The effect of surface composition on the functional properties of
milk powders
Nijdam, J.J. and Langrish, T.A.G.1
Chemical Engineering Department, University of Sydney,
NSW2006, Australia
Abstract
The migration of lactose, protein and fat within milk droplets and particles in a
spray dryer is investigated with a view to eventually modelling this process using
Computational Fluid Dynamics. Both protein and fat accumulate preferentially at the
surface of the milk particles as they dry, at the expense of lactose. This has repercussions
for the rate of particle agglomeration and wall deposition within the spray dryer, and the
functional properties of the dried powder, because the fat and lactose surface
concentrations affect the stickiness of the milk particles. The surface fat coverage, and
hence the particle stickiness, is particularly sensitive to small changes in fat content
1
Corresponding Author. Tel.: +61-2-9351-4568; fax: +61-2-9351-2854
Email address: timl@chem.eng.usyd.edu.au (Associate Professor Tim Langrish)
1
between 0 and 5%, which is likely to be important for the control of powder properties
and the operation of spray drying equipment in skim milk production. In addition, a
higher drying temperature favours the appearance of lactose over protein at the surface of
the milk particle. We postulate that higher temperatures hasten the formation of a surface
skin, which hinders the migration of surface-active protein towards the surface. Finally,
we have confirmed observations made by various other researchers on the morphological
evolution of a milk droplet as it dries, which involves the formation of a skin and a
vacuole, and the inflation and subsequent shrinkage of the particle.
Key words: Spray drying; caking; stickiness; particle morphology; drying temperature; Xray photoelectron spectrometry; agglomeration
1. Introduction
Milk is spray dried for easier storage, handling and transport. The spray drying
process involves atomising the milk within a flow of hot air, where water is progressively
evaporated from the droplets until dried milk particles are produced. The functional
properties of the resultant milk powder, such as particle size distribution, bulk density,
flowability and solubility, determine its storage, handling and transport capabilities.
There has been considerable work done in the past decade by various research groups
around the world to numerically model the spray drying process using Computational
Fluid Dynamics (CFD), with the ultimate aim of predicting these powder properties in
order to facilitate the design of improved spray dryers and new milk products. Thus far,
2
successful validation work has been carried out on various CFD models, which has
highlighted their ability to accurately predict 1) the transient and swirling airflow patterns
found in many spray dryers (Langrish et al., 2004; Guo et al., 2003), 2) the turbulent
dispersion of droplets and particles within a spray and hence their trajectories through the
dryer (Rüger et al., 2000; Berlemont et al., 1990; Nijdam et al., 2004a), and 3) the
coalescence and evaporation of droplets (Rüger et al., 2000; Nijdam et al., 2004 a, b).
How changes in the morphology of a droplet are modelled once sufficient
moisture is evaporated that a skin develops on its surface is less clear. Indeed, the
transformation of a milk droplet into a particle is rather complex. This process involves
the concentration of various milk components, such as lactose, fat and protein, at the
surface of the droplet as moisture evaporates, and the subsequent development of a skin.
It concerns the formation of a vacuole (a vapour bubble) within the droplet, once the
surface skin appears, that repeatedly inflates and deflates (Hassan and Mumford, 1993;
Walton, 2000; Hecht and King, 2000a), which affects the porosity of the particle.
Another important phenomenon is the migration of milk components through the aqueous
phase to the skin and on towards the surface, because the surface moisture, lactose and fat
concentrations affect the stickiness and agglomeration of particles, which influence the
particle size distribution, bulk density and flowability of the resultant milk powder.
Moreover, the presence of fat on the particle surfaces renders the milk powder
hydrophobic, such that its solubility in water is reduced, in addition to making it readily
susceptible to oxidation and subsequent rancidity (Pisecky, 1997).
Models have been developed to predict the evolution of a vacuole (inflation and
deflation cycle) within the droplet as it dries (Hecht and King, 2000b; Sano and Keey,
3
1982), although these models are not practical to simulate many particles simultaneously
within a spray dryer in addition to modelling the trajectories and drying of these particles.
