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Glass transition and the flowability and caking of powders containing amorphous lactose
Powder Technology 178 (2007) 119 – 128 www.elsevier.com/locate/powtec
Glass transition and the flowability and caking of powders containing amorphous lactose J.J. Fitzpatrick a,⁎, M. Hodnett a , M. Twomey a , P.S.M. Cerqueira a , J. O'Flynn a , Y.H. Roos b a
Department of Process and Chemical Engineering, University College, Cork, Ireland b Department of Food and Nutritional Sciences, University College, Cork, Ireland
Received 16 August 2005; received in revised form 11 November 2006; accepted 27 April 2007 Available online 5 May 2007
Abstract Many powders contain amorphous components, such as amorphous lactose in milk powders, which when given sufficient conditions of temperature and water content, will mobilise as a high viscosity flow making the particles sticky. This can lead to increased cohesiveness, powder caking and increased adhesion to surfaces. The transition from the glassy state is established by increasing the powder temperature to above its glass transition temperature which can be measured using differential scanning calorimetry. Exposing milk powder to over 10–20 °C above the lactose glass transition makes the powder more sticky, rendering it a lot more cohesive and also increases its adhesion to a stainless steel surface. This glass transition induced stickiness is time-dependent. Over time, crystallisation can take place converting the amorphous lactose into crystalline lactose. Furthermore, the caking behaviour of powders depends on the amount of component present in the amorphous state. Finally, this work presents an approach for applying the measured relationship between the glass transition and water content for predicting caking problems with powders containing amorphous components. © 2007 Elsevier B.V. All rights reserved. Keywords: Glass transition; Food powders; Powder flowability; Caking; Storage and handling
1. Introduction There is a large quantity and variety of materials produced industrially in powder form, and there is a need for information about their handling and processing characteristics. Powder flow properties are important in handling and processing operations [25,24,1,2]. Flow problems in hoppers and silos are commonplace problems for engineers and process operatives [3–5]. These problems may manifest themselves as unreliable inconsistent flow that reduces production rates. Cohesive attraction and frictional resistance developed between particles in a powder when consolidated must be overcome to make the powder flow. The cohesive attraction between particles in powders is very important in determining the minimum hopper opening size required to enable flow from hoppers and silos. A cohesive arch will form preventing discharge if ⁎ Corresponding author. Tel.: +353 21 4903089. E-mail address: [email protected] (J.J. Fitzpatrick). 0032-5910/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.powtec.2007.04.017
these attractions are not overcome by the gravity force acting due to the powder above. Wall friction is also a critical parameter in the design and operation of hoppers and silos for reliable discharge of bulk solids [6]. It is the frictional resistance to powder flow that exists between the powder and hopper/silo wall material. Wall friction is the dominant parameter in determining the minimum hopper angle (between the hopper wall and the horizontal) required to ensure mass flow, which is the desirable flow pattern for consistent reliable flow. Many food powders and food ingredient mixes are rendered complicated by the fact that they contain many different components, and this makes it difficult to predict their flow behaviour. Furthermore, during handling, storage, processing and distribution to the final consumer, the powders may experience a variety of temperatures and atmospheric humidities which may alter the handling behaviour and appearance of the powders. This is especially important if powders are transported to hotter, more humid climates, where a mix may cake solidly or liquefy from absorbing water. The consumer is usually very
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sensitive to any lumping, caking or difficulty in discharging the powder from its container. Many food powders contain amorphous glassy components, such as amorphous sugars. For example, many spray-dried dairy powders contain lactose in its amorphous state. Amorphous components are thermodynamically unstable and there exists a driving force for them to crystallise, however this requires that the molecules must be able to move. When an amorphous component is given sufficient conditions of temperature and water content, they can mobilise as a high viscosity flow, which can make them sticky and lead to caking [7]. Furthermore, molecular mobility enables amorphous components to crystallise over time. Glass transition measurements can be used to detect the onset of this mobility. Differential scanning calorimetry (DSC) is an established thermal analytical technique that can measure glass transition properties because glass transition induces a change in the specific heat of the material due to molecular mobility [8]. The glass transition occurs over a temperature range. This is often a relatively narrow range of 10 to 20 °C for amorphous sugars, however a much larger range, of for example 50 °C, may be expected for the glass transition of polymers in foods. Within this temperature range, the glass transition temperature can be referred to as the temperature initiating the onset of glass transition or the mid-point temperature of the change in specific heat capacity. The glass transitions of carbohydrates and proteins are affected by water, with greater water content resulting in decreased glass transition temperatures. Water acts as a plasticiser enabling the mobilisation of amorphous components. Consequently, a lower temperature is required to initiate the onset of the glass transition for higher water contents. Both higher temperature and water content can enable molecular mobility and the onset of the glass transition. Powders containing lactose in its amorphous state may crystallise, producing a discontinuous solid lactose phase within the particles. Crystallisation will only take place if the powder temperature is greater than its glass transition temperature (Tg), whereby the molecules have sufficient mobility to initiate crystallisation [9–12,8]. Tg is usually well above the storage temperature for most dry powders. However, lactose in its amorphous state is very hygroscopic and will readily sorb water from ambient air, and this increase in water will cause a significant plasticization and reduction in Tg. Crystallisation will occur if Tg is reduced to below the powder temperature. Crystallisation can be preceded by the formation of liquid bridges between powder particles, which leads to an increase in the cohesiveness and caking ability of the powder. For this reason, exposure of powders to higher temperatures and humidities, such as in tropical countries, can lead to the glass transition of amorphous components being exceeded, leading to powder handling and caking problems. Above the glass transition temperature (for a given water content) the powder particles become sticky [13,14] and this is often described by the sticky point temperature. It is generally accepted that the sticky point is about 10–20 °C above the glass transition onset temperature, especially for low molecular weight carbohydrates [15], as illustrated in Fig. 1. This sticki-
Fig. 1. Glass transition of lactose in skim milk powder.
ness is due to the molecular mobility of the amorphous components as a highly viscous flow between particle surfaces, making the powder a lot more cohesive. Furthermore, contacting the powder with a wall surface when the powder is in this sticky region will lead to increased adhesion between the powder and the surface. For example, in a spray drying process, if the surface temperature of drying droplets is greater than the Tg of an amorphous component and the particles contact the dryer wall, then the particles may stick onto the dryer wall causing coating of the wall surface over time [16]. The focus of this study was to investigate the application of glass transition measurements for assessing the cohesion/caking ability and wall friction characteristics of powders due to the presence of amorphous components, using amorphous lactose in dairy powders as a case-study. In particular, the objectives were: • To illustrate the relationship between the glass transition and the cohesion and caking of spray-dried skim-milk and whey permeate powders, which both contain amorphous lactose. • To investigate the influence of lactose glass transition on the adhesion/wall friction of skim-milk powder to a stainless steel surface. • To investigate the effect of the amount of amorphous lactose on the glass transition and caking of a mix of skim-milk powders containing varying amounts of amorphous and crystalline lactose. • To outline an approach for the application of glass transition measurements for predicting and preventing handling and caking problems in food powders, food ingredient mixes and other powders due to the presence of amorphous components. 2. Materials and methods 2.1. Milk powders Two 25 kg bags of commercial skim-milk powder (SMP) and one bag of lactose powder (LP) were donated by Dairygold, Mitchelstown, Ireland. A bag of commercial whey permeate powder (WPP) was donated by Kerry Ltd., Listowel, Ireland.
