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Stickiness curves of high fat dairy powders using the particle gun
ARTICLE IN PRESS
International Dairy Journal 17 (2007) 998–1005 www.elsevier.com/locate/idairyj
Stickiness curves of high fat dairy powders using the particle gun Anthony H. Paterson, Jenny Y. Zuo, John E. Bronlund, Rajesh Chatterjee Institute of Technology and Engineering, Massey University, Private Bag 11-222, Palmerston North, New Zealand Received 29 May 2006; accepted 9 November 2006
Abstract High fat (442%) dairy powders are inherently sticky due to their high levels of liquid surface fat. Incorrect operating conditions when spray drying these powders can rapidly lead to blockages. The particle gun was used to characterise the stickiness curves of high fat cream and cheese powders. Stickiness was shown to increase with increasing temperature to a maximum at 50 1C after which it decreased until no stickiness was observed above 68 1C. A dramatic increase in stickiness for the powders was found when the relative humidity of the air was increased past a certain critical point for each temperature. This was attributed to the lactose component of the powder exceeding its glass transition temperature by a critical amount. Best estimates of the (T Tg)crit. values for White Cheese Powder, Low Fat Cream Powder and High Fat Cream Powder were 28, 37 and 38 1C, respectively. r 2006 Elsevier Ltd. All rights reserved. Keywords: High fat dairy powders; Stickiness; Spray drying; Glass transition temperature; Caking; Lactose
1. Introduction Stickiness and caking of dairy powders is a significant problem during industrial scale spray drying operations. High-fat powders such as cream and cheese powders are particularly problematic to dry. The characterisation of powder stickiness as a function of temperature and relative humidity (RH) and identification of the underlying mechanisms of stickiness development offer considerable assistance toward the optimal operation of commercial spray driers for these products. Two main stickiness/caking mechanisms for dairy powders were identified by Foster, Bronlund, and Paterson (2005a); (1) powders that contain more than 42% total fat (greater than 1.95 g m 2 surface fat content), when exposed to a temperature above 40 1C where the fat becomes completely molten, form liquid bridges between the adjacent powder particles or between the particles and other surfaces upon contact; (2) when the particle temperature sufficiently exceeds the glass transition temperature of the amorphous sugar present in the powder, the amorphous sugar is able to flow far Corresponding author. Tel.: +64 6 350 5241; fax: +64 6 350 5604.
E-mail address: [email protected] (A.H. Paterson). 0958-6946/$ - see front matter r 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.idairyj.2006.11.001
enough to form liquid bridges between other particles or contacted surfaces. The surface composition of dairy powders has been studied by several authors (Fa¨ldt, Bergensta˚hl, & Carlsson, 1993; Fa¨ldt & Sjo¨holm, 1996; Kim, Chen, & Pearce, 2002) who have shown that the fat is disproportionately represented at the surface, with the fat content of skim milk, whole milk and cream powders having more fat on the surface than the average bulk composition. They used the electron spectroscopy for chemical analysis (ESCA) method to estimate the amount of surface fat on the powder in conjunction with a free fat extraction method. The ESCA analysis provides direct information about the powder surface independent of particle size and melting point of fat phase. In addition, the powder surface coverage of other components such as protein and carbohydrate are also detectable. Both the surface fat extraction and ESCA methods gave similar results. For powder with a higher fat content, the surface fat coverage was high and so was the free fat. Surface fat content was shown to be correlated to the total fat content and specific surface area by Foster et al. (2005a) for a range of dairy powders. The data were expressed in terms of the specific surface area in order to consider the particle size effects in the free fat extraction
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analysis. Foster et al. (2005a) used the blow test method to measure the stickiness caused by fat in dairy powders and demonstrated that significant levels of caking only occurred when the surface fat content was above 1.95 g m 2, corresponding to 42% total fat. These findings were confirmed empirically in that industrial spray drier operators consulted found that dairy powders with fat contents greater than 42% total fat were inherently more difficult to process and had lower run times. The blow tester measures the stickiness of particle to particle contact when the particles are sitting in a bed. This is quite different from the conditions the powders experience during manufacture in a spray drier where they travel through ducting at approximately 20 m s 1. Alternative tests that have been used for measuring the stickiness of dairy powders include the fluidised bed (Chatterjee, 2004; Toy, 2000), a cyclone tester (Boonyai, Bhandari, & Howes, 2004), the sticky-point temperature measured with a stirred cell (Downton, Flores-Luna, & King, 1982; Chuy & Labuza, 1994; Hennigs, Kockel, & Langrish, 2001; Wallack & King, 1988) and the particle gun (Chatterjee, 2004; Zuo, 2004; Zuo, Paterson, Bronlund, & Chatterjee, 2007). These tests generally either increase the temperature of a powder with fixed moisture content or increase the RH of the air being used to contact the particles at a constant temperature until the end point of the test is identified as the point where the particles stick together. The exception is the particle gun, which measures the percentage of particles fed into the apparatus that stick to a target plate when the powder is impacted against it. This is repeated at increasing values of air RH at a constant temperature. With this device it is possible to look at the increase in level of stickiness of a powder in conditions that exceed the end points measured with the other methods. By plotting the end point (of whichever test) as a function of temperature and RH or water activity of the powder, it is possible to produce a ‘‘stickiness curve’’ which shows the area of operation where the dairy powders can be considered sticky and the area where it should be safe to operate (Zuo et al., 2007). For the particle gun, the point at which the deposition starts to build has been taken as the sticky point of the powder and has been used to generate the stickiness curves of various dairy powders with low fat contents (o42%) (Zuo et al., 2007). Paterson, Bronlund, Zuo, and Chatterjee (2006) have shown that this sticky point can be characterised for a given set of air conditions for a particular powder by using the concept of a critical amount by which the glass transition temperature is exceeded by (T Tg)crit. In previous work on low fat powders, (T Tg)crit was found to be constant for a particular powder regardless of the temperature and RH conditions used to generate the sticky point (Foster, Bronlund, & Paterson, 2005b; Paterson, Bronlund, & Brooks, 2001; Paterson, Brooks, Foster, & Bronlund, 2005; Paterson et al., 2006; Zuo et al., 2007). This test shows that when the powders are sticky enough for them to adhere to the target plate, they are obviously sticky enough
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for more particles to build on the particles already adhered to the plate. Thus the particle gun measurement is a lumped parameter, combining both adhesion and cohesion. Attempts were made to use the fluidised bed with the high fat powders, but it was impossible to fluidise the beds at temperatures greater than 30 1C due to the surface fat stickiness mechanism. It was anticipated that the same problem would exist for the stirred cell and cyclone methods, so the particle gun method was chosen for this investigation. This paper outlines the generation of stickiness curves for three high fat dairy powders and uses both the stickiness curve and (T Tg)crit approaches given in Paterson et al. (2006) to analyse the data.
