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Chemical Engineering and Processing 46 (2007) 420–428

Development of stickiness of whey protein isolate andlactose droplets during convective drying

B. Adhikari a,∗, T. Howes a, A.K. Shrestha b, B.R. Bhandari b

a School of Engineering, The University of Queensland, Brisbane, Qld 4072, Australiab School of Land and Food Sciences, The University of Queensland, Brisbane, Qld 4072, Australia

Received 20 April 2006; received in revised form 30 June 2006; accepted 10 July 2006Available online 16 September 2006

bstract

The stickiness development of droplets of whey protein isolate (WPI), lactose and their mixture solutions was determined using an in situtickiness testing device at 24, 65 and 80 ◦C. Stainless steel, Teflon, glass and polyurethane probes were used. At room temperature, the presencef 0.5–1% (w/w) WPI greatly lowered the observed tensile strength of water and lactose solutions due to surface adsorption and led to a weakeningf the cohesive strength. At elevated temperatures, lactose droplets remained sticky showing cohesive failure until the surface was completelyovered with a thin crystal layer. WPI droplets formed a thin, smooth skin immediately on coming in contact with drying air. This surface becameon-sticky early in the course of drying due to the transformation of the surface to a glassy state. The skin forming and surface active nature of

PI was exploited to minimize the stickiness of honey in a pilot scale spray drying trial. Replacement of 5% (w/w) maltodextrin with WPI raised

he powder recovery of honey solids from 28% to 80% in a pilot scale drying test. At elevated temperature the magnitude of stickiness on probeaterials was in the order of glass > stainless steel > polyurethane > Teflon. The Teflon surface offered the lowest stickiness both at low and high

emperatures making it a suitable material to minimize stickiness through surface coating.2006 Elsevier B.V. All rights reserved.

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eywords: Stickiness; Tensile strength; Contact angle; WPI; Lactose; Teflon; P

. Introduction

Stickiness of liquid foods in processing equipment and pack-ges is a ubiquitous issue encountered in industry as well as invery day life. Stickiness leads to scale formation and foulingn thermal processes [1,2]. Stickiness of food powders duringroduction, handling and storage has long been recognised asmajor problem in powder making industries [3]. This prob-

em not only leads to processing difficulties such as frequentlant shutdowns and cleaning but also results in to low qualityroducts and fire hazards [4].

There is considerable demand for high value particulate prod-cts from natural foods such as powdered fruit juices, honey,hey permeates and vegetable soups, especially in develop-

ng countries where refrigerated-storage facilities are lacking5]. Furthermore, these products are important ingredients force-cream, yoghurt and non-alcoholic beverage manufactur-

∗ Corresponding author. Tel.: +61 7 336 59058; fax: +61 7 336 54199.E-mail address: bpadhikari@uq.edu.au (B. Adhikari).

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255-2701/$ – see front matter © 2006 Elsevier B.V. All rights reserved.oi:10.1016/j.cep.2006.07.014

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ng. However, the production of powders from these materialshrough drying is very difficult due to high concentrations ofugars, organic acids and minerals which have a greater ten-ency to stick to the equipment surfaces [6,7]. The nature of theontact surface can play an important role in food stickiness.

Michalsky et al. [8,9] have studied the adhesion behaviorf edible oil and food emulsions to glass, Teflon (PTFE), low-ensity polyethylene (LDPE), poly ethylene terephthalate (PET)nd stainless steel. They allowed the food samples to flow downn inclined substrate surface and measured the amount of sam-le remaining on the surface after the flow ceased. It was foundhat surface roughness, the yield stress of the sample and solidurface tension were the key factors responsible for adhesion.dhikari et al. [7] studied the surface stickiness of droplets of

ructose–maltodextrin and sucrose–maltodextrin mixtures dur-ng convective drying. They found that the presence of mal-odextrin improves the spray drying yield mainly due to its skin

orming property. However typically 40–60% (w/w) maltodex-rin solids need to be introduced before it is possible to converthese sugar solutions into powders even under mild spray dryingonditions [10].

