Food Preservation and Nitrogen Fixation : Science Cbse / Ncert Revision Notes


Food poisoning

An undesirable change in colour,  odour and taste in food brought about by microorganisms is called as food poisoning.  Food poisoning may cause infection, serious illness and even death.  Food has to be preserved in order to prevent it from being contaminated by microorganisms.

Food preservation

The process of treating food in order to slow down spoilage is called as food preservation.

Methods of food preservation

Chemical method of preservation is done by adding preservatives like sodium benzoate and sodium metabisulphite in jams and squashes.  

Food Preservation and Nitrogen Fixation, Science Cbse / Ncert Revision Notes, Class 8

Salting is done by using common salt. This absorbs moisture from food, making it dry and uninhabitable for microbes.

Preservation by sugar is done by adding sugar which reduces moisture in food,  inhibits the growth of microbes that spoil foodJams, jellies and squashes are preserved using sugar.

Preservation by oil is done by adding oil as bacteria and fungi cannot live in an environment of oil. Preservation by vinegar is done by adding vinegar as microorganisms cannot live in sour environment. Pasteurization is a method of preserving milk. Louis Pasteur discovered the process of pasteurization.

During pasteurization, milk is first heated to about 70 degrees Celsius for 15 to 30 seconds and then suddenly chilled. Sudden chilling of milk is done to prevent the growth of microbes.

Boiling and freezing are the two other methods used to preserve food.



Nitrogen fixation




Nitrogen is necessary for plants and animals to synthesize many components of their bodies. Nitrogen is an essential constituent of proteins, nucleic acids, chlorophyll and vitamins. Though 78 percent of our atmosphere is nitrogen, plants and animals cannot avail this component from air. Hence, nitrogen has to be fixed for them.


Process Of Nitrogen fixation

The process of conversion of atmospheric nitrogen into nitrogenous compounds and making it available for plants is called as nitrogen fixation. The organisms which fix nitrogen to plants are called as nitrogen fixers.

Most of the bacteria like Rhizobium living in the root nodules of legume plants, fix nitrogen from the atmosphere. They fix nitrogen to the plant they live in and in turn obtain shelter and nourishment from it.

Microorganisms in the soil convert atmospheric nitrogen into ammonium compounds, thereby enabling the survival of plants and animals.

Blue green algae can also fix nitrogen from the atmosphere. Fungi are the decomposers that degrade organic waste compounds into simple substances which are ready for reuse by plants.

The circulation of nitrogen between organisms and the atmosphere is called the nitrogen cycle. Microorganisms which act as nitogen fixers help in maintaining soil fertility.

Nitrogen is also fixed by lightning to a small extent. Microorganisms in soil decompose harmful and smelly substances making the environment clean.

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