Characterization of stickiness and cake formation in whole and skim milk powders

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1 Journal of Food Engineering 55 (2002) Characterization of stickiness and cake formation in whole and skim milk powders Necati Ozkan *, Nimali Walisinghe, Xiao Dong Chen Department of Chemical and Materials Engineering, School of Engineering, The University of Auckland, 20 Symonds Street, Auckland 92019, New Zealand Received 3 January 2002; accepted 4 March 2002 Abstract A viscometer technique based on the measurement of the torque required to turn a propeller inserted into milk powders and a penetration test based on the measurement of force required to penetrate milk powder compacts were used to characterize stickiness and cake formation in whole and skim milk powders. These simple techniques were suitable for determining the sticky point temperature (SPT) of the milk powders. Furthermore, the techniques provided useful information regarding the sticking behavior of the milk powders prior to the SPT. The stickiness and cake formation in the whole and skim milk powders were quite different due to the significant variation in the surface composition of these powders. Effects of moisture content, applied load, temperature, and time on the stickiness of the milk powders were also established. Ó 2002 Elsevier Science Ltd. All rights reserved. Keywords: Glass transition temperature; Milk powder; Viscometer; Penetration test; Surface composition 1. Introduction Milk powders, which are usually in the form of freeflowing agglomerates with a complicated particle size and shape, mainly consist of lactose, fat and protein. Characterization of surface properties of milk powders is important as these properties determine the behavior of powders during their storage, handling, transportation, and processing. Stickiness and cake formation in food powders have been recognized as significant problems. A direct interrelationship between the glass transition temperature and the stickiness (and subsequent cake formation) was already established for a wide range of food powders (Aquilera, del Valle, & Karel, 1995; Bhandari & Howes, 1999; Downton, Flores-Lune, & King, 1982; Lloyd, Chen, & Hargreaves, 1996; Peleg & Mannheim, 1977; Wallack & King, 1988). Milk powders produced by spray drying are predominantly amorphous and the glass transition temperature (T g ) is one of the distinctive properties of an amorphous material. Below the glass transition temperature, an amorphous material behaves as a glass with a very high * Corresponding author. Tel.: ; fax: x address: n.ozkan@auckland.ac.nz (N. Ozkan). viscosity (>10 12 Pa s) due to the limited molecular movement at low temperatures (Downton et al., 1982). Above the glass transition temperature, a glassy material starts to transform to a rubbery state and the viscosity of the material decreases considerably. The glass temperature of food materials is depressed significantly by water which is considered as a strong plasticiser in a food system (Bhandari & Howes, 1999). After spray drying, milk powders are sometimes transferred to bags at temperatures much higher than ambient, as a result, a certain temperature and moisture gradient could be created within the milk powder bags. Bronlund (1996) explored in details this phenomena for lactose powders (amorphous and crystalline) and established the relationships among the mass transfer, heat transfer, water content, amorphous lactose content, and crystallisation rate. The temperature changes and moisture migration within milk powder can also result in cake formation in the bags (Sukha, 2000). Furthermore, a bag of exported milk powder produced in New Zealand would be subjected to high temperature and humidity during a long distance travel through different climate zones, as a result, undesirable sticking and cake formation may occur in the milk powders. The reliable and sensitive measurement of the stickiness as influenced by moisture content, temperature, and consolidation /02/$ - see front matter Ó 2002 Elsevier Science Ltd. All rights reserved. PII: S (02)

2 294 N. Ozkan et al. / Journal of Food Engineering 55 (2002) pressure would provide data to establish the threshold conditions for the manufacturers to avoid. Various techniques were already developed to measure the stickiness in food powders. Lazar, Brown, Smith, Wong, and Lindquist (1956) developed a technique to measure the stickiness of amorphous food powders. In this technique, a test tube is packed with a powder sample and sealed using rotating mercury seal. Subsequently, the tube is immersed in a water bath. The bath temperature is slowly raised while the powder is intermittently stirred by hand with a small propeller embedded in the powder sample. At a certain temperature, which is a function of the moisture content of the powder, the force required to turn the stirrer increases sharply. The temperature at which this occurs is the caking temperature which is referred to as the sticky point temperature (SPT) of the powder (Downton et al., 1982; Wallack & King, 1988). Brennan, Herrera, and Jowitt (1971) slightly modified this technique and suggested a mechanical means of finding the SPT by measuring the current required to turn a stirrer. However, these mechanical stirring methods do not provide quantitative information about the strength of materials and they are not suitable to monitor the changes that occur in the powder at early stages of the stickiness process. As a result, comparisons between different materials are not possible and the effects of various parameters such as applied pressure and time cannot be studied (Papadakis & Bahu, 1992). The measurements of density and compressive strength of powder compacts were used to study the cohesion and caking of dairy powders (Lloyd et al., 1996; Rennie, Chen, Hargreaves, & Mackereth, 