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2.2. Particle size and water content Particle size distribution was measured by laser diffraction using the Malvern Mastersizer MSS with powder feeder unit (Worcestershire, UK). Each test was carried out in duplicate and the average mean particle size was reported. The difference in mean particle size of the duplicates was less than 5% of the average particle size. Water content (wet basis) was measured by weighing 3 g of a sample before and after drying in an oven (Sanyo-Gallenkamp, Loughborough, UK) at 105 °C for 3 days. Each test was carried out in triplicate. The difference in water content between the triplicates was within 0.6% w/w. If two of the water contents were close to each other, then the average of these two was reported, otherwise the average of the three was reported. 2.3. Glass transition Differential scanning calorimetry (DSC, Mettler Toledo 821e with N2 cooling, Greifensee, Switzerland) was used to measure lactose glass transition temperature range and the change in specific heat due to glass transition. Samples of dehydrated materials (4 to 15 mg) were prepared in preweighed DSC aluminium pans (40 μl; Mettler Toledo-27331, Greifensee, Switzerland), and the pans were then hermitically sealed. Duplicate samples of each material were analysed. An empty pan was used as a reference. The DSC was calibrated for temperature using n-hexane (melting point, − 95.0 °C), mercury (melting point, − 38.8 °C), water (melting point, 0.0 °C), gallium (melting point, 29.8 °C) and indium (melting point, 156.6 °C) and for heat flow using n-hexane (ΔHm, 151.8 J/g), mercury (ΔHm, 11.4 J/g), water (ΔHm, 334.5 J/g), gallium (ΔHm, 80 J/g), and indium (ΔHm, 28.45 J/g). All measurements were made at a scanning rate of 5 °C/min. An immediate rescan was made for each sample to verify the endothermic baseline shift associated with the glass transition. Glass transition temperatures were determined using STARe thermal analysis software, version 6.0 (Mettler Toledo, Greifensee, Switzerland). 2.4. Flow function measurement An annular shear cell was used for measuring the flow function and effective angle of internal friction and was the same as that described by Teunou et al. [17]. It had a fixed shearing rate of 7 mm/min and external and internal diameters of 164 mm and 120 mm, respectively. The milk powder was removed from its package and packed into the annular shear cell. The annular shear cell was then placed in a chamber, at a temperature of 20 °C, where the shear tests for measuring the instantaneous flow function were conducted. The procedure used to measure the instantaneous flow function was that recommended by the Standard Shear Test Technique (SSTT), using the Jenike shear cell [18]. Each flow function was measured in duplicate to assess reproducibility. If the flow functions did not plot on top of each other or close beside each other, the test was repeated a third time to determine which of the flow functions was valid. The flow index is defined as the inverse
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slope of the flow function. Jenike [19] used the flow index to classify powder flowability with lower values representing very cohesive and cohesive powders and higher values representing easy flow and free-flowing powders, and this classification was later extended by Tomas [20]. 2.5. Wall friction testing The wall yield locus of skim milk powder was measured using a Jenike shear cell (95 mm internal diameter) whereby the cylindrical base of the cell was replaced by a flat plate of stainless steel 304 commonly used in food processing. The wall yield locus was obtained by measuring the horizontal stress required to make the powder fail at the following normal stresses: 5.9, 4.4, 3.7, 3.0, 2.3 and 1.6 kPa. The procedure used was that recommended by the Standard Shear Test Technique (SSTT), using the Jenike shear cell. Each wall yield locus was measured in duplicate to assess reproducibility. If the wall yield loci did not plot on top of each other or close beside each other, the test was repeated a third time to determine which of the wall yield loci was valid. The failure shear stress at the maximum normal stress of 5.9 kPa is reported in this work as an index of the adhesion between the powder and the stainless steel coupon. To study the influence of temperature on wall friction characteristics of skim milk powder, duplicate pre-sheared rings on the stainless steel coupons containing the powder were placed in an oven for 6 h under controlled temperatures in the range of 20 to 130 °C. After incubation in the oven, one sample was removed from the incubator and the wall friction test was immediately carried out. At the end of the test, samples for water content measurement were taken from powder at the wall plate surface. This was repeated for the duplicate sample. The wall friction tests were performed rapidly after sample removal from the oven in order to minimise cooling of the sample. 2.6. Visual assessment of caking in controlled atmosphere chamber Visual assessment of the powder caking was carried out for powders after exposure to atmospheres at 76% relative humidity and 20 °C for a number of exposure times. The desired temperature was controlled using a Haake F3 heating/cooling system. Relative humidity was controlled by placing a tray of a saturated NaCl solution in the base of the sealed cabinet. There was a fan in the cabinet to maintain a uniform atmosphere, and the values of temperature and relative humidity in the cabinet were recorded by a thermometer and a hygrometer located inside the cabinet. The powder was placed in two trays inside the insulated chamber for the specified exposure time at a fixed relative humidity and temperature of 20 °C. The powder was spread out in the tray as a thin layer of about 1 cm thick to increase the rate of water sorption. After the exposure time, the two trays were then visually inspected for caking behaviour by moving a spatula through the powder/cake and assessing if the powder had caked or not and if the cake was a soft cake, strong cake or
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Fig. 2. Aluminium dish used in cake strength determination (with central hole and cover underneath to prevent powder falling through).
hard cake. At the end of the test, duplicate samples were taken for immediate preparation for thermogram measurement by differential scanning calorimetry. Triplicate samples were also taken for water content measurement. 2.7. Quantitative measurement of cake strength An empirical test for quantitatively measuring an index of cake strength was developed by the authors. It consisted of placing a measured amount of powder into a cylindrical aluminium dish which had a circular hole exactly in the centre of the dish with dimensions illustrated in Fig. 2. The dish had a cover placed underneath to prevent the powder falling through the hole. The powder was spread out across the dish in a consistent manner to reduce variability between tests due to packing procedure. SMP samples in the dishes were then exposed to different values of temperatures (10, 25 and 40 °C) and relative humidities (76% and 100% RH) over a number of time durations (2, 4, 6, 12, 24 h) to investigate how these conditions influence the cake strength of SMP. The relative humidities were achieved by placing a dish containing powder into a glass kilner jar, containing either saturated sodium chloride solution to obtain 76% RH or pure de-ionised water to obtain 100% RH, which were then sealed. The temperature within the jars was controlled by placing them in the controlled atmosphere chamber mentioned above.
Fig. 4. Flow functions of lactose powder (LP), whey permeate powder (WPP) and skim milk powder (SMP) at 20 °C.
Following exposure to a given temperature, relative humidity and time, the dish was removed from the kilner jar. The cover was removed from beneath the aluminium dish and the dish was centred below a rod attached to a JJ tester Lloyd Instruments, T-series, Fareham, UK). The rod had a diameter of 7.84 mm and was initially moved downwards to just above the top of the caked powder. The rod was then moved downward through the caked powder at a constant speed of 3 mm/min and the force versus displacement was measured using the JJ tester as the rod moved through the cake. This eventually caused the cake to fail with powder being ejected through the hole in the centre of the dish. A typical plot of force versus displacement is presented in Fig. 3. The peak force was used as a single parameter index of cake strength. A reproducibility study was carried out on one condition, which resulted in a mean peak force of 3.3 N and a standard deviation of 0.27 N. 3. Results and discussion 3.1. Glass transition and the cohesion/caking of dairy powders Lactose was in a crystalline form in the lactose powder (LP). Lactose was the major component in both WPP and SMP, however it was mainly in the amorphous form. Comparing the cohesiveness of these 3 powders (Fig. 4 and Table 1) shows that LP was very cohesive while WPP and SMP were easy flow. It is difficult to reconcile the major difference in cohesion between these 3 powders considering that lactose was the major component in each and that LP had a larger particle size. This maybe due to the crystalline nature of LP giving rise to greater frictional resistance between the particles, however the values Table 1 Powder physical and flow properties
Fig. 3. Typical force versus displacement curve for the cake strength tester.
Powder
Mean particle size (μm)
Water content (% w/w)
Effective angle of internal friction
Flow index
Flow classification
SMP LP WPP
53 157 98
4.7 – 3.8
52 49 49
6.3 1.6 5.9
Easy flow Very cohesive Easy flow
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for effective angle of internal friction do not justify this rationale. It may also be due to differences in surface water contents producing differences in cohesion due to liquid bridging. Both WPP and SMP have lactose in amorphous form because they are both spray-dried powders. Lactose in amorphous form is thermodynamically unstable and very hygroscopic, and if these lactose molecules gain mobility, they will tend to crystallise over time. Greater molecular mobility can be obtained at higher temperatures and water contents. The existence of a glass transition for the powder is an indication of molecular mobility due to amorphous components such as amorphous lactose. Experiments were carried out with LP, WPP and SMP whereby the powders were exposed to water in air at 76% relative humidity (RH) at 20 °C over a 48-hour period. DSC thermograms of the powders were measured over this time period as well as assessing the flowability/caking of the powders. The objective of this experimentation was to investigate how amorphous lactose content and water pick up from air influenced the flow/caking behaviour of the powders. Both WPP and SMP sorbed much water from the air, with WPP rapidly attaining a water content of around 10.5% after only 6 h, and SMP attained a water content of around 14.6% after 12 h. The water content of both these powders fluctuated around
Fig. 6. DSC thermograms of skim milk powder (SMP) exposed to 76% RH at 20 °C for a) 0 h; b) 12 h; c) 48 h.
