Heat sterilization is a unit operation used to destroy microbes and enzymes in food through heating at high temperatures for an extended period. There are two main methods - in-container sterilization and UHT processes. In-container sterilization involves heating food inside sealed containers like cans to achieve shelf stability at room temperature for over 6 months. The time required for sterilization depends on factors like the physical state and size of the food, container size, food pH, and heat resistance of microbes. UHT processes heat foods to even higher temperatures (132°C) for shorter times before aseptically filling into sterile containers to obtain shelf-stable products without refrigeration.

1. Y.Bavaneethan.
Lecturer
Department of Food Technology
SLGTI. Sri Lanka.11/27/2017 Y.BAVANEETHAN 1

2. Heat sterilisation
 Unit operation; foods are heated sufficiently at high
temperature & for long time to destroy microbial & enzyme
activity.
• a shelf life > 6 month at ambient temperatures.
 Severe heat treatment during the older process of in- container
sterilisation (canning);
• produce substantial changes in nutritional and sensory qualities
 More recent developments, introduced advanced technology
• Ohmic heating is to reduce the damage of nutrients &
sensory components.
 Two main procedure methods:
• Part 1: in-container heat sterilisation
• Part 2: UHT processes.11/27/2017 Y.BAVANEETHAN 2

3. In-container sterilization
physical state of
food
container
size
pH of the food
rate of heat penetration
into the food.
heat resistance of
micro-organisms or enzymes
How long to
sterilise a food?
The length of time required to sterilise a food. That is
influenced by:
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4. Heat resistance of micro-organisms
 The heat resistant, spore forming micro-organism –
Clostridium botulinum
•most dangerous pathogen.
•Under anaerobic conditions inside a sealed container
•This produce a powerful exotoxin- “botulin”.
Minimum requirement of heat sterilisation.
1. In low-acid foods(pH > 4.5)
• Destruction of C. botulinum is a minimum requirement of heat
sterilisation.
• Normally, Destruction of C. botulinum in low acid foods ensure,
• Destroyed all other heat resistant spoilage micro organism.
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5. 2. In acidic foods (pH 4.5–3.7),
– yeasts and fungi or heat-resistant enzymes are
used to establish Minimum requirement of
processing times and temperatures for
sterilization.
3. In more acidic foods (pH< 3.7),
– Destruction of enzyme inactivation is the
Minimum requirement of processing time and
temperature. (sometimes referred to as
pasteurisation).
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6. 11/27/2017 Y.BAVANEETHAN 6

7. Thermal Destruction of Microorganisms
 Heat is lethal to microorganisms, but each species has
own particular heat tolerance.
 The process is dependent both on the temperature of
exposure and the time required at this temperature to
destruction.
 Thermal destruction of micro-organisms simply
explained by D value.
 D values differ for different microbial species
– higher D value indicates greater heat resistance.
– 12 D value - Clostridium botulinum
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8. Decimal reduction time (D-value)
 Definition of D-value, which denotes the decimal
reduction time, this is the time required at a specific
temperature and under specified conditions to reduce
a microbial population by one decimal.
 The decimal reduction time is dependent on;
– the temperature
– the type of microorganism
– the composition of the medium containing the
microorganism.
 For example, a D value at 72°C of 1 minute means;
– that for each minute of processing at 72°C, the bacteria
population of the target microorganism will be reduced by 90%.
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9. • Here D value is 14 minutes (40-26) and would be representative of a process at 120°C.
[Specific temperature (120OC) and under specified conditions]
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10. The Z-value.
• The Z-value is the increase in temperature required to reduce
the decimal reduction time by one decimal. It is a measure of
the change in death rate with a change in temperature.
30/3
=10
(one decimal)
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11. The 12-D Process
 Canned foods are susceptible to the spores of the
organism Clostridium botulinum.
– causes botulism.
 These bacterial spores can survive many heat
treatment processes.
 Modern food production, canned foods are subjected
to a time/temperature process,
– That reduce the survival of most heat-resistant C.
botulinum spores
– by 12 logs or 12-D at 250℉ (121.1 OC)
 12 D values means 12 log cycle reduction of micro
organism. (10-12 reduction)
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12. • For example if there were 10,000 spores of a
species of spore in a can of food and a 12 D process
was given, the initial 10,000 spores (10 4 spores)
would be reduced to a theoretical 10-8 living spores
per can. ( 104 /1012 = 10-8)
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14. D-value is 1 minute at 121°C
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15. Example of how to use D and Z values in
Heat treatment Calculations
Raw milk at the processing plant has bacterial
population of 4x105/mL. It is to be processed
at 72°C for 210 seconds. The average D value
at 65°C for the mixed population is 7 min. The
Z value is 7°C. How many organisms will be
left after pasteurization?
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16. Types of Microorganisms Important in
Retorted Foods
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17. Rate of heat penetration
 The following factors are influences on the rate of heat
penetration:
1. Type of product:
– Liquid foods(peas in brine) convection heat transfer faster
than solid foods (by conduction).
2. Size of the container:
3. Agitation of the container:“control the cold point effect”
– Increases the rate of heat penetration in viscous or semi-
solid foods.
4. Temperature of the retort:
– higher temperature difference between food and heating
medium causes faster heat penetration
5. Type of container:
– Heat penetration is faster through metal than glass.11/27/2017 Y.BAVANEETHAN 17

