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Effect of powder properties and storage conditions on the flowability of milk powders with different fat contents
Journal of Food Engineering 64 (2004) 435–444 www.elsevier.com/locate/jfoodeng
Effect of powder properties and storage conditions on the flowability of milk powders with different fat contents J.J. Fitzpatrick b
a,*
, T. Iqbal a, C. Delaney a, T. Twomey a, M.K. Keogh
b
a Department of Process Engineering, University College, Cork, Ireland Teagasc, National Dairy Products Research Centre, Moorepark, Fermoy, Co. Cork, Ireland
Received 22 June 2003; accepted 11 November 2003
Abstract Consistent reliable flow of milk powders out of hoppers and silos is very important in their handling and processing. Shear cell techniques were applied in this work to measure and compare the flow properties of a commercial skim-milk powder (SMP), a whole milk powder (WMP) and a 73% high fat milk powder (HFP), and to investigate how storage temperature and exposure to moisture in air affected the flowability of these powders. These techniques were also applied to investigate how powder particle size and freefat content affected the flowability of a number of milk powders produced at pilot-scale. WMP and HFP were cohesive powders while SMP was easy flow, but SMP showed greater wall friction on the stainless steel material tested. Cohesion of SMP and WMP increased with storage temperature in the range of 5–25 C. Likewise, the cohesion of HFP increased from 5 to 20 C, but decreased at 30 and 40 C although it became very sticky at 60 C. Exposure of the powders to moisture in air at 46% relative humidity and 20 C showed a major increase in the cohesion of SMP, but had little effect on WMP and HFP. Decreasing particle size from 240 to 59 lm produced a major increase in cohesion of 26% fat milk powders. A similar effect was found with 1% fat milk powders, however decreasing particle size from 199 to 96 lm had no effect on the cohesion of 50% fat milk powders. Varying free-fat content had no major effect on the cohesion of 26% fat milk powders at 20 C. 2003 Elsevier Ltd. All rights reserved. Keywords: Powder flowability; Milk powders; Cohesion
1. Introduction There is a large quantity and variety of dairy ingredients produced industrially in powder form, and there is a need for information about their handling and processing characteristics. Powder property measurement is important because these properties intrinsically affect powder behaviour during storage, handling and processing. Powder flow properties are important in handling and processing operations, such as flow from hoppers and silos, transportation, mixing, compression and packaging (Knowlton, Carson, Klinzing, & Yang, 1994; Peleg, 1978). One of the major industrial powder problems is obtaining reliable and consistent flow out of hoppers and feeders without excessive spillage and dust generation. These problems are usually associated with
*
Corresponding author. Tel.: +353-21-4903089; fax: +353-214270249. E-mail address: j.fi[email protected] (J.J. Fitzpatrick). 0260-8774/$ - see front matter 2003 Elsevier Ltd. All rights reserved. doi:10.1016/j.jfoodeng.2003.11.011
the flow pattern inside the silo. The worst-case scenario is no flow. This can occur when the powder forms a cohesive arch across the opening, which has sufficient strength within the arch to be self-supporting. Mass flow is the ideal flow pattern where all the powder is in motion and moving downwards towards the opening. Funnel flow is where powder starts moving out through a central ‘‘funnel’’ that forms within the material, after which the powder against the walls collapse and move through the funnel. This process continues until the silo empties or until another no flow scenario occurs with the development of a stable rathole. Most flow problems are caused by a funnel flow pattern and can be cured by altering the pattern to mass flow (Johanson, 2002; Purutyan, Pittenger, & Carson, 1998). Measurement of powder flow properties is necessary for the design of mass flow hoppers. Jenike (1964) pioneered the application of shear cell techniques for measuring powder flow properties. In conjunction with the measured property data, he applied two-dimensional stress analysis in developing a
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Nomenclature SMP WMP HFP MCS UYS
skim milk powder whole milk powder high fat milk powder major consolidating stress (kPa) unconfined yield stress (kPa)
mathematical methodology for determining the minimum hopper angle and hopper opening size for mass flow from conical and wedge shaped hoppers. A hopper is the lower converging section of a silo and the hopper angle is the angle between the converging section and the horizontal. The measured flow properties used in this methodology are the flow function, the effective angle of internal friction and the angle of wall friction. The flow function is a plot of the unconfined yield stress of the powder versus major consolidating stress (Fig. 1), and represents the strength developed within a powder when consolidated, which must be overcome to make the powder flow. A flow function lying towards the bottom of the graph represents easy flow, and more difficult flow is represented as the flow functions move upwards in an anticlockwise direction. The flow index is defined as the inverse slope of the flow function. Jenike used the flow index to classify powder flowability with higher values representing easier flow. This was extended by Tomas and Schubert (1979) and is presented in Table 1. The angle of wall friction represents the adhesive strength between the powder and the silo wall material, the higher the angle the more difficult it is to move the powder along the wall surface. It is the angle between the horizontal and a straight-line from the origin intersecting the measured wall yield locus (Prescott, Ploof, & Carson, 1999), as illustrated in Fig. 2. The wall yield locus often has a positive Y -intercept, thus the angle of wall friction will vary with normal stress in the hopper, where it is higher at low stresses. Jenike’s mathematical methodology is the engineering standard practice for
Fig. 1. Flow functions: easy vs difficult flow.
RH qb qp de /w
relative humidity (%) powder bulk density (kg/m3 ) particle density (kg/m3 ) effective angle of internal friction () angle of wall friction ()
designing a hopper in terms of calculating the minimum hopper angle and opening size for mass flow. The angle of wall friction has a dominant effect in determining the minimum hopper angle required for mass flow. As a result, the hopper wall friction characteristic is critical in determining if funnel flow and its associated problems will occur in the silo. Changes in particle properties and storage conditions may influence the flowability of powders, sometimes even small changes can have significant effects. Particle size has a major influence on powder flowability. A powder maybe considered as having a particle size less than 200 lm, and as the size decreases below this, the flowability gets worse. One may not notice a major change in flowability as size is reduced from say 80–60 lm, however a noticeable disimprovement in flowability would be expected if the powder is reduced in size by an order of magnitude, for example, from 100 to 10 lm. This reduction in flowability at smaller particle size is due to the increased surface area per unit mass of powder. More surface area is available for cohesive forces, in particular, and frictional forces to resist flow. Intuitively, one would expect particle shape to affect flowability, as shape will influence the surface contacts between particles, however, there is not much reported work on the influence of shape on powder flowability. One recent paper describes the results of work which investigated how powder particle shape affected minimum hopper angle and outlet size required for mass flow (Bumiller, Carson, & Prescott, 2002). Powder moisture content usually has a significant impact on powder flowability. Increasing moisture content leads to reduced flowability due to the increase in liquid bridges and capillary forces acting between the powder particles (Scoville & Peleg, 1981). In addition, this may also lead to severe flowability problems due to powder caking. Storage conditions include storage temperature, exposure to relative humidity of air, storage time and consolidation. In general, varying the storage temperature from above freezing to 30 or 40 C does not usually have a major impact on powder flowability (Teunou & Fitzpatrick, 1999), provided no melting of components occurs or no component exceeds its glass transition temperature. For powders containing solid fats, an increase in temperature may cause some melting of fats,
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Table 1 Jenike classification of powder flowability by flow index Flowability
Hardened
Very cohesive
Cohesive
Easy flow
Free flowing
Flow index
<1
<2
<4
<10
>10
Fig. 2. Angle of wall friction.
