A watery or separated sauce is one of the few things that can ruin macaroni and cheese. The cheese sauce is susceptible to this problem at any stage of the cooking process, from the initial stove top preparation to the baking stage. It may even occur hours later, after you store the leftovers in the refrigerator. Knowing what causes the sauce to become watery empowers you to prevent the problem from occurring. In most cases, you can overcome watery mac and cheese with a straightforward solution.
There Is Too Little Fat
It’s best to use dairy products that contain at least some fat, because fat helps bind the sauce and prevent separation. While it’s possible to make cheese sauce with a reduced-fat product, such as skim milk or margarine, it is better to use whole milk or half-and-half, full-fat cheese and real butter. There really isn’t a solution for cheese sauce that is inconsistent due to lack of fat. It is best to start again with ingredients that are more likely to yield success.
It Needs Flour or Reduction
If the cheese sauce is watery -- or too thin -- it’s likely that there is just not enough flour. Add one teaspoon of flour to the sauce, whisking at a moderate speed to prevent clumping. Allow the sauce to gently simmer or “reduce” to cook out the excess liquid. Stir frequently to keep the mixture consistent. Repeat the process as needed to achieve the appropriate thickness.
Cheese May Be the Culprit
Even the best cheese sauce may initially look like milk with clumps of stringy melted cheese in it. Though it seems counterintuitive, the best solution to a watery sauce that won’t incorporate with the cheese is to add water. Reserve the water you boiled the pasta in -- the starch that cooks out of the pasta helps bind the sauce ingredients. If you already discarded the pasta water, regular warm water will suffice. Add one tablespoon of water to the sauce and whisk briskly. Repeat the process as needed until the ingredients are thoroughly incorporated. Taste the mixture to ensure you added enough cheese. If the sauce is well-balanced but still too thin, gently simmer the mixture to reduce it.
Poorly Drained Pasta or Overcooking
If you’ve added the pasta to the cheese sauce, then noticed water rising to the surface, the pasta may simply have brought excess water with it. Reduce the macaroni and cheese over low heat to cook the water out before baking it. If you notice a thin, clear liquid rising to the top of the casserole while it is baking, however, that's actually oil -- not water -- separating from the sauce because it has overcooked. Remove the casserole from the oven as soon as possible. Stirring the macaroni and cheese may redistribute the oil sufficiently, but you may need to prepare additional cheese sauce to re-balance the casserole.
Lingering Heat Became Condensation
If the casserole is still warm when you cover and refrigerate it, the lingering heat generates steam, which builds in the container as condensation. If enough condensation collects in the dish, the sauce may seem watery. Reheat the casserole uncovered in the microwave or oven to cook off the excess moisture. Stir the macaroni and cheese frequently to re-establish a consistent sauce.
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