Understanding emulsifiers won't just give you interesting facts to impress your friends with, it will also help you rescue a separated sauce and make a better Hollandaise.
Though about half water, the yolk of an egg contains a good amount of cholesterol. Like lecithin (an emollient derived from egg yolk), cholesterol is an emulsifier. Unlike lecithin, cholesterol stabilizes a water-in-oil emulsion, and destabilizes an oil-in-water emulsion, as in béarnaise sauce, for example. The properties of an emulsifier depend on its molecular structure.
It’s a general rule that whichever liquid a particular emulsifier is more soluble in, that liquid will constitute the continuous phase of the stable emulsion. The reason for this may be simply a matter of geometry. The egg yolk contains substances both favorable and unfavorable to the formation of emulsified sauces.
Adding cholesterol to a previously stable sauce can cause aggregation of the oil droplets, while lecithin could restore such separated sauces to smooth emulsions. Only the freshest eggs should be used in the preparation of emulsified sauces. Why? Because as an egg ages, its lecithin content slowly declines, while its cholesterol content remains unchanged.
The balance between the two, then, gets less favorable for oil-in-water emulsions as long as the egg is stored, and could conceivably reach the point at which such an emulsion would be impossible to maintain. Let a few fresh eggs from the same batch age for a week and you’ll discover that the old eggs will fail to emulsify the water and fat. When lecithin is beaten in, the sauce forms and stabilizes.
The most stable emulsion systems usually consist of blends of two or more emulsifiers, one portion having lipophilic tendencies (an attraction to fats), the other hydrophilic (an attraction to water). Only in relatively rare instances is a single emulsifier suitable.
There is general agreement that the ratio in eggs, which is said to be around 7 to 1, is actually very close to the point at which an oil-in-water emulsion will break and “invert” into a water-in-oil system. Hence the trickiness of emulsifying with egg yolks and the necessity of fresh eggs, so we really have no idea what are the optimum conditions for emulsified sauces. The question is probably pointless, since eggs, like soybeans, contain many potential emulsifiers, with cholesterol and lecithin only two of the more prominent.
The job of emulsification in mayonnaise, and almost surely in béarnaise as well, is accomplished by a whole range of substances, from individual (and invisible) molecules of cholesterol, lecithin and other emulsifiers, to those comparatively massive molecular aggregations that do show up in the microscope.
For the cook, it comes down to a couple of strategies for avoiding total failure. The irreversible curdling of egg proteins, which accompanies overheating, can be minimized and even avoided by using enough vinegar and white wine to keep the yolk liquid mixture distinctly acidic. So don’t substitute water for vinegar, and minimize the amount of egg white that accompanies the yolk into the pan. Second, successful incorporation of the butter oil as droplets depends on the natural emulsifiers contained in the yolk. Use the freshest eggs possible to avoid the deterioration of some of these substances. And if success is more important than complete fidelity to tradition, keep a small container of soy phosphatides, a.k.a. “lecithin,” hidden in the cabinet. A quick shake into a stubborn sauce may salvage it.
Rescuing a separated sauce
The rescue, or re-emulsification, of a broken mayonnaise is also applicable to the butter-based sauces, though the latter must be carefully heated to keep the butter melted. Once again, the problem is to get the oil droplets in a portion of the sauce down to the size and up to the concentration that they will almost automatically “mill” the remaining oil down. So a small amount of the sauce, including some of the yolk and vinegar phase, is put in a bowl with a little fresh yolk or water (and in the case of mayonnaise, some mustard; the seed particles are also excellent emulsifiers). These ingredients are beaten thoroughly to produce the initial emulsion, and then the rest of the sauce is very slowly dribbled in as the beating continues. All that this “rescue” amounts to is a repetition of the initial procedure, the only difference being that the water and oil phases are now partly mixed even before the beating begins.
An opaque, yellow, lemon-flavored sauce, made from egg yolks and butter. A reduction is often used to add flavor. Hollandaise is classified in the warm emulsion sauce category; it is not usually considered a grand sauce because it is made for use as required and does not have long holding capabilities because of its high egg yolk content.
Reduction: Used to add flavor
Egg yolks: Used to emulsify the butter
Butter: Adds flavor and consistency
Lemon: Acidity cuts fat taste of butter