- Email: [email protected]
Analysis of particle-gun-derived dairy powder stickiness curves
ARTICLE IN PRESS
International Dairy Journal 17 (2007) 860–865 www.elsevier.com/locate/idairyj
Analysis of particle-gun-derived dairy powder stickiness curves Anthony H. Paterson, John E. Bronlund, Jenny Y. Zuo, R. Chatterjee Institute of Technology and Engineering, Massey University, Private Bag 11-222, Palmerston North, New Zealand Received 28 August 2005; accepted 25 August 2006
Abstract The stickiness curves of a range of dairy powders were measured using a particle-gun rig. The stickiness curves for the powders were shown to run parallel but above the curve of the glass transition temperature (Tg) of amorphous lactose. By assuming that the amorphous lactose at the surface of the powder was in equilibrium with the exit conditions of the air from the particle gun, it was found that for any particular dairy powder sample, the amount of powder deposition measured on the particle-gun target disc collapsed into a single function of the temperature difference by which the amorphous lactose Tg at the surface was exceeded. The x-axis intercept of these plots was calculated and designated as (TTg)crit, characterizing the conditions for initiation of stickiness of the powder. The sensitivity of each powder to stickiness problems when placed in conditions where the critical TTg value at the surface is exceeded was quantified with the slope of the plot. These results show that it is the amorphous lactose component that is probably the main cause of stickiness in dairy powders and demonstrates how the particle-gun rig can be used to characterize the stickiness behaviour of powders over a wide range of conditions with two parameters. r 2006 Elsevier Ltd. All rights reserved. Keywords: Dairy powders; Stickiness tests; Blockages; Spray drying; Glass transition temperature
1. Introduction Sticking in drier exhaust air ducting and cyclone blockages are common problems in industrial driers and can limit spray drier run lengths. In order to run driers at maximum throughput, it is important to identify the point at which powders will become sticky enough to cause blockages. Numerous papers have investigated the powder conditions in which sticking or caking can occur during powder storage (White & Cakebread, 1966; Downton, FloresLuna, & King, 1982; Wallack & King, 1988; Chuy & Labuza, 1994; Jouppila & Roos, 1994a, b; Lloyd, Chen, & Hargreaves, 1996; Roos, Karel, & Kokini, 1996; Jouppila, Kansikas, & Roos, 1997; Hennigs, Kockel, & Langrish, 2001; Paterson, Bronlund, & Brooks, 2001; Paterson, Brooks, Foster, & Bronlund, 2005; Schuck et al., 2005). Bhandari, Datta, and Howes (1997), Hennigs et al. (2001) and Schuck et al. (2005) have used the concept of the 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.2006.08.013
temperature difference by which the glass transition temperature is exceeded (TTg) to describe the sticking behaviour of amorphous powders. Bhandari and Howes (1999) and Bhandari (2001) suggested that the critical viscosity of a sticky powder in sugar-rich foods is reached at a temperature 10–20 1C above the Tg of the sugar mixture and that the temperature of the surface of particles during spray drying should not reach 10–20 1C above the Tg although industrial experience shows that this condition is often exceeded with dairy powders. Keir (2001), Paterson et al. (2001), Foster (2002) and Paterson et al. (2005) used a blow tester to measure the degree of stickiness development with time and demonstrated that the rate of sticking was related to the TTg of the powder. It was not important what relative humidities (RHs) and temperature conditions were used to obtain that TTg value. They also concluded that the higher the TTg value, the faster the rate of sticking development. Under certain RH and temperature combinations the rate of stickiness development was so quick, that it could be regarded as instantaneous.
ARTICLE IN PRESS A.H. Paterson et al. / International Dairy Journal 17 (2007) 860–865
This phenomenon explains why storage-based stickiness experiments provide a much more conservative estimate for prevention of blockages within the drier during operation. The time available in the drier for liquid bridge formation during collisions with walls or other powder particles is short. For this reason in the drier, the powder surface must be much stickier than that required to cause caking during storage where time frames for bridge formation can be up to six orders of magnitude longer. Stickiness curves more relevant to industrial drier operation have been generated by a number of different experimental methods and reported in the literature (the fluidized rig, a stirred bed, a cyclone tester. or the sticky-point temperature) (Lazar, Brown, Smith, Wong, & Lindquist, 1956; Downton et al., 1982; Wallack & King, 1988; Chuy & Labuza, 1994; Hennigs et al., 2001; Boonyai, Bhandari, & Howes, 2004, 2005). These are usually presented as the powder temperature and air RH combinations that result in stickiness detection by whatever method was used in the study. Operation of the drier in conditions within these constraints should then avoid stickiness problems and therefore maximize drier run times. One recently developed method to do this is termed the particle gun method (Chatterjee, 2004; Zuo, 2004; Zuo, Paterson, Bronlund, & Chatterjee, 2006), where powder particles are brought into contact with air at controlled temperature and RH and impacted at a stainless steel plate. The fraction of the feed particles that stick to the plate is recorded as an index of the stickiness of the powder in those conditions. The apparatus was designed in this way to reproduce the sort of conditions (velocity, particle-wall impact time) that is present in commercial spray drier ducts and cyclones. In this paper, we present a new TTg-based analysis method for the treatment of stickiness data collected for a range of dairy powders using the particle-gun method. As a result of this analysis, we show how a powder stickiness curve can be characterized by two simple parameters that quantify the conditions required for stickiness in the drier and the sensitivity of the powder stickiness to drier operation outside these conditions.
