The Science of Separation

• Some sauces separate more distinctly than others. Most notably, sauces that feature fats such as heavy cream separate, because the fats don't naturally mix with the other ingredients. When you cook them together to make your sauce, they emulsify; this is when one of the ingredients is broken down into such fine molecules that it is dispersed throughout the other ingredient. Unlike ingredients that completely dissolve, such as salt, fats simply break apart into small particles. Over time, those particles find each other again, forming the big globs of separated fat in cream sauce that has been left sitting.

Stabilize Your Sauce

• To keep sauce from separating when you reheat it, prepare it with reheating in mind. When you initially prepare the sauce, start by mixing flour in with your melted butter. This adds a binding agent to your sauce that may make it thicker, but will also help prevent the ingredients from separating, especially when they're reheated. You can also experiment with adding more of an emulsifying agent such as egg, or more liquid such as water or wine, which dilutes the fats more.

Timing and Temperature

• You can keep any sauce from separating if you prepare it properly. Generally, this means preparing the sauce close to serving time. Whether you're making it from scratch or reheating it, sauce separates over time. Patience is also critical to preventing sauce from separating, especially when you're reheating; while microwaving your leftover sauce is the quickest way to heat it up, it won't do much to emulsify the ingredients that have already separated. Instead, you need to use a slower, more hands-on method.

Keep It Together

• When you reheat your sauce, take your time by using a saucepan over medium-low heat. Whisk the sauce thoroughly, both before you start heating it and as it heats, to ensure that the fats break up and can emulsify again. Adding milk to the sauce can also make it smoother and creamier; in sauces such as Alfredo, the ingredients can thicken as they separate, making it difficult to break them apart without adding more liquid. Noncream sauces, such as marinara, typically require only moderate stirring while they reheat.