1999). In these studies, dairy powders (lactose and milk powders) were consolidated in a metal die at various temperatures under a given compaction pressure. This concept provides quantitative information regarding the strength of caked powders and is useful for assessing the effects of various parameters on the cake formation. However, the limitation for this approach is that the strength data show considerable scatter due to the porous and granular nature of powder compacts and it may not be possible to measure the strength of very weak powder compacts due to handling difficulties for such compacts. Shear cell methods (Jenike shear cell (Jenike, 1964) and rotational split-level shear cell (Peschl, 1989)) have been successfully used to evaluate the flowability of powders and the caking properties of dry particulates. However, these methods are not suitable to characterize semi-solid food materials. Furthermore, it is very hard to adapt them to high temperature and high humidity systems (Adhikari, Howes, Bhandari, & Truong, 2001). Even though, some techniques have already been developed for characterizing stickiness and cake formation in powders, it would be useful to develop a simple and convenient technique for characterizing food powders in terms of their possible stickiness and cake formation. In this study, two simple techniques (torque measurement and penetration test) were devised and used to characterize the stickiness and cake formation in milk powders. The influence of various parameters such as composition (particularly surface composition), moisture content, and temperature of milk powders, applied compaction pressure, and time on the stickiness and cake formation of milk powders were also investigated. 2. Materials and methods 2.1. Milk powders Milk powders (skim milk (SMP) and whole milk (WMP)) produced at New Zealand Kiwi Dairies Ltd. were used in this study. The approximate compositions of these milk powders are given in Table 1 which were taken from the data given by Kim, Chen, and Pearce (in press) on the same powders. The scanning electron microscope (SEM) pictures of the milk powders, which are shown in Fig. 1, give an impression of the types of agglomerates that exist in the whole and skim milk powders. In order to investigate the effects of moisture content on the stickiness and cake formation of the milk powders, as-received milk powders were conditioned for about 12 h in a humidity chamber set at various relative humidity values. After conditioning the milk powders, they were immediately stored in sealed bags. The moisture content of the conditioned milk powders based on dry weight, determined using a moisture analyzer (Model ISO 9001), are given in Table 2. The notation adapted to describe the milk powders with various moisture contents is also given in Table 2. For instance, WMP-3.3 stands for the WMP with a moisture content of 3.3 wt.% Viscometer method (torque measurement) The rotational viscometer (Paar Physica-MC1, Germany) was used to evaluate the stickiness and cake Table 1 Approximate compositions for whole and skim milk powders Milk powder Lactose, wt.% Protein, wt.% Fat, wt.% Minerals, wt.% Water, wt.% WMP SMP

3 N. Ozkan et al. / Journal of Food Engineering 55 (2002) Fig. 1. SEM pictures of the milk powders: (a) SMP and (b) WMP. Table 2 Moisture contents of milk powders used in this study WMP SMP 2.6 wt.% (WMP-AR) 3.2 wt.% (SMP-AR) 3.3 wt.% (WMP-3.3) 4.5 wt.% (SMP-4.5) 4.7 wt.% (WMP-4.7) 6.1 wt.% (WMP-6.1) formation in milk powders. The schematic representation of the viscometer technique is shown in Fig. 2. The bath temperature and the torque required to rotate a home-made L shaped spindle (propeller or stirrer) inserted into milk powders were recorded using a data logging system connected to the viscometer. The size of the spindle (see Fig. 2) was optimized so that the torque measurements can be carried out without exceeding the instrument limit of 50 mn m. For the torque measurements, 52 g of milk powder was poured into the cylindrical cup with an inner diameter of 67 mm and tapped four times before fitting the cup and spindle to the viscometer. Two types of measurements could be taken: continuous and non-continuous measurements. For the continuous measurement, the lowest rotational speed (0.3 rpm) was used and the torque values were recorded as a function of temperature. The temperature of water jacket was increased linearly from 25 to 90 C using a heating rate of 1 C/min during the torque measurements. The continuous measurement was not adapted in this study since it was noticed that after the completion of one rotation, the spindle partially destroyed the milk powder compact, therefore, the torque values recorded after the completion of one or more rotations cannot represent the true strength of the junctions between the milk powder agglomerates. For the non-continuous measurement, the temperature of water jacket was set to a certain temperature and the cup containing 52 g milk powder was fixed into the viscometer. After keeping the milk powder in the cup for 20 min in order to obtain a uniform temperature, the torque measurements were recorded at 0.3 rpm for 40 s (one data point was recorded for every second). The average torque values were calculated using the last 20 data points. A typical result from the non-continuous measurement is shown in Fig. 3. For each average torque measurement, a new sample was used Penetration test and density measurements Using an aluminium die (see Fig. 4), the milk powders were consolidated at various temperatures and compaction pressures for a given time. The aluminium dies were also used to measure the tap density and consolidated density of the milk powders. The milk powders with different moisture contents were used (see Table 2). Fig. 2. Schematic illustration of the viscometer technique.