Fig. 5. DSC thermograms of whey permeate powder (WPP) exposed to 76% RH at 20 °C for a) 0 h; b) 6 h; c) 12 h.
these values for the remaining exposure time. The measured flow function of LP after 24-hour exposure showed only a small increase in cohesiveness, however both WPP and SMP had caked. DSC thermograms were measured for samples of the powders after the following exposure times: 0, 6, 12, 24 and 48 h. The DSC thermogram for LP did not change significantly over a 24-hour exposure time as there was little amorphous lactose present to cause glass transition changes. On the other hand, there were major changes observed from the DSC thermograms over a 48-hour exposure time for both WPP and SMP, as illustrated in Figs. 5 and 6, respectively, with glass transition property data presented in Table 2. For both powders at 0 h, there was a distinct glass transition, with glass transition temperatures of around 31 °C for WPP and 46 °C for SMP. After 6hour exposure, the glass transition temperature for WPP was reduced to − 10 °C, and after 12-hour exposure, the glass transition temperature for SMP was reduced to 5 °C. After 12 h, the glass transition no longer existed for WPP, and it took between 24 and 48 h for the glass transition to disappear for SMP. These changes in glass transition temperatures were expected and have been verified by others [12,11,10]. The time required for the glass transition to disappear was over 2 times
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Table 2 Glass transition properties of whey permeate powder (WPP) and skim milk powder (SMP) exposed to 76% relative humidity atmosphere (20 °C) for various exposure times Exposure time (h)
WPP 0 6 12 SMP 0 12 48 a
Water content (%)
Glass transition temperature range (°C)
ΔCp a (J g− 1 °C− 1)
Onset
Midpoint
Endpoint
6.2 10.6 10.5
23.4 − 10.8 –
30.7 −10.5 –
35.5 − 9.8 –
0.244 0.047 –
5.8 14.7 14.6
38.3 4.3
46.4 9
49.1 13 –
0.379 0.323 –
–
–
Change in specific heat due to glass transition.
longer for SMP than WPP. This may be due to the greater content of non-lactose components, such as proteins, present in SMP, which compete for the available water and inhibit the mobility of amorphous lactose molecules, which slows down its crystallisation. The reduction and disappearance of the glass transition, both in terms of temperature and change in specific heat, over time signifies the reduction in amorphous lactose content as it forms crystalline lactose. This was due to the rapid uptake of water over the first 6 to 12 h increasing the mobility of amorphous lactose molecules so that they could crystallise over time. [21] showed this crystallisation behaviour using electron microscopy. Consequently, the caking of both these powders may result from the formation of liquid bridges between the powder particles, which binds particles together to form a cake. Furthermore, as powder components dissolve in water, the liquid bridges will contain these components, which increase the viscosity of the liquid bridges making them stickier and forming a stronger cake. As water or thermal plasticization becomes excessive, lactose molecules form nuclei and crystallise randomly to various crystalline forms. As crystallisation proceeds sorbed water is lost, as the crystallised lactose is a lot less hygroscopic, and this leads to a reduction in liquid bridging which will influence cake strength. 3.2. Effect of glass transition on the caking of SMP Samples of SMP were exposed to a combination of temperatures, relative humidities and exposure times, and their cake strength was measured using the cake strength tester described in Section 2.7 above. After testing, samples of powder in the vicinity of the centre of the aluminium dish were taken for water content measurement. As expected, powder water content increased significantly with greater exposure time and greater relative humidity, as illustrated in Table 3. As already mentioned above, increased water content usually makes a powder more cohesive and may induce caking. Temperature had a less significant effect on water except for 40 °C at 100% RH which lead to by far the greatest water uptake. For most of the conditions, cake strength was low or negligible for lower exposure
times until after a given time the cake strength increased significantly, as illustrated in Table 3 and Fig. 7. Both relative humidity and temperature had major effects on cake strength once the powder started to cake, with higher relative humidity and higher temperature resulting in greater cake strength. Importantly, the lactose glass transition had a major influence on the development of cake strength, in addition to the presence of greater water. For a given measured caked sample water content, the lactose glass transition temperature was read from Fig. 1. The positive temperature difference between the powder exposure temperature minus the glass transition temperature [T–Tg]+ was calculated (if this was negative, it was assigned a zero value). Table 3 shows how [T–Tg]+ varies for the different experimental conditions. It can be seen from these data values that if [T–Tg]+ was 0 or less than 10 °C, then the cake strength was small or negligible, however, if [T–Tg]+ was greater than 20 °C, then cake strength increased to greater than 1 N. From this, it can be concluded that the lactose glass transition was influencing the initial development of cake strength. This makes sense because once the temperature of the amorphous lactose was increased above its glass transition and into its sticky temperature range, it became more sticky enabling it to stick particles together forming a cake. Furthermore, at 24-hour exposure, Table 3 shows a relationship between [T–Tg]+ and cake strength with higher cake strength at higher [T–Tg]+ for the experimental conditions tested. Furthermore, Paterson et al. [22] demonstrated that the cohesiveness of amorphous lactose powder over time was characterised by how much the powder temperature exceeded its glass transition temperature [T–Tg]+, and this was independent of how [T–Tg]+ was achieved. Table 3 Effect of temperature, relative humidity (RH) and exposure time on the water content, cake strength and [T–Tg]+ of skim milk powder (where T is the powder exposure temperature and Tg is the glass tranistion temperature of lactose at the powder water content. If T–Tg b 0, then [T–Tg]+ = 0) Time (h) 76% RH 76% RH 76% RH 100% RH 100% RH 100% RH 10 °C 25 °C 40 °C 10 °C 25 °C 40 °C 0 2 4 6 12 24 0 2 4 6 12 24 0 2 4 6 12 24
5.2 7 7.3 8 8.7 12.4 0 0.04 0.06 0.32 0.25 1.1 0 0 0 0 0 15
Water content (%w/w) 5.2 5.2 7.8 7.9 10.5 8 11.2 9.4 13 14 13.8 18.4 Cake strength (N) 0 0 0 0.04 0.03 0.15 0.03 1.06 0.1 0.4 1.9 0.7 1.8 2.2 1 1.9 3.5 1.8. [T–Tg]+ (°C) 0 0 0 0 0 0 3 20 0 12 38 2 30 41 22 38 51 40 5.2 6.4 7.5 8.8 12.2 14.4
5.2 7.2 9 11 14 17.9 0 0.1 0.23 2.7 3.1 3.1 0 0 14 25 37 53
5.2 9.6 13.9 15 18 22 0 1.7 4 4.9 5.4 7.8 0 0 52 56 69 88
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Fig. 7. Effect of temperature, relative humidity (RH) and exposure time on the cake strength of skim milk powder.
3.3. Effect of glass transition on the adhesion of SMP to a stainless steel surface Wall friction experiments were performed with SMP to investigate how temperature and glass transition influenced the adhesion of SMP onto a stainless steel surface. Powder samples were prepared for wall friction testing. They were placed in an oven for 6 h to attain thermal equilibrium at temperatures ranging from 20 °C to 130 °C. Then, they were removed from the oven and a wall friction test was performed immediately. Powders were disturbed using a spatula to visually assess if the powders had caked and if the cake was a soft, strong or hard cake samples were taken of powder at the wall surface for water content measurement. Results for the wall friction failure stress at a normal stress of 5.9 kPa for the various temperatures are presented in Table 4. The powder started to form a soft cake at about 70 °C, which was well above the glass transition for the corresponding water content, as obtained from Fig. 1. However, Table 4 shows no major increase in wall friction, in fact it reduced a little and then increased a little at higher temperatures. Overall, there appeared to be no relationship between increased adhesion to the stainless steel surface and glass transition. The reason for this may be because the powder may have become stickier sometime during the 6-hour exposure period giving rise to greater adhesion but the lactose crystallised sometime within the 6 h and the wall friction of the SMP with crystallised lactose was similar to the original SMP. Consequently, experiments were conducted to investigate the effect of exposure time in the oven on wall friction. Could greater adhesion be measured during this 6-hour period due to glass transition induced powder stickiness prior to lactose crystallisation? Preliminary experiments at 30-minute intervals showed that this was occurring. Then, experiments at 5-minute intervals were performed in the region where this effect was occurring. The experiments were performed at 2 different oven temperatures of 105 °C and 90 °C. As thermal equilibrium was not attained, the temperature of the powder in contact with the wall surface was measured using a thermocouple. Results for the wall friction failure stress at a normal stress of 5.9 kPa for the various exposure times at oven temperatures of 105 °C and
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90 °C are presented in Fig. 8, with corresponding temperature and water content data in Table 5. After 10-minute exposure to 105 °C, the wall friction started to increase from around 2.8 kPa up to a plateau of about 4.1 kPa after 20 min and remained at this level up to around 40 min before falling back to 3 kPa after 50 min. After 10-minute exposure, the powder temperature was 49 °C and this was above the glass transition temperature for its water content according to Fig. 1. Likewise, a similar trend occurred when exposing the powder to an oven temperature of 90 °C. What was happening was the powder became sticky once its temperature exceeded the lactose glass and entered the sticky temperature region. However, after a given time period, the powder lost its stickiness. This may be due to the lactose crystallising and loosing its molecular mobility and stickiness. It took longer for the powder to loose its stickiness at the lower oven temperature of 90 °C, as illustrated in Fig. 8, because the lower temperature reduced the rate of molecular mobility and crystallisation. These results show that the powder did adhere more strongly to the wall surface when amorphous lactose was above its glass transition and in the sticky zone. However, there is a time dependency effect because the molecular mobility conferring stickiness also initiated crystallisation which in-turn reduced mobility and stickiness over time and this may explain the reduction in measured wall friction. Furthermore, the powder caked and this may have influenced the wall friction behaviour of the powder because the powder was different physically. The powder caking mechanism is not clear. If the lactose has crystallised, then it is not due to sticky amorphous lactose. It appears also not to be due to liquid bridging because the water content of heated powder is low. It is most likely due to some form of solid bridge formation between the particles. 3.4. Effect of the amount of amorphous lactose on the caking of dairy ingredient mixes Experimentation was carried out to investigate how varying the amount of amorphous lactose influences the glass transition and caking behaviour of skim milk powder mixes. SMP was Table 4 Adhesion of skim milk powder onto a stainless steel surface as a function oven temperature after 6-hour incubation period Temperature (°C)
Water content (%)
Failure shear stress (kPa)
Caking behaviour (visual assessment)
20 30 40 50 60 70 80 90 100 110 120 130
6.0 – – – – 4.0 4.0 3.5 2.0 – – –
2.5 2.2 2.5 2.4 2.6 2.2 2.1 2.2 2.2 2.3 2.9 2.8
No cake No cake No cake No cake No cake Soft cake Soft cake Strong cake Strong cake Strong cake Hard cake Hard cake
(Failure shear stress is for a normal stress of 5.9 kPa).
