heat processing

1. PROCESSING OF MILK & MILK
PRODUCTS FST 710 - 3(2-1)

2. GENERAL ASPECTS OF PROCESSING
 Milk is a raw material in the manufacture of several food products.
 These products are predominantly made in dairy factories.
 Their mode of operation is dominated by the properties of the raw material.
 Some typical characteristics of the dairy industry are as follows:
 1. Milk is a liquid, and it is homogeneous (or it can readily be made
homogeneous). This implies that transport and storage are relatively simple
and it greatly facilitates the application of continuous processes.
 2. Milk properties vary according to source, season, and storage conditions,
and during keeping. This may imply that processes have to be adapted to the
variation in properties.

3. GENERAL ASPECTS OF PROCESSING
 3. Milk is highly perishable and the same is true of many intermediates
between raw milk and the final product. This requires strict control of hygiene
and storage conditions.
 4. Raw milk may contain pathogenic bacteria, and some of these can thrive in
milk. This also requires strict control of hygiene and the application of
stabilization processes.
 5. Generally, raw milk is delivered to the dairy throughout the year, but in
varying quantities (in some regions there is even no delivery during part of the
year). Because the milk must be processed within a few days at the most, this
generally implies that the processing capacity of a dairy cannot be fully used
during most of the year.

4. GENERAL ASPECTS OF PROCESSING
 6. Milk contains several components, and it can be separated in fractions in
various ways, e.g., in cream and skim milk, in powder and water, or in curd
and whey. Moreover, several physical transformations and fermentations can
be applied. This means that a wide variety of products can be made.
 7. Relatively small amounts of raw material (besides milk) are needed for the
manufacture of most milk products, but consumption of water and energy may
be high.
 8. One and the same unit operation can often be applied in the manufacture
of a range of products. This includes heat treatment, cooling, cream
separation, and homogenization.

5. GENERAL ASPECTS OF PROCESSING
Objectives. In the development of processes for the manufacture of food
products, several constraints have to be taken into account. These include
availability of materials, machinery, skilled staff, and specific knowledge, as well
as legal conditions. However, the objectives of the production process are of
paramount importance. The ensuing requirements can be grouped as follows:
 1. Safety of the product for the consumer. The health of the consumer can be
threatened by pathogenic bacteria (or their toxins) and by toxic or
carcinogenic substances. The first of these nearly always provides by far the
most serious hazard.

6. GENERAL ASPECTS OF PROCESSING
 2. Quality of the product. Apart from product safety, which may be considered
a quality aspect, this generally involves:
• Nutritional value.
• Eating quality: taste, odor, and mouthfeel.
• Appearance: color and texture.
• Usage properties, e.g., spreadability of butter, whippability of cream, and
dispersibility of milk powder; and, in general, ease of handling.
• Keeping quality or shelf life, i.e., the length of time a product can be kept
before it significantly decreases in quality or may have become a health
hazard.
 • Emotional values: a wide range of aspects, greatly varying among

7. GENERAL ASPECTS OF PROCESSING
The quality requirements vary widely among products, and even if they are the
same (e.g., the shelf life), different measures may be needed to meet them.
 3. Quality of the process. The process should be safe and convenient for the
staff involved as well as for other people in the vicinity. It should not cause
environmental problems, such as pollution, or excessive depletion of
exhaustible resources (e.g., energy and water).
 4. Expenses. Often, the necessity to maintain the processing costs within
limits is overriding. Concerns may include the price of raw materials (including
packaging), use of energy, equipment expenditure, and labor intensity, etc.
Also the flexibility and complexity of the process, with the ensuing probability
of making mistakes (resulting in poor quality or even the need to discard
products), may affect production costs. The same is true of the costs of
storage.

8. HEAT PROCESSING
 Heat treatment: This is generally the method of choice for liquid products. It is
active against microbes and enzymes. The method is convenient, flexible,
well-studied, and fairly inexpensive. The disadvantage is that undesirable
chemical reactions occur, especially at high heating intensity, for instance,
causing off-flavors.
 Mainly aimed at killing microorganisms and inactivating enzymes, or at
achieving some other, mainly chemical, changes.
 The results greatly depend on the intensity of the treatment, i.e., the
combination of temperature and duration of heating.
 Heat treatment may also cause undesirable changes, although desirability
may depend on the kind of product made and on its intended use.