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2. Materials and methods 2.1. Powder samples The stickiness characteristics of six dairy powders were investigated in this work. Two skim milk powders (SMP), two whole milk powders (WMP) and one milk protein concentrate (MPC) powder were supplied by Fonterra Co-operative Limited along with powder composition analysis results (see Table 1). An amorphous lactose powder was made on a lab-scale Anhydro Lab S.1 spray drier (Massey University, Palmerston North), using the method outlined by Lloyd et al. (1996) and viewed under a polarized microscope to ensure complete lack of any crystallinity. 2.2. The particle-gun rig The particle-gun rig consisted of a controlled humidity and temperature air supply system and a particle feeding system that ‘fires’ the particles onto a deposition plate at high velocity. Details of the design and control of the air supply system used are provided by O’Donnell, Bronlund, Brooks, and Paterson (2002). The barrel of the particle gun was developed by Crofskey (2000) and Chatterjee (2004) and consisted of a 10 mm-diameter glass tube, 1 m in length. Particles are manually introduced slowly at the top of the tube using a venturi funnel arrangement where they are accelerated up to 20 m s1 before being impacted against a 75 mm-diameter stainless steel disc. The target disc was placed 150 mm directly under the particle-gun tip. A typical data set was obtained by maintaining a constant temperature at the tip of the gun and then maintaining the RH at progressively increasing RHs, for each fresh sample of powder added. The amount of powder that adhered to the plate for each trial was weighted and expressed as a percentage of the powder put through the gun for that experiment, and the tip RH and temperature were recorded. Full details of the design and operation of the particle-gun rig can be found in Chatterjee (2004), Zuo (2004) and Zuo et al. (2006).
Table 1 Compositional data for the dairy powders used and their (TTg)crit results Milk powder
SMPa A Replicate SMP B A. Lactose WMP A WMP B MPC a
Fat %TB
Protein %TB
Lactose %TB
0.62
34.27
57.84
0.79 0 31.09 27.38 1.35
38.19 0 25.91 27.07 59.36
52.98 100 37.82 38.33 31.29
(TTg)crit (1C)
Slope %d/(TTg)
(TTg)crit (fitted to stickiness points) (1C)
37.9 40.9 39.7 22.8 37.98 33.72 47.46
3.04 2.95 0.34 1.58 0.78 0.42 0.34
37.3 40.9 37.9 24.7 37.5 34.0 47.7
SMP ¼ skim milk powder; WMP ¼ whole milk powder; MPC ¼ milk protein concentrate.
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To characterize each powder samples’ stickiness curve, the particle rig was set at the desired temperature and RH conditions. Once the rig had reached steady state, 25 g powder samples were slowly introduced into the feed funnel. The weights of the sample fed into the rig and the target disc weight were recorded before and after each run and these were used to calculate the extent of powder deposition using the following equation: % Deposition on plate Weight of deposit on target disc ¼ 100 . Weight of feed powder
ð1Þ
This procedure was repeated after adjusting the RH and/ or temperature of the air supply to the particle gun. Deposition measurements were made over the temperature range of 50–90 1C and RH range 5–50%. Replicate measurements were made on one powder sample (SMP-1) to demonstrate the level of repeatability achieved using the particle-gun apparatus. 2.4. Data analysis The raw data collected from the particle-gun rig were % deposition at a particular temperature and RH condition for the air at the tip of the gun. Stickiness curves were constructed by determining the x-axis intercept of the deposition versus RH plots by minimization of least-square residuals for levels of deposition over 0.5%. Each temperature, humidity combination represented by the intercept was presented on a RH versus temperature stickiness plot. The deposition levels measured on the particle-gun rig can be analysed in terms of the difference by which the glass transition temperature of amorphous lactose was exceeded by at the particle surface (TTg). It was assumed that the surface amorphous lactose of the particle had come to equilibrium with the surrounding air temperature and RH. The glass transition temperatures of amorphous lactose for each of the RH, temperature combinations of the exit air from the particle gun were predicted using the equation for amorphous lactose developed by Paterson et al. (2001) on the assumption that the RH of the air is equal to the water activity of the amorphous lactose at the surface. This allowed the deposition level to be plotted against the calculated TTg values. The x-axis intercept and slope of the plot were calculated using minimization of least-square residuals for points with greater than 0.5% deposition.
upwards. The data typically show very little deposition as the RH was increased at a particular temperature, until a critical RH value was reached. At this point, the deposition of the powder on the target disc was observed with levels that increased with increasing values of RH. The x-axis intercept can be used as an indication of the initial stickiness of the powder under the high velocity impact conditions simulated by the particle gun. This information can be plotted as a stickiness curve for the powder on a temperature versus RH graph, as shown in Fig. 2 which can be used as a guide by process operators to avoid outlet air conditions that will promote stickiness in ducts and cyclones. If the amorphous lactose glass transition temperature (Tg) line is plotted as a function of RH, it can be seen in Fig. 2 to run parallel but offset below the stickiness curve of the dairy powder. By using a least squares of the errors approach, it was possible to fit a temperature difference (TTg)crit value to the data points where (TTg)crit corresponds to the critical temperature above the Tg of amorphous lactose where the dairy powder surface becomes sticky enough to cause the particles to adhere to the target plate of the particle gun. In Fig. 2 it can be seen that (TTg)crit for the WMP A sample tested was 37.5 1C. 25 % Deposition on plate
2.3. Particle-gun operation
80.5 °C 72.2 °C 68.2 °C 60.5 °C
20 15 10 5 0 0
10
20
30
40
50
60
RH (%) Fig. 1. Particle-gun raw data. The % deposition of WMP A plotted against relative humidity of the exit air at constant temperatures.
