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Development of stickiness of whey protein isolate and lactose droplets during convective drying
Chemical Engineering and Processing 46 (2007) 420–428
Development of stickiness of whey protein isolate and lactose droplets during convective drying B. Adhikari a,∗ , T. Howes a , A.K. Shrestha b , B.R. Bhandari b a
b
School of Engineering, The University of Queensland, Brisbane, Qld 4072, Australia 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 2006 Available online 16 September 2006
Abstract The stickiness development of droplets of whey protein isolate (WPI), lactose and their mixture solutions was determined using an in situ stickiness testing device at 24, 65 and 80 ◦ C. Stainless steel, Teflon, glass and polyurethane probes were used. At room temperature, the presence of 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 weakening of the cohesive strength. At elevated temperatures, lactose droplets remained sticky showing cohesive failure until the surface was completely covered with a thin crystal layer. WPI droplets formed a thin, smooth skin immediately on coming in contact with drying air. This surface became non-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 WPI was exploited to minimize the stickiness of honey in a pilot scale spray drying trial. Replacement of 5% (w/w) maltodextrin with WPI raised the powder recovery of honey solids from 28% to 80% in a pilot scale drying test. At elevated temperature the magnitude of stickiness on probe materials was in the order of glass > stainless steel > polyurethane > Teflon. The Teflon surface offered the lowest stickiness both at low and high temperatures making it a suitable material to minimize stickiness through surface coating. © 2006 Elsevier B.V. All rights reserved. Keywords: Stickiness; Tensile strength; Contact angle; WPI; Lactose; Teflon; Polyurethane
1. Introduction Stickiness of liquid foods in processing equipment and packages is a ubiquitous issue encountered in industry as well as in every day life. Stickiness leads to scale formation and fouling in thermal processes [1,2]. Stickiness of food powders during production, handling and storage has long been recognised as a major problem in powder making industries [3]. This problem not only leads to processing difficulties such as frequent plant shutdowns and cleaning but also results in to low quality products and fire hazards [4]. There is considerable demand for high value particulate products from natural foods such as powdered fruit juices, honey, whey permeates and vegetable soups, especially in developing countries where refrigerated-storage facilities are lacking [5]. Furthermore, these products are important ingredients for ice-cream, yoghurt and non-alcoholic beverage manufactur-
∗
Corresponding author. Tel.: +61 7 336 59058; fax: +61 7 336 54199. E-mail address: [email protected] (B. Adhikari).
0255-2701/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.cep.2006.07.014
ing. However, the production of powders from these materials through drying is very difficult due to high concentrations of sugars, organic acids and minerals which have a greater tendency to stick to the equipment surfaces [6,7]. The nature of the contact surface can play an important role in food stickiness. Michalsky et al. [8,9] have studied the adhesion behavior of edible oil and food emulsions to glass, Teflon (PTFE), lowdensity polyethylene (LDPE), poly ethylene terephthalate (PET) and stainless steel. They allowed the food samples to flow down an inclined substrate surface and measured the amount of sample remaining on the surface after the flow ceased. It was found that surface roughness, the yield stress of the sample and solid surface tension were the key factors responsible for adhesion. Adhikari et al. [7] studied the surface stickiness of droplets of fructose–maltodextrin and sucrose–maltodextrin mixtures during convective drying. They found that the presence of maltodextrin improves the spray drying yield mainly due to its skin forming property. However typically 40–60% (w/w) maltodextrin solids need to be introduced before it is possible to convert these sugar solutions into powders even under mild spray drying conditions [10].
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One can think of two possible ways that might more effectively minimize the wall stickiness problem. Firstly, the dryer wall can be coated with materials that do not favor the sticking of the solutions/particles. Secondly the surface of the droplet/particles could be engineered in such as way that they resist coalescence when they come in contact with each other and also decreases their adherence to the dryer wall. The latter approach can greatly reduce the amount of additives required as a drying aid. Hence, this study aims at making a comparative study of the stickiness on stainless steel, glass, polyurethane and Teflon surfaces. Further, it also explores the possibility of using a protein solution to partially replace high molecular weight carbohydrates as drying aids, in order to manipulate the droplet surface property. 2. Materials and methods 2.1. Materials Spray dried lactose powder (Murray Goulburn Co-Ltd., Australia), hydrolyzed whey protein isolate (ALATAL 817TM , New Zealand Milk Powder, New Zealand) and maltodextrin of dextrose equivalent 6 (Roquette Freres, France) were used without further purification. They were vacuum dried (70 ◦ C 500 mbar) overnight and stored in desiccators over P2 O5 prior to solution preparation. Capilano floral honey (Capilano Honey Limited, Australia) was used. Dry solid content of honey was determined using AOAC recommended method [11]. A refractometer
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(RFM 340, Bellingham + Stanley Ltd., USA) was used in this purpose. 2.2. Methods 2.2.1. Contact angle Contact angle of the test solutions was measured using optical contact angle device (OCA 15 plusTM , Dataphysics, Germany). A 15 l of solution was used in all the tests. Sessile drops were formed on the surface of the test surfaces. The reported contact angle values represent the average of three readings. 2.2.2. Tensile strength Stickiness was measured using an in situ stickiness testing instrument. This instrument works on the principle of tack, that is, it mimics the feel when one touches a droplet surface. The working principle and the test protocols are given elsewhere [12]. The schematic diagram (Fig. 1) and the test procedure are briefly presented as follows. The probes (glass, stainless steel, polyurethane and Teflon) are attached to the shaft of the captive type linear actuator (Motor size 11 28H47, Hydon Switch and Instrument Inc., USA). The contact diameter of probes was 2.5 mm except that of glass which was 3.17 mm. The motor was driven by Intelligent Motion System driver (IM 483I). Its motion was controlled through LabView Software. The droplet holder is made up of Teflon solid cylinder (diameter 5 mm) which is mechanically attached to the weighing section of the precision load cell (±0.1 mg). A digital video camera
Fig. 1. Schematic diagram of the in situ stickiness testing instrument.
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and stereomicroscope (50 times magnification) imaging system monitors the approach, withdrawal and contact of the probe to the droplet surface. During the experiment, the stepper motor is driven downwards until the probe makes good contact with the droplet surface. The contact and withdrawal speed of the probe was maintained at 50 mm/min in all the trials. Once contact is established, the motor is subsequently withdrawn. The variation in the tensile force over time is recorded continuously to a PC. Digital images during the approach, contact and withdrawal are also recorded to the PC as well. The temperature of the droplet is recorded by inserting micro-thermocouples (T-type, Omega Engineering USA) to the droplet centre. The tensile strength, a peak force during the separation process normalized by the probe area, is taken as measure of stickiness of the droplet surface. The repeatability of this instrument was found to have two distinct regions. For the test conducted at room temperature, the standard deviation varied within 1–5% of mean values. The nature of the solids didn’t affect this spread. At elevated temperature, the standard variation was within 1–5% during the initial drying stages. However, as the solid content of the droplets increased, the repeatability decreased. At elevated temperature, the standard deviation varied within 5–10%. It was found that the higher the gradient in solids in droplets, the higher was the spread. The repeatability of this instrument is weaker when the surface of the droplet is structurally not uniform and rough. This happens towards the later stage of drying. Overall, this instrument measures the surface stickiness of drying droplets satisfactorily. The moisture history of the droplet was monitored through parallel experiments by placing the droplets on the droplet holder and monitoring the mass loss over time. 2.2.3. Spray drying A twin fluid nozzle spray dryer (SL 20, Saurin Company, Australia) with 3 l/h water evaporation capacity was used for the drying trials. The inlet and outlet temperatures of the dryer were maintained at 130 and 65 ◦ C, respectively. Powders were collected from the cyclone and spray dryer chamber by lightly sweeping the chamber wall. Honey/maltodextrin solid ratio of 50:50 was taken as the reference. A 5–10% of the maltodextrin solids were replaced with WPI solids and subsequently spray dried. Three runs were carried out and mean values are reported here. 2.2.4. Water activity and particle size distribution Water activity of the powders was measured using a water activity meter (AQUALAB 3TE, Decagon Company, USA). A Mastersizer 2000 (Malvern Company, USA) was used for par-
Fig. 2. Variation of tensile strength of lactose and whey protein isolate (WPI) at 24 ± 1 ◦ C. Probe: stainless steel, probe speed: 50 mm/min.
