Development of stickiness in amorphous lactose at constant T−Tg levels

ARTICLE IN PRESS

International Dairy Journal 15 (2005) 513–519 www.elsevier.com/locate/idairyj

Development of stickiness in amorphous lactose at constant T
Tg levels A.H.J. Paterson, G.F. Brooks, J.E. Bronlund, K.D. Foster Institute of Technology and Engineering, Massey University, Private Bag 11222, Palmerston North, New Zealand Received 2 September 2003; accepted 25 August 2004

Abstract The concept of the glass transition temperature ðT g Þ has been used to explain the rate of development of sticking in amorphous lactose. Stickiness development with time was measured using a blow tester. The parameter ðT
T g Þ was used to characterise the rate of stickiness development for a range of conditions (37–67 1C and 0.15
0.35 aw). At a given T
T g value the level of stickiness increased linearly with time, independently of how the T
T g condition was induced. r 2004 Elsevier Ltd. All rights reserved. Keywords: Dairy powders; Stickiness; Caking; Glass transition temperature; Lactose

1. Introduction Stickiness and caking in powders are closely related phenomena, to the extent that the two phrases are often used interchangeably to describe powder cohesion (Wallack & King, 1988; Chuy & Labuza, 1994; Paterson & Bronlund, 1997). Aguilera, del Valle, and Karel (1995) described caking in amorphous powders as the transformation of a powder into lumps and then an agglomerated solid. Stages in the caking process were defined, including bridging, agglomeration, compaction and liquefaction. Aguilera et al. (1995) assert that the caking of powders occurs ‘‘as a result of surface deformation and sticking at contact points between particles’’. Several papers have reported work on caking in a variety of food powders (Tsourouflis, Flink, & Karel, 1976; Downton, Flores-Luna, & King, 1982; Wallack & King, 1988; Lloyd, Chen, & Hargreaves, 1996; Bronlund, 1997; Paterson & Bronlund, 1997; Rennie, Chen, Hargreaves, & Mackereth, 1999). There have been several studies of the initial sticking behaviour in food powders leading to the formation of a solid caked product. These include Corresponding author. Fax: +64 6 350 5604.

E-mail address: [email protected] (A.H.J. Paterson). 0958-6946/$ - see front matter r 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.idairyj.2004.08.012

work on instant coffee and drink powders (Downton et al., 1982), milk powders (Paterson & Bronlund, 1997), fish protein hydrolyzate (Aguilera et al., 1995) and infant formulas (Chuy & Labuza, 1994). These works indicate that caking is a two-step process. For a powder to cake, the particles involved must first bridge together. This is referred to as sticking or lumping. The extent of stickiness before the solidification of these bridges will then determine the extent of caking in the powder. It is therefore desirable to have an understanding of the mechanisms that cause sticking between particles and how processing or storage conditions affect the rate of stickiness development. The work presented in this paper was aimed at characterising the rate of stickiness development in amorphous lactose powders under a range of processing conditions. The rates of cohesion development were then explained through application of the theories of viscous flow causing bridge formation between particles and the glass transition behaviour of amorphous lactose. 2. Theory In powders containing amorphous sugars, the formation of bridges between particles can be caused by the

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transformation of the amorphous surface of the powder into a rubber state, by exceeding the glass transition temperature (Peleg, 1993). A number of models have been put forward to explain/describe liquid bridging. Liquid bridging can be considered analogous to the fusion that occurs during sintering (Downton et al., 1982). Frenkel’s (1945) equation has been shown to be valid for describing the fusion process of spherical polymer beads of uniform size (Rosenzweig & Narkis, 1980, 1981—both cited in Wallack & King, 1988). Frenkel’s equation (Eq. (1)) relates the interparticle bridge radius with the time for coalescence of particles due to surface energy driven viscous flow: r2b ¼