18. 11/27/2017 Y.BAVANEETHAN 18

19. Why is this important?????
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20. Sterilization Process and Equipment
• The sterilization process in the canned product can be
subdivided into three phases.
– Phase 1 = (heating phase) the product temperature
is increased from ambient to the required
sterilization temperature.
– Phase 2 = (holding phase) This temperature is
maintained for a defined time.
– Phase 3 = (cooling phase) the temperature in the
can is decreased by introduction of cold water.
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21. Batch or Continuous type Autoclaves or
Retorts
 Batch retorts
• Vertical or horizontal
• Horizontal:
easier to load & unload and have facilities for
agitating ,but require more floor space.
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22. Continuous retorts
• Permit close control over processing conditions
• Produce more uniform products.
• Produce gradual changes in pressure inside cans
• less strain on can seams compared with batch equipment.
 Main disadvantages
• High in-process stock lost if a breakdown occurred
• Problems with metal corrosion & contamination by
thermophilic bacteria – due to inadequate preventative
measures.
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23. Continuous Hydrostatic Sterilizer
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24. Ultra high-temperature (UHT)/Aseptic
processes
Higher processing temperatures at shorter time are
possible. (132OC at 2 sec)
– into pre-sterilised containers in a sterile
atmosphere.
Liquid foods: milk, fruit juices and concentrates,
cream, yoghurt, wine, salad dressing, egg & ice cream
mix.
Foods with small discrete particles: cottage cheese, baby
foods, tomato products, fruit & vegetables, soups & rice
desserts.
High quality of UHT foods - Specially Chilled & frozen
foods
Shelf life at least 6 month without refrigeration.11/27/2017 Y.BAVANEETHAN 24

25. UHT Processing Characteristics
 Operation above 132ºC
 Heat transfer exposure for relatively small volume of
product to a large surface area product.
 Maintenance of turbulence in the product when, passes
over the heating surface
 Use of pumps to give constant delivery of product in
the heat exchanger
 Constant cleaning of heating surfaces to maintain high
rates of heat transfer
– reduce burning-on of the product.
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26.  UHT Equipment is classified according to the method
of heating:
Direct systems - steam injection
 Indirect systems - plate heat exchangers, tubular
heat exchangers
 Other systems - microwave, dielectric, ohmic
heating
• Main limitations of UHT
– High processing cost and complexity of the plant,
– Necessary to sterilize packaging materials and
associated pipe work and tanks.
– Need maintenance of sterile air and surfaces in
filling machines.
– The higher skill levels required to operation.
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27. Autoclave HeatStetilization
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28. Types of packaging for thermally processed
foods
• Retort(able) pouches
• Tetrapak
• Glass jars or bottles
• Plastic
• Cans (2-3 piece)
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29. Retortable pouch
3 layers ( laminate)
A. mylar or polypropylene
=> outside surface
B. aluminum –
=> barrier to gases and water vapor;
Hermatic container
C. polypropylene-
=>in contact with the food;
=>inert (prevent contact of food with aluminum)
=>heat resistant surface
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30. Heat Sterilization Effect on Food
• Purpose of heat sterilization:
– extend the shelf life of foods at ambient
temperatures
– minimizing the changes in nutritional value and
sensorial characteristics
• But compare with pasteurization, higher nutritional
and sensorial losses in heat commercial sterilization.
1. Colour
 Time–temperature combinations of sterilization effect
the pigments in foods.
– meats the red oxymyoglobin pigment is converted
to brown metmyoglobin.
– purplish myoglobin is converted to red–brown
myohaemichromogen11/27/2017 Y.BAVANEETHAN 30

31.  Sodium nitrite and sodium nitrate are added to meat
– reduce the risk of C. botulinum.
– Maintain red–pink coloration is due to nitric oxide
myoglobin and metmyoglobin nitrite.
 In fruits and vegetables Canning process,
– chlorophyll is converted to pheophytin.
– iron or tin react with anthocyanins to form a purple
pigment, AND
– colourless leucoanthocyanins form pink anthocyanin
complexes.
 Sterilized milk slight colour changes due to
caramelization and maillard browning.
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32. 2. Flavor and aroma
 In canned meats, Interactions between following
components produce more than 600 flavor.
– pyrolysis
– decarboxylation of amino acids
– Maillard reactions
– caramelisation of carbohydrates to furfural
– oxidation and decarboxylation of lipids.
 In fruits and vegetables form complex reactions;
– degradation, recombination and volatilization of
aldehydes, ketones, sugars, amino acids and organic acids.
 In milk - development of a cooked flavor.
– due to denaturation of whey proteins to form hydrogen
sulphide.11/27/2017 Y.BAVANEETHAN 32

33. 3. Texture or viscosity
 In canned meats, changes in texture;
– caused by coagulation and loss of water holding
capacity of proteins.
• which produces shrinkage and stiffening of
muscle tissues.
– Softening is caused by hydrolysis of collagen
– melting and dispersion of fats through the product.
 Polyphosphates added to canned meat,
– to bind water
– increases the tenderness of the product
– reduces shrinkage.
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34.  In fruits and vegetables, softening is caused by;
– hydrolysis of pectic materials, and
– gelatinization of starches and partial solubilisation of
hemicelluloses
 Calcium salts added to blancher water of fruits and
vegetable.
– to form insoluble calcium pectate that Increase the
firmness of the canned product
 Small changes in the viscosity of milk due to;
– modification of K-casein and,
– Increase the sensitivity to calcium precipitation and
coagulation.
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35. 4. Nutritional value
 Proteins are coagulated in canned meats and losses of
amino acids are 10–20%.
– Reductions in lysine and tryptophan
– reduces the biological value of the proteins by 6–9%.
– loss of thiamin (50–75%) and pantothenic acid (20–
35%).
 In canned fruits and vegetables,
– significant loss of water soluble vitamins, particularly
ascorbic acid.
 Sterilized soya–meat products may show increase
in nutritional value, due to decrease the stability
of the trypsin inhibitor in soy beans.
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36. Thank you
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