Slides explaining the different methods of food preservation. Informative for students studying AS or A2 Food Technology. A summary of preservative methods and short exam questions at the end.

1. FOOD TECHNOLOGY
FOOD PRESERVATION

2. WHY DO WE NEED TO PRESERVE FOOD?
 So that surplus foods from good harvests can be
stored and then used in times of shortage.
 So that we can enjoy seasonal fruits and
vegetables all year round.
 To enable it to be transported long distances
without it decaying.
 To allow it to be stored in the home for long periods
without going off.

3. CANNING
 Uses heat sterilisation to destroy micro-organisms that
cause food spoilage. The cans are heated which drives
out any air before it is sealed with a double seam to
prevent leakage or the re-entry of bacteria. Foods
preserved by this method have a long shelf life and can
be stored at ambient temperatures.
 The severity of the heat treatment can be lessened if the
food contains acid, salt or sugar. The most important pH
is 4.5, as below this the very dangerous organism
Clostridium botulinum is inhibited.
 Cans are cooled in chlorinated water to prevent any
possible contamination in cans which may have a slight
defect. This could infect the food and cause an outbreak
of food poisoning some time later.

4. BOTTLING
 Toughened glass is used because it can be taken to
high temperatures for heat treatment methods such as
bottling fruit and vegetables. It is easy to sterilise glass.
 The bottle (or jar) can be filled either with raw foods and
then heated slowly to boil the foods, destroying bacteria
and spores. Air is expelled through a specially designed
lid, and as the jar cools down, a vacuum is formed. The
sealed jar prevents the re-entry of bacteria. As there is
no oxygen present, bacteria are unable to multiply.
 Bottling can also be used for high temperature methods
such as jamming, pickling and chutney production.
Glass is non-reactive and therefore suitable for use with
acidic mixtures. Bottled food has a very long shelf life at
ambient temperatures.

5. PASTEURISATION/ UHT (ULTRA HEAT
TREATMENT)
 Food is heated to temperatures in excess of 100C to ensure
that spores are destroyed.
 In the case of milk the product is heated to not less than
132.2C and is packaged under aseptic conditions. It is similar
to pasteurised milk in terms of nutritive value and the heat is
so quick the colour and flavour changes are not so obvious as
in sterilised milk.
 The storage life is vastly longer however, for all products that
are heated there will be some loss of colour, flavour and
texture to a greater or lesser extent. Heat sensitive vitamins
e.g. thiamine, riboflavin, niacin and vitamin C will be affected.
 There’s a reduction of 10-20% of amino acid lysine in high
protein foods. Fruit juices may be fortified with additional
vitamin C.
 Packages are hermetically sealed. The packaging materials
used are robust and prevent entry of air unless opened or
pierced.

6. STERILISATION
 This process uses a temperature in excess of 100C in
order to destroy nearly all micro-organisms present in a
food. This is important as some micro-organisms can
form spores which have the ability to survive at high
temperatures.
 If the correct temperature is not reached there is the
possibility that the spores will germinate and grow and
food poisoning could result.
Some organisms can survive the sterilisation process if
not processed for enough time or a high enough
temperature, e.g. Clostridium botulinum.
 The product is packed in air-tight containers either before
or after heat treatment. If packaging follows heating, the
containers must be sterilised before use and filled under
aseptic conditions.
 Sterilising enables milk to be kept for 2-3 weeks
unopened, but results in a burnt, caramelised flavour and
browning.

7. FREEZE-DRYING
 Not to be confused with AFD (Accelerated freeze-drying)
 Rapid drying systems cause the outer edges and corners of
food to become dried out and rigid thus fix the shape of the
food pieces early in the process.
 Water is removed from the centre of the food to produce a
light honeycomb product, which readily rehydrates when
added to water (slow drying allows the product to shrink
further and produce a dense dried food, which is difficult to
rehydrate).
 As the food is porous the food is excellent for rehydration e.g.
coffee, meat, fruit and vegetables.
 The water-soluble vitamins may be lost from freeze-dried
foods. Some odours are lost as a result of this process.
 Out of all the drying processes, this is the most effective way
of maintaining the original characteristics of a product.
 This is a more costly process, but where colour, texture,
flavour, shape and reconstitutability properties are of
paramount importance (e.g. cuppa soups, snack pots, coffee),
this process gives the most organoleptic results.

8. SMOKING
 Process primarily used for meat and fish in combination with
salt to add flavour and delay spoilage.
 The degree of preservation is minimal with this process,
because high levels of salt and smoke make the food less
palatable and not so good for health reasons.
 Therefore, in order to preserve smoked products, additional
methods such as chilling and vacuum packing are used.
 Smoking is either hot or cold. In cold smoking the product
remains uncooked. The temperature doesn’t exceed 30C.
 Most products are then cooked, except salmon, which is eaten
raw. The organoleptic qualities are considered desirable.
 In hot smoking the temperature is allowed to rise so that the
food cooks. Smoked foods are often coloured yellowy as a
result of the process.
 They are partially dried and salty, so the texture and flavour
are distinctive. This is a positive organoleptic selling point for
these products.

9. SMOKING CONT...
Wood Smoking:
 It contains a number of substances that have anti-
microbal activity e.g. formaldehyde and higher
aldehydes, phenols and methanol, all of which are
highly inhibitory to micro-organisms.