2. Materials and methods Commercially produced White Cheese Powder (WCP) and High and Low Fat Cream Powder (HFCP, LFCP) samples and their compositions were provided by Fonterra New Zealand Ltd. Bulk composition values specified by the supplier are summarised in Table 1. WCP A was used for the high-temperature work and WCP B was used for the low-temperature work. Retention samples were used, so supplies of powder were limited and replicate runs could not be conducted. The particle-gun rig (Fig. 1) was developed to measure the point at which a powder becomes instantaneously sticky by changing the air conditions to which the powder was exposed to as it was fired onto a collection plate. The particle-gun rig was constructed in two parts: a constant humidity air supply system and the particle feeding system that enables the particle to be ‘‘fired’’ onto the target plate at the desired velocity. Details of the equipment are given in Zuo et al. (2007). Air, at a constant velocity of 20 m s 1 and constant RH and temperature was passed through the particle gun. Up to 25 g of the powder was fed by hand into the feed funnel at the top of the gun from where it was sucked into the barrel by a venturi effect and then ejected at the target plate. The percentage of the fed powder that was collected on the target plate was recorded. Values of less than 0.5% deposition were taken as not being significant. The RH of the air was increased between samples and steady-state conditions were achieved before commencing the next stickiness test. Four different temperatures over the range 60–80 1C were used to correspond to typical spray drier outlet air temperature Table 1 Compositional data for the dairy powders used Milk powder
Fat (%DBa)
Protein (%DB)
Lactose (%DB)
WCP A WCP B LFCP HFCP
42.1 43.0 55.8 71.8
21.2 19.5 16.2 12.3
27.9 29.0 24.4 13.3
a
DB means Dry Basis.
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Fig. 1. Schematic diagram of the particle gun apparatus showing the constant temperature and RH air supply rig with the particle gun on the right.
(a) Deposition on plate (%)
conditions. Work at lower temperatures (30–50 1C) is also reported in this paper. High velocities (20 m s 1) in the particle-gun rig were used, as this is typical of the velocities in industrial cyclones that are in the region of 20–45 m s 1(Masters, 1991). The final temperature and RH of the air were measured at the tip of the gun by a Rotronic HYGRO PALM, a portable humidity and temperature measuring instrument. The probe was calibrated before use against saturated salt solutions.
5.0 29.2 °C 33.7 °C 38.4 °C 42.0 °C 48.6 °C
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3.1. WCP (42.05%TS Fat) The stickiness results for WCP (Figs. 2(a) and (b)) were similar to those observed by Zuo et al. (2007) for other dairy powders. There was a small amount of initial deposition at about 0.5–2% for low temperatures and a constant level of about 0.5% for temperatures higher than 60 1C. This was higher than that observed for most low fat dairy powders which typically have 0–0.2% deposition up until the (T Tg)crit point is reached. For a particular air temperature, the measurement was repeated at increased RH values until the powder showed an increase in the fraction adhering to the target plate. The powder became progressively stickier as the RH of the air in the gun was increased further. At the RH values below the sticky point, a constant deposition at temperatures above 60 1C was observed. This is attributed to the surface fat becoming molten in the short time the particles are present in the air stream within the particle gun barrel. However, at temperatures below 50 1C there is a progressive increase in the level of deposition with
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80.5 °C 72.2 °C 68.2 °C 60.5 °C
70.0 Deposition on plate (%)
3. Results and discussion
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Fig. 2. (a) % deposition of White Cheese Powder stuck to the target plate as a function of air temperature and relative humidity at low temperatures (o50 1C). (b) % deposition of White Cheese Powder stuck to the target plate as a function of air temperature and relative humidity at high temperatures (460 1C).
increasing temperature as shown in Fig. 3. This is attributed to the increasing amount of liquid fat available at the surface of the particles. The maximum adhesion,
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Fig. 4. The stickiness curve for White Cheese Powder.
Fig. 3. The effect of temperature on the initial % deposition for White Cheese Powder.
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observed at around 50 1C, is thought to be because after all the surface fat is molten, (the maximum occurs well above the maximum fat melting temperature of 37 1C due to the dynamic heating process the particles are going through as they travel down the barrel) the surface fat then decreases in viscosity as the temperature is further increased, thus reducing the impact of fat bridging related stickiness. It is also evident in Figs. 2(a) and (b), that at a critical RH the powders become very sticky. This phenomena is consistent with the behaviour observed for low-fat powders and can be attributed to the amorphous lactose component of the powder (Zuo et al., 2007). The RH of the air at the point where the RH started to have a significant effect at a particular temperature were identified from Figs. 2(a) and (b) and are plotted in Fig. 4 as the stickiness curve. They were fitted by minimising the sum of the squared residuals using the Solver in Excel. The stickiness curve lies consistently above the Tg line for lactose calculated from the cubic equation for Tg from Paterson et al. (2005). To demonstrate this point the T Tg parameter was used to combine the effect of temperature and RH for the high-temperature results as shown in Fig. 5. The data for the different air temperatures, although more scattered than reported by Zuo et al. (2007) for low fat dairy powders, congregate around a single line with a common initial stickiness point. The initial stickiness point for the WCP was observed at a (T Tg)crit of 27.8 1C. A weakness of this work is that no replicate sets of data were collected due to powder availability. However, a measure of the error is shown by the scatter in the individual experimental points plotted that were independent batches of powder put through the particle gun. The limits of the (T Tg)crit value where the initiation of stickiness was observed were estimated from the 95% CI for the regression line of the % deposition against T Tg in Fig. 5 using MINITAB14. The 95% CI limits for (T Tg)crit were 24–30.5 1C. The low-temperature data, when analysed this way, showed more scatter and it was very difficult to see where the (T Tg)crit point was because the fat mechanism was
80.5 °C 72.2 °C 68.2 °C 60.5 °C
60 50 40 30 20
Slope = 2.255 R2 = 0.7418
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Fig. 5. % deposition on the plate plotted against T Tg for White Cheese powder for high-temperature conditions.
