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Fluidisation of whey powders above the glass-transition temperature
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Powder Technology 187 (2008) 53 – 61 www.elsevier.com/locate/powtec
Fluidisation of whey powders above the glass-transition temperature Justin Nijdam a,⁎, Alexander Ibach b , Matthias Kind b a
Chemical and Process Engineering Department, University of Canterbury, Private Bag 4800, Christchurch, New Zealand b Institut für Thermische Verfahrenstechnik, Universität Karlsruhe (TH), Postfach 6980, D-76128 Karlsruhe, Germany Received 5 July 2007; accepted 19 January 2008 Available online 2 February 2008
Abstract The fluidisation of partially-crystallised whey powder above the glass-transition temperature of lactose has been investigated, with the intent of crystallising the amorphous-lactose fraction in order to reduce the propensity of the powder to cake during storage. Partially-crystallised whey powder can be fluidised in a vibrated fluidised bed at temperatures of 25 to 40 °C above the glass-transition point of lactose, depending on the relative humidity of the air, before the powder becomes too sticky to fluidise. This temperature difference can be increased up to 80 °C by fluidising the powder with fine, relatively non-sticky, fully-crystallised whey powder in order to coat and protect the sticky partially-crystallised whey particulates during fluidisation. Despite this temperature-difference increase, the time required to crystallise the amorphous-lactose fraction in partiallycrystallised whey powder is not reduced sufficiently for this process to be viable in industry. An amorphous whey powder crystallisation process is likely to be more feasible, because the reduced salt and protein concentrations in this powder would ensure that lactose crystallisation is faster. Finally, this work has highlighted the potential of using the phenomenon of lactose plasticization above the glass-transition temperature and fines coating to improve the instant properties of milk-based powders. © 2008 Elsevier B.V. All rights reserved. Keywords: Sticky-point temperature; Caking; Dry coating; Crystallisation; Fluidised bed
1. Introduction Whey, a liquid by-product of the cheese manufacturing process, is normally converted for practical use to a powder within a spray dryer. This powder is comprised of approximately 75% lactose, 22% protein and salts, and a remainder consisting of lactic acid and fat [1]. The lactose portion of whey powder is usually in an amorphous state, due to rapid drying and solidification within the spray dryer. Amorphous lactose is very hygroscopic, and therefore readily adsorbs water from a humid environment. When sufficient water has been adsorbed, the lactose plasticizes, such that the whey powder becomes sticky and cakes together to form a product that is non-flowing and difficult to handle [2,3]. The temperature at which lactose begins to plasticize is known as the glass-transition temperature, which decreases as the moisture content of the powder increases. The common problem of whey-powder caking ⁎ Corresponding author. E-mail address: [email protected] (J. Nijdam). 0032-5910/$ - see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.powtec.2008.01.013
during storage results from a reduction in the glass-transition temperature below the ambient temperature, as water from the environment is absorbed by the amorphous lactose present. Plasticization also increases the mobility of individual lactose molecules, which re-organise progressively to form a stable structured crystalline form that is relatively non-hygroscopic [4]. Thus, the same process that causes the whey powder to cake together during storage in a humid environment is also responsible for the simultaneous stabilisation of the powder. It is possible to reduce the hygroscopic nature of lactose in whey powder by converting a portion of the lactose to the more stable crystalline form in a pre-crystallisation step before spray drying. This process involves seeding a supersaturated whey solution with fine lactose crystals to nucleate the crystallisation process, and allows the lactose to crystallise over a period of between 4 and 24 h [5,6]. According to Hynd [7], up to 85% of the lactose in whey can be crystallised in a carefully controlled pre-crystallisation step. Nevertheless, the whey powder produced by spray drying the resultant crystallised slurry remains hygroscopic to a certain extent, due to the continued
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presence of unstable amorphous lactose, which is formed as the soluble (non-crystallised) lactose portion of the slurry dries and solidifies. Thus, pre-crystallised whey powder still has a propensity to cake when exposed to sufficiently high humidities during storage, although to a lesser extent than amorphous whey powder. A number of workers have recommended that whey powder be treated in a crystallisation step following spray drying, in order to crystallise the amorphous-lactose fraction and thus limit the caking propensity of the powder. Saito [8] has suggested that whey powder containing amorphous lactose could be exposed to humid conditions in order to facilitate crystallisation, and the resultant powder, which would cake together during this process, could be ground down to produce a powder with a similar particle size distribution to the original powder. Ibach and Kind [9] have found temperatures and humidities at which such a post-crystallisation process for fully-amorphous whey powder could be conducted in a timeframe sufficiently short to be of industrial interest (of order minutes). Their results show that higher humidities and temperatures increase the crystallisation rate, although Maillard (or discolouration) reactions can affect the quality of the resultant powder when the temperature is too
high and/or the exposure time is too long. Hynd [7] has reported on an extension of the pre-crystallising and spray-drying processes, in which the powder leaves the spray dryer at a relatively high moisture content to allow continued crystallisation on a belt crystalliser. After crystallisation on the belt, the powder is subsequently dried in a vibrated fluidised bed where further crystallisation can also take place. The resultant powder consists of large agglomerates, and up to 95% of the lactose is in the crystal form. The aim of this paper is to attempt to fully crystallise whey powder using a vibrated fluidised bed through which hot humid air is passed, in a crystallisation process similar to that reported by Hynd [7], but excluding the crystallisation belt. Precrystallised whey powder has been chosen as the starting point for this process, since this powder is already produced in large quantities throughout Europe. The crystallisation times for amorphous whey powder found by Ibach and Kind [9] at various temperatures and humidities are used to provide an indication of the likely operating conditions required to completely crystallise partially-crystallised whey powder. In fact, Saito [8] has shown that partially-crystallised whey powder, which is produced by pre-crystallising and spray drying liquid whey, crystallises more
Fig. 1. A schematic diagram (not to scale) showing the bubble column, heaters and the vibrated fluidised bed.
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slowly than amorphous whey powder. His electron micrographs of pre-crystallised whey powder show a) spherical particles consisting of the concentrated constituents present in the liquid portion of the slurry at the end of the pre-crystallisation step (primarily soluble lactose, protein and salts), and b) lactose crystals covered with a thin layer of these same concentrated constituents. Apparently, the presence of the concentrated protein and salts retards the crystallisation of the amorphous lactose in pre-crystallised whey powder. Whey powder that has not been pre-crystallised does not have such high protein and salt concentrations, and therefore lactose crystallisation in this powder is more rapid. Nevertheless, the crystallisation times of Ibach and Kind [9] for amorphous whey powder provide a useful starting point for assessing the feasibility of an industrial process that fully crystallises partially-crystallised whey powder. 2. Methods and materials 2.1. Experimental set-up A vibrated fluidised bed (Fig. 1) has been constructed to fluidise whey powder with hot humid air, which is passed through a porous sintered-glass gas distributor (Robu, Germany) at the bottom of the bed. A bubble column (a tank containing distilled water, sparged with air) and a heater (a long pipe electrically heated using trace-heating tape) in series generate the hot humid air, which can have temperatures ranging from 20 to 100 °C, dew points ranging from 20 to 90 °C, and superficial velocities within the fluidised bed up to 0.8 m/s at ambient temperature. Hot dry air at similar temperatures and flows can be supplied to a jacket surrounding the round porous sintered-glass wall of the fluidised bed, where it passes through the porous wall into the fluidised bed. This hot dry air prevents sticky-particle wall fouling, by drying any particles in the vicinity of the wall to increase the glass-transition temperature of the lactose in these particles above the local air temperature. The inside diameter and height of the fluidised bed are 86 mm and 150 mm, respectively. The porous sintered-glass gas distributor and round wall have a nominal pore size of 16– 40 µm. A pneumatic linear vibrator (Netter Vibration, Germany) can be attached to the fluidised bed either vertically or horizontally, although we have found that a verticallyorientated vibrator provides the best fluidisation. Two vibrator models of different size are available for testing, NTS180 NF (small vibrator) and NTS250 NF (large vibrator), which, according to the manufacturer, provide nominal frequencies of 3120
Table 1 Composition of the partially-crystallised whey powder Component
Composition (wt.%)
Lactose – amorphous – crystalline Protein Fat
25.5 48.5 12 0.8
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Table 2 Particle size distributions of partially- and fully-crystallised whey powders and glass beads Partially-crystallised whey powders
D43 (μm) D32 (μm) D(v,0.1) (μm) D(v,0.5) (μm) D(v,0.9) (μm)
Fully-crystallised whey powders
Original (Zoma)
After heat-up fluidisation
b140 μm
N140 μm
172 85 60 157 308
211 134 117 202 326
114 77 57 112 174
218 155 128 212 323
Glass beads
88 82 75 88 103
