MILK PROCESSING

1. Principles of technological
processes in dairy processing

2. • Heat treatment
• Centrifugation
• Homogenisation
• Standardisation
• Deaeration
Basic treatments of liquid milk

3. Heat treatments
Purposes
• Food safety
• Inactivation of enzymes
• Prolonging of shelf life
• Changes of milk compounds (some cases)

4. Heat treatments
Food safety
• Milk as a source of infection (before heat
treatment)
• Perfect growth medium for microorganisms
• Spread of tuberculosis and typhus by milk
(previously)

5. Heat treatments
Food safety
• Pasteurisation
– Louis Pasteur – middle of 19th century
– Lethal effect of heat on microorganisms
– Use of heat treatment as a preservative
technique

6. Heat treatments
Pasteurisation of milk
• special type of heat treatment
• definition: “any heat treatment of milk
which secures the certain destruction of
Tubercle bacillus (Mycobacterium
tuberculosis) without markedly affecting
the physical and chemical properties of the
milk”

7. Heat treatments
Pasteurisation of milk
• None of the major pathogens in milk form
spores → fortune
• All common pathogenic organisms likely to
occur in milk: killed by relatively mild heat
treatment
– only a very slight effect on the physical and
chemical properties of milk

8. Heat treatments
Pasteurisation of milk
• Complete safety assured by heating milk
to 63 °C for 30 minutes
• Mycobacterium tuberculosis regarded as
the index organism for pasteurisation
– any heat treatment which destroys
Mycobacterium tuberculosis ensures
devastation of all other pathogens in milk

9. Heat treatments
Pasteurisation of milk
• Content of other substances and
microorganisms which may
– spoil the taste
– shorten the shelf life of various dairy products

10. Heat treatments
Secondary purpose of heat treatment
• destruction of as many as possible of these
other organisms and enzymatic systems
• requirement of more intense heat treatment
than is needed to inactivate the pathogens
• heat treatment applied as quickly as
possible after arrival at the dairy plant

11. Heat treatments
Time/temperature combination
• Very important
• Determination of the heat treatment
intensity

12. Heat treatment
Time/temperature combination
• Heat treatment: risk of adverse effects on
– appearance
– taste
– nutritional value of milk

13. Heat treatments
Phosphatase enzyme
• always present in raw milk
• destroyed by the temperature/time
combination necessary for efficient
pasteurization
• presence or absence is easily confirmed
(phosphatase test)
• absence of phosphatase → milk has been
adequately heated

14. Lethal effect curves and time/temperature
curves for destruction of some enzymes and
microorganims

15. Heat treatments
The main categories of heat treatment in the
dairy industry

16. Heat treatments
Indirect heating
• Transfer of heat from heating medium to
cold product on the other side of partition

17. Heat treatments
Direct heating
• heating medium (water) mixed with product
• heat transfer efficient for rapid heating
• use: sterilisation of milk by direct method
(steam injection or infusion of milk into
steam)
– long life milk production (UHT)
– removal of excess water from the product after
heat treatment

18. Heat treatments
The heat exchanger
• transfer heat by the indirect method
• heat transfer through the partition
• partition often corrugated to create more
turbulent flow → better heat transfer

19. Heat treatments
Plate heat exchanger
• pack of stainless
steel plates
clamped in a frame
• different stages of
treatment
– pre-heating, final
heating, cooling

20. Heat treatments

21. Heat treatments
• Heat exchanger in dairy industry

22. Centrifugation
• 4 % of fat in milk
• Lower density of fat globules compared to
water
• After 20–30 minutes from milking (milk
collection) → layer of cream on the surface
• Not proper method of fat collection
• Solution: centrifugation

23. Centrifugation
Centrifugal separator
• The milk enters the
disc stack through the
distribution holes
• Under the influence of
centrifugal force, the fat
globules begin to settle
in the separation
channels
– according to their
density

