Rancidity is the deterioration of fats and oils due to oxidation or hydrolysis, resulting in an unpleasant smell and taste. When fats are exposed to air (oxygen), light, moisture, or heat, they undergo chemical changes that produce aldehydes, ketones, and short-chain fatty acids — compounds responsible for the offensive odour of rancid food. Rancidity is prevented by antioxidants, nitrogen packaging, refrigeration, and airtight storage.
Rancidity is the deterioration of fats/oils by oxidation or hydrolysis, producing unpleasant smell.
Oxidative rancidity: free radical chain reaction of unsaturated fat C=C bonds with O₂.
Hydrolytic rancidity: ester bonds in fats split by water (lipases), releasing free fatty acids.
Butyric acid (from rancid butter) and aldehydes are the main malodour compounds.
Antioxidants (BHA, BHT, Vitamin E) prevent oxidative rancidity by scavenging free radicals.
Nitrogen flushing (MAP packaging) removes O₂ from packaging to prevent rancidity.
Unsaturated fats (polyunsaturated > monounsaturated) are more susceptible to rancidity.
Heat, light, metal ions (Cu²⁺, Fe²⁺), and moisture all accelerate rancidity.
Rancidity occurs by two main mechanisms:
Oxidative rancidity is a free radical chain reaction involving three stages:
Initiation (slow): R-CH=CH-R' + O₂ → R-CH•-CH-R' (free radical formed by H abstraction)
Propagation (fast, self-perpetuating): R• + O₂ → ROO• (peroxy radical) ROO• + RH → ROOH + R• (new radical formed) ROOH → RO• + •OH (hydroperoxide decomposition)
Termination: ROO• + ROO• → stable products (ROOR + O₂) R• + R• → R-R (stable dimer)
Final products:
The peroxide value (PV) measures the degree of oxidation in a fat sample. Acceptable PV < 10 meq O₂/kg fat.
Methods to prevent or slow rancidity:
Antioxidants (most effective for oxidative rancidity):
Nitrogen flushing / Modified Atmosphere Packaging (MAP):
Refrigeration: Slows all chemical reactions (lower temperature)
Airtight packaging: Prevents contact with O₂ and moisture
Metal chelating agents (EDTA): Bind pro-oxidant metal ions (Cu²⁺, Fe²⁺)
Hydrogenation of vegetable oils: Reduces C=C bonds, decreasing unsaturation (e.g., margarine from vegetable oil) — also prevents rancidity
Addition of salt: Reduces water activity in some foods
Common examples:
Health implications:
Rancidity is the chemical deterioration of fats and oils that produces a bad smell and taste. It occurs through oxidative rancidity (oxygen attacks C=C double bonds in unsaturated fats, forming aldehydes and ketones) or hydrolytic rancidity (water and lipases break ester bonds, releasing free fatty acids like butyric acid).
Oxidative rancidity is caused by atmospheric oxygen reacting with the double bonds in unsaturated fatty acids in a free radical chain reaction. It is accelerated by heat, light, metal ions (Cu²⁺, Fe²⁺), and is more rapid in polyunsaturated fats.
Rancidity is prevented by: (1) antioxidants like BHA, BHT, and Vitamin E, which scavenge free radicals; (2) nitrogen flushing of packaging to remove oxygen; (3) refrigeration; (4) airtight packaging; (5) adding chelating agents like EDTA to bind pro-oxidant metals.
Oxidative rancidity: caused by O₂ reacting with unsaturated fats via a free radical mechanism, producing aldehydes, ketones, and acids. Hydrolytic rancidity: caused by water (with lipase enzymes or acid/alkali) hydrolyzing ester bonds in triglycerides, releasing free fatty acids (especially butyric acid in butter).
Antioxidants are compounds that interrupt the free radical chain reaction of oxidative rancidity. BHA, BHT, and Vitamin E donate a hydrogen atom to peroxy radicals (ROO• + AH → ROOH + A•), forming stable, non-reactive radicals. This terminates the chain reaction and prevents further fat oxidation.
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