Fats are one of the most important ingredients in our bakes. They make light and airy cakes, moist muffins, flaky puff pastry, and fluffy bread. In this series of posts, we’ll dive deep into the roles of fat, including texture, flavor, cookie spread, and aeration. But before we explore fats’ interactions with other ingredients, we should first understand fats themselves.
Fats are lipids.
What bakers call “fats” belong to a broad category of molecules called lipids. These can be broadly defined as substances that do not dissolve in water. Along with proteins and carbohydrates like starch and sugar, lipids are one of the four broad classes of large molecules necessary for life. (The fourth are nucleic acids such as DNA.) In living organisms, lipids serve many functions. You can find them as energy reserves, cell membranes, signaling molecules, and more. Since our bodies cannot make all of these lipids, fats are an important part of a balanced diet.
In the kitchen, we use three kinds of lipids. Fats, such as butter, lard, and shortening, are solid at room temperature. Oils are liquid. Emulsifiers, including the lecithin in egg yolks and mono- or diglycerides in shortening, can be solid or liquid. They dissolve in both lipids and water, which makes them critical for combining water with oil and fat in our batters and doughs. We’ll talk more about emulsifiers in the next post.
For now, we’ll focus on fats and oils. They’re not always interchangeable in our recipes, but they do share a basic chemical structure that prevents them from dissolving in water. As we’ll see, small variations to this structure produce different types of fats and oils with a range of melting points, shelf lives, and impacts on our health. Because we often refer to all lipids as “fats” in baking, cooking, and nutrition, I will also use that terminology throughout the blog. If necessary, I will specify “solid fats” versus “liquid oils” or “oils.”
Fats are made from fatty acids.
Fatty acids
Fats and oils are made of fatty acids. One example, depicted in four different ways, is shown below. In living organisms, fatty acids are sources of energy. Molecularly, they’re chains of carbon atoms with a carboxylic acid (COO–) at the end. Each of the carbon atoms in the chain makes four bonds (represented by straight lines) to other carbons or to hydrogen. Think of each carbon atom as a person with four limbs. The carbons hold hands down the chain, and their two legs are each tied to a hydrogen.
Length and saturation
Fatty acids vary in a couple ways. First, as shown below, they are different lengths. A fatty acid can contain anywhere from four to thirty-six carbon atoms linking hands in a line. Second, the carbons in the fatty acid chain may form double bonds to each other. If we imagine our line of carbon people holding hands, a double bond occurs when two neighboring carbons link not just their arms, but their legs as well. They each lose a hydrogen to free up their legs, and the resulting fatty acid is unsaturated because it does not contain the maximum number of hydrogen atoms possible. In other words, if we unlinked the carbons’ legs, we could add more hydrogen onto an unsaturated fatty acid chain.
Saturated and unsaturated fats
If there are no double bonds along the chain, like in the figure above, the fatty acid is holding as many hydrogen atoms as possible. It’s saturated. Saturated fatty acids (what we call “saturated fats”) raise blood cholesterol and increase the risk of coronary heart disease, but unsaturated fatty acids (“unsaturated fats”) do not. Variations in fatty acid length and saturation have effects beyond health. For example, we’ll see that saturated fats tend to be solid, while unsaturated fats tends to be liquid. So oils are generally healthier than solid fats because of their lower saturated fat content. Although liquid oil cannot always be used in place of solid fat, we can adapt many recipes to use oil for more healthful baking.
Omega-3s
Another type of fat we often hear about are omega-3s. These are a specific type of polyunsaturated fatty acid (that is, a fatty acid with more than one double bond). Omega-3s, shown below, have their last double bond three carbons in from the end. (The last carbon is called the omega carbon after the last letter of the Greek alphabet.) More prevalent in our diets, but less well-known, are omega-6 fatty acids, which have their last double bond six carbons in from the end.
Foods contain both omega-3 and omega-6 fatty acids in different ratios. Nutritionists recommend that we eat no more than twice as much omega-6s as omega-3s to reduce the risk of cardiovascular disease, cancer, and inflammatory diseases. However, Western diets often contain a much higher ratio of omega-6s. Foods touted for their omega-3s, such as salmon and walnuts, simply contain a higher proportion of omega-3 fatty acids to omega-6 fatty acids.
Of the fats we use in the kitchen, canola oil has a low two-to-one ratio of omega-6s to omega-3s. Soybean oil (typically sold as “vegetable oil”) has a higher seven-to-one ratio. Neutral-flavored oils are typically interchangeable in baking, so to lower omega-6s, you might consider using canola oil where possible. Solid fats are mostly saturated fats, so they don’t contain many polyunsaturated omega-6s or omega-3s.
Triglycerides
In living organisms (and in our ingredients), fatty acid chains are packaged into triglycerides. As the prefix tri- suggests, they’re bundled into threes, held together by a molecule called glycerol (“-glyceride“). The entire structure is a triglyceride, shown below. The fats we bake with are mixed triglycerides, which means that each bundle contains three different fatty acids. And each type of fat is distinguishable by the types and proportions of fatty acids it contains. The fat in butter, for example, contains 68% saturated fat, 28% monounsaturated fat, and 4% polyunsaturated fat. Canola oil, on the other hand, only contains 7% saturated fat. Most of its fat is unsaturated, with 61% monounsaturated and 32% polyunsaturated.
Fats have different melting ranges.
These percentages affect properties like melting point. As we’ll discuss in a few posts, melting point is important for aeration, spread, and lamination in cakes, cookies, and puff pastry. Solid fats have high melting points, making them solid at room temperature, and they typically contain higher ratios of saturated fatty acids. Oils have low melting points, which makes them liquid at room temperature. They tend to contain more unsaturated fatty acids. (Notice this trend in the butter and canola oil percentages from above.) Why is this?
