Transcript Preservation FFL Jdt amended

Preservation
© Food – a fact of life 2009
Foundation
Principles of food preservation
The aim of preservation is to prevent food spoilage as a result of
growth of micro-organisms and breakdown of food by enzymes.
There are both short- and long-term methods of preservation.
As soon as food is harvested, slaughtered or manufactured into a product it
starts to change. This is caused by two main processes:
• autolysis – self destruction, caused by enzymes present in the food;
• microbial spoilage – caused by the growth of bacteria, yeasts and moulds.
Micro-organisms and enzymes need certain conditions to survive and
reproduce. These include:
•temperature
•oxygen
•food
•time
•moisture of water will slow or prevent decay
•pH level
© Food – a fact of life 2009
Some of the factors affecting the growth of micro - organisms can be
manipulated in different ways to prolong the life of the food product.
Temperature (high and low)
High temperatures prevent decay and destroy enzymes and proteins, killing the
organisms responsible for decay. Low temperatures slow decay by slowing the rate
of reaction of enzymes and will prevent growth and reproduction.
Low temperatures slow decay. Chilling or freezing the food to retard growth of
micro-organisms and inhibit enzyme activity.
Oxygen
Lack of oxygen will slow or prevent most forms of decay. Oxygen is needed for
respiration by organisms. Some bacteria can survive without oxygen, such as those
used in biogas generators. Food kept in an airtight container will deprive microorganisms of oxygen and prevent contamination.
Moisture
Lack of water will slow or prevent decay. Water is needed for transport and to
support reactions inside organisms. Decay organisms need water to digest their
food..Reducing the moisture content of the food to make water, (which is essential
for growth), unavailable to micro-organisms. Alternatively, placing food in a sugary
solution will make water unavailable for the growth of micro-organisms.
pH level
Placing food in an acidic or alkaline solution will inhibit the growth of microorganisms.
© Food – a fact of life 2009
Methods of Preservation
Traditional methods of food preservation began from the essential
need to store supplies when they were plentiful and to keep the
food fresh for as long as possible to last through the winter months.
Although food preservation has been in use for thousands of years, it is only
in the last two centuries that many of the ‘new’ food processing
techniques have been developed.
• High temperature
• Low temperature
• Removing moisture
• Use of chemicals
• Controlling the atmosphere
• Irradiation
.
© Food – a fact of life 2009
Temperature
Food can be preserved by removing one or more of the factors that bacteria and
fungi need to survive.
Methods of food preservation
Method
Examples
How it works
Vegetables
The sealed can is heated to kill
bacteria. When it cools, no more
bacteria can enter.
Ready made meals
The food is kept at a low
temperature (4°C) so bacteria
cannot reproduce (they're not
killed).
Vegetables and meat products
Bacteria cannot reproduce at low
temperatures and the water in the
food is frozen.
Drying
Fruit, flour, soups, pasta
Removing water prevents the
microorganisms from growing, or
digesting the food.
Adding salt or adding sugar
Jam, crisps, meat
Removes water by osmosis
Pickles
Changes the pH to prevent the
enzymes in the microorganisms
from working.
Canning
Cooling or chilling
Freezing
Adding vinegar
© Food – a fact of life 2009
High temperatures
High temperatures stop bacterial growth by killing the bacteria.
Heat processing is used for preserving all sorts of foods and food
products. Different methods include:
pasteurization
sterilization
UHT
Canning
Milk preservation
1. Milk which is pasteurized is heated up to 72°C for 15 seconds then rapidly
cooled to 10°C. Pasteurization only kills pathogenic bacteria so is not a longterm method of preservation.
2. Sterilized milk tastes and looks different to ordinary milk. The milk is bottled
and sealed with metal caps and then heated up to 104°C for 40 minutes or
113°C for 15 minutes. Sterilization kills nearly all micro-organisms and enzymes
and is therefore a long-term method of preserving milk. Sterilization preserves
food without the use of preservatives or refrigeration so is attractive to
consumers concerned with ‘artificial’ foods. Tetra Pak cartons are often used
as the packaging materials for sterilized products.
