Why are some fats solid and some liquid? Liquid fats are kinkier.

We encounter different types of fat when cooking a meal. Butter, lard, vegetable oil, olive oil, fats in meats, fats are everywhere. They are also versatile  sculpture media: butter art is a common fixture at state fairs.

But what is it exactly that makes butter solid? And why is vegetable oil liquid at the same temperature? They are both essentially the same thing: fats. What makes them different? The answer has to do with their chemistry.

Fats are long strings of carbon and hydrogen

Fats, like all the other foods that we eat, are just chemicals. Yummy, greasy, chemicals, but they obey the same laws of chemistry as everything else in the world.

Fats (or lipids, as they are called by nutritionists) are mostly made up of a chemical group called hydrocarbons. Its hard to think about, but the fat that you eat is made up of tiny little strings of carbon and hydrogen. The picture below is of a typical hydrocarbon: the black is carbon, and the white is hydrogen.

This is a picture of a typical saturated hydrocarbon.

What you should notice about this picture is that there is hydrogen is all over the carbons. There is no where else that you could stick a hydrogen on this molecule. Chemists would say that this is saturated with hydrogens. It turns out that, this is why the fat is a straight stick. The carbon can’t bend because the hydrogen gets in its way.

Unsaturated fats have less hydrogen, so they bend

Some organisms (especially plants) like to make lipids without all the hydrogens in them. These lipids are called unsaturated fats because they are not saturated with hydrogen. Because of this, the carbon chain bends, putting a kink or two in the shape.

This fat molecule has some kinks in it. The loss of hydrogens puts double bonds in the carbon. Double bonds have a different shape.

This is what you see in the fat content on the back of food nutrition labels: Saturated fats are straight. Mono-unsaturated fats have one kink, and poly-unsaturated fats have more kinks to them.

To be a solid, you have to pack tightly together

To think about what happens when you try to turn something into a solid, lets use the example of freezing liquid water. When water is warm (above 32 degrees Fahrenheit) water is a liquid. If you could zoom in really really close, you would see that the water molecules are zooming past each other with lots of space.

When the water starts getting colder, the molecules start moving slower. They eventually get so slow, that they begin to pack together. With water, when you get to 32 degrees, the water packs together so tightly that it becomes solid.

This is what happens to water when it freezes.

Every chemical has a different temperature that it decides to pack together into a solid. The harder it is to pack together, the colder it needs to be before it turns solid.

Kinky lipids are tougher to pack

Try to imagine packing together the lipids we saw above. If you were playing a game of tetris, which would ones would you want to work with?

The straight saturated lipids are a lot easier to pack together than the kinky unsaturated lipids. As a result, you need to bring the unsaturated lipids down much colder before they will become solid. For some lipids, this is the difference between being solid at room temperature (like lard) and being liquid (like vegetable oil).

You can see this every time you make bacon

Think of the last time you fried up a pan of some salty, crispy bacon.

You end up with a ton of liquid grease in the bottom of the pan. But this only lasts when it is hot. If you leave the grease in the pan to cool, it will turn solid. Sadly, this means that your bacon grease has lots of the unhealthy (but tasty) saturated fat. But it doesn’t stop me from sucking down half a pound without thinking (oops).

( yoda photo credit: Sweet One; tetris photo credit: k.steudel; bacon photo credit: nessguide)