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One can think of two possible ways that might more effec-ively minimize the wall stickiness problem. Firstly, the dryerall can be coated with materials that do not favor the stick-

ng of the solutions/particles. Secondly the surface of theroplet/particles could be engineered in such as way that theyesist coalescence when they come in contact with each othernd also decreases their adherence to the dryer wall. The latterpproach can greatly reduce the amount of additives required asdrying aid. Hence, this study aims at making a comparative

tudy of the stickiness on stainless steel, glass, polyurethane andeflon surfaces. Further, it also explores the possibility of usingprotein solution to partially replace high molecular weight

arbohydrates as drying aids, in order to manipulate the dropleturface property.

. Materials and methods

.1. Materials

Spray dried lactose powder (Murray Goulburn Co-Ltd., Aus-ralia), hydrolyzed whey protein isolate (ALATAL 817TM, Newealand Milk Powder, New Zealand) and maltodextrin of dex-

rose equivalent 6 (Roquette Freres, France) were used withouturther purification. They were vacuum dried (70 ◦C 500 mbar)

vernight and stored in desiccators over P2O5 prior to solutionreparation. Capilano floral honey (Capilano Honey Limited,ustralia) was used. Dry solid content of honey was deter-ined using AOAC recommended method [11]. A refractometer

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Fig. 1. Schematic diagram of the in si

and Processing 46 (2007) 420–428 421

RFM 340, Bellingham + Stanley Ltd., USA) was used in thisurpose.

.2. Methods

.2.1. Contact angleContact angle of the test solutions was measured using optical

ontact angle device (OCA 15 plusTM, Dataphysics, Germany).15 �l of solution was used in all the tests. Sessile drops were

ormed on the surface of the test surfaces. The reported contactngle values represent the average of three readings.

.2.2. Tensile strengthStickiness was measured using an in situ stickiness testing

nstrument. This instrument works on the principle of tack, thats, it mimics the feel when one touches a droplet surface. Theorking principle and the test protocols are given elsewhere

12]. The schematic diagram (Fig. 1) and the test procedure areriefly presented as follows. The probes (glass, stainless steel,olyurethane and Teflon) are attached to the shaft of the captiveype linear actuator (Motor size 11 28H47, Hydon Switch andnstrument Inc., USA). The contact diameter of probes was.5 mm except that of glass which was 3.17 mm. The motoras driven by Intelligent Motion System driver (IM 483I).

ts motion was controlled through LabView Software. Theroplet holder is made up of Teflon solid cylinder (diametermm) which is mechanically attached to the weighing sectionf the precision load cell (±0.1 mg). A digital video camera

tu stickiness testing instrument.

4 ering and Processing 46 (2007) 420–428

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22 B. Adhikari et al. / Chemical Engine

nd stereomicroscope (50 times magnification) imaging systemonitors the approach, withdrawal and contact of the probe to

he droplet surface. During the experiment, the stepper motor isriven downwards until the probe makes good contact with theroplet surface. The contact and withdrawal speed of the probeas maintained at 50 mm/min in all the trials. Once contact is

stablished, the motor is subsequently withdrawn. The variationn the tensile force over time is recorded continuously to a PC.igital images during the approach, contact and withdrawal

re also recorded to the PC as well. The temperature of theroplet is recorded by inserting micro-thermocouples (T-type,mega Engineering USA) to the droplet centre. The tensile

trength, a peak force during the separation process normalizedy the probe area, is taken as measure of stickiness of theroplet surface. The repeatability of this instrument was foundo have two distinct regions. For the test conducted at roomemperature, the standard deviation varied within 1–5% of meanalues. The nature of the solids didn’t affect this spread. Atlevated temperature, the standard variation was within 1–5%uring the initial drying stages. However, as the solid content ofhe droplets increased, the repeatability decreased. At elevatedemperature, the standard deviation varied within 5–10%. It wasound that the higher the gradient in solids in droplets, the higheras the spread. The repeatability of this instrument is weakerhen the surface of the droplet is structurally not uniform and

ough. This happens towards the later stage of drying. Overall,his instrument measures the surface stickiness of dryingroplets satisfactorily. The moisture history of the droplet wasonitored through parallel experiments by placing the droplets

n the droplet holder and monitoring the mass loss over time.

.2.3. Spray dryingA twin fluid nozzle spray dryer (SL 20, Saurin Company,

ustralia) with 3 l/h water evaporation capacity was used forhe drying trials. The inlet and outlet temperatures of the dryerere maintained at 130 and 65 ◦C, respectively. Powders were

ollected from the cyclone and spray dryer chamber by lightlyweeping the chamber wall. Honey/maltodextrin solid ratio of0:50 was taken as the reference. A 5–10% of the maltodextrinolids were replaced with WPI solids and subsequently sprayried. Three runs were carried out and mean values are reportedere.

