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The influence of temperature on the foaming of milk

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The effect of temperature (5-85 °C) on the foaming properties of cows' milk was investigated. The foaming properties of milk as a function of temperature varied considerably depending on fat content and the processing conditions used in manufacture. Skim milk foams were most stable when formed at 45 °C. Milk fat had a detrimental effect on foam formation and stability of whole milk especially in the temperature range 15-45 °C. The detrimental effects of milk fat on foaming properties were reduced by homogenization and ultra-high-temperature (UHT) treatment. No correlation was observed between foam formation and surface tension of whole milk in the temperature range15-45 °C. There was a pronounced difference in the bubble size distributions of whole milk and skim milk especially at half-life of the foams. Bubbles in whole milk foams were smaller and showed a higher degree of rupture as a result of coalescence than those in skim milk foams.
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The influence of temperature on the foaming of milk
Sapna Kamath
a
, Thom Huppertz
b
, Avis V. Houlihan
c
, Hilton C. Deeth
a
,
*
a
School of Land, Crop and Food Sciences, University of Queensland, Brisbane, Qld 4072, Australia
b
NIZO food research, P.O. Box 20, 6710 BA, Ede, The Netherlands
c
Innovative Food Technologies, Queensland Department of Primary Industries and Fisheries, 19 Hercules Street, Hamilton, Qld 4007, Australia
article info
Article history:
Received 18 June 2007
Received in revised form 27 April 2008
Accepted 2 May 2008
abstract
The effect of temperature (5–85
C) on the foaming properties of cows’ milk was investigated. The
foaming properties of milk as a function of temperature varied considerably depending on fat content
and the processing conditions used in manufacture. Skim milk foams were most stable when formed at
45
C. Milk fat had a detrimental effect on foam formation and stability of whole milk especially in the
temperature range 15–45
C. The detrimental effects of milk fat on foaming properties were reduced by
homogenization and ultra-high-temperature (UHT) treatment. No correlation was observed between
foam formation and surface tension of whole milk in the temperature range15–45
C. There was a pro-
nounced difference in the bubble size distributions of whole milk and skim milk especially at half-life of
the foams. Bubbles in whole milk foams were smaller and showed a higher degree of rupture as a result
of coalescence than those in skim milk foams.
Ó2008 Elsevier Ltd. All rights reserved.
1. Introduction
Ice cream, whipped cream and foamed beverages, such as cap-
puccino, are examples of dairy-based foams in which air bubbles
are, at least partially, enveloped and stabilized by milk components
(Anderson & Brooker, 1988). Dairy foams are appealing because of
their lightness, texture and mouth-feel. Milk proteins, due to their
physico-chemical characteristics and interactions with other com-
ponents of milk, largely determine the structure and stability of
most dairy-based foams (Damodaran, 1996; Kinsella, 1981).
Temperature plays an important role in determining the foam-
ing properties of milk by influencing the conformation of milk
proteins (Kilara & Harwalkar, 1996) and their distribution between
the serum and colloidal phases of milk (Downey & Murphy, 1970;
Law, 1996). Considerable research has been carried out on the in-
fluence of temperature on the foaming properties of milk proteins
in model systems (Mangino, Liao, Harper, Morr, & Zadow, 1987;
Morr, 1987; Phillips, Schulman, & Kinsella, 1990; Schmidt, Packard,
& Morris, 1984; de Wit, 1990). However, it is difficult to extrapolate
these results to reflect the foaming properties of milk over a broad
temperature range. Milk is a complex multi-component system of
which the foaming properties at any given temperature are influ-
enced by the interactions of components such as lipids and mineral
salts (Kinsella, 1984).
Some studies (Deeth & Smith, 1983; El-Rafey & Richardson,
1944; Mulder & Walstra, 1974; Sanmann & Ruehe, 1930; Ward,
1996; Ward, Goddard, Augustin, & McKinnon, 1997) have examined
the influence of temperature on the foaming properties of milk as
a composite system. These studies differed widely in the techniques
used to create foam and in the temperature range over which
foaming properties of milk were studied. It is therefore difficult to
arrive at a definite conclusion about the variation of foaming
properties of milk over a broad temperature range.
