the separation of an emulsion such as milk or mayonnaise. Cooks normally use the word when something has gone wrong, but the deliberate curdling of milk is part of the normal process of making cheese and other milk products. The term is also used vaguely for various misfortunes that produce a lumpy texture. Other words used in this context are flocculation, which means the formation of any kind of lumpy or fluffy masses and is understood here to mean a lumpiness that can be reversed; and coagulation, which means an irreversible hardening—if a sauce coagulates, the only thing to do is throw it away and start again. The term coagulation is normally used only of protein.
Foods that curdle in the true sense include all the emulsified sauces made with egg and oil; some cake batters; and milk, cream, and yoghurt, as well as any sauces, soups, or stews to which these are added—although in these cases separation may occur for more than one reason.
An emulsion is a mixture of two liquids in which one liquid forms tiny droplets suspended in the other. In cookery one liquid is generally an oil of some kind, the other water or some watery liquid such as vinegar. When the mixture curdles, the droplets run together into larger drops. If the mixture is left to stand, the two liquids may separate completely so that one floats as a layer on the other. This is a familiar event in vinaigrette dressing, which is an unstable, temporary emulsion. When separated it can be remixed simply by shaking hard in a jar with a lid.
Milk and mayonnaise are stable emulsions—at least over a time scale of hours or days. The droplets are maintained by an emulsifier, a substance which forms a protective boundary layer around them. Emulsifiers are sensitive to changes in their environment, and if the temperature or acidity of the mixture changes they may stop working. In this case more than shaking or stirring is required to re-form the emulsion.
Mayonnaise may curdle if the oil is added to the egg yolk faster than the two can be beaten together, so that there is no time for the physical work of beating to break the oil into small enough droplets. It may also separate if the ingredients are not all at the same temperature, or if an ingredient such as vinegar is added without adequate stirring, so that it forms regions of excessive acidity. Even when mayonnaise has been successfully made it may separate if put in the refrigerator, which makes the oil begin to solidify.
In all these cases the emulsion may be repaired by stirring it into another egg yolk. This must be done as gradually as when making the original mixture. If no more eggs are available, sometimes the mayonnaise can be reunited by stirring it into a spoonful of water, but, since there is much less emulsifier around when this method is used, it does not always work.
Milk does not consist only of fat and water; it also contains protein molecules suspended in the water. It can separate in two ways. In fresh, unhomogenized milk the droplets of fat float to the surface, forming a layer of ‘top of milk’. The milk can be remixed by shaking it. But when milk separates into semi-solid curds floating in clear whey it cannot be remixed. In this case not only the fat but some of the protein has separated, and the protein has begun to coagulate into a solid that cannot be liquefied again. Similarly, when milk is boiled the heat makes protein coagulate into a skin on the surface, which cannot be stirred back into the milk. The technical term for an irreversible change in protein is ‘denaturing’. This change can also be brought about by adding acid or alkali to milk.
Many hot dishes, especially sauces, soups, and stews, call for cream or yoghurt to finish. In some cases, most frequently when brought back to the boil, the milk protein will coagulate, producing a granular texture which cannot be rectified. Yoghurt and light or single cream are the most susceptible, and will be the more subject to the vice if there is acid (wine, say, or tomato) in the dish being cooked. Heavy or double cream, or crème fraîche, are much less unstable and will merrily boil to even thicker reductions.
Yoghurt and sour cream are less stable than fresh cream because of their acidity. However, yoghurt that is to be used in cooking can be prevented from curdling by first stirring in cornflour or egg white while it is slowly brought to the boil. These act as physical binders to thicken the mixture. It should also be noted that sheep's or goat's milk yoghurt, or strained yoghurt often called ‘Greek’, are more stable than plain yoghurt.
In this context, lumpiness in a white sauce needs a word of explanation. White sauce is partly an emulsion, because it is made with milk, so it can curdle like any other mixture made with milk. But it is also a ‘sol’, a dispersion of starch such as flour or cornflour in liquid, and it is this other aspect of the sauce which usually causes trouble. The major cause of lumpiness here and precautions to be taken are explained under starch.
Ralph Hancock is an encyclopedist with a special interest in food history and food science.