ticle size distribution determination. Tests were carried out in triplicate and mean values are reported. 3. Results and discussion 3.1. Droplet stickiness at room temperature The variation in tensile strength of droplets of lactose and WPI are presented in Fig. 2 which shows that the tensile strength of both the lactose and WPI solutions decreases with concentration. The decrease in tensile strength of 5% (w/w) lactose solution is merely 1.3% from that of pure water. The tensile strength of this solution continuously decreased and the final tensile strength of a 20% (w/w) lactose solution is 8% lower than that of pure water. Since pure lactose is known to be non-surface active and its viscosity also increases with concentration, it is expected that the tensile strength of this solution would increase. The decrease in concentration can be attributed to 0.5% (w/w) protein (as specified by the supplier) present in this material. There is an interesting trend regarding the variation of tensile strength of WPI solution with concentration (Fig. 2). At low concentrations, when the WPI concentration increased to 1% (w/w) the tensile strength reduced by 29% compared to pure water. The reduction in tensile strength continues until 5% (w/w) solid concentration which appears to be lowest point (33% reduction). Fig. 3 shows that the mode of failure at droplet–probe interface is cohesive. This means that droplet first undergoes necking and breaks within itself but not at the droplet-probe interface. This is probably due to lowering of the number of hydrogen bonds in water which is responsible for cohesive strength [13]. WPI acts to reduce the surface tension of
Fig. 3. Mode of failure of WPI solution at 24 ± 1 ◦ C.
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pure water quite remarkably indicating that WPI molecules are surface active and they migrate preferentially to the droplet–air interface and by doing so, lower the surface energy there. However, the adhesive strength at the droplet–probe interface has still remained much higher compared to the cohesive strength of the droplet molecules. This means that the presence of WPI in water facilitates the breaking of droplet within itself and consequently it will be harder to achieve a clean adhesive failure at the droplet–probe interface. This has great implication in processing, that is, protein molecules facilitate stickiness of food 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 it tended to increase slightly at highest concentration. However, the tensile strength of 25% (w/w) WPI solution is 66.34 Pa which is still well below the tensile strength of pure water (86.52 Pa) under test conditions. This shows that at concentrations greater than 20% (w/w) the presence of WPI solids brings about insignificant increase in cohesive strength of droplet. The small increment in cohesive strength might be due to the increased viscosity of the solutions. 3.2. Effect of probe materials on droplet stickiness Fig. 4 shows the variation of tensile strength of WPI droplets on different probes. The concentration of WPI was varied from 0 to 25% (w/w). This figure shows that the tensile strength of pure water on stainless steel and polyurethane are almost the same. The tensile strength of pure water on glass and Teflon surfaces are lower by 18.5% and 28%, respectively, compared to that on stainless steel. On the Teflon probe, it was observed that the water droplet retracted and only a small area was remained covered or wetted when the probe was completely withdrawn. This may be the reason why the observed tensile strength of water on a Teflon surface is so low. Furthermore, this can also be explained on the basis of Young’s equation given below: Wa = γLV (1 + cos θ)
(1)
where Wa is work of adhesion (J), γ LV the liquid surface tension (N/m) and θ is the contact angle. Eq. (1) explains that for a unit 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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Fig. 5. Contact angles of WPI solutions at different probe materials at 24 ± 0.5 ◦ C.
ues greater than zero. The contact angle (Fig. 5) of water on Teflon is 102.5◦ , which explains that the Teflon surface behaves as hydrophobic and hence the retraction of water droplet and the resultant reduction in tensile strength is expected. However, the contact angle of water on glass is much lower compared to the contact angle of water on stainless steel. This should result into greater wetting and hence a greater tensile strength upon separation. The much lower tensile strength of water droplet on glass (compared to stainless steel) even when it had greater wetttability seems contrary to Eq. (1). This might have resulted from the fact that, if the energy required to create a new surface within the droplet is lower than the energy required to achieve a clean separation at droplet probe–droplet interface then the mode of failure is cohesive (Fig. 3). When this occurs, stickiness cannot be explained from the surface tension related force alone. The slight increment in the contact angle at WPI concentration greater than 15% (w/w) can be attributed to increased viscosity. This increase in tensile strength when the contact angle has increased is contrary to Eq. (1) and proves the fact that there is a viscous force component in stickiness. Fig. 4 shows that the tensile strength of the WPI solutions on the probe (stainless steel, glass, polyurethane, Teflon) surfaces follows a similar trend. The tensile strength falls rapidly within 1% (w/w) of WPI concentration. The reduction in tensile strength continues to 5% (w/w) of WPI, albeit at much lower rate. The tensile strength remains constant from 5% to 20% (w/w) of WPI beyond which it shows a tendency to increase. This information shows that WPI acts as surfactant which sharply reduces the cohesive strength of the water molecules and a small quantity (≤1%, w/w) is sufficient to bring about substantial reduction in cohesive strength. This fact is also seen in a reduction in contact angle (Fig. 5). The mode of failure during the tensile strength measurements was cohesive in all these tests. Except for the Teflon surface, no retraction of droplet was observed during the probe withdrawal. Since there was no adhesive failure at the droplet–probe interface in these tests, then, one would expect that the tensile strength of the droplets should be independent of the probe material chosen. However, Fig. 4 indicates that there is significant effect of probe surfaces on the tensile strength or stickiness of the droplet.
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Fig. 6. Tensile strength of WPI/lactose mixture solutions at 24 ± 1 ◦ C.
Fig. 7. Contact angle of the WPI/lactose mixture drops on probe surfaces at 24 ± 1 ◦ C.
3.3. Stickiness of WPI/lactose mixtures The effect of addition of WPI on the tensile strength of lactose solution is given in Fig. 6. The WPI/lactose ratio has been varied from 0 to 1. The total solids in solution was kept at 25% (w/w) because it was difficult to consistently prepare an emulsion of WPI/lactose above this solid concentration and lower than this concentrations are not desired as feeds in spray drying operations. The tensile strength of these mixture droplets on different probe was also studied. As can be seen from this figure, the tensile strength of the lactose solution decreases to close to a minimum when the WPI fraction is 0.2. Except for the magnitude of the tensile strength, the effect of increment of WPI fraction in WPI/lactose solution is identical for all the probes. The tensile strength does not decrease much when the WPI fraction is increased above 0.2, rather it remains almost constant up to WPI fraction of 0.8 and then increases slightly beyond. As in case of pure WPI solution on different probe surfaces, the tensile strength of WPI/lactose solutions is the lowest for glass and the highest for stainless steel probe and the values obtained from the Teflon and polyurethane lie in between. The contact angle of the WPI/lactose mixtures decreased (Fig. 7) with an increase in WPI fraction. If the tensile strength is a function of surface force alone, then, according to Eq. (1), the tensile strength should increase with decrease in contact angle. This result demonstrates that the tensile strength of the WPI/lactose mixture is the result of complex interplay between surface and bulk forces. The magnitude of contact angle of WPI/lactose solution is Teflon > polyurethane > stainless steel > glass. This order does not occur in the tensile strength measurements where strength is the highest for stainless steel and the lowest for glass. The contact angle appears to increase with increase in WPI fraction on the glass surface. However, the maximum variation in contact angle in this case is 5.7◦ , which is not very far apart from the accuracy (±3◦ ) attainable in contact angel measurements [14]. Hence, the wetting behavior of pure lactose, WPI/lactose mixtures solutions up to 25% (w/w) solids on glass surface is close to that of water. Fig. 7 also corroborates with the results obtained from Figs. 2, 4 and 6 that
the tensile strength of droplets results from complex interplay between surface and bulk properties of materials. 3.4. Stickiness of WPI solutions at elevated temperatures 3.4.1. Temperature and moisture histories Moisture histories of WPI and lactose droplets at 65 and 80 ◦ C are presented in Fig. 8. This figure shows that in the 15 min of drying, lactose droplets dry faster compared to that of WPI. During this period, WPI droplet already forms a surface skin through which the rate of moisture removal is slowed down. After 15 min, the drying rate of lactose droplets slows down more and the WPI droplets dry faster than the lactose droplets. This may be due to the fact that the moisture diffusivity of lactose solution reduces strongly with an increase in solid concentration. The WPI droplet surface becomes rugged due to appearance of crests, troughs and wrinkles. All of these act to greatly increase the surface area. The increased surface area enhances evaporation and subsequently moisture removal. It was observed that a thin crust of crystals was formed on the surface of the lactose droplets which trapped the water within. The thickness of this crust increased over time. This is another reason why the lactose droplets dried slower.