and 2294 min− 1, respectively, and working moments of 0.1 and 0.36 cm kg, respectively, at an operating pressure of 2 bar. Trace heating is used on all pipes and connections between the bubble column and the fluidised bed. Moreover, the exterior of the bubble column has trace heating on the lid and on the side wall down to a point corresponding to the liquid level within the column. These measures ensure that the temperature of the adjacent inner surfaces is above the dew point so that no condensation occurs on these surfaces. Thus, the dew point of the humid air entering the fluidised bed should equal the water temperature within the bubble column, which we have confirmed using two dew point meters (Thermo-Hygrometer TH31 20D, Airflow Lufttechnik; Precision dew-point meter, Michell Instruments). Note that a conically-shaped baffle inside the bubble column between the ceiling and the water level prevents small droplets from being entrained by the airflow into the heating pipe. In addition, a concentric draft tube within the column draws air bubbles upwards and allows recirculation of water downwards through the annular section. Partially-crystallised whey powder has been supplied by Zoma, a local supplier in Germany. Note that this powder, which is otherwise known as pre-crystallised whey powder, was produced by pre-crystallising and subsequently spray drying liquid whey. We use the terminology “partially-crystallised” rather than “pre-crystallised” whey powder to make it clear that an amorphous-lactose portion still remains in this powder. The composition of the powder, which was provided by the supplier, is shown in Table 1. Various instruments were used to characterise this powder before and after it had been sieved, fluidised and/or crystallised. A scanning electron microscope (Gemini 1530, Leo) was used to examine the whey powders. Laser diffraction (Mastersizer S, Malvern) and digital-image processing (Camsizer, Retsch Technology) were used to measure the particle size distribution of the whey powder before and after fluidisation, respectively. Digital-image processing was found to be more appropriate than laser diffraction to measure the size of agglomerates formed during fluidisation of sticky whey powder, since there was no risk that the fragile agglomerates would break during the measurement using this technique. Table 2 gives the particle size distribution of the partially-crystallised whey powder supplied by Zoma, which was measured using laser diffraction. Fully-crystallised whey powder was produced by storing the partially-crystallised whey powder for at least one week in a
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desiccator with a saturated solution of KCl in distilled water to provide a very humid environment (relative humidity of 86%, ambient temperature) that encouraged crystallisation. This powder was gently ground periodically during storage to ensure that particulates did not cake together. A number of different experiments were carried out in order to find the limits of fluidisation of partially-crystallised whey powder, mixtures of partially- and fully-crystallised whey powders, and mixtures of glass beads (b120 µm diameter) and partially-crystallised whey powders. Descriptions of these experiments are given below. 2.2. Fluidisation limit of partially- and fully-crystallised whey powders Partially-crystallised whey powder was fluidised at various temperatures and humidities in order to find the humidity limit, for a given air temperature, above which the particles became too sticky to fluidise (Fig. 2). This humidity limit corresponds to the best possible operating conditions that result in the fastest crystallisation time for partially-crystallised whey powder within the fluidised bed built and tested in this work. Similar humidity limits were also found for fully-crystallised whey powder and glass beads (b 120 µm diameter) over the same range of temperatures. The height of each powder bed during fluidisation was approximately 10 mm. A minimum superficial air velocity through the fluidised bed of between 0.4 and 0.6 m/s was required in order to fluidise the powder at the humidity limit. An exact velocity cannot be provided here, since defining what constitutes reasonable fluidisation of sticky powders is rather subjective; fluidisation can range from sluggish to vigorous. However, we observed that, when fluidisation ceased after a slight increase in air humidity above the fluidisation limit given in Fig. 2, the bed could not be re-fluidised by a further increase in air velocity. Moreover, we found that neither the smaller nor the larger vibrator, operated at air pressures up to 6 bar, could refluidise a de-fluidised bed, which followed a similar increase in
Fig. 2. The upper limit on the relative humidity at which partially-crystallised and fully-crystallised whey powders can be fluidised.
air humidity above the fluidisation limit. Thus, this humidity limit represents a point above which no amount of additional mechanical energy, either in the form of airflow or vibration, can fluidise the whey powder in the fluidised bed tested in this work. The dry airflow through the porous sidewall of the fluidised bed during these experiments had an average superficial air velocity of approximately 0.03 m/s, which was observed to be sufficiently high to result in 1) good recirculation of particles in the vicinity of the wall back into the bulk of the fluidised powder, and 2) very little or no fouling of the wall, provided the fluidised bed was operated at or below the fluidisation limit given in Fig. 2. The ratio of the dry airflow through the sidewall (based on a 10 mm bed height during fluidisation) to the wet airflow through the gas distributor was less than 0.04. We assume that this ratio was sufficiently low that the dry airflow did not significantly affect the fluidisation limit found for the different powders. Note that the side wall and gas distributor flows resulted in a re-circulating toroidal flow pattern in the fluidised bed, with particles in the vicinity of the wall being drawn downwards, and particles in the central portion of the fluidised bed being forced upwards. Prior to fluidisation at a given temperature and humidity, the whey powder was pre-heated in the fluidised bed for a few minutes using hot dry air at the same temperature and velocity to prevent condensation of moisture on the particles when they were suddenly exposed to humid air. Finer particles were blown out of the fluidised bed by airflow entrainment during this heatup step. Table 2 shows the size distribution of the partiallycrystallised whey powder, sampled during the heat-up period when no further fines losses were observed. A comparison of the particle size distribution before and after the heat-up period shows a distinct increase in the particle size due to fines removal during initial fluidisation. 2.3. Fluidisation of mixtures of partially- and fully-crystallised whey powders and glass beads Mixtures of partially-crystallised and fully-crystallised whey powders were fluidised in an attempt to increase the maximum humidity, for a given air temperature, at which the whey powder could be fluidised, and thus minimise the time required to crystallise the amorphous-lactose portion of the powder. Two fully-crystallised whey powders with different size distributions (coarse and fine) were produced by sieving the original partially-crystallised whey powder using a 140 µm sieve, and crystallising the two resultant powders in a desiccator. These powders, whose particle size distributions are given in Table 2, were fluidised separately with partially-crystallised whey powder in order to ascertain the effect of particle size on the fluidisation of the mixtures. Partially-crystallised whey powder with the particle size distribution given in the second column of Table 2 (the powder produced after heat-up fluidisation) was used in these tests, so that the particle size distribution was the same as was used in determining the humidity limit for fluidisation of this powder. We chose an air temperature of 70 °C for most of the fluidisation experiments, because tests showed that air temperatures
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above 70 °C resulted in significant discolouration (due to Maillard reactions) of the whey powders fluidised for the lengths of time likely to be required to fully crystallise the powder (at least several minutes). The smaller vibrator operating at a pressure of 2 bar was used to provide the mechanical energy necessary to prevent the bed from de-fluidising, because tests showed that the larger vibrator did not significantly improve the observed quality of fluidisation. A superficial velocity in the fluidised bed of approximately 0.7 m/s was used to fluidise the powder mixtures, which is higher than the velocities used in the humidity-limit tests. The mixtures of partially- and fully-crystallised whey particles agglomerated almost immediately on contact with the humid air to form much larger particles, which required a higher velocity of 0.7 m/s to remain fluidised. The total weight of powder in each mixture was 40 g at the beginning of each test, which corresponded to a bed height during fluidisation of approximately 25 mm. We point out that this bed height was measured after the particles had agglomerated on contact with the humid air, which resulted in a significant increase in voidage of the fluidised bed. As with the previous experiments, the whey powder mixtures were pre-heated for a few minutes using hot dry air to prevent condensation of moisture on the particulates when they were suddenly exposed to humid air. A much lower air velocity of 0.25 m/s was used during the heat-up period, in order to minimise entrainment of fine particles out of the fluidised bed, which was especially important when the partially-crystallised and fine (b 140 µm) fully-crystallised whey powders were fluidised. 3. Results and discussion 3.1. Fluidisation limit of partially- and fully-crystallised whey powders Due to the presence of amorphous lactose, partially-crystallised whey powder is relatively sticky compared with fullycrystallised whey powder, when these powders are exposed to a humid environment. This is demonstrated in Fig. 2, which shows that fully-crystallised whey powder can be fluidised up to a relative humidity of approximately two times that of partiallycrystallised whey powder at any given air temperature tested in this work. Fig. 2 also shows the glass-transition curve found by Vuataz [10] for milk-based powders, including skim and whole milk powders, pure amorphous-lactose powder, and spray-dried whey powders with various degrees of pre-crystallised lactose. This curve implies that, when the humidity of the air is increased from a point below the curve to a point above the curve at a given air temperature, the lactose in any milk-based powder will begin to plasticize [3]. Clearly, the onset of plasticization of the lactose does not prevent the partially-crystallised whey powder from being fluidised. Indeed, the relative humidity can be increased by a further 15% above the glass-transition curve before fluidisation of this powder ceases. This is in agreement with observations made by Hennigs et al. [11], who found that, for a given relative humidity, the temperature of skim milk powder can be increased by nearly 25 °C above the glass-transition temperature of lactose before the powder becomes sticky. These workers define this so-called