4 296 N. Ozkan et al. / Journal of Food Engineering 55 (2002) system was placed in an oven set at various temperatures ranging from 20 to 70 C. After keeping the dies for a certain time in the oven, they were removed from the oven and left to cool down to room temperature. Subsequently, the height of the die with the punch (h c ) was measured again. From this height measurement, the density of consolidated milk powder was determined using Eq. (1), in this case h c is used instead of h t. While the powder compact was still in the die, the Instron testing machine (Model 5567, UK) was used to record the force required to penetrate the powder compact at a crosshead speed of 1mm/min using a cylindrical indenter with a diameter of 2 mm (see Fig. 4). 3. Experimental results and discussion 3.1. Viscometer method (torque measurements) Fig. 3. The torque measurements using the non-continuous viscometer technique. The solid lines represent the average torque values calculated using the last 20 data points. The height of empty die with the punch (h 0 ) was measured using a digital Vernier Calliper with a precision of 10 lm. Six grams of milk powder was placed in the die, after tapping four times to level the powder, the punch was inserted into die. By measuring the height of die with the milk powder and the punch (h t ) (see Fig. 4), the tap density of milk powder ðq tap Þ can be determined using the following expression W q tap ¼ ð1þ ðp=4þd 2 ðh t h 0 Þ where W is the mass of the milk powder and D is the inner diameter of the die. The tap density of the powder can be related to the packing efficiency of the milk powders. After the tap density measurements, a known load was placed on the top of the punch and the whole The values of the torque required to rotate the spindle inserted in the milk powders as a function of temperature for the as-received whole and skim milk powders are shown in Fig. 5. As can be seen from Fig. 5, a significant difference between these milk powders was recorded, which is largely due to difference between their surface compositions. The surface composition of the milk powders determined using the X-ray photoelectron spectroscopy (XPS) is given in Table 3 (Kim et al., in press). In the XPS measurements, the relative atomic concentration of carbon, oxygen, and nitrogen in the surface layer (10 nm) of the powder was analyzed. The details of the surface composition measurement are given elsewhere (Kim et al., in press). The surface composition of the WMP is mainly made of milk fats (98%), however, the surface composition of the SMP is mainly made of lactose and protein with a small amount of fat (18%). The surface composition and the bulk composition are quite different for both the WMP and SMP (comparing Tables 1 and 3). Because of the significant differences between the surface compositions of the WMP and SMP, the sticking behavior and free-flowing Fig. 4. The schematic representation of the die set (a, b) and the penetration test (c).

