The mechanism of digestion of fats or lipids, and the mechanism of action to prevent food spoilage
Question: Explain the mechanism of decomposition (rancidity) of fats or lipids.
The decomposition of lipids or fats occurs through a free-radical chain reaction triggered by atmospheric oxygen, a process known as autoxidation. The mechanism consists of the following steps:
Step 1: Initiation
In the presence of light, heat, or metal catalysts, the C–H bond of the lipid molecule undergoes homolytic fission (equal splitting). This generates a lipid free radical and a perhydrohydroxyl free radical.
[Here, L• = Any unsaturated organic/fatty acid radical]
Step 2: Propagation
In this step, the highly reactive lipid free radical (L•) rapidly reacts with atmospheric oxygen to produce a peroxylipid free radical.
Step 3: Chain Propagation and Autocatalysis
The peroxylipid free radical (L-O-O•) attacks another molecule of unsaturated fatty acid from the food, generating an unstable hydroperoxide molecule and a new lipid free radical. Finally, the unstable hydroperoxide breaks down further, re-generating highly active radical species that accelerate the chain reaction.
Through this continuous free-radical chain reaction, fats and oily food items decompose. This leads to the formation of volatile compounds like aldehydes and ketones, resulting in black spots, discoloration, and a foul, rancid odor.
Question: Explain the mechanism of preventing food spoilage (Rancidity prevention).
Food spoilage mainly occurs via oxidation and subsequent free-radical chain reactions that cause bad smells. Therefore, if we can intercept the oxidation process and terminate the chain reaction, food spoilage can be successfully prevented.
To achieve this, chemical substances called antioxidants are used. Antioxidants prevent oxidation by reacting with the highly reactive free radicals generated from fatty acids, converting them into less reactive and highly stable radicals.
Because the new radical formed from the antioxidant is stable, it cannot propagate the reaction further, effectively terminating the free-radical chain reaction. This is how antioxidants inhibit the oxidation of food and keep it fresh.
[Here, A• = Less reactive and highly stable antioxidant radical]