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sticky-point temperature as the temperature at which cohesion, caused by the formation of liquid bridges between individual particles, restricts movement of the powder, so that it is no longer free flowing. According to the glass-transition curve of Vuataz [10], the sticky-point temperature of the partially-crystallised whey powder tested in this work ranges from 25 °C above the glass-transition temperature at a relative humidity of 35% to 40 °C above the glass-transition temperature at a relative humidity of 20% (Fig. 2). The higher values of the sticky point temperatures found in this work for whey powder compared with those found by Hennigs et al. [11] for skim milk powder are likely to result from the different methods used to determine the sticky-point temperature. Hennigs et al. [11] used an impeller to stir the powder, and the sticky point corresponded to the temperature for a given humidity at which the torque required to turn the stirrer suddenly increased sharply. In the current work, the sticky point is defined as the temperature for a given humidity above which fluidisation ceases. In the experiments conducted in this work, an increase in the humidity above the fluidisation limit given in Fig. 2 normally resulted in particle agglomeration, followed by the formation of channels in the fluidised bed, through which air could bypass the bed, and subsequent de-fluidisation. Below the humidity limit, vibration collapsed the channels so that the air passed uniformly through the particle bed (rather than bypassing the bed through the channels), thus providing the mechanical energy required to rip particles apart when they tried to agglomerate. We found that only a modest amount of vibration was required to prevent channelling below the humidity limit, and that an airflow increase was the most effective method to prevent particle agglomeration. However, particle losses out of the fluidised bed due to airflow entrainment limited the extent to which the airflow could be increased. Above the humidity limit, no amount of vibration could prevent channelling, and thus airflow increases were also useless to prevent particle agglomeration and subsequent bed de-fluidisation. According to Ibach and Kind [9], crystallisation of amorphous whey powder at the humidity limit found for partially-crystallised whey powder (given in Fig. 2) would take at least 100 min, which is likely to be too long for an industrial-crystallisation process. As explained in the Introduction, we expect even longer crystallisation times for partially-crystallised whey powder, because the higher concentration of proteins and salts in the matrix further hinders the crystallisation of any amorphous lactose present in the matrix. Table 3 shows the crystallisation times expected if amorphous whey powder could be fluidised at the humidity limit found for fully-crystallised whey powder. These crystallisation times are of the order of several minutes, which is much more reasonable for an industrial process. These findings point to the possibility of fluidising fully and partially-crystallised whey powders together so that the mixture can be fluidised at a higher humidity to allow crystallisation of the partially-crystallised fraction within a more acceptable timeframe. This possibility will be explored in Section 3.2. Glass beads (b 120 µm) were fluidised to a maximum humidity of close to 90% over the same range of air temperatures tested for partially- and fully-crystallised whey powders. In fact,
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Table 3 Humidity limit for fluidisation of fully-crystallised whey powder, and the corresponding crystallisation time for amorphous whey powder according to Ibach and Kind [9] Air Humidity limit for Crystallisation time of amorphous whey temperature (°C) fluidisation (%) powder (Ibach and Kind [9]) (min) 50 60 70 80 90
70 68 61 55 48
16.1 13.6 11.6 13.2 10.7
we expect that the glass beads should fluidise at a relative humidity of close to 100%, given that they show no glasstransition behaviour over the range of air temperatures tested, and are not hygroscopic. Small temperature variations in the hot dry airflow through the side wall of the fluidised bed of up to 2 °C below the air temperature were most likely responsible for depressing the apparent maximum fluidisation humidity of the glass beads below 100%. These temperature variations allowed moisture to condense onto and hence wet the surfaces of the glass beads so that water bridges between individual beads could form. Nevertheless, these results indicate that nonhygroscopic powders can be fluidised at relative humidities approaching 100%, suggesting that the fully-crystallised whey powder, which fluidised at much lower relative humidities, is
somewhat hygroscopic and can become sticky, although clearly to a lesser extent than partially-crystallised whey powder. Note that the humidity limit for fully-crystallised whey powder shown in Fig. 2 was measured twice using whey powders that had been crystallised (in a desiccator containing a saturated solution of KCl) for one and two weeks, respectively. No difference in these curves was found, and we therefore assume that all amorphous lactose present in the partially-crystallised whey powder had crystallised in the desiccator. Moreover, a comparison of electron micrographs of the partially- and fullycrystallised whey powders shows clear crystalline structures on the surfaces of the fully-crystallised whey particles (Fig. 3), which suggests that any amorphous lactose present in the partially-crystallised whey powder had been subsequently crystallised within the desiccator. We therefore assume that the protein and salts rather than any residual amorphous lactose contributed to the hygroscopic and somewhat sticky nature of the fully-crystallised whey powder. 3.2. Fluidisation of mixtures of partially- and fully-crystallised whey powders and glass beads The aim of fluidising a mixture of partially- and fullycrystallised whey powders is to coat the sticky partially-crystallised whey particles with relatively non-sticky crystallised whey particles, so that the resultant agglomerates can be fluidised at higher humidities than would normally be possible when
Fig. 3. A full (left) and close-up (right) view of whey particles: a) partially-crystallised whey powder, b) fully-crystallised whey powder.
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fluidising partially-crystallised whey powder alone. This would allow faster crystallisation of the amorphous-lactose content in the whey powder. A similar technique, called dry-powder coating, has been adopted by Yang et al. [12] to improve the flowability of cohesive cornstarch powder by coating its particles, which are approximately 15 µm in diameter, with nanoparticles of silica. In the current work, a series of tests have been conducted to determine the highest ratio of partially- to fullycrystallised whey powder that can successfully be fluidised, and to show the advantage of using finer fully-crystallised whey powder to increase this ratio. The objective is to demonstrate whether an industrial whey-powder crystallisation process is feasible, in which a portion of the whey powder already fully crystallised in a fluidised bed is recycled and mixed with fresh partially-crystallised whey powder being fed into the fluidised bed. In the first test, partially-crystallised whey powder and the coarse (N 140 µm) fully-crystallised whey powder with different mass ratios were fluidised in order to determine the highest possible ratio of these powders that resulted in reasonable fluidisation. For a given air temperature, the humidity was varied between the humidity limits for the fluidisation of partially- and fully-crystallised powders (shown in Fig. 2) in order to find the highest humidity possible before fluidisation of the powder mixture ceased. Firstly, an air temperature and relative humidity of 70 °C and 61%, respectively, were tested, which correspond to the humidity limit of fluidisation for fully-crystallised whey powder. This fluidisation condition, which, according to the glass-transition curve of Vuataz [10] corresponds to a temperature of close to 80 °C above the glass-transition temperature of lactose in any milk powder, would result in the fastest crystallisation rate possible at an air temperature of 70 °C in the fluidised bed tested in this work. The highest fraction of partially-crystallised whey powder that could be fluidised with fully-crystallised whey powder at this condition was 25 wt.%. At this mass fraction, fluidisation was initially sluggish with only a modest amount of relative motion between particulates, which was a result of the plasticization of lactose (on contact with humid air) and subsequent agglomeration of the particulates. Moreover, fast channel formation and collapse were observed within the first 2 min of fluidisation. However, fluidisation became progressively more vigorous and channelling stopped during this period, and sustained vigorous fluidisation continued for 18 min more. Note that the total fluidisation time of 20 min for this mixture of partially- and fully-crystallised whey powders is nearly two times longer than the crystallisation time of apTable 4 Particle size distributions of agglomerated powders produced by fluidising mixtures of partially- and fully-crystallised whey powders at an air temperature and humidity of 70 °C and 61%, respectively
Full-crystallised whey particle size (μm) Partially-crystallised whey mass fraction (%) Dv,0.1 (μm) Dv,0.5 (μm) Dv,0.9 (μm)
Test 1
Test 2
Test 3
>140 25 390 640 920
b140 40 350 630 880
N140 40 340 720 1050
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Fig. 4. An agglomerated whey particulate produced by fluidising a mixture of partially- and fully-crystallised whey powder in the ratio 25:75 at a temperature and relative humidity of 70 °C and 61%, respectively.
