Question

Why does food spoil when kept for a very long time?

Answer 1

Oxidation is the process of addition of oxygen or removal of hydrogen. The spoiling of food is one of the types of oxidation. Food products contain various types of nutrients; among them are the fats and oils. The spoiled food is characterized by the discoloration and the bad smell and taste. This makes it perishable.

Complete answer:
Food is an essential necessity of all living beings. Food contains nutrients that make it palatable. Nutrients like carbohydrates, proteins and fats provide various functions in the human body. The fat and oils are essential food items that are used in the form of unsaturated fatty acids. They serve as a medium for the food to be palatable.
When food is kept for a long time, it is exposed to the atmosphere having oxygen. The unsaturated fats and oils then react with the oxygen and get oxidized to saturated fats. This results in the rancidity of food that creates foul smell and taste and discoloration sometimes.
Hence, food gets spoiled when kept for a long time as it gets rancid due to oxidation.

Note:
Some other atmospheric chemical reactions include corrosion, when iron gets rusted due to the reaction with moisture and oxygen, it forms a red coating of iron oxide. Another example is black coating of silver items due to the formation of silver sulphide on them. The green coating of copper substances is also due to corrosion as formation of copper hydroxide takes place.