© Food – a fact of life 2009
3. To produce UHT milk, the milk is heated to between 130°C and
150°C for 1-3 seconds in plate heat exchangers to ensure that
spores are destroyed. In the case of milk the product is heated to
not less than 132.2°C for one second and is packaged under
aseptic conditions.
• This method kills nearly all micro-organisms and enzymes but
there is little change in the flavour and colour of the food. It is
a long-term method of preservation and is usually used in
combination with aseptic packaging.
• Packages are hermetically sealed. The packaging materials
used are robust and prevent entry of air unless opened or
pierced. The products can be stored in ambient temperatures
for up to 6 months usually and do not require refrigeration.
• Until the package is opened the product remains in sterile
conditions and is therefore preserved.
© Food – a fact of life 2009
Canning and Bottling
Canning
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Food is packed into a can and then heat processed until the contents are
‘commercially sterile’ at 121°C by passing the can into a special steamer where a
vacuum is applied to draw the air from the can as the lid is sealed into place. A double seam is
made by rolling the metal edges together in the shape of a hook. Air is driven out by the steam,
preventing bacteria from surviving.
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 (bacteria free) 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.
Canned food has a very long shelf life at ambient temperatures.
Bottling
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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 – often referred to as a Kilner 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.
© Food – a fact of life 2009
Low Temperatures
Chilling
- A short-term method of preservation. Chilling preserves food by slowing
down the growth of bacteria.
- Raw ingredients and food products need to be chilled. Over the past 50
years chilling and freezing has become the most popular domestic method
of preserving food. This is mainly due to wider ownership of domestic
refrigerators and freezers and developments in technology, rather than the
discovery of new preservation principles.
- Chilling reduces the temperature to between 1ºC -4ºC.
- Chilling food cannot preserve a food indefinitely, but can reduce spoilage
caused by micro-organisms and enzymes.
What are cook-chill foods?
Processed food products that are chilled are usually called cook-chill foods.
They are prepared and cooked in the normal way before being blast chilled
(rapidly chilled to below 5°C (usually 2 °C) within 1½ hours using a special
chiller). The food is kept at a low temperature during transport, storage and
display up to when it is cooked.
Cook-chill foods can only be kept for a few days unless special packaging is
used. The food must be cooked to above 70°C and eaten within two hours.
© Food – a fact of life 2009
Freezing
Freezing was first used as a method of preserving food in China in
1800BC. It is a long-term method of preservation. It preserves
food by stopping the growth of bacteria. The bacteria remain dormant
until the food is defrosted. Raw ingredients and food products can be
frozen.
Reducing the temperature of the food to below – 18ºC reduces
the activity of the micro-organisms and enzymes.
Freezing also reduces the availability of water because ice crystals are
formed.
Quick and controlled freezing is needed so that large ice crystals do not form
and break the cell walls of the food. In modern methods of freezing, small
crystals are formed meaning that the food is much more likely to retain its
structure once it has been defrosted.
Disadvantages of freezing:
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Most food contains large amounts of water. When water is frozen, ice is formed.
Large ice crystals are formed when food is slowly frozen, this can damage the cell
structure of the food.
When the food defrosts, the water enclosed within the cells is released, e.g. cell
damage in soft fruits (strawberries) and the collapse of some colloidal systems in
food products, e.g. cream. Freezing food quickly can reduce the size of ice crystals.
When frozen, micro-organisms do not die, they simply become dormant, retarding
their growth.
Moulds can still grow in cold temperatures.
© Food – a fact of life 2009
Methods of freezing
Plate
In this method the food is prepared in the ordinary way and is packed into a flat
container, usually a cardboard-based container, often with polythene or wax lining (e.g.
beef burgers). The container is placed between flat, hollow refrigerated metal plates,
which are adjusted to press tightly to the pack. This method is slowed down if there are
any air gaps between the food and the metal plates. It too has been superseded by
other more efficient methods.
