Chemical Engineering Science 71 (2012) 75–84

Contents lists available at SciVerse ScienceDirect

Chemical Engineering Science

0009-25

doi:10.1

n Corr

Enginee

Australi

E-m

journal homepage: www.elsevier.com/locate/ces

A monodisperse spray dryer for milk powder: Modelling the formationof insoluble material

Samuel Rogers a, Yuan Fang a, Sean Xu Qi Lin b, Cordelia Selomulya a, Xiao Dong Chen a,c,n

a Department of Chemical Engineering, Monash University, Clayton Campus, Melbourne, Victoria 3800, Australiab COFCO Nutrition and Health Research Institute, Beijing, PR Chinac Department of Chemical and Biochemical Engineering, Xiamen University, Fujian, PR China

a r t i c l e i n f o

Article history:

Received 19 August 2011

Received in revised form

21 November 2011

Accepted 23 November 2011Available online 14 December 2011

Keywords:

Spray drying

Monodisperse droplets

Morphology

Dissolution

Food processing

Reaction engineering approach

09/$ - see front matter & 2011 Elsevier Ltd. A

016/j.ces.2011.11.041

esponding author at: Monash University Dep

ring, Monash University Clayton Campus Me

a. Tel.: þ61 3 99059344.

ail address: dong.chen@eng.monash.edu.au (X

a b s t r a c t

The use of monodisperse droplets in drying research allows far greater control of the droplet size and

temperature history than can be achieved in conventional lab-scale dryers. Since every powder particle

experiences identical, predictable conditions, the monodisperse dryer allows verification of droplet

drying models and investigation of phenomena such as particle morphology, solubility, wettability and

their governing factors. Insolubility in milk powder is largely caused by damage to proteins in hot

drying conditions. In the present work, monodisperse skim milk and milk protein powders were dried

in different heat conditions and the powder tested for insolubility index. The falling droplet drying was

modelled using the Reaction Engineering Approach (REA), which is a semi-empirical model of moisture

removal rate. This model was accurate in predicting final powder moisture contents. Previously

published correlations for insoluble material were added to the model. The experimental insolubility

was consistent with observations from a pilot-scale dryer study, but did not agree with proposed

reaction kinetics extrapolated from low temperature powder experiments. Powders clearly displayed

poor wettability and solubility at inlet air temperatures above 140 1C.

& 2011 Elsevier Ltd. All rights reserved.

1. Introduction

One of the advantages of spray drying food powders is thatwater can be removed quickly without the product being exces-sively heated. This is essential for many food powders containingprotein and fats that might otherwise undergo degradation andloss of quality. To reap the full benefits of the spray dryingprocess, the dryer should avoid overheating the powder withhigh air temperatures or excessive residence time. There havebeen numerous studies of powder quality reduction in variousstorage conditions (Celestino et al., 1997; Fitzpatrick et al., 2004).These are useful in determining the appropriate storage tempera-ture, humidity, and time-scale for powder products. The degrada-tion that can occur inside the dryer is more rapid and severe.Over-drying can lead to production of insoluble material, burntparticles and off-flavours.

The presence of insoluble material in milk powders is a goodindicator of powder damage, measured as an insolubility index(ISI) in ml of sediment from centrifugation after reconstitution(IDF Standard 129, 2005). The material recovered in the test is

ll rights reserved.

artment of Chemical

lbourne, Victoria 3800

. Dong Chen).

mainly denatured casein (Lampitt and Bushill, 1931), which isformed in a temperature dependent reaction, theorized to be firstorder in protein content (Nielsen et al., 1996). Proteins renderedinsoluble in spray drying have also been measured by reversephase HPLC (Anandharamakrishnan et al., 2008). Protein insolu-bility is a factor in powder wettability, although this property isalso affected by surface lactose and fat coverage (Millqvist-Furebyet al., 2001) among others. The effect of insoluble material onwettability is a key concern in drying of high protein specifica-tions such as milk protein concentrate (Gaiani et al., 2009).

To predict the extent to which the insoluble material reactionwill occur in given drying conditions, it is desirable to establishthe kinetic parameters. In several studies such as that by Nielsenet al. (1996), dry powder was tested for ISI after varying times ofexposure to temperatures over the range 50–90 1C. The insolublematerial formation reaction rates were used to establish anArrhenius temperature dependency. Kudo et al. (1990) andStraatsma et al. (1999) have demonstrated that the protein ismore susceptible to heat damage in a partially dried powder thanin dilute concentrate or fully dry powder. In the range 10–30 wt%moisture content (calculated on a wet basis), powder is mosttemperature sensitive (Walstra and Jenness, 1984). This observationis consistent with work that has shown improvements in solubilityby reducing dryer outlet air temperatures (Anandharamakrishnanet al., 2007; Gaiani et al., 2009). Straatsma et al. (1999) and later

