Do multiple double bonds in seed oil molecule lead to formation of many free radicals when we prepare food using heat? Could this effect be seen by adding a bit of iodine to a heated sample versus not heated?

 

 

Do multiple double bonds in seed oil molecule lead to formation of many free radicals when we prepare food using heat? Could this effect be seen by adding a bit of iodine to a heated sample versus not heated?

 

You want to know, why? Let’s try the explanation and the test.

Multiple double bonds in seed oil molecules lead to the formation of many free radicals during high-heat cooking. This effect can be indirectly observed with an iodine test, as the degree of unsaturation (double bonds) decreases in the heated oil.

How heat creates free radicals in seed oils

 

    Free-radical chain reaction: The process is known as lipid peroxidation and involves three steps: initiation, propagation, and termination.

        Initiation: High heat, along with light and oxygen, provides the energy to break a hydrogen atom from a carbon next to a double bond in a polyunsaturated fatty acid (PUFA). This creates a highly unstable lipid radical.

        Propagation: The lipid radical quickly reacts with an oxygen molecule to form a lipid peroxyl radical. This peroxyl radical then takes a hydrogen atom from another PUFA molecule, creating a new lipid radical and a hydroperoxide. This forms a destructive chain reaction.

        Termination: The chain reaction ends when two radicals react with each other or when the oxidizable oil is consumed.

    More double bonds, more radicals: PUFAs, common in many seed oils, are more susceptible to oxidation and free-radical formation because they have multiple double bonds. The hydrogen atoms on the carbons between the double bonds are particularly vulnerable to abstraction.

    Toxic byproducts: The breakdown of the PUFA chains creates new compounds, such as volatile aldehydes and ketones, which are responsible for the "off" flavors of rancid oil.

 

Using iodine to observe the effect of heating

The iodine value (or iodine number) test is a chemical method used to measure the amount of unsaturation in a fat or oil.

The test works as follows:

 

    Iodine reacts with double bonds: Iodine is added to an oil sample. The iodine reacts with and breaks the carbon-carbon double bonds, effectively adding itself across the bond until all double bonds have reacted.

    Color change: The number of iodine molecules consumed is directly proportional to the number of double bonds present. The disappearance of the iodine's brown color as it reacts with the double bonds is used to indicate the end point of the reaction.

 

Observing the effect of heating:

You could perform a simple experiment to observe the effect of heating on the unsaturation of seed oil:

 

    Prepare samples: Create two samples of the same seed oil.

    Heat one sample: Heat one sample of oil to a high temperature for a sustained period, such as frying food.

    Perform iodine test: Add drops of iodine to both the unheated and heated oil samples.

    Observe results: You would expect the heated sample to require fewer drops of iodine to produce a color change compared to the unheated sample. This is because the heat-induced formation of free radicals has already broken some of the double bonds in the oil, so there are fewer left to react with the iodine. The decrease in iodine value directly correlates with the amount of free-radical damage caused by heating.