Fig. 8. Moisture histories of lactose and WPI solution droplets at 65 and 80 ◦ C, 0.75 m/s air velocity and 2.5 ± 0.5 relative humidity.
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Fig. 9. Temperature histories of lactose and WPI solution droplets at 65 and 80 ◦ C, 0.75 m/s air velocity and 2.5 ± 0.5 relative humidity.
Fig. 9 presents the temperature histories of WPI and lactose droplets at 65 and 80 ◦ C. The temperature histories of both the WPI and lactose droplets bear similarities with that of their moisture histories. At the beginning, the droplet temperature of WPI increases more rapidly than the corresponding lactose droplet. However, once the rate of evaporation of lactose slows down, the temperature of droplets rise faster than that of WPI. It appears that crystallization cools down the droplets as exhibited remarkably by the temperature history of lactose droplet dried at 80 ◦ C. Once the crystallization occurred, the temperature of lactose droplets was lower than that of the WPI. Similar effect of crystallization of lactose on its drop temperature is seen on the plot at air temperature of 63 ◦ C. 3.4.2. Stickiness of WPI droplets at elevated temperatures The development of stickiness of WPI droplets at different probe surfaces at 65 ◦ C is presented in Fig. 10(a). The repeatability of the measurements, based on standard deviation, is shown through error bar. This varied between 5% and 8% of the measured mean values. The mode of failure when the WPI droplet attains a non-sticky state on stainless steel probe is given in Fig. 10(b). This mode of failure is common to all the probes when non-sticky state is attained. In case of a Teflon probe, a peak tensile strength is observed at droplet–probe interface at average moisture u = 2.43 (70.85%, w/w). At this point a clean adhesive failure occurred when the probe was pulled away. This is the point at which the cohesive strength of the droplet surface is close enough to achieve the adhesive strength at droplet–probe interface. A clean separation of the probe from the droplet surface occurs here. We wish to emphasize here that the peak tensile strength values reported here may be apparent peaks on the left (on x-axis) of a real peak. This is because it is very difficult to say if the measured peak value is the actual (maximum) peak value because it depends strongly on the surface moisture and temperature of the droplet. Nevertheless, the measured peaks should lie very close to the actual peaks in all cases. Hence, the conclusion drawn in this work will not be affected by this difficulty. The drop surface was completely no-sticky (similar to Fig. 10(b)) for the Teflon probe when the moisture content
Fig. 10. (a) Stickiness (tensile strength) of WPI solution droplets on different probe materials at 65 ◦ C, 0.75 air velocity, 2.5 ± 0.5 relative humidity and 50 mm/min probe speed. (b) Mode of failure at probe–WPI droplet interface at non-sticky state (65 ◦ C, 0.75 air velocity, 2.5 ± 0.5 relative humidity and 50 mm/min probe speed).
decreases to u = 2.2, that is, the solid concentration of the droplet was merely 31.2% (w/w) and the drying time was only 5 min. We applied 10 kPa of compressive pressure at the droplet surface to make sure that the surface was non-sticky. This extent of compressive pressure is recommended because the surface of food droplets behaves like pressure sensitive adhesive and also due to the fact that the droplet surface is not smooth [12,15]. There are two reasons why the WPI droplet surface becomes non-sticky in such an early stage of drying. Firstly, WPI droplet forms smooth skin at the surface immediately after coming into contact with the hot air and behaves like a flexible water pouch (Fig. 10(b)). The outer surface of the skin soon converts itself to non-sticky glassy material. Secondly, the Teflon surface is hydrophobic. It has to be noted that the temperature of the WPI droplet when its surface attained a non-sticky state was 45 ◦ C, well below the drying temperature of the air (65 ◦ C). The peak tensile strength of WPI droplets on polyurethane and stainless steel surfaces were achieved at u = 2.2 (68.75%, w/w) and with a corresponding drop temperature of 45 ◦ C. This suggests that the WPI droplets will stick to the polyurethane and stainless for a longer time compared to the Teflon surface. The magnitude of the peak tensile strength on the stainless steel was much higher compared to that of polyurethane. This indicates that the WPI droplet surface will have a greater tendency to stick on the stainless steel surface than the polyurethane at elevated temperature. The WPI droplet surface attained a complete nonsticky state when the average moisture was u = 1.27 (55.95%, w/w) and the corresponding drop temperature was 54.3 ◦ C both with polyurethane and stainless steel. In case of glass, the peak tensile strength when the droplet surface was cleanly separated was 4700 kPa which is the highest value among the probe
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materials. This means that the WPI surface has the greatest tendency to stick to the glass surface. However, the WPI surface attained non-sticky state with glass at the same moisture and temperature as in the case of stainless steel and polyurethane. The magnitude of the stickiness (peak tensile strength) of WPI droplet on probe materials is consistent with the contact angle results (Fig. 5). The magnitude of the contact angle is Teflon > polyurethane > stainless steel > glass. This means that the work of adhesion (Eq. (1)) or the tensile strength at the droplet–probe interface should be of the order of glass > stainless steel > polyurethane > Teflon. The stickiness of WPI drop surface is the greatest on the glass surface and the lowest on Teflon and that the value of the stainless steel and polyurethane fall in between with predicted order. A comparison of the peak tensile strength at 63 ◦ C with those obtained at room temperature shows that that for the surface forces of the adhesive (droplets) and adherend (probe surfaces) to be dominant (in stickiness), the adhesive mode of failure (Fig. 10(b)) is a must have criterion. Since the dominant mode of failure was cohesive (Fig. 3) during the tests at room temperature, the contact angle values (i.e. the surface forces) are not consistent with the development of surface stickiness. It is worth noting here that glass transition temperature (Tg ) of droplets is usually used to predict if it is likely to stick to equipment/packaging material or likely to cake upon storage and transportation. A practical rule is that if the droplet temperature is 20 ◦ C above its Tg , it will be sticky. Tg is a strong function of moisture content, and generally the mean moisture content is used to calculate it. It is very difficult to experimentally measure the Tg of anhydrous WPI and hence a calculated value of 153 ◦ C is taken [16]. At mean moisture content u = 1.27 (55.95%, w/w) the Tg of the WPI droplet will be −8 ◦ C or lower. The measured WPI droplet temperature is 53.4 ◦ C, which is at least 61.4 ◦ C above Tg . If we accept the Tg + 20 ◦ C criteria for stickiness, this droplet should be sticky. Quite contrary, the above results show that the drop does not stick even if compressive pressure of 10 kPa was used. Hence, for skin forming materials such as WPI, the behavior of the film or the surface properties rather than the bulk ones control the development of stickiness. Tensile strength of WPI at 80 ◦ C, 0.75 m/s air velocity and 2.5 ± 0.5% RH was studied to investigate the effect temperature and the results are presented in Fig. 11. The repeatability of the measurements is shown through error bars. The maximum spread (standard deviation), in these tests, was within 10% of the measured mean values. This figure shows that the peak tensile strength of droplets is the highest on the glass and lowest on the Teflon surfaces and the values for stainless steel and polyurethane fall in between in order. These results resemble the results obtained at 65 ◦ C and the same conclusion can be drawn. However, the moisture contents at which the WPI droplets exhibit peak stickiness and non-sticky state are lower compared to those at 65 ◦ C. Similarly, the droplet surface requires longer time to achieve clean adhesive failure and also to enter the non-sticky state. This is due to the fact that the droplet surface remains soft or plastic for longer time at higher temperature. Even at this drying temperature the droplet surface becomes completely non-sticky at average moisture u = 0.68 and
Fig. 11. Stickiness (tensile strength) of WPI solution droplets on different probe materials at 80 ◦ C, 0.75 air velocity, 2.5 ± 0.5 relative humidity and 50 mm/min probe speed.
drop temperature of 72 ◦ C, mainly due to formation of glassy skin at the droplet surface. 3.5. Comparison of stickiness of lactose and WPI droplets Fig. 12 compares the development of stickiness of WPI and lactose droplets at 65 and 80 ◦ C on stainless steel probes. This figure shows that the evolution of stickiness of WPI and lactose droplets is very different to each other. At room temperature the tensile strength of 25% (w/w) of WPI is 66 Pa compared to 79 Pa of lactose at the same solid concentration. Once the drying commences the cohesive strength of the WPI droplet increases very rapidly and exceeds the adhesive strength at droplet–probe interface at which a clean failure occurs for the first time. On further progress of drying the surface becomes glassy and enters the non-sticky regime fairly early in the drying process or at fairly high average moisture content. In the case of lactose, the tensile strength first decreases rather than increasing. It is interesting to observe that the tensile strength of lactose droplet at u = 0.49 (or 67%, w/w solids) is well below the tensile strength with droplet of 25% (w/w) solids at room temperature. This is because the cohesive strength of the
Fig. 12. Stickiness (tensile strength) of WPI and lactose solution droplets on stainless steel probe at 65 and 80 ◦ C, 0.75 air velocity, 2.5 ± 0.5 relative humidity and 50 mm/min probe speed.