A promising method for simulating the drying of a multitude of moist particles involves
the concept of the characteristic drying curve, which is an empirical, lumped parameter
approach in which the complexities of internal moisture transport are avoided by relating
the drying rate to the average moisture content of the particle (Harvie et al., 2002).
However, the clear disadvantage of this technique is that the surface moisture content and
temperature cannot be predicted, which are important variables for determining whether
the glass transition temperature of surface lactose has been exceeded, and therefore
whether the particle is sticky (Lloyd et al., 1996; Jouppila and Roos, 1994). This is likely
to be a key consideration when modelling particle agglomeration and wall deposition
within spray dryers. Straatsma et al. (1999) have overcome the shortcomings of the
characteristic drying curve method by developing a mathematical model to simulate the
internal diffusion of moisture within particles as they dry while simultaneously
calculating the trajectories of these particles through the spray dryer. Similar equations
can also be used to predict the diffusion of any other milk components within the
droplet/particle as it dries, although this is likely to be computationally expensive.
While the issue of skin and vacuole evolution and droplet/particle drying are
touched upon in this paper, we focus primarily on the migration of milk components
(lactose, fat and protein) to the surface of the milk particle during the spray drying
process. An understanding of these transport mechanisms is necessary in order to predict
the surface composition, and hence agglomeration of particles, and ultimately the
properties of the dried powder. The effect of surface fat concentration on the functional
4
properties of the milk powder is particularly studied in this paper, since very little work
has been published in the literature on this subject (Özkan et al., 2002). X-ray
photoelectron spectroscopy (XPS) is used to measure the surface composition of spray
dried milk powders with different fat contents according to the method developed by
Fäldt et al. (1993). In order to provide a measure of the agglomeration potential of
particles within the spray dryer, we investigate the effect of surface composition on the
caking ability of the powder using a standard sieving test. The particle size distribution of
the milk powders is measured using the laser diffraction technique, and the structure of
individual particles is studied by scanning electron microscopy.
5
2. Experimental
2.1 Sample Preparation
Skim and full-cream milk solutions were made to a concentration of 41.2%
(weight basis) by combining skim and full-cream milk powder (bought from a local
supplier) with distilled water at a constant temperature of 50 °C. Milk solutions with
various fat contents were then prepared by mixing different ratios of these skim and fullcream milk concentrates. The compositions of the milk solutions used are given in Table
1, although we are relying on the accuracy of the nutritional information provided on the
packaging to determine these compositions.
The milk concentrates were spray dried in a BUCHI Mini Spray Dryer B-290
(Switzerland), which operates co-currently with a two-fluid self-cleaning spray nozzle
having a cap orifice of diameter 1.5 mm. The BUCHI drying chamber is cylindrical and
vertically orientated with a length and diameter of approximately 500 mm and 150 mm,
respectively. During the experiments, the liquid feed to the nozzle was 8 ml/min and was
kept constant at 31 °C using a water bath. The flow of air from the nozzle was 440 L/hr,
while the flow of drying air was 38 m3/hr. Two inlet air temperatures, 120 °C and 200 °C,
were tested in order to show the effect of drying intensity on the surface composition.
These inlet air temperatures corresponded to outlet air temperatures of approximately
80 °C and 125 °C, respectively. After drying, the powder samples were transferred to
sealed bottles and refrigerated.
6
2.2 Electron Spectroscopy
In order to perform the surface composition analysis, it was assumed that the
solids in the milk were composed of only three components: lactose (carbohydrate),
protein and fat. These components typically amount to 95% of the total solids in milk,
with the remainder consisting of various minerals, and therefore a significant portion of
the milk composition was not taken into account during the analysis. In addition, it was
assumed that the protein consisted entirely of casein, which normally constitutes 86% of
the total proteins in milk, with the remainder consisting of whey proteins, such as
lactalbumin (Pisecky, 1997). Thus, the extracted surface compositions are not precise,
and the implications of this inaccuracy are discussed below.
