Transcript Document

Preserving
Food
© ORCA Education Limited 2005
New
Words
Scientific Principles
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Food preservation stops food going bad.
It prevents or delays spoilage and stops
the growth of harmful micro-organisms
such as bacteria, yeasts and moulds.
To survive and spread, micro-organisms
need warmth, food, moisture and time to
grow.
Preservation changes the environment of
the food and therefore prevents the
growth of micro-organisms.
Enzymes in food also cause deterioration
and must be destroyed.
In what foods do we use
micro-organisms? *
© ORCA Education Limited (2005)
and suppliers, all rights reserved
Why Preserve?
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Throughout history people have
found ways to preserve food, like
smoking or sun-drying this fish.
Ideally we would eat food fresh.
This is not possible as we need food
to last for when it is not available
fresh.
Preserving food has also improved
food safety.
Preservation lets us buy or collect
food when it is plentiful or cheap and
eat it out of season. *
© ORCA Education Limited (2005)
and suppliers, all rights reserved
Preservation Methods
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To preserve food like this fruit we need
to remove the conditions microorganisms need to survive and grow.
Our choices are:
to use high or low temperatures
reduce moisture (dehydration)
add chemicals to change the
acid/alkaline conditions or remove water
remove air to deprive micro-organisms
of oxygen and prevent further re-entry
by sealing it from the outside world
irradiation using new technology. *
© ORCA Education Limited (2005)
and suppliers, all rights reserved
The Use of Heat
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Most bacteria, yeasts, moulds and
enzymes are destroyed by heating to
100ºc.
The methods include:
Canning and bottling where heat
sterilization destroys micro-organisms
and enzymes and seals to prevent
contamination.
Pasteurisation which heats food to
72ºc and destroys many but not all
micro-organisms.
Ultra heat treatment (UHT) where high
temperatures are used to destroy all
Are tinned foods
bacteria e.g milk and fruit juices.
different to fresh foods?*
© ORCA Education Limited (2005)
and suppliers, all rights reserved
The Use of Low Temperature
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Low temperatures remove warmth and
moisture that micro-organisms need for
growth.
Food can be frozen or chilled.
Freezing stops the growth of microorganisms, slows down enzyme activity
and removes moisture by changing
water to ice crystals.
Chilling food slows down the rate at
which micro-organisms multiply and
also slows down enzyme activity.
Many prepared foods are cook-freeze or
cook-chill where they are cooked and
then frozen or quickly chilled.
© ORCA Education Limited (2005)
and suppliers, all rights reserved
Drying
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Drying or dehydration removes the
moisture micro-organisms need for
growth and reproduction.
This is a very old method of preservation
where fruit, vegetables, fish and meat
would be sun-dried or oven dried.
Today, mechanical methods used are:
spray drying for liquids such as milk
hot air beds for drying solid food such
as meat and pulses.
This freeze dryer freezes food
quickly then the tiny ice crystals
are removed by turning them to
steam by heat in a vacuum.
© ORCA Education Limited (2005)
and suppliers, all rights reserved
Chemical Methods
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Vinegar, salt, sugar and wood smoke have been used
for many centuries as preservatives.
Vinegar (acetic acid) is used in pickling, the strong acid
solution prevents the growth of micro-organisms.
Salt (sodium chloride) is used for meat and fish in a
solution (brine) or to coat the food. It reduces the
moisture content of the food.
Sugar is used in jams, canning and dried fruit. In
high concentration (60% for jams) it prevents
bacteria from growing.
Other chemical methods include the addition of
antioxidants, sulphur dioxide and nitrites.
Apart from the use of sugar what
other ways is the jam preserved? *
© ORCA Education Limited (2005)
and suppliers, all rights reserved
Removal of Air
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Canning and bottling use air removal
to preserve as well as sterilise.
Removal of air is also used in:
Vacuum packing where all air is removed from the
package. Food is kept in anaerobic conditions (no
oxygen) delaying the growth of micro-organisms.
Modified atmospheric packaging where most of the
oxygen is replaced with carbon dioxide and nitrogen.
Either method is often used to extend the shelf life
of meats, fish, bacon and cheese.
What other example might you
often eat that use modified
atmospheric packaging. *
© ORCA Education Limited (2005)
and suppliers, all rights reserved
Irradiation
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Irradiation is the process of
passing X rays from radioactive or
electron beams through food.
It prevents spoilage by destroying
micro-organisms.
The energy similar to ultraviolet
light does not make the food
radioactive but irradiated food must
be clearly labelled.
The purpose of irradiation is;
stop vegetables
from sprouting
destroy parasites in
meat e.g. pork
allow longer storage e.g.
shellfish and strawberries
© ORCA Education Limited (2005)
and suppliers, all rights reserved
delay ripening of fruit e.g.
mangoes and bananas
destroy insects that may damage
cereals, rice, etc. *
Strawberries
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There are several ways to preserve
any one kind of food.
Strawberries can be preserved by:
canning in sugar syrup or fruit juice
jamming
freezing
chilling
irradiation.
Some methods will retain the qualities
of the fresh strawberry in flavour,
texture, colour, appearance and
nutritional value, while others will not.*
© ORCA Education Limited (2005)
and suppliers, all rights reserved
Changes to Food
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How would the characteristics and
nutritional value of food like this be
changed by preservation?
Irradiation of the vegetables will lead
to loss of vitamins A,C,E and K.
If the fruit or maize are canned the
texture and appearance will change
but the nutritional value will remain.
Drying the apple will concentrate
flavour, reduce size and weight and
may increase sugar or salt content.
Jamming will add flavour and energy
value to the strawberries.
Sterilisation would change the
colour and taste of the milk. *
© ORCA Education Limited (2005)
and suppliers, all rights reserved
Vocabulary
Some words or terms to learn
spoilage
 preservation
 enzymes
 irradiation
 pasteurisation
 antioxidants
 shelf life
 parasites
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© ORCA Education Limited (2005)
and suppliers, all rights reserved
micro-organisms
 dehydration
 sterilization
 UHT
 cook freeze
 cook chilled
 freeze drying
 pickling
 vacuum packing
 modified atmospheric
packaging. *
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The Task
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You can enter here a task for your students.
© ORCA Education Limited (2005)
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