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Table 1 Powder 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) 50% (H), 45% (MD), 5% (WPI) 50% (H), 40% (MD), 10% (WPI)
28.4 ± 4.9 80.1 ± 4.1 82.2 ± 4.2
49.71 15.48 11.88
0.124 ± 0.003 0.183 ± 0.002 0.198 ± 0.002
H: honey; MD: maltodextrin; WPI: whey protein isolate.
lactose solution decreases with an increase in droplet temperature. This is expected because the fluidity of a given solution is higher at higher temperatures. This increased fluidity lowers the viscosity as well as cohesive strength of the solution and hence the tensile strength gets lowered. When the moisture content of the droplet decreases below u = 0.4 (solids > 70%, w/w) the tensile strength increases rapidly. This is because the effect of solid concentration starts dominating over the effect of temperature which are mutually opposing. At this point, numerous small crystals appeared on the droplet and they grew and became dominant. It was observed that the crystals accumulated on the surface to form a crust the thickness of which grew over time. The interior and non-crystallized part of the droplet was surrounded by the crystal crust. This crust was fragile in nature and broke when the probe made a contact. As the crust fractured in numerous locations (on contact) a thin layer of viscous solution then seeped out and contacted the probe. This phenomena was observed in all tests even when the moisture content u = 0.18 (85% solids, w/w). The mode of failure was observed to be varying between cohesive to adhesive-dominant. It was also observed that the failure was completely adhesive if the crystal crust was not broken at the surface. The effect of temperature on the magnitude of surface stickiness is clearly visible here below u = 0.49. The magnitude of tensile strength decreased with increase in temperature at a given solid concentration. This result also supports the earlier finding with WPI droplets that higher temperatures worsen the problem of stickiness during drying. The extraordinary difference in development of stickiness in WPI and lactose droplets comes from their nature. Lactose, being a disaccharide lacks the capacity to form a non-sticky skin at its surface; rather it crystallizes and somehow unsuccessfully acts to lower the stickiness. In spray drying, however, amorphous lactose is formed, which is usually considered non-sticky. On the other hand WPI due to its higher molecular weight forms a skin upon exposure to drying air which converts itself into glassy matrix that resists the stickiness. This skin forming behavior of WPI not only overcomes the sticky problem but also increases the drying rate by increasing the heat and mass transfer surface (Fig. 8) due to formation of folds and wrinkles at the surface.
shows that maltodextrin has to be added as a drying aid to bring the honey to maltodextrin ratio to less than 50:50 on a dry solid basis. Even this extent of drying aid resulted in a recovery of powder less than 50% (of solids in feed). This level of additive addition is not welcomed by consumers. Results from Section 3.1 above shows that WPI molecules are strong surfactants and preferentially migrate to a droplet–air interface. Subsequently, when the WPI droplet is subjected to elevated temperatures (Section 3.4.2), a thin, smooth and nonsticky skin is formed immediately after the commencement of drying. These observations can have two important implications. Firstly, the skin can resist coalescence when droplets come in contact with each other and also decrease their adherence to the dryer wall. Secondly, the proteins can be efficient drying aids because only a small amount will be required. Hence drying trials were carried out to evaluate the effectiveness of WPI as a drying aid. The product recovery and the stability of powders were taken as measures of drying effectiveness. Powders were collected from cyclone and also from the drying chamber by lightly sweeping the latter. The recoveries (%) were calculated based on the amount of total solids in the feed (Table 1). As shown in this table, when the honey to maltodextrin (solid) ratio was maintained at 50:50, the recovery was merely 28%. However, when 5% of the maltodextrin (from the above formulation) was replaced with WPI, a recovery of greater than 80% was achieved. When the amount of WPI was increased to 10% (maltodextrin 40%) the recovery increased only slightly (82%). It has to be noted that recovery in the laboratory scale spray dryer (as used here) is usually much lower compared to that of the industrial spray dryers [6]. The above trials have shown that small amount of WPI addition can bring about a great improvement in the product recovery and that the proteins can be used as smart drying aids. The effectiveness of WPI as drying aids is due to segregation of WPI molecules from honey solids (molecules) and the preferential migration of the former towards the droplet surface which is induced by the surface activity of protein molecules and their bigger size. The higher the magnitude of this segregation the greater will be the effectiveness of a protein as a drying aid. 4. Conclusions
3.6. Application of WPI as drying aid Honey is one of the most difficult materials to convert into powder form because its dry mass is comprised mainly of low molecular weight sugars. It has not been possible to produce honey power, in its pure form, using currently available powder production techniques [17]. The work of Bhandari et al. [6]
The stickiness development of WPI and lactose droplets was determined. The surface of the lactose droplet remained sticky with cohesive failure until the surface was completely covered with fragile crystal layer which fractured upon probe contact and allowed the thin solution layer to seep out to the probe even when the average moisture was lower than 16.67% (w/w). WPI
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droplets formed a thin, smooth skin immediately after coming in contact with the drying air. The tensile strength of this skin increased rapidly and peaked fairly early during drying process and became non-sticky due to transformation of the outer layer of the skin into a glassy material. Tests of WPI/lactose mixture droplets at room temperature showed that addition of 0.5–1% (w/w) WPI lowered the tensile strength of lactose solution mainly due to preferential migration of protein molecules to the surface and lowering of cohesive strength. The skin forming and surface active nature of WPI were exploited to minimize the stickiness of honey in a pilot scale spray drying trial. Replacement of 5% (w/w) maltodextrin with WPI raised the powder recovery of honey solids from 28% to 80%. Stickiness of the WPI on glass, Teflon and polyurethane surfaces was studied by modification of the probe surface. At elevated temperatures, the peak stickiness of the glass was the highest and on the Teflon was the lowest as would be predicted from contact angle information. The Teflon surface offered the lowest stickiness both at low and high temperatures making it a suitable material to minimize stickiness through surface coating. Acknowledgements The authors acknowledge Miss Charlotte Contassot’s help in experiments and the Dairy Ingredients Group of Australia (DIGA) and Australian Research Council (ARC) for financial support for this study. References [1] H. Xin, X.D. Chen, N. Ozkan, Removal of model foulant from metal surfaces, AIChE J. 5 (2005) 1961–1973. [2] D.I. Wilson, Challenges in cleaning: recent developments and future prospects, Heat Transfer Eng. 26 (2005) 54–59.