As we discussed in the context of sugar and water, solids form when molecules pack into a tight and organized crystal. If the molecules can’t pack together, they won’t solidify. This is why seawater doesn’t freeze at 32°F (0°C). The dissolved salt gets in between water molecules and prevents them from freezing into solid ice.
Saturation affects melting point.
If we consider the fatty acids we saw earlier, we can see that the shape of some fatty acids makes them easier to pack. Notice that all the saturated fatty acids are straight, with the carbon atoms standing in an orderly line. We can easily organize these saturated fats as shown below, like packing a truck full of rectangular boxes, so they crystallize and solidify. Butter, lard, shortening, and coconut oil all contain high percentages of saturated fats.
Oils, on the other hand, contain more unsaturated fats. Remember that unsaturated fatty acids are defined by double bonds, or carbons that have linked both an arm and a leg together. Because these carbons are so intricately connected, they’re angled differently than the single-bonded carbons. As a result, the double bond creates a kink in the fatty acid. Bent fatty acids cannot pack neatly into a crystal. It’d be like packing a truck full of irregularly shaped furniture. It’s hard to piece everything together efficiently. Thus, large amounts of unsaturated fats keep oils liquid at room temperature.
Length affects melting point.
Melting point is also determined by the length of a fatty acid. Since butter is solid at room temperature, for example, we know it contains a high proportion of saturated fatty acids. But why does butter soften before it actually melts?
The answer has to do with the variation in length of the saturated fatty acids. In butter, they range from long to short. The longer the fatty acids, the more surface area they have to stick together, the more tightly they’re held in their solid crystal. The shorter the fatty acids, the more easily they separate with heat. Thus, as butter warms, its shorter fats are already melting. The butter softens, but the stick maintains its shape because the longer fats remain solid. As the butter gets even warmer, the long fats finally melt, and the entire stick becomes liquid.
Trans fats
Another type of fat that’s worth mentioning, given their prevalence in the food industry and their impact on our health, are trans fats. Trans fats increase the risk of coronary heart disease because they increase bad cholesterol and decrease good cholesterol. They may also damage blood vessel walls.
So what are trans fats? “Trans” is a general term in chemistry used to refer to the placement of the atoms around a double bond. In naturally occurring unsaturated fatty acids, the hydrogen atoms are typically on the same side of the double bond, as shown on the right below. This is called the “cis” configuration. But the hydrogens could also be oriented on opposite sides of the double bond. This orientation, “trans,” occasionally occurs in nature. But the deleterious impact of trans fats on our health significantly increases with partially hydrogenated fats.
Partial hydrogenation
One method of processing fats, including shortening, margarine, lard, and sometimes oils, is called hydrogenation. It’s a chemical reaction that adds hydrogens to unsaturated fatty acids, essentially unlinking the legs of the double-bonded carbons and attaching hydrogens instead. As we’ve discussed, the more saturated the fat, the more solid it is. So manufacturers can use this method to adjust the consistency of the fat, making it more solid for creaming into cakes or laminating into croissants. But the more important purpose of hydrogenation is to increase the shelf life of the fat.
Unsaturated fatty acids have a shorter shelf life than saturated fatty acids because of their double bonds. Hydrogenation breaks those double bonds. But so does oxygen. Heat, light, or metal can all help oxygen break the double bonds in fatty acids. And while hydrogenation occurs under carefully controlled conditions that keep the fatty acid intact, oxygen breaks the fatty acid into two pieces. This process is called oxidative rancidity. The resulting fragments are actually what makes rancid oil smell and taste so bad.
In order to extend the shelf life of these unsaturated fats, manufacturers hydrogenate them to reduce the number of double bonds and the likelihood that the fatty acids break into rancid fragments. But if the fatty acids are fully saturated, the fat becomes too solid. The compromise is partial hydrogenation, shown below, where only some of the double bonds are removed. Unfortunately, in the remaining double bonds, hydrogenation switches the cis double bonds to the trans configuration. So although partially hydrogenated fats become more solid and stable, they contain more saturated and trans fats.
Alternatives to partial hydrogenation
Manufacturers have developed new processes that saturate fats without creating trans fats. Soybeans, for example, can be bred to contain fewer polyunsaturated fats, so their oil will also be more saturated. For solid fats, manufacturers can blend oils into fully hydrogenated fat. The resulting consistency is ideal for baking, and since the fat is fully hydrogenated, it’s completely saturated and there are no trans fatty acids. Manufacturers can also adjust the melting behavior of a fat with an enzyme called lipase, which detaches and reattaches fatty acids to different positions in a triglyceride.
Trans fats are banned in many places, but you can double-check the nutrition label for a food’s trans fat content. Since there is no specific need for partially hydrogenated fats in our baking, we can avoid them altogether and choose trans fat-free alternatives instead.
Conclusions
Our fats are made of fatty acids bundled into triglycerides. Variations in fatty acids and their proportions change properties of fats including melting behavior, stability, and their impact on our health. But we haven’t discussed the most important property of fats yet: their inability to dissolve in water. In the next post, we’ll focus on the chemical reasons behind this behavior. We’ll learn why water and fat don’t mix, why that’s a problem for our bakes, and how emulsifiers help us out.
References
Corriher, S. O. Bakewise; Scribner: New York, 2008.
Figoni, P. How Baking Works, 3rd ed.; John Wiley & Sons, Inc.: Hoboken, 2011.
Nelson, D. L.; Cox, M. M. Lehninger Principles of Biochemistry, 6th ed.; Freeman, W. H. & Company: New York, 2012.