9. HEAT PROCESSING
Objectives:
 Warranting the safety of the consumer: It specifically concerns killing of
pathogens like Mycobacterium tuberculosis, Coxiella burnetii, Staphylococcus
aureus, Salmonella species, Listeria monocytogenes, and Campylobacter
jejuni. It also concerns potentially pathogenic bacteria that may accidentally
enter the milk. A fairly moderate heat treatment kills all of these organisms.
 Increasing the keeping quality: It primarily concerns killing of spoilage
organisms and of their spores if present. Inactivation of enzymes, native to
milk or excreted by microorganisms, is also essential. Chemical deterioration
by autoxidation of lipids can be limited by intense heat treatment.

10. HEAT PROCESSING
 Establishing specific product properties: Examples are heating the milk before
evaporation to increase the coagulation stability of evaporated milk during its
sterilization inactivating bacterial inhibitors such as immunoglobulins and the
lactoperoxidase system to enhance the growth of starter bacteria, obtaining a
satisfactory consistency of yogurt coagulating serum proteins together with
casein during acidification of milk.

11. HEAT PROCESSING
Possible chemical and physical changes caused by heat treatment include:
 1. Gases, including CO2, are partly removed (if they are allowed to escape
from the heating equipment). Loss of O2 is important for the rate of oxidation
reactions during heating, and for the growth rate of some bacteria. The loss of
gases is reversible, but uptake from the air may take a long time.
 2. The amount of colloidal phosphate increases and the [Ca2+] decreases.
Again, the changes are reversible, though slowly (∼24 h).
 3. Lactose isomerizes and partly degrades to yield, for instance, lactulose and
organic acids.
 4. Phosphoric esters, those of casein in particular, are hydrolyzed.
Phospholipids are also split. Consequently, the amount of inorganic
phosphate increases.

12. HEAT PROCESSING
 5. The pH of the milk decreases, and the titratable acidity increases. All of
these changes depend somewhat on the prevailing conditions.
 6. Most of the serum proteins are denatured and thereby rendered insoluble.
 7. Part of the serum protein (especially of β-lactoglobulin) becomes covalently
bound to κ-casein and to some proteins of the fat globule membrane.
 8. Enzymes are inactivated.
 9. Reactions between protein and lactose occur, Maillard reactions in
particular. This involves loss of available lysine.
 10. Free sulfhydryl groups are formed. This causes, for instance, a decrease
of the redox potential.

13. HEAT PROCESSING
 11. Other reactions involving proteins occur.
 12. Casein micelles become aggregated. Aggregation may eventually lead to
coagulation.
 13. Several changes occur in the fat globule membrane, e.g., in its Cu
content.
 14. Acylglycerols are hydrolyzed and inter-esterified.
 15. Lactones and methyl ketones are formed from the fat.
 16. Some vitamins are degraded.

14. HEAT PROCESSING
Consequences of Heat Treatment:
 Bacterial growth rate of the organisms surviving, or added after heat
treatment, can be greatly affected, generally increased. This is mainly
because bacterial inhibitors are inactivated. Immunoglobulins are denatured
at relatively low intensity. Bacillus cereus is especially sensitive to IgM
(agglutinin). The lactoperoxidase system, especially affects lactic acid
bacteria. It is inactivated due to denaturation of lactoperoxidase. Lactoferrin,
especially affects Bacillus stearothermophilus; it needs conventional heat
sterilization to be inactivated. Bacteriophages can be inactivated, depending
on the heating intensity, and this is especially important for lactic
fermentations. Some stimulants can also be formed, e.g., formic acid, which
enhances growth of lactic acid bacteria, especially thermophilic ones; its
formation needs intense heat treatment.

15. HEAT PROCESSING
 Nutritive value decreases, at least for some nutrients.
 The flavor changes appreciably.
 Color may change. Heating milk at first makes it a little whiter, On increasing
the heating intensity, the color becomes brown.
 Viscosity may increase slightly and much more (if it happens). The latter
change especially occurs when concentrated milk is sterilized.
 Heat coagulation in evaporated milk is markedly decreased when the milk is
heated so that most of the serum protein is denatured before concentrating.
 Age gelation in sweetened condensed milk is also reduced when the milk is
intensely heated before concentrating.
 The rennetability of milk and the rate of syneresis of the rennet gel decrease.

16. HEAT PROCESSING
 Creaming tendency of the milk decreases.
 The proneness to autoxidation is affected in several ways.
 The composition of the surface layers of the fat globules formed during
homogenization or recombination is affected by the intensity of heating before
homogenization. This affects some product properties; for example, the
tendency to form homogenization clusters is increased.