5 N. Ozkan et al. / Journal of Food Engineering 55 (2002) Fig. 5. The values of torque as a function temperature for the asreceived WMP and SMP. The arrows indicate the approximate positions of the transition points (or the SPTs). The solid lines are the trend lines. Table 3 The surface composition of the milk powders assuming that they are composed of 3 main components (lactose, protein and fat) Powder Lactose (wt.%) Protein (wt.%) Fat (wt.%) WMP 2 98 SMP properties of these powders are expected to be different. It was observed that the SMP powder is more freeflowing than the WMP as expected since it is well known that the flowability of milk powders is reduced with increasing fat content. This observation was further established by the determination of the tap density of the milk powders (see Table 4). The average tap densities of the SMP are much higher than those of the WMP, again this is due to the fact that the free-flowing powders have better packing efficiency. The packing factors for the milk powders are also given in Table 4. Various researchers defined the SPT for various types of food powders as the temperature at which the force required to turn a propeller (stirrer) inserted into milk powders increases sharply (Downton et al., 1982; Wallack & King, 1988). Adapting the similar criteria, the SPTs for the WMP-AR and SMP-AR were estimated as approximately 57 and 65 C, respectively (see Fig. 5). Furthermore, the torque measurements using the viscometer allow a quantitative description for the stickiness behavior of the milk powders. However, it should be emphasized that the measured values will strongly depend on the geometry and the rotational speed of the spindle (stirrer). The torque values for the WMP-AR are larger than those for the SMP at low temperatures (<55 C) and they increase gradually with increasing temperature up to 65 C, indicating that there is a certain degree of stickiness between the WMP agglomerates at low temperatures. The formation of liquid bridges has been frequently reported as the reason for increasing strength in powders. If the powders are completely free-flowing, then there is only point contacts between the powders. The liquid bridges between the powders will be formed when a liquid appears at the point contacts (capillary condensation mechanism, melting of fats, and transformation from a glassy phase to a rubbery phase), exerting a significantly higher strength and cohesiveness of the powder mass. The increase in the torque values for the WMP-AR as a function of temperature at low temperatures (<65 C) is due to the formation of liquid phase starting at the point contacts (Rennie et al., 1999). This is particularly true in the WMP as we now know that the surface of the WMP mainly consists of free fat and the melting temperatures of the milk fats range from 40 to 40 C depending on their molecular chain length (Kleyn, 1992). A sharp increase in the torque value for the WMP-AR at high temperatures (>65 C) is probably due to the fact that the glass temperature of lactose is reached as a result the lactose is transformed to the rubbery phase contributing the formation of stronger bonds between the milk powders. It may also be possible that these lactose (rubbery phase) junctions become much stronger when some of the lactose is transformed to the crystalline lactose. It is in fact expected that the torque values for the SMP-AR is lower than those for the WMP-AR at low temperatures since the surface of the SMP is predominantly covered by lactose and protein, as a result, the melting of a small amount of fat does not contribute extensively to the stickiness in the SMP-AR at low temperatures. The torque values for the SMP-AR increases very sharply above 55 C since the glass Table 4 The average tap densities of the milk powders WMP-AR WMP-3.3 WMP-6.1 SMP-AR SMP-4.5 q tap (g/cm 3 ) a 0:597 0:012 0:601 0:010 0:498 0:018 0:653 0:004 0:654 0:001 Packing factor b a The average tap density of milk powder was calculated using Eq. (1). About 30 samples were used for each milk powder. b Packing factor is defined as the ratio of the average tap density to the particle density of milk powder. The particle densities of the SMP and WMP were 1.28 and 1.21 g/cm 3, respectively (Chen, 1994).