S. Rogers et al. / Chemical Engineering Science 71 (2012) 75–8476

Baldwin and Truong (2007) have produced materials with moisturecontents in this sensitive range. The nature of the samples rangedfrom humidified powder to gels and pastes. Straatsma et al. (1999)identified the thermo-sensitive range by testing these materials,then used a pilot-scale dryer to indirectly establish the kinetics ofthe reaction. Baldwin and Truong (2007) directly worked on thepowders and pastes to obtain kinetics over the range 1–50 wt%moisture content (for temperatures up to 55 1C). Both the studiescan be applied to the production of insoluble material within asingle particle in a spray dryer. The predictions cannot easily beverified with experiment, since a spray dryer contains a range ofparticle sizes that all experience different drying histories. Thedynamics of the spray and the drying air are difficult to measureexperimentally and the situation has been simulated using compu-tational fluid dynamics (Jin and Chen, 2009; Piatkowski andZbcinski, 2007, Woo et al., 2009). Many CFD simulations that includedrying use separate correlations to relate the flow conditions to thechanges in moisture content and temperature within each droplet.An ISI correlation could be included in the same way. The char-acteristic drying rate curve (Keey and Suzuki, 1974) and theReaction Engineering Approach or REA (Chen and Xie, 1997) aredroplet drying correlations that are specific to particular materials.To obtain the required data, it is necessary to measure the dryingbehaviour of a single droplet.

A number of researchers have isolated a droplet of milk andobserved its drying. It has been achieved by suspending a dropleton a thin filament or capillary (El-Sayed et al., 1990; Lin andGentry, 2003; Lin and Chen, 2002; Sano and Keey, 1982) as in theseminal droplet drying experiments of Ranz and Marshall (1952).Droplets have also been levitated above a hot surface using theLeidenfrost effect (Kudra et al., 1991; Tsapis et al., 2005). Theacoustic levitator has been used for droplets of a diameter close to400 mm (Schiffter and Lee, 2007; Verdurmen et al., 2004). In eachcase, the conditions experienced by the droplets have differencesfrom the conditions in a dryer, due to experimental limitationsincluding droplet size, air velocity and temperature measure-ment. Atomisers for industrial and pilot-spray dryers producedroplets of diameters ranging from 10 to 500 mm (Oakley, 1997).

A different approach to the isolation of a single drying dropletmakes use of monodisperse droplets generators (MDGs). Preciselycontrolled feed systems are used to produce a series of small droplets,each with identical volume. MDGs are a practical application of fluidbreakup theory that has been established over the last century(Eggers, 1997; Goedde and Yuen, 1969; Rayleigh, 1945). Devicescapable of producing monodisperse droplets in the micron size rangewere first developed for ink-jet printing to achieve fine imageresolution (Yoshimura et al., 1998). Many monodisperse dropletgenerators use a piezoelectric actuator to apply pressure pulses,ejecting small droplets from an orifice or modulating the breakupof a continuous jet of fluid (Brenn et al., 1997). Other designs usestrong electric fields (Meesters et al., 1992) or dispersing fluids(Xu et al., 2006). A distinction is made with MDGs in whether theyare ‘drop-on-demand’ or continuous. Many designs of MDG canachieve both, being capable of producing a single droplet whendesired, or regulating an existing liquid jet, causing it to disintegrateinto droplets that are monodisperse.

Producing monodisperse droplets and drying them in regulatedair drying conditions effectively removes the variable of droplet size.Each particle will experience the same drying history and largeenough amounts of powder can be produced to test for bulk proper-ties such as moisture content and insolubility. In previous work withMDGs, monodisperse droplets of a small size, containing varioussolutes, have been dried to observe particle morphology and compo-nent migration (Vehring et al., 2007). Skim milk and lactose solutionshave also been dried using MDGs, although the concentrations havebeen necessarily low (El-Sayed et al., 1990; Patel and Chen, 2007).

No one has yet worked with devices that can handle viscousand fouling process fluids such as milk concentrate and recon-stituted powder. To best model spray drying conditions, it isdesirable to use feed liquids similar to those in industry, since thekinetics of drying and insolubility are highly dependent on theinitial moisture content. Using a new monodisperse dropletnozzle and a dryer with a controlled airflow and temperatureprofile, we have produced uniform powder over a range oftemperatures and analysed the insolubility of this powder. Thematerials used were skim milk and milk protein concentratepowder (MPC85), a high protein product produced by ultra-filtration. The drying of a single droplet was modelled with aone-dimensional numerical integral calculation based on the REAwith insoluble material sub-models. The powder wettability wasalso examined for powders of varying insolubility.

