J. Dairy Sci. 92:5353–5356
doi:10.3168/jds.2009-2561
© American Dairy Science Association, 2009.
Hot topic: Sonication increases the heat stability of whey proteins
M. Ashokkumar,*1 J. Lee,* B. Zisu,† R. Bhaskarcharya,* M. Palmer,† and S. Kentish*
*Particulate Fluids Processing Centre, School of Chemistry and Department of Chemical and Biomolecular Engineering, University of Melbourne,
Victoria 3010, Australia
†Dairy Innovation Australia Limited, 671 Sneydes Road, Werribee 3030, Australia
ABSTRACT
The thickening or gelling of protein-based dairy
streams and ingredients upon exposure to heat has been
an ongoing problem in dairy processing for many decades. This phenomenon can restrict the range of dairy
product options and reduce manufacturing efficiencies
by limiting the type and extent of heat treatment that
can be used. In this report, we outline a novel approach
to overcoming this problem. The use of preheating
treatments to induce whey protein aggregate formation
in whey products is well known in the field. However,
we show that the application of ultrasound for a very
short duration after such a heating step breaks down
these aggregates and prevents their reformation on subsequent heating, thereby reducing the viscosity increase
that is usually associated with this process. This novel
technique has the potential to provide significant economic benefit to the dairy manufacturing industry.
Key words: ultrasound, whey protein, heat stability,
sonication
In the dairy industry, the stability of dairy proteins
toward heat treatment is a major processing issue. Exposure of whey proteins to temperatures in excess of
70°C causes denaturation, which in turn leads to protein
aggregation through both hydrophobic interactions and
the formation of intermolecular disulfide bonds (Wang
et al., 2006). This can result in excessive thickening
or gelling during processing of the dairy product and
later, upon storage (Morr and Richter, 1999). Several
pretreatment procedures have been developed to improve the heat stability of dairy proteins, including
forewarming (Deysher et al., 1929) and pH adjustment
(Singh, 2004). We have developed a simple and efficient
ultrasonic treatment that appears highly effective in
this regard.
Although ultrasonic processing technology appears
to work for several dairy systems, our research to date
has shown that effects on the heat stability of whey
Received July 12, 2009.
Accepted September 3, 2009.
1
Corresponding author: masho@unimelb.edu.au
protein solutions and concentrates are the most impressive. Whey, a by-product of cheese making, contains
significant amounts of β-lactoglobulin, α-lactalbumin,
and serum albumin, along with lactose and smaller
amounts of minerals, lipids, and lactic acid. In a dairy
factory, whey proteins are concentrated by membrane
filtration, evaporation, and spray drying for application
in a range of processed foods, including protein supplements and infant formulas. During this manufacturing
process, the aqueous whey protein solutions containing
significantly high levels (4 to 15% by weight) of protein
are subjected to heat treatment. Issues with thickening
and gelation of the protein solution ultimately limit
the extent to which heat treatment is applied, the total
solids concentration that can be used, or both.
The ultrasonic process involves a short preheat treatment followed by sonication of the whey protein solution
for a short time at 20 kHz. The general experimental
procedure is outlined in the scheme shown in Figure 1.
In a typical experiment, an aqueous solution containing
4 to 15% total protein (by weight) was preheated to
80°C and held for 1 min. The preheated solution was
then subjected to high-intensity, low-frequency ultrasound for less than 5 min. A post-heat-treatment in an
85°C water bath for 20 min was then used to assess the
effectiveness of the ultrasonic treatment.
In the specific experiments presented here, whey
protein concentrate (WPC) powder containing 80%
protein derived from industrial Cheddar cheese whey
was obtained from Warrnambool Cheese and Butter
Factory (Victoria, Australia). This powder was reconstituted to 6.4% protein (by weight) in deionized
water (>18.2 MΩ·cm, MilliQ, Millipore, North Ryde,
Australia) for 1 h and held overnight at <5°C. Whey
protein retentate containing 27% protein was obtained
as the retentate from the ultrafiltration of sweet whey
from the same source. This retentate was diluted to 4%
protein.
In bench-top sonication trials, preheating was carried
out on 200-g samples placed in sealed, stainless steel
tubular containers (73 mm diameter, 135 mm deep).
Each container was fitted with an automated stirring
arm and a thermometer. The apparatus was immersed
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ASHOKKUMAR ET AL.
Figure 1. A typical experimental plan for a protein solution; control (C), preheating (PreH), sonication (US), and postheating (PostH). Color
figure available in the online version of this article.
in a water bath preheated to 80°C. The solutions were
stirred continuously and reached 80°C in 120 ± 20 s.