10. DRYING
 Micro-organisms cannot live, grow and reproduce
without moisture.
 It is the removal of moisture by warmth or high
temperature. This is an effective method of
preservation.
 Foods can be dried using commercial methods e.g.
spray drying, freeze-drying, AFD, tunnel drying and
sunlight e.g. tomatoes and fruits or by oven drying
e.g. herbs, coffee and vegetables.
 Dried foods have a long shelf life and can be stored
at ambient temperatures.

11. VACUUM PACKING
 Foods are prepared and then packed into a pouch
or foil wrapper and the air is sucked out, forming a
vacuum inside the pack.
 Removing air takes away oxygen and moisture
which bacteria require in order to survive and
multiply.
 Some vacuum packs can be stored at ambient
temperatures e.g. coffee and have a medium shelf
life. Others stored at chill temperatures e.g. smoked
fish, cheese and cured meats. These have a short
shelf life.
 Organoleptic qualities are often well maintained
using this method.

12. MAP (MODIFIED ATMOSPHERE PACKAGING)
 Also known as ‘Controlled Atmosphere Packaging’
 Preserves food in sealed gas flushed packs
 In the packs oxygen level can be lowered or the carbon
dioxide level or nitrogen levels increased
 Carbon dioxide retards the growth of bacteria. Oxygen
helps retain the colour of the food e.g. meat stays red
and nitrogen is used to reduce the rate of oxidation. The
ratio of these gasses depends upon the food being
packaged.
 Food is prepared and placed in the container.
 Container is flushed with selected gas for the food type
and is hermetically sealed.
 MAP is often carried out in conjunction with chilling in
the case of chilled meals or prepared salads.

13. MAP CONT...
 Once the packaging is opened the food has a
normal shelf life and must be stored accordingly.
 The packaging used must not be reactive and
sufficiently strong to resist damage through piercing
or splitting.
 The most commonly used materials are plastics,
which may be ovenable, with a film cover.

14. IRRADIATION
 This method of preservation is where ionising radiations
are applied to foods to kill bacteria.
 There is said to be no risk of residual radioactivity and
the process should not be confused with radioactive
contamination or radioactivity.
 It is expensive to carry out and as yet is only permitted
in the UK for use with some vegetables, spices and
poultry products.
 It extends shelf life by delaying ripening (bananas),
inhibiting sprouting (potatoes and onions) and killing
moulds (strawberries).
 It reduces significantly the numbers of pathogenic
parasites in meat.
 No visible changes to irradiated food other than
reduction of undesirable factors above.
 At permitted doses of irradiation, the organoleptic
properties of most foods are unaffected.

15. SUMMARY OF PRESERVATION METHODS
Preservation Method Description
Irradiation Micro-organisms are killed as food
is bombarded with ionising
radiation. All irradiated food must
be labelled.
Modified Atmosphere Packaging
(MAP)
Foods are packed in a combination
of CO2, N and O2 gas instead of air
– this delays decay.
Additives Synthetic or natural substances
added to food in small quantities to
lengthen shelf life.
Chilling A short term method of preserving
food between 1 and 5C that slows
rate of decay.
Smoking Traditionally meat and fish were
hung over wood fires. This changes
colour and flavour of foods.

16. Preservation Method Description
Sugar Preservatives When fruit is boiled with sugar the
sugar stops the micro-organisms
multiplying.
Fermentation Alcohol or acids are used as
preservatives – beer, wine and soy
sauce.
Salting Coating food in salt or placing it in a
salt solution reduces the water
content and micro-organism growth
is retarded.
Bottling/Pickling Food is packed into glass jars and
covered in acidic, alcoholic, salt or
sugar solutions.
Spray Drying A fine spray of liquid is injected into
hot chambers. The water evaporates
leaving the solid part of the product.
Fluidised Bed Drying Warm air is blown upwards, directly
underneath the food causing it to
flow and remain separate.

17. Preservation Method Description
Canning Food is sterilised and placed in air
tight containers. The process is:
-Washing and preparation
-Blanching
-Filling the cans
-Sealing the cans
-Heating a retort (large pressure
cooker)
-Cooling
-Labelling
Roller Drying A liquid or paste form of the food is
uniformly spread over rollers or
drums which rotate slowly. The water
evaporates and the dried product is
scraped off.
Sun Drying Slow and only practical in hot, dry
climates.
Immersion Freezing Refrigerants are directly sprayed
onto food.

18. Preservation Method Description
Plate Freezing Food is prepared and packed
between flat hollow metal plates that
press tightly on the food.
Blast Freezing Blasting a stream of cold air (-40C)
over food.
Fluidised Bed Freezing The food floats on jets of cold air.
Scraped Heat Exchange Food is scraped off a cold surface.
This method reduces ice-crystal
formation producing a smooth end
product.
Cryogenic Freezing Liquid Nitrogen (-196C) or CO2 (-
78C) is sprayed onto small food
items.
Batch Pasteurisation The product is held at 62-63C for 30-
35 minutes.

19. Preservation Method Description
High Temperature/ Short Time
(HTST) Pasteurisation
The product is heated 72C for 15
seconds.
Sterilisation A severe form of heat processing
that destroys most micro-organisms.
The flavour and appearance of a
food product may change.
Ultra Heat Treatment (UHT) Food is heated to 130-150C for 1-3
seconds. The food is kept moving so
fewer changes occur.

20. SHORT EXAM QUESTIONS
 Why does dehydration stop the growth of micro-
organisms?
 Describe Roller Drying
 Write a sentence to describe the following methods
of freezing: Plate freezing, Scraped heat exchange,
Blast freezing
 What are the differences between UHT and
Sterilisation?
 What is MAP?