24. Centrifugation
Two products
• Cream with more than 35 % of fat
• Skim milk with less than 0.05 % of fat
• Also separation of sediment (unwanted
solid particles) during centrifugation

25. Standardisation of fat

26. Bactofugation
• Specially designed centrifuge (bactofuge)
used to separate microorganisms from
milk
• Improving the quality of milk
– especially when using heat-treatment of lower
temperatures
– e.g. during cheese production

27. Bactofugation
• Bacteria (esp. heat-
resistant spores):
significantly higher
density than milk
• bactofuge: useful
complement to
thermisation,
pasteurisation and
sterilisation

28. Homogenisation
• during 30 minutes after milking rise of fat
globules to the surface
• different fat content in the whole volume of
milk

29. Homogenisation
• Stokes´ Law: rising velocity of a particles
given by formula

30. Homogenisation
• Primarily: disruption of fat
globules into much smaller
ones
• Subsequently: smaller fat
globules in the whole
volume of milk during
storage

31. Homogenisation
• Disintegration of the original fat globules
achieved by a combination of contributing
factors such as turbulence and velocity
• Reduction of fat globule size from 3.5 μm
in diameter to below 1 μm
• Four- to six-fold increase in the fat/plasma
interfacial surface area

32. Homogenisation
• Homogenisation temperatures normally
applied: 55–80 °C
• homogenisation pressure: between 10 and
25 MPa (100–250 bar), depending on the
product

33. Homogenisation
Homogeniser
• milk forced though a narrow gap where the fat
globules are split

34. Homogenisation
• Energy demanding process: less than 1 %
of the energy is utilised for homogenisation
– pressure energy is converted into kinetic
energy
– part of energy is converted back to pressure
again after the device
– the other part is released as heat

35. Homogenisation
• Total homogenisation
– UHT milk and milk for production of fermented
milk product (e.g. yoghurt)
• Partial homogenisation
– only cream is homogenised and main body of
skim milk is not homogenised
– pasteurised (drinking) milk

36. Deaeration
• Milk contains from 6 to 9 % of gas
• Mainly CO2 and further O2 and N2
• Oxygen causes undesirable oxidation of
fat

37. Deaeration
Vacuum deaeration in the deaerator
• Pre-heated milk is fed to an expansion
vessel with vacuum
• Boilling point of milk is about 7 °C below
the pre-heated milk
• Water is vapoured and takes down gas
• Gas is taken away but water is condensed
and returns in the milk

38. Deaeration
• Flow of milk and air in
the vacuum deaerator
with built-in condeser

39. Pasteurised milk products

40. Pasteurised milk products
• Shelf-life of pasteurised milk is from 8 to 10
days at 5–7 °C in unopened package
• ESL milk – extend shelf life milk
– typical temperature/time program is 125–130 °C
for 2–4 seconds
– ultrapasteration
– shelf-life of products can be increased to 40
days

41. Pasteurised milk products
Packaging
• Enable efficient food distribution
• Maintain product hygiene
• Protect nutrients and flavour
• Reduce food spoilage and waste
• Increase food availability
• Convey product information

42. Long-life milk
In-container sterilisation
• Batch processing in autoclave
• Continuous processing systems such as
– Vertical hydrostatic towers
– Horizontal sterilisers

43. Long-life milk
UHT treatment
• Milk usually treated at 135–150 °C for a
few seconds
• Shelf-life of product is from months to
years

44. Long-life milk
UHT treatment
• Direct UHT system
– heating medium (stream) is condensed to milk
– need to be removed from milk by evaporation
in expansive chamber (vacuum, boiling point
of milk about 76 °C)
– homogenisation of milk after heat treatment –
aseptical homogenisators, increase cost

45. Long-life milk
UHT treatment
• Indirect UHT system
– heating medium is not condensed in milk, only
heat exchange through plate
– homogenisation before heat treatment

46. Thank you for your attention