2. Theory

2.1. Drying model

The drying of a single droplet was modelled by virtue of theknown droplet and particle size and the predictable airflows andtemperature profile in the column. Another paper by the sameauthors (Rogers et al., 2011) shows the measurements of airtemperatures throughout the drying chamber, which were usedto establish a profile. The modelled droplet decelerates to theslow surrounding airspeed (�0.1 m s�1) according to creeping-flow drag equations (Chuchottaworn et al., 1984) and then movesat terminal velocity down through the drying chamber. With thesurface turbulence known, the diffusion and heat transfer can bemodelled from droplet correlations. The heat and mass transfercoefficients are calculated from the following adaptation of theRanz-Marshall (1952) correlation by Chen (2005):

Nu¼ 2:04þ0:62 Re0:5Pr0:33ð1Þ

Sh¼ 1:63þ0:54 Re0:5Sc0:33ð2Þ

In many theoretical drying models, the radial moisture concen-tration profile is simulated within the drying droplet and a dynamicdiffusion is predicted. These models consider the moisture either tobe receding to the centre of the particle (Cheong et al., 1986) ormigrating to the surface, establishing diffusivity for water as afunction of temperature and moisture content (Handscomb andKraft, 2009; Sano and Keey, 1982). The mass and heat transfermodel used here is based on the work of Chen and Xie (1997),in which the transport of water to the surface is ‘lumped in’ with thereaction engineering approach (REA). For surface vapour concentra-tion rv,s (kg�m�3), the rate of mass transfer from the droplet is asfollows

msdX

dt¼�hmAðrv,s�rv,bÞ ð3Þ

where ms is the droplet dry mass (kg), hm is the mass transfercoefficient (m s�1), rv,b is the bulk vapour concentration (kg m�3)and A is the droplet surface area (m2). X is the average moisturecontent (kg kg�1), calculated on a dry basis. rv,s should approximatethe saturated vapour concentration rv,sat for droplets at high X. rv,sat

is dependent on the temperature at the droplet surface. As thedroplet temperature profile is assumed to be insignificant (Chen andPeng, 2005), rv,sat is evaluated at the droplet temperature, Td (K).The moisture content is reduced as drying proceeds and hence rv,s

deviates from the saturated value. The evaporation ‘reaction’ is fittedwith an Arrhenius temperature dependency and the activationenergy, DEv is calculated as a function of the average dropletmoisture content X. At lower X, more energy is required to remove

Fig. 1. Model predictions of droplet temperature profile and solids content over

time [Initial conditions: 200 mm droplet diameter, 20 wt% solids content, 160 1C

inlet air temperature, 20 1C droplet temperature] (the moisture content range

10–40 wt% is highlighted ie. 60–90 wt% solids content).

S. Rogers et al. / Chemical Engineering Science 71 (2012) 75–84 77

the remaining moisture, hence DEv becomes higher:

rv,s ¼ exp �DEv

RTd

� �rv,satðTdÞ ð4Þ

The relationship between X and DEv must be determined byexperiment for different drying materials. Even so, this simpleREA has proved to be a good approximation in many cases and itbypasses the need to determine effective mass diffusivity (whichis empirical anyway). In the current work, DEv was predicted fromX according to the findings for skim milk by Chen and Lin (2005):

DEv

DEv,b¼ 0:998 exp �1:405ðX�XeÞ

0:930h i

ð5Þ

where DEv,b is the equilibrium activation energy, evaluated at thebulk air conditions. Xe is the equilibrium moisture content, whichis calculated from desorption isotherms derived for skim milk,with the GAB (Guggenheim–Anderson–de Boer) model. Thisdetermines the point at which drying reaches equilibrium:

Xe ¼Ckmoaw

ð1�kawÞð1�kawþCkawÞð6Þ

where mo is the monolayer moisture content, and k and C arematerial dependent constants, correlations for which have beenestablished by Lin et al. (2005) for milk powders and Foster et al.(2005) for individual components of milk (lactose, whey protein,casein). The calculations of heat and mass transfer wereassembled in Microsoft Excel to produce a one dimensional finitedifference model, with a time step of 0.001 s. More informationabout this model is available in another publication (Rogers et al.,2011).

2.2. Insoluble material models

With the temperature and moisture content history of thepowders predicted, there is a unique opportunity to examine theeffects on powder quality, namely the solubility. Baldwin andTruong (2007) obtained rates of formation of insoluble material atconstant moisture contents and temperatures. In the currentwork, their data was fitted to the following equation (logisticscurve) as had been previously suggested (Kreyszig, 1979):

ISIðtÞ ¼a

1þðexpð�gtÞ=bÞ� � ð7Þ

where the parameter a is the maximum ISI reading that will bereached (related to the amount of protein available) and b is theISI value of the powder before heating. The exponential term gwas taken to be dependent on temperature and moisture contentby a standard Arrhenius relationship. For skim milk, the reactionis agreed to end near the point when 10 ml of insoluble materialis detectable in the centrifuge test, presumably when all heatsensitive protein is insoluble. For the first 5 ml, the reaction is notsignificantly limited by the lack of remaining protein and thereaction rate is approximately first order to ISI volume (ISI),where the reaction constant z is also temperature and moisturecontent dependent, derived from g. The situation is modelled withthe following differential equation for a first order reaction:

dðISIÞ

dt¼ z ISIð Þ ¼ koexp

�Ea

RTd

� �ðISIÞ ð8Þ

where ko, the pre-exponential factor, and Ea, the activation energyboth vary with moisture content. The reaction is only set to occurbetween moisture contents of 10 and 40 wt% (calculated on a wetbasis ie. kg water/kg total). The maximum rate occurs at 35 wt%.This modelling approach approximated the results of Baldwin andTruong and was used as a sub-model to predict ISI in thecurrent work.