The objective of the present study was to examine the effect of
temperature (5–85
C) on the foaming properties of raw whole milk,
homogenized pasteurized whole milk, pasteurized skim milk, UHT
homogenized whole milk, UHT skim milk. All foaming studies were
carried out using a specially designed foaming apparatus and the
patterns of foam formation, stability and bubble size distribution for
different types of milk wereestablished as a function of temperature.
2. Materials and methods
2.1. Materials
Raw bulk milk samples were collected from the Queensland
University dairy farm at Gatton, Queensland, Australia. Low-heat
skim milk powder was obtained from Murray Goulburn Co-Oper-
ative (Victoria, Australia).
2.2. Preparation of milks
Three lots each of homogenized pasteurized whole milk, pas-
teurized skim milk, UHT homogenized whole milk and UHT skim
milk were manufactured from three separate bulked raw milk
*Corresponding author.
E-mail address: h.deeth@uq.edu.au (H.C. Deeth).
Contents lists available at ScienceDirect
International Dairy Journal
journal homepage: www.elsevier.com/locate/idairyj
0958-6946/$ – see front matter Ó2008 Elsevier Ltd. All rights reserved.
doi:10.1016/j.idairyj.2008.05.001
International Dairy Journal 18 (2008) 994–1002
Author's personal copy
samples. The three raw bulk milk samples were collected one week
apart from a single herd. In each case, the homogenized pasteurized
whole milk, pasteurized skim milk, UHT homogenized whole milk
and UHT skim milk were made from the same raw bulk milk sample
on the same day as the milking. The temperature–time combina-
tion used for pasteurization was 75
C for 16 s. Homogenization
was carried out at pressures of 20 MPa (first stage) and 5 MPa
(second stage). UHT milks were produced in an APV UHT pilot
plant, using an indirect tubular mode of heating at 140
C for 6 s.
Reconstituted milk used to standardize the foaming apparatus was
prepared by dissolving low-heat skim milk powder in distilled
water at a concentration of 85 g L
1
. The reconstituted milk was left
to hydrate overnight at 4
C.
2.3. Determination of protein, fat, free fatty acid content and whey
protein denaturation
The fat and protein content of milk was determined using a FOSS
FT 120 Milkoscan (A/S Foss Electric, Hillerod, Denmark). All mea-
surements were conducted in triplicate after warming the milk
samples to 40
C. The free fatty acid content of all milks was de-
termined using the method described by Deeth, Fitzgerald, and
Wood (1975). The amount of undenatured whey protein in all milks
was determined using the method described by Elliott, Datta,
Amenu, and Deeth (2005).
2.4. Design of foaming apparatus
The foaming apparatus consisted of a sintered glass disc (1.0 cm
diameter; 0.5 cm thickness; porosity 4; Bibby Sterilin, Staffordshire,
UK) attached to one end of a glass tube (39 cm length; 0.5 mm
diameter). The other end of the glass tube was connected to
a pressurized air inlet via a flow meter (Dwyer Model MMF-50-PV,
Michigan City, USA; range 0.1–1.0 SCFH air) and a pressure gauge
(US Gauge, Chicago, USA; range 0–206.84 kPa) using rubber tubing.
The milk sample to be foamed was placed in a 250-mL low-form
measuring cylinder (250 mL 5 mL divisions; Hirschmann Class B;
DURAN
Ò
, Mainz, Germany). A diagrammatic representation of the
foaming apparatus is shown in Fig. 1.
2.5. Standardization of foaming apparatus
The foaming apparatus was standardized in terms of air pres-
sure, air flow rate, time of bubbling and volume of milk used for
foaming. Reconstituted low-heat skim milk powder (85 g solids L
1
)
was used to create foam for all standardization experiments.
Reconstituted milk made from low-heat skim milk powder was
used in order to eliminate the effect of the day-to-day vari-
ability of cows’ milk on the standardization of the foaming
apparatus. Repeatability of readings, in terms of initial foam
volume, was used to determine the combination of air pressure,
air flow rate, time of bubbling and volume of milk to be used in
foaming experiments. Repeatability of readings, sample size and
sensitivity of the foaming apparatus to changes in foam volume,
was determined by measuring the initial volume of foam
obtained by foaming milk at two different temperatures (5 and
85
C). This experiment was repeated 10 times using the same
measuring cylinder and the same lot of milk for all readings. To
determine if bubbling of air through milk caused large changes
in the temperature of milk during the process of foam forma-
tion, the temperature of the bulk liquid below the foam, im-
mediately after foaming was measured. The temperature of the
bulk liquid below the foam was not monitored during the
process of foam collapse.