.2.4. Water activity and particle size distributionWater activity of the powders was measured using a water

ctivity meter (AQUALAB 3TE, Decagon Company, USA). Aastersizer 2000 (Malvern Company, USA) was used for par-

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Fig. 3. Mode of failure of WP

ig. 2. Variation of tensile strength of lactose and whey protein isolate (WPI)t 24 ± 1 ◦C. Probe: stainless steel, probe speed: 50 mm/min.

icle size distribution determination. Tests were carried out inriplicate and mean values are reported.

. Results and discussion

.1. Droplet stickiness at room temperature

The variation in tensile strength of droplets of lactose andPI are presented in Fig. 2 which shows that the tensile strength

f both the lactose and WPI solutions decreases with concen-ration. The decrease in tensile strength of 5% (w/w) lactoseolution is merely 1.3% from that of pure water. The tensiletrength of this solution continuously decreased and the finalensile strength of a 20% (w/w) lactose solution is 8% lower thanhat of pure water. Since pure lactose is known to be non-surfacective and its viscosity also increases with concentration, it isxpected that the tensile strength of this solution would increase.he decrease in concentration can be attributed to 0.5% (w/w)rotein (as specified by the supplier) present in this material.

There is an interesting trend regarding the variation ofensile strength of WPI solution with concentration (Fig. 2).t low concentrations, when the WPI concentration increased

o 1% (w/w) the tensile strength reduced by 29% comparedo pure water. The reduction in tensile strength continues until% (w/w) solid concentration which appears to be lowestoint (33% reduction). Fig. 3 shows that the mode of failuret droplet–probe interface is cohesive. This means that droplet

rst undergoes necking and breaks within itself but not at theroplet-probe interface. This is probably due to lowering of theumber of hydrogen bonds in water which is responsible forohesive strength [13]. WPI acts to reduce the surface tension of

I solution at 24 ± 1 ◦C.

ering and Processing 46 (2007) 420–428 423

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ure water quite remarkably indicating that WPI molecules areurface active and they migrate preferentially to the droplet–airnterface and by doing so, lower the surface energy there.owever, the adhesive strength at the droplet–probe interfaceas still remained much higher compared to the cohesivetrength of the droplet molecules. This means that the presencef WPI in water facilitates the breaking of droplet within itselfnd consequently it will be harder to achieve a clean adhesiveailure at the droplet–probe interface. This has great implicationn processing, that is, protein molecules facilitate stickiness ofood materials on equipment surfaces and packages.

When the concentration of WPI is varied from 5 to 20%w/w), the tensile strength remained almost constant and itended to increase slightly at highest concentration. However,he tensile strength of 25% (w/w) WPI solution is 66.34 Pa whichs still well below the tensile strength of pure water (86.52 Pa)nder test conditions. This shows that at concentrations greaterhan 20% (w/w) the presence of WPI solids brings about insignif-cant increase in cohesive strength of droplet. The small incre-

ent in cohesive strength might be due to the increased viscosityf the solutions.

.2. Effect of probe materials on droplet stickiness

Fig. 4 shows the variation of tensile strength of WPI dropletsn different probes. The concentration of WPI was varied from 0o 25% (w/w). This figure shows that the tensile strength of pureater on stainless steel and polyurethane are almost the same.he tensile strength of pure water on glass and Teflon surfacesre lower by 18.5% and 28%, respectively, compared to that ontainless steel. On the Teflon probe, it was observed that the waterroplet retracted and only a small area was remained coveredr wetted when the probe was completely withdrawn. This maye the reason why the observed tensile strength of water on aeflon surface is so low. Furthermore, this can also be explainedn the basis of Young’s equation given below:

a = γLV(1 + cos θ) (1)

here Wa is work of adhesion (J), γLV the liquid surface ten-ion (N/m) and θ is the contact angle. Eq. (1) explains that forunit distance the force for adhesion would be lower for θ val-

Fig. 4. Stickiness of WPI solution on different probe materials.

