What makes a fat “saturated”?

Actually the most you can say of any particular fat is that it’s “high” in saturated fatty acids, since every fat, whether solid fat or oil, contains a mix of saturated and unsaturated fatty acid molecules. It’s the proportion of those fatty acids that’s important.

You can imagine a fatty acid molecule as a long, chain-like thing that resembles a line of pre-school teachers walking toddlers across a street (can you tell I have a 2-year old?). The teachers are carbon atoms and the kids are hydrogen atoms (a hydro-carbon chain). If every teacher in the line is holding two toddlers by the hand then that fatty acid is said to be saturated, full up with hydrogen atoms. I should add here that should you ever see a thing like this in real life, hurry up and enroll your kid in that pre-school, because the teacher-to-student ratio is unbeatable.

Anyway, if just one of the teachers in the line has a free hand, then the fatty acid is said to be mono-unsaturated. If more than one teacher has a free hand (or even two free hands) then that fatty acid chain is said to be poly-unsaturated.

The general rule of fats is that fats high in unsaturated fatty acids tend to be liquid at room temperature and the ones high in saturated fatty acids tend to be solid. The reason for that, because fully saturated hydrocarbon chains form straight lines. Chains that are missing one or more hydrogen atoms have all sorts of kinks and curves in them, and that’s a problem when it comes to crystal formation. Straight, uniform triglyceride molecules interlock well, like E-shaped LEGOs, and when they do so en masse you get crystals. The larger those crystals get the less the whole volume of fat flows and the firmer the fat gets. Triglyceride molecules with lots of crazy curves don’t interlock well and so remain separated and flowing. That fat is a liquid, which is to say, an oil.

But that’s not the only difference between saturated and unsaturated fats. Unsaturated fats, since they have molecular bonding sites — free hands — available, they can bond to other undesirable types of atoms (imagine unruly teenagers joining the line), altering or breaking the chain and leading to a rancid fat.

Thus you can see why food producers value fats that are high in saturated fatty acids. Not only are they firm at room temperature (which means they won’t soak into things or run out of a food before it’s eaten), they’re also highly durable (great for frying) and resistant to spoilage. All in all a very good deal for those who make foods, if a little hard on the waistlines of people who eat it.