2.6. Foamability and foam stability measurements
All foams in this study were produced in triplicate using the
foaming apparatus described in Section 2.5. In a 250 mL low-form
measuring cylinder, 50 mL milk at 5
C was placed and heated to
the desired temperature using a microwave oven (750 W; Pana-
sonic Model NN-5252, Sydney, Australia). Air at 34–42 kPa and
a flow rate of 2.4 mL s
1
was bubbled through the milk for 16 s via
the sintered glass disc.
Foamability was expressed as the total volume of foam in mL
obtained immediately after bubbling was stopped. Foam stability
was expressed in terms of aquantity called half-life, i.e., the time (in
minutes) required for the foam to collapse to half its original volume.
All foams were allowed to collapse at room temperature (22
C).
2.7. Surface tension measurements
Surface tension was measured by the Wilhelmy plate method
using a NIMA ST9000 tensiometer (Nima technology, Coventry,
UK). A platinum Wilhelmy plate (10.25 0.16 mm) was used for all
surface tension measurements. The tensiometer was calibrated as
per the procedure outlined in the NIMA ST9000 operating manual.
All glassware used for surface tension measurements was cleaned
Flow meter
Pressure gauge
Pressure
regulator
Milk (50 mL)
Sintered glass disc
(porosity 4)
Glass tube
Low form 250 mL measuring
c
y
linder
Air inlet
Fig. 1. Diagrammatic representation of foaming apparatus.
S. Kamath et al. / International Dairy Journal 18 (2008) 994–1002 995
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by overnight soaking in a solution containing 15% methylated
spirits, 15% acetone and 15% Decon 90
Ò
(Decon Laboratories Ltd,
East Sussex, UK). The glassware was then thoroughly rinsed with
tap water followed by rinsing with MilliQ water and dried in a hot-
air oven.
The surface tension of milk was measured at 10, 15, 25, 35 and
45
C. The milk samples (50 mL) at 5
C were placed in a pyrex glass
container (70 mm diameter) and heated to the desired temperature
using a hot plate on a low-heat setting with gentle stirring. Surface
tension measurements for a given milk and at a given temperature
were conducted in triplicate. The platinum plate was flamed prior
to each measurement.
2.8. Foam imaging and image analysis
Three lots of pasteurized homogenized whole milk and pas-
teurized skim milk (Section 2.2) were foamed at 45, 65 and 85
C
using the foaming apparatus and procedure described in Sections
2.5 and 2.7. The measuring cylinder containing the foam was placed
immediately under a microscope (Prism Optical, Eagle Farm,
Queensland, Australia) and the surface of the foam was photo-
graphed at 10 randomly chosen fields using a digital camera (Mi-
croPublisherÔ5.0 with Real Time Viewing 3.3 RTV, QImaging,
Barnaby, Canada) attached to the microscope using a zoom setting
of 0.7. The resultant spatial calibration as determined by a mi-
crometer slide was 0.2 pixels
m
m
1
. An Olympus LG-PS2 lamp (Ea-
gle Farm, Queensland, Australia) was used to illuminate the foam.
Images were captured and analysed using Image-Pro Plus 6.0
software (Media Cybernetics Inc, Bethesda, USA) at regular time
intervals up to the half-life of the foam. The diameters of all bubbles
in a given image were measured using the Image-Pro Plus software.
As most bubbles were not spherical, the diameter was taken to be
the longest length of a given bubble. Due to images of bubbles in
the sub-surface layer showing through the surface layer, mea-
surements could not be automated and the diameter of each bubble
was measured manually using the Image-Pro Plus software. The
appearance of bubbles was summarized in terms of the smoothed
distributions of the log-diameter of bubbles using Minitab 15
(Minitab Inc, State College PA, USA).
