INDIAN INSTITUTE OF TECHNOLOGY KHARAGPUR

AGRICULTURAL AND FOOD ENGINEERING DEPARTMENT

SEMINAR REPORT ON

FOULING OF HEAT EXCHANGERS IN DAIRY INDUSTRY.

Name:Vipul Chourasia

Roll no.: 20AG63R11

Date: 11/10/2020
Introduction
The dairy industry has been confronted with "fouling" of metal surfaces since plate
heat exchangers (PHEs) were introduced for pasteurizing and sterilizing milk in the
1930s. For most of the heat exchangers used, cleaning of the equipment at least
once per day is common practice. Recently, the fouling at relatively low
temperatures of other materials, such as rubber teat cups in milking machines and
membranes in separation processes, also have been found to limit processing time
and (intermediate) cleaning has become necessary as well. The costs of cleaning
are very high. In France alone, the total cost of fouling in the dairy industry in
1991 was estimated at 1000 million French francs. The fact that, during cleaning, a
process has to be temporarily halted makes many dairy operations rather
cumbersome. A large number of investigations to better understand the process of
fouling have already been performed with success by many reseachers, but a real
breakthrough--the complete control of fouling--has not been reached. This is
mainly due to the complexity of the dairy systems under consideration and a lack
of understanding of the mechanism of fouling.

Fouling related costs are: additional energy, lost productivity, additional

equipment, manpower, chemicals, environmental impact. Generally milk fouling is
so rapid that heat exchangers need to be cleaned every day to maintain production
capability and efficiency and meet strict hygiene standards. In comparison, the heat
exchangers in other major processing plants like petroleum, petrochemical etc need
to be cleaned only once or twice a year. According to Georgiadis, in the dairy
industry the cost due to the interruption in production can be dominant compared
with the cost due to reduction in performance efficiency. Along with the cost,
quality issues are equally important and in fact many times a shut down is required
due to the concerns of product quality/contamination instead of performance of the
heat exchanger.
MECHANISMS OF MILK FOULING

The fouling of heat exchangers may be defined as the accumulation of unwanted

deposits on heat transfer surfaces. Also, the deposit is usually hydrodynamically
rough so that there is an increased resistance to the flow of the fluid across the
deposit surface.The undesirable material maybe crystals, sediments, polymers,
cooking products, inorganic salts, biological growth corrosion products etc. In
dairy processing equipments the fouling (i.e., the formation of macroscopic layers
of foulants) is mainly caused by particle formation in the bulk of the liquid being
processed. These particles include both whey protein aggregates and calcium
phosphate particles.

Milk is a complicated biological fluid and contains a number of species. Its

average composition in weight % is: water – 87.5, total solids - 13.0, fat – 3.9,
lactose – 4.8, proteins – 3.4 (casein – 2.6, β-lactoglobulin (β-Lg) – 0.32, α-
lactalbumin (α-La) – 0.12), minerals – 0.8, and small quantities of other
miscellaneous species. Thermal response of these constituents generally differs
from each other. β-Lg has high heat sensitivity and figures prominently in the
fouling process. Milk fouling can be classified into two categories:

i) Type A (protein) fouling takes place for temperatures between 75oC and
110oC. The deposits are white, soft, and spongy (milk film) and their
composition is 50-70% proteins (mainly β-Lg), 30-40% minerals, and 4-
8% fat,
ii) Type B (mineral) fouling takes place at temperatures above 110oC. The
deposits are hard, compact, granular in structure, and grey in colour (milk
stone) and their composition is 70-80% minerals (mainly calcium
phosphate), 15-20% proteins, and 4-8% fat.

Whey proteins constitute just around 5% of the milk solids but they account for
more than 50% of the fouling deposits in type A fouling. β-Lg and α-La are the
two major whey proteins in the milk. Both are of globular nature and heat
sensitive, however, β-Lg is the dominant protein in heat induced fouling. The
caseins are resistant to thermal processing but do precipitate upon acidification.
Although the exact mechanisms and reactions between different milk components
are not yet fully understood, a relationship between the denaturation of native β-Lg
and fouling of heat exchangers has been established. Upon heating of milk, the
native proteins (β-Lg) first denature (unfold) and expose the core containing
reactive sulphydryl groups. The denatured protein molecules then react with the
similar or other types of protein molecules like casein, α-lactalbumin (α-La) and
form aggregates. The rate of fouling may be different for the denatured and
aggregated proteins. Being larger in size, the transport of the aggregated proteins
from the bulk to the heat transfer surfaces may be more difficult compared with the
denatured proteins, fouling occurs when the aggregation takes place next to the
heated surfaces. Delplace experimentally observed that only 3.6% of denatured β-
Lg was involved in deposit formation. Lalande found the figure to be around 5%.
However, it is not clear whether fouling was primarily caused by the aggregated
proteins or the denatured proteins deposited first on the heat transfer surfaces and
the aggregation took place subsequently.