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ig. 5. Contact angles of WPI solutions at different probe materials at4 ± 0.5 ◦C.

es greater than zero. The contact angle (Fig. 5) of water oneflon is 102.5◦, which explains that the Teflon surface behavess hydrophobic and hence the retraction of water droplet and theesultant reduction in tensile strength is expected. However, theontact angle of water on glass is much lower compared to theontact angle of water on stainless steel. This should result intoreater wetting and hence a greater tensile strength upon separa-ion. The much lower tensile strength of water droplet on glasscompared to stainless steel) even when it had greater wetttabil-ty seems contrary to Eq. (1). This might have resulted from theact that, if the energy required to create a new surface withinhe droplet is lower than the energy required to achieve a cleaneparation at droplet probe–droplet interface then the mode ofailure is cohesive (Fig. 3). When this occurs, stickiness can-ot be explained from the surface tension related force alone.he slight increment in the contact angle at WPI concentrationreater than 15% (w/w) can be attributed to increased viscos-ty. This increase in tensile strength when the contact angle hasncreased is contrary to Eq. (1) and proves the fact that there isviscous force component in stickiness.

Fig. 4 shows that the tensile strength of the WPI solutionsn the probe (stainless steel, glass, polyurethane, Teflon) sur-aces follows a similar trend. The tensile strength falls rapidlyithin 1% (w/w) of WPI concentration. The reduction in tensile

trength continues to 5% (w/w) of WPI, albeit at much lower rate.he tensile strength remains constant from 5% to 20% (w/w) ofPI beyond which it shows a tendency to increase. This infor-ation shows that WPI acts as surfactant which sharply reduces

he cohesive strength of the water molecules and a small quantity≤1%, w/w) is sufficient to bring about substantial reduction inohesive strength. This fact is also seen in a reduction in contactngle (Fig. 5).

The mode of failure during the tensile strength measurementsas cohesive in all these tests. Except for the Teflon surface, no

etraction of droplet was observed during the probe withdrawal.ince there was no adhesive failure at the droplet–probe interface

n these tests, then, one would expect that the tensile strength of

he droplets should be independent of the probe material cho-en. However, Fig. 4 indicates that there is significant effectf probe surfaces on the tensile strength or stickiness of theroplet.

424 B. Adhikari et al. / Chemical Engineering and Processing 46 (2007) 420–428

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adiwtdtsswimwas formed on the surface of the lactose droplets which trappedthe water within. The thickness of this crust increased over time.This is another reason why the lactose droplets dried slower.

Fig. 6. Tensile strength of WPI/lactose mixture solutions at 24 ± 1 ◦C.

.3. Stickiness of WPI/lactose mixtures

The effect of addition of WPI on the tensile strength of lactoseolution is given in Fig. 6. The WPI/lactose ratio has been variedrom 0 to 1. The total solids in solution was kept at 25% (w/w)ecause it was difficult to consistently prepare an emulsion ofPI/lactose above this solid concentration and lower than this

oncentrations are not desired as feeds in spray drying opera-ions. The tensile strength of these mixture droplets on differentrobe was also studied.

As can be seen from this figure, the tensile strength of theactose solution decreases to close to a minimum when the WPIraction is 0.2. Except for the magnitude of the tensile strength,he effect of increment of WPI fraction in WPI/lactose solu-ion is identical for all the probes. The tensile strength does notecrease much when the WPI fraction is increased above 0.2,ather it remains almost constant up to WPI fraction of 0.8 andhen increases slightly beyond. As in case of pure WPI solutionn different probe surfaces, the tensile strength of WPI/lactoseolutions is the lowest for glass and the highest for stainless steelrobe and the values obtained from the Teflon and polyurethaneie in between.

The contact angle of the WPI/lactose mixtures decreasedFig. 7) with an increase in WPI fraction. If the tensile strengths a function of surface force alone, then, according to Eq.1), the tensile strength should increase with decrease in con-act angle. This result demonstrates that the tensile strengthf the WPI/lactose mixture is the result of complex interplayetween surface and bulk forces. The magnitude of contact anglef WPI/lactose solution is Teflon > polyurethane > stainlessteel > glass. This order does not occur in the tensile strengtheasurements where strength is the highest for stainless steel

nd the lowest for glass. The contact angle appears to increaseith increase in WPI fraction on the glass surface. However, theaximum variation in contact angle in this case is 5.7◦, which

s not very far apart from the accuracy (±3◦) attainable in con-

act angel measurements [14]. Hence, the wetting behavior ofure lactose, WPI/lactose mixtures solutions up to 25% (w/w)olids on glass surface is close to that of water. Fig. 7 also cor-oborates with the results obtained from Figs. 2, 4 and 6 that

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ig. 7. Contact angle of the WPI/lactose mixture drops on probe surfaces at4 ± 1 ◦C.

he tensile strength of droplets results from complex interplayetween surface and bulk properties of materials.