6 298 N. Ozkan et al. / Journal of Food Engineering 55 (2002) transition temperature for the lactose is reached. The difference between the transition temperatures for the SMP-AR and WMP-AR is due to fact that the moisture content of the WMP-AR (2.7 wt.%) is lower than that of SMP-AR (3.2 wt.%). It is known that the glass temperature of the lactose is reduced with increasing its moisture content. The glass transition temperatures of lactose with moisture contents of 2.4, 4.3, and 5.9 wt.% were reported as 64, 43, and 33 C, respectively by Jouppila and Ross (1994). The torque values for the SMP is considerably larger than those for the WMP once the glass transition temperature for the SMP is exceeded suggesting that the strength of the bridges formed between the powders is different for the SMP and WMP. The bridges formed between the SMP are predominantly due to the rubbery lactose with a small contribution from the melting of fat. The bridges formed in the SMP are stronger than those formed in the WMP since the considerable amount of the liquid phase (fat melt) formed on the surface of the WMP which does not create strong junctions. Both lactose and proteins are strongly hygroscopic. While absorption of moisture by food products containing lactose such as milk powders is accompanied by increasing of stickiness, protein-rich products absorb moisture without the noticeable stickiness phenomena (Pisecky, 1999). Hence, it is thought that the contribution of proteins to the stickiness in the milk powders is not significant, especially for the WMP since the surface of the WMP consists of mainly fat (see Table 3). The results of the torque measurements for the WMP with various moisture contents (2.7, 3.3, and 6.1 wt.%) are shown in Fig. 6. The torque values for the WMP-6.1 Fig. 6. The values of torque as a function temperature for the WMP with various moisture contents. (The moisture content of WMP-AR is 2.6 wt.%.) The lines represent the trend lines. at low temperatures were slightly higher than those for the WMP-AR and WMP-3.3. It has been shown (Rennie et al., 1999) that increasing water content increases the cohesiveness. The SPT for the WMP-AR has already been reported as 65 C. When the moisture content of the WMP was increased to 3.3 wt.%, the SPT was reduced to approximately 55 C due to the lowering of the glass transition of lactose with increasing moisture content. However, it was not possible to estimate the SPT for the WMP-6.1 since there was no clear transition point. The non-appearance of a clear transition point for the WMP-6.1 is probably due to the fact that the junctions formed between milk powders are not very strong at high temperatures for the WMP-6.1. Once the junctions between the milk powders developed, the overall strength of the milk powder compacts depends on the total contact area of junctions and the strength of individual junctions between the milk powders. The strength of the milk powder compacts measured at high temperatures is controlled by two opposing factors: (i) increasing the total contact area of the junctions between milk powders with increasing temperature and (ii) decreasing the strength of the individual junctions with increasing temperature. It seems that the latter factor may neutralize the influence of the former factor for the WMP-6.1 at high temperatures, as a result, it was not possible to observe a clear transition. The strength of the rubbery lactose junctions in the WMP-6.1 will be much weaker since the viscosity of the rubbery phase is reduced with increasing moisture content and temperature. Therefore, the formation of the lactose junctions may not increase the strength of powder compacts at temperatures higher than the glass temperature even though the contact area of the junctions increases with increasing temperature. For the WMP-AR and WMP-3.3, the decrease in the strength of the individual junctions with increasing temperature was not sufficient enough to counteract the strength increase due to the increase in the total contact area for the junctions. In order to support this elucidation, first the WMP-6.1 was incubated at a given temperature for 20 min and was then cooled to 20 C, subsequently the torque measurement was carried out. When the temperature of the milk powders is increased, the liquid bridges (junctions) between the milk powders will be formed due to the melting of milk fats and the transformation of the glassy lactose to the rubbery lactose. Once these junctions formed at high temperatures, the strength of the individual junctions will increase significantly with decreasing temperature since the melted fat transforms into solid fats and the rubbery lactose transforms to the glassy lactose at 20 C. The torque measurements with cooling to 20 C and without cooling are shown in Fig. 7. As can be seen from Fig. 7, a clear transition point (around 37 C) was evident for the WMP-6.1 when the torque measurements were carried