The relative atomic concentrations of carbon, oxygen and nitrogen at the surface
of the milk powder samples were analysed using X-ray photoelectron spectroscopy
(XPS). These elemental ratios were converted into surface coverage ratios of lactose,
protein and fat by assuming a linear relationship between these elemental surface
compositions and the elemental compositions of each of the pure milk components
(lactose, protein and fat) making up the sample, according to the method described by
Fäldt et al. (1993) and subsequently adopted by Kim et al. (2003) for milk powders. We
have measured the following elemental compositions in the pure components: lactose carbon (56.7%) and oxygen (43.3%); casein - carbon (70.5%), oxygen (17.2%), nitrogen
(12.3%); fat - carbon (90.8%), oxygen (9.2%). We have also measured the elemental
composition of whey protein concentrate powder, which consists of milk proteins that
remain once casein has been removed, as follows: carbon (76.2%), oxygen (15.6%) and
7
nitrogen (8.2%). The whey protein tested in this work was not pure, containing additional
lactose (7.5%), fat (3.5%) and various other components (4.5%), which would tend to
augment the carbon concentration and reduce the nitrogen concentration. By making an
allowance for this impurity, we suggest that the whey protein and casein compositions are
not dissimilar, so that they can be lumped together in the analysis as ‘protein’. Thus, only
minerals are not accounted for properly in this work. However, these minerals contribute
only 5% of the overall composition of the milk, and this is not sufficient to affect the
conclusions drawn here. Note that we obtained the lactose (99% pure), anhydrous milk
fat (99% pure) and whey protein concentrate powder from Fonterra Co-operative Group
Limited (Temuka, New Zealand), and the casein (from bovine milk) from Sigma-Aldrich
(St Louis, MO, United States).
The samples of milk powder were manually pressed into pellets for the surface
analysis. X-ray photoelectron spectra were determined using a monochromatic Al K
source on an ESCALAB220i-XL instrument (VG Scientific, UK). The X-ray source was
operated at 120W, and a pass energy and step-size of 100 eV and 0.5 eV were used,
respectively. The take-off angle of the photoelectrons was perpendicular to the sample,
and the area analysed was a circular region of 0.5 mm in diameter. The pressure in the
vacuum chamber during the analysis was less than 1 x 10-9 mbar. The peak areas for the
important photoelectron emission signals (carbon, oxygen, nitrogen, sulphur, phosphate
etc) were measured and converted into relative elemental concentrations using Eclipse
software. The elemental concentrations of carbon, oxygen and nitrogen amounted to at
least 99% of all the elements registered, and thus the remaining elements were ignored in
the analysis. This indicates that the minerals (calcium, sodium and phosphate) in the milk
8
powder were not present at sufficiently high concentrations at the surface of the particles
to significantly affect the surface composition analysis.
2.3 Degree of Caking
The degree of caking was measured using the method described by Pisecky
(1997). A 1.5 g sample was first oven-dried for 1 hour at 102 °C. After cooling in a
desiccator, the sample was weighed, quickly transferred to a stainless steel sieve, and
shaken for 5 minutes in a shaking apparatus. The powder that passed through the sieve
was weighed, and the caking index was determined by the equation:
C
MT  M F
 100
MT
(1)
where M T is the total mass of powder and M F is the mass of fines that passed through
the sieve. Sieves with different apertures were previously tested in order to determine
which sieve gave the greatest caking sensitivity for the particle size distribution produced
by the BUCHI spray dryer. A sieve with an aperture of 106 um was used, although the
results for the 212 um tests are also reported. The sieving tests were carried out at an
ambient air temperature of 23.5 °C.
2.4 Particle Size Distribution
9
The particle size distribution of the milk powders was measured using a laser light
diffraction instrument, MasterSizer S (Malvern Instruments, Malvern, UK) according to
the method described by Pisecky (1997), in which a small sample of the powder is
suspended in isopropanol in a cuvette under magnetic agitation during the size
measurement. The particle size distribution was monitored every two minutes during
each measurement until successive readings became constant. This allowed the
agglomerates, which were most likely formed in the powder rather than within the drying
chamber where the particle number concentration was too low for particle collisions to
occur, to break up due to the shearing action of the magnetic agitator. The droplet size is
expressed as D(v, 0.5), the volume-weighted median diameter.