By comparison, the model of Straatsma et al. (1999) is ‘zerothorder’ (rate is independent of ISI), with a ‘relative’ Arrheniustemperature dependency, in which the temperature is offset by aconstant T0. This was placed at 348 K to best agree with theexperimental data. It is surmised that no powder was found tocontain insoluble material at lower temperatures in their study.The rate of reaction is given by the following equation

dISI

dt¼ koexp

�Ea

R

1

Tdþ

1

T0

� �� �ð9Þ

where the pre-exponential constant ko and the activation energyEa were set at 0.054 ml s�1 and 2.7�105 J mol�1, respectively.The moisture content dependence is lumped in this approach,with the above equation used only during moisture contents 30 to10 wt% in the drying history and stopping when the ISI reaches10 ml. This approach was used here as another sub-model topredict formation of insoluble material.

Some important model predictions for insolubility are shownin Fig. 1. The residence time is 6 s; a value that varies between4 and 8 s over the range of drying parameters used in this work.The residence times predicted by this model were reproduced in aseparate CFD analysis of this dryer (Woo et al., 2011). Themodelled temperature history suggests that the droplets spend1.4 s within the temperature sensitive range 40–10 wt%. Duringthis time, the temperature increases from 85 to 110 1C (with theinlet air temperature set to 160 1C).

3. Experimental methods

3.1. Materials

Commercial skim milk powder was provided by MurrayGoulburn Co-operative Co., Ltd. (Australia). Milk protein concen-trate powder (MPC85) was provided by Dairy Innovation AustraliaLtd. Powders were reconstituted at 501 in de-ionised water from aMilli-Q system from Millipore Corporation (Bedford, MA., USA).The specified composition of each powder before reconstitution isshown in Table 1 with compositions calculated on a wet basis.

Table 1Compositions of milk powders used in experiments (Chen and Lin, 2005; GEA

Filtration).

Moisture

(wt%)

Lactose

(wt%)

Fat

(wt%)

Protein

(wt%)

Mineral

(wt%)

Skim milk powder 3.8 49.8 0.6 36.5 9.3

MPC85 4.0 0.5 2.5 84.8 8.5

Fig. 2. Schematic of the monodisperse droplet nozzle inside the dryer inlet.

S. Rogers et al. / Chemical Engineering Science 71 (2012) 75–8478

3.2. Methods

3.2.1. Nozzle construction

The piezoelectric nozzles used for this work were assembledand tested to establish their ability to produce monodispersedroplets. Piezoelectric ceramic components were provided by APCInternational, Ltd (Mackeyville, PA., USA). Customised glasswarewas made by Monash Scientific Glass Blowing Services Pty. Ltd.,(Melbourne, Australia) where a capillary tube was drawn out intoa tapering funnel in the method described by Lee (2003). Theceramic components were attached to the glass nozzles usingepoxy resin and connected to the ‘Jet Drive’ pulse controller fromMicrofab Technologies Inc (Plano, TX., USA). Nozzles werechecked for function by their audible frequencies when pulsed.

3.2.2. Nozzle operation

The nozzle was attached to a separate feed reservoir with highpressure hoses and connectors. The temperature of the reservoirwas maintained at 50 1C using a resistance wire heating jacket.Various inline filters were used to prevent nozzle blockages(Swagelok Company, Solon, OH, USA). The reservoir and nozzlewere pressurised with compressed air to motivate flow. Nozzleswere run at varying pressures depending on the orifice size andthe fluid properties. Samples of feed exiting the nozzle weretested for solids content to ensure that the solids were notreduced in the filtration. The compressed air pressure wasregulated to control nozzle flow rate using a pressure controllerprovided by SMC Pneumatics (Australia) Pty Ltd. The flow ratewas measured by mass and stopwatch and the nozzle tip and jetwere photographed to observe the droplet formation regime. Thecamera used was a Nikon D90 DSLR with an AF 60 mm Micro lens(Nikon Australia Pty Ltd.) and a high speed flash to ‘freeze’ thedroplet motion and prevent motion blurring. With observation ofthe photographs, the piezoelectric pulsing frequency wasadjusted on the pulse controller to establish a monodispersedroplet mode. More details of this setup are given in a previouspublication (Rogers et al., 2011).