[3] K. Masters, Deposit-free spray drying: dream or reality? in: Drying 96Proceedings of the 10th International Drying Symposium (IDS’96), vol. 1, Krakow, Poland, 30 July–2 August, 1996, pp. 52–60. [4] S.E. Papadakis, R.E. Bahu, The sticky issues of drying Dry. Technol. 10 (1992) 817–837. [5] R.H.G. Rao, P.M. Gupta, Development of spray dried orange juice blended skim milk powder, Lait 82 (2002) 523–529. [6] B.R. Bhandari, N. Datta, T. Howes, Problems associated with spray drying of sugar-rich foods, Dry. Technol. 15 (1997) 671–684. [7] B. Adhikari, T. Howes, B.R. Bhandari, V. Troung, Effect of addition of maltodextrin on drying kinetics and stickiness of sugar and acid-rich foods during convective drying: experiments and modelling, J. Food Eng. 62 (2004) 53–68. [8] M.C. Michalsky, S. Desobry, J. Hardy, Adhesion of edible oils and food emulsions to rough surfaces, Food Sci. Technol. -LEB 31 (1998) 495–502. [9] M.C. Michalsky, S. Desobry, V. Babak, J. Hardy, Adhesion of food emulsions to packaging and equipment surfaces, Colloids Surf. A: Physiochem. Eng. Aspects 149 (1999) 107–121. [10] V. Truong, B.R. Bhandari, T. Howes, Optimization of co-current spray drying process of sugar-rich foods. Part I. Moisture and glass transition temperature profile during drying, J. Food Eng. 71 (2005) 55–65. [11] E.B. Wedmore, The accurate determination of the water content of honeys. Part I. Introduction and results, Bee World 36 (1955) 197–206. [12] B. Adhikari, T. Howes, B.R. Bhandari, V. Truong, In situ characterization of stickiness of sugar-rich foods using a linear actuator driven stickiness testing device, J. Food Eng. 58 (2003) 11–22. [13] K. Tsujii, Surface Activity: Principles, Phenomena, and Applications, Academic Press, Tokyo, 1998. [14] F.K. Skinner, Y. Rotenberg, A.W. Neumann, Contact-angle measurements from the contact diameter of sessile drops by means of a modified axisymmetric drop shape-analysis, J. Colloid Interf. Sci. 130 (1989) 25–34. [15] F.H. Hamond, Tack, in: D. Satas (Ed.), Handbook of Pressure Sensitive Adhesive Technology, Chapman & Hall, New York, 1989, pp. 38–60. [16] L.J. Burin, K. Jouppila, Y.H. Roos, J. Kansikas, M.D. Buera, Color formation in dehydrated modified whey powder systems as affected by compression and Tg , J. Agric. Food Chem. 48 (2000) 5263–5268. [17] B. Adhikari, T. Howes, D. Lecomte, B.R. Bhandari, A glass transition temperature approach for the prediction of the surface stickiness of a drying droplet during spray drying, Powder Technol. 149 (2005) 168–179.
Development of stickiness of whey protein isolate and lactose droplets during convective drying B. Adhikari a,∗ , T. Howes a , A.K. Shrestha b , B.R. Bhandari b a
b
School of Engineering, The University of Queensland, Brisbane, Qld 4072, Australia 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 2006 Available online 16 September 2006
Abstract The stickiness development of droplets of whey protein isolate (WPI), lactose and their mixture solutions was determined using an in situ stickiness testing device at 24, 65 and 80 ◦ C. Stainless steel, Teflon, glass and polyurethane probes were used. At room temperature, the presence of 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 weakening of the cohesive strength. At elevated temperatures, lactose droplets remained sticky showing cohesive failure until the surface was completely covered with a thin crystal layer. WPI droplets formed a thin, smooth skin immediately on coming in contact with drying air. This surface became non-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 WPI was exploited to minimize the stickiness of honey in a pilot scale spray drying trial. Replacement of 5% (w/w) maltodextrin with WPI raised the powder recovery of honey solids from 28% to 80% in a pilot scale drying test. At elevated temperature the magnitude of stickiness on probe materials was in the order of glass > stainless steel > polyurethane > Teflon. The Teflon surface offered the lowest stickiness both at low and high temperatures making it a suitable material to minimize stickiness through surface coating. © 2006 Elsevier B.V. All rights reserved. Keywords: Stickiness; Tensile strength; Contact angle; WPI; Lactose; Teflon; Polyurethane
1. Introduction Stickiness of liquid foods in processing equipment and packages is a ubiquitous issue encountered in industry as well as in every day life. Stickiness leads to scale formation and fouling in thermal processes [1,2]. Stickiness of food powders during production, handling and storage has long been recognised as a major problem in powder making industries [3]. This problem not only leads to processing difficulties such as frequent plant shutdowns and cleaning but also results in to low quality products and fire hazards [4]. There is considerable demand for high value particulate products from natural foods such as powdered fruit juices, honey, whey permeates and vegetable soups, especially in developing countries where refrigerated-storage facilities are lacking [5]. Furthermore, these products are important ingredients for ice-cream, yoghurt and non-alcoholic beverage manufactur-
∗
Corresponding author. Tel.: +61 7 336 59058; fax: +61 7 336 54199. E-mail address: [email protected] (B. Adhikari).
0255-2701/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.cep.2006.07.014
ing. However, the production of powders from these materials through drying is very difficult due to high concentrations of sugars, organic acids and minerals which have a greater tendency to stick to the equipment surfaces [6,7]. The nature of the contact surface can play an important role in food stickiness. Michalsky et al. [8,9] have studied the adhesion behavior of edible oil and food emulsions to glass, Teflon (PTFE), lowdensity polyethylene (LDPE), poly ethylene terephthalate (PET) and stainless steel. They allowed the food samples to flow down an inclined substrate surface and measured the amount of sample remaining on the surface after the flow ceased. It was found that surface roughness, the yield stress of the sample and solid surface tension were the key factors responsible for adhesion. Adhikari et al. [7] studied the surface stickiness of droplets of fructose–maltodextrin and sucrose–maltodextrin mixtures during convective drying. They found that the presence of maltodextrin improves the spray drying yield mainly due to its skin forming property. However typically 40–60% (w/w) maltodextrin solids need to be introduced before it is possible to convert these sugar solutions into powders even under mild spray drying conditions [10].
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One can think of two possible ways that might more effectively minimize the wall stickiness problem. Firstly, the dryer wall can be coated with materials that do not favor the sticking of the solutions/particles. Secondly the surface of the droplet/particles could be engineered in such as way that they resist coalescence when they come in contact with each other and also decreases their adherence to the dryer wall. The latter approach can greatly reduce the amount of additives required as a drying aid. Hence, this study aims at making a comparative study of the stickiness on stainless steel, glass, polyurethane and Teflon surfaces. Further, it also explores the possibility of using a protein solution to partially replace high molecular weight carbohydrates as drying aids, in order to manipulate the droplet surface property. 2. Materials and methods 2.1. Materials Spray dried lactose powder (Murray Goulburn Co-Ltd., Australia), hydrolyzed whey protein isolate (ALATAL 817TM , New Zealand Milk Powder, New Zealand) and maltodextrin of dextrose equivalent 6 (Roquette Freres, France) were used without further purification. They were vacuum dried (70 ◦ C 500 mbar) overnight and stored in desiccators over P2 O5 prior to solution preparation. Capilano floral honey (Capilano Honey Limited, Australia) was used. Dry solid content of honey was determined using AOAC recommended method [11]. A refractometer
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(RFM 340, Bellingham + Stanley Ltd., USA) was used in this purpose. 2.2. Methods 2.2.1. Contact angle Contact angle of the test solutions was measured using optical contact angle device (OCA 15 plusTM , Dataphysics, Germany). A 15 l of solution was used in all the tests. Sessile drops were formed on the surface of the test surfaces. The reported contact angle values represent the average of three readings. 2.2.2. Tensile strength Stickiness was measured using an in situ stickiness testing instrument. This instrument works on the principle of tack, that is, it mimics the feel when one touches a droplet surface. The working principle and the test protocols are given elsewhere [12]. The schematic diagram (Fig. 1) and the test procedure are briefly presented as follows. The probes (glass, stainless steel, polyurethane and Teflon) are attached to the shaft of the captive type linear actuator (Motor size 11 28H47, Hydon Switch and Instrument Inc., USA). The contact diameter of probes was 2.5 mm except that of glass which was 3.17 mm. The motor was driven by Intelligent Motion System driver (IM 483I). Its motion was controlled through LabView Software. The droplet holder is made up of Teflon solid cylinder (diameter 5 mm) which is mechanically attached to the weighing section of the precision load cell (±0.1 mg). A digital video camera
Fig. 1. Schematic diagram of the in situ stickiness testing instrument.