3Rp st , 2m

(1)

where rb is the bridge radius (m), Rp the particle radius (m), s the surface tension of the bridge material (N), t the exposure or contact time (s) and m the viscosity of the bridge material (Pa s). Using this equation Downton et al. (1982) predicted the critical viscosity for sticking of a 7:1 w/w sucrose/ fructose mixture in a short time period (1–10 s) to be between 106 and 108 Pa s. This critical viscosity range was confirmed experimentally using the sticky point test of Lazar, Brown, Smith, Wong, and Lindquist (1956). Using a very similar model, Wallack and King (1988) showed that for coffee extract powder and a maltodextrin/sucrose/fructose mixture, the predicted range of critical viscosities for sticking in a short time period was the same as those found by Downton et al. (1982). Eq. (1) shows that increased contact time and higher surface tension increases the bridge size, while greater viscosity or smaller particle sizes decrease the size of the liquid bridge built up in a given time. Stickiness was found to be related to moisture content, in that increasing the moisture content decreases the temperature at which powders become sticky (Tsourouflis et al., 1976; Downton et al., 1982; Wallack & King, 1988). The increased moisture lowers the glass transition temperature T g and therefore the temperature at which viscous flow occurs. Williams, Landel, and Ferry (1955) proposed the Williams–Landel–Ferry (WLF) equation (Eq. (2)) to relate the relaxation time of mechanical properties in amorphous materials to the temperature above glass transition. This equation has been used to describe the temperature dependence of viscosity for sugar solutions above the glass transition temperature (Soesanto & Williams, 1981; Downton et al., 1982). log

C 1 ðT
T g Þ m , ¼ mg C 2 þ ðT
T g Þ

(2)

where m is the viscosity of rubber amorphous material (Pa s), mg the viscosity at the glass transition point (Pa s),

C 1 a dimensionless constant, C 2 a constant (1C), T the temperature (1C) and T g the glass transition temperature (1C). Both Soesanto and Williams (1981) and Downton et al. (1982) found that the WLF model agreed well with experimentally measured viscosities of sugar solutions. If the rate of sticking is then limited by the viscosity as suggested by the Frenkel equation, it follows that stickiness development will depend only on T
T g and not on the actual temperature and moisture conditions the powder experiences directly. Roos and Karel (1991), Bhandari, Datta, and Howes (1997) and Dumoulin and Bimbenet (1998) have used the concept of the viscosity of the amorphous powders decreasing as the value of T
T g increases to describe the sticking behaviour of amorphous powders. The strength of a liquid bridge is proportional to the cross-sectional area ðpr2b Þ of that bridge. Therefore, when considering the Frenkel equation, strength is proportional to Dr2b =t: The Frenkel equation can be rearranged for r2b =t (Eq. (3)): r2b 3Rp s . ¼ 2m t

(3)

Rearranging the WLF equation for m; substituting into Eq. (3) and taking the log gives Eq. (4); !
2 rb C 1 ðT
T g Þ 3Rp s log . (4) þ ¼ log 2mg C 2 þ ðT
T g Þ t If the mechanism can be described by the above equation, then plotting logðr2b =tÞ versus ðT
T g Þ=ðC 2 þ T
T g Þ should yield a straight line with a slope equal to C 1 and an intercept equal to log½3Rp s=2mg : The theory suggests that the rate of development of stickiness in amorphous powders should be the above function of T
T g : Stickiness tests, such as the ‘‘stickypoint temperature’’ test (Lazar et al., 1956; Downton et al., 1982; Wallack & King, 1988; Chuy & Labuza, 1994), a viscometer inserted into a small fluidised bed of powder (Brooks, 2000; Hennigs, Kockel, & Langrish, 2001; Kockel, Allen, Hennigs, & Langrish, 2002; Ozmen & Langrish, 2002), observing the end of fluidisation in the bed (Toy, 2000; Thompson, Havea, & Pearce, 2001), the ‘‘surface caking temperature’’ ðT sc Þ (Chuy & Labuza, 1994), the caking index as used by Aguilera et al. (1995), the flowability as used by Aguilera et al. (1995), or the angle of repose (Aguilera et al., 1995), cannot easily measure the development of stickiness with time. Peleg (1993) also used the Frenkel equation to demonstrate the time-dependent nature of stickiness development. These tests measure a point where the particles become sticky enough to overcome the forces that the test procedure applies to separate or keep the particles apart. They do not enable the progression of the