proximately 12 min required for amorphous whey powder at the fluidisation conditions tested (Table 3). Table 4 gives the particle size distribution of the resultant powder, which shows that there was approximately a 3-fold increase in the particle size from 202 to 640 µm (based on Dv,0.5) due to agglomeration of sticky particles within the fluidised bed. Fig. 4 shows the highly porous nature of the whey agglomerates that were formed during fluidisation. A powder with such porous agglomerated particles would easily disperse in water, which is a desirable property of instant drinking powders. Thus, particle agglomeration due to lactose plasticization and stickiness above the glass-transition temperature can clearly be exploited to change the functional properties of lactose-containing milk powders. Note that the maximum mass fraction for good fluidisation of the partially- to fully-crystallised whey powders did not change significantly as the relative humidity was reduced from 61 to 50%, at an air temperature of 70 °C. Given the significant increase in crystallisation time that would be expected if the humidity were reduced by this amount (nearly 15 min for amorphous whey powder, according to the data of Ibach and Kind [9]), all further tests were carried out at the humidity limit for the fluidisation of fully-crystallised whey powder. In the second test, the same experiment was conducted as above, however the finer (b140 µm) fully-crystallised whey powder was fluidised with the partially-crystallised whey powder. A higher mass fraction of partially-crystallised whey powder of 40% was achieved, which produced similar fluidisation to the coarse-powder fluidisation described above. These results demonstrate that using a finer relatively non-sticky powder to coat and protect the partially-crystallised whey powder allows a higher proportion of partially-crystallised whey powder to be fluidised, which would increase the throughput of whey powder in an industrial-crystallisation process. Table 4 shows that the agglomerates were similar in size whether coarse or fine fullycrystallised whey powder was used. We hypothesise that the agglomerates were limited in the size they could grow due to attrition caused by violent collisions of agglomerates during fluidisation. Agglomerates of similar size were produced in both the coarse and fine-particle cases, because the intensities of fluidisation were similar in both cases. Less intense fluidisation
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due to lower local air velocities results in larger agglomerates, which we observed when we attempted to fluidise partiallycrystallised whey powder with the coarse (N 140 µm) fullycrystallised whey powder at a mass ratio of 40:60. In this case, fluidisation was very poor with severe channelling and very little relative motion of particles throughout the 20 min fluidisation period. The agglomerates, collected from regions of the fluidised bed where weak fluidisation was observed, were significantly larger (Table 4), because fluidisation was less intense due to lower air velocities through these regions of the bed caused by channel formation and airflow bypass in other regions of the bed. In the final test, glass beads (b 120 µm) were fluidised with partially-crystallised whey powder in order to demonstrate how the sticky whey particles were physically protected by nonhygroscopic particles during fluidisation. Partially-crystallised whey powder (10 wt.%) was fluidised with glass beads for 2 min using hot humid air at a temperature and relative humidity of 70 °C and 61%, respectively. This condition corresponded to the humidity limit for fluidisation of fully-crystallised whey powder, which is well above the humidity at which partiallycrystallised whey powder can be fluidised alone. As shown in Fig. 5, agglomerates of sticky partially-crystallised whey particulates surrounded by glass beads formed, which were relatively easy to fluidise. An energy-dispersive X-ray analysis (to identify the surface chemical composition of individual glass beads in the fluidised powder mixture) combined with a visual inspection of these beads using scanning electron microscopy have shown that no lactose was transferred to the surfaces of the glass beads during fluidisation. This indicates that the contact angle of the plasticized lactose was rather high, so that the so-called liquid bridges that formed between particles were tenuous and formed only weak bonds. Indeed, Fig. 6 suggests that the surfaces of the whey particles actually soften slightly, which allows particles that collide to press onto each other's surfaces. The weakness of the bonds between particles in agglomerates is bourn out by the observation that the whey agglomerates, produced in the first two tests, could easy be crushed between the fingers using minimal force. Whey agglomerates from the first two tests were crushed using a mortar and pestle, and the resultant powders were stored in a desiccator that contained a saturated KCl solution for one
Fig. 6. An agglomerated partially-crystallised whey and glass-bead particulate showing a crater where a glass bead had most likely been bonded to the whey particulate before falling off.
week to determine whether or not they had been fully crystallised during fluidisation. Powder caking during storage would indicate that amorphous lactose was still present in the powder at the end of fluidisation. These tests showed that the powders had in fact not crystallised during fluidisation. Apparently, significantly longer fluidisation times than 20 min are required to crystallise partially-crystallised whey powder, which indicates that, in the current form, this process is not feasible for crystallising this powder in an industrial process. We have already explained that longer crystallisation times are expected for partially-crystallised whey powder than for amorphous whey powder due to the presence of more concentrated salt and proteins in the former. The process described in this paper is likely to be more useful for crystallising amorphous whey powder than partially-crystallised whey powder, which has the advantage that no pre-crystallising step would be required. In this case, the crystallisation times given in Table 3 would be more applicable, which we consider are feasible for an industrial conversion process. Furthermore, diffusion of water vapour into the porous structure of each agglomerate towards the exposed internal surfaces would also have limited crystallisation during fluidisation to a certain extent. Finally, this work has demonstrated that large porous whey agglomerates can be produced by making use of the glass-transition point of lactose to bond particulates together in a fluidised bed. Relatively non-sticky powders such as sugars and cocoa could be adopted to coat and protect partially-crystallised whey powders so that the agglomerates are less prone to caking. Thus, the potential exists to create new instant food products that are relatively free flowing and which readily disperse in water. 4. Conclusions
Fig. 5. An agglomerated whey particulate produced by fluidising a mixture of partially-crystallised whey powder and glass beads in the ratio 10:90 at a temperature and relative humidity of 70 °C and 61%, respectively, for 2 min.
The initial aim of this work was to crystallise the amorphouslactose portion of partially-crystallised whey powder in a fluidised bed in order to reduce its propensity to cake during storage. Partially-crystallised whey powder can be fluidised at temperatures of 25 to 40 °C above the glass-transition point of lactose, depending on the relative humidity of the air. This temperature difference can be increased up to 80 °C by fluidising the powder with relatively non-sticky fully-crystallised whey powder,
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which would result in a significant reduction in the necessary crystallisation time. These non-sticky particles coat the sticky partially-crystallised whey particles during fluidisation, thus reducing the extent of particle agglomeration and subsequent channelling in the fluidised bed. Moreover, we have found that the use of finer non-sticky powders allows a higher proportion of the partially-crystallised whey powder to be fluidised, which would increase the throughput in an industrial-crystallisation process. However, the time required to crystallise the partiallycrystallised whey powder is still too long to be feasible in an industrial-crystallisation process, even for the most favourable fluidising conditions investigated here. Adopting a similar process to crystallise amorphous whey powders is likely to result in more reasonable crystallisation times. Finally, this work has highlighted the potential of using the phenomenon of lactose plasticization above the glass-transition temperature to produce agglomerated milk powders that are relatively free flowing and have a high porosity for easy dispersal in water. Acknowledgements This work has been supported by the Alexander von Humboldt Foundation (AvH), and by the Arbeitsgemeinschaft Judushieller Forschungsvereinigungen e.V. (AiF) in cooperation with Forschungskreis der Ernahrungsindustrie e.V. (FEI), all in Germany. References [1] R.J. Pearce, in: J.G. Zadow (Ed.), Whey and Lactose Processing, Elsevier Applied Science, London, 1992.
