Fats and Water Don’t Mix: An Introduction to Polarity

In the last post, we reviewed the basic chemical structure of fats and oils. They’re chains of carbon atoms called fatty acids bundled into triglycerides. Our ingredients contain unique ratios of fatty acids with varying lengths and saturations, and as a result, they have different melting points, stabilities, and effects on our health. However, in baking, the primary functions of all fats stem from their tendency to separate from water. In this post, we’ll explore why fat and water don’t mix, and then we’ll discuss how we can blend them together for homogenous batters.

Fats do not mix with water.

Fat and water do not mix. We see this when we whisk oil and vinegar into a vinaigrette, when lotion forms a barrier between our skin and the dry air, and when oil droplets dot the surface of soup. One word scientists use to describe fats (and other molecules that don’t dissolve in water) is hydrophobic: water-fearing. But they could just as well use the word lipophilic: fat-loving. Why, if a compound can’t dissolve in water, does it dissolve in fat? We’ve discussed this in the context of proteins and sugar, but because hydrophobicity is crucial to fats and their behavior, let’s review what it means to dissolve (or not) in water.

Water is polar.

Let’s start by taking a closer look at water. As shown below, water is a molecule of H2O: two atoms of hydrogen bonded to one molecule of oxygen. Think of the atoms like kids who are fighting for toys. In the case of a water molecule, the kids are not equal in strength. (Scientists call this strength electronegativity.) Oxygen is much stronger than the two hydrogens, and it will hold onto the toys for most of time. The hydrogens might wrest one away every so often, but oxygen wins it back.

The toys represent electrons, which are negatively charged particles. Because of its greater strength, the oxygen atom carries the electrons’ negative charge most of the time, and it develops a partial negative charge. The hydrogens are, for the most part, left devoid of electrons, so they develop partial positive charges. A molecule with an unequal electron distribution is polar, and it always contains electrical charges.

Water molecules (left) contain an oxygen atom (O) bonded to two hydrogen atoms (H). The oxygen holds the electrons (e) and acquires a partial negative (δ) charge. The hydrogens acquire a partial positive (δ+) charge. In ice (right), water molecules form a hydrogen-bonded lattice.

Just as the north and south poles on two magnets attract, positive and negative electric charges attract. The partial negative charge on a water molecule’s oxygen attracts positive charges. The partial positive charge on the hydrogens attracts negative charges. These attractions create hydrogen bonds. In water and ice, for example, the H2O molecules form hydrogen bonds to each other. But by definition, any polar molecule shares electrons unequally and carries electric charges that can form hydrogen bonds to water molecules. And if they form these bonds, they dissolve.

Polar molecules dissolve in water.

Let’s review some ingredients that dissolve in water. Sugar, as we’ve discussed, has several atoms available to form hydrogen bonds to water. They’re the ones hanging off the rings in the hydroxyl group (–OH) shown below. Ethanol, the compound that makes beverages alcoholic, contains a hydroxyl group as well, so our beer and vodka never separate into separate layers of water and alcohol. Same goes for acetic acid, which is diluted to make vinegar. We’ve also discussed the behavior of hydrophilic amino acids in proteins, which have partially charged atoms that form hydrogen bonds to water.

Other compounds that dissolve in water are ionic. They’re polar molecules taken to the extreme—the two atom kids are so unequal in strength that one of them takes the electron away from the other. The resulting ions carry electrical charge since one atom has an extra electron and the other is missing one. Table salt, for example, is called sodium chloride (NaCl) because it’s made of positive sodium ions (Na+) and negative chlorine ions (Cl). Similarly, baking soda (sodium bicarbonate) contains positive sodium ions and negative bicarbonate ions. The charged ions attract the polar water molecules, so salts dissolve in water as well.

Molecular structures of some compounds that dissolve in water. Red letters denote positively charged atoms; blue letters denote negatively charged atoms.
Baking soda dissolves in water. The positive sodium ion (Na+) attracts the negatively charged oxygen atoms in water, and the negative bicarbonate ion (HCO3) attracts positive hydrogen atoms.

Fats are nonpolar.

What about fats, then? If we look at a triglyceride, we can see that the only bonds are carbons to hydrogens, carbons to carbons, or carbons to oxygens. Carbon is pretty similar to hydrogen in terms of strength, so they share electron toys equally. Carbon is weaker than oxygen, but the oxygens are boxed in, so they can’t bully the rest of the atoms. Thus, triglycerides are nonpolar. The atoms share their electrons fairly equally, so they all remain neutrally charged.

Triglycerides are nonpolar. Electrons are equally shared, and there are no electrical charges.