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and stereomicroscope (50 times magnification) imaging system monitors the approach, withdrawal and contact of the probe to the droplet surface. During the experiment, the stepper motor is driven downwards until the probe makes good contact with the droplet surface. The contact and withdrawal speed of the probe was maintained at 50 mm/min in all the trials. Once contact is established, the motor is subsequently withdrawn. The variation in the tensile force over time is recorded continuously to a PC. Digital images during the approach, contact and withdrawal are also recorded to the PC as well. The temperature of the droplet is recorded by inserting micro-thermocouples (T-type, Omega Engineering USA) to the droplet centre. The tensile strength, a peak force during the separation process normalized by the probe area, is taken as measure of stickiness of the droplet surface. The repeatability of this instrument was found to have two distinct regions. For the test conducted at room temperature, the standard deviation varied within 1–5% of mean values. The nature of the solids didn’t affect this spread. At elevated temperature, the standard variation was within 1–5% during the initial drying stages. However, as the solid content of the droplets increased, the repeatability decreased. At elevated temperature, the standard deviation varied within 5–10%. It was found that the higher the gradient in solids in droplets, the higher was the spread. The repeatability of this instrument is weaker when the surface of the droplet is structurally not uniform and rough. This happens towards the later stage of drying. Overall, this instrument measures the surface stickiness of drying droplets satisfactorily. The moisture history of the droplet was monitored through parallel experiments by placing the droplets on the droplet holder and monitoring the mass loss over time. 2.2.3. Spray drying A twin fluid nozzle spray dryer (SL 20, Saurin Company, Australia) with 3 l/h water evaporation capacity was used for the drying trials. The inlet and outlet temperatures of the dryer were maintained at 130 and 65 ◦ C, respectively. Powders were collected from the cyclone and spray dryer chamber by lightly sweeping the chamber wall. Honey/maltodextrin solid ratio of 50:50 was taken as the reference. A 5–10% of the maltodextrin solids were replaced with WPI solids and subsequently spray dried. Three runs were carried out and mean values are reported here. 2.2.4. Water activity and particle size distribution Water activity of the powders was measured using a water activity meter (AQUALAB 3TE, Decagon Company, USA). A Mastersizer 2000 (Malvern Company, USA) was used for par-
Fig. 2. Variation of tensile strength of lactose and whey protein isolate (WPI) at 24 ± 1 ◦ C. Probe: stainless steel, probe speed: 50 mm/min.
ticle size distribution determination. Tests were carried out in triplicate and mean values are reported. 3. Results and discussion 3.1. Droplet stickiness at room temperature The variation in tensile strength of droplets of lactose and WPI are presented in Fig. 2 which shows that the tensile strength of both the lactose and WPI solutions decreases with concentration. The decrease in tensile strength of 5% (w/w) lactose solution is merely 1.3% from that of pure water. The tensile strength of this solution continuously decreased and the final tensile strength of a 20% (w/w) lactose solution is 8% lower than that of pure water. Since pure lactose is known to be non-surface active and its viscosity also increases with concentration, it is expected that the tensile strength of this solution would increase. The decrease in concentration can be attributed to 0.5% (w/w) protein (as specified by the supplier) present in this material. There is an interesting trend regarding the variation of tensile strength of WPI solution with concentration (Fig. 2). At low concentrations, when the WPI concentration increased to 1% (w/w) the tensile strength reduced by 29% compared to pure water. The reduction in tensile strength continues until 5% (w/w) solid concentration which appears to be lowest point (33% reduction). Fig. 3 shows that the mode of failure at droplet–probe interface is cohesive. This means that droplet first undergoes necking and breaks within itself but not at the droplet-probe interface. This is probably due to lowering of the number of hydrogen bonds in water which is responsible for cohesive strength [13]. WPI acts to reduce the surface tension of
Fig. 3. Mode of failure of WPI solution at 24 ± 1 ◦ C.
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pure water quite remarkably indicating that WPI molecules are surface active and they migrate preferentially to the droplet–air interface and by doing so, lower the surface energy there. However, the adhesive strength at the droplet–probe interface has still remained much higher compared to the cohesive strength of the droplet molecules. This means that the presence of WPI in water facilitates the breaking of droplet within itself and consequently it will be harder to achieve a clean adhesive failure at the droplet–probe interface. This has great implication in processing, that is, protein molecules facilitate stickiness of food 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 it tended to increase slightly at highest concentration. However, the tensile strength of 25% (w/w) WPI solution is 66.34 Pa which is still well below the tensile strength of pure water (86.52 Pa) under test conditions. This shows that at concentrations greater than 20% (w/w) the presence of WPI solids brings about insignificant increase in cohesive strength of droplet. The small increment in cohesive strength might be due to the increased viscosity of the solutions. 3.2. Effect of probe materials on droplet stickiness Fig. 4 shows the variation of tensile strength of WPI droplets on different probes. The concentration of WPI was varied from 0 to 25% (w/w). This figure shows that the tensile strength of pure water on stainless steel and polyurethane are almost the same. The tensile strength of pure water on glass and Teflon surfaces are lower by 18.5% and 28%, respectively, compared to that on stainless steel. On the Teflon probe, it was observed that the water droplet retracted and only a small area was remained covered or wetted when the probe was completely withdrawn. This may be the reason why the observed tensile strength of water on a Teflon surface is so low. Furthermore, this can also be explained on the basis of Young’s equation given below: Wa = γLV (1 + cos θ)
(1)
where Wa is work of adhesion (J), γ LV the liquid surface tension (N/m) and θ is the contact angle. Eq. (1) explains that for a unit 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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Fig. 5. Contact angles of WPI solutions at different probe materials at 24 ± 0.5 ◦ C.
ues greater than zero. The contact angle (Fig. 5) of water on Teflon is 102.5◦ , which explains that the Teflon surface behaves as hydrophobic and hence the retraction of water droplet and the resultant reduction in tensile strength is expected. However, the contact angle of water on glass is much lower compared to the contact angle of water on stainless steel. This should result into greater wetting and hence a greater tensile strength upon separation. The much lower tensile strength of water droplet on glass (compared to stainless steel) even when it had greater wetttability seems contrary to Eq. (1). This might have resulted from the fact that, if the energy required to create a new surface within the droplet is lower than the energy required to achieve a clean separation at droplet probe–droplet interface then the mode of failure is cohesive (Fig. 3). When this occurs, stickiness cannot be explained from the surface tension related force alone. The slight increment in the contact angle at WPI concentration greater than 15% (w/w) can be attributed to increased viscosity. This increase in tensile strength when the contact angle has increased is contrary to Eq. (1) and proves the fact that there is a viscous force component in stickiness. Fig. 4 shows that the tensile strength of the WPI solutions on the probe (stainless steel, glass, polyurethane, Teflon) surfaces follows a similar trend. The tensile strength falls rapidly within 1% (w/w) of WPI concentration. The reduction in tensile strength continues to 5% (w/w) of WPI, albeit at much lower rate. The tensile strength remains constant from 5% to 20% (w/w) of WPI beyond which it shows a tendency to increase. This information shows that WPI acts as surfactant which sharply reduces the cohesive strength of the water molecules and a small quantity (≤1%, w/w) is sufficient to bring about substantial reduction in cohesive strength. This fact is also seen in a reduction in contact angle (Fig. 5). The mode of failure during the tensile strength measurements was cohesive in all these tests. Except for the Teflon surface, no retraction of droplet was observed during the probe withdrawal. Since there was no adhesive failure at the droplet–probe interface in these tests, then, one would expect that the tensile strength of the droplets should be independent of the probe material chosen. However, Fig. 4 indicates that there is significant effect of probe surfaces on the tensile strength or stickiness of the droplet.
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Fig. 6. Tensile strength of WPI/lactose mixture solutions at 24 ± 1 ◦ C.
Fig. 7. Contact angle of the WPI/lactose mixture drops on probe surfaces at 24 ± 1 ◦ C.
3.3. Stickiness of WPI/lactose mixtures The effect of addition of WPI on the tensile strength of lactose solution is given in Fig. 6. The WPI/lactose ratio has been varied from 0 to 1. The total solids in solution was kept at 25% (w/w) because it was difficult to consistently prepare an emulsion of WPI/lactose above this solid concentration and lower than this concentrations are not desired as feeds in spray drying operations. The tensile strength of these mixture droplets on different probe was also studied. As can be seen from this figure, the tensile strength of the lactose solution decreases to close to a minimum when the WPI fraction is 0.2. Except for the magnitude of the tensile strength, the effect of increment of WPI fraction in WPI/lactose solution is identical for all the probes. The tensile strength does not decrease much when the WPI fraction is increased above 0.2, rather it remains almost constant up to WPI fraction of 0.8 and then increases slightly beyond. As in case of pure WPI solution on different probe surfaces, the tensile strength of WPI/lactose solutions is the lowest for glass and the highest for stainless steel probe and the values obtained from the Teflon and polyurethane lie in between. The contact angle of the WPI/lactose mixtures decreased (Fig. 7) with an increase in WPI fraction. If the tensile strength is a function of surface force alone, then, according to Eq. (1), the tensile strength should increase with decrease in contact angle. This result demonstrates that the tensile strength of the WPI/lactose mixture is the result of complex interplay between surface and bulk forces. The magnitude of contact angle of WPI/lactose solution is Teflon > polyurethane > stainless steel > glass. This order does not occur in the tensile strength measurements where strength is the highest for stainless steel and the lowest for glass. The contact angle appears to increase with increase in WPI fraction on the glass surface. However, the maximum variation in contact angle in this case is 5.7◦ , which is not very far apart from the accuracy (±3◦ ) attainable in contact angel measurements [14]. Hence, the wetting behavior of pure lactose, WPI/lactose mixtures solutions up to 25% (w/w) solids on glass surface is close to that of water. Fig. 7 also corroborates with the results obtained from Figs. 2, 4 and 6 that
the tensile strength of droplets results from complex interplay between surface and bulk properties of materials. 3.4. Stickiness of WPI solutions at elevated temperatures 3.4.1. Temperature and moisture histories Moisture histories of WPI and lactose droplets at 65 and 80 ◦ C are presented in Fig. 8. This figure shows that in the 15 min of drying, lactose droplets dry faster compared to that of WPI. During this period, WPI droplet already forms a surface skin through which the rate of moisture removal is slowed down. After 15 min, the drying rate of lactose droplets slows down more and the WPI droplets dry faster than the lactose droplets. This may be due to the fact that the moisture diffusivity of lactose solution reduces strongly with an increase in solid concentration. The WPI droplet surface becomes rugged due to appearance of crests, troughs and wrinkles. All of these act to greatly increase the surface area. The increased surface area enhances evaporation and subsequently moisture removal. It was observed that a thin crust of crystals was formed on the surface of the lactose droplets which trapped the water within. The thickness of this crust increased over time. This is another reason why the lactose droplets dried slower.