2.5 Scanning Electron Microscopy
Milk powder samples were attached to double-sided adhesive carbon tabs
mounted on SEM stubs, coated with gold/palladium, and examined with a Phillips
SEM505 (Philips Export BV, The Netherlands) operated between 3 and 4 kV.
3. Results and Discussion
The surface fat coverage is significantly higher than the average fat content, as
shown in Fig. 1, which suggests that fat is present at higher concentrations at the surface
of each milk particle than in the interior. For example, when the average fat content of the
milk powder is 5%, the surface fat coverage is in the vicinity of 35%. Thus, it appears
10
that fat is transported towards and accumulates at the surface of the milk particle during
drying, which results in a non-uniform distribution of fat throughout the solid matrix, as
previously demonstrated by Kim et al. (2003). Higher drying temperatures appear to
marginally favour the accumulation of fat at the surface, as shown by the slightly higher
surface fat coverage of the milk powders dried at 200 °C compared with 120 °C.
Differential Scanning Calorimetry (DSC) measurements by Jouppila and Roos (1994)
indicate that the melting point of fat in milk powder is between 10 and
30 °C. Given that the outlet temperature of the BUCHI spray dryer was at least 80 °C in
these tests, the fat was in a mobile fluid form throughout the spray drying process, and
was therefore readily transported to the surface of the droplets/particles.
Fig. 1 shows that a small change in the average fat content at low fat
concentrations results in a large change in the surface fat coverage. When the average fat
content is increased from 0 to 5%, the surface fat coverage increases from 0 to 35%.
However, the surface fat coverage is less affected by increases in the average fat content
at higher fat concentrations, only rising by a further 25% as the average fat content is
increased from 5% to 30%. It will be demonstrated below that this rapid change in
surface fat coverage at low average fat contents strongly affects the surface properties of
the milk particles, which is likely to have important repercussions for the production of
skim milk, where small changes in the fat content of skim milk concentrate fed to the
spray dryer can affect the agglomeration potential of the milk particles and the functional
properties of the dried powder. Fäldt and Bergenståhl (1996) have observed similar
trends in surface fat coverage with average fat content for emulsions of whey protein,
lactose and soybean oil.
11
Figs. 2 and 3 shows that protein has a higher surface concentration than lactose at
the lower drying temperature of 120 °C, even though there is more lactose than protein in
the powder by a factor of one-half, as shown in Table 1. This is in accordance with data
presented by Fäldt and Bergenståhl (1996), who explain that there is an accumulation of
surface-active protein at the air-water interfaces of the droplets, so that protein appears in
relatively high concentration on the dry powder surface. At the higher drying temperature
of 200 °C, this trend is reversed and more lactose appears at the surface of the powder
than protein, although the ratio of lactose to protein on the surface is still generally lower
than the average value in the powder, which further demonstrates the affinity of surfaceactive protein with the air-water interfaces of the droplets. We postulate that, at lower
drying temperatures, protein has more time to migrate to the surface of the droplet before
sufficient moisture is evaporated that a skin forms, which subsequently hinders further
transport of protein to the surface, especially when much of the solvent (in this case
water) is removed from the skin. At higher drying temperatures, less protein can migrate
towards the surface, because moisture is evaporated more quickly and therefore the
surface solidifies and immobilises the protein sooner. This is in contrast with fat, which is
in a mobile fluid form throughout the drying process, since the drying temperature is
above the melting point of fat.
Kim et al. (2003) have a different explanation for the apparent concentration of
fat and protein at the surface of a milk particle. They have stated that moisture content
gradients effectively concentrate the solutes (fat, protein and lactose) at the surface,
where the moisture content is lower, which causes Fickian diffusion of these solutes
towards the core of the milk particle. According to these workers, lower molecular
12
weight solutes, such as lactose, diffuse inwards more rapidly than higher molecular
weight solutes, such as protein and fat, which consequently concentrate at the surface.
