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A review of milk fouling on heat exchangersurfaces
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Areviewofmilkfoulingonheatexchangersurfaces
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DOI 10.1515/revce-2013-0003 Rev Chem Eng 2013; 29(3): 169–188
Emad Sadeghinezhad * , Salim Newaz Kazi , Ahmad Badarudin , Mohd Nashrul M. Zubair ,
Babak Lotfizadeh Dehkordi and Cheen Sean Oon
A review of milk fouling on heat exchanger surfaces
Abstract: Formation of deposits on heat exchanger sur-
faces during operation rapidly increases the thermal
resistance and reduces the operating service life. Product
quality is deteriorated by fouling, which causes reduc-
tion of proper heating. The chemistry of fouling from
milk fluids is qualitatively understood, and mathemati-
cal models for fouling at low temperatures exist, but the
behavior of systems at ultrahigh temperature processing
is still not clearly understood. The effect of whey protein
fouling on heat transfer performance and pressure drop in
heat exchangers was investigated by many researchers in
diversified fields. Among them, adding additives, electro-
magnetic means, treating of heat exchanger surfaces and
changing of heat exchanger configurations are notable.
The present review highlighted information about previ-
ous work on fouling, parameters influencing fouling and
its mitigation approach.
Keywords: dissolved salts; heat exchanger; milk fouling;
mitigation.
*Corresponding author: Emad Sadeghinezhad , Faculty of
Engineering, Department of Mechanical Engineering, University of
Malaya, 50603 Kuala Lumpur, Malaysia,
e-mail: [email protected]
Salim Newaz Kazi, Ahmad Badarudin, Mohd Nashrul M. Zubair, Babak Lotfizadeh Dehkordi and Cheen Sean Oon: Faculty of
Engineering , Department of Mechanical Engineering, University of
Malaya, 50603 Kuala Lumpur, Malaysia
1 Introduction Deposition of undesired materials on heat transfer sur-
faces are recognized as fouling. At present, milk fluid
fouling chemistry is qualitatively understood, and math-
ematical models for fouling at low temperatures exist,
but the behavior of systems at ultrahigh temperature
(UHT) processing is not clearly understood yet (Fryer et al.
1996b , Prakash et al. 2010 ). In the dairy industry, thermal
processing is an energy intensive-event, as every product
is heated there at least once (de Jong 1997 ). The problem
of heat exchanger fouling is a serious matter as it retards
heat transfer, enhances pressure drop and diminishes the
efficiency of the heat exchanger drop down, which ulti-
mately affects the economy of the process plant (Muller -
Steinhagen 1993 , Toyoda et al. 1994 , Gillham et al. 2000 ,
Hinrichs and Atamer 2011 ).
In the dairy industry, the costs due to interruption in
production can be dominating compared to the cost due
to reduction in performance efficiency (Georgiadis et al.
1998a,b ). Quality issues are also as equally important as
the cost, and in fact shutdown is required regularly con-
cerning product quality instead of performance of the heat
exchangers (Georgiadis et al. 1998b ).
Milk constituents are water, solids, fat, lactose, pro-
teins (casein, β -Lg, α -La), minerals and small quantities
of other miscellaneous constituents (Lyster 1970 , Lalande
et al. 1985 , Gotham et al. 1992 , Delplace et al. 1994 , Bylund
1995 ). Microbial growth prevents uninterrupted opera-
tion of heat exchanging equipment due to enhancement
of fouling, which causes forced shutdown (Lyster 1970 ,
Lalande et al. 1985 , Bylund 1995 ).
Denaturation of protein in heat exchangers starts
above 70 – 74 ° C and the first deposit layer is mostly
(usually < 5 μ m) mineral (Fryer and Belmar -Beiny 1991 ).
Many research interests highlighted the study of the
mechanism (de Jong 1997 , Delplace et al. 1997 ), deposi-
tion, modeling and comparison of deposition on different
surfaces to accumulate information for future reference
(de Jong et al. 1992 , Toyoda et al. 1994 , Chen et al. 2001 ).
Thus, the present paper attempts to accumulate informa-
tion on milk composition, fouling phenomena and mecha-
nism of fouling along with modeling approach (Elofsson
et al. 1996 , Visser and Jeurnink 1997 , de Jong et al. 1998 ).
2 Basic principles of fouling The effect of bulk temperatures of hot and cold streams of
liquid under turbulent flow is extended in the boundary
layer and generates good mixing (Bell and Mueller 2001 ).
Heat transfer equipment is often limited by fouling (Bott
1995 ). The effect of fouling on the heat transfer surface is
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170 E. Sadeghinezhad et al.: Milk fouling on heat exchanger surfaces
accounted for in the design by overall fouling resistance
R f , as presented by the following basic heat transfer equa-
tion (Eq. [1]):
0
1 1fR
U U− +
(1)
where U 0 and U are the clean and fouled overall heat
transfer coefficients, respectively (Bott 1995 ).
Figure 1 shows the ways in which the fouling resist-
ance may change. In heat exchangers used for food pro-
cessing, falling rate or linear fouling is usual. Constant-
rate fouling represents a linear enhancement of fouling
thickness with time, and the falling-rate fouling presents
fouling initiation similar to constant-rate fouling. It rep-
resents a linear progress up to a significant interval and
it also picks up a curved shape. Asymptotic fouling rep-
resents a general model of fouling progress, which could
be fitted by the equations for the curves. The three curves
(Figure 1) have initiation periods (induction), and these
periods are short and difficult to model so most math-
ematical models ignore them (Bott 1995 , Changani et al.
1997 , I. Tubman unpublished manuscript).
2.1 Mechanism of milk fouling
Calcium phosphate and whey protein are major components
in milk fouling deposit. Due to their heat sensitivity, both of
the components form insoluble aggregates in bulk solution.
The final deposit contains concentrated calcium phosphate
at the deposit and metal layer interface. There is induction
time in fouling phenomena at the initial stage after which
the fouling commences and a noticeable change is observed
(Belmar -Beiny and Fryer 1993 , Visser and Jeurnink 1997 ).
Visser and Jeurnink (1997) observed that the whey
protein fouling deposit proceeds in the same way as
thermally induced whey protein gelation. They also
reported that the major components in the fouling deposit
are calcium phosphate and whey protein. An investiga-
tion of fouling for a long period showed a higher propor-
tion of minerals in the deposits near the surface caused
by the diffusion of minerals through the earlier formed
deposits rather than minerals formed on the surface at
the beginning (Belmar -Beiny et al. 1993 ). Bulk and surface
processes govern fouling in a heat exchanger. A number
of stages guide the deposition, such as (Belmar -Beiny and
Fryer 1993 , Schreier and Fryer 1995 , Jeurnink et al. 1996a ):
1. aggregation and denaturation of proteins into the
bulk fluid,
2. migration of the aggregated proteins to the surface,
3. incorporation of proteins into the deposit layer due
to surface reactions and possible re-entrainment or
removal of deposits.
Buildup of minerals, protein aggregates and calcium
phosphate in the fouling deposit layer may not proceed
independently. Delsing and Hiddink (1983) experimen-
tally found that the presence of calcium ions is essential
for the growth of protein deposit layers.
2.1.1 Adsorption mechanism
Adsorption equilibrium can be reached when the concen-
tration of the adsorbate in the bulk solution is in dynamic
balance with that of the interface (Changani et al. 1997 ,
Mahdi et al. 2009 ). The adsorption equilibrium analysis
is the most important fundamental information used to
determine the capacity of adsorbent (Lalande et al. 1984 ,
Georgiadis and Macchietto 2000 ). Both the adsorption
capacity and the kinetic behavior of the adsorbent are of
great importance for the lab-scale and industrial-scale
applications (Mahdi et al. 2009 ). Kinetic analysis is a
useful tool to get the time required to reach the equilib-
rium regarding the completion of adsorption. The kinetic
process of adsorption is explained by several mathemati-
cal models where more than one mechanism may be
responsible for the rate-determining step (Polat 2009 ). The
interaction between the adsorbent-adsorbate is responsi-
ble for the nature of the equilibrium between them, and
the interaction is affected due to the resistances to mass
transfer in the boundary layer adjacent to the surface in
the establishment of equilibrium (Polat 2009 ).