.4. Stickiness of WPI solutions at elevated temperatures

.4.1. Temperature and moisture historiesMoisture histories of WPI and lactose droplets at 65 and 80 ◦C

re presented in Fig. 8. This figure shows that in the 15 min ofrying, lactose droplets dry faster compared to that of WPI. Dur-ng this period, WPI droplet already forms a surface skin throughhich the rate of moisture removal is slowed down. After 15 min,

he drying rate of lactose droplets slows down more and the WPIroplets dry faster than the lactose droplets. This may be due tohe fact that the moisture diffusivity of lactose solution reducestrongly with an increase in solid concentration. The WPI dropleturface becomes rugged due to appearance of crests, troughs andrinkles. All of these act to greatly increase the surface area. The

ncreased surface area enhances evaporation and subsequentlyoisture removal. It was observed that a thin crust of crystals

ig. 8. Moisture histories of lactose and WPI solution droplets at 65 and 80 ◦C,.75 m/s air velocity and 2.5 ± 0.5 relative humidity.

B. Adhikari et al. / Chemical Engineering and Processing 46 (2007) 420–428 425

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ig. 9. Temperature histories of lactose and WPI solution droplets at 65 and0 ◦C, 0.75 m/s air velocity and 2.5 ± 0.5 relative humidity.

Fig. 9 presents the temperature histories of WPI and lac-ose droplets at 65 and 80 ◦C. The temperature histories of bothhe WPI and lactose droplets bear similarities with that of their

oisture histories. At the beginning, the droplet temperaturef WPI increases more rapidly than the corresponding lactoseroplet. However, once the rate of evaporation of lactose slowsown, the temperature of droplets rise faster than that of WPI. Itppears that crystallization cools down the droplets as exhibitedemarkably by the temperature history of lactose droplet driedt 80 ◦C. Once the crystallization occurred, the temperature ofactose droplets was lower than that of the WPI. Similar effectf crystallization of lactose on its drop temperature is seen onhe plot at air temperature of 63 ◦C.

.4.2. Stickiness of WPI droplets at elevated temperaturesThe development of stickiness of WPI droplets at different

robe surfaces at 65 ◦C is presented in Fig. 10(a). The repeatabil-ty of the measurements, based on standard deviation, is shownhrough error bar. This varied between 5% and 8% of the mea-ured mean values. The mode of failure when the WPI dropletttains a non-sticky state on stainless steel probe is given inig. 10(b). This mode of failure is common to all the probeshen non-sticky state is attained. In case of a Teflon probe, aeak tensile strength is observed at droplet–probe interface atverage moisture u = 2.43 (70.85%, w/w). At this point a cleandhesive failure occurred when the probe was pulled away. Thiss the point at which the cohesive strength of the droplet surfaces close enough to achieve the adhesive strength at droplet–probenterface. A clean separation of the probe from the droplet sur-ace occurs here. We wish to emphasize here that the peak tensiletrength values reported here may be apparent peaks on the lefton x-axis) of a real peak. This is because it is very difficult toay if the measured peak value is the actual (maximum) peakalue because it depends strongly on the surface moisture andemperature of the droplet. Nevertheless, the measured peaks

hould lie very close to the actual peaks in all cases. Hence,he conclusion drawn in this work will not be affected by thisifficulty. The drop surface was completely no-sticky (similaro Fig. 10(b)) for the Teflon probe when the moisture content

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0 mm/min probe speed. (b) Mode of failure at probe–WPI droplet interfacet non-sticky state (65 ◦C, 0.75 air velocity, 2.5 ± 0.5 relative humidity and0 mm/min probe speed).