However, this transport mechanism cannot be rationalised with the observed increase in
surface lactose concentration at higher drying temperatures (Fig. 3), which should
enhance the Fickian diffusion of lactose towards the core, according to the StokesEinstein equation for diffusion of solutes in liquids (Kim et al., 2003), rather than retard it
as observed experimentally. We have proposed that surface affinity causes protein to
accumulate at the surface of a milk droplet, and that skin formation, which is more rapid
at higher drying temperatures, hinders this process. Note that protein accumulation at the
surface of a milk particle is important, because it affects the lactose surface coverage,
which is known to strongly influence the caking of milk powders during humid storage
when the glass transition temperature is exceeded (Jouppila and Roos, 1994).
There appears to be a strong correlation between the caking ability of the powder
and the surface fat coverage, as shown by the similarity of the shapes of the caking index
(Fig. 4) and surface fat coverage plots (Fig. 1). When the average fat content and,
consequently, the surface fat coverage of the milk powder is high, the powder is very
sticky at ambient temperature, so that it has a caking index approaching 100%, which
implies that very little milk powder passes through the sieve. The caking index remains
high even when the average fat content of the powder is reduced from 30% to 5%,
whether a 106 um or 212 um sieve is used. However, below 5% average fat content, there
is a sharp reduction in the surface fat coverage, as shown in Fig. 1, so that the stickiness
of the powder, and hence its caking ability, correspondingly diminishes quickly. The
individual milk particles are significantly smaller than the aperture of the sieves, as
13
shown in Fig. 5, which indicates that the milk powders should easily pass through the
sieves tested here, provided they do not stick and agglomerate. Özkan et al. (2002) have
explained that any fat present on the surface of individual particles forms weak bridges
between these particles, which help to bind them together to form larger agglomerates. In
the experiments presented here, it was observed that the individual milk particles in the
powders with high fat content balled together to form such agglomerates during the
sieving process, some of which were over 1 mm in diameter, which were too large to pass
through the sieve apertures. It is clear from Fig. 4 that the caking index changes very little
above 30% average fat content, because a large proportion of the surface of each particle
is coated in fat, irrespective of the average fat content of the particle, as shown in Fig. 1.
Note that the caking ability of the powder is independent of the drying temperature over
the range of temperatures tested, which is not surprising given the similarity in the
surface fat coverage curves for each of these temperatures, as shown in Fig. 1.
The particle size measurements shown in Fig. 5 suggest that the size of the milk
particles decreases as 1) the drying temperature decreases, and 2) the average fat content
of the powder increases. These trends are consistent with trends in the observed sizes of
the milk particles shown in the scanning electron micrographs of Figs. 6 and 7. Numerous
observations of shattered particles in these scanning electron micrographs indicate that
the particles are hollow, which implies that a vacuole (a vapour bubble) forms within a
particle soon after a skin develops on the surface, that inflates once the particle
temperature exceeds the local ambient boiling point and the vapour pressure within the
vacuole rises above the local ambient pressure. When the drying temperature is
sufficiently high, moisture is evaporated very quickly and the skin becomes dry and hard,
14
so that the hollow particle cannot deflate when vapour condenses within the vacuole as
the particle moves into cooler regions of the dryer. However, when the drying
temperature is lower, the skin remains moist and supple for longer so that the hollow
particle can deflate and shrivel as it cools. Thus, milk particles dried at 200 °C are
spherical and smooth (Fig. 6), while milk particles dried at 120 °C are smaller and have a
shrivelled appearance (Fig. 7). These concepts have been discussed previously by Hassan
and Mumford (1993), Walton (2000) and Hecht and King (2000a).
Finally, the milk particles may decrease in size as the fat content increases, as
shown in Fig. 5, due to a change in the surface tension of the milk concentrate fed to the
nozzle, which would alter the atomisation process. The possible influence of fat on the
mechanical properties of the skin, which may affect the inflation of the particles, is not
excluded either. The scanning electron micrographs (Figs. 6 and 7) clearly show that
there are larger numbers of small particles in the powders with higher average fat content,
which confirms the trend shown in Fig. 5, and that higher surface fat coverage helps to
bind milk particles together. The powders with very little fat content do not bind together
in this fashion, as shown in Figs. 6 and 7, which is consistent with the results of the
caking tests (Fig. 4), indicating that individual particles with very little surface fat pass
readily through a sieve, provided the apertures are adequately sized.
