Soy Lecithin: Why Is It In Everything?
Have you ever noticed soy lecithin on the ingredient statement of your pre-packaged food? It sometimes seems like it's in everything! It makes you wonder--how could one little additive be so pervasive? Well, because it's really useful.
What does it do?
First and foremost, it's used as an emulsifier, which means it makes oil and water mix together, which they ordinarily would never do. That's why you often see it in creamy salad dressings, mayonnaise, reduced-fat buttery spreads and other foods that have a hefty portion of oil.
It even helps to emulsify foods you've probably never thought of as emulsions (oil & water mixtures)--like chocolate. In chocolate, lecithin keeps the cocoa butter (fat) portion happy and stable, so it doesn't separate from the moisture, cocoa solids and dairy (in the case of milk chocolate) that make up the rest of the candy. Without lecithin, chocolate wouldn't be as velvety and smooth as we like it. And it would be more prone to fat bloom, the leading cause of gross-looking grayish streaky chocolate (still safe to eat though). A lot more foods than you realize benefit from lecithin's emulsifying powers (like Nature Valley peanut butter granola bars and flavored tea bags!).
But lecithin does more than just emulsify! In addition to that, it helps stabilize emulsions, which extends shelf life. It also reduces stickiness and is often used as a "releasing agent," which is integral to the effectiveness of non-stick cooking spray.
When added to bread dough, its anti-stickiness power makes doughs easier to work with, and they rise better because there's less sticky resistance for the gas bubbles to strain against. It's sometimes used as an anti-foaming agent during processing, but amazingly will cause a foam to form if whipped into a water-only (little or no fat) mixture! This is actually a common technique used by chefs who practice molecular gastronomy, and it's how I made this beautiful Caprese salad with basil foam:
Basil Foam: 1 cup water, 1 cup fresh basil, 2 g soy lecithin powder. Whip with immersion blender until foamy. Garnish with basil leaf.
Lecithin is also a surfactant, or wetting agent. Surfactants reduce the surface tension of liquids, which allows them to spread out faster and be absorbed quicker. Lecithin is often added to cake and other baking mixes so that water stirs in more easily, with fewer stubborn lumps in the batter. Due to its rich choline content, lecithin is also sold as a dietary supplement. Choline has been shown to reduce bad cholesterol, a major risk factor for heart disease.
How does it work?
Lecithin's versatility is remarkable, but how can one little molecule do so much? To answer that, we need to take a closer look at it. Let's start by looking at a related molecule: triglycerides, the most common form of fat in foods and in our bodies. Triglycerides consist of a glycerol backbone with three fatty acids attached. These fatty acids are oil soluble, meaning they'll dissolve in oil, but not water. The structure of lecithin is very similar, but instead of the third fatty acid, it has a phosphate group, which is water soluble, meaning it will dissolve in water, but not oil. Molecules like lecithin are referred to as phospholipids (phospho = phosphate, lipid = fat), and they are able to dissolve happily into mixtures that contain both water and oil.
In fact, their favorite place to be in oil-water mixtures is right at the border between the oil and the water, so they can poke their fatty acid tails into oil, while keeping their phosphate groups in water. Phospholipids like lecithin emulsify oil in water by forming little protective envelopes around tiny droplets of oil, as shown in the diagram below. The water-loving phosphate groups act as a sort of camouflage, allowing oil droplets that would normally never remain in water to stay for a long time. That's what keeps mayonnaise and salad dressings from separating into separate oil and water phases. The same principle applies to water-in-oil emulsions, where oil is the continuous (main) phase, and the water is dispersed in droplets throughout.