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Fig. 8. Adhesion of skim milk powder onto a stainless steel surface as a function of exposure time in an oven at temperatures of 90 °C and 105 °C. (Failure shear stress is for a normal stress of 5.9 kPa).
exposed to 55% relative humidity for 48 h to crystallise most of the lactose. DSC thermograms showed that most but not all of the lactose had crystallised. This was then mixed with the original SMP in proportions of 0.25:0.75; 0.5:0.5; and 0.75:0.25. The original SMP, the crystallised SMP and the three mixes were all exposed to an atmosphere of 76% relative humidity at 20 °C for 6 h. After 6 h, each powder was disturbed by a spatula and assessed visually for caking behaviour. Glass transition and water content measurements were also taken. As expected, the original SMP caked after the 6-hour exposure while the crystallised SMP did not cake. The mixes with 0.75 and 0.5 fraction of original SMP also caked while the 0.25 fraction did not cake. DSC thermograms showed that the glass transition temperature was less than 20 °C for the SMP and all 3 mixes after 6-hour exposure, as presented in Table 6, and the crystallised SMP did not have a glass transition temperature. The outcome from these experiments demonstrates that the amount of amorphous component present influences the caking behaviour of powders containing amorphous components. 3.5. Application of glass transition measurements for predicting powder caking An approach is presented in this section as to how glass transition versus water content for an amorphous component may be applied in predicting the likelihood of caking of a powder mix containing an amorphous component. DSC can be
performed to evaluate how glass transition temperature (Tg) varies with water content for the amorphous component in the ingredient mix, that is, a graph similar to Fig. 1 can be evaluated. As Tg depends on powder temperature and water content, it is necessary to develop unsteady-state heat and mass transfer models to predict how temperature and water content vary throughout a given geometry of powder over time, when the powder is exposed to defined temperature and relative humidity profiles. To initiate caking, the powder temperature needs to be greater than the onset glass transition temperature. Consequently, the temperature difference between the powder and the glass transition temperature [T–Tg]+ represents a driver for the onset of caking. Furthermore, it may be related to cake strength. For the given ingredient mix, experiments need to be performed to obtain data that can be applied in relating this driver to the onset of caking and cake strength versus time. For a given temperature and relative humidity time profiles for an ingredient mix, for example, during storage and transport, heat and mass transfer models can be used to predict the temperature and water content throughout the powder geometry over time. This information coupled with the Tg versus water relationship can be used to evaluate the driver [T–Tg]+ over time and geometry within the powder. This driver can then be used to predict the onset of caking and possibly the strength of the cake developing over time. This is schematically illustrated in Fig. 9. As mentioned in Section 3.4 above, the amount of amorphous component present will affect the caking behaviour of the powder mix. It will also affect the mass transfer model in particular, as more amorphous component present will render the powder with a greater affinity for absorbing water from air. Furthermore, the amount of amorphous component present may also affect how the glass transition temperature varies with water content. Haque and Roos [23] showed that the glass transition of amorphous lactose was influenced by the presence of proteins. Consequently, the amount of component in the amorphous state will influence the approach to predicting caking behaviour illustrated in Fig. 9. The composition of food ingredient mixes is usually well specified and the amount of amorphous component is known, however sometimes the amount present in the amorphous state may vary and thus
Table 6 Glass transition properties and caking behaviour of mixes of skim milk powder (SMP) and “crystallised” SMP exposed to 76% relative humidity atmosphere (20 °C) for 6 h % proportion of original SMP to crystallised SMP (% original:% crystallised)
Table 5 Variation in powder temperature and moisture content at wall surface over time exposure in an oven at fixed temperatures of 105 and 90 °C Exposure time (min)
0
10
20
30
40
Oven temperature = 105 °C Powder temperature (°C) 20 Water content (%) 6.2
61 6.0
84 5.0
91 –
94 4
Oven temperature = 90 °C Powder temperature (°C) 20 Water content (%) 6.1
60 6.0
76 5.3
82 –
84 4.5
50
85 –
60
85.5 –
70
86 3.5
0:100
25:75
50:50
75:25
100:0
Midpoint glass transition temperature (°C) Before exposure 10 – After 6 h none 7
– 0
– 12
55 10
Water content (%) Before exposure After 6 h
11.2 9.3
9.2 9.6
8.4 10.8
6.8 10.6
5.6 9.2
Caking behaviour After 6 h
No cake
No cake
Caked
Caked
Caked
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glass transition induced stickiness was time-dependent. Over time, crystallisation took place which converted the sticky amorphous lactose into crystalline form which was not sticky, and this may explain the reduced adhesion of the SMP onto the stainless steel wall surface after the initial increase. This work also demonstrated that the amount of component present in the amorphous state, in addition to storage temperature, water, glass transition and time, influences the caking behaviour of powders containing amorphous components, which are caking due to this glass transition induced caking mechanism. The powder temperature needs to be greater than the onset glass transition temperature for caking to occur due to the presence of an amorphous component. Consequently, the temperature difference between the powder and the glass transition temperature [T–Tg]+ represents a driver for greater molecular mobility, powder stickiness and caking. Heat and mass transfer models coupled with glass transition versus moisture data can be used to evaluate how this driver varies with the temperature and relative humidity to which the powder is exposed to over time. Once [T–Tg]+ is greater than a certain value, then the occurrence of caking is very likely. This approach can then be applied to predict if caking of an ingredient mix, due to the presence of an amorphous component, is likely to occur when the mix is exposed to specified environmental conditions. Furthermore, actions, such as improved packaging, could be integrated into the heat and mass transfer model to predict if these actions are likely to prevent the occurrence of caking. References Fig. 9. Schematic illustration of the application of glass transition and heat and mass transfer modelling for predicting the caking behaviour of a food ingredient mix containing a component in the amorphous state.
may not be known exactly. DSC has potential for assessing how much of the component is in the amorphous state. Finally, the transfer models can be applied to evaluate how packaging, handling procedures and controlled atmosphere storage influence powder temperature and moisture content in an effort to help prevent caking. 4. Conclusions Powders with amorphous components, such as amorphous lactose, may become sticky if the powder temperature is elevated above the components glass transition temperature and into the sticky temperature region. This can lead to the powder becoming much more cohesive and eventually caking, and can also cause a powder to adhere more to a surface. SMP is a hygroscopic powder and readily sorbs water from air. The increased water content may partially determine cake strength but it also reduces the lactose glass transition temperature Tg which may also contribute to cake strength if Tg is reduced below the powder temperature. In this work, it was shown that exposing SMP to a temperature above its glass transition and into the sticky zone made the powder more sticky, rendering it a lot more cohesive and susceptible to caking, and also increased its adhesion to a stainless steel surface. This
[1] S.R. de Silva, Characterisation of particulate materials — a challenge for the bulk solids fraternity, Powder Handling and Processing 12 (4) (2000) 355–362. [2] E. Ortega-Rivas, Review and research trends in food powder processing, Powder Handling and Processing 14 (1) (2003) 18–25. [3] H. Purutyan, B.H. Pittenger, J.W. Carson, Solve solids handling problems by retrofitting, Chemical Engineering Progress 94 (4) (1998) 27–39. [4] J.R. Johanson, Troubleshooting bins, hoppers and feeders, Chemical Engineering Progress 98 (5) (2002) 24–36. [5] E. McGee, Go with the flow, Chemical Engineer 755 (2004) 40–41. [6] J.K. Prescott, D.A. Ploof, J.W. Carson, Developing a better understanding of wall friction, Powder Processing and Handling 11 (1) (1999) 25–36. [7] J.M. Aguilera, J.M. del Valle, M. Karel, Caking phenomena in amorphous food powders, Trends in Food Science and Technology 6 (1995) 149–155. [8] Y.H. Roos, Thermal analysis, state transitions and food quality, Journal of Thermal Analysis and Calorimetry 71 (2003) 197–203. [9] Y.H. Roos, Phase Transitions in Foods, Academic Press, San Diego, CA, USA, 1995. [10] K. Jouppila, Y.H. Roos, Glass transitions and crystallization in milk powders, Journal of Dairy Science 77 (1994) 2907–2915. [11] K. Jouppila, J. Kansikas, Y.H. Roos, Glass transition, water plasticization and lactose crystallization in skim milk powders, Journal of Dairy Science 80 (1997) 3152–3160. [12] Y.H. Roos, Importance of glass transition and water activity to spray drying and stability of dairy powders, Lait 82 (2002) 475–484. [13] T. Kudra, Sticky region in drying — definition and identification, Drying Technology 21 (2003) 1457–1469. [14] P. Boonyai, B. Bhandari, T. Howes, Stickiness measurement techniques for food powders: a review, Powder Technology 145 (2004) 34–46. [15] Y.H. Roos, M. Karel, Phase transitions of mixtures of amorphous polysaccharides and sugars, Biotechnology Progress 7 (1991) 49–53.