3. Results and discussion
3.1. Protein, fat, free fatty acid content and whey protein
denaturation
The fat content of whole milks was 37 3gL
1
while the fat
content of all skim milks was <0.1%. The protein content of all milks
was 32 2gL
1
. The free fatty acid contents of all milks were
<1.0 mmol L
1
. The level of undenatured
b
-lactoglobulin in cows’
milk was reduced by <5% or >95%, while the level of undenatured
a
-lactalbumin was reduced by w5% or w80%, by pasteurization or
UHT treatment, respectively, with no notable difference in the ex-
tent of whey protein denaturation between skim and whole milks
treated at the same conditions.
3.2. Standardization of foaming apparatus
The temperature of the bulk liquid below the foam was not
changed by more than 1
C as a result of the foaming process. The
most repeatable results (Table 1), in terms of the initial foam vol-
ume, were obtained using the parameters outlined in Table 2. From
statistical analysis of readings obtained in Table 1, it was concluded
that a 5 mL difference in foam volume between two temperatures,
for a given milk, could be attributed to the difference in tempera-
ture treatment rather than apparatus variability, provided the
foams were produced in triplicate at each temperature.
3.3. Foamability and foam stability of cows’ milk as
a function of temperature
Figs. 2 and 3 show the average values of foamability and foam
stability, respectively, of raw whole, pasteurized homogenized
whole, pasteurized skim, UHT-treated homogenized whole and
UHT-treated skim milk foamed after equilibration at temperatures
in the range 5–85
C. The foamability of pasteurized and UHT-
treated skim milk increased progressively with increasing tem-
perature (Fig. 2). The foamability of whole milks decreased with
increasing temperature up to 25
C (for pasteurized and UHT-
treated whole milk) or 35
C (for raw whole milk). A similar min-
imum in foamability of pasteurized unhomogenized whole milk at
around 25
C was observed by Sanmann and Ruehe (1930). For all
whole milks, foamability fully recovered at temperatures 45
C,
and increased progressively with increasing temperature in the
range 45–85
C, where no notable differences in foamability be-
tween any of the milks were observed (Fig. 2). The minimum in the
temperature vs foamability profile of whole milk was most notable
in raw milk and least notable in UHT-treated milk (Fig. 2).
Values for foam stability showed considerable variation among
individual batches of milk, but the trends as a result of skimming,
heat treatment, homogenization and foaming temperature
remained the same. Whole milks formed extremely unstable foams
in the temperature range of 5–35
C(Fig. 3). Prins (1986) reported
that adding as little as 0.5% whole milk also greatly reduced stability
of foam formed from skim milk at 19
C. The stability of the foam
formed from raw or pasteurized homogenized whole milk increased
with increasing temperature in the range 35–85
C, with increases
in the latter being far greater than in the former. The stability of
foam made from UHT-treated homogenized whole milk was highest
when foaming was performed at 65
C(Fig. 3). The stability of foam
formed from pasteurized or UHT-treated skim milk increased with
increasing temperature of foaming up to 45
C, above which pro-
gressively less-stable foams were formed. At all temperatures
studied, foams formed from pasteurized skim milk were more sta-
ble than those formed from UHT-treated skim milk (Fig. 3).
Patterns of foamability (Fig. 2) and foam stability (Fig. 3)as
a function of temperature differed considerably between whole and
skim milks prepared from the same batch of raw whole milk, in-
dicating a large influence of the presence of milk fat on the foaming
properties of milk. Differences in foamability between whole and
skim milks were evident predominantly when milk was foamed at
a temperature in the range 5–35
C(Fig. 2), which coincides with
the temperature range in which the milk fat globules contain
Table 1
Summary of statistical results to determine repeatability of results
a
(in terms of
initial foam volume) using the foaming apparatus
5
C80
C
Average initial foam volume (mL) 40.0 70.0
S.D. (mL) 1.3 2.1
Coefficient of variation 3.6 3.2
a
Results are for ten measurements per temperature.