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3. Experimental methodology 3.1. Powder production and preconditioning Amorphous lactose was produced on a laboratoryscale spray drier (Anhydro Lab S.1) with an inlet air temperature of 200 1C and an outlet air temperature of 96 1C. No crystallinity could be seen in the recovered powder using a polarising microscope and the sample showed 100% amorphous lactose content using the gravimetric adsorption method of O’Donnell (1998). A sample of the amorphous lactose was first humidified in an expanded bed on an air supply rig (O’Donnell, Bronlund, Brooks, & Paterson, 2002). This took place for at least 24 h, and the bed was agitated at regular intervals. The sample was then removed and sealed in an airtight container; any headspace in the container was purged with the same air used to humidify the sample. The container was sealed in a plastic bag purged with the same air to further reduce the possibility of exposing the sample to ambient air. The moisture content of the amorphous lactose was determined by oven drying (120 1C for 24 h). The water activity was measured using a Hy-Cal RH sensor, calibrated against saturated salt solutions, inserted into the air space of the sealed plastic bag holding the powder and left until a constant reading was obtained. Brooks (2000) considered the literature values for T g and found much variation even at supposedly the same water activities. Many of the differences were traced to differences in scanning rates and/or differences in the way the moisture contents were measured. The most consistent literature data for T g of amorphous lactose as a function of water activity was collated and is shown here in Fig. 1 (Brooks, 2000). The cubic equation fitted by Brooks (2000), shown below as Eq. (5), was used to

estimate the glass transition temperature from the measured water activity of the amorphous lactose powders. T g ¼
530:66ðaw Þ3 þ 652:06ðaw Þ2
366:33aw þ 99:458 ð0oaw o0:575Þ.

ð5Þ

The relevant humidity of the air supply was set to match the water activity of the powder and the temperature of the air was set to give the desired ðT
T g Þ: 3.2. Stickiness determination using the blow tester The blow test rig and method are described in Paterson et al. (2001) and the blow tester is shown schematically in Fig. 2. The measured variable was the air flow value (L min
1) which resulted in a channel being carved into the bed. When the endpoint resulting in a measurement occurred at low flow rates, a large broad hole was usually formed. As the powder got stickier, and higher flow rates were required, the channels in the bed become narrower and shallower. The order that the bed segments for subsequent measurement within the given experimental conditions were used was randomised to eliminate any possible error that could result from performing measurements in adjacent segments.

Roos and Karel 1990

sinusoidal fit

Jouppila and Roos 1994

quadratic fit

Lloyd et al 1996

cubic fit

120

100

80 Tg (˚C)

formation of liquid bridges to be followed with time. Some of the tests involve exposing the powders to ambient conditions during the test, which could influence the results. For these reasons a new method using a blow test was developed. The blow tester, in its multiple test form, allows one sample to be subjected to a constant set of air conditions (temperature and relative humidity) and the forces holding the particles together are measured incrementally with time (Brooks, 2000; Paterson, Brooks, & Bronlund, 2001; Foster, 2002; Foster, Bronlund, & Paterson, 2005). In principle, the tester exposes the powder to a constant gentle downward flowing air flow during the whole period of the experiment; at specified time intervals the air flow through a blow nozzle is started and increased to a point at which a visible channel is blown in the powder bed. The flow rate required to form the channel is taken as the empirical measurement of stickiness.

515

60

40

20

0 0

0.1

0.2

0.3

0.4

0.5

0.6

Water activity

Fig. 1. Modelling of the glass transition temperature of amorphous lactose literature data for predicting the glass transition temperature as a function of water activity. – – sinusoidal fit (Roos and Karel, 1990), - - - quadratic fit (Jouppila and Roos, 1994), —— cubic fit (Lloyd et al., 1996).

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at the same conditions show that the blow test is subject to some variations, but that it is reproducible in terms of general stickiness trends. Fig. 4 shows the raw results for several different experiments at a T
T g of approximately 10 1C. While there is a degree of scatter amongst the data, the results from the different runs show similar slopes when the nature of the blow test and the uncertainty in estimating T g are considered. Fig. 5 shows the results obtained at T
T g  1 1C from experiments performed at 59, 49 and 31 1C. Inspection of this graph shows that at a temperature very close to T g ; little increase in stickiness was observed by the blow tester over the time of the experiment. The initial flow rate values of 3–3.5 L min
1 correspond to Fig. 2. Schematic diagram of blow tester used in this work. 18 T-Tg=12.3, 45.4˚C, 30.7%RH [Rep.1]

16

T-Tg=12.3, 45.4˚C, 30.7%RH, [Rep.2]

-1

Blow test endpoint [L.min ]

Once a channel in the selected segment was observed, the ball valve in the air line was turned off, immediately cutting off the supply of air. The flow meter and stopwatch readings were recorded on video as it was too difficult to observe simultaneously the endpoint of the experiment, the exact time of the measurement and the flow rate at the endpoint. The maximum flow rate before the air flow was cut off was recorded as the measured variable for the level of stickiness achieved at that time. Flow rates were measured to 0.5 L min
1, and time to the nearest second. The experiment lasted for approximately 7.5 h, with two replicate blow test readings being taken at each time.