which may produce viscous liquid bridges leading to increased cohesion. If the powder experiences a temperature below freezing, some of the water may freeze forming ice bridges between the powder particles resulting in caking. In addition, if liquefied fats are cooled, this will also result in solid bridge formation between particles leading to reduced flowability. The relative humidity of ambient air is usually a lot higher than the equilibrium relative humidity of most food powders, thus the powder may readily sorb moisture provided it is in intimate contact with air during handling, which can lead to increased cohesion and even caking. Many dairy powders are cohesive and many industrial silos storing them have mechanical and pneumatic discharge aids to help prevent arching and ratholing in an effort to maintain consistent reliable flow. There are not many reports in the literature on the flowability of dairy powders and how it is influenced by powder properties and storage conditions. The first major work was reported over 30 years ago by Buma (1971), who investigated the effect of particle size, free-fat content, temperature and moisture content on the cohesion of whole-milk powder (WMP). Cohesion was measured using an unconfined yield test, whereby a powder plug was first created by compacting a sample of powder and the pressure required to cause collapse of the unsupported plug was measured. Buma examined small particle sizes in the range of 20–40 lm and found a significant increase in cohesion at the smaller size. Cohesion is also expected to increase with increased free-fat content, especially at higher temperatures with greater fat liquefaction and liquid bridge formation. Buma determined free-fat content by solvent extraction, but found no correlation between free-fat content and
the cohesion of powders with similar particle size. Experiments were performed to investigate the effect of temperature (5, 20 and 40 C) on the cohesion of four different WMPs. The results showed a significant increase in cohesion between 5 and 20 C, but not at 40 C, except for one WMP which had the highest free-fat content. The increase in cohesion was attributed to fat liquefaction resulting in the formation of liquid bridges between the particles. Buma also conducted experiments to investigate the effect of moisture content on cohesion. The results showed a gradual reduction in cohesion from 2% to 5.5% moisture content followed by a sharp increase in cohesion above 6%. Rennie, Chen, Hargreaves, and Mackereth (1999) have also performed a study on the effect of composition, particle size, moisture and temperature on the cohesion of milk powders using an unconfined yield test similar to Buma. Their results show that the cohesion of WMP is much greater than skim-milk powder (SMP), and that reducing particle size from 200 to 80 lm produced a significant increase in cohesion for both powders. They showed that moisture content, even without the amorphous lactose transformation, affects cohesion with a sharp increase in cohesion of WMP above 6% moisture, similar to Buma’s results. Temperature also affects the cohesion of WMP, however this effect was also influenced by moisture content. WMP with 2.8% moisture showed a sharp increase in cohesion at around 45 C, while WMP with 1.8% moisture showed a sharp increase in cohesion at around 55 C. In this work, shear cell techniques were used to measure powder flow properties, including flow function and wall friction characteristics. The objective of this paper is to present results obtained from the application of these techniques for: • Comparing the flowability of three commercial milk powders with different milk-fat contents: SMP (0.9% fat), WMP (26% fat), and HFP (73% milk fat). • Investigating the effect of storage temperature and exposure to moisture in air on the flowability of the commercial powders. • Investigating the effect of particle size on the flowability of milk powders with 1%, 26% and 50% fat content produced on a pilot-scale. • Investigating the effect of free-fat content on the flowability of a milk powder with 26% fat content produced on a pilot-scale.
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2. Materials and methods 2.1. Milk powders Commercial SMP and WMP were donated by Dairygold in Mallow, Ireland, and a high fat powder (HFP) with 73% milk fat content was denotated by Kerry Ltd. in Listowel, Ireland. Teagasc Dairy Products Research Centre in Fermoy, Ireland supplied a number of powders produced in their pilot-scale spray driers. These were: Seven 26% fat milk powders with particle sizes ranging from 59 to 240 lm Three 1% fat milk powders with particle sizes ranging from 92 to 170 lm Three 50% fat milk powders with particle sizes ranging from 96 to 199 lm Seven 26% fat milk powders with free-fat contents ranging from 12.7% to 74.2%. 2.2. Physical properties • Particle size distribution was measured by laser diffraction using the Malvern Mastersizer MSS with powder feeder unit. • Moisture content (wet basis) was measured by weighing 3 g of a sample before and after drying in an oven at 105 C for 3 days. Each test was carried out in triplicate. • Bulk density was measured using an Engelsmann model A.-G. mechanical tapping device, where the volume of a given mass of powder after 1250 taps was measured to calculate the tapped bulk density. • Particle density was measured using a Micromeritis multivolume pycnometer model 1305 whose principle is gas (nitrogen) displacement. 2.3. Flow property measurement by shear cell tests 2.3.1. Flow function and effective angle of internal friction The annular shear cell (Fig. 3a) was used for measuring the flow function and effective angle of internal friction and is the same as that described by Teunou, Fitzpatrick, and Synnott (1999). It has a fixed shearing rate of 7 mm/min and external and internal diameters of 164 and 120 mm, respectively. The milk powder was removed from its package and packed into the annular shear cell. The annular shear cell was then placed in a chamber, at a temperature of 20 C, where the shear tests for measuring the instantaneous flow function were conducted. The procedure used to measure the instantaneous flow function is that recommended by the Standard Shear Test Technique (SSTT), using the Jenike shear cell (Institution of Chemical Engineers, 1989).
Fig. 3. Schematics of (a) annular shear cell used for measuring powder flow functions, and (b) Jenike shear cell used for measuring angle of wall friction.
To study the influence of temperature and relative humidity, the annular shear cell was enclosed in an insulated cabinet. The desired temperature was controlled using a Haake F3 heating/cooling system. 46% relative humidity was achieved by placing a tray of a saturated salt solution in the base of the sealed cabinet. There was a fan in the cabinet to maintain a uniform atmosphere, and the values of temperature and relative humidity in the cabinet were recorded by a thermometer and a hygrometer located inside the cabinet. There were two gauntlets, mounted on one side of the cabinet, to facilitate the experimenter manipulate powder within the controlled environment in the cabinet. For the temperature studies, the cell was packed, the lid was placed on top and the cell and powder were allowed equilibrate to the test temperature for 5 h prior to testing. For the relative humidity studies, the cell was packed and the free surface was exposed to 46% relative humidity for 18 h prior to placing the lid on top and starting the test. At the end of the test, samples were taken from the shearing region for moisture content measurement. 2.3.2. Angle of wall friction The wall yield locus of a powder was measured using a Jenike shear cell (95 mm internal diameter) whereby the cylindrical base of the cell was replaced by a flat plate of stainless steel 304 with a surface roughness of 0.2 lm Ra, as illustrated in Fig. 3b. The wall yield locus was obtained by measuring the horizontal stress required to make the powder fail at the following normal stresses: 5.9, 4.4, 3.7, 3.0, 2.3 and 1.6 kPa. The procedure used is that recommended by the Standard Shear Test Technique (SSTT), using the Jenike shear cell (Institution of Chemical Engineers, 1989). The angle of wall friction ð/w Þ reported is the angle formed with the horizontal by a line drawn from the origin to a point on the wall yield locus with a normal stress of 5.9 kPa. To study the influence of temperature on wall yield locus, the half-shear cell was first packed and then
J.J. Fitzpatrick et al. / Journal of Food Engineering 64 (2004) 435–444
placed in an incubator at the required temperature for 5 h. Then, it was removed from the incubator and the wall friction test was performed rapidly at ambient (20 C) in order to minimise cooling of the sample. Temperature measurements of powder in contact with the plate were initially performed using a thermocouple to monitor the temperature loss of a sample at 30 C during the measurement of a wall yield locus. The temperature dropped by about 2 C during the time required to measure the wall yield locus. More importantly, the temperature dropped by less than 1 C during the time required to measure the first failure stress at 5.9 kPa normal stress, which was the stress used in calculating the angle of wall friction presented in the results. To study the influence of 46% relative humidity, a sample of powder was spread out as thin layer in an open container and placed in the cabinet mentioned above where it was exposed for 18 h to an atmosphere at 46% relative humidity. At the end of this exposure time, samples were taken for moisture content measurement and the wall friction test, as described above, was performed.
439
4
UYS (kPa)
SMP WMP
3
HFP 2
1
0 0
2
4
(a)
6
MCS (kPa)
shear stress (kPa)
2 SMP WMP
1.5
HFP 1
0.5
0 0
2
(b)
4
6
normal stress (kPa)
Fig. 4. Comparison of the flowability of SMP, WMP, HFP (73% milk fat powder): (a) flow functions, (b) wall yield loci for 304 stainless steel.
