Pasteurization of milk
Is raw milk safe to consume?
• No. Raw milk is inherently dangerous. Raw milk may contain a whole host of
pathogens, including :
• Enterotoxigenic Staphylococcus aureus
• Campylobacter jejuni
• Salmonella species
• E. coli (EHEC) (ETEC)
• Listeria monocytogenes
• Mycobacterium tuberculosis
• Mycobacterium bovis
• Brucella species (abortus –cattle) (melitensis- goats)
• Coxiella burnetii
• Yersinia enterocolitica
• “Drinking raw milk or eating
raw milk products is like
playing Russian roulette with
your health.” John F. Sheehan
in FDA Consumer Sept/Oct.
2004
Hence, it is necessary to kill the pathogenic
organisms present in milk to stay healthy
History
• Pasteurization process was named after Louis Pasteur (1860s) who
discovered that heat treatment could inactivate spoilage microorganisms in
wine by applying heat at temperature below its boiling point and helped on
its preservation
• Pasteurization of milk was first done by Dr. Soxhlet of Germany in 1886
• This process was later applied to milk and remains the most important
operation in the processing of milk
• This process is a safer process and approved by FDA - US
What is pasteurization?
• Refers to the process of heating every particle of milk to certain time -
temperature combination in approved and properly operated equipment. After
pasteurization milk is immediately cooled to 5⁰C or below
• Aim and need – to render safer milk and to improve the keeping quality of milk
by destruction of almost all spoilage organism
• Objections – encourages slackening of effort for sanitary milk production
• May be used to mask low quality milk
• Reduces the nutrient value, cream line or cream volume
• Pasteurized milk will not clot with rennet
• Fails to destroy bacterial toxins in milk
Formulations of standards for pasteurization
Bacterial destruction – 100% for pathogens. Mycobacterium
tuberculosis, being considered as the most heat resistant
among pathogens, was chosen as the index organism for this
process. Any treatment which kills T.B cells also destroys all
other pathogens in milk
Cream line reduction - cream line or cream volume reduces
as the time - temperature increases
Phosphatase inactivation – pasteurization destroys
phosphatase enzyme completely.
Standards of pasteurization were such as to ensure
• Complete destruction of pathogens
• Negative phosphatase test
• Least damage to the cream line
Principles of heat exchange
• Rapid movement of film of fluids on both sides of HT surfaces
• Use of counter flow principles
• Use of metals of good conductivity
Consideration in heating milk
• Quick HT without cooked flavor
• Rate of HT depends on temp. gradient, renewal of surface films, Thermal
conductivity of HT wall, HT area etc.
• Initial temp. gradient is high but becomes smaller as milk reaches desired
pasteurization temp.
• Efficient renewal of surface films can eliminate use of metals of high thermal
conductivity
• Large HT area provides rapid HT
• Heating medium is hot water / steam
Methods of pasteurization
• In – the – bottle pasteurization
• Low Temperature and Long Time or batch pasteurization (LTLT)
• High Temperature Short Time or continuous flow pasteurization (HTST)
• Ultra Heat Treatment or Ultra High Temperature (UHT)
In-the-bottle pasteurization
• Bottles filled with raw milk and tightly sealed with special caps
• Time – temp. combination – 63-66⁰C for 30 mins
• Bottles passes through water sprays of low temperature
• Spray of water decreases temp. of both the product and the
bottle.
• Advantages – prevents possibility of post pasteurization
contamination
• Disadvantages – very slow HT, risk of bottle breakage, oversized
bottles should be used, needs spl. types of water tight caps
LTLT / Batch/holding pasteurization
• Traditional method of pasteurizing milk
• Process temperature is 62.8⁰C for 30 mins time is required
• Needs rapid cooling after heat treatment and it is indirect heating
system; heat moves through metal walls into the product for
heating and out of the product for cooling
• Milk sh. be cooled below 5 ⁰C
Types of LTLT pasteurizer
Water jacketed vat
• Made up of double walled around sides and bottom in which hot water or
steam under partial vacuum is sent for heating and cold water for cooling
• Outer wall is insulated to reduce heat loss and heat exchange takes place
through inner lining
• Temperature difference between heating water and milk sh. be kept minimum
and Milk is agitated by paddles/propellers
• While heating vat cover is kept open for escape of off flavors and closed when
holding milk
• Advantage – Flexible in use
Water spray type
• Water in the form of spray through perforated pipes
falls over the tank holding the product.
• The product is agitated by using paddles
• Rapid heat transfer occurs due to rapid movement of
film of spray water
Coil vat type
• Heating/ cooling medium is pumped using coil in a
horizontal or vertical positions, while the coil is turned
through the product
• The turning coil agitates the product but additional agitation
is also required foe effective HT.
