Food preservation by wanghonghx

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									Food preservation
    F.6 Chemical Project
    2.The need to preserve food
         3.Main principles of food
4.Technical principles of food
    5.Common techniques of food
If there is no food

            Food Spoiled
            Food Used

Food spoilage owing to microbial activities
Caused by yeasts,fungi or bacteria
either toxic in nature or produce toxic chemical
eg.bread mould

Food spoilage owing to chemical changes
Oxidation of glycerides-> peroxide
Oxidation of phenols-> browning of apple
Microbial activities caused by yeast, fungi,
bacteria etc..
Chemical and biochemical changes in food

How do the micro-organisms
spoil the food?
The micro-organisms are toxin in nature.
They produce toxin chemicals when they act
on the food.
   Examples of micro-organisms that
   spoiled food:
   Mould on bread
   Lactic acid bacteria in the souring mild
   Clostridium botulinum on meat.

Principles of browning of apple:
The phenols in apple is oxidized into
The diquinones will undergo polymerization and
finally a brown products were formed
1.Killing of micro-organisms
-micro-organisms found everywhere
-rapid growth in damp & warm environment
-Produce toxic substances ,harmful to man

2.Inhibition of microbial growth
-reproduce by simple cell division
-necessary to keep food in conditions that are not
favourable for microbial growth
3.Retardation of chemical or
biochemical changes

-food undergo chemical or biochemical in
storage even the absence of micro-

-Removal of air or water from food, and
the inhibition of enzyme activities are
common methods used to read the chemical
or biochemical changes in food.
Providing a favourable condition for the
growth of micro-organisms
Advantages of increasing or decreasing
the pH value
Extreme pH condition will inhabit the
growth of the micro-organism
The use of vinegar in pickling ,use of
lactic acid in yogurt
Since the micro-organism require a minimum a moisture
content of 20% to growth
Decreasing the moisture content inside the cells by
increasing the solute concentration of food,would inhabit
the microbial growth
Principle:Water potential of the micro-organism is
higher than that in the surroundings ,as water flow from
higher water potential to lower water potential ,the cells
run dry
Advantages:Micro-organism would not capable to
Examples:Putting the food into concentrated salt or
Providing a favourable conditions
microbial growth in food and leading
to hydrolytic reaction during which
food is spoiled.
Advantages of the removal of water:
Slowing down the microbial growth
rate.Suppressing chemical and
biochemical changes .
Examples:Drying and dehydration
Providing a favourable condition for microbial
growth in food
Advantages of decreasing temperature
Retarding the growth of micro-organism

Advantages of increasing temperature
a.Destroying the bacterial cells and the spores
b.Killing the micro-organisms

Examples:Heat treatment,freezing
Since many chemicals can inhabit the
microbial growth,the use of chemical ,
such as
    Anti-oxidant can be used to reduce
    the amount of dissolved oxygen in
    food .It can retard the chemical
    and biochemical changes
Heat kills micro-organisms, destroys enzyme
activity of micro-organisms in food.

Length of storage:
It depends on the heat treatment and upon
the type of food

Changes in quality:
The quality of food is maintained
E.g. the dried food products
such as biscuits, vegetables,
fruit, fish,shrimp and milk
  Water is unavailable for reactions to occur,
  and for micro-organisms to grow.

Length of storage:
This varies with the product, storage
conditions and the final moisture content
Changes in quality:
Often the reconstituted or rehydrated food is
comparable to fresh food
Radiation destroys enzymes
and micro-organisms inside the food
Length of storage:
It significantly increases the storage of any
food in comparison to its preservation by
conventional methods
Changes in quality:
Nutrient destruction is no greater than hat by
conventional methods
Chilling slows down microbial activities and
chemical changes resulting in spoilage

Length of storage:
After which time undesirable off- flavours
slowly develops

Changes in quality:
Freezing promotes the retention of
nutrients and does not destroy nutrients
In jam, fruits in heavy
sugar syrup.Food is kept in
a strong sugar solution.
  Water available for enzyme action and
  microbial growth is removed by osmosis.

  Length of storage:
  When properly bottled or canned, these
  fruits may be stored a year or more.

Changes in quality:
these foods contain more calories
than fresh fruit.
Salt   Fish
Water essential for enzyme action and
microbial growth is removed by osmosis.

Length of storage:
Most product can be stored three weeks to
several months.
Changes in quality:
Its greater the loss in food value, including
loss of vitamins and minerals
    Cooking destroys enzymes and micro-

 Length of storage:
 It can keep well for more than one year when
 properly canned

Changes in quality:
1. Colour and texture often different from fresh
2. No practical effect on proteins, carbohydrates
and fats
3. Vitamin C and B are often destroyed.
Most micro-organisms do not grow well in
acidic solutions

Length of storage:
Picking alone is not enough for long term
Changes in quality:
It adds an appealing flavour to the food
Sodium chloride ties up the water otherwise
used as a growth medium by micro-organism

Length of storage:
It mainly depends on the amount of chemicals
used and the food product.

           Changes in quality:
           The quality of food is generally
The added substances inactivate or kill
micro-organisms, retard chemical spoilage.

Length of storage:
It depends on the nature of chemical
used and the formed product.

Changes in quality:
Quality is maintained or enhanced.

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