Table 2
Summary of fixed parameters used for foaming experiments with the foaming
apparatus
Parameter
Volume of milk sample (mL) 50
Air pressure (kPa) 34–42
Air flow rate (mL s
1
) 2.4
Time of bubbling (s) 16
S. Kamath et al. / International Dairy Journal 18 (2008) 994–1002996
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a mixture of solid and liquid fats, and when milk fat globules are
most susceptible to partial coalescence; at >40
C, all milk fat is in
the liquid form (Mulder & Walstra, 1974). When globules contain-
ing solid and liquid fats are deformed near the foam lamella, the fat
crystals in the globules can pierce the thin film between the fat
globule and the air–serum interface, resulting in spreading of milk
fat globule membrane material and liquid fat over the air–serum
interface (Mulder & Walstra,1974). This spreading is detrimental to
foam formation and foam stability because fats readily adsorb on
the air–liquid interface but are not capable of forming the visco-
elastic interfacial layer required to stabilize the foam bubbles
(Walstra, Geurts, Noomen, Jellema, & vanBoekel, 1999). The
spreading of liquid fat can also destabilize foam bubbles by causing
liquid in the film to flow away from the spreading liquid fat, leading
to thinning, and ultimately rupture of the film (Walstra, 1996).
Support for the hypothesis that disruption of fat globules contain-
ing a mixture of solid and liquid fat impairs foamability at 5–35
C
comes from the observation that white clumps, which were readily
soluble in hexane and hence presumably fats, were found on the
surface of raw milk after foaming at 25
C. Above 40
C, the milk fat
globules contain fully liquid fat and are far less susceptible to dis-
ruption on deformation by the foam lamella (Mulder & Walstra,
1974).
The lower foamability at 5–35
C of raw whole milk than pas-
teurized or UHT-treated homogenized whole milk (Fig. 2) can be
related to the homogenization step applied to the UHT milks. The
decrease in fat globule size reduces the interfacial surface area over
which the liquid fat can spread on disruption and hence the
probability that it can spread sufficiently far to induce de-
stabilization of the liquid film stabilizing the foam bubbles (Prins,
1986). Furthermore, the size of milk fat crystals that can form is
greater in larger milk fat globules (Mulder & Walstra, 1974) so ho-
mogenization reduces the probability of crystals sufficiently large
to pierce the fat globule membrane being formed. Finally, homog-
enization also results in the adsorption of large amounts of caseins
on the fat–water interface, which increases the strength of the
membrane and reduces the probability of the fat crystals piercing it
(Melsen & Walstra,1989). The better foamability at 5–35
C in UHT-
treated milk than in pasteurized whole milk (Fig. 2) may be due to
the heat-induced association of whey proteins with the casein on
the surface of homogenized milk fat globules (Huppertz & Kelly,
2006), which further stabilizes the membrane against disruption.
The increase in foamability of skim milks or whole milks with
increasing temperature in the range 5–85
C or 35–85
C, re-
spectively (Fig. 2), is at least partially due to the decrease in vis-
cosity of milk with increasing temperature (Fernandez-Martin,
1972 ) enabling protein molecules to migrate more rapidly to the
air–serum interface of milk foams (Kinsella, 1981; Patino, Delgado,
& Fernandez, 1995). Furthermore, the surface tension of the milk
decreases with increasing temperature (Bertsch, 1983), and this
decrease in surface tension is conducive to improved foamability.
The foam stability of raw milk and pasteurized homogenized
whole milk increased progressively with increase in the tempera-
ture at which milk was foamed above 35
C(Fig. 3). This charac-
teristic pattern is likely to be due to re-crystallization of globular
milk fat trapped in the foam lamella, during storage of foams at
room temperature for foam stability measurements. The temper-
ature of foam formed from whole milks was 38, 50 and 63
C, when
the foams were formed from milk at 45, 65 and 85
C, respectively.
Therefore, the re-formation of fat crystals would occur later in
foams formed at 85
C, resulting in foams having the highest sta-
bility. Furthermore, surface tension will increase when the foams
cool down during storage, inducing further instability. The lower
foam stability of raw milk when compared to pasteurized homog-
enized whole milk could be because fat globules in raw milk con-
tain larger crystals and a thinner membrane when compared with
pasteurized homogenized whole milk and are therefore more
prone to disruption on deformation of the foam lamella.