The interaction between proteins and adsorbents
do not occur instantaneously. Heat and mass transfer is
controlled by the adsorption rate. The controlling mecha-
nisms of adsorption rate are explained by mathematical
Foul
ing
resi
stan
ce
Possibleinductionperiod
Foulingperiod
Constant-rate fouling
Falling-rate fouling
Asymptotic fouling
Time
Figure 1 Possible ways in which the fouling resistance can evolve
with time (Bott 1995 , Changani et al. 1997 ). Reproduced with per-
mission from © Elsevier.
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E. Sadeghinezhad et al.: Milk fouling on heat exchanger surfaces 171
models. These models describing the adsorption data
have been explained by adsorption reaction models and
adsorption diffusion models (Polat 2009 ).
The forces that play a role in the attachment of the
calcium phosphate to a heated metal surface are the Lif-
shitz-van der Waals (LW) interaction forces, the electro-
static double-layer interaction forces (EL), the Lewis acid/
base interaction forces (AB) and the Brownian motion
(Br). Van Oss divided the total surface energy of a surface
into four components, among them the electron donor
component ( γ − ) being the most often used for character-
izing components of solid surfaces (VanOss 1994; VanOss
et al. 1997 , Wu and Nancollas 1998 , Rosmaninho et al. 2001 ).
After formation of the initial layer on the solid surface,
other particles move from the bulk liquid and adhere on
the top of the initial layer and develop a more or less struc-
tured and compact layer of deposits. The structure of the
formed layer depends on the first layer of construction,
which depends mainly on the surface properties of the
particles and ions present in the solution that contribute
to the growth of the deposition (precipitation/crystalliza-
tion kinetics). The interactions of these factors also deter-
mine the resistance to removal of fouling deposits (Krause
1993 , Bott 1994 , Jeurnink et al. 1996a,b , Amjad 1998 , Visser
1999 , Santos et al. 2004 , Rosmaninho and Melo 2006 ).
2.1.2 Causes of protein aggregation
The β -Lg protein has a global structure that is held
together by S S bonds and one non-exposed internal free
SH group. The β -Lg starts to unfold with the rise in tem-
perature. The free thiol group is therefore liberated from
the β -Lg and the bulk solution, and then the molecule
enters into an activated state, making it possible to react
with another β -Lg molecule. Thus, a radical chain grows
to form an aggregate that is able to deposit on the heat
transfer surface. The rate of deposition was found related
to the concentration of activated molecules in the solu-
tion, which could be calculated by using the model of
denaturation and aggregation of the β -Lg (Lalande and
Reno 1988 , Delplace et al. 1994 ).
Research showed that the two major whey proteins,
α -La and β -Lg, become unstable at temperatures above
65 ° C. When heated above this temperature, protein dena-
turation occurs, resulting in protein aggregation and pre-
cipitation. The response to thermal treatment varies with
the types of protein. They precipitate in different propor-
tions and make the separation possible. Therefore, heat-
induced aggregation and precipitation is an important
treatment in the manufacturing process of many dairy
Thermalboundarylayer
Bulk fluid
Reaction
AdhesionTransfer
Mass
N D A
N* D* A*
Wall
δT
Figure 2 Model of protein and salt deposition on the heat
exchanger surface (Georgiadis and Macchietto 2000 , Youcef et al.
2009). Reproduced with permission from © Elsevier.
products and is used to modify functional properties
with the goal of ensuring the safety of the food products.
The rate at which whey proteins aggregate is controlled
by process conditions like protein concentration, pH and
temperature and the presence of other components (Polat
2009 ).
The kinetics of deposition, different physical and
chemical parameters, quantitative analysis of deposi-
tion and fouling resistance are yet to be fully understood
(Lalande and Reno 1988 , Visser and Jeurnink 1997 , Mahdi
et al. 2009 ).
3 Mineral deposition In addition to the protein fouling, deposition of calcium
phosphate also takes place, which represents an inverse
solubility relation with temperature. During the preheat-
ing process, the ionic product becomes high, following
the inverse solubility concentration limits. Jeurnink and
Brinkman (1994) stated that salts dissolved in milk deposit
in the form of crystals on the surface of the heat exchanger.
Calcium phosphate may also precipitate in the core flow.
Ultimately, in all the cases deposition was formed on the
tested stainless steel wall of the heat exchanger plates as
shown in Figure 2 (Mahdi et al. 2009 ).
In milk fouling, the constituents of deposition proceed
though a complex process in which both whey protein
aggregation and calcium phosphate formation in the bulk
fluid are to be accounted for.
Details of the fouling mechanism can be described in
the following steps (Mahdi et al. 2009 ):
1. Straight adsorption of a protein monolayer occurs on
the heat exchanger surface even at room temperature.
2. Formation of activated β -Lg molecules in the bulk
solution at temperatures higher than 65 ° C. The β -Lg
aggregates (tenths of nanometers) and calcium
phosphate particles are formed.
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172 E. Sadeghinezhad et al.: Milk fouling on heat exchanger surfaces
3. These foulant particles formed in the bulk are con-
tinuously transported to the heated surface. However,
some activated molecules could be deactivated during
this phase due to some reactions with other molecules
in the bulk that transform the particles to an active
state insufficient to create fouling.
4. Deposition of activated molecules by adsorption on
the heat exchanger surface. Calcium ions entrapped
in the protein deposit may help to stabilize these
structures.
5. At relatively high temperatures (above 85 ° C), the main
deposit component is calcium phosphate, which
offers an open network structure where small protein
aggregates can be entrapped.
3.1 Dairy material deposition
The composition and structure of fouling (Peny and Green
1985 ) vary greatly with the processing conditions, which
significantly affect their removal processes as well (Xin
2003 ).
3.1.1 Milk composition
Milk composition is dependent on its source and hence
may not be possible to change (Table 1 ). Variations in milk
fouling are also dependent on differences in its composi-
tion (Burton 1967 , Belmar -Beiny et al. 1993 , de Jong 1997 ,
Bansal and Chen 2006 ). Fouling is enhanced with the
increase in protein concentration (Toyoda et al. 1994 ,
Changani et al. 1997 ). Some researchers reported that the
heat stability of milk proteins decreases with the reduc-
tion in pH (Foster et al. 1989 , Gotham 1990 , Xiong 1992 ,
Corredig and Dalgleish 1996 , de Jong et al. 1998 ). The
chemistry of the protein reaction in fouling is further dis-
cussed by some authors (Changani et al. 1997 , Visser and
Jeurnink 1997 , Christian et al. 2002 ). Roefs and deKruif
(1994) reported that the increase or decrease in the milk
calcium content lowers the heat stability and causes more
fouling in comparison to normal milk. Milk fat has little
effect on fouling (Nicorescu et al. 2009 ).
Additives (minerals, vitamins, enzymes and synthe-
sis additives) (Burton 1968 , Changani et al. 1997 , de Jong
1997 , Guo et al. 2010 ) may retard fouling by enhancing the
heat stability of milk, but addition of any ingredient in
milk may not be permitted in many countries (Lyster 1970 ,
Skudder et al. 1981 , Changani et al. 1997 ).
Reduction of fouling in the heat exchangers can be
achieved by preheating of milk in rising tubes, which
causes denaturation and aggregation of proteins before
transferring to the heating section (Bell and Sanders 1944 ,
Burton 1968 , Mottar and Moermans 1988 , Foster et al.