The native proteins may adsorb on the heat transfer surface at low temperatures i.e.
below 70oC with coverage of less than 5 mg/m2 but this does not result in any
fouling. According to Fryer and Belmar-Beiny, protein denaturation in heat
exchangers starts only at temperatures above 70-74oC. They also reported that the
first deposit layer (usually < 5μm) is largely mineral. According to Visser and
Jeurnink mainly the proteins form the first layer. Analysis of deposits after fouling
for an extended period usually shows that the deposits near the surface contain a
higher proportion of minerals. This is caused by the diffusion of minerals through
the deposits to the surface rather than minerals forming firstly on the surface.
Belmar-Beiny and Fryer analysed deposits with contact heating times down to 4 s
and found that the first layer was made of proteinaceous material. Fouling in a heat
exchanger depends on bulk and surface processes. The deposition is a result of a
number of stages:

i) denaturation and aggregation of proteins in the bulk

ii) transport of the aggregated proteins to the surface

iii) surface reactions resulting in incorporation of protein into the deposit layer and

iv) possible re-entrainment or removal of deposits.

The step controlling the overall fouling hence may either be related to
physical/chemical changes in the proteins or the mass transfer of the proteins
between the fluid and the heat transfer surface. In some cases, it may be a
combination of both. Schreier and Fryer proposed that fouling was dependent on
the bulk and surface reactions and not on the mass transfer. It was also proposed
that the fouling rate was independent of the concentration of foulant in the liquid.

The Major Whey Proteins

The two major proteins in the whey fraction of milk are β-LG and a-lactalbumin
(α-LA). Both proteins are of a globular nature and sensitive to heat. β-
Lactoglobulin is the dominant protein, and so the emphasis in fouling studies is on
the behavior of this protein because this protein is also more heat sensitive than is
α-LA. In this review, we will therefore limit ourselves to the properties and the
behavior of β-LG. There are two major genetic variants of β-LG in western cattle,
variant A and variant B, with a slight difference in amino acid composition and in
thermal behavior. β- LG-B, the dominant variant used in model studies, contains
162 amino acid residues and has a molecular weight of 18.277.

pH Effects on β-LG

Native β-LG in aqueous solution undergoes a series of associations depending on

the pH.

At pH < 2.0 and room temperature, β-LG is monomeric owing to strong

electrostatic repulsive forces. Upon heating in aqueous solution, initially compact
irregular particles, 0.2 µm in diameter, are formed. These particles then fuse
together into aggregates 0.5-1 µm in size, linked together into chains less than 0.3
µm. Fouling at this pH will be limited owing to the strong electrostatic repulsion
between the molecules themselves and with regard to stainless steel, which has an
isoelectric pH of about 4.0.

At pH = 4.65 β-LG, close to its isoelectric point (pH = 5.13), is octameric at room
temperature. Upon heating, the protein starts to aggregate and ultimately
precipitates. Fouling will be severe at this pH because no electrostatic barrier
against deposition is present.

In the pH range of 5.5-6.5, β-LG dissolved in distilled water is present as a dimer.

The dimensions of the doplet are: 17.9 A (1.79 nm) for the radius of the two
composing monomers for the length of the particle. Upon heating, this dimer
dissociates into a monomer at 323 K (50°C).

At temperatures above 333 K (60°C), the buried SH-group is exposed to the

solution by the unfolding of the protein and becomes reactive. When heating is
prolonged, aggregation and particle growth sets in. These particles are linked
together into a chainlike fibrous structure when acid precipitated or at neutral pH
into a turbid gel when the concentration is sufficiently high. The formation of these
particles is of direct relevance for the fouling behavior of whey protein-containing
systems, owing to their accumulation at the heating surface.
 Fig. The structure of β-LG at different pHs.