2.4. Free-fat content measurement The free-fat content was determined using a solvent extraction technique where 10 g of powder was mixed gently with CCl4 for 15 min at ambient temperature according to the procedure described by A/S Niro Atomizer (1978). The tests were conducted in duplicate.
3. Results and discussion 3.1. Comparison of the flowability of commercial milk powders with different fat contents The measured flow functions for SMP, WMP and the 73% high fat powder (HFP) are presented in Fig. 4a. The cohesion developed within the SMP is much less than that of WMP, and this is in agreement with work presented by Rennie et al. (1999). The WMP and HFP have similar flow functions and are considered as very cohesive powders by their flow index given in Table 2 as classified in Table 1. On the other hand, SMP is classified as an easy flow powder. As a result, WMP and HFP
are much more susceptible to cohesive arching than SMP. Surface composition of the powder particles is expected to play an important role in its flow behaviour because flowability involves overcoming the surface attractions between powder particles. Recent work by Kim, Chen, and Pearce (2002) applied electron spectroscopy for chemical analysis to measure the surface composition of 4 industrial spray dried milk powders, including SMP (1% fat), WMP (26.5% fat) and a cream powder (71.5% fat). They showed that the surface fat content of these powders was much higher than their bulk average compositions, with the surface fat contents of SMP, WMP and cream powder being 18%, 98% and 99%, respectively. The surface fat content of SMP is a lot less than both the WMP and cream powder and this may explain its’ lower cohesiveness. The surfaces of both the WMP and the cream powder are nearly totally covered with fat, and this may explain the similarity of the flow functions of WMP and HFP measured in this work.
Table 2 Physical and flow properties of commercial milk powders at ambient conditions (20 C) Powder
SMP WMP HFP
Physical properties
Flow properties
Fat content (%w/w)
Mean particle size (lm)
Moisture content (%w/w)
Bulk density (kg/m3 )
Particle density (kg/m3 )
Flow index
de ()
/w ()
0.9 26 73
53 99 76
4.7 3.3 2
646 627 433
1133 1180 934
6.1 1.45 1.78
51.5 48 50
15.4 11 12
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Hopper wall friction characteristic is very important as this has a major role in determining if mass or funnel flow will occur in the silo. The wall yield locus of the three powders is presented in Fig. 4b and the corresponding angles of wall friction (at 5.9 kPa normal pressure) are presented in Table 2. The wall friction values obtained for the stainless steel wall material tested are considered low to medium, considering angles of wall friction, presented by Fitzpatrick, Barringer, and Iqbal (2004) for 13 food powders, varied from 12 to 27. Eventhough the SMP is considered an easier flow powder than either WMP or HFP based on flow index, it adheres more strongly to the stainless steel wall material tested. As a result, a steeper hopper angle is required to obtain mass flow for the SMP than either of the other two powders.
4
UYS (kPa)
5°C 15°C
3
25°C 2 1 0 0
2
4
6
MCS (kPa)
(a) 4
UYS (kPa)
5°C 15°C
3
25°C 2
1
3.2. Effect of storage temperature
0 0
4
6
MCS (kPa) 4
5°C
UYS (kPa)
The effect of storage temperature on the flow functions of the three commercial milk powders is shown in Fig. 5 and the corresponding flow index is presented in Table 3. For SMP, there is a small increase in the cohesiveness at 25 C. This is likely due to increased thermoplasticity of components at higher temperature, especially lactose. The effect of temperature on the cohesiveness of WMP is more pronounced. Buma (1971) also showed an increase in the cohesion of WMP with temperature from 5 to 20 C. This increase in cohesion at higher temperature is likely due to the partial melting of milk fat resulting in the formation of liquid bridges between particles causing an increase in cohesion due to capillary forces. Furthermore, when WMP enters a silo at elevated temperatures and is allowed to cool to ambient, it may undergo solidification of fats leading to the formation of solid bridges. This can greatly increase the powder cohesiveness and may lead to caking.
2
(b)
20°C
3
30°C 40°C
2
1
0 0
(c)
2
4
6
MCS (kPa)
Fig. 5. Effect of storage temperature on the flow functions of SMP, WMP and HFP.
Due to the high fat content of HFP, it was initially expected that temperature would have an even more pronounced effect. Increasing the temperature from 5 to
Table 3 Effect of storage temperature on the flowability of SMP, WMP and HFP Powder temperature (C)
Moisture before (%w/w)
Moisture after (%w/w)
Flow index
de ()
/w ()
SMP 5 15 25
4.7 4.7 4.7
6.3 6.1 4
52 51 50
14.4 15.9 16.3
WMP 5 15 25
3.3 3.3 3.3
2.3 1.76 1.38
43 48 51
10.7 10.7 12
HFP 5 20 30 40
2 2 2 2
2.2 1.74 2.43 2.35
44 50 50 47
14.1 12.3 12.1 12.6
2.1 2 1.8 1.66
J.J. Fitzpatrick et al. / Journal of Food Engineering 64 (2004) 435–444
3.3. Effect of exposure to moisture in air The three powders were exposed to moisture in air at 46% relative humidity (20 C) over an 18 h period. All three powders picked up moisture from the air, as shown in Table 4. The HFP powder picked up a lot less moisture than SMP and WMP, which is probably due to its high fat content. The effect of this moisture increase on the flow functions of the powders is illustrated in Fig. 6 and the corresponding flow index is presented in Table 4. The cohesiveness of WMP and HFP was not significantly affected, however there was a large increase in the cohesiveness of SMP. Many food powders containing lactose in its amorphous state may crystallise producing solid crystal bridges between the particles. Crystallisation will only take place if the powder temperature is greater than its glass transition temperature ðTg Þ, whereby the molecules have sufficient mobility to initiate crystallisation (Roos, 1995; Jouppila & Roos, 1994; Jouppila, Kansikas, & Roos, 1997). Tg is usually well above the storage temperature for most dry powders. However, lactose in its amorphous state is very hygroscopic and will readily sorb moisture from ambient air,
Table 4 Effect of exposure to moisture in air at 46% relative humidity (RH) and 20 C for 18 h on the flowability of SMP, WMP and HFP Moisture content (%w/w)
Flow index
de ()
3 WMPinstantaneous
2
WMP-46%RH 1 HFPinstantaneous
0 0
2
4
HFP-46%RH
6
MCS (kPa) Fig. 6. Effect of exposure to moisture in air at 46% relative humidity (RH) and 20 C on the flow functions of SMP, WMP, HFP (73% milk fat powder).
and this increase in moisture will cause a significant reduction in Tg . Crystallisation will initiate if Tg is reduced below the powder temperature, resulting in solid bridges between powder particles, which can greatly increase the cohesiveness of the powder. As SMP has the highest lactose content and the highest moisture after the 18 h exposure, this mechanism may explain the increase in cohesion. However, measurement of glass transition properties using, for example differential scanning calorimetry, would be required to confirm this. Moisture sorption had only a small effect on the wall friction of each powder with small increases at higher moistures, as shown in Table 4. Effective angles of internal friction also increased a little at higher moistures. 3.4. Effect of particle size Milk powders, with 1%, 26% and 50% milk fat content, were produced on a pilot-scale tall form drier. Powders with different mean particle sizes were produced using different size pressure nozzles. The flow properties of each of these powders were measured. Fig. 7 illustrates the effect of particle size on the flow functions of the milk powders with 1%, 26% and 50% milk fat content. For the 26% fat powder, reducing particle size from 239 to 59 lm had a significant effect on powder 4
/w ()
59 micron 3
SMP Instantaneous 46% RH
4.7 7.5
6.1 3
51.5 56
15.4 16.5
WMP Instantaneous 46% RH
3.3 6.9
1.45 1.6
48 52
11 13.4
HFP Instantaneous 46% RH
SMP-46%RH
UYS (kPa)
Powder
SMPinstantaneous
4
UYS (kPa)
20 C increased the cohesiveness of HFP, however increasing the temperature further to 30 and 40 C had the reverse affect which was unexpected, as illustrated in Fig. 5. On the other hand, it is interesting to note that Buma (1971) showed no increase in cohesion with temperature from 20 to 40 C for WMP. Tests were also performed on HFP at 60 C, however the powder became very sticky at this temperature making it very difficult to pack the powder, which gave results that were not reproducible. Storage temperature did not have a major influence on wall friction, as presented in Table 3. For both SMP and WMP, there was a small increase in wall friction at the higher temperature, while for HFP, the highest wall friction was at the lowest temperature.