Temperature(°C)
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100 90 80 70 60 50 40 30 20 10 0
Stickiness curve
(T-Tg)crit =37.5 °C Tg lactose curve
0
10
20
30
40
50
RH%
3. Results and discussions Fig. 1 demonstrates a typical set of results for a sample of WMP (WMP A) when the air temperature was kept constant and the RH of the air was gradually step changed
Fig. 2. The stickiness curve for WMP A from four experimental points fitted by a least squares of error method to a curve (TTg)crit above the Tg line of amorphous lactose. The points are the intercepts of the % deposition on plate lines on the x-axis on Fig. 1 for the various temperatures.
ARTICLE IN PRESS A.H. Paterson et al. / International Dairy Journal 17 (2007) 860–865
% Deposition
These results suggested that a plot of deposition level against (TTg) should collapse the data into a single line, as shown in Fig. 3, where all the data from different temperatures and RH combinations for the same powders fall onto one line. Duplicate measurements of a SMP (SMP A) are shown in Fig. 4 along with the result found for SMP B. The data points in Figs. 3 and 4 clearly demonstrate that there is no temperature trend in the data when they are plotted in this way. This result shows that there is one common critical surface temperature difference above the glass transition temperature of amorphous lactose, (TTg)crit, at which initiation of stickiness for the powder occurs. This means that stickiness is not a direct function of temperature or RH, but rather the combination of these, resulting in a surface (TTg)crit for the powder. The data in Fig. 3 were fitted by a least-squares regression line and the x-intercept found giving a second estimate of the surface (TTg)crit value for the powder, which gave a value of 38.0 1C for WMP A. This result is, within the experimental
20 18 16 14 12 10 8 6 4 2 0
80.5 °C 72.2 °C 68.2 °C 60.5 °C
Slope = 0.78 R^2 =0.7336
0
10
20
30 38.0 40 T-Tg
50
60
70
Fig. 3. The % deposition of WMP A plotted against TTg, where Tg has been calculated using the relative humidity of the exit air from the particle gun as the surface water activity of the particle. Four different temperature data sets have been plotted.
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error, the same as that obtained (37.5 1C) by fitting a Tg+(TTg)crit line to the stickiness data plot shown in Fig. 2. The results for the duplicated sample measurements show that there is uncertainty in the particle-gun stickiness method resulting in a small difference in the x-axis intercept. It is clear from Fig. 4 that the level of uncertainty in the measurements is smaller than the differences observed among different powders. Fig. 5 presents the results for the dairy powders measured in this work. It can be seen that significantly different surface (TTg)crit values were observed for the different powders. However, when different powders exceed the surface (TTg)crit point by a given amount, they can still behave quite differently, explaining why some powders have reputations of being difficult to dry while others are less sensitive. Figs. 4 and 5 can be used to identify the temperature differences above Tg of amorphous lactose where a given % deposition will occur, based on the slope of the trend line. The slope of the trend line is a measure of powder surface stickiness sensitivity with respect to temperature and RH changes above (TTg)crit. The larger the slope, the more sensitive the powder is to such changes, suggesting that small changes in temperature and/or RH above the surface (TTg)crit results in large impacts on the stickiness of powder deposition in drier ducting and cyclones. The surface (TTg)crit and slope values for the powder samples determined from the deposition/TTg plots are summarized in Table 1. The surface (TTg)crit values for the two SMPs and WMPs studied varied from 37.9 to 40.9 1C. The slopes of the deposition/TTg curves, however, varied significantly from powder to powder (ranging from 0.34%d (TTg)1 to 3.04%d (TTg)1), with SMP B having a distinctively lower slope than the other skim milk powder tested, making it a lot less sticky than the other skim milk powder at equivalent conditions above their
60 50
SMP A WMP B MPC Lactose
35 % Deposition on plate
% Deposition on plate
40
40 30 20 10
30 25 20 15 10 5
37.9 0
0 0
10
20
30
39.740 T-Tg (°C)
40.950
60
Fig. 4. The % deposition of SMP A (with a replicate) and SMP B plotted against TTg, where Tg has been calculated using the relative humidity of the exit air from the particle gun as the surface water activity of the particle. Four different temperature data sets have been plotted for each powder sample.
0
10
20
30
40
50
60
70
T-Tg (°C) Fig. 5. The % deposition of SMP A K, WMP B ’, MPC m, and amorphous lactose ~, plotted against TTg, where Tg has been calculated using the relative humidity of the exit air from the particle gun as the surface water activity of the particle. Four different temperature data sets have been plotted for each powder.
