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Theory:

Effect of an oxidation reaction in real life.
  
Rancidity:
The oxidation of oils and fats present in food material resulting in bad smell and taste is called rancidity.
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Rancid cheesecakes
 
When unsaturated (oil/fat) components of food materials are exposed to sunlight, they can be changed into hydroperoxides, which split down into esters, volatile aldehydes, ketones, alcohols, and hydrocarbons (some of which have unpleasant odours).
 
Oil becomes rancid (rancid oil) due to the decomposition of fats, or sometimes milk becomes rancid due to not heating it in the humid atmosphere.
 
It is also possible to prevent rancidity through the use of some preventive measures.
 
Rancidity is classified into three types. These are
  • Hydrolytic Rancidity
  • Oxidative Rancidity
  • Microbial Rancidity
Hydrolytic rancidity is also known as hydrolysis/enzymatic oxidation. This reaction takes place more rapidly in the presence of an enzyme (lipase), heat, and moisture, leading to the formation of glycerol and free fatty acids.
Oxidative rancidity, also known as autoxidation, happens when oxygen is absorbed from the environment. Most oils and fats will break down and degrade in the presence of oxygen or ultraviolet (UV) radiation, forming various other compounds.
Microbial rancidity is a process in which micro-organisms such as bacteria, fungi and yeast use their enzymes to break down chemical structures in the oil or fat to produce unwanted smells and tastes.
Have you ever tasted or smelled fat/oil containing foods left for a long time?
 
Rancidity can occur in a variety of places in our daily life. For example, when butter is left out in an open atmosphere, the smell and taste changes, resulting in rancidity. Rancidity includes vitamins in foods that are degraded, resulting in a diminish in nutritional value.
 
Methods to prevent rancidity:
  
Oils and fats become rancid and lose their smell and taste when they oxidize. Antioxidants (BHT, BHA, vitamin E, and vitamin C) are typically added to foods containing fats and oils to prevent oxidation. We can slow food oxidation by storing it in airtight containers. Do you know that chips manufacturers usually flush bags of chips with gas like nitrogen to prevent the chips from getting oxidized!