7 N. Ozkan et al. / Journal of Food Engineering 55 (2002) Fig. 7. The values of torque as a function temperature for the WMP The torque measurements were carried out at 20 C after incubating the powders at a given temperature for 20 min and without cooling the powders. at 20 C. The torque measurements for the WMP with lower moisture contents (2.7 and 3.3 wt.%) were not carried out at 20 C. If the torque measurements for these powders were carried out at 20 C after incubating them at high temperatures, the torque values would have been much larger Penetration test The penetration force, F, against the depth of penetration (or displacement), d, curves for the SMP at various temperatures are given in Fig. 8. For the penetration tests, the milk powders were consolidated in the aluminium dies using a constant pressure of 6.25 kpa at various temperatures for 3 h, subsequently, the dies were removed from the oven and left to cool down to room temperature. All the penetration experiments were carried out at room temperature. Each measurement was carried out twice. The force against displacement data for the milk compacts formed at low temperatures were quite noisy. However, the force against displacement curves at high temperatures were quite smooth indicating that the milk powders were fused to each other completely. To establish the interrelationship between the penetration force and temperature, the values of the penetration force at a penetration depth of 1 mm were used. The justification for choosing the force at the penetration depth of 1 mm as a criterion is that most of the milk powder compacts did not crack (break) at this penetration depth and at this penetration depth, the value of penetration force for extensively caked milk powders reached to nearly 200 N which is the upper Fig. 8. The penetration force against the depth of penetration plots for the SMP at various temperatures. The arrows indicate the approximate positions of the penetration force at the penetration depth of 1 mm. Thick solid line is the trend line for the plot at 40 C. limit of the load cell used in this study. The penetration force (at d ¼ 1 mm) as a function of temperature for the SMP-AR and WMP-AR is shown in Fig. 9. From the penetration tests, the SPTs for the SMP-AR and WMP- AR were determined as approximately 55 and 60 C, respectively. A good agreement between the results from the penetration test and the viscometer method was obtained. The SPTs determined using the penetration test are slightly lower than those determined using the viscometer technique. The reasons for obtaining a little lower SPTs using the penetration test are as follows: (i) the penetration tests were carried out at room temperature after incubating the milk powders in the oven at a certain temperature and subsequently cooling them to room temperature and (ii) a consolidation pressure of 6.25 kpa was applied to the milk powders for the penetration tests. All these might amplified the effect of sticking. No pressure was applied to the milk powders for the torque measurements. From Fig. 9(a), it may appear that there is no difference between the SMP-AR and WMP-AR at low temperatures, however, when the penetration force data was represented using a logarithmic scale as shown in Fig. 9(b). It is possible to see the differences between these milk powders at low temperatures. As can be seen from Fig. 9(b), the penetration forces for the SMP-AR are smaller than those for the WMP-AR at lower temperatures, indicating the free-flowing nature of the SMP-AR. However, much larger penetration forces were recorded for the SMP- AR above 50 C, again this is due to the plasticization of the lactose. The same observations had already been reported using the results of the torque measurements.

8 300 N. Ozkan et al. / Journal of Food Engineering 55 (2002) Fig. 9. (a) The penetration force (d ¼ 1 mm) against temperature plots for the as-received WMP and SMP. The arrows indicate the approximate positions of the SPT. The solid lines are the trend lines. (b) Semi-logarithmic the penetration force (d ¼ 1 mm) against temperature plots for the asreceived WMP and SMP. As anticipated, the SPT of the SMP was lowered with increasing moisture content. The SPTs of the SMP-AR and the SMP-4.5 were determined as 55 and 43 C, respectively from the penetration force (d ¼ 1 mm) against temperature plots (see Fig. 10) Effects of consolidation pressure and time The penetration test was also used to investigate the effects of consolidation pressure and time. In order to investigate the influence of consolidation pressure, the milk powders were consolidated at various compaction pressures at a certain temperature (40 or 70 C) for 3 h. The penetration force against displacement (depth of penetration) curves for the WMP with various moisture contents consolidated at 40 C and 6.3 kpa are shown in Fig. 11. The penetration force data for the WMP-AR was very noisy, indicating that the WMP-AR did not plasticize at 40 C. The penetration force for all WMP increased with increasing consolidation pressure (see Fig. 12). The values of penetration force for the WMP- AR were quite low suggesting that only weak junctions between the milk powders are formed when some of the milk fats melted at 40 C. With increasing consolidation pressure, the density of the milk powder compacts increased, as a result, the total contact area of the junctions increased resulting in larger penetration forces. When the moisture content of the WMP was increased, the penetration forces were increased significantly, Fig. 10. The penetration force (d ¼ 1 mm) against temperature plots for the SMP-AR and SMP-4.5. Fig. 11. The penetration force against depth of penetration (displacement) curves for the WMP with various moisture contents consolidated at 6.3 kpa and 40 C. The arrows indicate the approximate positions of the penetration force at the penetration depth of 1 mm.