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[2] J.M. Aguilera, J.M. Del Valle, M. Karel, Caking phenomena in amorphous food powders, Trends in Food Science & Technology 6 (1995) 149–154. [3] R.J. Lloyd, X.D. Chen, J.B. Hargreaves, Glass transition and caking of spray-dried lactose, International Journal of Food Science & Technology 31 (1996) 305–311. [4] M.K. Haque, Y.H. Roos, Water plasticization and crystallization of lactose in spray-dried lactose/protein mixtures, Food Engineering and Physical Properties 69 (1) (2004) 23–29. [5] P.F. Fox, P.L.H. McSweeney, Dairy Chemistry and Biochemistry, Blackie Academic & Professional, London, 1998. [6] V.H. Holsinger, Physical and chemical properties of lactose, in: P.F. Fox (Ed.), Advances Dairy Chemistry Volume 3, Chapman & Hall, London, 1997. [7] J. Hynd, Drying of whey, Journal of the Society of Dairy Technology 33 (2) (1980) 52–54. [8] Z. Saito, Lactose crystallization in commercial whey powders and in spraydried lactose, Food Microstructure 7 (1988) 75–81. [9] A. Ibach, M. Kind, Crystallization kinetics of amorphous lactose, wheypermeate and whey powders, Carbohydrate Research 342 (2007) 1357–1365. [10] G. Vuataz, The phase diagram of milk: a new tool for optimising the drying process, Lait 82 (2002) 485–500. [11] C. Hennigs, T.K. Kockel, T.A.G. Langrish, New measurements of the sticky behaviour of skim milk powder, Drying Technology 19 (3&4) (2001) 471–484. [12] J. Yang, A. Sliva, A. Banerjee, R.N. Dave, R. Pfeffer, Dry particle coating for improving the flowability of cohesive powders, Powder Technology 158 (2005) 21–33.
Powder Technology 187 (2008) 53 – 61 www.elsevier.com/locate/powtec
Fluidisation of whey powders above the glass-transition temperature Justin Nijdam a,⁎, Alexander Ibach b , Matthias Kind b a
Chemical and Process Engineering Department, University of Canterbury, Private Bag 4800, Christchurch, New Zealand b Institut für Thermische Verfahrenstechnik, Universität Karlsruhe (TH), Postfach 6980, D-76128 Karlsruhe, Germany Received 5 July 2007; accepted 19 January 2008 Available online 2 February 2008
Abstract The fluidisation of partially-crystallised whey powder above the glass-transition temperature of lactose has been investigated, with the intent of crystallising the amorphous-lactose fraction in order to reduce the propensity of the powder to cake during storage. Partially-crystallised whey powder can be fluidised in a vibrated fluidised bed at temperatures of 25 to 40 °C above the glass-transition point of lactose, depending on the relative humidity of the air, before the powder becomes too sticky to fluidise. This temperature difference can be increased up to 80 °C by fluidising the powder with fine, relatively non-sticky, fully-crystallised whey powder in order to coat and protect the sticky partially-crystallised whey particulates during fluidisation. Despite this temperature-difference increase, the time required to crystallise the amorphous-lactose fraction in partiallycrystallised whey powder is not reduced sufficiently for this process to be viable in industry. An amorphous whey powder crystallisation process is likely to be more feasible, because the reduced salt and protein concentrations in this powder would ensure that lactose crystallisation is faster. Finally, this work has highlighted the potential of using the phenomenon of lactose plasticization above the glass-transition temperature and fines coating to improve the instant properties of milk-based powders. © 2008 Elsevier B.V. All rights reserved. Keywords: Sticky-point temperature; Caking; Dry coating; Crystallisation; Fluidised bed
1. Introduction Whey, a liquid by-product of the cheese manufacturing process, is normally converted for practical use to a powder within a spray dryer. This powder is comprised of approximately 75% lactose, 22% protein and salts, and a remainder consisting of lactic acid and fat [1]. The lactose portion of whey powder is usually in an amorphous state, due to rapid drying and solidification within the spray dryer. Amorphous lactose is very hygroscopic, and therefore readily adsorbs water from a humid environment. When sufficient water has been adsorbed, the lactose plasticizes, such that the whey powder becomes sticky and cakes together to form a product that is non-flowing and difficult to handle [2,3]. The temperature at which lactose begins to plasticize is known as the glass-transition temperature, which decreases as the moisture content of the powder increases. The common problem of whey-powder caking ⁎ Corresponding author. E-mail address: [email protected] (J. Nijdam). 0032-5910/$ - see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.powtec.2008.01.013
during storage results from a reduction in the glass-transition temperature below the ambient temperature, as water from the environment is absorbed by the amorphous lactose present. Plasticization also increases the mobility of individual lactose molecules, which re-organise progressively to form a stable structured crystalline form that is relatively non-hygroscopic [4]. Thus, the same process that causes the whey powder to cake together during storage in a humid environment is also responsible for the simultaneous stabilisation of the powder. It is possible to reduce the hygroscopic nature of lactose in whey powder by converting a portion of the lactose to the more stable crystalline form in a pre-crystallisation step before spray drying. This process involves seeding a supersaturated whey solution with fine lactose crystals to nucleate the crystallisation process, and allows the lactose to crystallise over a period of between 4 and 24 h [5,6]. According to Hynd [7], up to 85% of the lactose in whey can be crystallised in a carefully controlled pre-crystallisation step. Nevertheless, the whey powder produced by spray drying the resultant crystallised slurry remains hygroscopic to a certain extent, due to the continued
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presence of unstable amorphous lactose, which is formed as the soluble (non-crystallised) lactose portion of the slurry dries and solidifies. Thus, pre-crystallised whey powder still has a propensity to cake when exposed to sufficiently high humidities during storage, although to a lesser extent than amorphous whey powder. A number of workers have recommended that whey powder be treated in a crystallisation step following spray drying, in order to crystallise the amorphous-lactose fraction and thus limit the caking propensity of the powder. Saito [8] has suggested that whey powder containing amorphous lactose could be exposed to humid conditions in order to facilitate crystallisation, and the resultant powder, which would cake together during this process, could be ground down to produce a powder with a similar particle size distribution to the original powder. Ibach and Kind [9] have found temperatures and humidities at which such a post-crystallisation process for fully-amorphous whey powder could be conducted in a timeframe sufficiently short to be of industrial interest (of order minutes). Their results show that higher humidities and temperatures increase the crystallisation rate, although Maillard (or discolouration) reactions can affect the quality of the resultant powder when the temperature is too
high and/or the exposure time is too long. Hynd [7] has reported on an extension of the pre-crystallising and spray-drying processes, in which the powder leaves the spray dryer at a relatively high moisture content to allow continued crystallisation on a belt crystalliser. After crystallisation on the belt, the powder is subsequently dried in a vibrated fluidised bed where further crystallisation can also take place. The resultant powder consists of large agglomerates, and up to 95% of the lactose is in the crystal form. The aim of this paper is to attempt to fully crystallise whey powder using a vibrated fluidised bed through which hot humid air is passed, in a crystallisation process similar to that reported by Hynd [7], but excluding the crystallisation belt. Precrystallised whey powder has been chosen as the starting point for this process, since this powder is already produced in large quantities throughout Europe. The crystallisation times for amorphous whey powder found by Ibach and Kind [9] at various temperatures and humidities are used to provide an indication of the likely operating conditions required to completely crystallise partially-crystallised whey powder. In fact, Saito [8] has shown that partially-crystallised whey powder, which is produced by pre-crystallising and spray drying liquid whey, crystallises more
Fig. 1. A schematic diagram (not to scale) showing the bubble column, heaters and the vibrated fluidised bed.