1989 ). Reconstituted milk generates much less fouling,
approximately 25 % , when β -Lg is denatured during the
production of milk powders (Changani et al. 1997 , Visser
and Jeurnink 1997 ). Changani et al. (1997) reported that
the reconstituted milk contains 9 % less calcium, which
would have resulted in less fouling. In contrast, Newstead
and coworkers observed increase of UHT fouling rates of
the recombined milk with the increase of preheating treat-
ment (preheating temperature × preheating time) (Fryer
et al. 1996a,b , Fung et al. 1998 , Newstead et al. 1998 , Xin
2003 , Riera et al. 2013 , Boxler et al. in press ).
Figure 3 indicates that more deposit is initially formed
on surfaces having higher γ − (Modler 2000 , Rosmaninho
et al. 2005 ). The composition of whey is considerably more
variable than that of milk and is highly dependent on the
type of cheese being manufactured, seasonality, culture
and rennet selection, manufacturing procedures and type
of equipment used. These differences in composition and
functionality are a challenge to the manufacturers (Regester
and Smithers 1991 , Mahdi et al. 2009 , Modler 2009 ).
Caseins of protein precipitate upon acidification and
generate resistance to thermal processing (Visser and
Jeurnink 1997 ). The native proteins ( β -Lg) first denature
(unfold) and expose the core containing reactive sulfhy-
dryl groups with the heating of milk. In the denatured
state, protein molecules react with similar or other types
of protein molecules like casein and α -La and form aggre-
gates (Treybal 1981 , Dalgleish 1990 , Changani et al. 1997 ,
Chen 2000 , Modler 2009 , Espina et al. 2010 ).
The extreme variability (Toyoda et al. 1994 ) of the
composition of fouling layers published in the literature is
highlighted by the selection in Table 2 .
Table 1 Types of milk deposits formed at different processing temperature ranges (Xin 2003 ).
Classification Temperature ( ° C) Process Composition (wt % )
Protein Mineral Fat
Type A 75 – 110 Pasteurization 50 – 70 30 – 50 4 – 8
Type B 110 – 140 UHT treatment 15 – 20 70 – 80 4 – 8
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E. Sadeghinezhad et al.: Milk fouling on heat exchanger surfaces 173
3.1.2 Classification of milk fouling deposits
The formation of fouling and the extent and nature of depo-
sits in dairy fluid processing is influenced by many factors,
such as the processing temperature, seasonal changes, age,
pH, air content and pretreatment as well as heat exchanger
geometry (Burton 1968 , Lalande and Ren é 1988 , Yoon and
Lund 1989 , Changani et al. 1997 , Visser and Jeurnink 1997 ).
Milk deposits are classified in different ways depend-
ing on the chosen criteria. From the cleaning point of
view, it is useful to classify typical soils according to
their nature and structure. On the basis of the processing
temperature range, Lyster (1965) and Burton (1968) classi-
fied milk deposits as type A and type B. The typical com-
positions of type A and B fouling are summarized in Table
3 (Lyster 1965 , Burton 1968 ).
There are significant differences noticed between the
deposits and the raw milk. Detailed information can be
obtained from the reported compositional analysis of milk
deposits (Tissier et al. 1984 , Lalande et al. 1985 , Jeurnink
and Brinkman 1994 ). Lyster (1965) reported that although
calcium and phosphate are only 30 % by weight of the
mineral content in raw milk, they typically formed 90 % by
weight of the mineral content of milk fouling deposits. The
mass ratio of calcium to phosphate is found to be 1.5 – 1.6,
indicating the presence of hydroxyapatite [Ca 5 OH(PO
4 )
3 ],
the least soluble calcium phosphate complex (Lyster 1965 ,
Lalande et al. 1985 ). Proteinaceous deposits (type A) are
the dominant factor in most food processing plants, and
they are normally more difficult to remove compared to
mineral deposits (van Asselt et al. 2005 ). The low density
of proteinaceous deposit can induce a high pressure drop
and a high thermal resistance across processing equip-
ment (Lyster 1965 , Fickak et al. 2011 ).
3.2 Factors affecting milk fouling
Milk fouling in heat exchangers is affected by several
factors, which can be broadly classified into different
major categories. Table 4 summarizes important aspects
2.5 70°C
44°C
1.5
Dep
osit
(mg)
0.5
0.0TiN14 TiN11 TiN10TiN12 TiN13TiN15
2.0
1.0
Figure 3 Deposit mass formed after 15 min of deposition, which is
the first mass detected at 44 ° C and 70 ° C. Surfaces are placed from
left to right in an increasing order of their surface energy, more
precisely their γ - parameter (Rosmaninho et al. 2005 ).
Table 2 Basic research on aspects of fouling mechanisms (Bansal and Chen 2006 ).
Observation References
Protein unfolding or denaturation step is
reversible
de Wit and Swinkles (1980), Anema and McKenna (1996) , Changani et al. (1997) ,
Chen et al. (1998a), Polat (2009)
Protein denaturation is irreversible Ruegg et al. (1977) , Lalande et al. (1985) , Arnebrant et al. (1987) , Gotham et al.
(1992) , Roefs and deKruif (1994) , Karlsson et al. (1996) , Polat (2009)
Protein aggregation is irreversible Mulvihill and Donovan (1987) , Anema and McKenna (1996) , Changani et al. (1997) ,
Chen et al. (1998a)
Protein denaturation is the governing reaction Lalande et al. (1985) , Hege and Kessler (1986) , Arnebrant et al. (1987) , Kessler and
Beyer (1991) , de Jong et al. (1992)
Protein aggregation is the governing reaction Lalande and Ren é (1988) , Gotham et al. (1992) , Delplace et al. (1997)
Formation of protein aggregates enable fouling
reduction
de Jong et al. (1992) , Delplace et al. (1997) , van Asselt et al. (2005)
Only protein aggregates cause fouling (modeled
the milk fouling process based on the assumption
that only aggregated proteins resulted in fouling)
Toyoda et al. (1994)
Fouling is considered to depend on protein
reactions only
de Jong and van der Linden (1992), de Jong et al. (1992) , Belmar -Beiny et al. (1993) ,
Delplace et al. (1994, 1997) , Schreier and Fryer (1995) , Grijspeerdt et al. (2004) ,
Nema and Datta (2005) , Sahoo et al. (2005)
Fouling is dependent on mass transfer as well as
bulk and surface reactions
Toyoda et al. (1994) , Chen et al. (1998a, 2000, 2001), Georgiadis et al. (1998a,b),
Georgiadis and Macchietto (2000) , Bansal and Chen (2005) , Bansal and Chen
(2005) , Bansal and Chen (2006)
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174 E. Sadeghinezhad et al.: Milk fouling on heat exchanger surfaces
of the fouling investigation and also shows the composi-
tion of fouling layers in different occasions (Bansal and
Chen 2005 , Bansal and Chen 2006 ).
3.2.1 Low-temperature milk deposits
In dairy processing plants, the soil also forms on cooled
surfaces. Formation of low-temperature soil is quite dif-
ferent from that of the heat-induced fouling deposit. Kane
and Middlemiss (1985) found that a soil with a much more
open structure and much larger fat content is formed at
low temperature, and its forming process is also different.
