If you’re anything like me, when you go shopping for cooking oil you pick whatever’s cheapest on the shelf. Olive oil, vegetable oil, it’s all just going in the pan to cook eggs in anyway. (I can hear my foodie friends crying now.) However, not all oils are created equal. We’ve all heard about the horrors of trans-fats, but what is a trans fat? Are there cis fats? What about just fat fats?

Because saturated fats have more hydrogen bonds, they have more energy… which is why your dietician told you to cut back on the butter

There are four types of oils: saturated, polyunsaturated, monounsaturated, and trans fats. Fatty acids are long carbon chains with a carboxyl group, characterized by one carbon double bonded to an oxygen and singly bonded to an -OH group, on one end.

“Saturated” comes from the type of carbon chain that the fat has: if it only has single bonds, it is saturated (each carbon is “saturated” with its maximum number of bonds). Examples of saturated fats are butter and coconut oil. Because saturated fats have more hydrogen bonds, they have more energy. (A carbon double bond has more energy than a carbon single bond, however, to have a double bond, you lose two carbon-hydrogen bonds. The net energy loss from the missing C-H bonds is greater than the energy you gain from having a double bond, so the unsaturated carbon chain has less energy overall.) Because they have more energy, they are also higher in calories, which is why your dietician told you to cut back on the butter.

If the carbon chain has any double bonds, it is “unsaturated.” Unsaturated fats can have one or more double bonds. Monounsaturated fats (those with one double bond) include oil olive, while polyunsaturated fats (more than one double bond) include safflower oil and canola oil.

Unsaturated fats are also more likely to be liquids: the double bonds make “kinks” in the carbon chain. Because of these kinks, the acyl groups can’t pack together as well, and a result of this is that they melt at lower temperatures, thus being liquid at room temperature.

How do trans fats fit into this? A trans fat is a type of unsaturated fat: the name “trans” comes from the fact that the carbons on a double bond are diagonal to each other (trans) instead of next to each other (cis). Most natural unsaturated fats, such as those we discussed above, have a cis-configuration. Trans fats are also known as “partially hydrogenated oils,” because they are industrially made by adding hydrogen to vegetable oils, which are polyunsaturated. By adding hydrogen to these unsaturated vegetable oils, industrial chemists hoped to make a vegetable oil more solid (one example of which is crisco) at room temperature and less vulnerable to oxidation by saturating it with hydrogens. However, when this process is not done to completion, some of the cis double bonds turn into trans bonds instead.

Scientists have observed that consuming fats with at least one trans-double bond increases blood cholesterol. But why? The jury’s still out on this one: some research suggests that trans-fats competitively inhibit (see here) the metabolism of other types of fats, which could contribute to heart disease. The search for understanding the metabolic effects of trans-fats on the body, however, is still ongoing.

If your oil starts smoking, take it off the heat instead of pushing forward with cooking

An oil is oxidized when it reacts with the oxygen in the air. Double bonds in unsaturated fats can be cleaved by the oxygen, producing potentially toxic compounds like ketones and aldehydes. The practical effect of this is that the oil becomes rancid, and it’s no longer good. This is why Europeans or fancy hotels leave butter (a saturated fat) out at room temperature for days at a time, but if you leave a bottle of peanut oil out open in the sun, it might quickly go bad. Some oils, like olive oil, have anti-oxidants, either naturally or industrially added as a preservative. Anti-oxidants prevent oxidation by being oxidized instead of the fatty acid, so your bottle of EVOO will last longer than other unsaturated fats (See research data here).

So, now that you understand the difference between different oils, why is it important to use different oils for different things while cooking?

Because they have different chemical structures, oils respond differently to heat. As a general rule, oils with a longer fatty acid chains (like butter) stand up better to heat, while those with shorter chains (like olive oil) have lower smoke points. The smoke point is simply the point at which the oil starts to degrade and release volatile products in the form of smoke. The oil breaks down into glycerol and free fatty acids, and the glycerol breaks down further into toxic compounds, including acrolein and various aldehydes. Acrolein in particular is carcinogenic in large quantities and contributes to the burnt-food-taste. A 2010 study from the journal Food Chemistry suggested that the production of these compounds was directly related with time and heat: so, if your oil starts smoking, take it off the heat instead of pushing forward with cooking: your body and your taste buds will appreciate it. Check out our graphic above for hints about which oils to use when!