ecreases to u = 2.2, that is, the solid concentration of the dropletas merely 31.2% (w/w) and the drying time was only 5 min.e applied 10 kPa of compressive pressure at the droplet surface

o make sure that the surface was non-sticky. This extent of com-ressive pressure is recommended because the surface of foodroplets behaves like pressure sensitive adhesive and also due tohe fact that the droplet surface is not smooth [12,15]. There arewo reasons why the WPI droplet surface becomes non-sticky inuch an early stage of drying. Firstly, WPI droplet forms smoothkin at the surface immediately after coming into contact withhe hot air and behaves like a flexible water pouch (Fig. 10(b)).he outer surface of the skin soon converts itself to non-stickylassy material. Secondly, the Teflon surface is hydrophobic. Itas to be noted that the temperature of the WPI droplet whents surface attained a non-sticky state was 45 ◦C, well below therying temperature of the air (65 ◦C).

The peak tensile strength of WPI droplets on polyurethanend stainless steel surfaces were achieved at u = 2.2 (68.75%,/w) and with a corresponding drop temperature of 45 ◦C. This

uggests that the WPI droplets will stick to the polyurethane andtainless for a longer time compared to the Teflon surface. Theagnitude of the peak tensile strength on the stainless steel wasuch higher compared to that of polyurethane. This indicates

hat the WPI droplet surface will have a greater tendency to stickn the stainless steel surface than the polyurethane at elevatedemperature. The WPI droplet surface attained a complete non-ticky state when the average moisture was u = 1.27 (55.95%,

/w) and the corresponding drop temperature was 54.3 ◦C bothith polyurethane and stainless steel. In case of glass, the peak

ensile strength when the droplet surface was cleanly separatedas 4700 kPa which is the highest value among the probe

4 ering and Processing 46 (2007) 420–428

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rather than increasing. It is interesting to observe that the tensilestrength of lactose droplet at u = 0.49 (or 67%, w/w solids) iswell below the tensile strength with droplet of 25% (w/w) solidsat room temperature. This is because the cohesive strength of the

26 B. Adhikari et al. / Chemical Engine

aterials. This means that the WPI surface has the greatestendency to stick to the glass surface. However, the WPI surfacettained non-sticky state with glass at the same moisture andemperature as in the case of stainless steel and polyurethane.

The magnitude of the stickiness (peak tensile strength) ofPI droplet on probe materials is consistent with the contact

ngle results (Fig. 5). The magnitude of the contact angle iseflon > polyurethane > stainless steel > glass. This means that

he work of adhesion (Eq. (1)) or the tensile strength at theroplet–probe interface should be of the order of glass > stainlessteel > polyurethane > Teflon. The stickiness of WPI drop sur-ace is the greatest on the glass surface and the lowest on Teflonnd that the value of the stainless steel and polyurethane fall inetween with predicted order. A comparison of the peak ten-ile strength at 63 ◦C with those obtained at room temperaturehows that that for the surface forces of the adhesive (droplets)nd adherend (probe surfaces) to be dominant (in stickiness),he adhesive mode of failure (Fig. 10(b)) is a must have crite-ion. Since the dominant mode of failure was cohesive (Fig. 3)uring the tests at room temperature, the contact angle valuesi.e. the surface forces) are not consistent with the developmentf surface stickiness.

It is worth noting here that glass transition temperature (Tg)f droplets is usually used to predict if it is likely to stick toquipment/packaging material or likely to cake upon storage andransportation. A practical rule is that if the droplet temperatures 20 ◦C above its Tg, it will be sticky. Tg is a strong functionf moisture content, and generally the mean moisture content issed to calculate it. It is very difficult to experimentally measurehe Tg of anhydrous WPI and hence a calculated value of 153 ◦Cs taken [16]. At mean moisture content u = 1.27 (55.95%, w/w)he Tg of the WPI droplet will be −8 ◦C or lower. The measured

PI droplet temperature is 53.4 ◦C, which is at least 61.4 ◦Cbove Tg. If we accept the Tg + 20 ◦C criteria for stickiness, thisroplet should be sticky. Quite contrary, the above results showhat the drop does not stick even if compressive pressure of0 kPa was used. Hence, for skin forming materials such as WPI,he behavior of the film or the surface properties rather than theulk ones control the development of stickiness.