3.2.2 Effect of whey protein concentration
An important functional property of whey proteins is
their ability, under appropriate conditions, to form heat-
induced gel structures capable of immobilizing large
quantities of water and other food components (Kuhn
and Foegeding 1991 ). During formation of heat-induced
whey protein gels (HIWPGs) only a fraction of the whey
proteins are aggregated (Lyster 1965 , Burton 1968 , Verheul
and Roefs 1998a ,b, Fickak et al. 2011 ). HIWPG with high
protein concentration (Belmar -Beiny et al. 1993 , Schreier
et al. 1994 , Delplace and Leutiet 1995 , Fryer et al. 1996,
Davies et al. 1997 , Gillham et al. 1999 , Chen 2000 , Xin
et al. 2002a,b ) tends to form faster due to the increasing
rate of aggregation and the decreasing coagulation time
(Sharma and Hill 1983 , Mleko 1999 , Ndoye et al. 2013 ). It
has also been reported that increasing the protein concen-
tration increases the firmness and the aggregate size of
whey protein concentrate (WPC) gels, which accelerates
the gelation process (Mleko 1999 ). Furthermore, Puyol
et al. (2001) have found that whey protein isolate (WPI)
gels with high protein concentration tend to form at lower
temperature (Mulvihill et al. 1990 , Langton and Hermans-
son 1992 , Verheul and Roefs 1998). Some researchers
(Fryer et al. 1996a , Visser 1997 , Wilson et al. 1999, 2002 ,
Chen et al. 2004 , Fickak et al. 2011 ) have concluded that
the high protein concentration can greatly influence the
formation mechanisms of whey protein gels; however, the
effect of protein concentration on the fouling and clean-
ing of dairy heat exchanger surface has not been dem-
onstrated quantitatively by them (Bird and Fryer 1991 ,
Gillham et al. 1999 , Gillham et al. 2000 , Puyol et al. 2001 ,
Xin et al. 2002b , Mercad é -Prieto and Chen 2006 , Fickak
et al. 2011 , Ndoye et al. 2013 ).
In fact, aggregated whey protein molecules dominate
the basic structure of the fouling deposits. Because of
this and also the complex nature of milk deposits, many
researchers (Belmar -Beiny and Fryer 1993 , Belmar -Beiny
et al. 1993 , Schreier et al. 1994 , Davies et al. 1997 , Xin et al.
2002a,b , Fickak et al. 2011 ) found HIWPGs to be a reliable
model system for investigating milk fouling and cleaning.
HIWPGs also contain a small quantity of minerals and
have been found to have the same nature of type A milk
deposits as described by Lyster (1965) and Burton (1968) ,
where proteins represent more than 60 wt % of the deposit
mass. Formation of the HIWPG deposits results from the
aggregation of whey proteins upon heating. Furthermore,
a study by Puyol et al. (2001) found that WPI gels with
high protein concentration tend to form at lower tempera-
ture. Some researchers presented dissolution or “ clean-
ing ” mechanisms based on the breakdown of protein
aggregates, diffusion of small oligomers throughout the
swollen layer, and disentanglement of large aggregates
next to the gel-solvent boundary layer. However, little is
known about the effect of protein concentration within
fouling and heat-induced whey protein gel on their dis-
solution (Fickak et al. 2011 ).
Fickak et al. (2011) studied the effect of whey protein
concentration on the fouling and cleaning behaviors of a
pilot-scale heat exchanger. They assessed the influence of
the properties of surfaces (based on stainless steel) on the
fouling behavior of different milk components (calcium
phosphate and whey proteins), complex milk systems
[fouling model fluid (FMF)] and milk-related bacteria. The
formation and dissolution rate of HIWPG is influenced
by the whey protein concentration. It was found that
the structure of HIWPG became more rigid with increas-
ing protein concentration. The dissolution rate of HIWPG
decreased almost linearly with the increase in protein
concentration (Fickak et al. 2011 ). In all cases the surface
material practically influenced the fouling behavior,
although in different ways they influenced the deposition
and the cleaning phases (Rosmaninho et al. 2007b ).
Table 3 Average composition of milk (Bansal and Chen 2006).
Constituents Average concentration (wt % )
Water 87.5
Total solids 13
Proteins 3.4
Lactose 4.8
Minerals 0.8
Fat 3.9
Proteins 3.4
Casein 2.6
β -Lg 0.32
α -La 0.12
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E. Sadeghinezhad et al.: Milk fouling on heat exchanger surfaces 175
Table 4 Composition of fouling layers from a selection of studies (Bennett 2007).
References Milk type Milk temperature ( ° C) Equipment Composition ( % dry basis)
Lyster (1965) Whole 85 Plate heat exchanger (regenerative section) Protein: 60
Mineral: 25
Fat: 12
Lalande et al. (1984) Whole 65 – 70 Plate heat exchanger (regenerative section) Protein: 50
Mineral: 40
Fat: 1
Whole 120 – 138 Plate heat exchanger (regenerative section) Protein: 15
Mineral: 75
Fat: 3
Fung et al. (1998) Whole 4 – 90 Tubular heat exchanger Protein: 32
Mineral: 5
Fat: 50
Whole (damaged) 4 – 90 Tubular heat exchanger Protein: 32
Mineral: 4
Fat: 49
Tissier et al. (1984) Whole 72 Pasteurizer Protein: 50
Mineral: 15
Fat: 25
Whole 90 Sterilizer Protein: 50
Mineral: 40
Fat: 1
Whole 138 Sterilizer Protein: 12
Mineral: 75
Fat: 3
Yoon and Lund (1989) Whole 88 Plate heat exchanger (preheat) Protein: 43
Mineral: 45
Fat: ND
Whole 120 Plate heat exchanger (sterilizer) Protein: 45
Mineral: 40
Fat: ND
Calvo and Rafael (1995) Whole 80 Plate heat exchanger (heating) Protein: 52
Mineral: 9
Fat: 23
Grandison (1988) Whole 110 – 140 Plate heat exchanger (regenerative and heating) Protein: 19 – 44
Mineral: 57 – 20
Fat: 1 – 28
Jeurnink et al. (1989) Whole 85 Tubular heat exchanger Protein: 64
Mineral: 18
Fat: 15
Whole 120 Tubular heat exchanger Protein: 43
Mineral: 49
Fat: 3
Delsing and Hiddink (1983) Skim 76 Tubular heat exchanger Protein: 78
Mineral: 17
Fat: –
Jeurnink and de Kruif (1995) Skim 85 Plate heat exchanger Protein: 44
Mineral: 45
Fat: –
Skudder et al. (1986) Whole 80 – 110 Plate heat exchanger (regenerative) Protein: 51
Mineral: 20
Fat: 6
Whole 110 – 140 Plate heat exchanger (heating) Protein: 22
Mineral: 53
Fat: 5
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176 E. Sadeghinezhad et al.: Milk fouling on heat exchanger surfaces
3.3 Heat exchanger characteristics and operating conditions
Some significant operating parameters, such as air
content, velocity or turbulence and temperature, could
be varied in a heat exchanger of a milk processing plant
(Lalande et al. 1985 , Hege and Kessler 1986 , Arnebrant
et al. 1987 , Lalande and Reno 1988 , Kessler and Beyer 1991 ,
Gotham et al. 1992 , Belmar -Beiny and Fryer 1993 , Belmar -
Beiny et al. 1993 , Schreier and Fryer 1995 ). The presence
of air in milk enhances fouling (Burton 1968 , de Jong 1997 ,
de Jong et al. 1998 ). It is reported that fouling is enhanced
when air bubbles are formed on heat transfer surfaces,
which then become a nuclei for deposit formation (Burton
1968 , de Jong 1997 ). Heating of milk decreases its dis-
solved air content, resulting in reduction of the pressure
of the flowing process liquid (de Jong 1997 , de Jong et al.
1998 , Muthukumaran et al. 2011 ).
Enhanced turbulence in flow retards fouling deposi-
tion (Belmar -Beiny and Fryer 1993 , Belmar -Beiny et al.