9 N. Ozkan et al. / Journal of Food Engineering 55 (2002) Fig. 12. The penetration force (d ¼ 1 mm) against compaction pressure for the WMP with various moisture contents. demonstrating that much stronger junctions between milk powders are formed because of the plasticization of lactose (that is, some of the glassy lactose is transformed to rubbery lactose). The values of penetration force for the WMP-6.1 were much smaller than those for the WMP-4.7 (see Fig. 12). This observation was not expected since the extent of plasticization for the milk powders increases with increasing moisture content. In order to elucidate this unexpected result, it is necessary to look at the packing factors of the milk powder compacts which are given in Table 4. The packing factor of the WMP-6.1 is much lower than that of the WMP- AR and WPM-4.7. The total contact area of the junctions between the milk powders will be much smaller for these compacts with low packing factors, therefore, the overall strength of these milk powder compacts will not be very high. Furthermore, the viscosity of the rubbery phase (lactose) will decrease with increasing moisture content, therefore, the individual junctions would be weakened with increasing moisture content. In order to prove this point, the penetration force at d ¼ 1mmasa function of density of milk compacts is illustrated in Fig. 13. At a given density, the penetration force decreases with the increasing moisture content of the milk powder. It appears that the initial tap density of the milk powder has a large effect on the penetration force. Assuming that the similar tap density difference exists in the milk powder compacts used for the torque measurements, the same behavior was not observed for the torque measurements (see Fig. 6). The possible explanations for this disagreement is that the initial density of Fig. 13. The interrelationship between the penetration force (d ¼ 1 mm) and the density of the WMP with various moisture contents. The solid lines are the trend lines. the milk powder compacts does not have a large influence on the torque measurement and/or the density differences in the milk powder compacts used for the torque measurements were not significant. The influence of the initial density of the milk powder compacts on the torque measurements was not investigated in this study. The penetration force against depth of penetration curves for the SMP-AR consolidated using various compaction pressures at 70 C are shown in Fig. 14(a). The penetration force increased with increasing compaction pressure because the density of the powders compacts increases with increasing pressure as a result the total contact area of the junctions between milk powders increases. Fig. 14(a) also shows the reproducibility of the experimental data (all the measurements were duplicated). The penetration force (d ¼ 1 mm) (the average of two measurements) against compaction pressure results for the SMP-AR at 40 and 70 C are shown in Fig. 14(b). The values of penetration force (d ¼ 1 mm) for the SMP-AR at 40 C were very low since the lactose in the SMP-AR is not transformed to the rubbery phase at 40 C. However, the values of the penetration force for the SMP-AR at 70 C were very high, indicating that the cakes formed at 70 C were very strong. Again this observation was expected because the lactose in the SMP-AR transforms to the rubbery phase, as a result, it is possible to form strong junctions and these junctions are further strengthened due to the crystallization of the lactose at 70 C. The SEM micrographs of the SMP and WMP consolidated at 70 C are given in Fig. 15. A considerable amount of needle

10 302 N. Ozkan et al. / Journal of Food Engineering 55 (2002) Fig. 14. (a) The penetration force against displacement at 70 C for the SMP-AR. (b) The penetration force against compaction pressure at 40 and 70 C for the SMP-AR. Fig. 15. The SEM pictures for (a) the SMP-AR and (b) the WMP-AR. Both the SMP-AR and the WMP-AR were incubated at 70 C for 3 h. like lactose crystals was observed on the surface of the SMP after 3 h of incubation at 70 C, which confirms that the lactose is transformed from the rubbery state to the crystalline phase. However, it was not possible to observe lactose crystals on the surface of WMP due to the fact that the surface of the WMP was largely covered with a thin layer of milk fats (Kim et al., in press). In order to illustrate the effect of time on the stickiness of the milk powders, the WMP-AR and SMP-AR were consolidated at a compaction pressure of 6.25 kpa for 1, 3, and 12 h at 40 and 70 C. The results of the penetration force (d ¼ 1 mm) against time for the WMP-AR and SMP-AR are shown in Fig. 16. At 40 C, the values of the penetration force (d ¼ 1 mm) for both the SMP and WMP were very low and did not increase with time. However, the values of the penetration force increased with increasing time for both the SMP and WMP at 70 C. These observations further confirm the hypothesis that the viscous flow occurs in amorphous food powders at temperatures higher than the T g. The increase in the penetration force values at high temperatures (>T g ) with increasing time was due to two factors: (i) the contact area of the junctions between the milk powder increases due to the viscous flow and (ii) the Fig. 16. The penetration force (d ¼ 1 mm) against time at 40 and 70 C for the SMP-AR and WMP-AR.

