(i) Fats and oils when come into the contact of moist air at room temperature, undergo two types of reactions:
hydrolysis and oxidation.
These reactions make them rancid and they give odour due to the formation of small volatile fatty acid molecules by the breakage of ester bonds.
This can be prevented by keeping the fats and oils at low temperature in the refrigerator or by removing the air and moisture around them. Nitrogen is an inert gas, hence it acts as a antioxidant and prevents the oils and fats from oxidation. Thus, oil and fats are flushed with nitrogen to prevent their oxidation and stop odour. Food items stay fresher longer.
(ii) An olfactory indicator is a substance whose smell varies depending on whether it is mixed with an acidic or basic solution. Olfactory indicators can be used in the laboratory to test whether solution is a base or an acid, a process called olfactory titration. It works on the principle that when an acidic or base is added to it, its characteristic smell cannot be detected. All, vanilla, onion and clove, can be used as olfactory indicators.
Vanilla: The smell of vanilla vanishes with base, but its smell does not vanish with an acid.
Onion: Juice of onion loses its smell when added with base and it does not change its smell with acid.
Clove: In bases, its characteristic smell cannot be detected.