The higher foam stability of UHT-treated homogenized whole
milk, compared to raw whole milk and pasteurized homogenized
whole milk, at temperatures 45
C is likely to be due to the heat-
induced association of whey proteins with caseins on the surface of
the milk fat globules, further strengthening the milk fat globules
against disruption on re-formation of fat crystals during cooling of
the foams. Moreover UHT milk has a higher viscosity than raw
whole milk and pasteurized homogenized whole milk resulting in
slower drainage and higher foam stability. Unlike raw whole milk
and pasteurized homogenized whole milk which showed a pro-
gressive increase in foam stability with increasing temperature, the
foam stability of UHT whole milk peaked at 65
C, following which
0
10
20
30
40
50
60
70
80
90
0 20406080100
Temperature
(
°C
)
Foamability (mL)
Fig. 2. Average foamability of cows’ milk as a function of temperature (5–85 C): (B)
raw whole milk, (C) pasteurized homogenized whole milk, (,) UHT homogenized
whole milk, (-) UHT skim milk, (
6
) pasteurized skim milk. The error bars presented
are the pooled standard errors for individual milks. The same standard errors are
applied at each data point for a given milk.
0
50
100
150
200
250
300
350
400
Half-life (mins)
020406080100
Temperature
(
°C
)
Fig. 3. Average foam stability of cows’ milk as a function of temperature (5–85 C): (B)
raw whole milk, (C) pasteurized homogenized whole milk, (,) UHT homogenized
whole milk, (-) UHT skim milk, (
6
) pasteurized skim milk. The error bars presented
are the pooled standard errors for individual milks. The same standard errors are
applied at each data point for a given milk.
S. Kamath et al. / International Dairy Journal 18 (2008) 994–1002 997
Author's personal copy
there was a drop in foam stability at 75 and 85
C. This trend was
reproducible for the three lots of milks studied; the reasons for this
were not investigated.
Skim milk foams were more stable than foams formed by whole
milks, especially when formed from milk below 45
C(Fig. 3). The
stability of foams formed from skim milks was highest when milk
was warmed to 45
C prior to foaming, and least when foam was
formed from milk at 5
C. The level of dissociated casein in milk
increases as the temperature of milk decreases from 45 to 5
C, with
b
-casein accounting for 60% of the dissociated caseins at 5
C(Rose,
196 8), and it is well known that
b
-casein is highly surface active but
forms unstable foams (Graham & Phillips, 1979). At 45
C, 95% of
caseins are in the form of micelles (Singh, 1995) and it appears that
the presence of higher amounts of micellar casein at 45
C increases
the stability of skim milk foams. Walstra et al. (1999) stated that
skim milk foams are most stable at around 40
C with the foam
being largely stabilized by casein. The decline in the stability of
foams formed from skim milk pre-warmed to temperatures >45
C
could be due to the dissociation of caseins, which occurs especially
above 60
C(Rollema & Brinkhuis, 1989). Furthermore, milk vis-
cosity decreases with increasing temperature, as a result of which
drainage is faster and foam stability is reduced.
3.4. Surface tension measurements
The average surface tension values of raw whole milk, pas-
teurized homogenized whole milk, pasteurized skim milk, UHT-
treated homogenized whole milk and UHT-treated skim milk in the
temperature range 5–45
C are shown in Fig. 4. The surface tension
values for raw whole milk, pasteurized homogenized whole milk
and pasteurized skim milk were in good agreement with those
obtained by Watson (1958) for similar milks over this temperature.
Watson (1958) observed that the surface tension of pasteurized
homogenized whole milk in the temperature range 15–40
Cav-
eraged about 3 mN m
1
higher than that of raw whole milk and
36
41
46
51
56
61
Surface tension (mN m-1)
01020304050
Temperature
(
°C
)
Fig. 4. Surface tension of cows’ milk in the temperature range 5–45 C: (B) raw whole
milk, (C) pasteurized homogenized whole milk, (,) UHT homogenized whole milk,
(-) UHT skim milk, (
6
) pasteurized skim milk. Values are a mean of three determi-
nations s.d. using three separate lots of milk.
Fig. 5. Images of foam formed at 45 C from (a) pasteurized homogenized whole milk immediately after foaming (b) pasteurized homogenized whole milk at half-life (c) pas-
teurized skim milk immediately after foaming (d) pasteurized skim milk at half-life. Bar ¼1000
m
m.
S. Kamath et al. / International Dairy Journal 18 (2008) 994–1002998
Author's personal copy
was lower than that of skim milk. A similar trend was seen in the
present study (Fig. 4).