1993 , Santos et al. 2001, 2003 ). Paterson and Fryer (1988)
and Changani et al. (1997) reported that the thickness and
subsequently the volume of laminar sublayer decreases
with the increasing velocity of flowing milk, which retards
the depositions of foulant on heat transfer surfaces. Other
researchers (Rakes et al. 1986 , Paterson and Fryer 1988 ,
Changani et al. 1997 ) also have informed that the higher
flow velocities also promote deposit re-entrainment due
to enhanced fluid shear stresses.
Belmar -Beiny et al. (1993) concluded that higher tur-
bulence and different flow characteristics in fact generate
a shorter induction period in plate heat exchangers com-
pared to tubular heat exchangers (Bradley and Fryer 1992 ,
de Jong et al. 1992 , Delplace et al. 1994 , Toyoda et al. 1994 ,
Changani et al. 1997 , Chen et al. 1998a, 2001 , van Asselt
et al. 2005 ).
In a heat exchanger, the temperature of milk is proba-
bly the most important factor that is controlling the fouling
(Burton 1968 , Kessler and Beyer 1991 , Belmar -Beiny et al.
1993 , Toyoda et al. 1994 , Corredig and Dalgleish 1996 ,
Elofsson et al. 1996 , Jeurnink et al. 1996a , Santos et al.
2003 ). Higher fouling results from increase of milk tem-
perature. Milk fouling can be classified into two catego-
ries: type A and type B (Burton 1968 , Lund and Bixby 1975 ,
Changani et al. 1997 , Visser and Jeurnink 1997 ). The nature
of fouling changes from type A to type B (Table 1) at tem-
peratures exceeding 110 ° C (Burton 1968 ).
Bulk temperature (average of the inlet and outlet
temperatures of the milk) and the temperature dif-
ference between bulk and surface are both important
for fouling. Chen and Bala (1998) studied the effect of
surface and bulk temperatures on the fouling of whole
milk, skim milk and whey protein. They observed that
on initiating fouling, the surface temperature was the
most important factor. They also noticed no fouling at
the surface temperature of < 68 ° C, even when the bulk
temperature was up to 84 ° C (Chen and Bala 1998 , Chen
et al. 2001 ). Chen et al. (2001) predicted that mixing
caused by inline mixers can reduce fouling substan-
tially, but no information was provided about precipita-
tion in bulk (precipitate of calcium phosphate coming
out from the solution).
In dairy and other food processing industries, plate
heat exchangers are extensively used. Delplace et al.
(1994) informed that plate heat exchangers are prone
to fouling due to their narrow flow channels and high
References Milk type Milk temperature ( ° C) Equipment Composition ( % dry basis)
Ma et al. (1998) Whole 85 Tubular heat exchanger Protein: 20
Mineral: 4
Fat: 45
Skim 85 Tubular heat exchanger Protein: 64
Mineral: 13
Fat: –
Johnson and Roland (1940) Whole 82 Tubular heat exchanger Protein: 35
Mineral: 5
Fat: 52
Truong (2001) Whole 110 Downstream from direct steam injector (rig) Protein: 39
Mineral: 8
Fat: 39
Whole 105 Downstream from direct steam injector (plant) Protein: 63
Mineral: 20
Fat: 3
(Table 4 Continued)
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E. Sadeghinezhad et al.: Milk fouling on heat exchanger surfaces 177
surface temperature. Complete mitigation of milk fouling
in a heat exchanger is difficult, as the temperature of the
heat transfer surface needs to be considerably higher than
the bulk temperature for efficient heat transfer (Zaida
et al. 1987 , Metaxas and Meredith 1988 , Thompson and
Thompson 1990 , Kindle et al. 1996 , Sieber et al. 1996 ,
Villamiel et al. 1996 , de Jong 1997 , Delplace et al. 1997 , de
Jong et al. 1998 ).
Direct resistance heating (ohmic heating) is a novel
heat treatment process where electrical current is passed
through the milk to generate heat, which pasteurizes or
sterilizes milk (deAlwis and Fryer 1990 , Quarini 1995 ). In
recent years this technology has been in use after being
abandoned for a major part of the 20th century. APV Inter-
national Ltd. (England) developed commercial ohmic
heating units for continuous sterilization of food prod-
ucts (Skudder and Biss 1987 , Fryer et al. 1993 , Ayadi et al.
2003a,b, 2004 ).
3.4 Effect of microorganisms
The adhesion of microorganisms to the surface is
enhanced by the formation of fouling layer resulting in
biofouling. The deposits become nutrients for microor-
ganisms, ensuring their growth. Most of the processes in
industry are carried out at temperatures below 100 ° C. For
instance, pasteurization is commonly achieved by heating
milk at 72 ° C for 15 s in a continuous flow system. Just the
pathogenic bacteria along with some vegetative cells are
killed at this temperature. A higher temperature of 85 ° C is
required to kill the remaining vegetative cells. Spores are
resistant to large amounts of heat and remain active well
beyond this temperature (Bott 1993 ).
Biofouling develops in a heat exchanger, either by
microorganism deposition or biofilm formation, which
raises serious quality concerns. The effect of biofouling
in dairy plants has been investigated by Flint and cow-
orkers (Bott 1993 , Flint and Hartley 1996 , Flint et al. 1997,
1999, 2000 ). They informed that biofouling occurs in two
different mechanisms: accumulation of microorganisms
on the heat transfer surfaces and attachment of microor-
ganisms on the deposit layer formed on the heat transfer
surfaces.
The deposit layer of microorganisms not only affects
the product quality but also influences the fouling process
(Yoo et al. 2006 ). Hydrodynamic forces drive the bacteria
and release them to the process fluid, which increases
the bacterial concentration at the downstream. This may
cause bacterial growth in areas not conducive to biofoul-
ing (Chen et al. 1998b , Yoo and Chen 2002 ).
4 Effect of surface material on milk fouling
It has been found that the deposit growth appears to be
dependent mainly on the interactions between the fluid
and the surface, and the nature of the surface becomes
unimportant once the first layer is formed (Dupeyrat
et al. 1987 , Yoon and Lund 1989 ). Different coatings on the
surface had no effect on the amount of deposit formed,
but they had an effect on the strength of adhesion (Boxler
et al. in press, Britten et al. 1988 ). The magnitude of the
soil-substrate adhesion force, hence the cleaning pro-
cesses, may be altered by changing the nature and condi-
tions of the surface by surface modification methods, such
as coating, electrochemical polishing, chemical treatment
and magnetic field (Kittaka 1974 , Koopal 1985 , Nassauer
1985 , Britten et al. 1988 , Petermeier et al. 2002 , Xin 2003 ,
Premathilaka et al. 2006 ).
4.1 Ferrous and nonferrous surfaces
The most common types among the materials used in
process equipment in the food industry are different
grades of stainless steel. Several techniques have been
applied to modify its surface properties with the aim of
reducing the buildup of unwanted deposits (fouling).
The characteristics of some of those techniques were
published (Santos et al. 2004 ), where the description of
their role on fouling caused by several milk components
and dairy products are incorporated (Rosmaninho et al.
2001, 2003, 2005 ). Surface energy is one among several
surface parameters affecting and controlling the fouling
process. Reactive sputtering technique was used to obtain
a number of stainless steel materials with similar surface
composition and morphology having variable surface
energy values for conducting performance investigation
of those materials. The objective of some studies was to
estimate the calcium phosphate component in the main
mineral constituent of deposits from milk (Jeurnink et al.
1996a , Visser 1999 ) and the role of the surface energy on
fouling buildup and its cleaning along with finding out a
better way of characterization of fouling caused by milk
(Rosmaninho et al. 2005 , Kukulka and Leising 2010 ).