• Disadvantage – coil are difficult to clean
High Temperature Short Time pasteurization - HTST
• First developed in UK by A.P.V.Co (1922)
• A modern method which handles large volumes of milk
• Provides continuous flow of milk and heated to 72⁰C for at least 15s and cooled to 5⁰C
Advantages
• Quicker method of heat treatment and maintains the quality of both raw and
pasteurized milk
• Less floor space required
• Lower initial cost
• More efficient use of labour
• Easily cleaned and sanitized
• Reduced milk losses
• Development of thermophiles is not a problem
• Disadvantages
• Not suitable for smaller quantity of milk
• Gasket damage occurs
• Complete drainage is not possible
• Greater accumulation of milk stone in the heating section
• Milk flow – balance tank, pump, regenerative heating,
holding, regenerative cooling and cooling by chill water or
brine and incorporation of filters and clarifiers as desired
Functions of the important parts
Float controlled balance tank (FCBT) – maintains
constant head of milk for feeding the raw milk pump
Pump
1. Centrifugal pump with a flow control device to
deliver constant output, placed after FCBT
2. Rotary positive pump – placed between the
regenerator and heater
Plates – Plate Heat Exchanger
• Used to heat milk below the boiling point
• It is a compact, simple, easily cleaned and inspected unit
• Plates are used for heating, cooling, regeneration and holding
• Plates are supported by press b/w a terminal block in each
cooling and heating section
• Heat moves from a warm to cold medium through SS plates
• Space b/w plates 3 mm by a non absorbent rubber gasket
The plates are numbered and assembled
Properly to provide uniform HT
• Ports are provided at the top and bottom of the
plates to permit heating and cooling medium without
mixing
Regeneration (Heating)
• The raw and incoming milk is partially and indirectly
heated by hot outgoing milk
• Economy of HTST process increases
• Filter – placed after regeneration unit, consists of 40
90 mesh cloth, usually cylindrical in shape, two filters
are used to provide continuous operation
• Holding – heated milk is hold for a specific time-
temp. combination
• Flow diversion valve - Routes the milk after heat
treatment Properly pasteurized milk flows forward
through the unit
• Unpasteurized milk is automatically diverted to FCBT
for reprocessing
• Regeneration cooling – Pasteurized hot outgoing milk
is partially and indirectly cooled by the incoming milk
• Adds economy of the process
• Control panel – Contains instruments and control
mechanisms
• Hot water set – circulates hot water through the
heating section
• Automatic control devices
• Steam pressure controller
• Water temperature controller
• Milk temp. recorder
Pressure in the system
Pressurized milk – 15 psi
Raw milk – 14 psi
Heating/cooling medium – 12 -13 psi
Holding time set – the flow time of the fastest
particle of milk at a prescribed temp. through the
holding section
UHT pasteurization
• It is a continuous process, and the product is packaged after sterilization into a
sterile container
• Typical time temperature combination for UHT milk is 1-3s and 130 -150 ⁰C
and milk is cooled to 7 ⁰C .
• Shelf life of the milk is 6-9 month without refrigeration
• It is also used for soy milk, cream, fruit juices,
yoghurt, wine, soups and honey
• Better product quality
• Shelf life of the milk is 2 – 3 weeks
• Nutrient and quality of the milk is preserved
• Plate type pasteurizer is used for this purpose
• Other products – juice and egg products (low viscous liquids)
The process results in increased retention of:
• nutrients - due to short time;
• colour - as there is no non-enzymic browning;
• flavour - as there is no non-enzymic browning;
• texture - since there is no denaturing of proteins.
UHT treatment was developed to kill or inactivate all micro-organisms
without causing as much damage to the product as sterilisation. Milk
may taste cooked and will be slightly brown in colour.
Scraped heat exchanger for pasteurisation
• Products which are semi-solid, or contain lumps or
particles over 12mm in size may be pasteurised by
scraped-surface heat exchangers, microwave or
direct steam injection.
• Used for highly viscous fluids
Other types of pasteurization
• Electric pasteurization
• Vacuum pasteurization – vacreation, Vacuum 37-50
mmHg and 71 - 82°C
• Stassanization - 74°C for 7s in tubular HE
• Flash pasteurization – same as HTST
• Uperization – same as UHT , 150°C for a fraction of
seconds
Factors affecting pasteurisation
Variables which affect the time and temperature at
which the pasteurisation process is carried out
include:
• food type;
• viscosity of the product;
• pH of the product;
• particle size;
• equipment used;
• method used.