still prominent. Subtracting off the deposition caused by the fat mechanism (i.e. the result of Fig. 3 at each temperature) resulted in Fig. 6 which shows that there is an increase in the slope of the % deposition versus T Tg as temperature is increased up to 50 1C. This effect is evident in Fig. 2(a) but not for the high-temperature results in Fig. 2(b) or Fig. 4. Our interpretation of this is that at the lower temperatures the fat mechanism still accounts for a major percentage of the stickiness mechanism and that this affects the slope of Fig. 6 above the critical (T Tg) point. 3.2. LFCP Fig. 7(a) shows the powder depositions on the target plate are dependent on temperature and RH of the air going through the particle gun for the LFCP. At a constant air temperature and increasing humidity, a point was reached where there was a dramatic increase in the deposition of the powder on the target plate, indicating where there is a change in stickiness mechanism. For each temperature, a similar trend line was observed and the intercept of the trend line with the line of initial deposition was identified to be the start of amorphous lactose related stickiness at that particular temperature and RH condition.
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Adjusted deposition on plate (%)
1002 10 9 8 7 6 5 4 3 2 1 0 -1
29.2 °C 33.7 °C 38.4 °C 42.0 °C 48.6 °C 29.2 °C 33.7 °C 38.4 °C 42.0 °C 48.6 °C
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Fig. 6. % deposition minus the initial fat deposition effect of White Cheese Powder stuck to the target plate as a function of air temperature and relative humidity at low temperatures (o50 1C).
Deposition on plate (%)
(a) 50 80.5 °C 72.2 °C 68.2 °C 60.5 °C
45 40 35 30 25 20 15
the temperature range 60–80 1C. In this temperature range, all the fat on the surface will be molten, and therefore, a constant deposition was observed. This is in contrast with low fat dairy powders, such as whole milk powder or skim milk powder, where the initial deposition was virtually zero as reported in Zuo (2004), Paterson et al. (2006), and Zuo et al. (2007). In these cases, amorphous lactose was the only mechanism responsible for the powder stickiness. With LFCP, both fat and amorphous lactose contribute to the powder stickiness. The lactose mechanism can be distinctively seen in Fig. 8, with the different temperature data being reduced to one line when plotted against T Tg. The critical T Tg was identified to be 36.9 1C, where the powder particles start to stick on the plate over and above the initial deposition. This (T Tg)crit value of 36.9 1C was found under various combinations of temperature and RH conditions. The 95% CI limits for (T Tg)crit were found to be 34.8–38.4 1C from the 95% CI for the fitted line using MINTAB.14 Fig. 9 shows the stickiness curve of Tg+35.9 1C was the best fit from the cubic equation of Tg for amorphous lactose. This value corresponds to the point where there is additional deposition over and above the fat-melting mechanism when the powder was exposed to high process temperatures. The fat mechanism is only
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Fig. 8. % deposition on the plate plotted against T Tg for Low Fat Cream Powder for high-temperature conditions.
Relative humidity (%)
Increasing the temperature reduced the RH at which this point occurred. Since the powder is classified as one of the high-fat powders, it was expected that an initial deposition would be seen. This is shown in Fig. 7(b) when the % deposition scale has been expanded. Fig. 7(b) illustrates the effect of fat on initial deposition. Clearly, an initial deposition of 0.5% has occurred within
100 90 Temperature (°C)
Fig. 7. (a) % deposition of Low Fat Cream Powder stuck to the target plate as a function of air temperature and relative humidity at high temperatures (460 1C). (b) % deposition of Low Fat Cream Powder stuck to the target plate as a function of air temperature and relative humidity at high temperatures (460 1C). Expanded scale.
Stickiness curve
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T-Tg crit. = 35.9 °C
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Tg lactose curve
20 10 0 0
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Realtive humidity (%)
Fig. 9. The stickiness curve for Low Fat Cream Powder.
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dependent on the temperature, not the RH. The (T Tg)crit values obtained from Figs. 8 and 9 are the same within experimental error. They are significantly different from the (T Tg)crit. value obtained for WCP and this is assumed to be due to the
Deposition on plate (%)
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29.1° C
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Fig. 10. (a) % deposition of High Fat Cream Powder stuck to the target plate as a function of air temperature and relative humidity at low temperatures (o50 1C). (b) % deposition of High Fat Cream Powder stuck to the target plate as a function of air temperature and relative humidity at high temperatures (460 1C).
70 65 60 55 50 45 40 35 30 25 20 15 10 5 0
1003
effect of fat on the surface interfering with the lactose stickiness mechanism. 3.3. HFCP Fig. 10(a) shows the results obtained when HFCP was put through the particle gun at low temperatures (o50 1C) and Fig. 10(b) gives the results for high temperatures (460 1C). The initial adhesion observed is due to fat liquid bridging. This effect is highlighted in Fig. 3 where, in a similar manner observed for WCP, it can be seen that the amount of adhesion is affected by the temperature of the air in the particle gun. The powder also goes through a maximum stickiness due to fat liquid bridging at about 50 1C. The volume effect of fat at the surface is evidenced by the increased stickiness of HFCP over both WCP and LFCP. This is because the high fat content in the bulk leads to a higher surface fat content (Buma, 1971a, 1971b; Fa¨ldt & Bergensta˚hl, 1994; Foster et al., 2005a). The amorphous lactose effect is also obvious in Figs. 10(a) and (b). A T Tg plot (Fig. 11) was used to combine the temperature and RH effects on powder deposits for the high-temperature data. The data points are quite spread and the 95% CI limits for (T Tg)crit were found to be 16–29 1C from the 95% CI for the fitted line using MINTAB14. Although the amorphous lactose effect is observed, there was major deposition due to the molten fat mechanism and this has significantly increased the scatter in the data. The molten-fat effect has also distorted the (T Tg)crit point as calculated using the data in Fig. 10 because the lactose effect obviously starts where the RH starts to affect the result but the fitting technique extrapolates the data to the 0% deposition line without taking into account the fat effect. Fig. 12 is the data from Fig. 11 re-plotted with the effect of fat on the deposition subtracted off. Using the 95% CI for the fitted line, the 95% CI for (T Tg)crit was found to be 18–32 1C.