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surface (TTg)crit values. The reason for this is unknown and cannot be attributed to bulk composition as these were only slightly different. One possible reason for this phenomenon could be that the surface compositions of the powders differ, even though the bulk compositions are the same. Differences between surface and bulk composition have been reported by Fa¨ldt and Bergensta˚hl (1996) and Kim, Chen, and Pearce (2005). Despite having similar surface (TTg)crit values, the differences in slopes of the lines in Fig. 5 show that SMPs and WMPs behave quite differently in conditions above this value. In general, the slopes of SMPs are much higher than WMPs; this means that at any particular condition that exceeds the stickiness curve, WMP deposits are lighter than SMP. This could be due to the lower lactose content of WMP compared with SMP, which means that, despite the two types of powders having similar (TTg)crit values, the SMP will build deposits faster than WMP under similar conditions and hence can be regarded as a ‘‘stickier’’ powder. The effect of powder composition on stickiness behaviour can be further seen from the results for MPC and pure amorphous lactose, shown in Table 1 and Fig. 5 with MPC exhibiting a very high surface (TTg)crit value of 47.7 1C. The surface (TTg)crit value measured for amorphous lactose (22.8 1C) was in reasonable agreement with the value of 25 1C obtained by Brooks (2000) by blow testing and the value of 26.2 1C obtained by Chatterjee (2004) using the particle gun at low temperatures (less than 48 1C) and fitting a Tg+(TTg)crit curve through the stickiness points at the various temperatures. Re-analysis of the data from Chatterjee (2004) using the % deposition versus TTg plot approach showed considerable scatter in these results with (TTg)crit value varying between 18 and 27 1C. As can be seen from the results in Table 1, there is a trend between lactose content of the powders tested and the critical surface (TTg) value, suggesting further work with a wider range of powder compositions should be undertaken in the future. There is no trend observed between powder composition and the slope of the deposition/ (TTg) plot. Further investigations are required in order to identify the cause of these differences in stickiness behaviour. 4. Conclusions This work has reported the results of the initiation of stickiness point for various dairy powders as measured by using a particle-gun rig. A plot of % deposition versus the temperature difference above the Tg of amorphous lactose, (TTg) successfully combined the temperature and relative humidity (RH) factors and shows that it is the amorphous lactose in dairy powders that causes the sticking problems during processing. The particle-gun method allows the stickiness characteristics of a powder to be characterized in two parameters. The surface (TTg)crit value summarizes
the whole range of RH and temperature conditions at which initiation of stickiness occurs. The slope of the trend line in the deposition/TTg plot shows the propensity of the powder to stick in response to the temperature and/or RH changes in conditions beyond the critical TTg value of the powder. The surface (TTg)crit value appears to be a function of the amount of amorphous lactose level of the powder and should be investigated in future work. The differences in slope of the deposition/TTg curves are not a function of bulk composition and may instead indicate different surface compositions or morphologies among powders. This phenomenon should also be the subject of future work. Acknowledgements The authors wish to thank TechNZ and Fonterra for their financial and material support, without which this project could have been completed. The advice and help of Dr. T. Truong is also acknowledged. References Bhandari, B. (2001). Glass transition in relation to stickiness during spray drying. Food Technology International, 64–68. Bhandari, B. R., Datta, N., & Howes, T. (1997). Problems associated with spray drying of sugar-rich foods. Drying Technology, 15(2), 671–684. Bhandari, B. R., & Howes, T. (1999). Implication of glass transition for the drying and stability of dried foods. Journal of Food Engineering, 40(1–2), 71–79. Boonyai, P., Bhandari, B., & Howes, T. (2004). Stickiness measurement techniques for food powders: A review. Powder Technology, 145, 34–46. Boonyai, P., Bhandari, B., & Howes, T. T. I. (2005). Measurement of glass–rubber transition temperature of skim milk powder by static mechanical test. Drying Technology, 23(7), 1499–1514. Brooks, G. F. (2000). The sticking and crystallisation of amorphous lactose. Palmerston North, New Zealand: M.E., Massey University. Chatterjee, R. (2004). Characterising stickiness of dairy powders. Palmerston North, New Zealand: M.E., Massey University. 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. Crofskey, C. M. (2000). Investigation into the caking problems associated with spray dried cream powders 55 and 70. Final Year Research project report, Massey University, Palmerston North, New Zealand. 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. (1996). Spray-dried whey protein/lactose/ soybean oil emulsions. 1. Surface composition and particle structure. Food Hydrocolloids, 10(4), 421–429. Foster, K. D. (2002). The prediction of sticking in dairy powders. Ph.D., Massey University, Palmerston North, New Zealand. 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. Jouppila, K., Kansikas, J., & Roos, Y. H. (1997). Glass transition, water plasticization, and lactose crystallization in skim milk powder. Journal of Dairy Science, 80, 3152–3160. Jouppila, K., & Roos, Y. H. (1994a). Glass transitions and crystallisation in milk powders. Journal of Dairy Science, 77(10), 2907–2915.
ARTICLE IN PRESS A.H. Paterson et al. / International Dairy Journal 17 (2007) 860–865 Jouppila, K., & Roos, Y. H. (1994b). Water sorption and time-dependent phenomena of milk powders. Journal of Dairy Science, 77, 1798–1808. Keir, T. (2001). Milk powder stickiness: A preliminary study. M.Tech., Massey University, Palmerston North, New Zealand. Kim, E. H.-J., Chen, X. D., & Pearce, D. (2005). Melting characteristics of fat present on the surface of industrial spray-dried dairy powders. Colloids and Surfaces B: Biointerfaces, 42, 1–8. Lazar, M. E., Brown, A. H., Smith, G. S., Wong, F. F., & Lindquist, F. E. (1956). Experimental production of tomato powder by spray drying. Food Technology, 10, 129–137. Lloyd, R. J., Chen, X. D., & Hargreaves, J. B. (1996). Glass transition and caking of spray-dried lactose. International Journal of Food Science and Technology, 31(4), 305–311. O’Donnell, A. M., Bronlund, J. E., Brooks, G. F., & Paterson, A. H. J. (2002). A constant humidity air supply system for pilot scale applications. International Journal of Food Science and Technology, 37, 369–374. 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.