441
69 micron 150 micron
2
191 micron 212 micron 220 micron
1
239 micron 0 0
2
4
6
8
MCS (kPa) 2 2.8
1.78 1.61
50 52
12.3 12.6
Fig. 7. Effect of particle size on the flow functions of milk powder with 26% milk fat content.
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Table 5 Effect of particle size on the flow properties of milk powders with 1%, 26% and 50% fat content Mean particle size (lm)
Moisture content (%w/w)
Flow index
de ()
/w ()
4.8 5.3
49 51 60 57 57 49
7.3 11.4 7.5 7.1 7.8 8 6.4 11 12 11.5
26% fat powder 59 69 150 191 212 220 239
6.7 6.4 5.3 5.3
1.9 1.7 2.5 3.4 3.3 5 10.9
1% fat powder 92 141 170
7.8 7.6 7.2
3.6 6.8 6.3
57 61 60
50% fat powder 96 161 199
3.4 3.5 3.6
2.35 2.2 2.45
41.5 43.5 44.5
cohesion by reducing the flow index from over 10 (freeflow) to just below 2 (very cohesive), as presented in Table 5. This is an expected result as reducing particle size increases the contact area between particles allowing greater interaction between cohesive forces. Likewise, for the 1% fat powder, reducing particle size from 170 to 92 lm reduced the flow index from 6.3 to 3.6. The flow index of these powders may appear unusually low for a low fat powder when compared to the commercial SMP above, however it should be noted that the moisture content of these powders is much higher thus rendering them more cohesive. For the 50% fat powder, decreasing particle size from 199 to 96 lm had no effect on the cohesion between the particles as indicated by flow index (Table 5) which was an unexpected result. Maybe the cohesiveness derived from the high fat content has a more dominant effect than reducing particle size within the range studied. The measured angles of wall friction for each of the powders were lower than expected when compared to those for the commercial powders above, however the 1% fat powder had higher wall friction than those with higher fat content. Particle size had no significant effect on the wall friction of any of the powders, as presented
8 8.2 8.4
in Table 5, and the effective angle of internal friction tended to increase with particle size. 3.5. Effect of free-fat content The physical properties of the seven 26% fat powders with free-fat content varying from 12.6% to 74.2% are similar in terms of particle size, moisture content and bulk density as presented in Table 6. The measured flow functions of the powders are illustrated in Fig. 8. Six of the seven flow functions are very close to each other with the 12.7% free-fat powder being further below the rest. There was no relationship between free-fat content and powder cohesion for these six powders, however the 12.7% free-fat powder is the lowest free-fat powder and is the least cohesive. All in all, these measurements show that free-fat content in the range of 13–74% has no major influence on the cohesion of 26% fat powder at 20 C. Buma also found no correlation between free-fat content and the cohesion of WMP with similar mean particle size. The free-fat content measured will depend on the solvent extraction process, including extraction time and temperature. In addition to the surface free-fat, solvent
Table 6 Physical properties of 26% fat powders with varying free-fat content Powder
Free-fat content (%w/w)
Mean particle size (lm)
Moisture content (%w/w)
Bulk density (kg/m3 )
12.6% 13.2% 30.6% 47.9% 49.3% 58.8% 74.2%
12.6 13.2 30.6 47.9 49.3 58.8 74.2
83 58 47 56 55 69 68
4.5 3.7 3 3 3 3.9 3.8
660 650 590 610 630 640 630
ff ff ff ff ff ff ff
J.J. Fitzpatrick et al. / Journal of Food Engineering 64 (2004) 435–444 4 12.6%ff 13.2%ff
UYS (kPa)
3
30.6%ff 47.9%ff
2
49.3%ff 58.8%ff
1
74.2%ff 0 0
2
4
6
MCS (kPa) Fig. 8. Effect of free-fat content on the flow functions of 26% fat milk powders.
extraction will extract free-fat from within the bulk of the powder particles. As powder flowability depends on surface composition, it is the surface free-fat content that is likely to play a key role in determining powder flowability and stickiness. Kim et al. (2002) found that the outer surface of industrial spray-dried WMP is largely covered by free (unprotected) fat and that fat globules protected by protein are located underneath this surface free-fat. As industrial spray-dried WMP has low free-fat content and its’ surface is nearly totally covered by free-fat, then increasing the powder free-fat content will not contribute to additional surface coverage with free-fat. This may explain why increase in freefat content had no significant effect on the flow functions of the WMP powders measured in this work.
4. Conclusions The commercial WMP and HFP can be classified as cohesive or very cohesive and are much more cohesive than SMP. They are thus more likely to give cohesive arching problems. On the other hand, SMP has greater wall friction, which suggests that a steeper hopper angle is required to obtain mass flow from a hopper constructed of the stainless steel material tested. For both SMP and WMP, powder cohesiveness tended to increase with storage temperature in the range of 5–25 C, in particular, the WMP. For HFP, cohesion increased from 5 to 20 C, however it decreased at 30 and 40 C although the powder became very sticky at 60 C. Exposure to moisture in air at 46% relative humidity and 20 C had a major effect on the cohesiveness of SMP but had little effect on either WMP or HFP. This may be due to the high lactose content of SMP and the ability of amorphous lactose to undergo glass transitions resulting in the formation of solid crystal bridges between powder particles. Storage temperature and exposure to moisture tests had little affect on the wall friction of each powder. There was a large increase in the cohesion of a 26% fat milk powder as particle size decreased from 240 to 59 lm. This is most likely due to the increased surface area
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per unit mass at smaller particle sizes. A similar effect was measured for a 1% fat milk powder, however particle size within the range of 96–199 lm appeared to have no effect on the cohesion of a 50% fat powder. Free-fat content, ranging from 13% to 74%, had no significant effect on the cohesion of a 26% fat milk powder at 20 C. Surface free-fat content is likely to play a key role in determining powder flowability, and as the surface of WMP is nearly totally covered by freefat, then increasing the powder free-fat content will not contribute to additional surface coverage with freefat.
References A/S Niro Atomizer, Copenhagen, Denmark (1978). Determination of free-fat on the surface of milk powder particles. In I. Haugaard Sorensen, J. Knag, J. Pisecky, V. Westergaard (Eds.), Analytical methods for dry milk products (fourth ed.), De Forenede Trykkerier A/S, Copenhagen, Denmark, p. 46. Buma, T. J. (1971). Free fat in spray-dried whole milk 5. Cohesion. Determination, influence of particle size, moisture content and freefat content. Netherlands Milk and Dairy Journal, 25, 107–122. Bumiller, M., Carson, J., & Prescott, J. (2002). Particle shape analysis of powders and the effect of shape on powder flow. In Fourth world congress on particle technology, Sydney, Australia. Fitzpatrick, J. J., Barringer, S. A., & Iqbal, T. (2004). Flow property measurement of food powders and sensitivity of Jenike’s hopper design methodology to the measured values. Journal of Food Engineering, 61, 399–405. Institution of Chemical Engineers (1989). Standard shear testing technique for particulate solids using the Jenike shear cell, UK. Jenike, A. W. (1964). Storage and flow of solids. Bulletin 123, Engineering Experiment Station, University of Utah, USA. Johanson, J. R. (2002). Troubleshooting bins, hoppers and feeders. Chemical Engineering Progress (April), 24–36. Jouppila, K., & Roos, Y. H. (1994). Glass transitions and crystallization in milk powders. Journal of Dairy Science, 77, 2907. Jouppila, K., Kansikas, J., & Roos, Y. H. (1997). Glass transition, water plasticization and lactose crystallization in skim milk powders. Journal of Dairy Science, 80, 3152–3160. Kim, E. H. J., Chen, X. D., & Pearce, D. (2002). Surface characterization of four industrial spray-dried powders in relation to chemical composition, structure and wetting property. Colloids and Surfaces B: Biointerfaces, 26, 197–212. Knowlton, T. M., Carson, J. W., Klinzing, G. E., & Yang, W. C. (1994). The importance of storage, transfer and collection. Chemical Engineering Progress, 90, 44–54. Peleg, M. (1978). Flowability of food powders and methods for its evaluation––a review. Journal of Food Process Engineering, 1, 303– 328. Prescott, J. K., Ploof, D. A., & Carson, J. W. (1999). Developing a better understanding of wall friction. Powder Processing and Handling, 11(1), 25–36. Purutyan, H., Pittenger, B. H., & Carson, J. W. (1998). Solve solids handling problems by retrofitting. Chemical Engineering Progress (April), 27–39. Rennie, P. R., Chen, X. D., Hargreaves, C., & Mackereth, A. R. (1999). A study of the cohesion of dairy powders. Journal of Food Engineering, 39, 277–284. Roos, Y. H. (1995). Phase transitions in foods. San Diego, CA, USA: Academic Press.