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[16] 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 Technology 149 (2005) 168–179. [17] E. Teunou, J.J. Fitzpatrick, E.C. Synnott, Characterisation of food powder flowability, Journal of Food Engineering 39 (1999) 31–37. [18] Institution of Chemical Engineers, Standard Shear Testing Technique for Particulate Solids Using the Jenike Shear Cell, Rugby, UK, 1989. [19] A.W. Jenike, Storage and Flow of Solids, Bulletin, vol. 123, Utah Engineering Experiment Station, University of Utah, Salt Lake City, USA, 1964. [20] J. Tomas, H. Schubert, Particle characterisation, Partec, vol. 79, Nurnberg, Germany, 1979, pp. 301–319. [21] H.M. Lai, S. Schmidt, Lactose crystallization in skim milk powder observed by hydrodynamic equilibria, scanning electron microscopy and nuclear magnetic resonance, Journal of Food Science 55 (1990) 994–999.
[22] A.H.J. Paterson, G.F. Brooks, J.E. Bronlund, K.D. Foster, Development of stickiness in amorphous lactose at constant T–Tg levels, International Dairy Journal 15 (2005) 513–519. [23] K.M. Haque, Y.H. Roos, Water plasticization and crystallization of lactose in spray-dried lactose/protein mixtures, Journal of Food Science — Food Engineering and Physical Properties 69 (1) (2004) 23–29. [24] T.M. Knowlton, J.W. Carson, G.E. Klinzing, W.C. Yang, The importance of storage, transfer and collection, Chemical Engineering Progress 90 (4) (1994) 44–54. [25] M. Peleg, Flowability of food powders and methods for its evaluation — a review, Journal of Food Process Engineering 1 (1978) 303–328.
Glass transition and the flowability and caking of powders containing amorphous lactose J.J. Fitzpatrick a,⁎, M. Hodnett a , M. Twomey a , P.S.M. Cerqueira a , J. O'Flynn a , Y.H. Roos b a
Department of Process and Chemical Engineering, University College, Cork, Ireland b Department of Food and Nutritional Sciences, University College, Cork, Ireland
Received 16 August 2005; received in revised form 11 November 2006; accepted 27 April 2007 Available online 5 May 2007
Abstract Many powders contain amorphous components, such as amorphous lactose in milk powders, which when given sufficient conditions of temperature and water content, will mobilise as a high viscosity flow making the particles sticky. This can lead to increased cohesiveness, powder caking and increased adhesion to surfaces. The transition from the glassy state is established by increasing the powder temperature to above its glass transition temperature which can be measured using differential scanning calorimetry. Exposing milk powder to over 10–20 °C above the lactose glass transition makes the powder more sticky, rendering it a lot more cohesive and also increases its adhesion to a stainless steel surface. This glass transition induced stickiness is time-dependent. Over time, crystallisation can take place converting the amorphous lactose into crystalline lactose. Furthermore, the caking behaviour of powders depends on the amount of component present in the amorphous state. Finally, this work presents an approach for applying the measured relationship between the glass transition and water content for predicting caking problems with powders containing amorphous components. © 2007 Elsevier B.V. All rights reserved. Keywords: Glass transition; Food powders; Powder flowability; Caking; Storage and handling
1. Introduction There is a large quantity and variety of materials produced industrially in powder form, and there is a need for information about their handling and processing characteristics. Powder flow properties are important in handling and processing operations [25,24,1,2]. Flow problems in hoppers and silos are commonplace problems for engineers and process operatives [3–5]. These problems may manifest themselves as unreliable inconsistent flow that reduces production rates. Cohesive attraction and frictional resistance developed between particles in a powder when consolidated must be overcome to make the powder flow. The cohesive attraction between particles in powders is very important in determining the minimum hopper opening size required to enable flow from hoppers and silos. A cohesive arch will form preventing discharge if ⁎ Corresponding author. Tel.: +353 21 4903089. E-mail address: [email protected] (J.J. Fitzpatrick). 0032-5910/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.powtec.2007.04.017
these attractions are not overcome by the gravity force acting due to the powder above. Wall friction is also a critical parameter in the design and operation of hoppers and silos for reliable discharge of bulk solids [6]. It is the frictional resistance to powder flow that exists between the powder and hopper/silo wall material. Wall friction is the dominant parameter in determining the minimum hopper angle (between the hopper wall and the horizontal) required to ensure mass flow, which is the desirable flow pattern for consistent reliable flow. Many food powders and food ingredient mixes are rendered complicated by the fact that they contain many different components, and this makes it difficult to predict their flow behaviour. Furthermore, during handling, storage, processing and distribution to the final consumer, the powders may experience a variety of temperatures and atmospheric humidities which may alter the handling behaviour and appearance of the powders. This is especially important if powders are transported to hotter, more humid climates, where a mix may cake solidly or liquefy from absorbing water. The consumer is usually very
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sensitive to any lumping, caking or difficulty in discharging the powder from its container. Many food powders contain amorphous glassy components, such as amorphous sugars. For example, many spray-dried dairy powders contain lactose in its amorphous state. Amorphous components are thermodynamically unstable and there exists a driving force for them to crystallise, however this requires that the molecules must be able to move. When an amorphous component is given sufficient conditions of temperature and water content, they can mobilise as a high viscosity flow, which can make them sticky and lead to caking [7]. Furthermore, molecular mobility enables amorphous components to crystallise over time. Glass transition measurements can be used to detect the onset of this mobility. Differential scanning calorimetry (DSC) is an established thermal analytical technique that can measure glass transition properties because glass transition induces a change in the specific heat of the material due to molecular mobility [8]. The glass transition occurs over a temperature range. This is often a relatively narrow range of 10 to 20 °C for amorphous sugars, however a much larger range, of for example 50 °C, may be expected for the glass transition of polymers in foods. Within this temperature range, the glass transition temperature can be referred to as the temperature initiating the onset of glass transition or the mid-point temperature of the change in specific heat capacity. The glass transitions of carbohydrates and proteins are affected by water, with greater water content resulting in decreased glass transition temperatures. Water acts as a plasticiser enabling the mobilisation of amorphous components. Consequently, a lower temperature is required to initiate the onset of the glass transition for higher water contents. Both higher temperature and water content can enable molecular mobility and the onset of the glass transition. Powders containing lactose in its amorphous state may crystallise, producing a discontinuous solid lactose phase within the particles. Crystallisation will only take place if the powder temperature is greater than its glass transition temperature (Tg), whereby the molecules have sufficient mobility to initiate crystallisation [9–12,8]. Tg is usually well above the storage temperature for most dry powders. However, lactose in its amorphous state is very hygroscopic and will readily sorb water from ambient air, and this increase in water will cause a significant plasticization and reduction in Tg. Crystallisation will occur if Tg is reduced to below the powder temperature. Crystallisation can be preceded by the formation of liquid bridges between powder particles, which leads to an increase in the cohesiveness and caking ability of the powder. For this reason, exposure of powders to higher temperatures and humidities, such as in tropical countries, can lead to the glass transition of amorphous components being exceeded, leading to powder handling and caking problems. Above the glass transition temperature (for a given water content) the powder particles become sticky [13,14] and this is often described by the sticky point temperature. It is generally accepted that the sticky point is about 10–20 °C above the glass transition onset temperature, especially for low molecular weight carbohydrates [15], as illustrated in Fig. 1. This sticki-
Fig. 1. Glass transition of lactose in skim milk powder.
ness is due to the molecular mobility of the amorphous components as a highly viscous flow between particle surfaces, making the powder a lot more cohesive. Furthermore, contacting the powder with a wall surface when the powder is in this sticky region will lead to increased adhesion between the powder and the surface. For example, in a spray drying process, if the surface temperature of drying droplets is greater than the Tg of an amorphous component and the particles contact the dryer wall, then the particles may stick onto the dryer wall causing coating of the wall surface over time [16]. The focus of this study was to investigate the application of glass transition measurements for assessing the cohesion/caking ability and wall friction characteristics of powders due to the presence of amorphous components, using amorphous lactose in dairy powders as a case-study. In particular, the objectives were: • To illustrate the relationship between the glass transition and the cohesion and caking of spray-dried skim-milk and whey permeate powders, which both contain amorphous lactose. • To investigate the influence of lactose glass transition on the adhesion/wall friction of skim-milk powder to a stainless steel surface. • To investigate the effect of the amount of amorphous lactose on the glass transition and caking of a mix of skim-milk powders containing varying amounts of amorphous and crystalline lactose. • To outline an approach for the application of glass transition measurements for predicting and preventing handling and caking problems in food powders, food ingredient mixes and other powders due to the presence of amorphous components. 2. Materials and methods 2.1. Milk powders Two 25 kg bags of commercial skim-milk powder (SMP) and one bag of lactose powder (LP) were donated by Dairygold, Mitchelstown, Ireland. A bag of commercial whey permeate powder (WPP) was donated by Kerry Ltd., Listowel, Ireland.