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Paterson, A. H. J., Brooks, G. F., Foster, K. D., & Bronlund, J. E. (2005). The development of stickiness in amorphous lactose at constant TTg levels. International Dairy Journal, 15, 513–519. Roos, Y. H., Karel, M., & Kokini, J. L. (1996). Glass transitions in lowmoisture and frozen foods: Effects on shelf life and quality. Food Technology, 50(11), 95–108. Schuck, P., Blanchard, E., Dolivet, A., Mejean, S., Onillon, E., & Jeantet, R. (2005). Water activity and glass transition in dairy ingredients. Le Lait, 85(4/5), 295–304. Wallack, D. A., & King, C. J. (1988). Sticking and agglomeration of hygroscopic, amorphous carbohydrate and food powders. Biotechnology Progress, 4(1), 31–35. White, G. W., & Cakebread, S. H. (1966). The glassy state in certain sugar-containing food products. Journal of Food Technology, 1, 73–82. Zuo, J. Y. (2004). The stickiness curve of dairy powders. M.Tech., Massey University, Palmerston North, New Zealand. Zuo, J. Y., Paterson, A. H. J., Bronlund, J., & Chatterjee, R. (2006). Using a particle-gun to measure initiation of stickiness of dairy powders. International Dairy Journal, in press.
International Dairy Journal 17 (2007) 860–865 www.elsevier.com/locate/idairyj
Analysis of particle-gun-derived dairy powder stickiness curves Anthony H. Paterson, John E. Bronlund, Jenny Y. Zuo, R. Chatterjee Institute of Technology and Engineering, Massey University, Private Bag 11-222, Palmerston North, New Zealand Received 28 August 2005; accepted 25 August 2006
Abstract The stickiness curves of a range of dairy powders were measured using a particle-gun rig. The stickiness curves for the powders were shown to run parallel but above the curve of the glass transition temperature (Tg) of amorphous lactose. By assuming that the amorphous lactose at the surface of the powder was in equilibrium with the exit conditions of the air from the particle gun, it was found that for any particular dairy powder sample, the amount of powder deposition measured on the particle-gun target disc collapsed into a single function of the temperature difference by which the amorphous lactose Tg at the surface was exceeded. The x-axis intercept of these plots was calculated and designated as (TTg)crit, characterizing the conditions for initiation of stickiness of the powder. The sensitivity of each powder to stickiness problems when placed in conditions where the critical TTg value at the surface is exceeded was quantified with the slope of the plot. These results show that it is the amorphous lactose component that is probably the main cause of stickiness in dairy powders and demonstrates how the particle-gun rig can be used to characterize the stickiness behaviour of powders over a wide range of conditions with two parameters. r 2006 Elsevier Ltd. All rights reserved. Keywords: Dairy powders; Stickiness tests; Blockages; Spray drying; Glass transition temperature
1. Introduction Sticking in drier exhaust air ducting and cyclone blockages are common problems in industrial driers and can limit spray drier run lengths. In order to run driers at maximum throughput, it is important to identify the point at which powders will become sticky enough to cause blockages. Numerous papers have investigated the powder conditions in which sticking or caking can occur during powder storage (White & Cakebread, 1966; Downton, FloresLuna, & King, 1982; Wallack & King, 1988; Chuy & Labuza, 1994; Jouppila & Roos, 1994a, b; Lloyd, Chen, & Hargreaves, 1996; Roos, Karel, & Kokini, 1996; Jouppila, Kansikas, & Roos, 1997; Hennigs, Kockel, & Langrish, 2001; Paterson, Bronlund, & Brooks, 2001; Paterson, Brooks, Foster, & Bronlund, 2005; Schuck et al., 2005). Bhandari, Datta, and Howes (1997), Hennigs et al. (2001) and Schuck et al. (2005) have used the concept of the 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.2006.08.013
temperature difference by which the glass transition temperature is exceeded (TTg) to describe the sticking behaviour of amorphous powders. Bhandari and Howes (1999) and Bhandari (2001) suggested that the critical viscosity of a sticky powder in sugar-rich foods is reached at a temperature 10–20 1C above the Tg of the sugar mixture and that the temperature of the surface of particles during spray drying should not reach 10–20 1C above the Tg although industrial experience shows that this condition is often exceeded with dairy powders. Keir (2001), Paterson et al. (2001), Foster (2002) and Paterson et al. (2005) used a blow tester to measure the degree of stickiness development with time and demonstrated that the rate of sticking was related to the TTg of the powder. It was not important what relative humidities (RHs) and temperature conditions were used to obtain that TTg value. They also concluded that the higher the TTg value, the faster the rate of sticking development. Under certain RH and temperature combinations the rate of stickiness development was so quick, that it could be regarded as instantaneous.
