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Question : Comprehension:

Read the following passage and answer the questions.

All foodstuffs change. Green fruits become ripe and eggs go bad or rotten. It is the enzymes naturally present in each food which bring about the complex chemical changes that lead first to ripening and then to rotting.

How does one control the activity of enzymes, bacteria, yeasts and moulds in food? All of them require air, moisture and a certain temperature, usually somewhere near the body temperature, to be active. Depriving them of one or more of these will suppress them. All microorganisms can be killed by heat sterilisation. This simply means heating the food to high enough temperatures by boiling, deep or shallow frying, roasting, baking and, for milk particularly, pasteurisation. After such sterilisation, if the food is sealed in airtight containers, it can be kept for a long time. A certain water content in food is necessary for microorganisms to be active. Drying in the sun is a simple way of bringing down the moisture level so low that the enzymes and microorganisms cannot flourish.

Some chemicals can suppress undesirable activity. The addition of salt, vinegar, spices and oil or sugar syrup are other ways of preventing foods from going bad. If heating kills, freezing inactivates the enzymes and microorganisms. This is what happens in a refrigerator, in which fruits, vegetables and milk can be kept for fairly long periods. For meat and fish, even colder temperatures, below freezing point, are necessary for preservation. All foods which are kept cold in this way, once taken out and returned to room temperature, are again subject to spoilage and change.

Question:
The main theme of the passage is:

Option 1: Chemicals in foodstuff

Option 2: Ripening of fruit

Option 3: Production of micro-organisms

Option 4: Preservation of food