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Scoville, E., & Peleg, M. (1981). Evaluation of the effect of liquid bridges on the bulk properties of model powders. Journal of Food Science, 46, 174–177. Teunou, E., Fitzpatrick, J. J., & Synnott, E. C. (1999). Characterisation of food powder flowability. Journal of Food Engineering, 39, 31–37.
Teunou, E., & Fitzpatrick, J. J. (1999). Effect of relative humidity and temperature on food powder flowability. Journal of Food Engineering, 42(2), 109–116. Tomas, J., & Schubert, H. (1979). Particle characterisation, Partec 79 (pp. 301–319). Nurnberg, Germany.
Effect of powder properties and storage conditions on the flowability of milk powders with different fat contents J.J. Fitzpatrick b
a,*
, T. Iqbal a, C. Delaney a, T. Twomey a, M.K. Keogh
b
a Department of Process Engineering, University College, Cork, Ireland Teagasc, National Dairy Products Research Centre, Moorepark, Fermoy, Co. Cork, Ireland
Received 22 June 2003; accepted 11 November 2003
Abstract Consistent reliable flow of milk powders out of hoppers and silos is very important in their handling and processing. Shear cell techniques were applied in this work to measure and compare the flow properties of a commercial skim-milk powder (SMP), a whole milk powder (WMP) and a 73% high fat milk powder (HFP), and to investigate how storage temperature and exposure to moisture in air affected the flowability of these powders. These techniques were also applied to investigate how powder particle size and freefat content affected the flowability of a number of milk powders produced at pilot-scale. WMP and HFP were cohesive powders while SMP was easy flow, but SMP showed greater wall friction on the stainless steel material tested. Cohesion of SMP and WMP increased with storage temperature in the range of 5–25 C. Likewise, the cohesion of HFP increased from 5 to 20 C, but decreased at 30 and 40 C although it became very sticky at 60 C. Exposure of the powders to moisture in air at 46% relative humidity and 20 C showed a major increase in the cohesion of SMP, but had little effect on WMP and HFP. Decreasing particle size from 240 to 59 lm produced a major increase in cohesion of 26% fat milk powders. A similar effect was found with 1% fat milk powders, however decreasing particle size from 199 to 96 lm had no effect on the cohesion of 50% fat milk powders. Varying free-fat content had no major effect on the cohesion of 26% fat milk powders at 20 C. 2003 Elsevier Ltd. All rights reserved. Keywords: Powder flowability; Milk powders; Cohesion
1. Introduction There is a large quantity and variety of dairy ingredients produced industrially in powder form, and there is a need for information about their handling and processing characteristics. Powder property measurement is important because these properties intrinsically affect powder behaviour during storage, handling and processing. Powder flow properties are important in handling and processing operations, such as flow from hoppers and silos, transportation, mixing, compression and packaging (Knowlton, Carson, Klinzing, & Yang, 1994; Peleg, 1978). One of the major industrial powder problems is obtaining reliable and consistent flow out of hoppers and feeders without excessive spillage and dust generation. These problems are usually associated with
*
Corresponding author. Tel.: +353-21-4903089; fax: +353-214270249. E-mail address: j.fi[email protected] (J.J. Fitzpatrick). 0260-8774/$ - see front matter 2003 Elsevier Ltd. All rights reserved. doi:10.1016/j.jfoodeng.2003.11.011
the flow pattern inside the silo. The worst-case scenario is no flow. This can occur when the powder forms a cohesive arch across the opening, which has sufficient strength within the arch to be self-supporting. Mass flow is the ideal flow pattern where all the powder is in motion and moving downwards towards the opening. Funnel flow is where powder starts moving out through a central ‘‘funnel’’ that forms within the material, after which the powder against the walls collapse and move through the funnel. This process continues until the silo empties or until another no flow scenario occurs with the development of a stable rathole. Most flow problems are caused by a funnel flow pattern and can be cured by altering the pattern to mass flow (Johanson, 2002; Purutyan, Pittenger, & Carson, 1998). Measurement of powder flow properties is necessary for the design of mass flow hoppers. Jenike (1964) pioneered the application of shear cell techniques for measuring powder flow properties. In conjunction with the measured property data, he applied two-dimensional stress analysis in developing a
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Nomenclature SMP WMP HFP MCS UYS
skim milk powder whole milk powder high fat milk powder major consolidating stress (kPa) unconfined yield stress (kPa)
mathematical methodology for determining the minimum hopper angle and hopper opening size for mass flow from conical and wedge shaped hoppers. A hopper is the lower converging section of a silo and the hopper angle is the angle between the converging section and the horizontal. The measured flow properties used in this methodology are the flow function, the effective angle of internal friction and the angle of wall friction. The flow function is a plot of the unconfined yield stress of the powder versus major consolidating stress (Fig. 1), and represents the strength developed within a powder when consolidated, which must be overcome to make the powder flow. A flow function lying towards the bottom of the graph represents easy flow, and more difficult flow is represented as the flow functions move upwards in an anticlockwise direction. The flow index is defined as the inverse slope of the flow function. Jenike used the flow index to classify powder flowability with higher values representing easier flow. This was extended by Tomas and Schubert (1979) and is presented in Table 1. The angle of wall friction represents the adhesive strength between the powder and the silo wall material, the higher the angle the more difficult it is to move the powder along the wall surface. It is the angle between the horizontal and a straight-line from the origin intersecting the measured wall yield locus (Prescott, Ploof, & Carson, 1999), as illustrated in Fig. 2. The wall yield locus often has a positive Y -intercept, thus the angle of wall friction will vary with normal stress in the hopper, where it is higher at low stresses. Jenike’s mathematical methodology is the engineering standard practice for
Fig. 1. Flow functions: easy vs difficult flow.