80.5 °C 72.2 °C 68.2 °C 60.5 °C
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Fig. 11. % deposition on the plate plotted against T Tg for High Fat Cream Powder for high temperature conditions (460 1C).
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1004 70
80.5 °C 72.2 °C
60 Adjusted deposition (%)
68.2 °C 60.5 °C
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Fig. 12. % deposition of High Fat Cream powder adjusted by subtracting the average fat deposition from the data at RH values below where the lactose has an effect, for high temperatures (460 1C).
100 Tg-Lactose Inital lactose stickiness Tg+37.6°C
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Fig. 13. The stickiness curve for High Fat Cream Powder.
The points at which the lactose mechanism was initiated can be plotted on a temperature versus RH plot, representing the exit air conditions from a spray drier, in the form of a stickiness curve. This is done in Fig. 13 for HFCP. A least squares of errors approach gives a value of (T Tg)crit for HFCP of 37.6 1C. The curve is not a good fit and this is attributed to the interference of the fat on the lactose stickiness mechanism. 4. Conclusions This work has focused on the stickiness of high fat dairy powders. Molten fat has a significant effect, causing smearing and stickiness problems during processing. A linear relationship between the initial level of % deposition and temperature over the range of 30–50 1C shows the
effect of having more fat available at the surface. At higher temperatures the level of stickiness fell as temperature was increased. This was attributed to the lowering of the viscosity of the surface fat. However, the amorphous lactose effect observed in lowfat powders was still present for all high-fat powders studied in this work. All powders showed an increase in stickiness once a critical RH value, at each air temperature, was exceeded. It has been demonstrated that this effect can be attributed to the amorphous lactose content of the powders and that each powder exhibits a critical (T Tg) value that describes the point of initiation of the lactose stickiness mechanism. These points can be plotted as a stickiness curve for the powder which can be used by the operators to avoid the regions of operation where the powders become much stickier.
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Acknowledgements The authors wish to thank TechNZ and Fonterra for their financial and material support, without which this project could not have been completed. The advice and help of Dr. T. Truong is also acknowledged. References Boonyai, P., Bhandari, B., & Howes, T. (2004). Stickiness measurement techniques for food powders: A review. Powder Technology, 145, 34–46. Buma, T. J. (1971a). Free fat in spray-dried whole milk. 2. An evaulation of methods for the determination of free-fat content. Netherlands Milk and Dairy Journal, 25, 42–52. Buma, T. J. (1971b). Free fat in spray-dried whole milk. 4. Significance of free fat for other properties of practical importance. Netherlands Milk and Dairy Journal, 25, 88–106. Chatterjee, R. (2004). Characterising stickiness of dairy powders. M.E. thesis, Massey University, Palmerston North, New Zealand. Chuy, L. E., & Labuza, T. P. (1994). Caking and stickiness of dairy-based food powders as related to glass transition. Journal of Food Science, 59(1), 43–46. Downton, G. E., Flores-Luna, J. L., & King, C. J. (1982). Mechanism of stickiness in hygroscopic, amorphous powders. Industrial and Engineering Chemistry Fundamentals, 21, 447–451. Fa¨ldt, P., & Bergensta˚hl, B. (1994). The surface composition of spraydried protein lactose powders. Colloid Surface A, 90(2–3), 183–190. Fa¨ldt, P., Bergensta˚hl, B., & Carlsson, G. (1993). The surface coverage of fat on food powders analysed by ESCA (Electron Spectroscopy for Chemical Analysis). Food Structure, 12, 225–234. Fa¨ldt, P., & Sjo¨holm, I. (1996). Characterization of spray-dried whole milk. Milchwissenschaft, 51(2), 88–92.
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Foster, K. D., Bronlund, J. E., & Paterson, A. H. J. (2005a). The contribution of milk fat towards the caking of dairy powders. International Dairy Journal, 15(1), 85–91. Foster, K. D., Bronlund, J. E., & Paterson, A. H. J. (2005b). Glass transition related cohesion of amorphous sugar powders. Journal of Food Engineering, 77(4), 997–1006. Hennigs, C., Kockel, T. K., & Langrish, T. A. G. (2001). New measurements of the sticky behavior of skim milk powder. Drying Technology, 19(3–4), 471–484. Kim, E. H. J., Chen, X. D., & Pearce, D. (2002). On the mechanisms of surface formation and surface compositions of industrial milk powders. Drying Technology, 21(2), 265–278. Masters, K. (1991). Spray drying handbook (5th ed.). Harlow, England: Longman Scientific & Technical. Paterson, A. H. J., Bronlund, J. E., & Brooks, G. F. (2001). The Blow Test for measuring the stickiness of powders. Paper presented at the conference of food engineering. Reno, Nevada, USA. Paterson, A. H. J., Brooks, G. F., Foster, K. D., & Bronlund, J. E. (2005). The development of stickiness in amorphous lactose at constant T Tg levels. International Dairy Journal, 15(5), 513–519. Paterson, A. H. J., Bronlund, J. E., Zuo, J. Y., & Chatterjee, R. (2006). Analysis of particle gun derived dairy powder stickiness curves. International Dairy Journal, in press, doi:10.1016/j.jdairyj.2006. 08.013. Toy, K. I. M. (2000). Stickiness rig: A procedure to characterise powder stickiness. Fonterra, Palmerston North. Wallack, D. A., & King, C. J. (1988). Sticking and agglomeration of hygroscopic, amorphous carbohydrate and food powders. Biotechnology Progress, 4(1), 31–35. Zuo, J. Y. (2004). The stickiness curve of dairy powders. M.Tech. thesis, Massey University, Palmerston North, New Zealand. Zuo, J. Y., Paterson, A. H. J., Bronlund, J. E., & Chatterjee, R. (2007). Using a particle-gun to measure initiation of stickiness of dairy powders. International Dairy Journal, 17(3), 268–273.