17. HEAT PROCESSING
Heating Intensity:
 Thermalization: This is a heat treatment of lower intensity than low
pasteurization, usually 20 s at 60 to 69°C. The purpose is to kill bacteria,
especially psychrotrophs, as several of these produce heat-resistant lipases
and proteinases that may eventually cause deterioration of milk products.
Except for the killing of many vegetative microorganisms and the partial
inactivation of some enzymes, thermalization causes almost no irreversible
changes in the milk.

18. HEAT PROCESSING
 Low pasteurization: This is a heat treatment of such intensity that the
enzyme alkaline phosphatase of milk is inactivated. It may be realized by
heating for 30 min at 63°C or for 15 sec at 72°C. Almost all pathogens that
can be present in milk are killed; it specifically concerns Mycobacterium
tuberculosis, a relatively heat-resistant organism that formerly was among the
most dangerous pathogens. All yeasts and molds and most, but not all,
vegetative bacteria are killed. Some species of Microbacterium that grow
slowly in milk are not killed. Furthermore, some enzymes are inactivated but
by no means all of them. The flavor of milk is hardly altered, little or no serum
protein is denatured, and cold agglutination and bacteriostatic properties
remain virtually intact. A more intense heat treatment is, however, often
applied (e.g., 20 s at 75°C). This causes, for instance, denaturation of
immunoglobulins (hence, decrease in cold agglutination and bacteriostatic
activity) and sometimes a perceptible change in the flavor of milk.

19. HEAT PROCESSING
 High pasteurization: This heat treatment is such that the activity of the
enzyme lactoperoxidase is destroyed, for which 20 s at 85°C suffices.
However, higher temperatures, up to 100°C, are sometimes applied. Virtually
all vegetative microorganisms are killed but not bacterial spores. Most
enzymes are inactivated, but milk proteinase (plasmin) and some bacterial
proteinases and lipases are not or incompletely inactivated. Most of the
bacteriostatic properties of milk are destroyed. Denaturation of part of the
serum proteins occurs. A distinct cooked flavor develops; a gassy flavor
develops in cream. There are no significant changes in nutritive value, with
the exception of loss of vitamin C. The stability of the product with regard to
autoxidation of fat is increased. Except for protein denaturation, irreversible
chemical reactions occur only to a limited extent.

20. HEAT PROCESSING
 Sterilization: This heat treatment is meant to kill all microorganisms including
the bacterial spores. To that end, 30 min at 110°C (in-bottle sterilization), 30
sec at 130°C, or 1 s at 145°C usually suffices. The latter two are examples of
so-called UHT (ultra-high-temperature, short time) treatment. Some other
effects of each of these heat treatments are different. Heating for 30 min at
110°C inactivates all milk enzymes, but not all bacterial lipases and
proteinases are fully inactivated; it causes extensive Maillard reactions,
leading to browning, formation of a sterilized milk flavor, and some loss of
available lysine; it reduces the content of some vitamins; causes considerable
changes in the proteins including casein; and decreases the pH of the milk by
about 0.2 unit. Upon heating for 1 s at 145°C chemical reactions hardly occur,
most serum proteins remain unchanged, and only a weak cooked flavor
develops. It does not inactivate all enzymes, e.g., plasmin is hardly affected
and some bacterial lipases and proteinases not at all, and therefore such a

21. HEAT PROCESSING
Heat Transfer Theory:
 Temperature difference
 Heat flow from warmer to colder
 Heat flow is rapid when temperature difference is great
 Heat can be transferred in 3 ways
 Conduction: Transfer of thermal energy through solid bodies and through layers of
liquid at rest.
 Convection: Heat transfer that occurs when particles with high heat content are
mixed with cold particles and transfer their heat by conduction.
 Radiation: Emission of heat from a body which has accumulated thermal energy.

22. HEAT PROCESSING
Heat Transfer Principles:
 Direct Heating: heating medium is mixed with the product.
 To heat water: Steam is injected directly into the water and transfer heat to the water
 To heat products such as curd in manufacture of certain types of cheese (by mixing
hot water with curd) and to sterilize milk by direct method (steam injection or infusion
of milk into steam).
 It is efficient method for rapid heating.
 Steam injection heating causes disruption of fat globules and some coagulation of
protein. Homogenization (aseptic) at high pressure re-disperses the coagulum.
Without homogenization, the product gives an inhomogeneous astringent or non
smooth sensation in the mouth, and a sediment tends to form on storage.
 Mixing with heating medium makes strict demands on the quality of heating medium.
 Direct heating is forbidden in some countries on the basis that it introduces foreign
matter into the product.