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slowly than amorphous whey powder. His electron micrographs of pre-crystallised whey powder show a) spherical particles consisting of the concentrated constituents present in the liquid portion of the slurry at the end of the pre-crystallisation step (primarily soluble lactose, protein and salts), and b) lactose crystals covered with a thin layer of these same concentrated constituents. Apparently, the presence of the concentrated protein and salts retards the crystallisation of the amorphous lactose in pre-crystallised whey powder. Whey powder that has not been pre-crystallised does not have such high protein and salt concentrations, and therefore lactose crystallisation in this powder is more rapid. Nevertheless, the crystallisation times of Ibach and Kind [9] for amorphous whey powder provide a useful starting point for assessing the feasibility of an industrial process that fully crystallises partially-crystallised whey powder. 2. Methods and materials 2.1. Experimental set-up A vibrated fluidised bed (Fig. 1) has been constructed to fluidise whey powder with hot humid air, which is passed through a porous sintered-glass gas distributor (Robu, Germany) at the bottom of the bed. A bubble column (a tank containing distilled water, sparged with air) and a heater (a long pipe electrically heated using trace-heating tape) in series generate the hot humid air, which can have temperatures ranging from 20 to 100 °C, dew points ranging from 20 to 90 °C, and superficial velocities within the fluidised bed up to 0.8 m/s at ambient temperature. Hot dry air at similar temperatures and flows can be supplied to a jacket surrounding the round porous sintered-glass wall of the fluidised bed, where it passes through the porous wall into the fluidised bed. This hot dry air prevents sticky-particle wall fouling, by drying any particles in the vicinity of the wall to increase the glass-transition temperature of the lactose in these particles above the local air temperature. The inside diameter and height of the fluidised bed are 86 mm and 150 mm, respectively. The porous sintered-glass gas distributor and round wall have a nominal pore size of 16– 40 µm. A pneumatic linear vibrator (Netter Vibration, Germany) can be attached to the fluidised bed either vertically or horizontally, although we have found that a verticallyorientated vibrator provides the best fluidisation. Two vibrator models of different size are available for testing, NTS180 NF (small vibrator) and NTS250 NF (large vibrator), which, according to the manufacturer, provide nominal frequencies of 3120
Table 1 Composition of the partially-crystallised whey powder Component
Composition (wt.%)
Lactose – amorphous – crystalline Protein Fat
25.5 48.5 12 0.8
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Table 2 Particle size distributions of partially- and fully-crystallised whey powders and glass beads Partially-crystallised whey powders
D43 (μm) D32 (μm) D(v,0.1) (μm) D(v,0.5) (μm) D(v,0.9) (μm)
Fully-crystallised whey powders
Original (Zoma)
After heat-up fluidisation
b140 μm
N140 μm
172 85 60 157 308
211 134 117 202 326
114 77 57 112 174
218 155 128 212 323
Glass beads
88 82 75 88 103
and 2294 min− 1, respectively, and working moments of 0.1 and 0.36 cm kg, respectively, at an operating pressure of 2 bar. Trace heating is used on all pipes and connections between the bubble column and the fluidised bed. Moreover, the exterior of the bubble column has trace heating on the lid and on the side wall down to a point corresponding to the liquid level within the column. These measures ensure that the temperature of the adjacent inner surfaces is above the dew point so that no condensation occurs on these surfaces. Thus, the dew point of the humid air entering the fluidised bed should equal the water temperature within the bubble column, which we have confirmed using two dew point meters (Thermo-Hygrometer TH31 20D, Airflow Lufttechnik; Precision dew-point meter, Michell Instruments). Note that a conically-shaped baffle inside the bubble column between the ceiling and the water level prevents small droplets from being entrained by the airflow into the heating pipe. In addition, a concentric draft tube within the column draws air bubbles upwards and allows recirculation of water downwards through the annular section. Partially-crystallised whey powder has been supplied by Zoma, a local supplier in Germany. Note that this powder, which is otherwise known as pre-crystallised whey powder, was produced by pre-crystallising and subsequently spray drying liquid whey. We use the terminology “partially-crystallised” rather than “pre-crystallised” whey powder to make it clear that an amorphous-lactose portion still remains in this powder. The composition of the powder, which was provided by the supplier, is shown in Table 1. Various instruments were used to characterise this powder before and after it had been sieved, fluidised and/or crystallised. A scanning electron microscope (Gemini 1530, Leo) was used to examine the whey powders. Laser diffraction (Mastersizer S, Malvern) and digital-image processing (Camsizer, Retsch Technology) were used to measure the particle size distribution of the whey powder before and after fluidisation, respectively. Digital-image processing was found to be more appropriate than laser diffraction to measure the size of agglomerates formed during fluidisation of sticky whey powder, since there was no risk that the fragile agglomerates would break during the measurement using this technique. Table 2 gives the particle size distribution of the partially-crystallised whey powder supplied by Zoma, which was measured using laser diffraction. Fully-crystallised whey powder was produced by storing the partially-crystallised whey powder for at least one week in a
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desiccator with a saturated solution of KCl in distilled water to provide a very humid environment (relative humidity of 86%, ambient temperature) that encouraged crystallisation. This powder was gently ground periodically during storage to ensure that particulates did not cake together. A number of different experiments were carried out in order to find the limits of fluidisation of partially-crystallised whey powder, mixtures of partially- and fully-crystallised whey powders, and mixtures of glass beads (b120 µm diameter) and partially-crystallised whey powders. Descriptions of these experiments are given below. 2.2. Fluidisation limit of partially- and fully-crystallised whey powders Partially-crystallised whey powder was fluidised at various temperatures and humidities in order to find the humidity limit, for a given air temperature, above which the particles became too sticky to fluidise (Fig. 2). This humidity limit corresponds to the best possible operating conditions that result in the fastest crystallisation time for partially-crystallised whey powder within the fluidised bed built and tested in this work. Similar humidity limits were also found for fully-crystallised whey powder and glass beads (b 120 µm diameter) over the same range of temperatures. The height of each powder bed during fluidisation was approximately 10 mm. A minimum superficial air velocity through the fluidised bed of between 0.4 and 0.6 m/s was required in order to fluidise the powder at the humidity limit. An exact velocity cannot be provided here, since defining what constitutes reasonable fluidisation of sticky powders is rather subjective; fluidisation can range from sluggish to vigorous. However, we observed that, when fluidisation ceased after a slight increase in air humidity above the fluidisation limit given in Fig. 2, the bed could not be re-fluidised by a further increase in air velocity. Moreover, we found that neither the smaller nor the larger vibrator, operated at air pressures up to 6 bar, could refluidise a de-fluidised bed, which followed a similar increase in
Fig. 2. The upper limit on the relative humidity at which partially-crystallised and fully-crystallised whey powders can be fluidised.
air humidity above the fluidisation limit. Thus, this humidity limit represents a point above which no amount of additional mechanical energy, either in the form of airflow or vibration, can fluidise the whey powder in the fluidised bed tested in this work. The dry airflow through the porous sidewall of the fluidised bed during these experiments had an average superficial air velocity of approximately 0.03 m/s, which was observed to be sufficiently high to result in 1) good recirculation of particles in the vicinity of the wall back into the bulk of the fluidised powder, and 2) very little or no fouling of the wall, provided the fluidised bed was operated at or below the fluidisation limit given in Fig. 2. The ratio of the dry airflow through the sidewall (based on a 10 mm bed height during fluidisation) to the wet airflow through the gas distributor was less than 0.04. We assume that this ratio was sufficiently low that the dry airflow did not significantly affect the fluidisation limit found for the different powders. Note that the side wall and gas distributor flows resulted in a re-circulating toroidal flow pattern in the fluidised bed, with particles in the vicinity of the wall being drawn downwards, and particles in the central portion of the fluidised bed being forced upwards. Prior to fluidisation at a given temperature and humidity, the whey powder was pre-heated in the fluidised bed for a few minutes using hot dry air at the same temperature and velocity to prevent condensation of moisture on the particles when they were suddenly exposed to humid air. Finer particles were blown out of the fluidised bed by airflow entrainment during this heatup step. Table 2 shows the size distribution of the partiallycrystallised whey powder, sampled during the heat-up period when no further fines losses were observed. A comparison of the particle size distribution before and after the heat-up period shows a distinct increase in the particle size due to fines removal during initial fluidisation. 2.3. Fluidisation of mixtures of partially- and fully-crystallised whey powders and glass beads Mixtures of partially-crystallised and fully-crystallised whey powders were fluidised in an attempt to increase the maximum humidity, for a given air temperature, at which the whey powder could be fluidised, and thus minimise the time required to crystallise the amorphous-lactose portion of the powder. Two fully-crystallised whey powders with different size distributions (coarse and fine) were produced by sieving the original partially-crystallised whey powder using a 140 µm sieve, and crystallising the two resultant powders in a desiccator. These powders, whose particle size distributions are given in Table 2, were fluidised separately with partially-crystallised whey powder in order to ascertain the effect of particle size on the fluidisation of the mixtures. Partially-crystallised whey powder with the particle size distribution given in the second column of Table 2 (the powder produced after heat-up fluidisation) was used in these tests, so that the particle size distribution was the same as was used in determining the humidity limit for fluidisation of this powder. We chose an air temperature of 70 °C for most of the fluidisation experiments, because tests showed that air temperatures
