a blend of two liquids where one forms tiny droplets which are evenly dispersed in the other. It is not strictly a mixture, because the two liquids do not actually mix. The technical term for combinations of this kind is a colloid. The blend may be stable, although in practice—and especially in cookery—emulsions often separate.
Common emulsions used in cookery are milk, cream, and butter, and made sauces such as mayonnaise. Butter is not usually thought of as a liquid, but it is a genuine emulsion. To understand why, it is necessary to examine the structure of these substances in some detail.
In emulsions found in foodstuffs the two liquids are usually an oil or fat of some kind, and water or a watery liquid. There are two ways the liquids can combine. The oil may form droplets suspended in water, or the water may form droplets suspended in oil. Technically, the liquid which forms droplets is known as the disperse(d) phase, and the liquid in which these drops are scattered is known as the dispersion medium. Milk is an obvious example of an oil-in-water emulsion, with a small amount of semi-solid butterfat dispersed in a larger amount of water. (It also contains proteins which are dispersed in the water.) Cream is similar but with much more oil; very thick cream contains almost 50% fat.
There is a limit to how much oil an oil-in-water emulsion will hold. Thick cream is close to this limit; it is therefore not very stable. When cream is churned to make butter, the agitation breaks up the water into droplets. Quite suddenly—if you are using a hand churn you can feel it happening—the droplets of butterfat merge together; the result is a blend in which droplets of water are dispersed in continuous butterfat. The disperse phase and the dispersion medium have changed places, a process known as phase inversion. The new emulsion cannot hold as much water as the original cream did. The extra water separates out as buttermilk. Butter is about 80% fat.
When an emulsion is formed (or, as above, reformed), an essential part of the process is mechanical work. One has to stir hard to make mayonnaise. Milk as it comes from the cow is not a totally stable emulsion; the butterfat slowly rises to the surface to form ‘top of milk’. It can be made completely stable (at least, until bacteria make it ‘go off’) by putting more work into it—by the process of homogenization, in which it is forced through tiny holes which break the fat droplets into smaller droplets which are held more firmly in place and do not float upwards.
What holds the droplets in place in an emulsion? Oil and water do not mix because they are fundamentally different substances, not only in their obvious characteristics but also on a molecular scale. The molecules of water are ‘polar’: they have an unequal distribution of electric charge, one side of the molecule being positively charged, the other negatively. Many other substances also have polar molecules with unbalanced charges. Oil molecules, in contrast, are not polar; their charges are evenly balanced.
A positively charged object attracts a negatively charged one and vice versa, but two objects with similar charges repel each other. When two polar molecules are brought near each other, they tend to align themselves so that the positive side of one is facing the negative side of the other. Non-polar molecules such as those of oil do not feel these forces, but they are slightly attracted to each other by ‘van der Waals forces’, the very weak forces which attract all molecules to each other (they are named after J. D. van der Waals (1837–1923), a Dutch physicist). Polar molecules also feel van der Waals forces, but these are easily outweighed by the much stronger forces stemming from their electric charge.
If an attempt is made to blend water and oil, no matter how hard they are stirred together to mix them, the polar water molecules will tend to move together because of their electrical charges, and the non-polar oil molecules will tend to move together because of van der Waals forces. The two liquids will therefore separate.
Something else is needed to hold an emulsion together; this is known as an emulsifier. It is a substance whose molecules have one polar end and one non-polar end. The polar end is strongly attracted to a water molecule; the non-polar end is weakly attracted to an oil molecule. These molecules can form a layer at the boundary of a region of oil and a region of water, with their polar ends facing the water and their non-polar ends facing the oil. If, in a blend of oil and water, a droplet of oil is completely surrounded by a layer of emulsifier in this way, it will be able to stay suspended in the water and will not merge with other oil droplets.
Emulsifiers belong to a group of substances called ‘surfactants’, short for surface active agents. The best-known surfactant is soap, whose molecules surround particles of greasy dirt and hold them suspended in water, so that they float away out of the article being washed. Many foodstuffs display surface action to some extent and may be used as emulsifiers. Egg yolk contains a powerful emulsifier, lecithin, and a less strong one, cholesterol. The grains of some finely powdered ingredients also have this property, including mustard powder—which is why, when making mayonnaise, mustard is added to the egg yolk before mixing in the oil.
The food industry uses many other emulsifying substances, usually derived from natural foodstuffs. These include alginates, made from seaweed; gums and pectin extracted from various plants; and various modified kinds of cellulose. Their purpose is not only to bind emulsions but also to retain water in food, so that it does not go stale quickly. In foods sold in the EU emulsifiers are given E numbers beginning with 3 or 4.
The making of mayonnaise also demonstrates another fact about the formation of emulsions, the ‘seeding’ effect. As soon as a small amount of emulsion has formed, the remainder of the ingredients will blend much more easily. At first the oil is added drop by drop, but once the emulsion has started to take one can trickle it in in a thin stream, as long as one keeps stirring hard.
Because emulsions depend on the attraction between polar molecules, they can fail to form or break down if their watery component contains dissolved substances that carry a lot of electric charge. For example, acids provide large amounts of positively charged hydrogen atoms. Adding vinegar to mayonnaise can make it curdle, unless it is dribbled in gradually while the mixture is stirred so that no regions of high acidity can form. alkalis (such as bicarbonate of soda) and excessive amounts of salt can also destroy emulsions. The breakdown of emulsions, and what to do about it, are described under curdling.
Ralph Hancock is an encyclopedist with a special interest in food history and food science.