The surface tension of pasteurized homogenized whole milk
and UHT homogenized whole milk was higher than that of raw
whole milk (Fig. 4). The increase in surface tension as a result of
homogenization is believed to be due to coating of fat globules with
casein and a consequent decrease in the amount of casein available
for adsorption at the air–water interface (Michalski & Briard, 2003;
Sherbon, 1988). The surface tension values of raw whole milk,
pasteurized homogenized whole milk and UHT homogenized
whole milk were lower than those of pasteurized skim milk and
UHT skim milk. This is because the surface tension of milk de-
creases with increasing fat content up to 4% fat (Watson, 1958).
The change of surface tension with temperature between 15 and
25
C was similar for both whole and skim milks (Fig. 4). Therefore
the change does not reflect the changing state of milk fat and the
variation in foaming properties in this temperature range (Section
3.3). This could be because the surface tension of milk is not gov-
erned solely by its triglyceride content. Proteins, phospholipids and
free fatty acids also play an important role in determining the
surface properties of milk (Sherbon, 1988) and their effect may
mask any effect of the changing state of fat on the surface tension of
milk at 15 and 25
C.
3.5. Effect of temperature on the bubble size distribution
of milk foam
The surface of foams formed by pasteurized homogenized
whole milk and pasteurized skim milk was observed under
a light microscope and bubble sizes quantified to determine
whether the presence of fat had an effect on the appearance and
bubble size distribution of milk foam. The collapse of foams
formed from pasteurized homogenized whole milk at 5 and 25
C
was too rapid for representative and reproducible images to be
obtained. Figs. 5–7 show representative microscopic images of
the surface of foams formed from pasteurized homogenized
whole milk and pasteurized skim milk at 45, 65 and 85
C, im-
mediately after foam formation and at the half-life of the foams.
Visually, the bubbles of fresh foams appeared more or less uni-
form in size with well-defined lamellae. Bubbles in foam formed
from the whole milk appeared smaller than those formed from
the skim milk (Figs. 5–7). These observations were confirmed by
the measured sizes of the surface bubbles. The size distributions
in fresh foams were narrower and the average sizes of the bub-
bles were smaller in foams formed from whole milk compared
with those of foams formed from skim milk (Fig. 9). The smaller
average bubble size of foams formed from whole milk could be
due to the higher viscosity of this milk when compared to skim
milk. Higher viscosity promotes smaller bubble size, due to de-
creased rates of coalescence (Laakkonen, Moilanen, & Aittamaa,
2005).
At the half-life, foam formed from whole milk contained a few
large bubbles and a large number of small bubbles (Figs. 5–7). This
was reflected in a shift in the distributions of the measured bubble
sizes to the left in Fig. 9. Coalescence of bubbles was frequently
observed and was followed by immediate rupture of the coalesced
bubbles. An example of bubble coalescence in whole milks is
shown in Fig. 8. Rupture of bubbles usually led to the flow of
Fig. 6. Images of foam formed at 65 C from (a) pasteurized homogenized whole milk immediately after foaming (b) pasteurized homogenized whole milk at half-life (c) pas-
teurized skim milk immediately after foaming (d) pasteurized skim milk at half-life. Bar ¼1000
m
m.
S. Kamath et al. / International Dairy Journal 18 (2008) 994–1002 999
Author's personal copy
clusters of smaller bubbles towards the area of rupture (Fig. 8). The
lamellae, especially around very small bubbles, were poorly de-
fined with the bubbles appearing to float in areas of liquid in some
areas (Fig. 8). These areas of liquid are formed as a result of
coalescence of bubbles at the surface with bubbles in the un-
derlying layers of foam and subsequent bubble rupture. In whole
milk this is attributable to spreading of liquid fat on to the surface
of bubbles when foams were produced at temperatures up to 40
C
(Section 3.4). Moreover the spreading liquid fat could displace
adsorbed proteins at the air–water interface of bubbles and lead to
increased disproportionation where large bubbles grow at the
expense of smaller bubbles (Prins, 1991; Walstra, 1996). Co-
alescence and rupture of large bubbles and decrease in the size of
smaller bubbles as a result of disproportionation could explain the
preponderance of smaller bubbles at the half-life of the foams and
the shift in size distribution towards the left in Fig. 9 for foams
formed from the whole milk.