RH qb qp de /w
relative humidity (%) powder bulk density (kg/m3 ) particle density (kg/m3 ) effective angle of internal friction () angle of wall friction ()
designing a hopper in terms of calculating the minimum hopper angle and opening size for mass flow. The angle of wall friction has a dominant effect in determining the minimum hopper angle required for mass flow. As a result, the hopper wall friction characteristic is critical in determining if funnel flow and its associated problems will occur in the silo. Changes in particle properties and storage conditions may influence the flowability of powders, sometimes even small changes can have significant effects. Particle size has a major influence on powder flowability. A powder maybe considered as having a particle size less than 200 lm, and as the size decreases below this, the flowability gets worse. One may not notice a major change in flowability as size is reduced from say 80–60 lm, however a noticeable disimprovement in flowability would be expected if the powder is reduced in size by an order of magnitude, for example, from 100 to 10 lm. This reduction in flowability at smaller particle size is due to the increased surface area per unit mass of powder. More surface area is available for cohesive forces, in particular, and frictional forces to resist flow. Intuitively, one would expect particle shape to affect flowability, as shape will influence the surface contacts between particles, however, there is not much reported work on the influence of shape on powder flowability. One recent paper describes the results of work which investigated how powder particle shape affected minimum hopper angle and outlet size required for mass flow (Bumiller, Carson, & Prescott, 2002). Powder moisture content usually has a significant impact on powder flowability. Increasing moisture content leads to reduced flowability due to the increase in liquid bridges and capillary forces acting between the powder particles (Scoville & Peleg, 1981). In addition, this may also lead to severe flowability problems due to powder caking. Storage conditions include storage temperature, exposure to relative humidity of air, storage time and consolidation. In general, varying the storage temperature from above freezing to 30 or 40 C does not usually have a major impact on powder flowability (Teunou & Fitzpatrick, 1999), provided no melting of components occurs or no component exceeds its glass transition temperature. For powders containing solid fats, an increase in temperature may cause some melting of fats,
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Table 1 Jenike classification of powder flowability by flow index Flowability
Hardened
Very cohesive
Cohesive
Easy flow
Free flowing
Flow index
<1
<2
<4
<10
>10
Fig. 2. Angle of wall friction.
which may produce viscous liquid bridges leading to increased cohesion. If the powder experiences a temperature below freezing, some of the water may freeze forming ice bridges between the powder particles resulting in caking. In addition, if liquefied fats are cooled, this will also result in solid bridge formation between particles leading to reduced flowability. The relative humidity of ambient air is usually a lot higher than the equilibrium relative humidity of most food powders, thus the powder may readily sorb moisture provided it is in intimate contact with air during handling, which can lead to increased cohesion and even caking. Many dairy powders are cohesive and many industrial silos storing them have mechanical and pneumatic discharge aids to help prevent arching and ratholing in an effort to maintain consistent reliable flow. There are not many reports in the literature on the flowability of dairy powders and how it is influenced by powder properties and storage conditions. The first major work was reported over 30 years ago by Buma (1971), who investigated the effect of particle size, free-fat content, temperature and moisture content on the cohesion of whole-milk powder (WMP). Cohesion was measured using an unconfined yield test, whereby a powder plug was first created by compacting a sample of powder and the pressure required to cause collapse of the unsupported plug was measured. Buma examined small particle sizes in the range of 20–40 lm and found a significant increase in cohesion at the smaller size. Cohesion is also expected to increase with increased free-fat content, especially at higher temperatures with greater fat liquefaction and liquid bridge formation. Buma determined free-fat content by solvent extraction, but found no correlation between free-fat content and
the cohesion of powders with similar particle size. Experiments were performed to investigate the effect of temperature (5, 20 and 40 C) on the cohesion of four different WMPs. The results showed a significant increase in cohesion between 5 and 20 C, but not at 40 C, except for one WMP which had the highest free-fat content. The increase in cohesion was attributed to fat liquefaction resulting in the formation of liquid bridges between the particles. Buma also conducted experiments to investigate the effect of moisture content on cohesion. The results showed a gradual reduction in cohesion from 2% to 5.5% moisture content followed by a sharp increase in cohesion above 6%. Rennie, Chen, Hargreaves, and Mackereth (1999) have also performed a study on the effect of composition, particle size, moisture and temperature on the cohesion of milk powders using an unconfined yield test similar to Buma. Their results show that the cohesion of WMP is much greater than skim-milk powder (SMP), and that reducing particle size from 200 to 80 lm produced a significant increase in cohesion for both powders. They showed that moisture content, even without the amorphous lactose transformation, affects cohesion with a sharp increase in cohesion of WMP above 6% moisture, similar to Buma’s results. Temperature also affects the cohesion of WMP, however this effect was also influenced by moisture content. WMP with 2.8% moisture showed a sharp increase in cohesion at around 45 C, while WMP with 1.8% moisture showed a sharp increase in cohesion at around 55 C. In this work, shear cell techniques were used to measure powder flow properties, including flow function and wall friction characteristics. The objective of this paper is to present results obtained from the application of these techniques for: • Comparing the flowability of three commercial milk powders with different milk-fat contents: SMP (0.9% fat), WMP (26% fat), and HFP (73% milk fat). • Investigating the effect of storage temperature and exposure to moisture in air on the flowability of the commercial powders. • Investigating the effect of particle size on the flowability of milk powders with 1%, 26% and 50% fat content produced on a pilot-scale. • Investigating the effect of free-fat content on the flowability of a milk powder with 26% fat content produced on a pilot-scale.
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2. Materials and methods 2.1. Milk powders Commercial SMP and WMP were donated by Dairygold in Mallow, Ireland, and a high fat powder (HFP) with 73% milk fat content was denotated by Kerry Ltd. in Listowel, Ireland. Teagasc Dairy Products Research Centre in Fermoy, Ireland supplied a number of powders produced in their pilot-scale spray driers. These were: Seven 26% fat milk powders with particle sizes ranging from 59 to 240 lm Three 1% fat milk powders with particle sizes ranging from 92 to 170 lm Three 50% fat milk powders with particle sizes ranging from 96 to 199 lm Seven 26% fat milk powders with free-fat contents ranging from 12.7% to 74.2%. 2.2. Physical properties • Particle size distribution was measured by laser diffraction using the Malvern Mastersizer MSS with powder feeder unit. • Moisture content (wet basis) was measured by weighing 3 g of a sample before and after drying in an oven at 105 C for 3 days. Each test was carried out in triplicate. • Bulk density was measured using an Engelsmann model A.-G. mechanical tapping device, where the volume of a given mass of powder after 1250 taps was measured to calculate the tapped bulk density. • Particle density was measured using a Micromeritis multivolume pycnometer model 1305 whose principle is gas (nitrogen) displacement. 2.3. Flow property measurement by shear cell tests 2.3.1. Flow function and effective angle of internal friction The annular shear cell (Fig. 3a) was used for measuring the flow function and effective angle of internal friction and is the same as that described by Teunou, Fitzpatrick, and Synnott (1999). It has a fixed shearing rate of 7 mm/min and external and internal diameters of 164 and 120 mm, respectively. The milk powder was removed from its package and packed into the annular shear cell. The annular shear cell was then placed in a chamber, at a temperature of 20 C, where the shear tests for measuring the instantaneous flow function were conducted. The procedure used to measure the instantaneous flow function is that recommended by the Standard Shear Test Technique (SSTT), using the Jenike shear cell (Institution of Chemical Engineers, 1989).
Fig. 3. Schematics of (a) annular shear cell used for measuring powder flow functions, and (b) Jenike shear cell used for measuring angle of wall friction.
To study the influence of temperature and relative humidity, the annular shear cell was enclosed in an insulated cabinet. The desired temperature was controlled using a Haake F3 heating/cooling system. 46% relative humidity was achieved by placing a tray of a saturated salt solution in the base of the sealed cabinet. There was a fan in the cabinet to maintain a uniform atmosphere, and the values of temperature and relative humidity in the cabinet were recorded by a thermometer and a hygrometer located inside the cabinet. There were two gauntlets, mounted on one side of the cabinet, to facilitate the experimenter manipulate powder within the controlled environment in the cabinet. For the temperature studies, the cell was packed, the lid was placed on top and the cell and powder were allowed equilibrate to the test temperature for 5 h prior to testing. For the relative humidity studies, the cell was packed and the free surface was exposed to 46% relative humidity for 18 h prior to placing the lid on top and starting the test. At the end of the test, samples were taken from the shearing region for moisture content measurement. 2.3.2. Angle of wall friction The wall yield locus of a powder was measured using a Jenike shear cell (95 mm internal diameter) whereby the cylindrical base of the cell was replaced by a flat plate of stainless steel 304 with a surface roughness of 0.2 lm Ra, as illustrated in Fig. 3b. The wall yield locus was obtained by measuring the horizontal stress required to make the powder fail at the following normal stresses: 5.9, 4.4, 3.7, 3.0, 2.3 and 1.6 kPa. The procedure used is that recommended by the Standard Shear Test Technique (SSTT), using the Jenike shear cell (Institution of Chemical Engineers, 1989). The angle of wall friction ð/w Þ reported is the angle formed with the horizontal by a line drawn from the origin to a point on the wall yield locus with a normal stress of 5.9 kPa. To study the influence of temperature on wall yield locus, the half-shear cell was first packed and then
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placed in an incubator at the required temperature for 5 h. Then, it was removed from the incubator and the wall friction test was performed rapidly at ambient (20 C) in order to minimise cooling of the sample. Temperature measurements of powder in contact with the plate were initially performed using a thermocouple to monitor the temperature loss of a sample at 30 C during the measurement of a wall yield locus. The temperature dropped by about 2 C during the time required to measure the wall yield locus. More importantly, the temperature dropped by less than 1 C during the time required to measure the first failure stress at 5.9 kPa normal stress, which was the stress used in calculating the angle of wall friction presented in the results. To study the influence of 46% relative humidity, a sample of powder was spread out as thin layer in an open container and placed in the cabinet mentioned above where it was exposed for 18 h to an atmosphere at 46% relative humidity. At the end of this exposure time, samples were taken for moisture content measurement and the wall friction test, as described above, was performed.