ARTICLE IN PRESS A.H. Paterson et al. / International Dairy Journal 17 (2007) 860–865
This phenomenon explains why storage-based stickiness experiments provide a much more conservative estimate for prevention of blockages within the drier during operation. The time available in the drier for liquid bridge formation during collisions with walls or other powder particles is short. For this reason in the drier, the powder surface must be much stickier than that required to cause caking during storage where time frames for bridge formation can be up to six orders of magnitude longer. Stickiness curves more relevant to industrial drier operation have been generated by a number of different experimental methods and reported in the literature (the fluidized rig, a stirred bed, a cyclone tester. or the sticky-point temperature) (Lazar, Brown, Smith, Wong, & Lindquist, 1956; Downton et al., 1982; Wallack & King, 1988; Chuy & Labuza, 1994; Hennigs et al., 2001; Boonyai, Bhandari, & Howes, 2004, 2005). These are usually presented as the powder temperature and air RH combinations that result in stickiness detection by whatever method was used in the study. Operation of the drier in conditions within these constraints should then avoid stickiness problems and therefore maximize drier run times. One recently developed method to do this is termed the particle gun method (Chatterjee, 2004; Zuo, 2004; Zuo, Paterson, Bronlund, & Chatterjee, 2006), where powder particles are brought into contact with air at controlled temperature and RH and impacted at a stainless steel plate. The fraction of the feed particles that stick to the plate is recorded as an index of the stickiness of the powder in those conditions. The apparatus was designed in this way to reproduce the sort of conditions (velocity, particle-wall impact time) that is present in commercial spray drier ducts and cyclones. In this paper, we present a new TTg-based analysis method for the treatment of stickiness data collected for a range of dairy powders using the particle-gun method. As a result of this analysis, we show how a powder stickiness curve can be characterized by two simple parameters that quantify the conditions required for stickiness in the drier and the sensitivity of the powder stickiness to drier operation outside these conditions.
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2. Materials and methods 2.1. Powder samples The stickiness characteristics of six dairy powders were investigated in this work. Two skim milk powders (SMP), two whole milk powders (WMP) and one milk protein concentrate (MPC) powder were supplied by Fonterra Co-operative Limited along with powder composition analysis results (see Table 1). An amorphous lactose powder was made on a lab-scale Anhydro Lab S.1 spray drier (Massey University, Palmerston North), using the method outlined by Lloyd et al. (1996) and viewed under a polarized microscope to ensure complete lack of any crystallinity. 2.2. The particle-gun rig The particle-gun rig consisted of a controlled humidity and temperature air supply system and a particle feeding system that ‘fires’ the particles onto a deposition plate at high velocity. Details of the design and control of the air supply system used are provided by O’Donnell, Bronlund, Brooks, and Paterson (2002). The barrel of the particle gun was developed by Crofskey (2000) and Chatterjee (2004) and consisted of a 10 mm-diameter glass tube, 1 m in length. Particles are manually introduced slowly at the top of the tube using a venturi funnel arrangement where they are accelerated up to 20 m s1 before being impacted against a 75 mm-diameter stainless steel disc. The target disc was placed 150 mm directly under the particle-gun tip. A typical data set was obtained by maintaining a constant temperature at the tip of the gun and then maintaining the RH at progressively increasing RHs, for each fresh sample of powder added. The amount of powder that adhered to the plate for each trial was weighted and expressed as a percentage of the powder put through the gun for that experiment, and the tip RH and temperature were recorded. Full details of the design and operation of the particle-gun rig can be found in Chatterjee (2004), Zuo (2004) and Zuo et al. (2006).
Table 1 Compositional data for the dairy powders used and their (TTg)crit results Milk powder
SMPa A Replicate SMP B A. Lactose WMP A WMP B MPC a
Fat %TB
Protein %TB
Lactose %TB
0.62
34.27
57.84
0.79 0 31.09 27.38 1.35
38.19 0 25.91 27.07 59.36
52.98 100 37.82 38.33 31.29
(TTg)crit (1C)
Slope %d/(TTg)
(TTg)crit (fitted to stickiness points) (1C)
37.9 40.9 39.7 22.8 37.98 33.72 47.46
3.04 2.95 0.34 1.58 0.78 0.42 0.34
37.3 40.9 37.9 24.7 37.5 34.0 47.7
SMP ¼ skim milk powder; WMP ¼ whole milk powder; MPC ¼ milk protein concentrate.
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To characterize each powder samples’ stickiness curve, the particle rig was set at the desired temperature and RH conditions. Once the rig had reached steady state, 25 g powder samples were slowly introduced into the feed funnel. The weights of the sample fed into the rig and the target disc weight were recorded before and after each run and these were used to calculate the extent of powder deposition using the following equation: % Deposition on plate Weight of deposit on target disc ¼ 100 . Weight of feed powder
ð1Þ
This procedure was repeated after adjusting the RH and/ or temperature of the air supply to the particle gun. Deposition measurements were made over the temperature range of 50–90 1C and RH range 5–50%. Replicate measurements were made on one powder sample (SMP-1) to demonstrate the level of repeatability achieved using the particle-gun apparatus. 2.4. Data analysis The raw data collected from the particle-gun rig were % deposition at a particular temperature and RH condition for the air at the tip of the gun. Stickiness curves were constructed by determining the x-axis intercept of the deposition versus RH plots by minimization of least-square residuals for levels of deposition over 0.5%. Each temperature, humidity combination represented by the intercept was presented on a RH versus temperature stickiness plot. The deposition levels measured on the particle-gun rig can be analysed in terms of the difference by which the glass transition temperature of amorphous lactose was exceeded by at the particle surface (TTg). It was assumed that the surface amorphous lactose of the particle had come to equilibrium with the surrounding air temperature and RH. The glass transition temperatures of amorphous lactose for each of the RH, temperature combinations of the exit air from the particle gun were predicted using the equation for amorphous lactose developed by Paterson et al. (2001) on the assumption that the RH of the air is equal to the water activity of the amorphous lactose at the surface. This allowed the deposition level to be plotted against the calculated TTg values. The x-axis intercept and slope of the plot were calculated using minimization of least-square residuals for points with greater than 0.5% deposition.
