HOW TO MAKE A LIASON
Liason is a mixture of egg yolks and cream, which is used to add just a slight thickness to a sauce or a dish. Since the egg yolks have their own special properties, they also add some body and flavor to the finished dish, as well as some yellow golden color. The texture conveyed by a liason is therefore smooth and creamy, with a nice shine as a bi-product. However, it's not always the result that many people get when they use the liason, and that's because the egg yolk's properties are easily destroyed by improper handling. The big point of importance here is to mix the egg yolks into the mixture at a proper rate and temperature, and we will describe how to do it soon. For now, let's consider some basic facts behind the process of cooking with liason.
The problem described above correlates with the fact that egg yolks coagulate at lower temperatures than the hot sauce or dish is normally cooked at. In normal cases, an egg yolk will coagulate at 65 C/149 F. However, if we add cream to it and mix these two ingredients, the coagulation temperature will raise by about 20 C to 85 C/185 F (it can be slightly lower than that). This is of course fortunate, since we want more heat-tolerant egg yolks to use in a hot liquid. But that's as far as we can go with a liason - 85 C. However, this is not entirely bad, as long as we keep in mind one very important fact: we need to add the hot liquid TO the liason, not the other way round. In addition, we need to add it slowly while continuously stirring, simply to avoid a dramatic heat shock to the egg yolks - the heat should be distributed quickly throughout the entire mixture so that it all cools down a bit. This is known as tempering. Once everything is blended together, the dish or sauce is put back on the stove (low heat) and gently warmed up during continuous stirring, just until it slightly thickens. It is important now to not heat it up too much, not above 85 C/185 F, or the egg yolks will curdle!
A basic formula for making a liason is 1 part egg yolk to 3 parts cream. In terms of simpler calculations, take one egg yolk for each 80 ml/2.7 fl oz of cream. Mix these two ingredients evenly and completely. Then, start adding the hot liquid to the liason, while continuously whipping. You are now tempering the liason. Add the hot liquid in small portions or the egg yolks will curdle! When you've added enough of the liquid, so that the liason has reached a hot temperature, you can start to adding it to the hot dish (not above 85 C/185 F though!); do it slowly and mix continuously. Stop when the dish/sauce is thickened and serve immediately. For more on proportions between liason and rest of the dish just follow the recipe.