The mechanism of deposition can be separated into
several steps (Ruegg et al. 1977 , Lalande et al. 1985 , Hege
and Kessler 1986 , Arnebrant et al. 1987 , Mulvihill and
Donovan 1987 , Lalande and Ren é 1988 , Kessler and Beyer
1991 , Gotham et al. 1992 , Roefs and deKruif 1994 , Toyoda
et al. 1994 , Anema and McKenna 1996 , Karlsson et al.
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178 E. Sadeghinezhad et al.: Milk fouling on heat exchanger surfaces
1996 , Changani et al. 1997 ). In the initial stage, calcium
phosphate particles form in the bulk due to heating. On
arrival at the vicinity of the heated surface, these particles
could be attached to the surface depending on the forces
established between the foulants and the surface. Depo-
sition on the surface depends on the interaction forces
between the particles and the surface and surface proper-
ties of the particles and of the metal support (Rosmaninho
et al. 2005 ).
4.1.1 TiN surfaces
Fouling depends on several surface properties like rough-
ness, surface composition and surface energy (Mullin
1993, 2001 ). Different stainless-steel-based surfaces with a
wide range of surface energy values, having similar rough-
ness and qualitative chemical compositions, were used to
investigate the influence of the surface energy on the dep-
osition process (Mullin 1993 , Bott 1994 , Rosmaninho et al.
2002 , Rosmaninho and Melo 2006 ). The investigated sur-
faces were 316 2R (bright annealed) based and prepared
by the technique of reactive sputtering coating (with
different proportions of titanium (Ti) and N in an argon
atmosphere). In this technique, reactive sputtering of Ti
(99.7 % purity) with an unbalanced magnetron cathode
was used for coating on the stainless steel surfaces. Nitro-
gen (99.998 % N 2 by volume) was used as the reaction gas
(Rosmaninho et al. 2007a ).
In their work, Rosmaninho et al. (2007a) tested all
the surfaces under varying operational parameters in the
same modification technique. Consequently, the differ-
ent TiN-sputtered surfaces had similar morphology and
surface composition, although with varying proportions
of Ti and N and different surface energy properties. The
surface used for investigation could be distinguished and
characterized on the basis of their surface energy values
as shown in Table 5 .
With reference to Table 5, the most distinguishing
factor among the surfaces is the electron-donor com-
ponent ( γ − ), and therefore this could be the characteri-
zing parameter for the surfaces considered in the work
(Rosmaninho et al. 2007a ).
The mass of calcium phosphate deposits on different
surfaces at two temperatures (44 ° C and 70 ° C) after 15-,
30-, 45-, 60-, 120- and 240-min time intervals are evalu-
ated. The deposition trend of calcium phosphate was
qualitatively very similar for all the surfaces, and in all
individual cases it was similar to an overall linear growth
(Rosmaninho et al. 2007a ).
The residual deposit mass after cleaning (with water
at the same temperature of the deposit formation) gen-
erally increased with the γ − value of the surface (Britten
et al. 1988 , Changani et al. 1997 ). More deposit remains
at the higher-energy surfaces except at the highest-energy
surface (not yet explainable), where a small decrease
was detected (Zhao et al. 2005 , Rosmaninho et al. 2007a ,
Kananeh et al. 2010 ).
The trends presented by the researchers indicate that
the more deposit is initially formed on surfaces having
higher γ − (Figure 3). This dependency was informed in
previous research with an indication of relation between
affinities of the surfaces to nucleation of fouling depos-
its. Furthermore the combination of size and number of
the first aggregates of the foulant from calcium phosphate
formed on the surface is dependent on the surface energy
values ( γ − component) (Rosmaninho et al. 2005, 2007a,b ).
The effect of whey protein ( β -Lg) on the fouling
pattern of calcium phosphate of a simulated solution
(milk) was studied and evaluated by Rosmaninho and
Melo (2007) . They mainly considered fouling depend-
ence on the surface energy of different modified surfaces
used for the study of deposition of materials. The depo-
sition curves obtained in the presence and absence of
protein were considerably different. The investigators also
observed two growth periods at different time spans in
the presence of the whey protein. The appearance of the
second growth period after the delay time was dependent
on the type of surface where fouling developed, more pre-
cisely on its roughness, surface composition and surface
energy values (Rosmaninho and Melo 2007 ).
4.2 Surface treatment of heat exchangers in the dairy industry
Antifouling coatings based on nanocomposites (Table 6 )
have been used to decrease fouling on plate and frame
heat exchangers used in a food processing plant (Kananeh
Table 5 Surface energy components of the TiN sputtered surfaces
(Rosmaninho et al. 2007a).
Surface a γ LW (mJ/m 2 ) γ + (mJ/m 2 ) γ - (mJ/m 2 ) γ TOT (mJ/m 2 )
TiN 1 43.2 (0.1) 0.7 (0.0) 55.3 (0.0) 55.70 (0.1)
TiN 2 43.6 (0.2) 1.3 (0.0) 23.0 (1.8) 54.3 (0.2)
TiN 3 43.4 (0.1) 1.0 (0.2) 46.2 (4.6) 56.7 (0.4)
TiN 4 43.4 (0.1) 1.3 (0.1) 18.3 (2.7) 53.2 (0.4)
TiN 5 42.80 (0.9) 1.0 (0.9) 26.0 (2.2) 53.0 (0.6)
a All the surfaces became covered by a similar layer of TiN, which
makes them different on the basis of their surface energy, and as
such the surfaces were named TiN 1, TiN 2, TiN 3, TiN 4 and TiN 5.
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E. Sadeghinezhad et al.: Milk fouling on heat exchanger surfaces 179
et al. 2009 ). The low-energy surfaces led to a hydropho-
bic and an oleophobic effect. In the heating section of a
pasteurizer, four different coated plates were equipped
in order to study fouling. The pasteurizer was operated
with a 10 % whey protein solution, which was heated up
to 85 ° C. Three coated samples of stainless steel accu-
mulated a reduced fouling amount in comparison to the
uncoated stainless steel plates. The electropolished plates
showed about 64 % of each deposited surface cleaned by
cleaning in place (CIP) at a time interval in comparison to
the standard SS plates, whereas the coated plates showed
approximately 30 % cleaned at that time span (Kananeh et
al. 2009 , Dowling et al. 2010 ).
The influence of the surface energy on the stainless
steel substratum at similar roughness was studied by
Rosmaninho and Melo (2007) to investigate the deposi-
tion process. In order to change their surface energy, the
stainless steel 316 2R-based materials were subjected to
three types of surface modification processes such as ion
implantation (Santos et al. 2004 , Rosmaninho and Melo
2006 ), plasma chemical vapor deposition coating, nickel-
phosphor-polytetrafluorethylene (Ni-P-PTFE) coating and
nanocomposite coating (Beuf et al. 2003a,b , Rosmaninho
and Melo 2006, 2007, 2008 , Augustin et al. 2007 , K ü ck
et al. 2007 , Rosmaninho et al. 2007a,b , Kananeh et al.
2009 , 2010, Ozden and Puri 2010 ).
1. Ni-P-PTFE surface was the most promising one for
nonmicrobiological deposits (calcium phosphate,
β -Lg and FMF milk-based product). It generally
accumulates less deposit buildup and in all cases was
the easiest to clean.
2. Considering food contamination, on the basis of the
data obtained from adhesion the ion implanted (TiC)
surface appeared to be the most suitable surface,
which also carries less spores after the cleaning
process. Looking only at the number of adhered
spores, the Ni-P-PTFE and the xylan surfaces did
not appear to be as good as previously obtained TiC
surfaces (Rosmaninho et al. 2007b ).
To minimize milk fouling effects, Ni-P-PTFE coating is not
the most appropriate surface treatment against fouling,
but ion implantation is preferable, as proven by many
researchers. This remark is supported by the character-
istics provided by the Ni-P-PTFE surface modification
method, which is systematically different from the other
methods as reported by previous authors (Santos et al.
