One of the fundamental skills for home cooks and professional chefs alike is preparing foods ahead of time whenever possible. Some foods can be kept hot until you need them, while others must be chilled and then reheated. Time plays a part in that decision. For example if you've made a batch of rich, cheesy Mornay sauce, it can stay hot briefly but not overnight.
Making Mornay Sauce
Mornay sauce is one of many that are derived from a base, or "mother" sauce, called Bechamel. Bechamel is the fundamental savory white sauce, made by flavoring hot milk with a hint of onion and nutmeg, then thickening it with a roux of flour and butter. The basic mixture is transformed into Mornay sauce by adding shredded Gruyere and Parmesan, two intensely flavorful cheeses. A small amount of hot cream can be added for richness or to thin the sauce, then it's finished by whisking in a small amount of butter.
The Problem of Cheese
The cheeses add rich flavors to the sauce, and also help thicken it as their proteins are diluted and spread throughout the liquid. However, that presents a problem for the cook. The cheese proteins are very sensitive to high temperatures, which is why most Mornay recipes instruct cooks to let the Bechamel cool for a few moments before the cheese is added. If the sauce becomes too hot after the cheese is added, the cheese proteins will contract and shrink together, forming tough, grainy curds in the sauce.
This makes holding a Mornay sauce problematic over anything more than a short period. It must be kept at a temperature above 140 degrees Fahrenheit to remain food safe, because at lower temperatures bacteria would flourish in the sauce. Yet at temperatures beginning around 155 F to 160 F, the proteins in the sauce will contract and cause it to "break." That leaves a very narrow temperature range for holding the sauce, within the margin of error for most restaurant equipment and definitely problematic for a home cook armed only with a double boiler.
There are better options for advance preparation. One is to prepare the sauce ahead of time, then heat it very gently over a double boiler before your meal is ready. It requires slow heating and frequent stirring, so unless you have help it can put a crimp in your meal plans. A better alternative is to prepare a basic Bechamel sauce ahead of time, warming it in a saucepan or your microwave oven. Once it's fully heated you can convert it to Mornay sauce at the last minute by stirring in the shredded cheese, cream and butter.
- On Cooking: A Textbook of Culinary Fundamentals; Sarah Labensky, et al.
- On Food and Cooking: The Science and Lore of the Kitchen; Harold McGee
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