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above 70 °C resulted in significant discolouration (due to Maillard reactions) of the whey powders fluidised for the lengths of time likely to be required to fully crystallise the powder (at least several minutes). The smaller vibrator operating at a pressure of 2 bar was used to provide the mechanical energy necessary to prevent the bed from de-fluidising, because tests showed that the larger vibrator did not significantly improve the observed quality of fluidisation. A superficial velocity in the fluidised bed of approximately 0.7 m/s was used to fluidise the powder mixtures, which is higher than the velocities used in the humidity-limit tests. The mixtures of partially- and fully-crystallised whey particles agglomerated almost immediately on contact with the humid air to form much larger particles, which required a higher velocity of 0.7 m/s to remain fluidised. The total weight of powder in each mixture was 40 g at the beginning of each test, which corresponded to a bed height during fluidisation of approximately 25 mm. We point out that this bed height was measured after the particles had agglomerated on contact with the humid air, which resulted in a significant increase in voidage of the fluidised bed. As with the previous experiments, the whey powder mixtures were pre-heated for a few minutes using hot dry air to prevent condensation of moisture on the particulates when they were suddenly exposed to humid air. A much lower air velocity of 0.25 m/s was used during the heat-up period, in order to minimise entrainment of fine particles out of the fluidised bed, which was especially important when the partially-crystallised and fine (b 140 µm) fully-crystallised whey powders were fluidised. 3. Results and discussion 3.1. Fluidisation limit of partially- and fully-crystallised whey powders Due to the presence of amorphous lactose, partially-crystallised whey powder is relatively sticky compared with fullycrystallised whey powder, when these powders are exposed to a humid environment. This is demonstrated in Fig. 2, which shows that fully-crystallised whey powder can be fluidised up to a relative humidity of approximately two times that of partiallycrystallised whey powder at any given air temperature tested in this work. Fig. 2 also shows the glass-transition curve found by Vuataz [10] for milk-based powders, including skim and whole milk powders, pure amorphous-lactose powder, and spray-dried whey powders with various degrees of pre-crystallised lactose. This curve implies that, when the humidity of the air is increased from a point below the curve to a point above the curve at a given air temperature, the lactose in any milk-based powder will begin to plasticize [3]. Clearly, the onset of plasticization of the lactose does not prevent the partially-crystallised whey powder from being fluidised. Indeed, the relative humidity can be increased by a further 15% above the glass-transition curve before fluidisation of this powder ceases. This is in agreement with observations made by Hennigs et al. [11], who found that, for a given relative humidity, the temperature of skim milk powder can be increased by nearly 25 °C above the glass-transition temperature of lactose before the powder becomes sticky. These workers define this so-called
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sticky-point temperature as the temperature at which cohesion, caused by the formation of liquid bridges between individual particles, restricts movement of the powder, so that it is no longer free flowing. According to the glass-transition curve of Vuataz [10], the sticky-point temperature of the partially-crystallised whey powder tested in this work ranges from 25 °C above the glass-transition temperature at a relative humidity of 35% to 40 °C above the glass-transition temperature at a relative humidity of 20% (Fig. 2). The higher values of the sticky point temperatures found in this work for whey powder compared with those found by Hennigs et al. [11] for skim milk powder are likely to result from the different methods used to determine the sticky-point temperature. Hennigs et al. [11] used an impeller to stir the powder, and the sticky point corresponded to the temperature for a given humidity at which the torque required to turn the stirrer suddenly increased sharply. In the current work, the sticky point is defined as the temperature for a given humidity above which fluidisation ceases. In the experiments conducted in this work, an increase in the humidity above the fluidisation limit given in Fig. 2 normally resulted in particle agglomeration, followed by the formation of channels in the fluidised bed, through which air could bypass the bed, and subsequent de-fluidisation. Below the humidity limit, vibration collapsed the channels so that the air passed uniformly through the particle bed (rather than bypassing the bed through the channels), thus providing the mechanical energy required to rip particles apart when they tried to agglomerate. We found that only a modest amount of vibration was required to prevent channelling below the humidity limit, and that an airflow increase was the most effective method to prevent particle agglomeration. However, particle losses out of the fluidised bed due to airflow entrainment limited the extent to which the airflow could be increased. Above the humidity limit, no amount of vibration could prevent channelling, and thus airflow increases were also useless to prevent particle agglomeration and subsequent bed de-fluidisation. According to Ibach and Kind [9], crystallisation of amorphous whey powder at the humidity limit found for partially-crystallised whey powder (given in Fig. 2) would take at least 100 min, which is likely to be too long for an industrial-crystallisation process. As explained in the Introduction, we expect even longer crystallisation times for partially-crystallised whey powder, because the higher concentration of proteins and salts in the matrix further hinders the crystallisation of any amorphous lactose present in the matrix. Table 3 shows the crystallisation times expected if amorphous whey powder could be fluidised at the humidity limit found for fully-crystallised whey powder. These crystallisation times are of the order of several minutes, which is much more reasonable for an industrial process. These findings point to the possibility of fluidising fully and partially-crystallised whey powders together so that the mixture can be fluidised at a higher humidity to allow crystallisation of the partially-crystallised fraction within a more acceptable timeframe. This possibility will be explored in Section 3.2. Glass beads (b 120 µm) were fluidised to a maximum humidity of close to 90% over the same range of air temperatures tested for partially- and fully-crystallised whey powders. In fact,
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Table 3 Humidity limit for fluidisation of fully-crystallised whey powder, and the corresponding crystallisation time for amorphous whey powder according to Ibach and Kind [9] Air Humidity limit for Crystallisation time of amorphous whey temperature (°C) fluidisation (%) powder (Ibach and Kind [9]) (min) 50 60 70 80 90
70 68 61 55 48
16.1 13.6 11.6 13.2 10.7
we expect that the glass beads should fluidise at a relative humidity of close to 100%, given that they show no glasstransition behaviour over the range of air temperatures tested, and are not hygroscopic. Small temperature variations in the hot dry airflow through the side wall of the fluidised bed of up to 2 °C below the air temperature were most likely responsible for depressing the apparent maximum fluidisation humidity of the glass beads below 100%. These temperature variations allowed moisture to condense onto and hence wet the surfaces of the glass beads so that water bridges between individual beads could form. Nevertheless, these results indicate that nonhygroscopic powders can be fluidised at relative humidities approaching 100%, suggesting that the fully-crystallised whey powder, which fluidised at much lower relative humidities, is
somewhat hygroscopic and can become sticky, although clearly to a lesser extent than partially-crystallised whey powder. Note that the humidity limit for fully-crystallised whey powder shown in Fig. 2 was measured twice using whey powders that had been crystallised (in a desiccator containing a saturated solution of KCl) for one and two weeks, respectively. No difference in these curves was found, and we therefore assume that all amorphous lactose present in the partially-crystallised whey powder had crystallised in the desiccator. Moreover, a comparison of electron micrographs of the partially- and fullycrystallised whey powders shows clear crystalline structures on the surfaces of the fully-crystallised whey particles (Fig. 3), which suggests that any amorphous lactose present in the partially-crystallised whey powder had been subsequently crystallised within the desiccator. We therefore assume that the protein and salts rather than any residual amorphous lactose contributed to the hygroscopic and somewhat sticky nature of the fully-crystallised whey powder. 3.2. Fluidisation of mixtures of partially- and fully-crystallised whey powders and glass beads The aim of fluidising a mixture of partially- and fullycrystallised whey powders is to coat the sticky partially-crystallised whey particles with relatively non-sticky crystallised whey particles, so that the resultant agglomerates can be fluidised at higher humidities than would normally be possible when
Fig. 3. A full (left) and close-up (right) view of whey particles: a) partially-crystallised whey powder, b) fully-crystallised whey powder.
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fluidising partially-crystallised whey powder alone. This would allow faster crystallisation of the amorphous-lactose content in the whey powder. A similar technique, called dry-powder coating, has been adopted by Yang et al. [12] to improve the flowability of cohesive cornstarch powder by coating its particles, which are approximately 15 µm in diameter, with nanoparticles of silica. In the current work, a series of tests have been conducted to determine the highest ratio of partially- to fullycrystallised whey powder that can successfully be fluidised, and to show the advantage of using finer fully-crystallised whey powder to increase this ratio. The objective is to demonstrate whether an industrial whey-powder crystallisation process is feasible, in which a portion of the whey powder already fully crystallised in a fluidised bed is recycled and mixed with fresh partially-crystallised whey powder being fed into the fluidised bed. In the first test, partially-crystallised whey powder and the coarse (N 140 µm) fully-crystallised whey powder with different mass ratios were fluidised in order to determine the highest possible ratio of these powders that resulted in reasonable fluidisation. For a given air temperature, the humidity was varied between the humidity limits for the fluidisation of partially- and fully-crystallised powders (shown in Fig. 2) in order to find the highest humidity possible before fluidisation of the powder mixture ceased. Firstly, an air temperature and relative humidity of 70 °C and 61%, respectively, were tested, which correspond to the humidity limit of fluidisation for fully-crystallised whey powder. This fluidisation condition, which, according to the glass-transition curve of Vuataz [10] corresponds to a temperature of close to 80 °C above the glass-transition temperature of lactose in any milk powder, would result in the fastest crystallisation rate possible at an air temperature of 70 °C in the fluidised bed tested in this work. The highest fraction of partially-crystallised whey powder that could be fluidised with fully-crystallised whey powder at this condition was 25 wt.%. At this mass fraction, fluidisation was initially sluggish with only a modest amount of relative motion between particulates, which was a result of the plasticization of lactose (on contact with humid air) and subsequent agglomeration of the particulates. Moreover, fast channel formation and collapse were observed within the first 2 min of fluidisation. However, fluidisation became progressively more vigorous and channelling stopped during this period, and sustained vigorous fluidisation continued for 18 min more. Note that the total fluidisation time of 20 min for this mixture of partially- and fully-crystallised whey powders is nearly two times longer than the crystallisation time of apTable 4 Particle size distributions of agglomerated powders produced by fluidising mixtures of partially- and fully-crystallised whey powders at an air temperature and humidity of 70 °C and 61%, respectively
Full-crystallised whey particle size (μm) Partially-crystallised whey mass fraction (%) Dv,0.1 (μm) Dv,0.5 (μm) Dv,0.9 (μm)
Test 1
Test 2
Test 3
>140 25 390 640 920
b140 40 350 630 880
N140 40 340 720 1050
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Fig. 4. An agglomerated whey particulate produced by fluidising a mixture of partially- and fully-crystallised whey powder in the ratio 25:75 at a temperature and relative humidity of 70 °C and 61%, respectively.