4. Conclusions
Both fat and lactose have a strong influence on the stickiness of milk particles.
The stickiness of these particles is particularly sensitive to small changes in the fat
15
content between 0 and 5%, which is due to a rapid change in surface fat coverage from
0 to 35%. Moreover, it appears that protein accumulates at the surface of a milk
droplet/particle at the expense of lactose during drying (possibly due to the surface-active
nature of the protein), which will also affect particle stickiness when the glass transition
temperature of lactose is exceeded. Thus, the experimental results highlight the
importance of predicting the transport of each of these milk components within a
droplet/particle as moisture is evaporated in order to properly model particulate
agglomeration and wall deposition in a spray dryer, and to estimate the properties of the
spray-dried milk powder. We have confirmed observations made by various other
researchers on the morphological evolution of a milk droplet as it dries, which involves
the formation of a skin and a vacuole, and the inflation and subsequent shrinkage of the
particle.
Acknowledgements
We wish to thank Mr. K. Kota (Chemical Engineering Department, University of
Sydney) and Dr Bill Bin Gong (School of Chemistry, University of New South Wales)
for their technical assistance, Dr James Winchester (Fonterra Co-operative Group
Limited, Temuka, New Zealand) for his academic support, and Mr John Gabites
(Fonterra Co-operative Group Limited, Temuka, New Zealand) for providing the lactose,
whey protein concentrate and anhydrous milk fat. This work has been supported by an
Australian Research Council Discovery Grant.
16
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20
Surface Fat Coverage (%)
70
60
50
40
30
20
120 °C
200 °C
10
0
0
5
10
15
20
25
Average Fat Content (% weight)
30
Fig. 1. The surface fat coverage of milk powders with different average fat contents spray
dried at 120 °C and 200 °C.
21
Surface Protein Coverage (%)
60
50
40
30
20
120 °C
10
200 °C
0
0
5
10
15
20
25
Average Fat Content (% weight)
30
Fig. 2. The surface protein coverage of milk powders with different average fat contents
spray dried at 120 °C and 200 °C.
Surface Lactose Coverage (%)
60
50
40
30
20
10
120 °C
200 °C
0
0
5
10
15
20
25
Average Fat Content (% weight)
30
Fig. 3. The surface lactose coverage of milk powders with different average fat contents
spray dried at 120 °C and 200 °C.
22
100
Caking Index (%)
80
60
40
120 °C (106 um sieve)
200 °C (106 um sieve)
20
200 °C (212 um sieve)
0
0
5
10
15
20
25
30
Average Fat Content (% )
Volume weighted median diameter,
D(v,0.5), (um)
Fig. 4. The effect of the average fat content on the degree of caking for milk powders
spray dried at 120 °C and 200 °C.
35
120°C
200 °C
30
25
20
15
10
5
0
0
5
10
15
20
25
30
Average Fat Content (% )
Fig. 5. The volume weighted median diameter D(v,0.5) of milk powders with different
average fat contents spray dried at 120 °C and 200 °C.
23
(a) Skim milk powder (low fat content)
(b) Full-cream milk powder (high fat content)
Fig. 6. Scanning electron micrographs of milk powders spray dried at 200 °C.
24
(a) Skim milk powder (low fat content)
(b) Full-cream milk powder (high fat content)
Fig. 7. Scanning electron micrographs of milk powders spray dried at 120 °C.
25
Milk Solution
1 (skim)
2
3
4
5
6 (full-cream)
Fat
(%solids)
1.1
1.8
3.4
6.7
14.0
29.8
Protein
(%solids)
39.3
39.0
38.4
37.0
33.9
27.3
Lactose
(%solids)
59.6
59.2
58.3
56.3
52.1
43.0
Table 1. Composition of milk solutions.
26