2004 , Rosmaninho et al. 2007b ).
Rosmaninho et al. (2007b) found that the Ni-P-PTFE
coating is the only modification method that could clearly
change the topography of unmodified steel surfaces. On
masking the grain boundaries at the surface, these types
of coatings create the thickest layer on the stainless steel
surface (10 μ m, against 0.2 – 2.5 μ m for all the other sur-
faces). They have also reported that the thickness of the
Ni-P-PTFE layer is small, which did not create a significant
effect on the thermal resistance of the heat transfer wall
(Rosmaninho and Melo 2008 ).
Kananeh et al. (2010) reported that the fouling of gas-
keted plate heat exchangers in milk production has been
reduced by the use of nanocomposite coatings. However,
an antifouling coating with low surface energy (low wet-
tability) led to a hydrophobic and oleophobic effect. They
have also investigated the performance of a number of
coatings and surface treatments of heat exchangers used
in milk processing. Certain polyurethane-coated plates
and tubes received thinner deposit layer compared to that
of standard uncoated stainless steel plates and tubes.
They achieved 70 % reduction in cleaning time of coated
plates in comparison to that of the standard stainless steel
one. Plates coated with different nanocomposites as well
as electropolishing were installed in the heating section
of the pasteurizer. Polyurethane-coated plates exhibited
the thinnest deposit layer due to the lowest total surface
energy (Kananeh et al. 2010 , Barish and Goddard 2013 ).
Electropolished plates also present a reduced deposit
buildup in comparison to the standard stainless steel
plates and were almost similar to the coated plates.
Kananeh et al. (2010) observed 30 % reduction in CIP time
of electropolished plates in comparison to standard stain-
less steel plates.
The results from an investigation in a pilot plant
reveal that the coatings must be further developed so that
they can withstand the thermal and mechanical stresses
that arise in industrial operation (Kananeh et al. 2010 ).
5 Milk fouling removal Xin (2003) reported that the cleaning of the milk process-
ing surfaces is an essential stage to remove the undesired
Table 6 Fouling surface sample specifications used in the experi-
ments conducted by Kananeh et al. (2009) .
Substrate Material Thickness ( μ m)
SS Stainless steel –
EP Electrically polished stainless steel –
A2 Epoxy resin-based coating of INM 83.7
A9 Polyurethane-based coating of INM 53.0
A10 Polyurethane-based coating of INM 85.2
A67 Polyurethane-based coating of INM 27.6
PTFE Teflon 22.5
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180 E. Sadeghinezhad et al.: Milk fouling on heat exchanger surfaces
materials. The cost of the cleaning process is very high
due to the high frequency of cleaning, hygienic require-
ment, chemicals, energy, water, product losses, labor and
downtime. Pritchard et al. (1988) estimated that up to 42 %
of the production time is spent in the cleaning process
of the dairy industry. Standard cleaning procedures for
fouling deposits are well used in the CIP system. The pro-
cedure is based on a controlled automatic circulation of
formulated detergents in a certain section of a plant. An
optimized CIP process can reduce the costs of cleaning as
well as the negative environmental impacts. The CIP time
of all coated plates was reduced. Xin (2003) emphasized
an investigation for developing a mathematical model to
optimize cleaning processes.
At operation temperature lower than 100 ° C during
pasteurization, protein is the main constituent of milk
fouling deposits as a result of the heat-induced protein
denaturation and aggregation reaction (Lalande et al.
1985 , deWit et al. 1986 , Grasshoff 1989 ). Due to the high
heat transfer resistance and the difficulty of removal, pro-
teinaceous fouling is a major concern for cleaning pro-
cesses (Xin 2003 , Nigam et al. 2008 , Fickak et al. 2011 ).
In a typical cleaning study, a test setup may be
installed to develop fouling deposits and then it could
be cleaned by selected cleaning solutions. Concentrated
whey protein was used to study fouling formation but the
preparation was not precisely controlled, which triggered
variable solutions. In a similar fouling procedure, signifi-
cant variations of water content and amount of fouling
have been reported (Gotham 1990 , Gillham 1997 ), which
makes the comparison of cleaning results very difficult
due to the sensitivity of the cleaning rate on the composi-
tion and properties of fouling (Xin 2003 , Zulewska et al.
2009 , Li et al. 2013 ).
The fouled heat exchanger surfaces were cleaned by
three-stage cleaning method. In the first stage, the whey
protein solution was drained and the system was washed
with water at a velocity of 10.423 cm/s (for approx. 10 min)
until there were no protein traces left in the stream
water. The rinsing efficiency was estimated using infor-
mation on the turbidity of flowing stream. The rinsing
process was stopped at 0.5 – 1 NTU (number of transfer
units) turbidity of cleaning water (USEPA 2001 , Fickak
et al. 2011 ).
Then, a cleaning solution (50 L of NaOH at 0.5 wt % )
was used to clean the remaining deposits. During clean-
ing, the temperature of the cleaning solution was main-
tained constant at 60 ± 0.5 ° C. In the process of CIP, the
cleaning solution was recirculated through the system
(Fickak et al. 2011 ). The CIP solution was drained after the
complete removal of the fouling layer (Fickak et al. 2011 ).
Many monitoring methods have been developed for
cleaning research. There is still lack of accurate online
methods for suitable cleaning kinetics of fouling. Explora-
tion of the fundamental cleaning mechanisms and opti-
mization of the cleaning process are still a big challenge
for researchers in this field (Bell and Sanders 1944 , Burton
1968 , Arnebrant et al. 1987 , Foster et al. 1989 , de Jong
et al. 1992 , de Jong 1997 , Petermeier et al. 2002 , Xin 2003 ,
Grijspeerdt et al. 2004 , van Asselt et al. 2005 , Bansal and
Chen 2006 , Jun et al. 2008 , Dowling et al. 2010 , Espina
et al. 2010 ).
6 Model for milk fouling Mathematical models for fouling at low temperatures
exist, but the behavior of systems at UHT is still unclear.
Heat transfer coefficients and pressure drops were meas-
ured during fouling in all sections of the heat exchanger
(Fryer et al. 1996b , Jun and Puri 2005a , De Bonis and
Ruocco 2009 , Mahdi et al. 2009 ).
Nema amd Datta (2005) developed a model that can
be used to control steam temperature or pressure in a
helical triple-tube heat exchanger to overcome the reduc-
tion in milk outlet temperature due to fouling. They raised
the temperature of the wall gradually to counter heat
losses due to fouling (Chen and Bala 1998 , Chen et al.
2001 , Balsubramanian et al. 2008 ). It is reported that the
major contribution to the overall cost due to interruption
of production comes from cleaning. Thus, the duration
of the heating cycle tends to be maximum at the optimal
solution (Toyoda et al. 1994 ). Their proposed model may
be useful for predicting the steam temperature or increase
of pressure required for compensating for the reduction in
milk outlet temperature as affected by fouling in a tubular
heat exchanger. It could be implemented for commercial
UHT milk sterilizers with suitable modifications (Nema
and Datta 2005 , Hooper et al. 2006 , Boxler et al. 2013 ).
Georgiadis and Macchietto (2000) used a fouling
model that relies on the β -Lg reaction scheme as shown in
Figure 3. The model was adapted from the work of Toyoda
and Fryer (1997) and was first proposed by de Jong et al.