Above pH = 6.5, only limited coagulation occurs when β-LG is heat denatured
(e.g., after 10 min at 80°C); at neutral pH and low protein concentrations, the
solution even remains crystal clear. The change in thermal behavior at pH = 6.5, is
due to an increased thiol activity.

At pH = 7.5, the N-R transition (leads to the monomeric form upon raising the pH
to 8.0, and aggregation upon heating is inhibited. Because most dairy fluids are
processed in the pH range between 6.0 and 7.0, knowledge of the temperature
effect on the properties of β-LG in this range is of direct relevance for
understanding the various processes taking place upon heating these fluids. In
particular, this is the case for the aggregation and gelation behavior of this protein
because of the accumulation of protein aggregates formed in the bulk solution at
the heating surface whereby the critical (bulk) gelation concentration may be
reached.
At high pHs (8 and above), when the formation of large aggregates is prohibitied,
fouling of whey proteins accordingly will be substantially diminished. The same
could be achieved by an alkaline shock (10 min at pH = 11), which inhibits the
formation of large aggregates and enhances denaturation through irreversible
exposure of two carboxyl groups.

Factors Affecting Milk Fouling

Milk fouling may be unavoidable practically and continuous efforts are required to
minimise it. Fouling in a heat exchanger depends on various parameters like heat
transfer method, hydraulic and thermal conditions, heat transfer surface
characteristics, type and quality of milk along with its processing history etc. The
factors affecting milk fouling in heat exchangers can be broadly classified into five
major categories:

− Milk composition

− Operating conditions in heat exchangers

− Type and characteristics of heat exchangers

− Presence of micro-organisms

− Location of fouling

Milk composition

The composition of milk depends on its source and hence may not be possible to
change. A seasonal variation in milk fouling depends on differences in its
composition. Increasing the protein concentration results in higher fouling . The
heat stability of milk proteins decreases with a reduction in pH. The calcium ions
present in the milk decrease the denaturation temperature of β-Lg, promote
aggregation by attaching to β-Lg, and enhance the deposition by forming bridges
between the proteins adsorbed on the heat transfer surface and aggregates formed
in the bulk. It has also been reported that increasing or decreasing the calcium
content of milk compared with normal milk lowers the heat stability and caused
more fouling. The fat present in milk has little effect on fouling. Additives may
reduce fouling by enhancing heat stability of milk but they may not be permitted in
many countries. Holding milk for 24 hours before processing results in less fouling
although further ageing increases fouling. Prolonged storage of milk for a few days
at 5oC may enhance fouling due to the action of proteolytic enzymes.

Operating conditions in heat exchangers

Important operating parameters that can be varied in a heat exchanger are: air
content, velocity/turbulence, and temperature. The presence of air in milk enhances
fouling. However, fouling is enhanced only when air bubbles are formed on heat
transfer surfaces, which then act as nuclei for deposit formation. The solubility of
air in milk decreases with heating as well as a reduction in the pressure. Also the
formation of air bubbles is enhanced by mechanical forces induced by valves,
expansion vessels, free-falling streams etc. Fouling decreased with increasing
turbulence. According to Paterson and Fryer the thickness and subsequently the
volume of laminar sublayer decrease with increasing velocity and as a result the
amount of foulant depositing on the heat transfer surface decreases. Higher flow
velocities also promote deposit re-entrainment through increased fluid shear
stresses. Higher turbulence and different flow characteristics are in fact found to
result in smaller induction period in plate heat exchangers compared with tubular
heat exchangers. The reason for this may be the presence of low velocity zones
near the contact points between the adjacent plates. The use of pulsatile flow was
found to mitigate fouling when only the wall region near the heat transfer surface
was hot enough to cause the protein denaturation and aggregation reactions. The
reason was that the fluid spent less time near the wall due to higher mixing. The
pulsations however, enhanced fouling when the bulk fluid was also hot enough for
the protein reactions to take place. Temperature of milk in a heat exchanger is
probably the single most important factor controlling fouling. Increasing the
temperature results in higher fouling. Beyond 110oC, the nature of fouling will
change from type A to type B. It is worth mentioning that both the absolute
temperature and temperature difference are important for fouling.
Type and characteristics of heat exchangers:

Plate heat exchangers are commonly used in dairy industry as they offer
advantages of superior heat transfer performance, lower temperature gradient, ease
of maintenance etc. However, plate heat exchangers are prone to fouling due to
their narrow flow channels. Also milk fouling in a heat exchanger is difficult to
completely eliminate, as temperature of the heat transfer surface needs to be
considerably higher than the bulk temperature.

