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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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