439
4
UYS (kPa)
SMP WMP
3
HFP 2
1
0 0
2
4
(a)
6
MCS (kPa)
shear stress (kPa)
2 SMP WMP
1.5
HFP 1
0.5
0 0
2
(b)
4
6
normal stress (kPa)
Fig. 4. Comparison of the flowability of SMP, WMP, HFP (73% milk fat powder): (a) flow functions, (b) wall yield loci for 304 stainless steel.
2.4. Free-fat content measurement The free-fat content was determined using a solvent extraction technique where 10 g of powder was mixed gently with CCl4 for 15 min at ambient temperature according to the procedure described by A/S Niro Atomizer (1978). The tests were conducted in duplicate.
3. Results and discussion 3.1. Comparison of the flowability of commercial milk powders with different fat contents The measured flow functions for SMP, WMP and the 73% high fat powder (HFP) are presented in Fig. 4a. The cohesion developed within the SMP is much less than that of WMP, and this is in agreement with work presented by Rennie et al. (1999). The WMP and HFP have similar flow functions and are considered as very cohesive powders by their flow index given in Table 2 as classified in Table 1. On the other hand, SMP is classified as an easy flow powder. As a result, WMP and HFP
are much more susceptible to cohesive arching than SMP. Surface composition of the powder particles is expected to play an important role in its flow behaviour because flowability involves overcoming the surface attractions between powder particles. Recent work by Kim, Chen, and Pearce (2002) applied electron spectroscopy for chemical analysis to measure the surface composition of 4 industrial spray dried milk powders, including SMP (1% fat), WMP (26.5% fat) and a cream powder (71.5% fat). They showed that the surface fat content of these powders was much higher than their bulk average compositions, with the surface fat contents of SMP, WMP and cream powder being 18%, 98% and 99%, respectively. The surface fat content of SMP is a lot less than both the WMP and cream powder and this may explain its’ lower cohesiveness. The surfaces of both the WMP and the cream powder are nearly totally covered with fat, and this may explain the similarity of the flow functions of WMP and HFP measured in this work.
Table 2 Physical and flow properties of commercial milk powders at ambient conditions (20 C) Powder
SMP WMP HFP
Physical properties
Flow properties
Fat content (%w/w)
Mean particle size (lm)
Moisture content (%w/w)
Bulk density (kg/m3 )
Particle density (kg/m3 )
Flow index
de ()
/w ()
0.9 26 73
53 99 76
4.7 3.3 2
646 627 433
1133 1180 934
6.1 1.45 1.78
51.5 48 50
15.4 11 12
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Hopper wall friction characteristic is very important as this has a major role in determining if mass or funnel flow will occur in the silo. The wall yield locus of the three powders is presented in Fig. 4b and the corresponding angles of wall friction (at 5.9 kPa normal pressure) are presented in Table 2. The wall friction values obtained for the stainless steel wall material tested are considered low to medium, considering angles of wall friction, presented by Fitzpatrick, Barringer, and Iqbal (2004) for 13 food powders, varied from 12 to 27. Eventhough the SMP is considered an easier flow powder than either WMP or HFP based on flow index, it adheres more strongly to the stainless steel wall material tested. As a result, a steeper hopper angle is required to obtain mass flow for the SMP than either of the other two powders.
4
UYS (kPa)
5°C 15°C
3
25°C 2 1 0 0
2
4
6
MCS (kPa)
(a) 4
UYS (kPa)
5°C 15°C
3
25°C 2
1
3.2. Effect of storage temperature
0 0
4
6
MCS (kPa) 4
5°C
UYS (kPa)
The effect of storage temperature on the flow functions of the three commercial milk powders is shown in Fig. 5 and the corresponding flow index is presented in Table 3. For SMP, there is a small increase in the cohesiveness at 25 C. This is likely due to increased thermoplasticity of components at higher temperature, especially lactose. The effect of temperature on the cohesiveness of WMP is more pronounced. Buma (1971) also showed an increase in the cohesion of WMP with temperature from 5 to 20 C. This increase in cohesion at higher temperature is likely due to the partial melting of milk fat resulting in the formation of liquid bridges between particles causing an increase in cohesion due to capillary forces. Furthermore, when WMP enters a silo at elevated temperatures and is allowed to cool to ambient, it may undergo solidification of fats leading to the formation of solid bridges. This can greatly increase the powder cohesiveness and may lead to caking.
2
(b)
20°C
3
30°C 40°C
2
1
0 0
(c)
2
4
6
MCS (kPa)
Fig. 5. Effect of storage temperature on the flow functions of SMP, WMP and HFP.
Due to the high fat content of HFP, it was initially expected that temperature would have an even more pronounced effect. Increasing the temperature from 5 to
Table 3 Effect of storage temperature on the flowability of SMP, WMP and HFP Powder temperature (C)
Moisture before (%w/w)
Moisture after (%w/w)
Flow index
de ()
/w ()
SMP 5 15 25
4.7 4.7 4.7
6.3 6.1 4
52 51 50
14.4 15.9 16.3
WMP 5 15 25
3.3 3.3 3.3
2.3 1.76 1.38
43 48 51
10.7 10.7 12
HFP 5 20 30 40
2 2 2 2
2.2 1.74 2.43 2.35
44 50 50 47
14.1 12.3 12.1 12.6
2.1 2 1.8 1.66
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3.3. Effect of exposure to moisture in air The three powders were exposed to moisture in air at 46% relative humidity (20 C) over an 18 h period. All three powders picked up moisture from the air, as shown in Table 4. The HFP powder picked up a lot less moisture than SMP and WMP, which is probably due to its high fat content. The effect of this moisture increase on the flow functions of the powders is illustrated in Fig. 6 and the corresponding flow index is presented in Table 4. The cohesiveness of WMP and HFP was not significantly affected, however there was a large increase in the cohesiveness of SMP. Many food powders containing lactose in its amorphous state may crystallise producing solid crystal bridges between the particles. Crystallisation will only take place if the powder temperature is greater than its glass transition temperature ðTg Þ, whereby the molecules have sufficient mobility to initiate crystallisation (Roos, 1995; Jouppila & Roos, 1994; Jouppila, Kansikas, & Roos, 1997). Tg is usually well above the storage temperature for most dry powders. However, lactose in its amorphous state is very hygroscopic and will readily sorb moisture from ambient air,
Table 4 Effect of exposure to moisture in air at 46% relative humidity (RH) and 20 C for 18 h on the flowability of SMP, WMP and HFP Moisture content (%w/w)
Flow index
de ()
3 WMPinstantaneous
2
WMP-46%RH 1 HFPinstantaneous
0 0
2
4
HFP-46%RH
6
MCS (kPa) Fig. 6. Effect of exposure to moisture in air at 46% relative humidity (RH) and 20 C on the flow functions of SMP, WMP, HFP (73% milk fat powder).