(1992) . Above 65 ° C, β -Lg of milk becomes thermally unsta-
ble and (i) undergoes molecular denaturation and exposes
the reactive sulfhydryl (-SH) groups and (ii) polymerizes
irreversibly to produce insoluble particles in aggregated
form (de Jong et al. 1992 , Toyoda and Fryer 1997 , Georgi-
adis and Macchietto 2000 ). In the study of fouling, the
key step is to understand the interrelationship between
the chemical reactions that give rise to deposition and the
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E. Sadeghinezhad et al.: Milk fouling on heat exchanger surfaces 181
fluid mechanics associated with the heat transfer equip-
ment. The reaction scheme is described as follows (Geor-
giadis and Macchietto 2000 , Wallh ä u ß er et al. 2012 ):
1. In both the bulk and the thermal boundary layers in
the milk, the proteins in a first-order reaction and
the native protein N are transformed to denatured
protein D. The denatured protein is then reacts to form
aggregated protein A in a second-order reaction.
2. From each protein, the mass transfer between the
bulk and the thermal boundary layer takes place.
3. The aggregated protein is specifically deposited on
the wall. The concentration of aggregated protein
in the thermal boundary layer is proportional to the
deposition rate.
4. The fouling resistance to heat transfer is proportional
to the thickness of the deposit (Fryer et al. 1996b ,
Georgiadis et al. 1998a , Robbins et al. 1999 , Georgiadis
and Macchietto 2000 , Grijspeerdt et al. 2003, 2004 ,
Jun and Puri 2005a,b , Bansal and Chen 2006 , Mahdi
et al. 2009 , Wallh ä u ß er et al. 2012 ).
The reaction rate constants are expressed in the common
form as:
K = K o exp(- E / RT ) (2)
For the two reactions the preexponential factors K o and
the activation energies E are taken from de Jong et al.
(1992) and presented in Table 7 (Georgiadis and Macchi-
etto 2000 ) and Table 8 (Mahdi et al. 2009 ).
Mahdi et al. (2009) showed that fouling is highly
dependent on the various process operating conditions.
The different parameters seem to affect the phenomenon
more specifically in the flow direction rather than in the
width direction. The mass of deposit depends mainly on
milk temperature and time of processing. Moreover, it
is observed that the fouling extent is strongly related to
the Reynolds number and is inversely proportional to the
velocity (Jun and Puri 2005a,b , COMSOL Multiphysics
2007 , De Bonis and Ruocco 2009 , Choi et al. 2012 ).
7 Summary In the dairy industry, thermal processing is an energy-
intensive act. Heat exchanger fouling diminishes heat
transfer, enhances pressure drop and reduces efficiency,
which ultimately affects the economy of a processing
plant. Product quality could be deteriorated due to fouling
and lack of proper heating. The chemistry of milk fouling
fluids is qualitatively understood but still needs intensive
research to gather quantitative information. Heat trans-
fer coefficient and pressure drop studies through fouling
were conducted by many researchers for most types of
heat exchangers. Many research works were conducted
to explore the mitigation of fouling on heated surfaces.
Among them, electromagnetic means, surface modifi-
cations and changing of heat exchanger configurations
are notable. The present review highlighted information
about previous work on fouling and influencing param-
eters of the fouling factor including mitigation approach.
Extended research on fouling will enable researchers to
find ways to mitigate fouling in heat exchangers of milk
and food processing industries. However, it is found that
surface modification toward low total surface energy is an
appropriate approach of fouling mitigation.
Nomenclature AB Acid/base interaction forces
Br Brownian motion
CFD Computational fl uid dynamics
CIP Cleaning in place
EL Electrostatic double-layer interaction forces
FMF Fouling model fl uid
HIWPG Heat-induced whey protein gels
LW Lifshitz-van der Waals interaction forces
Ni-P-PTFE Nickel-phosphor-polytetrafl uorethylene
NTU Number of transfer units
R Ideal gas constant (kJ/mol ° C)
R f Fouling resistance
SMUF Simulated milk ultra fi ltrate
UHT Ultrahigh temperature
WPC Whey protein concentrate
WPI Whey protein isolate
E Activation energy (kJ/mol)
K No
Interchange coeffi cient (1/s)
K Do
Eff ective reaction rate constant in the fl uidized state
(m 3 /kg s)
Table 7 Kinetic data for the reactions of β -Lg (de Jong et al. 1992 ,
Georgiadis and Macchietto 2000 ).
E N (kJ/mol) K No (1/s) E D (kJ/mol) K Do (m 3 /kg s)
261 312 3.37 × 10 37 1.36 × 10 43
Table 8 Kinetic parameters for the fouling reaction scheme (Mahdi
et al. 2009 ).
T ( ° C) E (J/mol) ln ( K o )
Native 70 – 90 2.614 × 10 5 86.41
Denatured 70 – 90 3.37 × 10 37 89.40
Aggregation 70 – 90 2.885 × 10 5 91.32
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182 E. Sadeghinezhad et al.: Milk fouling on heat exchanger surfaces
α -La α -Lactalbumin
β -Lg β -Lactoglobulin
γ Surface energy (mJ/m 2 )
γ LW Lifshitz-van der Waals component (mJ/m 2 )
γ AB Acid-base component (mJ/m 2 )
γ - Electron donor (mJ/m 2 )
γ + Electron acceptor (mJ/m 2 )
Acknowledgments: The authors gratefully acknowledge
High Impact Research Grant UM.C/HIR/MOHE/ENG/45
and University of Malaya, Malaysia, for support to con-
duct this research work.
Received January 23, 2013; accepted March 7, 2013
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Mr. E. Sadeghinezhad received his Bachelor of Engineering degree
from University of Shahid Bahonr, Kerman, and Master of Engi-
neering Science from University of Malaya, Malaysia. He is now
pursuing his PhD in the field of heat transfer, fouling and thermo-
dynamics from the University of Malaya. His major interests are
heat transfer fluids, numerical and experimental multiphase flow,
fouling and its mitigation and heat-exchanging devices.
Dr. S.N. Kazi has 18 years of engineering service experience in
petrochemical industries. His academic background includes a BSc
in Mechanical Engineering, Masters in Mechanical Engineering and
PhD in Chemical and Materials Engineering. He has 49 technical
papers published in national and international journals and confer-
ence proceedings. He has worked as a consultant for Crown Agents
Services Ltd. At present he is working as an academic in the Depart-
ment of Mechanical Engineering, Faculty of Engineering, University
of Malaya, Kuala Lumpur, Malaysia.
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188 E. Sadeghinezhad et al.: Milk fouling on heat exchanger surfaces
Dr. Ahmad Badaruddin received his MEng from the University of
London, UK, and PhD from Cranfield University, UK, in turbulence
modeling. His area of expertise includes computational fluid dynam-
ics (hybrid flow solver development, large eddy simulation, synthetic
turbulence, heat transfer with nanofluids, multiphase flow with phase
change, building and environment). At present he is working as an
academic staff at the Department of Mechanical Engineering, Faculty
of Engineering, University of Malaya, Kuala Lumpur, Malaysia.
Mr. M. Nashrul received his Bachelor of Engineering degree from
University of Malaya, Malaysia, and Master of Engineering Science
(Research) from Monash University, Australia. He is now pursu-
ing his PhD in the field of heat transfer and thermodynamics from
University of Malaya. His major interests are heat transfer fluids,
numerical and experimental multiphase flow, fouling and its mitiga-
tion and heat-exchanging devices.
Babak Lotfizadeh Dehkordi received his BSc in Mechanical Engi-
neering from the Shahrekod University, Iran, in 2009. He studied
thermophysical properties of nanofluids, which is a bridge between
heat transfer and nanotechnology (material), and finished his
master ’ s at the University of Malaya in 2011. At present, he is con-
ducting research on developing nanofluids, augmentation of heat
transfer and renewable energy.
Mr. Oon Cheen Sean graduated from the University Malaysia Perlis
with a major in mechanical engineering in 2010. He received the
Graduate Engineer award from the Board of Engineering Malaysia
in 2011. He received the Fellowship Award for further studies and
obtained his MEng in Mechanical Engineering from University of
Malaya. He has published more than six technical papers on heat
transfer research.
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