notes prepared by subhankar Karmakar
Preservation of food:
The process in which the food materials are given a suitable physical or chemical treatment to prevent their spoilage is called food preservation. Some of the methods for preserving foods are:
1. Sun drying or dehydration
2. Heating
3. Cooling or refrigeration
4. Deep freezing
5. Addition of common salt
6. Addition of sugar
7. Addition of mustard oil and vinegar
8. Use of special chemical preservatives like sodium meta-bi-sulphite, sodium benzoate and citric acid.
9. Pasteurisation
10. Packing food in airtight packets.
1. Preservation of food by sun drying or dehydration:
Water content of the food is removed by sun drying. It is also known as dehydration.
Vegetables like spinach, methi leaves, cauliflower, peas are preserved in our homes by the sun drying method.
2. Preservation of food by heating:
Microorganisms in the food can be killed by heating the food. We boil milk to prevent it from spoilage.
3. Preservation of food by cooling or refrigeration:
The growth of microorganisms can be inhibited by keeping low temperature.
The food materials like milk, kneaded flour, cooked food, fresh fruits and vegetables are kept in a cool place like refrigerator to prevent their spoilage.
4. Preservation of food by deep freezing:
At sub zero (below zero degree centigrade) temperature the growth of microorganism are completely stopped.
Deep freezing method is used for the preservation of foods like meat, fish and their products; fruits and vegetables.
5. Preservation of food by adding common salt:
common salt prevents the growth of food spoilage microorganisms due to which it is used to preserve a number of food materials. It is one of the oldest practice for preservation of food like meat and fish.
common salt is also used to preserve fruits such as raw mangoes lemon and amla as well as tamarind.
6. Preservation of food by adding sugar:
Sugar is used as a preservative in making jams and jellies from fruits. The fruits which are preserved in the form of jams and jellies by using sugar as preservative are Apple, ripe mango, Orange, strawberry, pineapple and guava etc.
7. Preservation of food by mustard oil and vinegar:
Mustard oil and vinegar are widely used as preservatives for the preservation of fruits and vegetables in the form of pickles.
Fruits like raw mango, amla and lemon are preserved using mustard oil and vinegar in the form of their pickles.
8. Preservation of food by using special chemicals as preservatives:
Sodium metabisulphite, sodium benzoate and citric acid are used as chemical preservatives. To preserve foods such as jams, jellies, juices and squashes sodium metabisulphite and sodium benzoate are used. To preserve confectionery citric acid is used as a preservative.
9. Preservation of food by pasteurisation:
Pasteurisation involves the process of heating followed by quick cooling. Packaged milk is always pasteurized.
10. Preservation of food by packing and air tight packets:
This days dry fruits and even vegetables are sold in sealed airtight packets to prevent the attack of microorganisms on them. This helps the dry fruits and vegetables to remained unspoiled for a longer time.
NITROGEN FIXATION:
The process of converting nitrogen gas of atmosphere or air into compounds of nitrogen which can be used by the plants is called nitrogen fixation.
The nitrogen gas of atmosphere or air can be fixed or converted into nitrogen compounds-
1. By certain nitrogen fixing bacteria present in the soil,
2. By Rhizobium bacteria present in the root nodules of leguminous plants,
3. Buy blue-green algae,
4. By lightning.
Rhizobium Bacteria:
The nitrogen fixing rhizobium bacteria live in in the root nodules of leguminous plants like peas, beans etc, also known as legumes.
Nitrogen fixation by lightning:
Nitrogen gas of atmosphere also gets fixed through the action of lightning in the sky. When lightning takes place in the sky during thunderstorm, a high temperature is produced in the atmosphere. At this high temperature, nitrogen gas of air combines with oxygen gas of air to form nitrogen compounds. These nitrogen compounds dissolve in rain water, fall to earth with rain water and go to the soil. Nitrogen of atmosphere can also be fixed by artificial methods.
THE NITROGEN CYCLE:
The circulation of nitrogen element through living things like plants and animals and non living environment like air soil and water, is called nitrogen cycle in nature.
"Nitrogen is required by both plants and animals for their growth and development. Nitrogen is an essential component of proteins, which make up the bodies of plant and animals. Nitrogen is also present in chlorophyll, nucleic acids and vitamins. The same nitrogen element is circulated again and again through living things like plants and animals and non living things like air soil and water. This whole process is known as nitrogen cycle."
The main steps of nitrogen cycle:
I. The atmosphere or air contains nitrogen gas. The nitrogen fixing bacteria present in the soil and in the root nodules of leguminous plants, blue green algae and lightning in the sky fix nitrogen gas from the atmosphere and convert it into compounds of nitrogen which go into soil.
II. The plant take compounds of nitrogen from the soil for their growth. The plants absorb the nitrogen compounds from the soil through their roots. The plants convert the compounds of nitrogen into plant proteins and other organic compounds which make up the body of plants.
III. The plants are eaten by the animals as food. Animals convert plant proteins into animal proteins and other organic compounds which make up their body. Some animals also eat other animals to obtained nitrogen compound. Thus, animals obtain nitrogen compounds by eating plants as well as other animals.
IV. When plants and animals die, the complex nitrogen compounds like proteins etc. present in their dead bodies are decomposed and converted into simple compounds of nitrogen by certain bacteria and fungi present in the soil. Animal excretions like urine etc. are also converted into simple compounds of nitrogen. All the simple compounds of nitrogen formed in this way go into the soil. In this way, the compounds of nitrogen which were taken by the plants from the soil during their growth are returned to the soil. From the soil these nitrogen compounds are again absorbed by the new plants for their growth and this part of nitrogen cycle is repeated endlessly.
V. Some of the compounds of nitrogen formed from the decay of dead plants and animals, are decomposed by denitrifying bacteria present in the soil to form nitrogen gas. This nitrogen gas goes back into the atmosphere from where it initially came. This process is the reverse of fixation of nitrogen. In this way the nitrogen gas which was removed from the atmosphere during fixation is put back into the atmosphere.
From the atmosphere nitrogen gas is used again during nitrogen fixation and the nitrogen cycle is repeated in nature again and again. As a result of nitrogen cycle in nature, the percentage of nitrogen gas in the atmosphere or their remains constant.