3.2.3. Dryer operation

The main drying chamber was made from aluminium spiralducting (diameter 0.6 m, length 3.5 m), insulated with fibreglasslagging. Hot air was introduced with variable temperature heatguns (Robert Bosch (Australia) Pty Ltd.) at the top of chamber. Thetotal flow rate of air was approximately 1000 L min�1, giving anair to feed ratio of 500:1 (kg:kg). The combined airflow from theguns was passed through an air dispersion plate with small holesto evenly distribute the airflow throughout the column. The airvelocity was measured to be 0.1 m s�1 directly below the disper-sion plate. The velocity was undetectably small elsewhere in thechamber. Thermocouples at five positions inside the dryingchamber were used to monitor the vertical temperature profile.More detailed temperature profiles for use in modelling wereestablished by moving a thermocouple wire along the centre ofthe dryer. After the dryer had heated to steady state, the nozzle

(with feed already flowing, i.e., immediately after frequencyadjustment) was introduced to a central inlet at the top of thedryer. The hollow walls of the inlet were pumped with coolingwater, to keep the tip cool and prevent the performance of thetemperature sensitive piezo-ceramic from varying. The nozzleassembly is shown in Fig. 2. Powder was collected as it fell fromthe conical base of the drying chamber, depositing on paper20 cm below the edge of the drier (the paper surface was alwayswithin 10 1C of room temperature). The conical walls werevibrated to prevent powder from settling on the incline.

3.2.4. Insoluble material test, moisture content and wettability

Since the powder samples were only around 10 g in size, theamount used in quality tests was less than suggested in thestandards. Powder samples were tested for moisture contentusing a scaled down variant of GEA Niro Method No. A1a.

(IDF Standard 026, 2004), with 0.2 g of powder oven dried at102 1C for 2 h until constant weight was reached. The solids contentof reconstituted milk samples was established using the samemethod, with a thin layer of liquid (3 ml) placed on the weighingdish. Wettability was measured for skim milk powders by staticwetting times based on GEA Niro Method No. A6a. and the method ofFreudig et al. (1999). 100 ml of water at 25 1C was placed in a250 ml beaker and 1 g of powder was tipped from a weighing dishlevel with the beaker rim. The time for all powder to visibly sinkbeneath the surface was measured. Trials were run in triplicate.

The insolubility test was conducted with reference to GEA Niro

Method No. A3a (IDF Standard 129, 2005). Monodisperse skimmilk powders were reconstituted, 5 g into 100 ml of water at20 1C (the GEA Niro method calls for 10 g into 100 ml). Thesolution was stirred for 5 min then decanted into 15 ml centrifugetubes. These were centrifuged at 1000 rpm for 5 min and thesupernatant tipped off. The pellet volume was read and then wasdried to constant weight at 50 1C for 24 h. In cases where thevolume was too low to read (TLTR), the dry weight was used tointerpolate an ISI volume. The protocol for milk protein concen-trate powders used 3 g of powder to 150 ml, with water at 50 1Cand the solution stirred for 30 min.

S. Rogers et al. / Chemical Engineering Science 71 (2012) 75–84 79

3.2.5. Scanning electron microscope and light microscope

Scanning Electron Microscope (SEM) images were taken usinga JSM 840A Scanning Microscope from Jeol Ltd., Japan, which ishoused in the Monash Centre for Electron Microscopy, MonashUniversity. Samples were coated by ion sputtering with gold. Thelight microscope used was a Motic B Series, with accompanyingcomputer software Motic Image Plus provided by Motic.

4. Results and discussion

To examine the development of insoluble material in spraydrying, experiments were carried out on skim milk and milkprotein concentrate. Both powders were reconstituted to 20 wt%solids content. The solutions were filtered and jetted through anozzle with a 75 mm orifice. The stable droplet breakup from thenozzle was photographed (Fig. 3). The diameter of the dropletswas measured from the photographs with reference to the nozzletip of known size. The droplet size for the milk protein concen-trate shown in Fig. 3 was estimated at 17073 mm, with anapplied pulse frequency of 10 kHz. The droplet breakup wasoperated as a continuous jet, with a short region of connectedthreads after the nozzle tip which broke up into discrete droplets.Flow rates ranged from 1 to 2 g min�1.

Fig. 3. Photograph of 75 mm nozzle tip and jet of monodisperse droplets of

reconstituted milk protein concentrate (20 wt% solids) (the nozzle is pointed

downwards).

Fig. 4. Uniform skim milk powder dried at varying inlet air temperatures (Ta); (a) Micr

(ii) Ta¼159 1C.