Foods: flat products such as burgers and fish fingers
Blast
A blast-freezer is a large cabinet in which a fan has been introduced to move the air
over the product. Moving air readily takes up heat and loses it again. The air reaches a
temperature of -25ºC or lower and should move with a velocity of 400m/minute. The
blast freezer is generally a batch system. Continuous systems which have been
developed have more throughputs.
Foods: irregular shaped foods such as vegetables and prawns, chicken
Cryogenic
Food is immersed in or sprayed with very cold liquefied gases such as nitrogen and
carbon dioxide. It is a rapid method of freezing which can cause thermal shock to some
foods because of the sudden contraction. Liquid nitrogen is sprayed onto food on a
conveyor belt in a tunnel.
Foods: delicate foods such as raspberries and small items like prawns
© Food – a fact of life 2009
Drying
Drying is a long-term method of preservation and the removal of moisture by
warmth or high temperature. Since bacteria need moisture to grow and
reproduce, removing the water stops their growth, thereby preserving the
food. It is probably the oldest method of preservation
Modern commercial methods such as:
• spray drying
• freeze drying
• roller drying
• accelerated freeze drying (AFD),
• tunnel drying
• fluidised bed drying
• oven drying, e.g. herbs, coffee, tea, vegetables
Traditional methods of removing moisture are by sun drying: tomatoes, dates,
figs, chillis, peppers and sultanas are dried in this way. This method needs heat
and some wind to dry out the food before spoilage happens. It is a slow
process and not a effective long term method. Dried foods can be stored at
ambient temperatures.
• Modern methods of removing moisture include
© Food – a fact of life 2009
• Accelerated freeze-drying (AFD) is the most modern method of drying. The
food is dried under vacuum at reduced pressure. It is then frozen before
being rapidly heated. Since the ice changes to water vapour (steam)
without passing through the liquid stage, most of the flavour, colour and
texture of the food is preserved. The food also does not shrink as much as in
other methods of drying.
Foods: fruit, vegetables, chicken, omelettes, herbs and even dog food!
• Spray drying. Liquids are spray-dried in a funnel-shaped chamber; solid
foods are dried in a continuous tunnel; less robust foods are dried in a fixed
cabinet.
© Food – a fact of life 2009
Irradiation
• This is a method of preservation where ionising radiations are applied to
foods to kill bacteria (gamma rays / beams of electrons).
• There is said to be no risk of residual radioactivity and this process
should not be confused with radioactive contamination or
radioactivity, though some consumers are still concerned about
possible, yet undiscovered effects.
• Opponents of food irradiation sometimes state that large-scale
irradiation would increase processing, transportation, and handling
times for fruits and vegetables thus contributing to a negative
ecological balance compared to locally grown foods.
• Food labelling regulations are not consistent across the world, so
consumers may not be 100% sure that they are consuming nonirradiated foods.
• Irradiation has the potential to reduce microbial spoilage, insect
damage and the need for chemical additives to improve keeping
qualities, flavour etc. Often used in prawns, strawberries and potatoes
(to prevent them sprouting).
• It is expensive to carry out and as yet is only permitted in the UK for use
with
some
vegetables,
spices and poultry products.
© Food
– a fact
of life
2009
Modified Atmosphere Packaging (MAP)
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Modified Atmosphere Packaging is also called Controlled Atmosphere.
It is a method of storage where the oxygen level can be lowered or the carbon
dioxide level or nitrogen levels increased.
In this form of packaging, the packs are gas flushed, reducing food spoilage and
prolonging shelf life.
Nitrogen prevents rancidity in fatty foods such as nuts.
Oxygen is required in packaging raw meat to preserve its red 'bloom'. If oxygen levels
are too low, bacteria will respire anaerobically, producing toxins.
MAP has to be used in conjunction with temperature control in storing fresh foods
(below 5°C).
It is a very good medium for storing fresh fruits, vegetables, meats and fish. Spoilage
is reduced and shelf-life is increased.
