Download File - Stage 6 Food Technology Syllabus

Processes (aka Techniques) of Food Preservation

Syllabus Link: Preservation Processes, including canning, drying, pasteurising, freezing
and fermenting.
1. Chemical Methods

Use of Chemical Additives
-

There are preservatives that are added directly to the food product.
If the concentration of additive is high enough, the micro-organisms will be killed.
Examples;
o Benzoic Acid used in fruit juices and pickled vegetables.
o Nitrates used in processed ham.
o Sulfur Dioxide used in pre-sliced carrots and apples to control enzymic
browning.
Smoking
-
The smoking process combines heat to create smoke. The smoke dries the moisture in
meat products and destroys bacteria and prevents enzymic browning.

Curing and Salting
-
Curing uses the combination of nitrates and salt for preservation.
The main method of preservation is the action of the salt-binding moisture. The salt
draws the moisture from the food (process known as osmosis), thus, preserving it.
Deli meats, e.g salami, are preserved in this way.
2. Reducing Water Activity
Micro-organisms and enzymes require moisture for growth. The principle behind moisture
control is to remove water content so it is not available for microbial and enzyme growth.
Manufacturers aim to dry products so that they have a moisture content of 5-6% because
moulds and bacteria can grow in foods with a moisture content as little as 13-20%.

Spray Drying
-

Spray drying is the removal of water under controlled conditions in a spray drier.
Milk and eggs are dried to powders using this method.
Vacuum Dehydration

Water is removed from liquid foods by the application of heat in a vacuum.
Evaporated milk and condensed milk are producing using this process.
Freeze Drying
-
Freeze drying is an expensive method that is not commonly used.
It is used to make instant coffee, tea, soup and dried mushrooms.
Process:
1. Product is quickly frozen so that it becomes a solid.
2. Placed onto shallow trays and exposed to a gentle heat.
3. The gentle heat causes the ice crystals to sublimate (turn from a solid to a vapour
without passing through the liquid phase), leaving dehydrated particles.
3. Raising Temperature
Most micro-organisms and enzymes are destroyed when exposed to temperatures above 60
degrees celsius.

Canning/Bottling
-
Canning is a heat treatment process that aims to heat the food to the point where the
food is sterilized yet still edible.
Food is sealed into sterile containers to prevent micro-organisms entering after
treatment of the food.
Enzymes are also destroyed in this process.
There are TWO types of canning processes:
1. Aseptic Canning: Foods are first heated and then placed in a sterile container and
sealed (faster process)
2. Conventional Canning: Foods are placed in a container, sealed and then heated.
-
-
In both processes, the cans are heated in retorts (large pressure cookers) for the length
of time needed to destroy the micro-organisms. Temperatures used are between 105
and 140 degrees Celsius.
After heat processing, the cans are cooled rapidly to avoid the ‘critical temperature
zone’ (5-60 degrees Celsius).
Canning is effective because it involves 2 preservation principles – temperature
control and exclusion of air. The air is removed from the can by a vacuum. However,
some foods are not suitable for the canning process because their quality suffers e.g
texture and colour.
When buying canned foods, ensure the ends of the cans are slightly concave and they
are free from bulges and dents. Bulges indicate that the micro-organisms within the
can have produced a gas due to reacting with oxygen (air).
Some acid foods can corrode containers and cause a chemical reaction that produces
hydrogen gas. Cans used for high-acid foods have lacquered insides (white lining inside
cans of apricots).

Pasteurisation
-
Pasteurisation is the process where a food item is sterilized by heating it to a high
temperature in a short time.
It kills or reduces heat-resistant micro-organisms.
Food Examples: Milk, Fruit Juice, Beer, Wine.
Today, the most common pasteurization method is HTST (High Temperature, Short
Time).
Milk is pasteurized using a heat exchanger.
Advantage: no significant change in the flavour or texture.
-
72 Degrees Celsius
15 Seconds

Ultra Heat Treatment (UHT)
-
This process is used for UHT Milk. This milk does not require refrigeration for storage.
It can be stored in the cupboard for up to 6 months.
This process kills all heat-resistant bacteria.
Disadvantage: May be a slight change in flavour.
-
140 Degrees Celsius
3-5 Seconds

Blanching
-
This is a process of immersing vegetables in boiling water and then immersing them in
cold water to stop the cooking process.
This destroys enzymic activity and pathogenic micro-organisms.
-
4. Lowering Temperature

Chilling
-
Chilling slows the growth and activity of enzymes and micro-organisms.
It is the storage of foods above freezing points and below 15 degrees Celsius.
Foods preserved using this process include; fruits, vegetables, meat, fish, milk and
cheese (perishable foods).
Below 15 Degrees Celsius.

Freezing
-
Freezing is the storage of foods between -15 and -30 degrees Celsius.
Freezing makes enzymes and micro-organisms dormant.
It changes water to ice so that it is not available for microbial growth.
The slower the process of freezing, the larger the ice crystals that can form.
-15 and -30 Degrees Celsius
5. Biological Methods

Fermentation
-
Fermentation occurs when carbohydrates are converted to acids or alcohol through
the action of micro-organisms.
Used in the production of alcoholic beverages such as; wine, cider, beer.
This happens when the yeast acts upon the sugars in honey, grapes, grains or fruits,
converting them to alcohol.
Carbon dioxide is given off as a by-product of this process.
Also used to make yoghurt. The bacteria feeds on the carbohydrates (sugar), multiplies
and creates lactic acid. The pH of the food lowers, flavour sours and the environment
becomes unfavourable.
-