Cooking Fats 101: What's Up With Saturated and Unsaturated Fats?

Getting to know your fats can be a slippery business. If wading through the myriad bottles on supermarket shelves wasn't already a daunting task, then the latest word on saturated fats is enough to turn everything on its head. In this series, we'll be talking about what makes fats special, how to tell them apart, and how to pick the best one for the job.

Last week, we categorized fats in terms of smoke point. Today, we're taking a different approach, and delving into the magical world of molecules. The big question:

What's up with saturated and unsaturated fats?

On a molecular level, all fats are composed of triglycerides—a compound of three fatty acids bound to a molecule of glycerol.

Wait, sorry, did your eyes just glaze over? Yeah, mine too. Let's start over.

Meet Bob. Bob is a fatty acid.

Like you, me, and this ridiculously cute puppy, Bob is made up of all kinds of smaller stuck-together sciencey bits. In Bob's case, those bits are essentially a long chain of carbon molecules all linked together, with a couple of oxygen molecules tacked onto the end. Each of his carbon molecules has the ability to link to up to two hydrogen atoms. And depending on how he goes about it, you'll get three different types of Bob:

1. Saturated: Every carbon atom along the chain is linked to two hydrogen atoms .
2. Monounsaturated: Two adjacent carbon atoms are missing a hydrogen atom and instead form a double bond with each other.
3. Polyunsaturated: Two or more sets of carbon atoms form double bonds with each other.

Here's the thing: every time a fatty acid has one of these double bonds in it, the long, normally straight chain gets a little bend in it. Bob gets a little kinky, if you will, and the more double bonds he has, the kinkier he gets. Why do we care about Bob's, erm, proclivities? Time to talk cooking fats.

Let's take another stab at that first sentence: on a molecular level, all fats are a compound of three fatty acids Bobs bound to a molecule of glycerol (a.k.a. a sugar alcohol compound). When we talk about cooking fats being saturated or unsaturated, we're actually talking about the proportion of saturated to unsaturated molecules. Most natural oils and fats contain different types of fats—canola and light olive oils have relatively low proportions of saturated fat molecules, so we call them "unsaturated." Butter and vegetable shortening, on the other hand, have a very high proportion of saturated fat molecules to unsaturated fat molecules, so we refer to them as "highly saturated fats."

Saturated Bob Fats

So, we know saturated fats are packed to the gills with hydrogen atoms (single-bond Bobs). Though most saturated fats are animal-derived (think butter, lard, and tallow), you'll also spot highly saturated vegetable fats, like coconut and palm oils. They're also easy to recognize, because they're solid at room temperature. Why?

It all boils down to our Bobs: those straight, saturated fat molecules are easier to stack into a stable structure, while bendy, kinky unsaturated fats tumble and fall. Imagine playing two games of Jenga; one with your average wooden blocks, and another with a bucket of plastic soldiers. This is useful because you can actually tell just how saturated a fat is by noting how firm and cloudy it gets at room temperature. That's why low-saturation fats—collectively classified as oils—are completely liquid; moderately saturated fats like chicken or duck fats are solids; lard runs nice and firm; and the most highly saturated ruminant animal fats, like beef or sheep, are as hard as candle wax.

Because their molecular structure is relatively strong, saturated products are often referred to as stable fats, meaning that they can be stored for longer periods of time and are less likely to oxidize or degrade when introduced to other ingredients when you're cooking. That said, especially when using them in higher volumes, as for deep-frying, keep in mind that they'll solidify again as they cool, leaving you with a waxy coating in your mouth.

I've heard that saturated fats are bad for your heart and should be avoided as much as possible. Is this true?

For decades, saturated fats have been associated with the irreversible build-up of LDL, or "bad" cholesterol, leading to a widespread belief that eating a diet high in saturated fat leads to health complications including serious heart and circulatory system diseases. Now...researchers and medical professionals aren't so sure, and it turns out that saturated fats may not be quite as scary as we all once thought. Perhaps not even particularly scary at all.

Of course, that's today, and I'm no doctor; speak to yours if you have serious questions on this matter.

Unsaturated Fats

Unsaturated fats feature one or more double bonds between the carbon atoms in the oil's fatty acids. This type of bond is particularly loose and exhibits a tendency to recombine with other compounds, such as water or metals. The more double bonds present in the molecular structure of an unsaturated oil, the more easily that oil will degrade or go rancid. In that sense, oils with a higher proportion of monounsaturated fats are more stable—and thus more reusable—than those packed with polyunsaturated molecules.

Monounsaturated oils include most of your flavorful, lower smoke point products, such as virgin and extra-virgin olive oil, that are unsuitable for high-temperature cooking. Meanwhile, a number of polyunsaturated vegetable oils, such as safflower, may have higher smoke points, but they also oxidize more rapidly and deteriorate relatively easily. When it comes to stocking your pantry, it's advisable to look for blended oils—corn, soy, peanut, and vegetable oils, for instance —that have a more even distribution of mono- and polyunsaturated fats. They'll have consistently higher smoke points and longer shelf lives than their brethren.

How does saturation affect baking?

When it comes to baking, fats wear a few different hats, and, crucially, there a few jobs that only your solid fats can do.

Aerates: Saturated fats can be manipulated to contain a lot of air, (usually by beating or creaming), lightening frostings and batters.
Flavors: Hopefully this one's pretty self-explanatory.
Shortens and tenderizes: Fats can interfere with the development of gluten—the network of proteins that is created when flour and water are combined and kneaded into dough. Without the addition of fats, gluten will form a long, rubbery matrix, which is why bakers refer to adding fat to the mix as "shortening." Solid fats, like vegetable shortening or lard, can actually be cut into flour for more flaky, tender results; an option that liquid fats simply can't offer.

Speaking of shortening, what is it?

Both shortening and margarine are partially hydrogenated vegetable fats: vegetable oils that have been chemically treated to essentially make saturated fats out of unsaturated ones by restructuring the formation of bonds in the fatty acid chain. One side effect of that industrial process is the creation of trans fats.

And trans fats are...?

In natural fat molecules, or cis fats, missing hydrogen atoms will be absent from the same sides of the chain. In a trans fat, the carbon chain is basically all twisted up, so that those missing slots wind up on opposite sides. Check it out:

Like saturated fats, trans fats have long been believed detrimental to cardiovascular health and the build-up of LDL cholesterol. Unlike saturated fats...nobody's arguing otherwise.

More Cooking Fats 101

• What's a Smoke Point and Why Does it Matter?
• What Makes Fats and Oils Essential to Cooking?