Fig. 8. Moisture histories of lactose and WPI solution droplets at 65 and 80 ◦ C, 0.75 m/s air velocity and 2.5 ± 0.5 relative humidity.
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Fig. 9. Temperature histories of lactose and WPI solution droplets at 65 and 80 ◦ C, 0.75 m/s air velocity and 2.5 ± 0.5 relative humidity.
Fig. 9 presents the temperature histories of WPI and lactose droplets at 65 and 80 ◦ C. The temperature histories of both the WPI and lactose droplets bear similarities with that of their moisture histories. At the beginning, the droplet temperature of WPI increases more rapidly than the corresponding lactose droplet. However, once the rate of evaporation of lactose slows down, the temperature of droplets rise faster than that of WPI. It appears that crystallization cools down the droplets as exhibited remarkably by the temperature history of lactose droplet dried at 80 ◦ C. Once the crystallization occurred, the temperature of lactose droplets was lower than that of the WPI. Similar effect of crystallization of lactose on its drop temperature is seen on the plot at air temperature of 63 ◦ C. 3.4.2. Stickiness of WPI droplets at elevated temperatures The development of stickiness of WPI droplets at different probe surfaces at 65 ◦ C is presented in Fig. 10(a). The repeatability of the measurements, based on standard deviation, is shown through error bar. This varied between 5% and 8% of the measured mean values. The mode of failure when the WPI droplet attains a non-sticky state on stainless steel probe is given in Fig. 10(b). This mode of failure is common to all the probes when non-sticky state is attained. In case of a Teflon probe, a peak tensile strength is observed at droplet–probe interface at average moisture u = 2.43 (70.85%, w/w). At this point a clean adhesive failure occurred when the probe was pulled away. This is the point at which the cohesive strength of the droplet surface is close enough to achieve the adhesive strength at droplet–probe interface. A clean separation of the probe from the droplet surface occurs here. We wish to emphasize here that the peak tensile strength values reported here may be apparent peaks on the left (on x-axis) of a real peak. This is because it is very difficult to say if the measured peak value is the actual (maximum) peak value because it depends strongly on the surface moisture and temperature of the droplet. Nevertheless, the measured peaks should lie very close to the actual peaks in all cases. Hence, the conclusion drawn in this work will not be affected by this difficulty. The drop surface was completely no-sticky (similar to Fig. 10(b)) for the Teflon probe when the moisture content
Fig. 10. (a) Stickiness (tensile strength) of WPI solution droplets on different probe materials at 65 ◦ C, 0.75 air velocity, 2.5 ± 0.5 relative humidity and 50 mm/min probe speed. (b) Mode of failure at probe–WPI droplet interface at non-sticky state (65 ◦ C, 0.75 air velocity, 2.5 ± 0.5 relative humidity and 50 mm/min probe speed).
decreases to u = 2.2, that is, the solid concentration of the droplet was merely 31.2% (w/w) and the drying time was only 5 min. We applied 10 kPa of compressive pressure at the droplet surface to make sure that the surface was non-sticky. This extent of compressive pressure is recommended because the surface of food droplets behaves like pressure sensitive adhesive and also due to the fact that the droplet surface is not smooth [12,15]. There are two reasons why the WPI droplet surface becomes non-sticky in such an early stage of drying. Firstly, WPI droplet forms smooth skin at the surface immediately after coming into contact with the hot air and behaves like a flexible water pouch (Fig. 10(b)). The outer surface of the skin soon converts itself to non-sticky glassy material. Secondly, the Teflon surface is hydrophobic. It has to be noted that the temperature of the WPI droplet when its surface attained a non-sticky state was 45 ◦ C, well below the drying temperature of the air (65 ◦ C). The peak tensile strength of WPI droplets on polyurethane and stainless steel surfaces were achieved at u = 2.2 (68.75%, w/w) and with a corresponding drop temperature of 45 ◦ C. This suggests that the WPI droplets will stick to the polyurethane and stainless for a longer time compared to the Teflon surface. The magnitude of the peak tensile strength on the stainless steel was much higher compared to that of polyurethane. This indicates that the WPI droplet surface will have a greater tendency to stick on the stainless steel surface than the polyurethane at elevated temperature. The WPI droplet surface attained a complete nonsticky state when the average moisture was u = 1.27 (55.95%, w/w) and the corresponding drop temperature was 54.3 ◦ C both with polyurethane and stainless steel. In case of glass, the peak tensile strength when the droplet surface was cleanly separated was 4700 kPa which is the highest value among the probe
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materials. This means that the WPI surface has the greatest tendency to stick to the glass surface. However, the WPI surface attained non-sticky state with glass at the same moisture and temperature as in the case of stainless steel and polyurethane. The magnitude of the stickiness (peak tensile strength) of WPI droplet on probe materials is consistent with the contact angle results (Fig. 5). The magnitude of the contact angle is Teflon > polyurethane > stainless steel > glass. This means that the work of adhesion (Eq. (1)) or the tensile strength at the droplet–probe interface should be of the order of glass > stainless steel > polyurethane > Teflon. The stickiness of WPI drop surface is the greatest on the glass surface and the lowest on Teflon and that the value of the stainless steel and polyurethane fall in between with predicted order. A comparison of the peak tensile strength at 63 ◦ C with those obtained at room temperature shows that that for the surface forces of the adhesive (droplets) and adherend (probe surfaces) to be dominant (in stickiness), the adhesive mode of failure (Fig. 10(b)) is a must have criterion. Since the dominant mode of failure was cohesive (Fig. 3) during the tests at room temperature, the contact angle values (i.e. the surface forces) are not consistent with the development of surface stickiness. It is worth noting here that glass transition temperature (Tg ) of droplets is usually used to predict if it is likely to stick to equipment/packaging material or likely to cake upon storage and transportation. A practical rule is that if the droplet temperature is 20 ◦ C above its Tg , it will be sticky. Tg is a strong function of moisture content, and generally the mean moisture content is used to calculate it. It is very difficult to experimentally measure the Tg of anhydrous WPI and hence a calculated value of 153 ◦ C is taken [16]. At mean moisture content u = 1.27 (55.95%, w/w) the Tg of the WPI droplet will be −8 ◦ C or lower. The measured WPI droplet temperature is 53.4 ◦ C, which is at least 61.4 ◦ C above Tg . If we accept the Tg + 20 ◦ C criteria for stickiness, this droplet should be sticky. Quite contrary, the above results show that the drop does not stick even if compressive pressure of 10 kPa was used. Hence, for skin forming materials such as WPI, the behavior of the film or the surface properties rather than the bulk ones control the development of stickiness. Tensile strength of WPI at 80 ◦ C, 0.75 m/s air velocity and 2.5 ± 0.5% RH was studied to investigate the effect temperature and the results are presented in Fig. 11. The repeatability of the measurements is shown through error bars. The maximum spread (standard deviation), in these tests, was within 10% of the measured mean values. This figure shows that the peak tensile strength of droplets is the highest on the glass and lowest on the Teflon surfaces and the values for stainless steel and polyurethane fall in between in order. These results resemble the results obtained at 65 ◦ C and the same conclusion can be drawn. However, the moisture contents at which the WPI droplets exhibit peak stickiness and non-sticky state are lower compared to those at 65 ◦ C. Similarly, the droplet surface requires longer time to achieve clean adhesive failure and also to enter the non-sticky state. This is due to the fact that the droplet surface remains soft or plastic for longer time at higher temperature. Even at this drying temperature the droplet surface becomes completely non-sticky at average moisture u = 0.68 and
Fig. 11. Stickiness (tensile strength) of WPI solution droplets on different probe materials at 80 ◦ C, 0.75 air velocity, 2.5 ± 0.5 relative humidity and 50 mm/min probe speed.