The yields of powder produced were on average 79% (thetheoretical maximum assumed for completely dried powder withknown nozzle flow rate and feed solids content), giving produc-tion rates of up to 20 g per hour. Powder deposits on the drierwalls account for the remainder. Fig. 4 shows images of severaluniform skim milk powders produced in this study. The morphol-ogy can be described as ‘buckled’ (Fig. 4a and c), likely due toinward drying stresses on the particle (Keey, 1992; Tsapis et al.,2005), while many of the particles dried at higher temperaturesappear to have ‘puffed’ (Fig. 4b and d) possibly because of theexpansion of boiling vapour (Sano and Keey, 1982). Particlemorphology in these experiments has been discussed in anotherpaper by the same author (Rogers et al., 2011). Particles areclearly similar in size and precise measurements were made withimage analysis. The open source software package Image J

(Ferreira and Rasband, 2010) was used for particle image recogni-tion, predicting diameters from the area occupied in each photo-graph, assuming that each particle was essentially spherical.A histogram of particle sizes from this method is shown inFig. 5. Some outliers on the histogram are likely to be artefacts,caused by two or more particles close together being resolved as asingle larger particle by the software. The majority of particles areclustered around the mean of 106 mm.

Spray drying was carried out at a number of different inlet airtemperatures for the two milk products. The droplet and particlesize were measured for each, as well as the final moisture content(calculated on a wet basis) and insolubility properties. Data fromeach drying run is summarised in Table 2. The inlet temperatureswere chosen to encompass the range of dryness. When usinglower temperatures (Ta4120 1C) the particles were sticky to thetouch and exhibited some cracking, while at higher temperatures(Ta4160 1C) the powder was visibly yellowed. The temperatureranges are rough estimates, since the property transitions werenot well defined.

Because of difficulties with operation of the nozzle, it was notpractical to produce identically sized droplets from run to run.

oscope photographs: (i) Ta¼111 1C (ii) Ta¼159 1C; (b) SEM images: (i) Ta¼135 1C,

S. Rogers et al. / Chemical Engineering Science 71 (2012) 75–8480

The droplet size was, however, consistent throughout each run, assize was measured before and after. The model was used topredict final moisture contents at each temperature. Severalinitial droplet sizes were used and the results are shown withthe experimental points in Fig. 6. The experimental error barsreflect the variability of two consecutive moisture contentmeasurements.

According to the model, drying is heavily dependent on initialdroplet size. The measured sizes of the droplets also had someuncertainty (Table 2). Despite these sources of error, the agree-ment with the data was fairly good in the middle of thetemperature range. The experimental findings may have under-stated the true moisture contents for low drying temperaturesbecause wet particles were prone to stick to the dryer walls, withonly dryer ones being collected.

Each of the drying experiments was run for long enough toproduce sufficient sample sizes for ISI testing. Powders werereconstituted for the centrifuge test of insoluble material. Thereason for the lower concentrations and longer stirring times inthe MPC method was that milk protein powder is slow to dissolverelative to other dairy powders. The same was found to be true ofthe uniform milk protein powder dried in this experiment.Solutions of reconstituted powder for each sample were centri-fuged in quadruplicate and the results averaged Table 3

The attempt to make all the protein insoluble by oven dryingovernight was successful, since the mass of dried insoluble mate-rial was very similar to the protein compositions �30 and 85 wt%of the total solids for skim milk and milk protein concentrate

Fig. 5. Histogram of particle diameters for uniform powder dried from 20 wt%

solids reconstituted skim milk powder (Sample size: �1000 particles).

Table 2Results of drying reconstituted skim milk and milk protein concentrate at varying tem

Feed Inlet

temperature (1C)

Outlet

temperature (1C)

SMP reconstituted (20 wt%) 90 30

111 31

135 34

149 36

157 37

165 40

183 45

MPC 85 reconstituted (20 wt%) 77 30

107 30

155 37

178 43

respectively. The ISI volumes for SMP given in Table 3 requirescaling by 6.7, since in this variation of the ISI test, half the amountof powder was used in reconstitution and smaller centrifuge tubeswere used. For example, the most insoluble powder that was driedhere would scale to an ISI of 9.3 ml. Generally, the maximum ISIvalues observed for damaged skim milk powder are around 10 ml(Baldwin and Truong, 2007), where correctly dried powder canhave less than 0.1 ml. For MPC, the scaling required is 16.7(different amounts used). ISI volumes for MPC powder (whendried to specification) have been measured at 9.4 ml (Thomaset al., 2004). For all tests, volumes that were too low to read (TLTR)were extrapolated from the masses of the dried centrifuge pellets,assuming the overnight dried results represented the maximumvalue. The results for SMP are shown in Figs. 7 and 8, alongsidemodelling results using the correlation proposed by Straatsmaet al. (1999). These results are also shown in Table 3.

The Straatsma model accurately predicted the onset of insolublematerial production, but overestimated the final values at highertemperatures. Within the error limits of the model and the data, it isa highly credible correlation. The similarity of the results to thepredictions also suggests that the modelled temperature profiles aregenerally accurate. The parameters given by Straatsma wereadjusted to better fit the data obtained from the present work, asshown in Fig. 7. A convincing fit was obtained by reducing theactivation energy to 10�105 kJ mol�1, which is similar to thatobtained from experiments on dry powder by Nielsen et al. (1996).

peratures.