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2.2. Particle size and water content Particle size distribution was measured by laser diffraction using the Malvern Mastersizer MSS with powder feeder unit (Worcestershire, UK). Each test was carried out in duplicate and the average mean particle size was reported. The difference in mean particle size of the duplicates was less than 5% of the average particle size. Water content (wet basis) was measured by weighing 3 g of a sample before and after drying in an oven (Sanyo-Gallenkamp, Loughborough, UK) at 105 °C for 3 days. Each test was carried out in triplicate. The difference in water content between the triplicates was within 0.6% w/w. If two of the water contents were close to each other, then the average of these two was reported, otherwise the average of the three was reported. 2.3. Glass transition Differential scanning calorimetry (DSC, Mettler Toledo 821e with N2 cooling, Greifensee, Switzerland) was used to measure lactose glass transition temperature range and the change in specific heat due to glass transition. Samples of dehydrated materials (4 to 15 mg) were prepared in preweighed DSC aluminium pans (40 μl; Mettler Toledo-27331, Greifensee, Switzerland), and the pans were then hermitically sealed. Duplicate samples of each material were analysed. An empty pan was used as a reference. The DSC was calibrated for temperature using n-hexane (melting point, − 95.0 °C), mercury (melting point, − 38.8 °C), water (melting point, 0.0 °C), gallium (melting point, 29.8 °C) and indium (melting point, 156.6 °C) and for heat flow using n-hexane (ΔHm, 151.8 J/g), mercury (ΔHm, 11.4 J/g), water (ΔHm, 334.5 J/g), gallium (ΔHm, 80 J/g), and indium (ΔHm, 28.45 J/g). All measurements were made at a scanning rate of 5 °C/min. An immediate rescan was made for each sample to verify the endothermic baseline shift associated with the glass transition. Glass transition temperatures were determined using STARe thermal analysis software, version 6.0 (Mettler Toledo, Greifensee, Switzerland). 2.4. Flow function measurement An annular shear cell was used for measuring the flow function and effective angle of internal friction and was the same as that described by Teunou et al. [17]. It had a fixed shearing rate of 7 mm/min and external and internal diameters of 164 mm and 120 mm, respectively. The milk powder was removed from its package and packed into the annular shear cell. The annular shear cell was then placed in a chamber, at a temperature of 20 °C, where the shear tests for measuring the instantaneous flow function were conducted. The procedure used to measure the instantaneous flow function was that recommended by the Standard Shear Test Technique (SSTT), using the Jenike shear cell [18]. Each flow function was measured in duplicate to assess reproducibility. If the flow functions did not plot on top of each other or close beside each other, the test was repeated a third time to determine which of the flow functions was valid. The flow index is defined as the inverse
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slope of the flow function. Jenike [19] used the flow index to classify powder flowability with lower values representing very cohesive and cohesive powders and higher values representing easy flow and free-flowing powders, and this classification was later extended by Tomas [20]. 2.5. Wall friction testing The wall yield locus of skim milk powder was measured using a Jenike shear cell (95 mm internal diameter) whereby the cylindrical base of the cell was replaced by a flat plate of stainless steel 304 commonly used in food processing. The wall yield locus was obtained by measuring the horizontal stress required to make the powder fail at the following normal stresses: 5.9, 4.4, 3.7, 3.0, 2.3 and 1.6 kPa. The procedure used was that recommended by the Standard Shear Test Technique (SSTT), using the Jenike shear cell. Each wall yield locus was measured in duplicate to assess reproducibility. If the wall yield loci did not plot on top of each other or close beside each other, the test was repeated a third time to determine which of the wall yield loci was valid. The failure shear stress at the maximum normal stress of 5.9 kPa is reported in this work as an index of the adhesion between the powder and the stainless steel coupon. To study the influence of temperature on wall friction characteristics of skim milk powder, duplicate pre-sheared rings on the stainless steel coupons containing the powder were placed in an oven for 6 h under controlled temperatures in the range of 20 to 130 °C. After incubation in the oven, one sample was removed from the incubator and the wall friction test was immediately carried out. At the end of the test, samples for water content measurement were taken from powder at the wall plate surface. This was repeated for the duplicate sample. The wall friction tests were performed rapidly after sample removal from the oven in order to minimise cooling of the sample. 2.6. Visual assessment of caking in controlled atmosphere chamber Visual assessment of the powder caking was carried out for powders after exposure to atmospheres at 76% relative humidity and 20 °C for a number of exposure times. The desired temperature was controlled using a Haake F3 heating/cooling system. Relative humidity was controlled by placing a tray of a saturated NaCl solution in the base of the sealed cabinet. There was a fan in the cabinet to maintain a uniform atmosphere, and the values of temperature and relative humidity in the cabinet were recorded by a thermometer and a hygrometer located inside the cabinet. The powder was placed in two trays inside the insulated chamber for the specified exposure time at a fixed relative humidity and temperature of 20 °C. The powder was spread out in the tray as a thin layer of about 1 cm thick to increase the rate of water sorption. After the exposure time, the two trays were then visually inspected for caking behaviour by moving a spatula through the powder/cake and assessing if the powder had caked or not and if the cake was a soft cake, strong cake or
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Fig. 2. Aluminium dish used in cake strength determination (with central hole and cover underneath to prevent powder falling through).
hard cake. At the end of the test, duplicate samples were taken for immediate preparation for thermogram measurement by differential scanning calorimetry. Triplicate samples were also taken for water content measurement. 2.7. Quantitative measurement of cake strength An empirical test for quantitatively measuring an index of cake strength was developed by the authors. It consisted of placing a measured amount of powder into a cylindrical aluminium dish which had a circular hole exactly in the centre of the dish with dimensions illustrated in Fig. 2. The dish had a cover placed underneath to prevent the powder falling through the hole. The powder was spread out across the dish in a consistent manner to reduce variability between tests due to packing procedure. SMP samples in the dishes were then exposed to different values of temperatures (10, 25 and 40 °C) and relative humidities (76% and 100% RH) over a number of time durations (2, 4, 6, 12, 24 h) to investigate how these conditions influence the cake strength of SMP. The relative humidities were achieved by placing a dish containing powder into a glass kilner jar, containing either saturated sodium chloride solution to obtain 76% RH or pure de-ionised water to obtain 100% RH, which were then sealed. The temperature within the jars was controlled by placing them in the controlled atmosphere chamber mentioned above.
Fig. 4. Flow functions of lactose powder (LP), whey permeate powder (WPP) and skim milk powder (SMP) at 20 °C.
Following exposure to a given temperature, relative humidity and time, the dish was removed from the kilner jar. The cover was removed from beneath the aluminium dish and the dish was centred below a rod attached to a JJ tester Lloyd Instruments, T-series, Fareham, UK). The rod had a diameter of 7.84 mm and was initially moved downwards to just above the top of the caked powder. The rod was then moved downward through the caked powder at a constant speed of 3 mm/min and the force versus displacement was measured using the JJ tester as the rod moved through the cake. This eventually caused the cake to fail with powder being ejected through the hole in the centre of the dish. A typical plot of force versus displacement is presented in Fig. 3. The peak force was used as a single parameter index of cake strength. A reproducibility study was carried out on one condition, which resulted in a mean peak force of 3.3 N and a standard deviation of 0.27 N. 3. Results and discussion 3.1. Glass transition and the cohesion/caking of dairy powders Lactose was in a crystalline form in the lactose powder (LP). Lactose was the major component in both WPP and SMP, however it was mainly in the amorphous form. Comparing the cohesiveness of these 3 powders (Fig. 4 and Table 1) shows that LP was very cohesive while WPP and SMP were easy flow. It is difficult to reconcile the major difference in cohesion between these 3 powders considering that lactose was the major component in each and that LP had a larger particle size. This maybe due to the crystalline nature of LP giving rise to greater frictional resistance between the particles, however the values Table 1 Powder physical and flow properties
Fig. 3. Typical force versus displacement curve for the cake strength tester.
Powder
Mean particle size (μm)
Water content (% w/w)
Effective angle of internal friction
Flow index
Flow classification
SMP LP WPP
53 157 98
4.7 – 3.8
52 49 49
6.3 1.6 5.9
Easy flow Very cohesive Easy flow
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for effective angle of internal friction do not justify this rationale. It may also be due to differences in surface water contents producing differences in cohesion due to liquid bridging. Both WPP and SMP have lactose in amorphous form because they are both spray-dried powders. Lactose in amorphous form is thermodynamically unstable and very hygroscopic, and if these lactose molecules gain mobility, they will tend to crystallise over time. Greater molecular mobility can be obtained at higher temperatures and water contents. The existence of a glass transition for the powder is an indication of molecular mobility due to amorphous components such as amorphous lactose. Experiments were carried out with LP, WPP and SMP whereby the powders were exposed to water in air at 76% relative humidity (RH) at 20 °C over a 48-hour period. DSC thermograms of the powders were measured over this time period as well as assessing the flowability/caking of the powders. The objective of this experimentation was to investigate how amorphous lactose content and water pick up from air influenced the flow/caking behaviour of the powders. Both WPP and SMP sorbed much water from the air, with WPP rapidly attaining a water content of around 10.5% after only 6 h, and SMP attained a water content of around 14.6% after 12 h. The water content of both these powders fluctuated around
Fig. 6. DSC thermograms of skim milk powder (SMP) exposed to 76% RH at 20 °C for a) 0 h; b) 12 h; c) 48 h.