upwards. The data typically show very little deposition as the RH was increased at a particular temperature, until a critical RH value was reached. At this point, the deposition of the powder on the target disc was observed with levels that increased with increasing values of RH. The x-axis intercept can be used as an indication of the initial stickiness of the powder under the high velocity impact conditions simulated by the particle gun. This information can be plotted as a stickiness curve for the powder on a temperature versus RH graph, as shown in Fig. 2 which can be used as a guide by process operators to avoid outlet air conditions that will promote stickiness in ducts and cyclones. If the amorphous lactose glass transition temperature (Tg) line is plotted as a function of RH, it can be seen in Fig. 2 to run parallel but offset below the stickiness curve of the dairy powder. By using a least squares of the errors approach, it was possible to fit a temperature difference (TTg)crit value to the data points where (TTg)crit corresponds to the critical temperature above the Tg of amorphous lactose where the dairy powder surface becomes sticky enough to cause the particles to adhere to the target plate of the particle gun. In Fig. 2 it can be seen that (TTg)crit for the WMP A sample tested was 37.5 1C. 25 % Deposition on plate
2.3. Particle-gun operation
80.5 °C 72.2 °C 68.2 °C 60.5 °C
20 15 10 5 0 0
10
20
30
40
50
60
RH (%) Fig. 1. Particle-gun raw data. The % deposition of WMP A plotted against relative humidity of the exit air at constant temperatures.
Temperature(°C)
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100 90 80 70 60 50 40 30 20 10 0
Stickiness curve
(T-Tg)crit =37.5 °C Tg lactose curve
0
10
20
30
40
50
RH%
3. Results and discussions Fig. 1 demonstrates a typical set of results for a sample of WMP (WMP A) when the air temperature was kept constant and the RH of the air was gradually step changed
Fig. 2. The stickiness curve for WMP A from four experimental points fitted by a least squares of error method to a curve (TTg)crit above the Tg line of amorphous lactose. The points are the intercepts of the % deposition on plate lines on the x-axis on Fig. 1 for the various temperatures.
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% Deposition
These results suggested that a plot of deposition level against (TTg) should collapse the data into a single line, as shown in Fig. 3, where all the data from different temperatures and RH combinations for the same powders fall onto one line. Duplicate measurements of a SMP (SMP A) are shown in Fig. 4 along with the result found for SMP B. The data points in Figs. 3 and 4 clearly demonstrate that there is no temperature trend in the data when they are plotted in this way. This result shows that there is one common critical surface temperature difference above the glass transition temperature of amorphous lactose, (TTg)crit, at which initiation of stickiness for the powder occurs. This means that stickiness is not a direct function of temperature or RH, but rather the combination of these, resulting in a surface (TTg)crit for the powder. The data in Fig. 3 were fitted by a least-squares regression line and the x-intercept found giving a second estimate of the surface (TTg)crit value for the powder, which gave a value of 38.0 1C for WMP A. This result is, within the experimental
20 18 16 14 12 10 8 6 4 2 0
80.5 °C 72.2 °C 68.2 °C 60.5 °C
Slope = 0.78 R^2 =0.7336
0
10
20
30 38.0 40 T-Tg
50
60
70
Fig. 3. The % deposition of WMP A plotted against TTg, where Tg has been calculated using the relative humidity of the exit air from the particle gun as the surface water activity of the particle. Four different temperature data sets have been plotted.
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error, the same as that obtained (37.5 1C) by fitting a Tg+(TTg)crit line to the stickiness data plot shown in Fig. 2. The results for the duplicated sample measurements show that there is uncertainty in the particle-gun stickiness method resulting in a small difference in the x-axis intercept. It is clear from Fig. 4 that the level of uncertainty in the measurements is smaller than the differences observed among different powders. Fig. 5 presents the results for the dairy powders measured in this work. It can be seen that significantly different surface (TTg)crit values were observed for the different powders. However, when different powders exceed the surface (TTg)crit point by a given amount, they can still behave quite differently, explaining why some powders have reputations of being difficult to dry while others are less sensitive. Figs. 4 and 5 can be used to identify the temperature differences above Tg of amorphous lactose where a given % deposition will occur, based on the slope of the trend line. The slope of the trend line is a measure of powder surface stickiness sensitivity with respect to temperature and RH changes above (TTg)crit. The larger the slope, the more sensitive the powder is to such changes, suggesting that small changes in temperature and/or RH above the surface (TTg)crit results in large impacts on the stickiness of powder deposition in drier ducting and cyclones. The surface (TTg)crit and slope values for the powder samples determined from the deposition/TTg plots are summarized in Table 1. The surface (TTg)crit values for the two SMPs and WMPs studied varied from 37.9 to 40.9 1C. The slopes of the deposition/TTg curves, however, varied significantly from powder to powder (ranging from 0.34%d (TTg)1 to 3.04%d (TTg)1), with SMP B having a distinctively lower slope than the other skim milk powder tested, making it a lot less sticky than the other skim milk powder at equivalent conditions above their
60 50
SMP A WMP B MPC Lactose
35 % Deposition on plate
% Deposition on plate
40
40 30 20 10
30 25 20 15 10 5
37.9 0
0 0
10
20
30
39.740 T-Tg (°C)
40.950
60
Fig. 4. The % deposition of SMP A (with a replicate) and SMP B plotted against TTg, where Tg has been calculated using the relative humidity of the exit air from the particle gun as the surface water activity of the particle. Four different temperature data sets have been plotted for each powder sample.