proximately 12 min required for amorphous whey powder at the fluidisation conditions tested (Table 3). Table 4 gives the particle size distribution of the resultant powder, which shows that there was approximately a 3-fold increase in the particle size from 202 to 640 µm (based on Dv,0.5) due to agglomeration of sticky particles within the fluidised bed. Fig. 4 shows the highly porous nature of the whey agglomerates that were formed during fluidisation. A powder with such porous agglomerated particles would easily disperse in water, which is a desirable property of instant drinking powders. Thus, particle agglomeration due to lactose plasticization and stickiness above the glass-transition temperature can clearly be exploited to change the functional properties of lactose-containing milk powders. Note that the maximum mass fraction for good fluidisation of the partially- to fully-crystallised whey powders did not change significantly as the relative humidity was reduced from 61 to 50%, at an air temperature of 70 °C. Given the significant increase in crystallisation time that would be expected if the humidity were reduced by this amount (nearly 15 min for amorphous whey powder, according to the data of Ibach and Kind [9]), all further tests were carried out at the humidity limit for the fluidisation of fully-crystallised whey powder. In the second test, the same experiment was conducted as above, however the finer (b140 µm) fully-crystallised whey powder was fluidised with the partially-crystallised whey powder. A higher mass fraction of partially-crystallised whey powder of 40% was achieved, which produced similar fluidisation to the coarse-powder fluidisation described above. These results demonstrate that using a finer relatively non-sticky powder to coat and protect the partially-crystallised whey powder allows a higher proportion of partially-crystallised whey powder to be fluidised, which would increase the throughput of whey powder in an industrial-crystallisation process. Table 4 shows that the agglomerates were similar in size whether coarse or fine fullycrystallised whey powder was used. We hypothesise that the agglomerates were limited in the size they could grow due to attrition caused by violent collisions of agglomerates during fluidisation. Agglomerates of similar size were produced in both the coarse and fine-particle cases, because the intensities of fluidisation were similar in both cases. Less intense fluidisation
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due to lower local air velocities results in larger agglomerates, which we observed when we attempted to fluidise partiallycrystallised whey powder with the coarse (N 140 µm) fullycrystallised whey powder at a mass ratio of 40:60. In this case, fluidisation was very poor with severe channelling and very little relative motion of particles throughout the 20 min fluidisation period. The agglomerates, collected from regions of the fluidised bed where weak fluidisation was observed, were significantly larger (Table 4), because fluidisation was less intense due to lower air velocities through these regions of the bed caused by channel formation and airflow bypass in other regions of the bed. In the final test, glass beads (b 120 µm) were fluidised with partially-crystallised whey powder in order to demonstrate how the sticky whey particles were physically protected by nonhygroscopic particles during fluidisation. Partially-crystallised whey powder (10 wt.%) was fluidised with glass beads for 2 min using hot humid air at a temperature and relative humidity of 70 °C and 61%, respectively. This condition corresponded to the humidity limit for fluidisation of fully-crystallised whey powder, which is well above the humidity at which partiallycrystallised whey powder can be fluidised alone. As shown in Fig. 5, agglomerates of sticky partially-crystallised whey particulates surrounded by glass beads formed, which were relatively easy to fluidise. An energy-dispersive X-ray analysis (to identify the surface chemical composition of individual glass beads in the fluidised powder mixture) combined with a visual inspection of these beads using scanning electron microscopy have shown that no lactose was transferred to the surfaces of the glass beads during fluidisation. This indicates that the contact angle of the plasticized lactose was rather high, so that the so-called liquid bridges that formed between particles were tenuous and formed only weak bonds. Indeed, Fig. 6 suggests that the surfaces of the whey particles actually soften slightly, which allows particles that collide to press onto each other's surfaces. The weakness of the bonds between particles in agglomerates is bourn out by the observation that the whey agglomerates, produced in the first two tests, could easy be crushed between the fingers using minimal force. Whey agglomerates from the first two tests were crushed using a mortar and pestle, and the resultant powders were stored in a desiccator that contained a saturated KCl solution for one
Fig. 6. An agglomerated partially-crystallised whey and glass-bead particulate showing a crater where a glass bead had most likely been bonded to the whey particulate before falling off.
week to determine whether or not they had been fully crystallised during fluidisation. Powder caking during storage would indicate that amorphous lactose was still present in the powder at the end of fluidisation. These tests showed that the powders had in fact not crystallised during fluidisation. Apparently, significantly longer fluidisation times than 20 min are required to crystallise partially-crystallised whey powder, which indicates that, in the current form, this process is not feasible for crystallising this powder in an industrial process. We have already explained that longer crystallisation times are expected for partially-crystallised whey powder than for amorphous whey powder due to the presence of more concentrated salt and proteins in the former. The process described in this paper is likely to be more useful for crystallising amorphous whey powder than partially-crystallised whey powder, which has the advantage that no pre-crystallising step would be required. In this case, the crystallisation times given in Table 3 would be more applicable, which we consider are feasible for an industrial conversion process. Furthermore, diffusion of water vapour into the porous structure of each agglomerate towards the exposed internal surfaces would also have limited crystallisation during fluidisation to a certain extent. Finally, this work has demonstrated that large porous whey agglomerates can be produced by making use of the glass-transition point of lactose to bond particulates together in a fluidised bed. Relatively non-sticky powders such as sugars and cocoa could be adopted to coat and protect partially-crystallised whey powders so that the agglomerates are less prone to caking. Thus, the potential exists to create new instant food products that are relatively free flowing and which readily disperse in water. 4. Conclusions
Fig. 5. An agglomerated whey particulate produced by fluidising a mixture of partially-crystallised whey powder and glass beads in the ratio 10:90 at a temperature and relative humidity of 70 °C and 61%, respectively, for 2 min.
The initial aim of this work was to crystallise the amorphouslactose portion of partially-crystallised whey powder in a fluidised bed in order to reduce its propensity to cake during storage. Partially-crystallised whey powder can be fluidised at temperatures of 25 to 40 °C above the glass-transition point of lactose, depending on the relative humidity of the air. This temperature difference can be increased up to 80 °C by fluidising the powder with relatively non-sticky fully-crystallised whey powder,
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which would result in a significant reduction in the necessary crystallisation time. These non-sticky particles coat the sticky partially-crystallised whey particles during fluidisation, thus reducing the extent of particle agglomeration and subsequent channelling in the fluidised bed. Moreover, we have found that the use of finer non-sticky powders allows a higher proportion of the partially-crystallised whey powder to be fluidised, which would increase the throughput in an industrial-crystallisation process. However, the time required to crystallise the partiallycrystallised whey powder is still too long to be feasible in an industrial-crystallisation process, even for the most favourable fluidising conditions investigated here. Adopting a similar process to crystallise amorphous whey powders is likely to result in more reasonable crystallisation times. Finally, this work has highlighted the potential of using the phenomenon of lactose plasticization above the glass-transition temperature to produce agglomerated milk powders that are relatively free flowing and have a high porosity for easy dispersal in water. Acknowledgements This work has been supported by the Alexander von Humboldt Foundation (AvH), and by the Arbeitsgemeinschaft Judushieller Forschungsvereinigungen e.V. (AiF) in cooperation with Forschungskreis der Ernahrungsindustrie e.V. (FEI), all in Germany. References [1] R.J. Pearce, in: J.G. Zadow (Ed.), Whey and Lactose Processing, Elsevier Applied Science, London, 1992.
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