Some fats are very resistant to fatty acid peroxidation, whereas others are vulnerable. These reactions play an important role in our health, and can be affected greatly by the temperature at which the fatty acids are. Thus, cooking with the correct fats can help to lower your intake of damaged fats.
Here I will explain the chemistry and implications of it on health.
Lipid Peroxidation 101
To begin with, fatty acids are composed of two parts:
- A hydrocarbon tail, which is hydrophobic, aka fat soluble (butane used as an example below). This chain can be anywhere from 4 to 28 carbons long and naturally occurring fatty acids only contain tails with an even number of carbons.
- A carboxylic acid group, which is hydrophilic, aka water soluble. (shown below) *The "R" group simply means any carbon chain.
|Carboxylic acid group|
Put together, these two above examples make butyric acid, a fatty acid commonly found in butter.
Carbon atoms will form 4 bonds usually, and as it pertains to this discussion, they always do.
Fatty acids can be broken into two main groups:
- Saturated: Every carbon in the hydrocarbon tail has as many hydrogens bonded to it as possible(aka the carbons are 'saturated' with hydrogens), meaning there are no carbon-carbon double bonds.
- Unsaturated: There are one or more carbon-carbon double bonds(C=C), resulting in some carbons that are 'unsaturated', in that they don't have as many hydrogens on them as could be possible. In nutrition, this category is often divided further into monounsaturated fatty acids (MUFAs, containing only one C=C bond) and polyunsaturated fatty acids (PUFAs, containing two or more C=C bonds)
Double bonds change a few things about fatty acids, including the structure(which affects melting point) and the rate of lipid peroxidation.
Stearic Acid, a saturated fat commonly found in animal products such as beef. Note the straight nature of the hydrocarbon chain; this lends itself to fitting together well with other fatty acid molecules, so that they can pack densely together, which results in a higher melting point. Saturated fats are usually solid at room temperature.
Oleic acid (the main MUFA in olive oil), an 18-carbon MUFA is shown below. It has one C=C bond (notice the C=C bonded carbons only have 1 hydrogen attached to each of them, rather than 2 as is normal on saturated carbons)
Also, notice how the C=C bond makes a kink in the chain, making this type of fatty acid less straight than saturated fatty acids. This kink is responsible for the difference in the melting point between things like butter (mostly saturated) and vegetable oil (mostly unsaturated). The more C=C bonds a fatty acid has, the more kinks in the chain and therefore the molecules cannot layer together as well. This reduces the inter(between)molecular forces, and lowers the melting point.
Carbon-carbon single bonds can rotate freely around the bond axis, but carbon-carbon double bonds are locked in position (the double bond has to be broken in order for it to rotate).
Here is an example of a polyunsaturated fatty acid, Docosahexaenoic acid (aka DHA), a major component of fish oil, with 6 double bonds. Its structure doesn't allow for the molecules to pack tightly, and therefore it has a lower melting point than more saturated fatty acids of the same length(increasing length raises melting point due to increased Van der Waals forces).
|Docosahexaenoic Acid (DHA)|
Now that we know how double bonds affect structure, what do they have to do with cooking?
It turns out, that the hydrogens on the carbon adjacent to the C=C bond, referred to as allylic hydrogens, are highly susceptible to reacting by free radical mechanisms. When a hydrogen is taken, there is left a lone electron (this is what we call a radical) on the carbon where the hydrogen was. Because the free radical formed next to a C=C bond is surprisingly stable; it doesn’t take that as much energy to cause the formation of a free radical as it would without the presence of a double bond.
The double bond can shift between two resonance states (essentially states of electron distribution), and stabilize the fatty acid radical by spreading out the distribution of the free radical over two carbons instead of one. Essentially, there is only a half free radical on each carbon that shares the radical, and this sharing stabilizes the radical, lowering the energy needed to achieve this state.
By lowering the energy needed to reach the radical state, the reaction occurs at a much higher rate.
This is where cooking comes in. It is accepted that in general, the rate of a reaction in chemistry about doubles for every 10 degree Celsius increase in temperature. This is because the rate of collisions between molecules about doubles for each 10 degree increase in temperature.
In cooking, the increased heat causes a rapid increase in the formation of fatty acid free radicals by the process shown below. The lipid radical continues to react (propagation) with other lipids until either two radical react (highly unlikely unless radicals are present in very high concentrations) or it reacts with an antioxidant.
The consumption of lipids that have undergone peroxidation will result in the absorption of these compounds. These compounds continue to react with fatty acids in your body, causing major damage to cell membranes, hormones, cholesterol, and more.
The production of free radicals increases oxidative stress on the body, which is known to play a key role in the progression many chronic inflammatory conditions, including diabetes, cancer, heart disease, obesity, etc. Oxidative stress increases inflammation in the body ,(inflammation increases oxidative stress as well), which also reduces insulin sensitivity,,, a key marker in diabetes.
- Saturated fats do not undergo lipid peroxidation at anywhere near the rates that unsaturated fats do.
- You do not want to eat fats that have undergone lipid peroxidation
- Cooking increases the rate of lipid peroxidation
For a practical guide on what fats to choose, check out FAQs: What Are Safe Cooking Fats & Oils?
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3. Oxidative stress and diseases - Wikipedia
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