Tensile strength of WPI at 80 ◦C, 0.75 m/s air velocity and.5 ± 0.5% RH was studied to investigate the effect tempera-ure and the results are presented in Fig. 11. The repeatabilityf the measurements is shown through error bars. The max-mum spread (standard deviation), in these tests, was within0% of the measured mean values. This figure shows that theeak tensile strength of droplets is the highest on the glassnd lowest on the Teflon surfaces and the values for stainlessteel and polyurethane fall in between in order. These resultsesemble the results obtained at 65 ◦C and the same conclu-ion can be drawn. However, the moisture contents at which the

PI droplets exhibit peak stickiness and non-sticky state areower compared to those at 65 ◦C. Similarly, the droplet sur-ace requires longer time to achieve clean adhesive failure and

lso to enter the non-sticky state. This is due to the fact that theroplet surface remains soft or plastic for longer time at higheremperature. Even at this drying temperature the droplet surfaceecomes completely non-sticky at average moisture u = 0.68 and

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robe speed.

rop temperature of 72 ◦C, mainly due to formation of glassykin at the droplet surface.

.5. Comparison of stickiness of lactose and WPI droplets

Fig. 12 compares the development of stickiness of WPI andactose droplets at 65 and 80 ◦C on stainless steel probes. Thisgure shows that the evolution of stickiness of WPI and lactoseroplets is very different to each other. At room temperaturehe tensile strength of 25% (w/w) of WPI is 66 Pa compared to9 Pa of lactose at the same solid concentration. Once the dryingommences the cohesive strength of the WPI droplet increasesery rapidly and exceeds the adhesive strength at droplet–probenterface at which a clean failure occurs for the first time. Onurther progress of drying the surface becomes glassy and entershe non-sticky regime fairly early in the drying process or atairly high average moisture content.

In the case of lactose, the tensile strength first decreases

ig. 12. Stickiness (tensile strength) of WPI and lactose solution droplets ontainless steel probe at 65 and 80 ◦C, 0.75 air velocity, 2.5 ± 0.5 relative humiditynd 50 mm/min probe speed.

B. Adhikari et al. / Chemical Engineering and Processing 46 (2007) 420–428 427

Table 1Powder recovery, water activity and mean particle size of honey powders with different amount of WPI

Sample Powder recovery (%) Particle size D (V, 0.5) (�m) Water activity (24.9 ± 0.1 ◦C)

50% (H), 50% (MD), 0% (WPI) 28.4 ± 4.9 49.71 0.124 ± 0.00350% (H), 45% (MD), 5% (WPI) 80.1 ± 4.1 15.48 0.183 ± 0.0025

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0% (H), 40% (MD), 10% (WPI) 82.2 ± 4.2

: honey; MD: maltodextrin; WPI: whey protein isolate.

actose solution decreases with an increase in droplet tempera-ure. This is expected because the fluidity of a given solutions higher at higher temperatures. This increased fluidity lowershe viscosity as well as cohesive strength of the solution andence the tensile strength gets lowered. When the moisture con-ent of the droplet decreases below u = 0.4 (solids > 70%, w/w)he tensile strength increases rapidly. This is because the effectf solid concentration starts dominating over the effect of tem-erature which are mutually opposing. At this point, numerousmall crystals appeared on the droplet and they grew and becameominant. It was observed that the crystals accumulated on theurface to form a crust the thickness of which grew over time.he interior and non-crystallized part of the droplet was sur-

ounded by the crystal crust. This crust was fragile in naturend broke when the probe made a contact. As the crust frac-ured in numerous locations (on contact) a thin layer of viscousolution then seeped out and contacted the probe. This phenom-na was observed in all tests even when the moisture content= 0.18 (85% solids, w/w). The mode of failure was observed

o be varying between cohesive to adhesive-dominant. It waslso observed that the failure was completely adhesive if therystal crust was not broken at the surface. The effect of tem-erature on the magnitude of surface stickiness is clearly visibleere below u = 0.49. The magnitude of tensile strength decreasedith increase in temperature at a given solid concentration. This

esult also supports the earlier finding with WPI droplets thatigher temperatures worsen the problem of stickiness duringrying.