Mean droplet

size (mm)

Mean particle

size (mm)

Mean moisture

content (wt%)

19174 11774 10.671

195713 112720 9.370.2

200724 138711 7.770.3

20776 112722 3.370.1

18679 95714 3.270.2

206712 13877 3.070.4

18177 11079 1.970.3

20174 112712 8.670.4

230721 13776 6.272

20177 143723 3.570.3

20775 131722 2.970.1

Fig. 6. Wet basis final moisture contents for uniform skim milk powders dried at

different temperatures compared with model predictions (for 200 mm initial

droplet diameter) (Results for 190, 210 and 220 mm are shown to demonstrate

the effect of droplet size on the model).

Table 3Results of the insoluble material tests.

Material Inlet

temperature (1C)

Insoluble

mass (%)

ISI volume

(ml)

Scaled ISI

volume (ml)

Model ISI volume

(using same inlet

temperature) (ml)

Original MPC85 071 0 (TLTR)

Original MPC85 dried overnight 9177 170.5

Uniform milk 77 071 0 (TLTR)

Protein concentrate 107 171 0 (TLTR)

Powder 155 2074 No results

178 5374 No results

Original SMP 272 0 (TLTR) 0

Original SMP dried overnight 3177 1.970.5 12.7

Uniform skim milk powder 90 171 0 (TLTR) 0 0.01

111 270.1 0 (TLTR) 0 0.01

134 270.2 0 (TLTR) 0 0.4

150 674 0.2570.5 1.7 10

157 1976 0.8070.5 5.4 10

165 15.970.2 0.7570.5 5.0 10

183 2477 1.470.5 9.4 10

Fig. 7. Extrapolated ISI volumes for monodisperse skim milk powder dried at

different inlet temperatures compared with Straatsma model predictions.

S. Rogers et al. / Chemical Engineering Science 71 (2012) 75–84 81

The addition of a reference temperature is still required

dISI

dt¼ koexp

�Ea

R

1

Tdþ

1

T0

� �� �¼ 0:064exp

�10n105

R

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The data from Baldwin and Truong (2007) and the subsequentprojections suggested a comparatively slow rate of insolublematerial production. The predictions were much lower than theISI findings of this study. They had mentioned that by theirextrapolation, ‘The time to produce a high degree of insolublepowder (ADMI SI of 6 ml) at 100 1C is only 10 s (Baldwin andTruong, 2007). In this work, powder at 100 1C became significantlyinsoluble in only 1.4 s (as in the temperature profile shown in Fig. 1).At the highest drying temperature used here, the time spent in thesensitive range was only 1 s, although the particle experienced

hotter temperatures than 100 1C during this time. Their data wasobtained from experiments at much lower temperatures (almost50 1C difference) and perhaps there are unforeseen occurrences inspray drying conditions. Particles may be approaching or exceedingtemperatures at which the water will boil.

Selected uniform skim milk powders were tested for wettabilityby the static wetting test. The results are shown in Table 4 with thewetting time of industrial SMP. The industrial powder had a Sautermean diameter of 60754 mm (as measured by microscope imagerecognition.) The wetting may be faster for industrial powder becauseof the small particles present. There is an obvious increase in thewetting time for the uniform powder dried at 165 1C, which had anISI of 6.2 ml. The partially insoluble 150 1C powder (ISI of 2 ml) wasno less wettable than the powders with low ISI volumes. According toprevious research, the surface lipid content can be reduced at highertemperatures for milk protein concentrates (Gaiani et al., 2009).Surface lipids can affect wetting even for low fat products such asskim milk. The effect of reducing surface fat is to improve wettability(Millqvist-Fureby et al., 2001), so this may oppose the wettabilityreduction from heat damage insolubility at higher temperatures.Since there was a large increase in wetting time for severely insolublepowders, the surface fat effect may be less significant. Surface analysisof the monodisperse powders could elucidate this in future work.

The milk protein concentrate powder dried at higher inlettemperatures was also insoluble. When reconstituted, the powderremained in discrete particles, swelling slightly, which suggestedthat the material on the surface was completely insoluble andperhaps had gelled (Fig. 8b). Three morphologies were observedduring the drying trials. Particles dried at the lowest temperatureswere smooth and spherical with a single ‘sink-hole’, reminiscentof a pitted olive, (Fig. 8c and d) where hotter temperaturesproduced the ‘buckled’ morphology (Fig. 8a and b) and eventually‘puffed’ as with the skim milk (not shown). This difference mayhave been related to a thicker or stronger shell having formedduring the slower drying. A similar morphology effect wasobserved in the drying of glycoproteins by Vehring (2008) whenthe transport properties of the feed components were adjusted.

From Fig. 9, it seems that increasing amounts of the protein willbecome insoluble when the powder is dried at inlet temperaturesover 150 1C, similarly to the skim milk powders. No REA data forMPC has been obtained to date and there are also no insolubilitymodels. At the highest inlet temperature used here, the protein hadnot fully become insoluble. It would seem that there is a wide rangeof drying temperatures that can produce partially insoluble MPC.