The samples were held at this temperature for a further
1 min and then transferred to an ice bath for rapid
cooling to 20 to 25°C. The preheating step for the
pilot-scale trials was deliberately designed to match
the bench-top sonication trials as closely as possible. A
temperature-controlled, steam-jacketed kettle containing approximately 40 L was used with manual stirring.
The heating temperature was set to achieve a come-up
time to 80°C of 120 ± 30 s, similar to the bench-top
process. The whey protein retentate was then held at
this temperature for a further 1 min before being manually transferred to stainless steel containers and rapidly
cooled to 20 to 25°C using an ice water bath.
In bench-top sonication trials, 60-mL samples were
sonicated in a custom-made glass vessel equipped with
a cooling jacket, using a 20-kHz, 450 W ultrasonic horn
(19 mm diameter, Branson Sonifier 450, Danbury, CT)
at an amplitude of 50% for between 5 s and 20 min.
The actual power delivered to the solution was determined using calorimetry to be 31 W. During sonication
chilled water was circulated continuously to maintain
the sample temperature below 10°C. In the pilot-scale
trials whey protein solutions (<20°C) were pumped at
a flow rate of 300 mL/min through a 20-kHz, 1 kW
flow-through sonication unit (Hielscher UIP 1000hd,
Teltow, Germany). The sonication was carried out at
60% amplitude, giving a calorimetric power delivery
of 270 W or 54 J/mL. Temperature in the sonication
unit was maintained by circulating ice water through
an external jacket using a peristaltic pump.
For postheating, aliquots of sonicated and unsonicated solutions were placed either as 1-mL samples in
1.5-mL Eppendorf tubes or as 33-g samples in 70-mL
polypropylene sample containers. These were then immersed in an 85°C water bath for 20 min. The temperature reached 70°C within the first 10 min of this
postheat cycle and hence the samples were exposed to
temperatures >70°C for at least 10 min. The containers
were then transferred to an ice bath for rapid cooling.
Some samples were frozen at −18°C and then freezedried in a Dynavac freeze dryer, Model FD-5 (Dynavac
Journal of Dairy Science Vol. 92 No. 11, 2009
freeze dryer, model FD-5, Melbourne, Australia). The
freeze-dried samples were gently milled to produce
flakes, which were stored in tightly sealed plastic containers at room temperature.
The size distribution of protein agglomerates was
determined at room temperature (approximately 22°C)
using a Malvern Mastersizer 2000 laser diffraction
system (Malvern Instruments Ltd., Malvern, UK).
Samples were shaken vigorously by hand and dispersed
directly into circulating deionized water to obtain an
obscuration of 10%. Measurements were made using a
refractive index of 1.456 for the protein agglomerates
and 1.33 for the dispersant (H2O) with an absorption
coefficient of 0.001. Mie theory (Van De Hulst, 1957)
was used to analyze the data, taken as the average of 3
measurements.
The viscosity of the treated samples was measured
using a Universal Stress Rheometer SR5 (Rheometric
Scientific Inc., Piscataway, NJ). A controlled steady
stress sweep was carried out using 40-mm parallel plate
geometry with a set gap of 1 mm at 25°C. The stress
was varied to obtain the viscosity data between the
shear rates of 50 to 200 s−1. The data presented in
Figure 2 is at a shear rate of 100 s−1.
Figure 2 shows typical experimental results observed
with a reconstituted WPC (6.4% protein by weight).
The viscosity of the preheated sample increased significantly compared with that of the control sample.
This viscosity then increased further upon a second
heat treatment. At higher protein concentrations, these
heat treatments can lead to gel formation. However,
the sample subjected to sonication for even 5 s showed
a significant decrease in viscosity, and the postheating
process had only a minimal effect on the viscosity of
the solution. Similar results were achieved for solutions of whey protein retentate sampled directly from
an industrial separation process. As can be seen in
Figure 3, continuous flow through processing provided
similar changes in the viscosity of the solution caused
by sonication. Results at other protein concentrations
were also similar. This functionality (low viscosity) was
preserved even after freeze or spray drying and then
reconstitution into aqueous solution (Figure 3).
HOT TOPIC: SONICATION INCREASES HEAT STABILITY
Figure 2. The effects on solution viscosity for a 6.4% protein (by
weight) solution reconstituted from whey protein concentrate and sonicated with a 20-kHz horn at a calorimetric power of 31 W in a batch
mode. Dark gray (blue) bars represent solutions without sonication;
light gray (pink) bars indicate sonicated solutions. PreH = preheating;
PostH = postheating; US = sonication. Color figure available in the
online version of this article.