and this increase in moisture will cause a significant reduction in Tg . Crystallisation will initiate if Tg is reduced below the powder temperature, resulting in solid bridges between powder particles, which can greatly increase the cohesiveness of the powder. As SMP has the highest lactose content and the highest moisture after the 18 h exposure, this mechanism may explain the increase in cohesion. However, measurement of glass transition properties using, for example differential scanning calorimetry, would be required to confirm this. Moisture sorption had only a small effect on the wall friction of each powder with small increases at higher moistures, as shown in Table 4. Effective angles of internal friction also increased a little at higher moistures. 3.4. Effect of particle size Milk powders, with 1%, 26% and 50% milk fat content, were produced on a pilot-scale tall form drier. Powders with different mean particle sizes were produced using different size pressure nozzles. The flow properties of each of these powders were measured. Fig. 7 illustrates the effect of particle size on the flow functions of the milk powders with 1%, 26% and 50% milk fat content. For the 26% fat powder, reducing particle size from 239 to 59 lm had a significant effect on powder 4
/w ()
59 micron 3
SMP Instantaneous 46% RH
4.7 7.5
6.1 3
51.5 56
15.4 16.5
WMP Instantaneous 46% RH
3.3 6.9
1.45 1.6
48 52
11 13.4
HFP Instantaneous 46% RH
SMP-46%RH
UYS (kPa)
Powder
SMPinstantaneous
4
UYS (kPa)
20 C increased the cohesiveness of HFP, however increasing the temperature further to 30 and 40 C had the reverse affect which was unexpected, as illustrated in Fig. 5. On the other hand, it is interesting to note that Buma (1971) showed no increase in cohesion with temperature from 20 to 40 C for WMP. Tests were also performed on HFP at 60 C, however the powder became very sticky at this temperature making it very difficult to pack the powder, which gave results that were not reproducible. Storage temperature did not have a major influence on wall friction, as presented in Table 3. For both SMP and WMP, there was a small increase in wall friction at the higher temperature, while for HFP, the highest wall friction was at the lowest temperature.
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69 micron 150 micron
2
191 micron 212 micron 220 micron
1
239 micron 0 0
2
4
6
8
MCS (kPa) 2 2.8
1.78 1.61
50 52
12.3 12.6
Fig. 7. Effect of particle size on the flow functions of milk powder with 26% milk fat content.
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Table 5 Effect of particle size on the flow properties of milk powders with 1%, 26% and 50% fat content Mean particle size (lm)
Moisture content (%w/w)
Flow index
de ()
/w ()
4.8 5.3
49 51 60 57 57 49
7.3 11.4 7.5 7.1 7.8 8 6.4 11 12 11.5
26% fat powder 59 69 150 191 212 220 239
6.7 6.4 5.3 5.3
1.9 1.7 2.5 3.4 3.3 5 10.9
1% fat powder 92 141 170
7.8 7.6 7.2
3.6 6.8 6.3
57 61 60
50% fat powder 96 161 199
3.4 3.5 3.6
2.35 2.2 2.45
41.5 43.5 44.5
cohesion by reducing the flow index from over 10 (freeflow) to just below 2 (very cohesive), as presented in Table 5. This is an expected result as reducing particle size increases the contact area between particles allowing greater interaction between cohesive forces. Likewise, for the 1% fat powder, reducing particle size from 170 to 92 lm reduced the flow index from 6.3 to 3.6. The flow index of these powders may appear unusually low for a low fat powder when compared to the commercial SMP above, however it should be noted that the moisture content of these powders is much higher thus rendering them more cohesive. For the 50% fat powder, decreasing particle size from 199 to 96 lm had no effect on the cohesion between the particles as indicated by flow index (Table 5) which was an unexpected result. Maybe the cohesiveness derived from the high fat content has a more dominant effect than reducing particle size within the range studied. The measured angles of wall friction for each of the powders were lower than expected when compared to those for the commercial powders above, however the 1% fat powder had higher wall friction than those with higher fat content. Particle size had no significant effect on the wall friction of any of the powders, as presented
8 8.2 8.4
in Table 5, and the effective angle of internal friction tended to increase with particle size. 3.5. Effect of free-fat content The physical properties of the seven 26% fat powders with free-fat content varying from 12.6% to 74.2% are similar in terms of particle size, moisture content and bulk density as presented in Table 6. The measured flow functions of the powders are illustrated in Fig. 8. Six of the seven flow functions are very close to each other with the 12.7% free-fat powder being further below the rest. There was no relationship between free-fat content and powder cohesion for these six powders, however the 12.7% free-fat powder is the lowest free-fat powder and is the least cohesive. All in all, these measurements show that free-fat content in the range of 13–74% has no major influence on the cohesion of 26% fat powder at 20 C. Buma also found no correlation between free-fat content and the cohesion of WMP with similar mean particle size. The free-fat content measured will depend on the solvent extraction process, including extraction time and temperature. In addition to the surface free-fat, solvent
Table 6 Physical properties of 26% fat powders with varying free-fat content Powder
Free-fat content (%w/w)
Mean particle size (lm)
Moisture content (%w/w)
Bulk density (kg/m3 )
12.6% 13.2% 30.6% 47.9% 49.3% 58.8% 74.2%
12.6 13.2 30.6 47.9 49.3 58.8 74.2
83 58 47 56 55 69 68
4.5 3.7 3 3 3 3.9 3.8
660 650 590 610 630 640 630
ff ff ff ff ff ff ff
J.J. Fitzpatrick et al. / Journal of Food Engineering 64 (2004) 435–444 4 12.6%ff 13.2%ff
UYS (kPa)
3
30.6%ff 47.9%ff
2
49.3%ff 58.8%ff
1
74.2%ff 0 0
2
4
6
MCS (kPa) Fig. 8. Effect of free-fat content on the flow functions of 26% fat milk powders.
extraction will extract free-fat from within the bulk of the powder particles. As powder flowability depends on surface composition, it is the surface free-fat content that is likely to play a key role in determining powder flowability and stickiness. Kim et al. (2002) found that the outer surface of industrial spray-dried WMP is largely covered by free (unprotected) fat and that fat globules protected by protein are located underneath this surface free-fat. As industrial spray-dried WMP has low free-fat content and its’ surface is nearly totally covered by free-fat, then increasing the powder free-fat content will not contribute to additional surface coverage with free-fat. This may explain why increase in freefat content had no significant effect on the flow functions of the WMP powders measured in this work.
4. Conclusions The commercial WMP and HFP can be classified as cohesive or very cohesive and are much more cohesive than SMP. They are thus more likely to give cohesive arching problems. On the other hand, SMP has greater wall friction, which suggests that a steeper hopper angle is required to obtain mass flow from a hopper constructed of the stainless steel material tested. For both SMP and WMP, powder cohesiveness tended to increase with storage temperature in the range of 5–25 C, in particular, the WMP. For HFP, cohesion increased from 5 to 20 C, however it decreased at 30 and 40 C although the powder became very sticky at 60 C. Exposure to moisture in air at 46% relative humidity and 20 C had a major effect on the cohesiveness of SMP but had little effect on either WMP or HFP. This may be due to the high lactose content of SMP and the ability of amorphous lactose to undergo glass transitions resulting in the formation of solid crystal bridges between powder particles. Storage temperature and exposure to moisture tests had little affect on the wall friction of each powder. There was a large increase in the cohesion of a 26% fat milk powder as particle size decreased from 240 to 59 lm. This is most likely due to the increased surface area
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per unit mass at smaller particle sizes. A similar effect was measured for a 1% fat milk powder, however particle size within the range of 96–199 lm appeared to have no effect on the cohesion of a 50% fat powder. Free-fat content, ranging from 13% to 74%, had no significant effect on the cohesion of a 26% fat milk powder at 20 C. Surface free-fat content is likely to play a key role in determining powder flowability, and as the surface of WMP is nearly totally covered by freefat, then increasing the powder free-fat content will not contribute to additional surface coverage with freefat.
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