0
10
20
30
40
50
60
70
T-Tg (°C) Fig. 5. The % deposition of SMP A K, WMP B ’, MPC m, and amorphous lactose ~, plotted against TTg, where Tg has been calculated using the relative humidity of the exit air from the particle gun as the surface water activity of the particle. Four different temperature data sets have been plotted for each powder.
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surface (TTg)crit values. The reason for this is unknown and cannot be attributed to bulk composition as these were only slightly different. One possible reason for this phenomenon could be that the surface compositions of the powders differ, even though the bulk compositions are the same. Differences between surface and bulk composition have been reported by Fa¨ldt and Bergensta˚hl (1996) and Kim, Chen, and Pearce (2005). Despite having similar surface (TTg)crit values, the differences in slopes of the lines in Fig. 5 show that SMPs and WMPs behave quite differently in conditions above this value. In general, the slopes of SMPs are much higher than WMPs; this means that at any particular condition that exceeds the stickiness curve, WMP deposits are lighter than SMP. This could be due to the lower lactose content of WMP compared with SMP, which means that, despite the two types of powders having similar (TTg)crit values, the SMP will build deposits faster than WMP under similar conditions and hence can be regarded as a ‘‘stickier’’ powder. The effect of powder composition on stickiness behaviour can be further seen from the results for MPC and pure amorphous lactose, shown in Table 1 and Fig. 5 with MPC exhibiting a very high surface (TTg)crit value of 47.7 1C. The surface (TTg)crit value measured for amorphous lactose (22.8 1C) was in reasonable agreement with the value of 25 1C obtained by Brooks (2000) by blow testing and the value of 26.2 1C obtained by Chatterjee (2004) using the particle gun at low temperatures (less than 48 1C) and fitting a Tg+(TTg)crit curve through the stickiness points at the various temperatures. Re-analysis of the data from Chatterjee (2004) using the % deposition versus TTg plot approach showed considerable scatter in these results with (TTg)crit value varying between 18 and 27 1C. As can be seen from the results in Table 1, there is a trend between lactose content of the powders tested and the critical surface (TTg) value, suggesting further work with a wider range of powder compositions should be undertaken in the future. There is no trend observed between powder composition and the slope of the deposition/ (TTg) plot. Further investigations are required in order to identify the cause of these differences in stickiness behaviour. 4. Conclusions This work has reported the results of the initiation of stickiness point for various dairy powders as measured by using a particle-gun rig. A plot of % deposition versus the temperature difference above the Tg of amorphous lactose, (TTg) successfully combined the temperature and relative humidity (RH) factors and shows that it is the amorphous lactose in dairy powders that causes the sticking problems during processing. The particle-gun method allows the stickiness characteristics of a powder to be characterized in two parameters. The surface (TTg)crit value summarizes
the whole range of RH and temperature conditions at which initiation of stickiness occurs. The slope of the trend line in the deposition/TTg plot shows the propensity of the powder to stick in response to the temperature and/or RH changes in conditions beyond the critical TTg value of the powder. The surface (TTg)crit value appears to be a function of the amount of amorphous lactose level of the powder and should be investigated in future work. The differences in slope of the deposition/TTg curves are not a function of bulk composition and may instead indicate different surface compositions or morphologies among powders. This phenomenon should also be the subject of future work. Acknowledgements The authors wish to thank TechNZ and Fonterra for their financial and material support, without which this project could have been completed. The advice and help of Dr. T. Truong is also acknowledged. References Bhandari, B. (2001). Glass transition in relation to stickiness during spray drying. Food Technology International, 64–68. Bhandari, B. R., Datta, N., & Howes, T. (1997). Problems associated with spray drying of sugar-rich foods. Drying Technology, 15(2), 671–684. Bhandari, B. R., & Howes, T. (1999). Implication of glass transition for the drying and stability of dried foods. Journal of Food Engineering, 40(1–2), 71–79. Boonyai, P., Bhandari, B., & Howes, T. (2004). Stickiness measurement techniques for food powders: A review. Powder Technology, 145, 34–46. Boonyai, P., Bhandari, B., & Howes, T. T. I. (2005). Measurement of glass–rubber transition temperature of skim milk powder by static mechanical test. Drying Technology, 23(7), 1499–1514. Brooks, G. F. (2000). The sticking and crystallisation of amorphous lactose. Palmerston North, New Zealand: M.E., Massey University. Chatterjee, R. (2004). Characterising stickiness of dairy powders. Palmerston North, New Zealand: M.E., Massey University. 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. Crofskey, C. M. (2000). Investigation into the caking problems associated with spray dried cream powders 55 and 70. Final Year Research project report, Massey University, Palmerston North, New Zealand. 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. (1996). Spray-dried whey protein/lactose/ soybean oil emulsions. 1. Surface composition and particle structure. Food Hydrocolloids, 10(4), 421–429. Foster, K. D. (2002). The prediction of sticking in dairy powders. Ph.D., Massey University, Palmerston North, New Zealand. 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. Jouppila, K., Kansikas, J., & Roos, Y. H. (1997). Glass transition, water plasticization, and lactose crystallization in skim milk powder. Journal of Dairy Science, 80, 3152–3160. Jouppila, K., & Roos, Y. H. (1994a). Glass transitions and crystallisation in milk powders. Journal of Dairy Science, 77(10), 2907–2915.
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