It is important to note that these effects were observed only when sonication was used following a heat
treatment. Other researchers observe protein aggregation (Stathopulos et al., 2004; Gülseren et al., 2007)
and viscosity increases (Kresic et al., 2008) in similar
dairy solutions when sonication alone is used. Indeed, in
the present work, sonication in the absence of preheat
treatment resulted in gel formation when a relatively
severe treatment was applied. For example, a 4% protein solution sonicated in the bench-top cell for 60 min
gelled upon exposure to an 80°C water bath for 12 min,
whereas the unsonicated control sample did not gel in
20 min.
Ultrasound is known to generate acoustic cavitation
in liquids: the bubble nuclei present in the liquid medium grow and implode because of the pressure fluctuations caused by ultrasound. During acoustic cavitation,
the liquid medium is subjected to extreme forces that
include shear, turbulence, microstreaming, and heat.
In addition, highly reactive radicals are generated
(Ashokkumar and Mason, 2007). Considering these
ultrasound-induced cavitation effects, the observed viscosity changes might have been caused by the physical
or chemical effects of acoustic cavitation.
To investigate possible chemical effects due to radical generation, further experiments were carried out
in which reconstituted WPC solutions were sonicated
over a range of frequencies (20 kHz to 1 MHz). Previous studies (Ashokkumar et al., 2008) have shown that
reactive radical generation is significantly greater at
frequencies above 20 kHz. In this study, the WPC solutions sonicated at 20 kHz showed significant viscosity
reduction, whereas those sonicated at higher frequencies
did not show any viscosity reduction; this indicates that
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the observed viscosity reduction is caused primarily by
physical forces generated during acoustic cavitation.
Figure 4 shows that the protein aggregate size increased with heat treatment but reduced significantly
under sonication. Similar observations were made for
bench-top experiments involving WPC and across a
range of protein concentrations. This again suggests
that physical effects are responsible for the observed viscosity changes. Other authors have shown that similar
changes in whey protein aggregate size may be induced
by physical shear processes such as microparticulation,
as used in the manufacture of Simplesse (Nutra Sweet
Co. Ltd., St. Louis, IL) (Taylor and Fryer, 1994; Sanchez et al., 1997; Onwulata et al., 2002; Iordache and
Jelen, 2003). However, to the best of our knowledge,
there is only one comparable observation of increased
heat stability (Dissanayake and Vasiljevic, 2009). These
authors observed an increase in the heat coagulation
time from 14.5 to 102.5 s following exposure of a 10%
protein whey retentate to 90°C for 20 min and microfluidization for 5 passes at 140 MPa.
Although the mechanism is still not fully understood
at the molecular level, we speculate that the observed
viscosity reduction in whey protein solutions is primarily caused by the shear forces generated during acoustic
cavitation. The initial increase in solution viscosity
after heat treatment is due to heat-induced aggregation of whey proteins. During sonication, the shear
forces generated by acoustic cavitation disrupt the hydrophobic interactions or the intermolecular disulfide
bonds. It is known that the physical forces generated
during acoustic cavitation are strong enough to break
polymer chains (Price and Smith, 1993). Because further aggregation of protein particles occurs to a much
Figure 3. The viscosity of a whey protein retentate collected from
the factory floor, diluted to 4% protein (by weight), and sonicated using a flow through a 20-kHz horn transducer at a calorimetric power
of 270 W and a flow rate of 300 mL/min, after freeze drying and reconstitution to 15% solids. PreH = preheating; US = sonication. Color
figure available in the online version of this article.
Journal of Dairy Science Vol. 92 No. 11, 2009
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ASHOKKUMAR ET AL.
REFERENCES
Figure 4. The agglomerate particle size distribution for the same
whey protein retentate (Figure 3) before freeze drying. PreH = preheating; PostH = postheating; US = sonication. Color figure available
in the online version of this article.
lesser extent during the post-heat-treatment process,
we speculate that the functional groups responsible
for such interparticle interactions such as free thiols
are also deactivated during the heat treatment and
sonication sequence. Other researchers have shown that
heat-induced protein denaturation can cause irreversible changes to protein structure (Fox and McSweeney,
1998). Similarly, Gülseren et al. (2007) proposed that
sonication can alter the functional properties of bovine
serum albumin through the formation of an ultrasonically induced state that differs from a thermally, mechanically, or solvent induced state.
These remarkable effects form the basis of a completely new approach to controlling the viscosity and
heat stability of milk proteins, which is expected to find
wide application in the dairy processing industry. Further investigations on protein conformational changes
as a function of ultrasound-induced shear forces will
give more insight into the mechanism of action, which
forms the basis of our continuing research. Ultimately,
the relationship between the extent of denaturation of
the major whey proteins and their heat stability after
ultrasonication treatment will need to be determined.
ACKNOWLEDGMENTS
This research was funded by Dairy Innovation Australia (Werribee) and the Australian Research Council
(ARC-LP grant; Canberra).
Journal of Dairy Science Vol. 92 No. 11, 2009
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