Class 8 Science

Food Preservation

Techniques to prolong the shelf life of a food are called food preservation. We know that microbes proliferate very fast in presence of food, moisture, oxygen and ambient temperature. All the methods of food preservation are aimed at ruling out the food, moisture, oxygen and ambient temperature for microbes so that they won’t proliferate. Some common methods of food preservation are as follows:

Food Poisoning: Some bacteria produce a toxic substance in food. Consuming a food item with toxic substance can result in food poisoning. Food poisoning is a serious case and needs immediate hospitalization. Lack of timely care in case of food poisoning may prove fatal.

Sun Drying

This is a traditional method of food preservation. Sun-drying helps in removing moisture from food. Grains are dried in sun before being stored. Many vegetables are also sun-dried so that they can be used in off season.

Chemical Method of Food Preservation

Some chemicals prevent the growth of microorganisms in food. These chemicals are used in pickles and jams to preserve them. Salt, acids and oil are also used as food preservatives. Examples: sodium benzoate and sodium metabisulphate.

Preservation by common salt

When a food item is kept in plenty of salt, water from food comes out because of osmosis. It results in dehydration of the food item. Absence of moisture helps to prevent the growth of microbes. Fish, meat and pickles are preserved by adding salt.

Preservation by Sugar

Sugar preserves food by reducing moisture in food. Jam, jelly and squash are preserved by adding sugar.

Preservation by Oil and Vinegar

The layer of oil on top of food prevents oxygen from entering the food. Some microbes do not survive in absence of oxygen. Some microbes cannot survive in acidic environment and thus vinegar is an effective food preservative.

Heat and Cold Treatment

Most of the organisms can survive within a particular range of temperature. When the temperature becomes too low or too high, organisms cannot survive. Heating a food item or freezing a food items thus helps in preservation. In most of the households, milk is boiled twice or thrice in a day to prolong its shelf life.


Milk is pasteurized before being packed. Pasteurization involves heating the milk to 70°C for about 15 to 30 seconds and is then quickly cooled down. Pasteurization helps in killing the microbes which may be present in milk. This method was discovered by Louis Pasteur.

Storage and Packing

Some food items are stored in air-tight packets so that oxygen is not available for proliferation of microbes. Some food items are packed in cans; along with some preservatives. Oily food; such as potato chips; is packed in air-tight packets which are filled with nitrogen gas. Nitrogen gas prevents the oily food from becoming rancid.

Nitrogen Fixation

You may have read that 78% of atmosphere is composed of nitrogen. But green plants cannot utilize gaseous nitrogen. Gaseous nitrogen needs to be converted into compounds of nitrogen so that plants can utilize nitrogen. The process of converting nitrogen into compounds which can be taken up by green plants is called nitrogen fixation. Following are the main steps of nitrogen fixation:

Nitrogen Cycle: The cycle of events by which nitrogen is channelized into the living world and back to atmosphere is called nitrogen cycle.