Fig. 8. Uniform milk protein concentrate powder dried at varying inlet air temperatures (Ta); (a) Microscope photographs (i) Ta¼178 1C (ii) Ta¼178 1C (powder

reconstituted in water), (iii) Ta¼77 1C, (b) SEM image Ta¼77 1C.

Table 4Results of wetting tests for skim milk powders dried at different temperatures.

Material Inlet temperature (1C) Wetting time (min)

SMP n/a 51

Uniform skim milk powder 90 2710

111 572

135 575

150 271

165 61711

Fig. 9. Percentage of total material undissolved in monodisperse milk protein

powders dried at varying inlet temperatures.

S. Rogers et al. / Chemical Engineering Science 71 (2012) 75–8482

Changes in powder residence time may therefore have a significanteffect on the ISI of MPC powder. It has been demonstrated, in worksoon to be published by the same authors (Fang et al., 2011), thatthe monodisperse milk protein powder experiences a drop indissolution rate at inlet temperatures above 130 1C, while it remainsas soluble as the original powder when dried at lower temperatures.

Although this study focused on inlet air temperatures, if the outletair temperature could be independently controlled it might beexpected to have a large effect, as powder passes the sensitivetemperature range towards the end of the drying history. Somepublications have confirmed this (Anandharamakrishnan et al., 2007;Gaiani et al., 2009).

5. Conclusions

In these findings, it is suggested that skim milk powderscan become partially insoluble in the region of inlet temperature150–180 1C and will reach maximum insolubility at higher tempera-tures. The milk protein concentrate began to produce insolublematerial within a similar temperature range, but required a hotterdrying temperature to make the entire heat-sensitive protein fractioninsoluble. It would seem that powders are not damaged below acertain temperature, but in conditions above this point there is a widescope for partial damage. Changes in residence time or particle sizemight therefore have a significant effect on quality. The Straatsmamodel (1999) performed well when appended to the model in thisexperiment, successfully predicting the temperature ‘threshold’ afterwhich a significant ISI is produced for skim milk. An improvedcorrelation, with adjusted parameters, has been presented in thiswork. The extrapolation of insoluble material production rates fromwork on powders at low temperature (Baldwin and Truong, 2007) hasfailed to produce sensible results when used in conjunction with adrying model. It may be that there are factors are involved that arenot described by the simple Arrhenius temperature dependency.Further work to properly characterise the rate of ISI production isrequired.

Some peripheral observations in this work may be of interest,such as the morphologies of the powders dried, which are discussedin another work by the same authors (Rogers et al., 2011). Alsonoticed was the solubility of partially heat damaged milk proteinconcentrates, where the entire surface remains essentially intactupon reconstitution with some soluble material possibly beingpresent in the centre. The wettability of skim milk powders was

S. Rogers et al. / Chemical Engineering Science 71 (2012) 75–84 83

found to be reduced simultaneously with the increase in insolublematerial. The drying of monodisperse droplets has been provenfeasible with reconstituted milk products to produce reproducibledrying data. Theoretical drying models and correlations fromsimplified experiments (such as suspended droplets) can now betested in a situation similar to an industrial spray dryer. This dryercan be operated at over a wide range of temperatures and feedsolids contents while still producing adequately dried powder. Hereis has been used to examine the effects of inlet temperature onmoisture content, insolubility and wettability.

Nomenclature

a parameter for logistics curve of ISI over timeb parameter for logistics curve of ISI over timeg parameter for logistics curve of ISI over timeDEv apparent activation energy, J kmol�1

DEv,b equilibrium activation energy, J kmol�1

rv,b bulk (air) vapour concentration, kg m�3

rv,s surface vapour concentration, kg m�3

rv,sat saturated vapour concentration, vA droplet surface area, m2

aw bulk water activityC material dependant constant for GAB modelEa activation energy for arrhenius equation of insoluble

material production rate, J mol�1

hm external mass transfer coefficient, m s�1

ISI volume of insoluble material in 10 g of reconstitutedpowder, ml

k material dependant constant for GAB modelko pre-exponential constant for arrhenius equation of inso-

luble material production rate, ml s�1

m droplet mass, kgmo monolayer moisture contentms droplet dry mass, kgNu Nusselt numberPr Prandtl numberR Gas constant, J kmol�1 K�1

Re Reynolds numberSc Schmidt numberSh Sherwood numbert time, sTa air inlet temperature, KTd droplet average temperature, KTs droplet surface temperature, KTo temperature offset in Straatsma modelX droplet average moisture content on dry basis

(moisture/solids), kg kg�1

Xe equilibrium moisture content corresponding to the bulk(air) condition on dry basis, kg kg�1

z reaction constant for first order equation for insolublematerial production

Acknowledgments

Dr. Samuel Rogers gratefully acknowledges the financial sup-port from the Geoffrey Gardiner Foundation and Dairy InnovationAustralia. The authors thank Anja Tomic and David Yang, finalyear students who helped with drying and powder analysis.

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