moisture sorption peak heights of all the experimental conditions (Figure 7) for skim milk powders with respect to the

sorption peak heights of amorphous SMP from conventional spray drying (base case)

Milk powders

Average peakheight, %

(in sorption)

Improvement, %(after fluidizedbed processing) Comment

Peak height,Mass change, %

Raw powder (1) 6.32 NA Used as basement forcurve 4.

Processed in three-stagefluidized bed (4)

0.83 87 Crystallized frompowder type 1.

Partially crystallizedfrom Humid Loop (3)

4.07 NA (36) Used as basement forcurve 5.

Processed in two-stage fluidizedbed from Humid Loop (5)

0.34 92 Crystallized frompowder type 3.

Corresponding numbers to the curves in Figure 7.Improvement by partial crystallization in a Humid Loop spray drying.



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the recrystallization of anhydrous lactose crystals (andpolymorphs) in the moisture-induced crystallization pro-cess. The moisture expelled from the powders went towardsthe recrystallization of lactose crystals and was not used tocrystallize the amorphous portion. By comparisons, theXRD shows the amount of the a-lactose monohydrateform on its own.

Crystallization of amorphous powders in a fluidized beddryer=crystallizer with hot humid air is an interactionbetween drying and crystallization. Using high temperatureprocessing air causes particles to desorb moisture at lowercorresponding water activities. The process includes sorp-tion, crystallization, and then desorption. Therefore, theinitial moisture content, the moisture sorption rate, thedrying rate, and the final moisture content govern the par-ticle structure and crystal configuration and the crystalliza-tion rate. Powders could have egg-shell structures withamorphous cores and crystalline surfaces,[45] mostlyamorphous, mostly crystalline,[4] or with crystallized coresand amorphous surfaces. The different parameters for crys-tallization and drying under the upper limits of fluidizationcould be used to give different amounts of crystallinity andparticle structures. Figure 10 shows some different crystal-line particle structures that could be produced using thistechnique. The egg-shell structure in Figure 10A that wereproduced by a single stage fluidized-bed drying=crystalliza-tion at T 50C, RH 60%, and t 60 min had very lowinitial moisture content.[45] Therefore the processing atrelatively high temperature and humidity crystallized fewouter layers of particles and the bulk remains amorphous.The 8.8% initial=bulk moisture content of the materialfrom Humid Loop spray drying provides very low Tg for

the materials inside the particles and processing by hotair caused crystalline core for the powders that thecross-section has been shown in Figure 10B. The large lac-tose crystals on the surface come from the low viscosity=high mobility of wet materials and the nucleation in theHumid Loop spray drying.

CONCLUSIONS

Processed milk powders show less moisture sorption andmore lactose crystals, leading to an improvement in thedegree of amorphicity of spray-dried milk powder bypost-processing in a two- or three-stage fluidized beddryer=crystallizer. Crystallized lactose appeared to beformed, and the processed milk powders were heavily tex-tured (crystallized on the surface). The surface textureimprovement and lower moisture sorption from theenvironment mean that the powders have more stabilityand better flow, which could potentially be a feasible sol-ution for flow problems associated with handling, bulktransfer, and hopper design in relation to these powders.The multiple-stage fluidized bed system showed distinctivepotential to crystallize lactose significantly in skim milkpowder using an industrial-feasible process. This shortcrystallization process could be carried out in a lower pro-cessing time if the multiple-stage fluidized bed crystallizer iscombined with a spray dryer operating with a Humid

Loop.

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