23. HEAT PROCESSING
 Indirect Heating: Most common method in dairies
 Partition is placed between the heating medium and the product.
 Hot water flows on the one side of partition and cold milk on the other side.
 A heat exchanger is used to transfer heat by the indirect method.
 Countercurrent Flow: The temperature difference between two liquids is
best utilized if they flow in opposite directions.
 During the passage the product is gradually heated.
 Concurrent Flow: Both liquids enter the heat exchanger from the same end
and flow in same direction.

24. HEAT PROCESSING
 Regenerative Heating and Cooling: Also called as heat recovery.
 The heat of pasteurized milk is utilized to warm the cold milk.
 The incoming cold milk is preheated by the outgoing hot milk, which is
spontaneously precooled.
 This saves heating and cooling energy.
 In this way as much as 94-95% of the heat contents of the pasteurized milk
can be recycled.

25. HEAT PROCESSING
Different Types of Heat Exchangers:
 Plate Heat Exchanger: Consist of a pack of stainless steel plates clamped in a
frame.
 The plates are corrugated in pattern designed for optimum heat transfer.
 Because of the large heating surface per unit volume of liquid that is to be heated,
the difference between the temperature of the heating agent and the temperature of
the liquid to be heated can be small.
 Warming and cooling proceed rapidly in plate heat exchangers
 The energy consumption (for heating and cooling) can be relatively small because
heat can be regenerated.
 The plates are shaped in such a way as to greatly enhance turbulence in the liquid.
This enhances heat transfer and diminishes fouling.

26. HEAT PROCESSING
 As a disadvantage, leakage can occur, for instance, due to an imperfect or
worn-out rubber gasket between two plates. If as yet unpasteurized milk in the
regeneration section leaks into the milk that has passed the holder section,
contamination with undesirable bacteria may occur.
 There is only a very small distance between plates and this precludes the
heat treatment of highly viscous liquids because a high pressure would be
needed to overcome the high flow resistance.
 liquids containing small particles, say > 50 μm, and liquids that cause heavy
fouling also give problems.

27. UNIVERSITY OF AGRICULTURE FAISALABAD – UAF

28. HEAT PROCESSING
 Tubular Heat Exchanger: Consist of tubes instead of plates.
 Generally have a smaller heating surface per unit volume of liquid to be
heated than plate heat exchangers.
 To restrict fouling and enhance heat transfer, high flow rates are used, which
necessitates high pressures. But this causes no problems because tubes are
much stronger than plates; some tubular heat exchangers even have no
sealing gaskets but have (spirally bent) concentric tubes.
 Tubular heat exchangers can readily be applied to obtain very high
temperatures (e.g., 150°C). Accordingly, they are excellently fit for indirect
UHT treatment.
 In modern heat exchangers, the milk may be in counterflow with water
throughout the apparatus

29. UNIVERSITY OF AGRICULTURE FAISALABAD – UAF

30. HEAT PROCESSING
 Scraped-Surface Heat Exchanger: Consist of a cylinder through which the
product is pumped in countercurrent flow to the service medium in the
surrounding jacket.
 The product enters the vertical cylinder through the lower port and
continuously flow upward through the cylinder.
 The rotating blades continually remove the product from the cylinder wall.
 It is designed for heating and cooling viscous, sticky and lumpy products.

31. UNIVERSITY OF AGRICULTURE FAISALABAD – UAF

32. DIRECT HEATING TECHNOLOGIES
 Characterized by the direct contact of the treated product with the heating
medium, for example steam, whereas in indirect systems the heat is always
transferred indirectly through the wall of a heat exchanger.
 There are two different technologies of direct heating used for UHT
processing of dairy products - steam injection and steam infusion.
 In steam injection, steam is injected into the product and in steam infusion the
UHT processed product is introduced into a steam filled vessel.

33.  The purpose of UHT processing plant is to heat the product to the sterilization
temperature (in the range 135 - 150ºC), hold it there for a few seconds, and
then cool it to a suitable filling temperature.
 Direct systems use steam which is directly mixed with the product under
pressure and the vapour is later removed by use of vacuum cooling and
indirect systems where the final heating medium is hot water or steam
separated from the processed product by a heat conducting barrier made of
stainless steel, preventing their direct contact.

34.  The most important advantage of direct heating technology is very fast
heating and in this way minimizing the rate of chemical changes. It takes just
a few tenths of a second to reach the sterilizing temperature.
 This is the main reason why direct systems are used to ensure superior
product quality. However, it is very difficult to recover the energy used in direct
heating and the operation is becoming very costly. This is why direct heating
systems are always a combination of direct and indirect heating.