The heat transfer surface to which the deposits stick also affects fouling. Stainless
steel is the standard material used in the dairy industry. Factors that may affect
fouling of a stainless steel surface are: presence of a chromium oxide or passive
layer, surface energy, surface microstructure (roughness), presence of active sites
and type of stainless steel used.

Presence of Micro-organisms:

The formation of deposits promotes the adhesion of micro-organisms to the surface

resulting in bio-fouling. Furthermore, the deposits provide nutrients to
microorganisms ensuring their growth. Also most of the processes in industry are
carried out at temperatures below 100°C. For example, pasteurisation is generally
achieved by heating of milk at 72°C for 15 seconds in a continuous flow system.
At this temperature only the pathogenic bacteria along with some vegetative cells
are killed. A higher temperature of 85°C is required to kill the remaining
vegetative cells. Spores are much more heat-resistant and remain active well
beyond this temperature. However, their inactivation is important for the products
with longer shelf life.

Bio-fouling, either micro-organisms deposition or biofilm formation, in a heat

exchanger raises serious quality concerns. Bio-fouling takes place through two
different mechanisms:

 Deposition of micro-organisms directly on the heat transfer surfaces of the

heat exchanger, and
 Deposition/attachment/entrapment of micro-organisms on/in the deposit
layer forming on the heat transfer surfaces.
With the supply of nutrients by the deposits, bacteria multiply. The presence of
micro-organisms in the process stream and/or deposit layer does not only affect the
product quality, it influences the fouling process as well.

Location of fouling:

Protein denaturation and aggregation reactions take place as soon as milk is

processed thermally. The relative amounts of the denatured and aggregated
proteins depend on factors like operating conditions, type and design of heat
exchanger, properties of the heat transfer surface etc.

The use of an efficient technology may help to mitigate fouling within a heat
exchanger; however the processed milk at the exit of the heat exchanger would still
have large amounts of the denatured and aggregated proteins.

This would result in severe fouling at various locations further downstream. Hence
controlling fouling within the heat exchanger may not yield effective results and an
overall strategy may need to be developed to study the fouling process over the
entire setup.

Conclusion:

The major components in fouling are calcium phosphate ions and whey proteins.
Both components form insoluble aggregates in the bulk of the liquid as a result of
their heat sensitivity. β-LG, the major whey protein, plays a dominating role in
fouling through its heat sensitivity. Its thermal properties and behavior are strongly
dependent on pH and the presence of other components such as calcium ions.
Upon heating at neutral pH, the molecule initially unfolds and exposes its free thiol
group. This free thiol group makes the molecule reactive toward other β-LG
molecules, both in the nonreactive and in the reactive state. As a result of this
reactivity, the protein will form complexes in the form of aggregates, ultimately
leading to gelation at the heating surface, provided the protein concentration is
sufficiently high, as a result of accumulation of proteinaceous material upon
adsorption. The process of whey protein denaturation resulting in aggregation and
ultimately gelation is controlled by a range of parameters, including pH, calcium
concentration, and temperature. The determining factor in bulk fouling is found to
be the formation of activated aggregates in a limited size range in the bulk of the
liquid and the subsequent deposition. Knowledge of whey protein particle
formation therefore seems crucial in understanding and in controlling fouling.A
significant amount of research has been done on fouling; however the underlying
fouling mechanisms are not yet fully understood. A part of the problem is the
complex nature of milk and the dairy processes.

References:

Belmar-Beiny M. T. & Fryer P. J. (1993) Preliminary stages of fouling from whey

protein solutions. Journal of Dairy Research 60 467-483.

Lalande M., Tissier J.-P. & Corrieu G. (1985) Fouling of heat transfer surfaces
related to β-lactoglobulin denaturation during heat processing of milk.
Biotechnology Progress 1 (2) 131-139.

Petermeier H., Benning R., Delgado A., Kulozik U., Hinrichs J. & Becker T.
(2002) Hybrid model of the fouling process in tubular heat exchangers for the dairy
industry. Journal of Food Engineering 55 9-17

Rakes P. A., Swartzel K. R. & Jones V. A. (1986) Deposition of dairy protein-

containing fluids on heat exchanger surfaces. Biotechnology Progress 2 (4) 210-
217.