11 N. Ozkan et al. / Journal of Food Engineering 55 (2002) partial crystallization of lactose takes place at high temperatures. 4. Conclusions The SPT of the milk powders decreased with increasing moisture content of the powders. The SPTs for the WMP-AR, WMP-3.3, and WMP-6.1 were determined as 65, 55, and 37 C, respectively using the viscometer technique. Furthermore, it was possible to monitor the changes in the milk powders at lower temperature (i.e. prior to the SPT). A significant difference between the WMP and SMP was observed. The values of the torque and the penetration force for the SMP were lower than those for the WMP prior to the SPT. However, once the SPT is reached, the values of the torque and the penetration force for the SMP became much larger than those for the WMP. The differences between the SMP and WMP is mainly due to the significant variation in the surface composition of these milk powders. The surface of the WMP was almost completely covered with a very thin layer of free milk fats even though the concentration of fat in the WMP is about 27 wt.%. The stickiness occurred in the WMP at low temperatures is mainly due to the melting of the milk fats resulting in the formation of relatively weak junctions within the milk powder. The contribution of moisture may also be possible especially for the milk powders with a high moisture content. The junctions between the powders become quite strong only at temperatures higher than the glass transition temperature of lactose. Even though, the stickiness occurred at relatively low temperatures for the WMP-6.1, a strong cake formation was not observed in the WMP-6.1 since the average tap density of the WMP-6.1 was very low and the junctions in the WMP-6.1 were weakened due to the plasticization of lactose by water (i.e. the viscosity of lactose was reduced considerably). The SMP cakes formed at temperatures higher than the glass transition temperature of lactose were very strong because the junctions formed between the SMPs were predominantly made of lactose and these junctions further strengthened especially when the rubbery lactose transformed to the crystalline phase. In general, it has been shown that the viscometer technique and the penetration test are suitable for characterizing the stickiness and cake formation in milk powders. References Adhikari, B., Howes, T., Bhandari, B. R., & Truong, V. (2001). Stickiness in foods: a review of mechanisms and test methods. International Journal of Food Properties, 4(1), Aquilera, J. M., del Valle, J. M., & Karel, M. (1995). Caking phenomena in amorphous food powders. Trends in Food Science and Technology, 6, Bhandari, B. R., & Howes, T. (1999). Implication of glass transition for the drying and stability of dried foods. Journal of Food Engineering, 40, Brennan, J. G., Herrera, J., & Jowitt, R. (1971). A study of the factors affecting the spray drying of concentrated orange juice, on a laboratory scale. Food Technology, 6, Bronlund, J. (1996). Caking of spray-dried lactose powders. Ph.D. Thesis, Massey University. Chen, X. D. (1994). Towards a comprehensive model based control of milk drying processes. Drying Technology, 12(5), Downton, G. E., Flores-Lune, J. L., & King, C. J. (1982). Mechanism of stickiness in hydroscopic, amorphous powders. Industrial and Engineering Chemistry Fundamentals, 21, Jenike, A. W. (1964). Storage and flow of solids. Bulletin 123, Utah Engineering Experimental Station, University of Utah. Jouppila, K., & Ross, Y. H. (1994). Glass transitions and crystallization in milk powders. Journal of Dairy Science, 77, Kim, E. H. J., Chen, X. D., Pearce, D. (in press). Surface characterization of four industrial spray-dried powders in relation to chemical composition, structure, and wetting property. Colloids & Surfaces, Biointerfaces. Kleyn, D. H. (1992). Textural aspects of butter. Food Technology, 46, Lazar, W. 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, 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, Papadakis, S. E., & Bahu, R. E. (1992). The sticky issues of drying. Drying Technology, 10(4), Peleg, M., & Mannheim, C. H. (1977). The mechanism of caking of powdered onion. Journal of Food Processing and Preservation, 1, Peschl, I. A. S. Z. (1989). Quality control of powders for industrial application. Powder Handling Process, 1(4), Pisecky, J. (1999). Handbook of Milk Powder Manufacture (p. 184). Niro A/S, Copenhagen, Denmark. Rennie, P. R., Chen, X. D., Hargreaves, J. B., & Mackereth, A. R. (1999). A study of the cohesion of dairy powders. Journal of Food Engineering, 39, Sukha, P. R. (2000). Heat and mass transfer in packed food powders in relation to lumping and caking. Fourth Year Research Project, Department of Chemical and Materials Engineering, the University of Auckland. Wallack, D. A., & King, C. J. (1988). Sticking and agglomeration of hygroscopic, amorphous carbohydrate and food powders. Biotechnology Progress, 4(1),

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