By contrast, foams formed from pasteurized skim milk at 45, 65
and 85
C became coarser during storage (Figs. 5–7). Thus at the
half-life of the foam, the distributions of the measured bubble sizes
became very broad and shifted to a higher average bubble size
compared to the those of the corresponding fresh foams (Fig. 9)
suggesting coalescence of bubbles as the foams aged. Unlike
pasteurized homogenized whole milk, the foam lamellae around
smaller bubbles remained well defined and there were no pro-
nounced areas of liquid observed at the surface of the foam
(Figs. 5–7) suggesting that rupture of bubbles was not frequent; this
could explain why foams formed by skim milks were more stable
than those formed from whole milks.
4. Conclusion
The foaming properties of milk vary considerably with the
temperature at which the milk is foamed. The differences in
Fig. 7. Images of foam formed at 85 C from (a) pasteurized homogenized whole milk immediately after foaming (b) pasteurized homogenized whole milk at half-life (c) pas-
teurized skim milk immediately after foaming (d) pasteurized skim milk at half-life. Bar ¼1000
m
m.
Fig. 8. Enlarged image of foam at half-life formed from milk at 65 C. C ¼coalescence,
S¼streaming of bubbles towards ruptured bubbles and L ¼area of liquid at the surface
of foam as a result of bubble rupture. Bar ¼1000
m
m.
S. Kamath et al. / International Dairy Journal 18 (2008) 994–10021000
Author's personal copy
patterns of foamability and foam stability between skim milks and
whole milks suggest that the effect of temperature on the foaming
properties of milk is influenced by the fat portion of the milk, and
by the processing conditions used during its manufacture. Skim
milks show a pronounced foam stability peak at 45
C. Foams
formed by skim milks are generally more stable than foams formed
by whole milks, especially at temperatures below 45
C.
The effect of milk fat on the foaming properties of whole milks is
largely determined by the physical state of milk fat and thereby the
temperature at which milk is foamed. The destabilising effect of
milk fat on foam formation and stability is most pronounced when
the fat globules contain both solid and liquid fat i.e., in the tem-
perature range 5–35
C. The change in the physical state of milk fat
over the temperature range of 5–45
C and its pronounced effect on
the foaming properties of whole milks, do not reflect the surface
tension values of milk in this temperature range. Foams produced
from skim milk and whole milk differ considerably in their ap-
pearance and bubble size distributions with whole milk foams
showing smaller sized bubbles and higher rates of bubble rupture
as a result of coalescence during storage.
The foaming patterns of pasteurized skim milk which contain
very small amounts of fat are a reflection of the foaming behaviour
of its protein fraction. A study of the foaming properties of the
individual protein fractions of skim milk coupled with a study of
the role of milk proteins in stabilizing the air–serum interface of
skim milk foams might provide further explanation for the patterns
of foamability and foam stability observed for skim milks.
Acknowledgements
The authors gratefully acknowledge Dairy Australia for financial
support and Parmalat Australia for milk analyses.
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4.03.63.22.82.42.01.6
16
a
14
12
10
8
6
4
2
0
log diameter
Percent
2.450 0.2665
2.374 0.3235
2.714 0.2877
2.997 0.4393
Mean StDev
2.550 0.2512
2.532 0.2995
2.699 0.2676
2.917 0.4448
Mean StDev
2.558 0.2721
2.509 0.3014
2.797 0.2996
2.892 0.4215
Mean StDev
A
B
C
D
bA
B
C
D
3.93.63.33.02.72.42.11.8
16
14
12
10
8
6
4
2
0
log diameter
Percent
cA
B
C
D
3.63.22.82.42.01.6
16
14
12
10
8
6
4
2
0
lo
g
diameter
Percent
Fig. 9. Distributions of bubble sizes of foams formed from pasteurized homogenized
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A¼pasteurized homogenized whole milk, fresh foam, B ¼pasteurized homogenized
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S. Kamath et al. / International Dairy Journal 18 (2008) 994–1002100 2
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Vita. Thesis (Ph. D.)--University of Illinois, 1929. Includes bibliographical references (leaves 68-70).