One egg yolk, a squirt of white vinegar and lemon juice, half a teaspoon of dry mustard, a cup of vegetable oil and salt and pepper to taste. You toss all the ingredients in the food processor except the oil and whiz them till they’re mixed. Then you slowly drizzle in the oil with the machine on. You remember that the oil must be added extremely sparingly at first until it comes together, then poured in a small but steady stream. Voila! Suddenly, you have mayonnaise. Proud of your culinary accomplishment, you place the mayo in the fridge. That’s out of the way and now you can read the Sunday paper with your coffee till lunchtime.
Lunchtime arrives and you assemble the sandwich necessities. But when you retrieve the mayo, you notice something is wrong. Instead of a smooth, creamy consistency, there are little puddles of oil separated from the primary mass. The mayonnaise “broke” as we in the culinary world would say. What went wrong and can you fix it? Take out your notebooks class; Emulsions 101 is now in session.
Mayonnaise, milk, cream, butter, Hollandaise sauce, vinaigrettes, sauce Béarnaise, etc., are all emulsions. An emulsion is a blending of fat, usually oil or dairy fat, and a liquid, usually water based, whereby tiny, even microscopic droplets of the one are dispersed in the other. The tinier the droplets the creamier and more stable the emulsion. The problem with emulsions is their constituents are resistant to intermingling. Oil and water do not mix. Once brought together their chemical properties exert notable effort to separate and recombine into their component parts. But there are two types of ammo in this shotgun wedding to make the marriage work.
The first is agitation. If you combine oil and vinegar in a jar and shake it vigorously you’ll establish an emulsion. However, this fusion is temporary. Left to its own devices it will separate in no time. The oil will float to the top with the water below. However, there are substances, called emulsifiers, which will maintain the wedlock. The two most common emulsifiers are mustard and egg yolk. Mustard coats the oil droplets and inhibits them from recombining. But egg yolk is even better. Egg yolk contains lecithin and because of its molecular structure, it adheres to both the water and the oil, and places a nearly permanent barrier between them.
But nothing is perfect and a variety of factors can still land the antagonistic oil and water in divorce court. First, if the oil or fat is added too quickly at the beginning, it will inundate the water medium and the emulsion will not form properly. Secondly, too much agitation, (as in any relationship), will backfire and cause it to separate. Third, extremes of heat or cold, will dissect the union. (For the mayo, normal refrigerator temperature is OK but if it is too close to freezing, it’s splitsville). Finally, adding all of the vinegar/lemon juice at the onset will facilitate the emulsion, but at the expense of forming an unstable one, (since too much initially will prevent the oil droplets from becoming as tiny as they should). Thus, the compromise is adding a little at the start to help the emulsion form and then the remainder after all the oil is incorporated.
Therefore, our lazy Sunday cook’s mayo could have separated from 1) adding all the vinegar and lemon juice at the beginning, 2) over-agitating it in the food processor, (which is why traditional chefs favor whisking it by hand), or 3) placing it in a fridge that was too cold.
So then, how do we get the broken mayo partners to renew their vows? Simple, we send them to marriage counseling to work on the basis of their relationship, i.e., we start a new emulsion. Whisk an egg yolk with a pinch of dry mustard in a bowl. Then, slowly mix in the broken mayo, whisking constantly. If you use a whisk, as opposed to a food processor, your arm would fall off before you over-agitated it so you needn’t worry about that. In the culinary world, some marriages can be saved.