14 12 10 8 6 4 2 0 0

100

200

300

400

500

Time (min)

18

T-Tg=9.1, 37.3˚C, 35.2%RH T-Tg=12.3, 45.4˚C, 30.7%RH T-Tg=12.3, 45.4˚C, 30.7%RH T-Tg=11.2, 68.8˚C, 14.9%RH T-Tg=11.4, 40.4˚C, 34.4%RH

16 -1

The air flow required to blow a channel in the powder, at a particular point in time, has been taken as a direct measurement of the stickiness of the powder at that time. Fig. 3 shows a comparison of two replicate runs. The replicate measurements made at each time during each run are also shown. The gradual increase in air flow rate required to blow a channel in the powder bed as the time the bed spends above T g increases, demonstrates that stickiness is a time-dependent phenomenon. It confirms that the bridges between the particles take time to develop. Fig. 3 also shows that while the results are very similar, there were some variations in the observation between replicates. Possible reasons for the scatter about the lines include imperfections in the bed surface and fluctuations in the conditions of the experiment. The intercept differences indicate that there was significant variation in the powder exposure times before the start of the experiment, while the differences in slopes probably represent the differences caused by other factors such as packing density. Overall, the two runs

Fig. 3. Blow test endpoints for a bed of amorphous lactose exposed to T
T g ¼ 12:3 1C conditions plotted against time of exposure. Both runs were performed under near-identical conditions.

Blow test endpoint [L.min ]

4. Results and discussion

14 12 10 8 6 4 2 0 0

100

200

300

400

500

Time [min]

Fig. 4. Blow test endpoints for a bed of amorphous lactose exposed to T
T g ¼ 10 1C conditions induced in different ways and plotted against the time of exposure.

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T-Tg=21.9, 70.4˚C, 19.7%RH

20 18

14 -1

Blow test endpoint [L.min ]

Blow test endpoint [L.min-1 ]

22

T-Tg=1.2, 58.8˚C, 14.9%RH T-Tg=0.5, 49.0˚C, 19.7%RH T-Tg=0.8, 31.3˚C, 33%RH

16

517

12 10 8 6

T-Tg=0.8 T-Tg=1.2

4 T-Tg=0.5

2

16 14 12 10 8 6 4

0 0

100

200

300

400

500

Time [min]

Fig. 5. Blow test endpoints for a bed of amorphous lactose exposed to T
T g ¼ 1 1C conditions induced in different ways and plotted against the time of exposure.

2 0 0

50

100

150

200

250

300

350

400

Time [min] Fig. 6. Blow test endpoints for a bed of amorphous lactose exposed to a T
T g ¼ 22 1C condition over time.

free flowing powder, as measured with amorphous lactose powder held below its T g : Towards the later stages of the runs, the experiments at 59 and 31 1C showed small increases to 5–6 L min
1 in air flow rate required to blow a channel in the bed, indicating that some liquid bridge formation was occurring, even at these very low T
T g values. The statistical student ‘‘t’’-test showed that the slopes of the fitted least-squares linear lines were significantly different from zero ðPp0:05Þ: Fig. 6 shows the results of an experiment performed at a T
T g of 21.9 1C at 70 1C and 20% RH. Compared with the experiments at T
T g  10 1C; this experiment started at a higher blow test reading of about 7 L min
1 after the T
T g condition had been reached, and the powder became sticky at a faster rate. The slope of the least-squares regression linear line (0.039) was significantly ðPp00001Þ different from the average slope of 0.0192 for the T
T g ¼ 10 1C: Attempts to measure the rate of stickiness development when the T
T g exceeded 25 1C were unsuccessful, as the stickiness developed so fast that the blow tester could not blow a channel in the powder bed at any air flow rate by the time the rig had been reassembled (approximately 30 s). Figs. 4 and 5 show experiments at the same approximate T
T g values (10 and 1 1C, respectively). These T
T g values were obtained using a variety of different temperature and RH conditions. At low T
T g values, data obtained at 59 1C and 15% RH [T
T g ¼ 1:2 1C] closely follow data obtained at 31 1C and 33% RH [T
T g ¼ 0:8 1C]. At higher T
T g values, data obtained at 69 1C and 15% RH [T
T g ¼ 11:2 1C] show reasonable agreement with the rest of the data, obtained at temperatures from 37 to 45 1C and 31 to 35% RH.