Fig. 5. DSC thermograms of whey permeate powder (WPP) exposed to 76% RH at 20 °C for a) 0 h; b) 6 h; c) 12 h.
these values for the remaining exposure time. The measured flow function of LP after 24-hour exposure showed only a small increase in cohesiveness, however both WPP and SMP had caked. DSC thermograms were measured for samples of the powders after the following exposure times: 0, 6, 12, 24 and 48 h. The DSC thermogram for LP did not change significantly over a 24-hour exposure time as there was little amorphous lactose present to cause glass transition changes. On the other hand, there were major changes observed from the DSC thermograms over a 48-hour exposure time for both WPP and SMP, as illustrated in Figs. 5 and 6, respectively, with glass transition property data presented in Table 2. For both powders at 0 h, there was a distinct glass transition, with glass transition temperatures of around 31 °C for WPP and 46 °C for SMP. After 6hour exposure, the glass transition temperature for WPP was reduced to − 10 °C, and after 12-hour exposure, the glass transition temperature for SMP was reduced to 5 °C. After 12 h, the glass transition no longer existed for WPP, and it took between 24 and 48 h for the glass transition to disappear for SMP. These changes in glass transition temperatures were expected and have been verified by others [12,11,10]. The time required for the glass transition to disappear was over 2 times
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Table 2 Glass transition properties of whey permeate powder (WPP) and skim milk powder (SMP) exposed to 76% relative humidity atmosphere (20 °C) for various exposure times Exposure time (h)
WPP 0 6 12 SMP 0 12 48 a
Water content (%)
Glass transition temperature range (°C)
ΔCp a (J g− 1 °C− 1)
Onset
Midpoint
Endpoint
6.2 10.6 10.5
23.4 − 10.8 –
30.7 −10.5 –
35.5 − 9.8 –
0.244 0.047 –
5.8 14.7 14.6
38.3 4.3
46.4 9
49.1 13 –
0.379 0.323 –
–
–
Change in specific heat due to glass transition.
longer for SMP than WPP. This may be due to the greater content of non-lactose components, such as proteins, present in SMP, which compete for the available water and inhibit the mobility of amorphous lactose molecules, which slows down its crystallisation. The reduction and disappearance of the glass transition, both in terms of temperature and change in specific heat, over time signifies the reduction in amorphous lactose content as it forms crystalline lactose. This was due to the rapid uptake of water over the first 6 to 12 h increasing the mobility of amorphous lactose molecules so that they could crystallise over time. [21] showed this crystallisation behaviour using electron microscopy. Consequently, the caking of both these powders may result from the formation of liquid bridges between the powder particles, which binds particles together to form a cake. Furthermore, as powder components dissolve in water, the liquid bridges will contain these components, which increase the viscosity of the liquid bridges making them stickier and forming a stronger cake. As water or thermal plasticization becomes excessive, lactose molecules form nuclei and crystallise randomly to various crystalline forms. As crystallisation proceeds sorbed water is lost, as the crystallised lactose is a lot less hygroscopic, and this leads to a reduction in liquid bridging which will influence cake strength. 3.2. Effect of glass transition on the caking of SMP Samples of SMP were exposed to a combination of temperatures, relative humidities and exposure times, and their cake strength was measured using the cake strength tester described in Section 2.7 above. After testing, samples of powder in the vicinity of the centre of the aluminium dish were taken for water content measurement. As expected, powder water content increased significantly with greater exposure time and greater relative humidity, as illustrated in Table 3. As already mentioned above, increased water content usually makes a powder more cohesive and may induce caking. Temperature had a less significant effect on water except for 40 °C at 100% RH which lead to by far the greatest water uptake. For most of the conditions, cake strength was low or negligible for lower exposure
times until after a given time the cake strength increased significantly, as illustrated in Table 3 and Fig. 7. Both relative humidity and temperature had major effects on cake strength once the powder started to cake, with higher relative humidity and higher temperature resulting in greater cake strength. Importantly, the lactose glass transition had a major influence on the development of cake strength, in addition to the presence of greater water. For a given measured caked sample water content, the lactose glass transition temperature was read from Fig. 1. The positive temperature difference between the powder exposure temperature minus the glass transition temperature [T–Tg]+ was calculated (if this was negative, it was assigned a zero value). Table 3 shows how [T–Tg]+ varies for the different experimental conditions. It can be seen from these data values that if [T–Tg]+ was 0 or less than 10 °C, then the cake strength was small or negligible, however, if [T–Tg]+ was greater than 20 °C, then cake strength increased to greater than 1 N. From this, it can be concluded that the lactose glass transition was influencing the initial development of cake strength. This makes sense because once the temperature of the amorphous lactose was increased above its glass transition and into its sticky temperature range, it became more sticky enabling it to stick particles together forming a cake. Furthermore, at 24-hour exposure, Table 3 shows a relationship between [T–Tg]+ and cake strength with higher cake strength at higher [T–Tg]+ for the experimental conditions tested. Furthermore, Paterson et al. [22] demonstrated that the cohesiveness of amorphous lactose powder over time was characterised by how much the powder temperature exceeded its glass transition temperature [T–Tg]+, and this was independent of how [T–Tg]+ was achieved. Table 3 Effect of temperature, relative humidity (RH) and exposure time on the water content, cake strength and [T–Tg]+ of skim milk powder (where T is the powder exposure temperature and Tg is the glass tranistion temperature of lactose at the powder water content. If T–Tg b 0, then [T–Tg]+ = 0) Time (h) 76% RH 76% RH 76% RH 100% RH 100% RH 100% RH 10 °C 25 °C 40 °C 10 °C 25 °C 40 °C 0 2 4 6 12 24 0 2 4 6 12 24 0 2 4 6 12 24
5.2 7 7.3 8 8.7 12.4 0 0.04 0.06 0.32 0.25 1.1 0 0 0 0 0 15
Water content (%w/w) 5.2 5.2 7.8 7.9 10.5 8 11.2 9.4 13 14 13.8 18.4 Cake strength (N) 0 0 0 0.04 0.03 0.15 0.03 1.06 0.1 0.4 1.9 0.7 1.8 2.2 1 1.9 3.5 1.8. [T–Tg]+ (°C) 0 0 0 0 0 0 3 20 0 12 38 2 30 41 22 38 51 40 5.2 6.4 7.5 8.8 12.2 14.4
5.2 7.2 9 11 14 17.9 0 0.1 0.23 2.7 3.1 3.1 0 0 14 25 37 53
5.2 9.6 13.9 15 18 22 0 1.7 4 4.9 5.4 7.8 0 0 52 56 69 88
J.J. Fitzpatrick et al. / Powder Technology 178 (2007) 119–128
Fig. 7. Effect of temperature, relative humidity (RH) and exposure time on the cake strength of skim milk powder.