drop temperature of 72 ◦ C, mainly due to formation of glassy skin at the droplet surface. 3.5. Comparison of stickiness of lactose and WPI droplets Fig. 12 compares the development of stickiness of WPI and lactose droplets at 65 and 80 ◦ C on stainless steel probes. This figure shows that the evolution of stickiness of WPI and lactose droplets is very different to each other. At room temperature the tensile strength of 25% (w/w) of WPI is 66 Pa compared to 79 Pa of lactose at the same solid concentration. Once the drying commences the cohesive strength of the WPI droplet increases very rapidly and exceeds the adhesive strength at droplet–probe interface at which a clean failure occurs for the first time. On further progress of drying the surface becomes glassy and enters the non-sticky regime fairly early in the drying process or at fairly high average moisture content. In the case of lactose, the tensile strength first decreases rather than increasing. It is interesting to observe that the tensile strength of lactose droplet at u = 0.49 (or 67%, w/w solids) is well below the tensile strength with droplet of 25% (w/w) solids at room temperature. This is because the cohesive strength of the
Fig. 12. Stickiness (tensile strength) of WPI and lactose solution droplets on stainless steel probe at 65 and 80 ◦ C, 0.75 air velocity, 2.5 ± 0.5 relative humidity and 50 mm/min probe speed.
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Table 1 Powder 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) 50% (H), 45% (MD), 5% (WPI) 50% (H), 40% (MD), 10% (WPI)
28.4 ± 4.9 80.1 ± 4.1 82.2 ± 4.2
49.71 15.48 11.88
0.124 ± 0.003 0.183 ± 0.002 0.198 ± 0.002
H: honey; MD: maltodextrin; WPI: whey protein isolate.
lactose solution decreases with an increase in droplet temperature. This is expected because the fluidity of a given solution is higher at higher temperatures. This increased fluidity lowers the viscosity as well as cohesive strength of the solution and hence the tensile strength gets lowered. When the moisture content of the droplet decreases below u = 0.4 (solids > 70%, w/w) the tensile strength increases rapidly. This is because the effect of solid concentration starts dominating over the effect of temperature which are mutually opposing. At this point, numerous small crystals appeared on the droplet and they grew and became dominant. It was observed that the crystals accumulated on the surface to form a crust the thickness of which grew over time. The interior and non-crystallized part of the droplet was surrounded by the crystal crust. This crust was fragile in nature and broke when the probe made a contact. As the crust fractured in numerous locations (on contact) a thin layer of viscous solution then seeped out and contacted the probe. This phenomena was observed in all tests even when the moisture content u = 0.18 (85% solids, w/w). The mode of failure was observed to be varying between cohesive to adhesive-dominant. It was also observed that the failure was completely adhesive if the crystal crust was not broken at the surface. The effect of temperature on the magnitude of surface stickiness is clearly visible here below u = 0.49. The magnitude of tensile strength decreased with increase in temperature at a given solid concentration. This result also supports the earlier finding with WPI droplets that higher temperatures worsen the problem of stickiness during drying. The extraordinary difference in development of stickiness in WPI and lactose droplets comes from their nature. Lactose, being a disaccharide lacks the capacity to form a non-sticky skin at its surface; rather it crystallizes and somehow unsuccessfully acts to lower the stickiness. In spray drying, however, amorphous lactose is formed, which is usually considered non-sticky. On the other hand WPI due to its higher molecular weight forms a skin upon exposure to drying air which converts itself into glassy matrix that resists the stickiness. This skin forming behavior of WPI not only overcomes the sticky problem but also increases the drying rate by increasing the heat and mass transfer surface (Fig. 8) due to formation of folds and wrinkles at the surface.
shows that maltodextrin has to be added as a drying aid to bring the honey to maltodextrin ratio to less than 50:50 on a dry solid basis. Even this extent of drying aid resulted in a recovery of powder less than 50% (of solids in feed). This level of additive addition is not welcomed by consumers. Results from Section 3.1 above shows that WPI molecules are strong surfactants and preferentially migrate to a droplet–air interface. Subsequently, when the WPI droplet is subjected to elevated temperatures (Section 3.4.2), a thin, smooth and nonsticky skin is formed immediately after the commencement of drying. These observations can have two important implications. Firstly, the skin can resist coalescence when droplets come in contact with each other and also decrease their adherence to the dryer wall. Secondly, the proteins can be efficient drying aids because only a small amount will be required. Hence drying trials were carried out to evaluate the effectiveness of WPI as a drying aid. The product recovery and the stability of powders were taken as measures of drying effectiveness. Powders were collected from cyclone and also from the drying chamber by lightly sweeping the latter. The recoveries (%) were calculated based on the amount of total solids in the feed (Table 1). As shown in this table, when the honey to maltodextrin (solid) ratio was maintained at 50:50, the recovery was merely 28%. However, when 5% of the maltodextrin (from the above formulation) was replaced with WPI, a recovery of greater than 80% was achieved. When the amount of WPI was increased to 10% (maltodextrin 40%) the recovery increased only slightly (82%). It has to be noted that recovery in the laboratory scale spray dryer (as used here) is usually much lower compared to that of the industrial spray dryers [6]. The above trials have shown that small amount of WPI addition can bring about a great improvement in the product recovery and that the proteins can be used as smart drying aids. The effectiveness of WPI as drying aids is due to segregation of WPI molecules from honey solids (molecules) and the preferential migration of the former towards the droplet surface which is induced by the surface activity of protein molecules and their bigger size. The higher the magnitude of this segregation the greater will be the effectiveness of a protein as a drying aid. 4. Conclusions
3.6. Application of WPI as drying aid Honey is one of the most difficult materials to convert into powder form because its dry mass is comprised mainly of low molecular weight sugars. It has not been possible to produce honey power, in its pure form, using currently available powder production techniques [17]. The work of Bhandari et al. [6]
The stickiness development of WPI and lactose droplets was determined. The surface of the lactose droplet remained sticky with cohesive failure until the surface was completely covered with fragile crystal layer which fractured upon probe contact and allowed the thin solution layer to seep out to the probe even when the average moisture was lower than 16.67% (w/w). WPI
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droplets formed a thin, smooth skin immediately after coming in contact with the drying air. The tensile strength of this skin increased rapidly and peaked fairly early during drying process and became non-sticky due to transformation of the outer layer of the skin into a glassy material. Tests of WPI/lactose mixture droplets at room temperature showed that addition of 0.5–1% (w/w) WPI lowered the tensile strength of lactose solution mainly due to preferential migration of protein molecules to the surface and lowering of cohesive strength. The skin forming and surface active nature of WPI were exploited to minimize the stickiness of honey in a pilot scale spray drying trial. Replacement of 5% (w/w) maltodextrin with WPI raised the powder recovery of honey solids from 28% to 80%. Stickiness of the WPI on glass, Teflon and polyurethane surfaces was studied by modification of the probe surface. At elevated temperatures, the peak stickiness of the glass was the highest and on the Teflon was the lowest as would be predicted from contact angle information. The Teflon surface offered the lowest stickiness both at low and high temperatures making it a suitable material to minimize stickiness through surface coating. Acknowledgements The authors acknowledge Miss Charlotte Contassot’s help in experiments and the Dairy Ingredients Group of Australia (DIGA) and Australian Research Council (ARC) for financial support for this study. References [1] H. Xin, X.D. Chen, N. Ozkan, Removal of model foulant from metal surfaces, AIChE J. 5 (2005) 1961–1973. [2] D.I. Wilson, Challenges in cleaning: recent developments and future prospects, Heat Transfer Eng. 26 (2005) 54–59.
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