International Dairy Journal 17 (2007) 998–1005 www.elsevier.com/locate/idairyj
Stickiness curves of high fat dairy powders using the particle gun Anthony H. Paterson, Jenny Y. Zuo, John E. Bronlund, Rajesh Chatterjee Institute of Technology and Engineering, Massey University, Private Bag 11-222, Palmerston North, New Zealand Received 29 May 2006; accepted 9 November 2006
Abstract High fat (442%) dairy powders are inherently sticky due to their high levels of liquid surface fat. Incorrect operating conditions when spray drying these powders can rapidly lead to blockages. The particle gun was used to characterise the stickiness curves of high fat cream and cheese powders. Stickiness was shown to increase with increasing temperature to a maximum at 50 1C after which it decreased until no stickiness was observed above 68 1C. A dramatic increase in stickiness for the powders was found when the relative humidity of the air was increased past a certain critical point for each temperature. This was attributed to the lactose component of the powder exceeding its glass transition temperature by a critical amount. Best estimates of the (T Tg)crit. values for White Cheese Powder, Low Fat Cream Powder and High Fat Cream Powder were 28, 37 and 38 1C, respectively. r 2006 Elsevier Ltd. All rights reserved. Keywords: High fat dairy powders; Stickiness; Spray drying; Glass transition temperature; Caking; Lactose
1. Introduction Stickiness and caking of dairy powders is a significant problem during industrial scale spray drying operations. High-fat powders such as cream and cheese powders are particularly problematic to dry. The characterisation of powder stickiness as a function of temperature and relative humidity (RH) and identification of the underlying mechanisms of stickiness development offer considerable assistance toward the optimal operation of commercial spray driers for these products. Two main stickiness/caking mechanisms for dairy powders were identified by Foster, Bronlund, and Paterson (2005a); (1) powders that contain more than 42% total fat (greater than 1.95 g m 2 surface fat content), when exposed to a temperature above 40 1C where the fat becomes completely molten, form liquid bridges between the adjacent powder particles or between the particles and other surfaces upon contact; (2) when the particle temperature sufficiently exceeds the glass transition temperature of the amorphous sugar present in the powder, the amorphous sugar is able to flow far Corresponding author. Tel.: +64 6 350 5241; fax: +64 6 350 5604.
E-mail address: [email protected] (A.H. Paterson). 0958-6946/$ - see front matter r 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.idairyj.2006.11.001
enough to form liquid bridges between other particles or contacted surfaces. The surface composition of dairy powders has been studied by several authors (Fa¨ldt, Bergensta˚hl, & Carlsson, 1993; Fa¨ldt & Sjo¨holm, 1996; Kim, Chen, & Pearce, 2002) who have shown that the fat is disproportionately represented at the surface, with the fat content of skim milk, whole milk and cream powders having more fat on the surface than the average bulk composition. They used the electron spectroscopy for chemical analysis (ESCA) method to estimate the amount of surface fat on the powder in conjunction with a free fat extraction method. The ESCA analysis provides direct information about the powder surface independent of particle size and melting point of fat phase. In addition, the powder surface coverage of other components such as protein and carbohydrate are also detectable. Both the surface fat extraction and ESCA methods gave similar results. For powder with a higher fat content, the surface fat coverage was high and so was the free fat. Surface fat content was shown to be correlated to the total fat content and specific surface area by Foster et al. (2005a) for a range of dairy powders. The data were expressed in terms of the specific surface area in order to consider the particle size effects in the free fat extraction
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analysis. Foster et al. (2005a) used the blow test method to measure the stickiness caused by fat in dairy powders and demonstrated that significant levels of caking only occurred when the surface fat content was above 1.95 g m 2, corresponding to 42% total fat. These findings were confirmed empirically in that industrial spray drier operators consulted found that dairy powders with fat contents greater than 42% total fat were inherently more difficult to process and had lower run times. The blow tester measures the stickiness of particle to particle contact when the particles are sitting in a bed. This is quite different from the conditions the powders experience during manufacture in a spray drier where they travel through ducting at approximately 20 m s 1. Alternative tests that have been used for measuring the stickiness of dairy powders include the fluidised bed (Chatterjee, 2004; Toy, 2000), a cyclone tester (Boonyai, Bhandari, & Howes, 2004), the sticky-point temperature measured with a stirred cell (Downton, Flores-Luna, & King, 1982; Chuy & Labuza, 1994; Hennigs, Kockel, & Langrish, 2001; Wallack & King, 1988) and the particle gun (Chatterjee, 2004; Zuo, 2004; Zuo, Paterson, Bronlund, & Chatterjee, 2007). These tests generally either increase the temperature of a powder with fixed moisture content or increase the RH of the air being used to contact the particles at a constant temperature until the end point of the test is identified as the point where the particles stick together. The exception is the particle gun, which measures the percentage of particles fed into the apparatus that stick to a target plate when the powder is impacted against it. This is repeated at increasing values of air RH at a constant temperature. With this device it is possible to look at the increase in level of stickiness of a powder in conditions that exceed the end points measured with the other methods. By plotting the end point (of whichever test) as a function of temperature and RH or water activity of the powder, it is possible to produce a ‘‘stickiness curve’’ which shows the area of operation where the dairy powders can be considered sticky and the area where it should be safe to operate (Zuo et al., 2007). For the particle gun, the point at which the deposition starts to build has been taken as the sticky point of the powder and has been used to generate the stickiness curves of various dairy powders with low fat contents (o42%) (Zuo et al., 2007). Paterson, Bronlund, Zuo, and Chatterjee (2006) have shown that this sticky point can be characterised for a given set of air conditions for a particular powder by using the concept of a critical amount by which the glass transition temperature is exceeded by (T Tg)crit. In previous work on low fat powders, (T Tg)crit was found to be constant for a particular powder regardless of the temperature and RH conditions used to generate the sticky point (Foster, Bronlund, & Paterson, 2005b; Paterson, Bronlund, & Brooks, 2001; Paterson, Brooks, Foster, & Bronlund, 2005; Paterson et al., 2006; Zuo et al., 2007). This test shows that when the powders are sticky enough for them to adhere to the target plate, they are obviously sticky enough
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for more particles to build on the particles already adhered to the plate. Thus the particle gun measurement is a lumped parameter, combining both adhesion and cohesion. Attempts were made to use the fluidised bed with the high fat powders, but it was impossible to fluidise the beds at temperatures greater than 30 1C due to the surface fat stickiness mechanism. It was anticipated that the same problem would exist for the stirred cell and cyclone methods, so the particle gun method was chosen for this investigation. This paper outlines the generation of stickiness curves for three high fat dairy powders and uses both the stickiness curve and (T Tg)crit approaches given in Paterson et al. (2006) to analyse the data.