3.3. Effect of glass transition on the adhesion of SMP to a stainless steel surface Wall friction experiments were performed with SMP to investigate how temperature and glass transition influenced the adhesion of SMP onto a stainless steel surface. Powder samples were prepared for wall friction testing. They were placed in an oven for 6 h to attain thermal equilibrium at temperatures ranging from 20 °C to 130 °C. Then, they were removed from the oven and a wall friction test was performed immediately. Powders were disturbed using a spatula to visually assess if the powders had caked and if the cake was a soft, strong or hard cake samples were taken of powder at the wall surface for water content measurement. Results for the wall friction failure stress at a normal stress of 5.9 kPa for the various temperatures are presented in Table 4. The powder started to form a soft cake at about 70 °C, which was well above the glass transition for the corresponding water content, as obtained from Fig. 1. However, Table 4 shows no major increase in wall friction, in fact it reduced a little and then increased a little at higher temperatures. Overall, there appeared to be no relationship between increased adhesion to the stainless steel surface and glass transition. The reason for this may be because the powder may have become stickier sometime during the 6-hour exposure period giving rise to greater adhesion but the lactose crystallised sometime within the 6 h and the wall friction of the SMP with crystallised lactose was similar to the original SMP. Consequently, experiments were conducted to investigate the effect of exposure time in the oven on wall friction. Could greater adhesion be measured during this 6-hour period due to glass transition induced powder stickiness prior to lactose crystallisation? Preliminary experiments at 30-minute intervals showed that this was occurring. Then, experiments at 5-minute intervals were performed in the region where this effect was occurring. The experiments were performed at 2 different oven temperatures of 105 °C and 90 °C. As thermal equilibrium was not attained, the temperature of the powder in contact with the wall surface was measured using a thermocouple. Results for the wall friction failure stress at a normal stress of 5.9 kPa for the various exposure times at oven temperatures of 105 °C and
125
90 °C are presented in Fig. 8, with corresponding temperature and water content data in Table 5. After 10-minute exposure to 105 °C, the wall friction started to increase from around 2.8 kPa up to a plateau of about 4.1 kPa after 20 min and remained at this level up to around 40 min before falling back to 3 kPa after 50 min. After 10-minute exposure, the powder temperature was 49 °C and this was above the glass transition temperature for its water content according to Fig. 1. Likewise, a similar trend occurred when exposing the powder to an oven temperature of 90 °C. What was happening was the powder became sticky once its temperature exceeded the lactose glass and entered the sticky temperature region. However, after a given time period, the powder lost its stickiness. This may be due to the lactose crystallising and loosing its molecular mobility and stickiness. It took longer for the powder to loose its stickiness at the lower oven temperature of 90 °C, as illustrated in Fig. 8, because the lower temperature reduced the rate of molecular mobility and crystallisation. These results show that the powder did adhere more strongly to the wall surface when amorphous lactose was above its glass transition and in the sticky zone. However, there is a time dependency effect because the molecular mobility conferring stickiness also initiated crystallisation which in-turn reduced mobility and stickiness over time and this may explain the reduction in measured wall friction. Furthermore, the powder caked and this may have influenced the wall friction behaviour of the powder because the powder was different physically. The powder caking mechanism is not clear. If the lactose has crystallised, then it is not due to sticky amorphous lactose. It appears also not to be due to liquid bridging because the water content of heated powder is low. It is most likely due to some form of solid bridge formation between the particles. 3.4. Effect of the amount of amorphous lactose on the caking of dairy ingredient mixes Experimentation was carried out to investigate how varying the amount of amorphous lactose influences the glass transition and caking behaviour of skim milk powder mixes. SMP was Table 4 Adhesion of skim milk powder onto a stainless steel surface as a function oven temperature after 6-hour incubation period Temperature (°C)
Water content (%)
Failure shear stress (kPa)
Caking behaviour (visual assessment)
20 30 40 50 60 70 80 90 100 110 120 130
6.0 – – – – 4.0 4.0 3.5 2.0 – – –
2.5 2.2 2.5 2.4 2.6 2.2 2.1 2.2 2.2 2.3 2.9 2.8
No cake No cake No cake No cake No cake Soft cake Soft cake Strong cake Strong cake Strong cake Hard cake Hard cake
(Failure shear stress is for a normal stress of 5.9 kPa).
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Fig. 8. Adhesion of skim milk powder onto a stainless steel surface as a function of exposure time in an oven at temperatures of 90 °C and 105 °C. (Failure shear stress is for a normal stress of 5.9 kPa).
exposed to 55% relative humidity for 48 h to crystallise most of the lactose. DSC thermograms showed that most but not all of the lactose had crystallised. This was then mixed with the original SMP in proportions of 0.25:0.75; 0.5:0.5; and 0.75:0.25. The original SMP, the crystallised SMP and the three mixes were all exposed to an atmosphere of 76% relative humidity at 20 °C for 6 h. After 6 h, each powder was disturbed by a spatula and assessed visually for caking behaviour. Glass transition and water content measurements were also taken. As expected, the original SMP caked after the 6-hour exposure while the crystallised SMP did not cake. The mixes with 0.75 and 0.5 fraction of original SMP also caked while the 0.25 fraction did not cake. DSC thermograms showed that the glass transition temperature was less than 20 °C for the SMP and all 3 mixes after 6-hour exposure, as presented in Table 6, and the crystallised SMP did not have a glass transition temperature. The outcome from these experiments demonstrates that the amount of amorphous component present influences the caking behaviour of powders containing amorphous components. 3.5. Application of glass transition measurements for predicting powder caking An approach is presented in this section as to how glass transition versus water content for an amorphous component may be applied in predicting the likelihood of caking of a powder mix containing an amorphous component. DSC can be
performed to evaluate how glass transition temperature (Tg) varies with water content for the amorphous component in the ingredient mix, that is, a graph similar to Fig. 1 can be evaluated. As Tg depends on powder temperature and water content, it is necessary to develop unsteady-state heat and mass transfer models to predict how temperature and water content vary throughout a given geometry of powder over time, when the powder is exposed to defined temperature and relative humidity profiles. To initiate caking, the powder temperature needs to be greater than the onset glass transition temperature. Consequently, the temperature difference between the powder and the glass transition temperature [T–Tg]+ represents a driver for the onset of caking. Furthermore, it may be related to cake strength. For the given ingredient mix, experiments need to be performed to obtain data that can be applied in relating this driver to the onset of caking and cake strength versus time. For a given temperature and relative humidity time profiles for an ingredient mix, for example, during storage and transport, heat and mass transfer models can be used to predict the temperature and water content throughout the powder geometry over time. This information coupled with the Tg versus water relationship can be used to evaluate the driver [T–Tg]+ over time and geometry within the powder. This driver can then be used to predict the onset of caking and possibly the strength of the cake developing over time. This is schematically illustrated in Fig. 9. As mentioned in Section 3.4 above, the amount of amorphous component present will affect the caking behaviour of the powder mix. It will also affect the mass transfer model in particular, as more amorphous component present will render the powder with a greater affinity for absorbing water from air. Furthermore, the amount of amorphous component present may also affect how the glass transition temperature varies with water content. Haque and Roos [23] showed that the glass transition of amorphous lactose was influenced by the presence of proteins. Consequently, the amount of component in the amorphous state will influence the approach to predicting caking behaviour illustrated in Fig. 9. The composition of food ingredient mixes is usually well specified and the amount of amorphous component is known, however sometimes the amount present in the amorphous state may vary and thus
Table 6 Glass transition properties and caking behaviour of mixes of skim milk powder (SMP) and “crystallised” SMP exposed to 76% relative humidity atmosphere (20 °C) for 6 h % proportion of original SMP to crystallised SMP (% original:% crystallised)
Table 5 Variation in powder temperature and moisture content at wall surface over time exposure in an oven at fixed temperatures of 105 and 90 °C Exposure time (min)
0
10
20
30
40
Oven temperature = 105 °C Powder temperature (°C) 20 Water content (%) 6.2
61 6.0
84 5.0
91 –
94 4
Oven temperature = 90 °C Powder temperature (°C) 20 Water content (%) 6.1
60 6.0
76 5.3
82 –
84 4.5
50
85 –
60
85.5 –
70
86 3.5
0:100
25:75
50:50
75:25
100:0
Midpoint glass transition temperature (°C) Before exposure 10 – After 6 h none 7
– 0
– 12
55 10
Water content (%) Before exposure After 6 h
11.2 9.3
9.2 9.6
8.4 10.8
6.8 10.6
5.6 9.2
Caking behaviour After 6 h
No cake
No cake
Caked
Caked
Caked
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glass transition induced stickiness was time-dependent. Over time, crystallisation took place which converted the sticky amorphous lactose into crystalline form which was not sticky, and this may explain the reduced adhesion of the SMP onto the stainless steel wall surface after the initial increase. This work also demonstrated that the amount of component present in the amorphous state, in addition to storage temperature, water, glass transition and time, influences the caking behaviour of powders containing amorphous components, which are caking due to this glass transition induced caking mechanism. The powder temperature needs to be greater than the onset glass transition temperature for caking to occur due to the presence of an amorphous component. Consequently, the temperature difference between the powder and the glass transition temperature [T–Tg]+ represents a driver for greater molecular mobility, powder stickiness and caking. Heat and mass transfer models coupled with glass transition versus moisture data can be used to evaluate how this driver varies with the temperature and relative humidity to which the powder is exposed to over time. Once [T–Tg]+ is greater than a certain value, then the occurrence of caking is very likely. This approach can then be applied to predict if caking of an ingredient mix, due to the presence of an amorphous component, is likely to occur when the mix is exposed to specified environmental conditions. Furthermore, actions, such as improved packaging, could be integrated into the heat and mass transfer model to predict if these actions are likely to prevent the occurrence of caking. References Fig. 9. Schematic illustration of the application of glass transition and heat and mass transfer modelling for predicting the caking behaviour of a food ingredient mix containing a component in the amorphous state.
may not be known exactly. DSC has potential for assessing how much of the component is in the amorphous state. Finally, the transfer models can be applied to evaluate how packaging, handling procedures and controlled atmosphere storage influence powder temperature and moisture content in an effort to help prevent caking. 4. Conclusions Powders with amorphous components, such as amorphous lactose, may become sticky if the powder temperature is elevated above the components glass transition temperature and into the sticky temperature region. This can lead to the powder becoming much more cohesive and eventually caking, and can also cause a powder to adhere more to a surface. SMP is a hygroscopic powder and readily sorbs water from air. The increased water content may partially determine cake strength but it also reduces the lactose glass transition temperature Tg which may also contribute to cake strength if Tg is reduced below the powder temperature. In this work, it was shown that exposing SMP to a temperature above its glass transition and into the sticky zone made the powder more sticky, rendering it a lot more cohesive and susceptible to caking, and also increased its adhesion to a stainless steel surface. This
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