These neutral charges don’t attract the partial charges of polar molecules like water. Putting them together would be like trying to stick a magnet onto a wooden surface. It won’t work because the wood isn’t magnetic. So because fats don’t attract the partial charges in water, fat and water don’t mix. Instead, uncharged nonpolar molecules hang out together, separate from the charged polar molecules.

Imagine a pile of magnets and wood. Even if we tried to mix them, the magnets would all stick to each other, and the wood would remain separate. This is why oil and water form two distinct layers even after we whisk them together. The polar water molecules, like the magnets, stick together, leaving the nonpolar oil molecules to form their own layer. As we’ll see in future posts, the separation of fat and water is crucial in laminated dough, where fat or oil separate layers of dough that ultimately form flakes. The lack of attraction among the oil molecules also explains why fat gets so much hotter than water, and why fats are so slippery.

In a mixture of fat and water, water molecules form hydrogen bonds to each other (right). Fat molecules are nonpolar (left), so they repel water.

Like dissolves like.

So if a substance doesn’t dissolve in water, it’s probably nonpolar. It doesn’t carry any charges. But that’s exactly the type of substance that dissolves in fats. In general, nonpolar compounds do not dissolve in water, but they do dissolve in fats. As my 10th-grade chemistry teacher taught us, “like dissolves like.” Polar water dissolves polar compounds because they’re all charged, and nonpolar fats dissolve nonpolar compounds because they’re not charged. It is important to note, however, that polarity exists on a spectrum. So while this applies to most mixtures we create in the kitchen, it’s not always so clear-cut with compounds we might find in a chemistry laboratory.

Emulsifiers combine water and fat

In fact, some substances are both polar and nonpolar. One portion of the molecule is electrically charged and forms hydrogen bonds with water, and another portion does not. In the kitchen, we call these compounds emulsifiers. We can find them in egg yolks, dairy products, and shortening. They’re crucial for blending watery liquids, like milk, water, and egg whites, with fatty ingredients, like butter and oil. They also hold air. This is important in most batters and doughs, where we want all of our ingredients to be mixed homogenously.

Emulsifiers come in all sorts of shapes and sizes, but let’s take a closer look at one type that may look and sound familiar: mono- and diglycerides, shown below. These are the same as our triglyceride fats, except they’re missing one or two fatty acids. Our bodies remove fatty acids from triglycerides for energy using an enzyme called lipase, and fatty acids also separate with heat. Notice how this exposes part of the glycerol. Now there’s a hydroxyl (–OH) that sticks out, just like the ones in water, sugar, ethanol, and acetic acid. It carries electrical charges and forms hydrogen bonds to water. But the fatty acid chain(s) are still nonpolar, so they repel water.

Mono- and diglycerides are emulsifiers. They contain polar portions (–OH) and nonpolar portions (fatty acid chains).

So mono- and diglycerides have a polar portion (the exposed glycerol) and a nonpolar portion (the fatty acids). When the emulsifier is in a mixture of both oil and water, the molecules orient themselves so that each portion interacts with the right liquid. The nonpolar fatty acids face the nonpolar oil, and the polar glycerol faces the polar water. As you can see below, emulsifiers surround droplets of oil and dissolve them in water. They do the same thing with air. The fatty acids stay away from the water and stick into the air bubble, while the glycerol interacts with water.

Emulsifiers surround oil or air in water by orienting their nonpolar portions away from water and their polar portions toward water.

Soap is an emulsifier, too. When soap encounters grease, it positions its nonpolar, hydrophobic portion toward the fat and packages it into little droplets. When we rinse with water, the soap dissolves because of its polar, hydrophilic portion, and both the soap and the fat droplets wash away.

Emulsifiers in the kitchen

Most of our recipes contain watery liquids, fatty ingredients, and air. Emulsifiers are crucial for combining all three and keeping them blended throughout the mixing and baking processes. If you crack an egg into a creamed butter-and-sugar mixture while making cookies or cake, or into cream cheese while making cheesecake, you may notice that it takes a minute to incorporate the egg. In fact, recipes sometimes add eggs one at a time to ensure they get mixed evenly. This is because the watery whites don’t mix well with the fatty butter or cream cheese. It’s only when the yolk breaks and releases its emulsifiers that the egg white blends smoothly into the fat. Emulsifiers also have other effects in our bakes. which we’ll discuss more in the upcoming posts.

Conclusions

The defining characteristic of a lipid, its inability to dissolve in water, is a result of its nonpolar chemical structure. This is beneficial when it comes to frying oil and flaky pastry, but it’s undesirable for cake batter. Some compounds, called emulsifiers, have both a polar and a nonpolar portion, so they dissolve in both water and fat. Emulsifiers help us blend watery and fatty ingredients in our recipes. In the next post, we’ll start to take a look at how fats and emulsifiers interact with other ingredients to create delicious baked goods.



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.

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