The general agreement of the experiments at different temperature and humidity conditions, but with similar T
T g values, demonstrates that the main factor in determining the sticking behaviour of amorphous lactose is the magnitude of the temperature increase above T g : If the initial rate of change in caking strength can be considered to describe the rate at which liquid bridges form between particles ðr2b =tÞ; then the rate of sticking obtained from Figs. 3 to 6 can be used to describe ðr2b =tÞ: Eq. (4) suggests that a plot of the log (rate of sticking) versus ðT
T g Þ=ðC 2 þ T
T g Þ should be a straight line. This is shown in Fig. 7. The values for C 1 and C 2 obtained are those that gave the best fit of the data to the Frenkel/WLF model when using a least-squares error fitting procedure. The value for C 1 obtained was 1.6, which is different from the universal constant of
17.44 proposed by Williams et al. (1955). The constant C 2 was found to be 3.5 1C, which also differs from the universal constant of 51.6. The strong linear fit ðR2 ¼ 0:9647Þ obtained indicates that the general viscositybased mechanism used for the Frenkel model and the WLF model describes the amorphous lactose system well with the specific constants C 1 and C 2 : Figs. 3–6 show that it takes a different amount of time for a given level of stickiness to develop within a powder bed under different environmental conditions. In order to relate the blow tester results to practical observations, Table 1 was compiled, which describes the extent of stickiness in the powder at different blow test endpoint levels. Our results support the notion that it is the magnitude of difference by which the temperature of the powder exceeds T g rather than how it exceeds the T g ; (that is, it

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-2

log(Rate of stickiness development) [L.min ]

0

-0.5

-1

-1.5

-2

-2.5

-3 0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

(T-Tg)/(C2+T-Tg)

Fig. 7. The log of the rate of stickiness development, as measured by the slope of the blow test lines, plotted against ðT
T g Þ=ðC 2 þ T
T g Þ:

Table 1 Observations of powder condition at endpoint of the blow test Flow rate at test endpoint (L min
1) 0–3.5 4–6.5 7–9.5 10–12.5 13–15.5 16–18.5 19–22

Observations Free flowing powder. Some binding between particles, but easily disturbed. When disturbed powder has no lumps. When disturbed powder forms lumps which are fragile and easy to break up. More difficult to disturb. Larger lumps with more strength formed. Hard to break up bed, lumps have significant strength. Lumps are larger (whole segments) and difficult to crush. Limit of tester. Bed very difficult to break up. Very hard large lumps formed.

does not matter what combination of RH and temperature are used to achieve the T
T g value) that determines the rate of stickiness development. This is important for understanding how storage and processing conditions impact on the level of stickiness experienced. Our results confirm that conditions that can cause powders to stick during storage can be tolerated during processing where there is insufficient time for the sticky bridges to form. This work has shown that for pure amorphous lactose powders, T g must be exceeded by 25 1C for the powder to be instantaneously sticky and hence causing the powder to stick during processing. However, values of temperature even just above T g will be sufficient to cause the same powder to cake during storage.

lactose can be characterised by the magnitude of difference the powder exceeds T g ; regardless of which combination of water activity and temperature are used to achieve this critical T
T g level. Amorphous lactose, at the same value of T
T g but at different temperatures and water activities, showed similar relationships between the level of cohesiveness of the powder with time. Amorphous lactose became stickier much faster at higher values of T
T g : It was found that at conditions higher than T
T g ¼ 25 1C; pure amorphous lactose became very sticky almost instantaneously. Even very short contact times, such as those experienced in an industrial fluidised bed drier, are sufficient to lead to particles sticking together under these conditions for pure amorphous lactose powders.

5. Conclusion Acknowledgement The multiple reading blow test allows the development of stickiness with time over a wide range of rates to be followed. The sticking behaviour of amorphous

We thank the New Zealand Dairy Board for the funding they provided for this project.

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