2. Materials and methods Commercially produced White Cheese Powder (WCP) and High and Low Fat Cream Powder (HFCP, LFCP) samples and their compositions were provided by Fonterra New Zealand Ltd. Bulk composition values specified by the supplier are summarised in Table 1. WCP A was used for the high-temperature work and WCP B was used for the low-temperature work. Retention samples were used, so supplies of powder were limited and replicate runs could not be conducted. The particle-gun rig (Fig. 1) was developed to measure the point at which a powder becomes instantaneously sticky by changing the air conditions to which the powder was exposed to as it was fired onto a collection plate. The particle-gun rig was constructed in two parts: a constant humidity air supply system and the particle feeding system that enables the particle to be ‘‘fired’’ onto the target plate at the desired velocity. Details of the equipment are given in Zuo et al. (2007). Air, at a constant velocity of 20 m s 1 and constant RH and temperature was passed through the particle gun. Up to 25 g of the powder was fed by hand into the feed funnel at the top of the gun from where it was sucked into the barrel by a venturi effect and then ejected at the target plate. The percentage of the fed powder that was collected on the target plate was recorded. Values of less than 0.5% deposition were taken as not being significant. The RH of the air was increased between samples and steady-state conditions were achieved before commencing the next stickiness test. Four different temperatures over the range 60–80 1C were used to correspond to typical spray drier outlet air temperature Table 1 Compositional data for the dairy powders used Milk powder
Fat (%DBa)
Protein (%DB)
Lactose (%DB)
WCP A WCP B LFCP HFCP
42.1 43.0 55.8 71.8
21.2 19.5 16.2 12.3
27.9 29.0 24.4 13.3
a
DB means Dry Basis.
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Fig. 1. Schematic diagram of the particle gun apparatus showing the constant temperature and RH air supply rig with the particle gun on the right.
(a) Deposition on plate (%)
conditions. Work at lower temperatures (30–50 1C) is also reported in this paper. High velocities (20 m s 1) in the particle-gun rig were used, as this is typical of the velocities in industrial cyclones that are in the region of 20–45 m s 1(Masters, 1991). The final temperature and RH of the air were measured at the tip of the gun by a Rotronic HYGRO PALM, a portable humidity and temperature measuring instrument. The probe was calibrated before use against saturated salt solutions.
5.0 29.2 °C 33.7 °C 38.4 °C 42.0 °C 48.6 °C
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3.1. WCP (42.05%TS Fat) The stickiness results for WCP (Figs. 2(a) and (b)) were similar to those observed by Zuo et al. (2007) for other dairy powders. There was a small amount of initial deposition at about 0.5–2% for low temperatures and a constant level of about 0.5% for temperatures higher than 60 1C. This was higher than that observed for most low fat dairy powders which typically have 0–0.2% deposition up until the (T Tg)crit point is reached. For a particular air temperature, the measurement was repeated at increased RH values until the powder showed an increase in the fraction adhering to the target plate. The powder became progressively stickier as the RH of the air in the gun was increased further. At the RH values below the sticky point, a constant deposition at temperatures above 60 1C was observed. This is attributed to the surface fat becoming molten in the short time the particles are present in the air stream within the particle gun barrel. However, at temperatures below 50 1C there is a progressive increase in the level of deposition with
(b) 80.0
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70.0 Deposition on plate (%)
3. Results and discussion
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Fig. 2. (a) % deposition of White Cheese Powder stuck to the target plate as a function of air temperature and relative humidity at low temperatures (o50 1C). (b) % deposition of White Cheese Powder stuck to the target plate as a function of air temperature and relative humidity at high temperatures (460 1C).
increasing temperature as shown in Fig. 3. This is attributed to the increasing amount of liquid fat available at the surface of the particles. The maximum adhesion,
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Fig. 4. The stickiness curve for White Cheese Powder.
Fig. 3. The effect of temperature on the initial % deposition for White Cheese Powder.
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observed at around 50 1C, is thought to be because after all the surface fat is molten, (the maximum occurs well above the maximum fat melting temperature of 37 1C due to the dynamic heating process the particles are going through as they travel down the barrel) the surface fat then decreases in viscosity as the temperature is further increased, thus reducing the impact of fat bridging related stickiness. It is also evident in Figs. 2(a) and (b), that at a critical RH the powders become very sticky. This phenomena is consistent with the behaviour observed for low-fat powders and can be attributed to the amorphous lactose component of the powder (Zuo et al., 2007). The RH of the air at the point where the RH started to have a significant effect at a particular temperature were identified from Figs. 2(a) and (b) and are plotted in Fig. 4 as the stickiness curve. They were fitted by minimising the sum of the squared residuals using the Solver in Excel. The stickiness curve lies consistently above the Tg line for lactose calculated from the cubic equation for Tg from Paterson et al. (2005). To demonstrate this point the T Tg parameter was used to combine the effect of temperature and RH for the high-temperature results as shown in Fig. 5. The data for the different air temperatures, although more scattered than reported by Zuo et al. (2007) for low fat dairy powders, congregate around a single line with a common initial stickiness point. The initial stickiness point for the WCP was observed at a (T Tg)crit of 27.8 1C. A weakness of this work is that no replicate sets of data were collected due to powder availability. However, a measure of the error is shown by the scatter in the individual experimental points plotted that were independent batches of powder put through the particle gun. The limits of the (T Tg)crit value where the initiation of stickiness was observed were estimated from the 95% CI for the regression line of the % deposition against T Tg in Fig. 5 using MINITAB14. The 95% CI limits for (T Tg)crit were 24–30.5 1C. The low-temperature data, when analysed this way, showed more scatter and it was very difficult to see where the (T Tg)crit point was because the fat mechanism was
80.5 °C 72.2 °C 68.2 °C 60.5 °C
60 50 40 30 20
Slope = 2.255 R2 = 0.7418
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Fig. 5. % deposition on the plate plotted against T Tg for White Cheese powder for high-temperature conditions.
