What is the Difference between an Emulsion, Gravy, and a Sauce in Cooking?

The word emulsify is too often misused as a synonym for blend or thicken. It is not. Restaurant menus love to call any thick sauce an emulsion. It is not. Chefs like to say they’re emulsifying a sauce by using a roux . They’re not.

Many substances, including flour, cornstarch, gelatin, pectin, okra, egg, and even pureed banana, will thicken a soup, custard, jam, gravy, or sauce. But when you use them you are not emulsifying anything. An emulsion is a very specific kind of mixture of two liquids that don’t ordinarily mix, one suspended in the form of tiny droplets within the other.

The prototypical kitchen example of a true emulsion is mayonnaise, in which the mutual loathing between oil and water (the latter existing within the vinegar or lemon juice and egg white) has been overcome by two things: the brute force of being beaten together, and the action of a special chemical ingredient called an emulsifier. Only when this combination of physical and chemical powers is operating will oil and water mix and stay mixed in the form of a true emulsion.

As on that blind date we’ve all suffered through at one time or another, there are simply no attractive forces between a water molecule and an oil molecule. So even if you shake a bottle of vegetable oil and vinegar until they appear to have coalesced into a homogeneous whole, they will sooner or later, usually sooner, separate into two distinct layers. You will have failed to make a stable emulsion.

At most, you will have made a colloidal suspension, in which the oil has been broken down into such tiny micro-droplets or globules that they are kept suspended in the vinegar by the constant bombardment of water molecules from all directions. But this marriage is doomed to failure. No matter how much muscle power you put into shaking your vinaigrette dressing, even with the assistance of a governor of California, the oil globules will eventually bump into one another and reunite into a coherent, separate layer. Again, no permanent emulsion.

We can foil the reunification of the oil globules by adding a secret ingredient called an emulsifying agent or an emulsifier. Emulsifiers are made of snakelike molecules that have long, oil-loving (lipophilic) tails and water-loving (hydrophilic) heads. Their lipophilic tails burrow into the oil globules, leaving their hydrophilic heads sticking out like thousands of cloves studding a ham. The “cloves” attract a cloak of water molecules because they contain positive and negative charges that pull on the water molecules’ slightly negative and positive parts. (Water molecules are dipoles.) The resulting cloak of water molecules disguises the oil droplet as a water lover and prevents other oil droplets from attempting to unite with it. Because the emulsifier cloaks all the oil droplets in this way, they will not coalesce even if they bump into one another. They remain individually suspended. Now we have a true emulsion.

Where can we find those secret agents called emulsifiers? An excellent one is lecithin, a phosphorus-containing, fat-like chemical (a phospholipid) found in egg yolks. The phosphorus ends of its molecules are hydrophilic and their other ends are lipophilic. In mayonnaise, they emulsify the oil and vinegar into a permanently stable, homogeneous sauce.

Because we make mayonnaise from a small amount of vinegar or lemon juice (water) and a large amount, about eight times as much of oil, it may be hard to believe that all that oil has been crowded into that small amount of water. Many people are thus led to believe that mayonnaise is a suspension of tiny water droplets in oil, rather than a suspension of tiny oil droplets in water. But in fact there are so many oil globules that they aren’t actually suspended in the vinegar so much as coated with a very thin film of water, as in a bucket of wet peas, which tend to stick together because of the water’s surface tension. That’s why mayonnaise is so thick.

It is true, however, that a suspension of tiny water droplets in oil, the opposite of mayonnaise, is also classified as an emulsion. Butter and margarine, for example, are emulsions of water in oil.

When making mayonnaise with a whisk, we must dribble the oil into the acid-and-egg mixture very slowly to ensure that every added bit of oil is promptly reduced to colloidal-sized globules. If the oil drops are too big to remain suspended, they will coalesce while floating up to the top, where they will form a separate layer, defeating the entire purpose. After a while the oil can be added a little faster, because relatively large droplets will be quickly surrounded by millions of already emulsifier-coated colloidal globules that will “insulate” them from one another, keeping them apart temporarily until they themselves get whacked down to colloidal size.

When making mayonnaise in a blender, on the other hand, we can add a small amount of oil directly to the vinegar-and-egg mixture just before turning on the machine. The blender blades are much quicker than a whisk at chopping the oil globules down to colloidal size, so quick that the globules don’t have time to coalesce.

Commercial dressings and other foods contain a wide variety of emulsifiers to keep their complex mixtures of carbohydrates, fats, proteins, and water together in a stable form. Some of the emulsifiers that you may see on ingredient labels are mono and diglycerides, polyglycerol esters, propylene glycol esters, and sugar esters of fatty acids. And, of course, lecithin.

Foods are often thickened by substances such as gelatin, starches, and gums, including agar, acacia, xanthin, and carrageenan.

But thickeners aren’t emulsifiers. Thickeners work by making the watery part so viscous that even relatively large oil droplets can’t rise through it to coalesce into a layer.

Any old trick to keep a food homogeneous. Who wants to eat oily puddles mixed with globs of watery paste?