Have you ever had a custard turn into wet scrambled eggs, or a cheese sauce separate into a greasy, grainy mess? In both cases, the problem is protein that has been overcooked. Understanding the behavior of proteins during cooking is one of the most fundamental skills for success in the kitchen.
What are proteins? They are long molecular chains, made up of chemical groups called amino acids, containing atoms of carbon, oxygen, hydrogen and nitrogen. Generally, they are corkscrew shaped and may bend over onto themselves to form loops. Imagine something like a bunch of long coils with little magnets along their sides. Initially, the coils (proteins) are individual units, gently folded over onto themselves so that the magnets (chemical bonds) hold them in ‘U’ or ‘S’ or other shapes.
As proteins are heated, these bonds open up and the proteins straighten out, or ‘denature’. Since the proteins are in a solution, such as the milk in the custard, they are free to move around. As they do, the bonds will reconnect, this time with near by proteins rather than with themselves. The proteins then make a web of connections and the liquid thickens or coagulates. As it thickens, the protein web is able to trap the water and other ingredients and hold them in suspension.
But, if you overcook the proteins, then the bonds between and within the proteins tighten up. The proteins pull closer together and start to squeeze out the water. In the custard, the eggs scramble. In the cheese sauce, the milk proteins in the cheese curdle and force out the liquid and fat.
The most critical part of making successful sauces and custards is controlling the heat in such a way as to prevent curdling. Protein molecules will start to denature and begin to bond with each other somewhere around 120°F (50°C). This process will continue up until about 180°F (82°C), at which point the proteins will become overcooked and the dish will curdle. These temperatures will vary depending on what other ingredients are used. Acid denatures or “cooks” proteins at low temperatures, for example when fish or other seafood is marinated in lime juice to make ceviche.
The presence of starch, fat, salt and other ingredients can also affect the temperature at which curdling happens. Stirred custard made with cornstarch or flour will thicken at a much higher temperature and might even come to a gentle boil without harm. The starch gets in the way of the protein bonding and helps to prevent curdling. This is also why high fat dairy products, like heavy cream, are used in pan sauces. The fat in the cream helps to prevent curdling. With low fat dairy products in pan sauces, you also need to use a starch.
Cooks will frequently tire of waiting for a custard to thicken and will raise the heat under the pan. If you do this, you run the risk of shooting right past the temperature where the sauce or custard is thickened and overcooking it, so it is best to be patient. For delicate sauces and custards, a double boiler can help moderate the temperature to avoid problems. Once the sauce or custard is completely cooked, it is a good idea to transfer it to another dish to prevent the heat remaining in the pan from causing a disaster.
Patience, attention and technique are the keys to making smooth sauces and custards containing eggs or dairy products.