still prominent. Subtracting off the deposition caused by the fat mechanism (i.e. the result of Fig. 3 at each temperature) resulted in Fig. 6 which shows that there is an increase in the slope of the % deposition versus T Tg as temperature is increased up to 50 1C. This effect is evident in Fig. 2(a) but not for the high-temperature results in Fig. 2(b) or Fig. 4. Our interpretation of this is that at the lower temperatures the fat mechanism still accounts for a major percentage of the stickiness mechanism and that this affects the slope of Fig. 6 above the critical (T Tg) point. 3.2. LFCP Fig. 7(a) shows the powder depositions on the target plate are dependent on temperature and RH of the air going through the particle gun for the LFCP. At a constant air temperature and increasing humidity, a point was reached where there was a dramatic increase in the deposition of the powder on the target plate, indicating where there is a change in stickiness mechanism. For each temperature, a similar trend line was observed and the intercept of the trend line with the line of initial deposition was identified to be the start of amorphous lactose related stickiness at that particular temperature and RH condition.
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Adjusted deposition on plate (%)
1002 10 9 8 7 6 5 4 3 2 1 0 -1
29.2 °C 33.7 °C 38.4 °C 42.0 °C 48.6 °C 29.2 °C 33.7 °C 38.4 °C 42.0 °C 48.6 °C
-20
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Fig. 6. % deposition minus the initial fat deposition effect of White Cheese Powder stuck to the target plate as a function of air temperature and relative humidity at low temperatures (o50 1C).
Deposition on plate (%)
(a) 50 80.5 °C 72.2 °C 68.2 °C 60.5 °C
45 40 35 30 25 20 15
the temperature range 60–80 1C. In this temperature range, all the fat on the surface will be molten, and therefore, a constant deposition was observed. This is in contrast with low fat dairy powders, such as whole milk powder or skim milk powder, where the initial deposition was virtually zero as reported in Zuo (2004), Paterson et al. (2006), and Zuo et al. (2007). In these cases, amorphous lactose was the only mechanism responsible for the powder stickiness. With LFCP, both fat and amorphous lactose contribute to the powder stickiness. The lactose mechanism can be distinctively seen in Fig. 8, with the different temperature data being reduced to one line when plotted against T Tg. The critical T Tg was identified to be 36.9 1C, where the powder particles start to stick on the plate over and above the initial deposition. This (T Tg)crit value of 36.9 1C was found under various combinations of temperature and RH conditions. The 95% CI limits for (T Tg)crit were found to be 34.8–38.4 1C from the 95% CI for the fitted line using MINTAB.14 Fig. 9 shows the stickiness curve of Tg+35.9 1C was the best fit from the cubic equation of Tg for amorphous lactose. This value corresponds to the point where there is additional deposition over and above the fat-melting mechanism when the powder was exposed to high process temperatures. The fat mechanism is only
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Fig. 8. % deposition on the plate plotted against T Tg for Low Fat Cream Powder for high-temperature conditions.
Relative humidity (%)
Increasing the temperature reduced the RH at which this point occurred. Since the powder is classified as one of the high-fat powders, it was expected that an initial deposition would be seen. This is shown in Fig. 7(b) when the % deposition scale has been expanded. Fig. 7(b) illustrates the effect of fat on initial deposition. Clearly, an initial deposition of 0.5% has occurred within
100 90 Temperature (°C)
Fig. 7. (a) % deposition of Low Fat Cream Powder stuck to the target plate as a function of air temperature and relative humidity at high temperatures (460 1C). (b) % deposition of Low Fat Cream Powder stuck to the target plate as a function of air temperature and relative humidity at high temperatures (460 1C). Expanded scale.
Stickiness curve
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T-Tg crit. = 35.9 °C
30
Tg lactose curve
20 10 0 0
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Fig. 9. The stickiness curve for Low Fat Cream Powder.
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dependent on the temperature, not the RH. The (T Tg)crit values obtained from Figs. 8 and 9 are the same within experimental error. They are significantly different from the (T Tg)crit. value obtained for WCP and this is assumed to be due to the
Deposition on plate (%)
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Fig. 10. (a) % deposition of High Fat Cream Powder stuck to the target plate as a function of air temperature and relative humidity at low temperatures (o50 1C). (b) % deposition of High Fat Cream Powder stuck to the target plate as a function of air temperature and relative humidity at high temperatures (460 1C).
70 65 60 55 50 45 40 35 30 25 20 15 10 5 0
1003
effect of fat on the surface interfering with the lactose stickiness mechanism. 3.3. HFCP Fig. 10(a) shows the results obtained when HFCP was put through the particle gun at low temperatures (o50 1C) and Fig. 10(b) gives the results for high temperatures (460 1C). The initial adhesion observed is due to fat liquid bridging. This effect is highlighted in Fig. 3 where, in a similar manner observed for WCP, it can be seen that the amount of adhesion is affected by the temperature of the air in the particle gun. The powder also goes through a maximum stickiness due to fat liquid bridging at about 50 1C. The volume effect of fat at the surface is evidenced by the increased stickiness of HFCP over both WCP and LFCP. This is because the high fat content in the bulk leads to a higher surface fat content (Buma, 1971a, 1971b; Fa¨ldt & Bergensta˚hl, 1994; Foster et al., 2005a). The amorphous lactose effect is also obvious in Figs. 10(a) and (b). A T Tg plot (Fig. 11) was used to combine the temperature and RH effects on powder deposits for the high-temperature data. The data points are quite spread and the 95% CI limits for (T Tg)crit were found to be 16–29 1C from the 95% CI for the fitted line using MINTAB14. Although the amorphous lactose effect is observed, there was major deposition due to the molten fat mechanism and this has significantly increased the scatter in the data. The molten-fat effect has also distorted the (T Tg)crit point as calculated using the data in Fig. 10 because the lactose effect obviously starts where the RH starts to affect the result but the fitting technique extrapolates the data to the 0% deposition line without taking into account the fat effect. Fig. 12 is the data from Fig. 11 re-plotted with the effect of fat on the deposition subtracted off. Using the 95% CI for the fitted line, the 95% CI for (T Tg)crit was found to be 18–32 1C.
