Lesson Notes By Weeks and Term - Senior Secondary School 2

Preservation of foods






TOPIC: Preservation of foods

Definition, Reasons and Methods.


Definition of food preservation:

Food preservation can be defined as a way of protecting foods from spoilage or undesirable changes�

Reasons for food Preservation

Food is preserved to prevent natural and microbial decay, by modifying the conditions that favor�

enzyme activity, and the growth of micro-organisms.

In the past food was preserved to provide a store of food when there was no other source of food.��

But today food is preserved for the following reasons.

  1. To add variety to the diet by making foods available out of season.
  2. To make use of food when it is cheap and plentiful and store it for later use.
  3. To vary the diet by preserving food in ways that makes a new product out of the food. (E.g. by pickling, jam making)

Methods of Preservation

Food decay can be slowed by:

  1. Heating
  2. Removal of moisture
  3. Removal of air
  4. Reduction of temperature
  5. Addition of a chemical preservation

  1. Heat Preservation:� Heating destroys micro-organisms and enzymes activity.� Most bacterial, yeasts, moulds and enzymes are destroyed by heating at 1000C. The main methods of heat treatment are:

��� Sterilization, Pasteurization canning and bottling

Sterilization kills harmful and souring bacteria more completely than pasteurization foods that are preserved using those methods are (1) milk � � � (2) canned or bottled foods� (3) milk products,� (4) Fruit juices � � � (5) vegetable juices� � � � (6) Beer� � (7) Vinegar� � (8) wine

Canning and bottling:

Today canning is one of the most widely used methods of preservation.� A huge variety of foods are canned and bottled.� Both methods rely on heat sterilization to destroy microbes and enzymes and sealing to prevent contamination during storage.

  1. Removal of moisture�

��� (Dehydration)

��� Micro-organisms depend on water for growth and reproduction Dehydrating foods will therefore stop or slows down the action of micro-organisms which are responsible for food decay.

  1. Removal of air

��� Foods can be wrapped in an impermeable plastic film and the air removed under a vacuum.� This will prevent the entry of micro-organism until the seal is broken.

  1. Reduction of temperature�

��� When the temperature is reduced, the activities of most micro-organisms are slowed down, until they become dormant (inactive) and growth and multiplication cease.� Some types are destroyed at low temperature

  1. Addition of a chemical preservative substance called preservatives can be added to food to inhibit the action of enzymes and either destroy or inactivate micro-organisms which may contaminate the food.

��� There are many different chemical preservatives in use some of them are;

  1. Sodium Chloride (common salt) foods preserved in salt include;

��� Cheese, Sausages, dried fish canned vegetables (in brine) bacon.

  1. Acids:

��� Some acids are used as preservatives e.g.

��� Ethanoic acid (vinegar)

��� Citric acid

��� Lactic acid

��� Tartaric acid

  1. Sulphur dioxide

��� Foods preserved with sulphur dioxide include�


Fruit for jam making

Flour for biscuits

Fruit juice




Soft drinks

Tomato puree


Sugar is an effective preservative & adds flavor and energy value to foods.� It is used to preserve many foods including fruit in jam making canned & bottled fruits.

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