Vacuum Packing
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The food is placed in plastic packaging and the air around the food is then sucked
out and the plastic bag sealed.
The food is now in anaerobic conditions.
Once the package is opened the food needs to be stored in appropriate storage
conditions.
Bacon, coffee and fish are most commonly packaged in this way.
© Food – a fact of life 2009
Chemical methods
Sugar
Sugar reduces microbial activity due to its dehydrating effect.
The water available to the microorganisms is reduced by osmosis as
the sugar solution is more concentrated than the cytoplasm inside the
cells of the microorganism.
Therefore water passes out of the cell and the cell becomes
dehydrated.
The initial boiling of the fruit will destroy the enzymes and microorganisms (but not spores), preventing spoilage later on.
The high concentration of sugar added during the jam making process
makes the water unavailable thus reducing the microbial activity
through dehydration effect.
Jam jars are normally heated before the jam is added destroys the
micro-organisms found in the jars.
© Food – a fact of life 2009
Salt
Salt is used often in solution as brine.
This method is used in preserving meats, such as ham or bacon, and is
known as curing.
Many foods are stored in brine solutions, in cans or jars or sealed
sachets, such as olives, tuna fish.
The solution is usually 25% sodium chloride, 1% potassium nitrate and
0.1% sodium nitrate.
The salt content acts in the same way as the sugar, through the action
of osmosis.
Coating food in salt or placing it in a salt solution (brine) reduces the
moisture content of the food, i.e. reduces the availability of water.
With little moisture, micro-organism growth is retarded.
However, the taste of the food may change considerably.
© Food – a fact of life 2009
Vinegar (Pickling)
Vinegar (acetic acid) can be used to pickle foods such as vegetables
or eggs.
The lowered pH provides unfavourable conditions for microorganisms
to reproduce.
Most microorganisms grow bets if the pH of the food is between 6.6
and 7.5 (neutral).
Bacteria are less acid-tolerant than moulds and yeasts.
The initial boiling of the ingredients will destroy enzymes and microorganisms (but not spores), preventing spoilage later on.
Vegetables and fruits are covered in vinegar and other ingredients,
often including spices. The high concentration of acid inhibits
bacterial growth and multiplication.
Pickle/or chutney jars are normally heated before the product is
added to destroy micro-organisms found in the jars.
© Food – a fact of life 2009
Smoke
• Smoke contains antimicrobial substances which
reduce food spoilage.
• The preservative effect comes from phenols and
acids and aldehydes in the smoke (which produce a
thin tar like film over the surface).
• It also causes surface dehydration of the product,
and the high temperature and lack of oxygen if ‘hot
smoked’ can kill some microbes.
• Smoking is often used on foods which have already
been cured (partially dried through the use of salt).
• Foods that are preserved through smoking are
normally meat, fish, and cheese.
© Food – a fact of life 2009
Advantages and disadvantages to environment of
preserving fresh foods by the methods like UHT, vacuum
packing, drying, canning
NEGATIVES
• Litter from packaging causing an eyesore, pollution and physical hazards.
• Tins rusting and posing risks from sharp edges
• Pollution caused by factories that make the packaging
• Use of natural resources to make packaging
• The possibility that the packaging cannot be recycled, e.g. tetra-packs
• The creation of a society that expects packaged food rather than being willing to buy food at
source, e.g. milk from the milk man, using recyclable glass bottles.
• The costs added to these products as a result of the methods used and the materials consumed
• Vacuum packs may require refrigeration and will thus have a shorter shelf life.
POSITIVES
• None of these methods require refrigeration, so all of them are environmentally friendly in terms of
fuel consumption
• They can all be kept at ambient temperatures
• They are easy to stack and to store
• They can be kept for a long time, meaning fewer shopping trips required
• They are easy to prepare, some being ready to eat on opening
• Canning, Drying and UHT are safe methods of preservation, with low risks attached
© Food – a fact of life 2009