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Fig. 11. % deposition on the plate plotted against T Tg for High Fat Cream Powder for high temperature conditions (460 1C).
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80.5 °C 72.2 °C
60 Adjusted deposition (%)
68.2 °C 60.5 °C
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Fig. 12. % deposition of High Fat Cream powder adjusted by subtracting the average fat deposition from the data at RH values below where the lactose has an effect, for high temperatures (460 1C).
100 Tg-Lactose Inital lactose stickiness Tg+37.6°C
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Fig. 13. The stickiness curve for High Fat Cream Powder.
The points at which the lactose mechanism was initiated can be plotted on a temperature versus RH plot, representing the exit air conditions from a spray drier, in the form of a stickiness curve. This is done in Fig. 13 for HFCP. A least squares of errors approach gives a value of (T Tg)crit for HFCP of 37.6 1C. The curve is not a good fit and this is attributed to the interference of the fat on the lactose stickiness mechanism. 4. Conclusions This work has focused on the stickiness of high fat dairy powders. Molten fat has a significant effect, causing smearing and stickiness problems during processing. A linear relationship between the initial level of % deposition and temperature over the range of 30–50 1C shows the
effect of having more fat available at the surface. At higher temperatures the level of stickiness fell as temperature was increased. This was attributed to the lowering of the viscosity of the surface fat. However, the amorphous lactose effect observed in lowfat powders was still present for all high-fat powders studied in this work. All powders showed an increase in stickiness once a critical RH value, at each air temperature, was exceeded. It has been demonstrated that this effect can be attributed to the amorphous lactose content of the powders and that each powder exhibits a critical (T Tg) value that describes the point of initiation of the lactose stickiness mechanism. These points can be plotted as a stickiness curve for the powder which can be used by the operators to avoid the regions of operation where the powders become much stickier.
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Acknowledgements The authors wish to thank TechNZ and Fonterra for their financial and material support, without which this project could not have been completed. The advice and help of Dr. T. Truong is also acknowledged. References Boonyai, P., Bhandari, B., & Howes, T. (2004). Stickiness measurement techniques for food powders: A review. Powder Technology, 145, 34–46. Buma, T. J. (1971a). Free fat in spray-dried whole milk. 2. An evaulation of methods for the determination of free-fat content. Netherlands Milk and Dairy Journal, 25, 42–52. Buma, T. J. (1971b). Free fat in spray-dried whole milk. 4. Significance of free fat for other properties of practical importance. Netherlands Milk and Dairy Journal, 25, 88–106. Chatterjee, R. (2004). Characterising stickiness of dairy powders. M.E. thesis, Massey University, Palmerston North, New Zealand. Chuy, L. E., & Labuza, T. P. (1994). Caking and stickiness of dairy-based food powders as related to glass transition. Journal of Food Science, 59(1), 43–46. Downton, G. E., Flores-Luna, J. L., & King, C. J. (1982). Mechanism of stickiness in hygroscopic, amorphous powders. Industrial and Engineering Chemistry Fundamentals, 21, 447–451. Fa¨ldt, P., & Bergensta˚hl, B. (1994). The surface composition of spraydried protein lactose powders. Colloid Surface A, 90(2–3), 183–190. Fa¨ldt, P., Bergensta˚hl, B., & Carlsson, G. (1993). The surface coverage of fat on food powders analysed by ESCA (Electron Spectroscopy for Chemical Analysis). Food Structure, 12, 225–234. Fa¨ldt, P., & Sjo¨holm, I. (1996). Characterization of spray-dried whole milk. Milchwissenschaft, 51(2), 88–92.
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Foster, K. D., Bronlund, J. E., & Paterson, A. H. J. (2005a). The contribution of milk fat towards the caking of dairy powders. International Dairy Journal, 15(1), 85–91. Foster, K. D., Bronlund, J. E., & Paterson, A. H. J. (2005b). Glass transition related cohesion of amorphous sugar powders. Journal of Food Engineering, 77(4), 997–1006. Hennigs, C., Kockel, T. K., & Langrish, T. A. G. (2001). New measurements of the sticky behavior of skim milk powder. Drying Technology, 19(3–4), 471–484. Kim, E. H. J., Chen, X. D., & Pearce, D. (2002). On the mechanisms of surface formation and surface compositions of industrial milk powders. Drying Technology, 21(2), 265–278. Masters, K. (1991). Spray drying handbook (5th ed.). Harlow, England: Longman Scientific & Technical. Paterson, A. H. J., Bronlund, J. E., & Brooks, G. F. (2001). The Blow Test for measuring the stickiness of powders. Paper presented at the conference of food engineering. Reno, Nevada, USA. Paterson, A. H. J., Brooks, G. F., Foster, K. D., & Bronlund, J. E. (2005). The development of stickiness in amorphous lactose at constant T Tg levels. International Dairy Journal, 15(5), 513–519. Paterson, A. H. J., Bronlund, J. E., Zuo, J. Y., & Chatterjee, R. (2006). Analysis of particle gun derived dairy powder stickiness curves. International Dairy Journal, in press, doi:10.1016/j.jdairyj.2006. 08.013. Toy, K. I. M. (2000). Stickiness rig: A procedure to characterise powder stickiness. Fonterra, Palmerston North. Wallack, D. A., & King, C. J. (1988). Sticking and agglomeration of hygroscopic, amorphous carbohydrate and food powders. Biotechnology Progress, 4(1), 31–35. Zuo, J. Y. (2004). The stickiness curve of dairy powders. M.Tech. thesis, Massey University, Palmerston North, New Zealand. Zuo, J. Y., Paterson, A. H. J., Bronlund, J. E., & Chatterjee, R. (2007). Using a particle-gun to measure initiation of stickiness of dairy powders. International Dairy Journal, 17(3), 268–273.