Rancidification is the process of complete or incomplete oxidation or hydrolysis of fats and oils when exposed to air, light, or moisture or by bacterial action, resulting in unpleasant taste and odor. Specifically, it is the hydrolysis or autoxidation of fats into short-chain aldehydes and ketones, which are objectionable in taste and odor. When these processes occur in food, undesirable odors and flavors can result.

1. Chemical reactions &
Rancidity of Fats

2. Saponification
• Saponification is a process that involves the conversion of fat, oil, or
lipid, into soap and alcohol by the action of heat in the presence of
aqueous alkali (e.g. NaOH).
• Soaps are salts of fatty acids.

3. Rancidity
• It refers to the spoilage of a food in such a way that it becomes undesirable (and
usually unsafe) for consumption.
• When people say that a food has "gone bad," what they're usually talking about
is rancidity.
• It is the process of complete or incomplete oxidation or hydrolysis of fats and oils,
when exposed to air or light or moisture or enzymes, resulting in unpleasant taste
and odor.

4. Oil Reversion
• It is the breakdown (oxidation) of an essential fatty acid linolenic acid, found in
certain vegetable oils, leading to an undesirable flavor change prior to the start of
actual rancidity.
• Therefore, before the actual decay of the oil one can "taste" the difference and predict
that the oil may run bad in future.
• A reversed oil/fat once detected can help save ourselves from consuming a potentially
bad fat/oil.
• That's because a rancid oil can have damaging health effects such as cancer, heart
disease etc.
• Example is reversion of Soybean oil. Soybean oil has a triglyceride composition rich
in monounsaturated (23% oleic acid) and polyunsaturated fatty acids (57% linoleic
acid; 7% linolenic acid).
• The linolenic acid contains three unsaturated bond and which can be easily undergone
oxidation process. Because of the presence of high content of linolenic and linolenic
acid, oil undergoes oxidative degradation and develops color change and fishy or
painty smell. This process is called oil reversion.

5. Mechanism of Oil Reversion
• Oxidation is the primary mechanism involved in the oil reversion.
• When it comes to oxidation, linolenic acid is about ten times more
vulnerable than linoleic acid and about one hundred times more than
oleic acid.
• The oxidation of double bonds is a radical-driven process.

6. Radical reactions typically have three steps
1. The initiation reaction, where an energy source (heat; light) generates
a radical on the fatty acid.
2. A propagation step with oxygen giving rise to peroxides, which react
with more unsaturated fatty acid, creating new radicals.
3. A termination reaction, where two radicals interact forming a new
single bond.

7. Types of Rancidity
1. Oxidative Rancidity
• The specific types of rancidity involving oxygen damage to foods are called
“oxidative rancidity.”
• During the process, oxygen molecules interact with the structure of the oil
and damage its natural structure in a way that can change its odor, its taste,
and its safety for consumption,
• i.e. fat is oxidized and decomposes into compounds with shorter carbon
chains such as fatty acids, aldehydes, and ketones all of which are volatile
and contribute to the unpleasant odor of rancid fats.
• Oxidative rancidity leads to the formation of both unpalatable and toxic
compounds.

8. Types of Rancidity
2. Hydrolytic Rancidity:
• Fats are enzymatically hydrolyzed, the release of free fatty acids from
glycerides, cause some rancid odor. This process is called hydroiytic
rancidity.
• Hydrolysis will split fatty acid chains away from the glycerol
backbone in glycerides.
• These free fatty acids can then undergo further auto-oxidation leads to
oxidative rancidity.

9. Factors Affecting Rancidity and Reversion
1. Oxidation:
• Primary cause of rancidity.
• Oxidation primarily occurs with unsaturated fats by a free radical-
mediated process.
• These chemical processes can generate highly reactive molecules in
rancid foods and oils, which are responsible for producing unpleasant
and noxious odors and flavors.
• This process is called auto-oxidation or oxidative rancidity.

10. 2. Hydrolysis:
• Triglycerides react with water under appropriate condition to form
diglycerides and free fatty acid residues.
• Diglycerides later combine with water to form monoglycerides and
fatty acids.
• Finally the monoglycerides completely hydrolysed to form glycerol
and fatty acids.
• This process is called hydrolytic rancidity.

11. 3. Presence of Microorganisms – Microbial Lipase:
• Certain microorganisms can produce the hydrolytic enzyme called
lipase, which directly interferes the hydrolysis of triglycerides and
produce glycerol and fatty acids.
• These fatty acids undergo auto- oxidation to form rancid.
• The microbial lipase requires suitable pH and other conditions for its
activity upon fats and oil.

12. 4. Presence of Unsaturation in Fatty Acid Chain:
• When a fatty substance is exposed to air, its unsaturated components
are converted into hydroperoxides, which break down into volatile
aldehydes, esters, alcohols, ketones, and hydrocarbons, some of which
have disagreeable odours.
• Butter becomes rancid and liberates volatile and foul-smelling acids,
particularly butyric acid.

13. 5. Polyunsaturation:
• The more polyunsaturated a fat is, the faster it will go rancid.
Vegetable oils have to become several times more rancid than animal
fats.
• Presence of polyunsaturation in oils and fats makes them more
susceptible to rancidity than monosaturated and other types of
saturated fatty acids.

14. 6. Chemical Structure of Oils and Fats:
• If oils and fats are chemically more complex and consists more
number of double bond, more number of carboxyl or hydroxyl groups,
then the chances of become rancid is high.
• The double bonds found in fats and oils play a role in auto-oxidation.
Oils with a high degree of unsaturation are most susceptible to auto-
oxidation.

15. 7. Temperature and pH:
• These are the important factor which influences the food items rich in
fat and oils become rancid.
• Suitable temperature and alkaline pH are required for the hydrolytic
action of microbial lipase.
• Temperature and pH indirectly influence the auto-oxidation and
hydrolysis.

16. 8. Heat and Light:
• Presence of heat and light accelerate the rate of reaction of fats with
oxygen, i.e., heat accelerates auto-oxidation.
• Heat and light act as the energy source for the production of free
radical in rancidity and reversion of oils and fats.

17. Prevention of Rancidity
1. Addition of Antioxidants:
Antioxidants retard oxidation primarily by either hydrogen or electron
donation. Natural antioxidants include flavonoids, polyphenols, ascorbic acid
(vitamin C) and tocopherols (vitamin E).
2. Addition of Sequestering Agents:
Sequestering agents bind metals, thus preventing them from catalyzing auto-
oxidation. Examples of sequestering agents include EDTA (ethylene diamine
tetra acetic acid) and citric acid.
3. Proper Storage of Fats and Oil Food:
Another method for preventing rancidity of food is the proper storage,
keeping away from the action of oxygen. Rancidification can be decreased by
storing fats and oils in a cool, dark place with little exposure to oxygen or free
radicals, since heat and light accelerate the rate of reaction of fats with
oxygen.