The extraordinary difference in development of stickiness inPI and lactose droplets comes from their nature. Lactose, being

disaccharide lacks the capacity to form a non-sticky skin at itsurface; rather it crystallizes and somehow unsuccessfully actso lower the stickiness. In spray drying, however, amorphousactose is formed, which is usually considered non-sticky. Onhe other hand WPI due to its higher molecular weight forms akin upon exposure to drying air which converts itself into glassyatrix that resists the stickiness. This skin forming behavior ofPI not only overcomes the sticky problem but also increases

he drying rate by increasing the heat and mass transfer surfaceFig. 8) due to formation of folds and wrinkles at the surface.

.6. Application of WPI as drying aid

Honey is one of the most difficult materials to convert into

owder form because its dry mass is comprised mainly of lowolecular weight sugars. It has not been possible to produce

oney power, in its pure form, using currently available pow-er production techniques [17]. The work of Bhandari et al. [6]

wwaw

11.88 0.198 ± 0.002

hows that maltodextrin has to be added as a drying aid to bringhe honey to maltodextrin ratio to less than 50:50 on a dry solidasis. Even this extent of drying aid resulted in a recovery ofowder less than 50% (of solids in feed). This level of additiveddition is not welcomed by consumers.

Results from Section 3.1 above shows that WPI moleculesre strong surfactants and preferentially migrate to a droplet–airnterface. Subsequently, when the WPI droplet is subjected tolevated temperatures (Section 3.4.2), a thin, smooth and non-ticky skin is formed immediately after the commencement ofrying. These observations can have two important implications.irstly, the skin can resist coalescence when droplets come inontact with each other and also decrease their adherence to theryer wall. Secondly, the proteins can be efficient drying aidsecause only a small amount will be required.

Hence drying trials were carried out to evaluate the effec-iveness of WPI as a drying aid. The product recovery and thetability of powders were taken as measures of drying effective-ess. Powders were collected from cyclone and also from therying chamber by lightly sweeping the latter. The recoveries%) were calculated based on the amount of total solids in theeed (Table 1). As shown in this table, when the honey to mal-odextrin (solid) ratio was maintained at 50:50, the recovery was

erely 28%. However, when 5% of the maltodextrin (from thebove formulation) was replaced with WPI, a recovery of greaterhan 80% was achieved. When the amount of WPI was increasedo 10% (maltodextrin 40%) the recovery increased only slightly82%). It has to be noted that recovery in the laboratory scalepray dryer (as used here) is usually much lower compared tohat of the industrial spray dryers [6].

The above trials have shown that small amount of WPI addi-ion can bring about a great improvement in the product recoverynd that the proteins can be used as smart drying aids. The effec-iveness of WPI as drying aids is due to segregation of WPI

olecules from honey solids (molecules) and the preferentialigration of the former towards the droplet surface which is

nduced by the surface activity of protein molecules and theirigger size. The higher the magnitude of this segregation thereater will be the effectiveness of a protein as a drying aid.

. Conclusions

The stickiness development of WPI and lactose droplets wasetermined. The surface of the lactose droplet remained sticky

ith cohesive failure until the surface was completely coveredith fragile crystal layer which fractured upon probe contact

nd allowed the thin solution layer to seep out to the probe evenhen the average moisture was lower than 16.67% (w/w). WPI

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roplets formed a thin, smooth skin immediately after comingn contact with the drying air. The tensile strength of this skinncreased rapidly and peaked fairly early during drying pro-ess and became non-sticky due to transformation of the outerayer of the skin into a glassy material. Tests of WPI/lactose

ixture droplets at room temperature showed that addition of.5–1% (w/w) WPI lowered the tensile strength of lactose solu-ion mainly due to preferential migration of protein molecules tohe surface and lowering of cohesive strength. The skin formingnd surface active nature of WPI were exploited to minimize thetickiness of honey in a pilot scale spray drying trial. Replace-ent of 5% (w/w) maltodextrin with WPI raised the powder

ecovery of honey solids from 28% to 80%. Stickiness of thePI on glass, Teflon and polyurethane surfaces was studied byodification of the probe surface. At elevated temperatures, the

eak stickiness of the glass was the highest and on the Teflonas the lowest as would be predicted from contact angle infor-ation. The Teflon surface offered the lowest stickiness both

t low and high temperatures making it a suitable material toinimize stickiness through surface coating.

cknowledgements

The authors acknowledge Miss Charlotte Contassot’s helpn experiments and the Dairy Ingredients Group of AustraliaDIGA) and Australian Research Council (ARC) for financialupport for this study.

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