Heating pasta sauce sounds simple enough, as if you could just pour it in a pan and cook it until hot. In fact, that's how you do it, except for one crucial detail: you have to it heat it gently until it reaches 165 degrees Fahrenheit. Pasta sauce almost always comprises tomatoes or cream, both of which are sensitive to heat. If tomato or cream sauce scorches in the slightest, or exceeds 200 F, it develops a burnt taste you can't cover up. So, pour the sauce in a pan and heat it, but do so gently.
Tomato Sauce
Transfer the tomato sauce to a heavy-bottomed saucepan and place it on the stove over medium-low heat.
Pour a scant amount of hot water or stock in the sauce. Stir the sauce occasionally, scraping the bottom of the pan as little as possible.
Check the temperature of the sauce after about 10 minutes; it should measure about 165 F or above.
Check the consistency of the sauce and add a bit of water or stock to thin it out, if needed. Taste the sauce and adjust the seasonings to taste.
Cream Sauce
Transfer the cream sauce to a heavy-bottomed saucepan and set it on the stove over medium-low heat.
Pour a tablespoon or two of cream in the sauce and stir. The fat in the cream will help the sauce hold together while it heats.
Heat the sauce until it reaches 165 F, stirring frequently.
If you see the sauce separate during heating, add a teaspoon of cold water to it and take it off the burner. Stir vigorously until the sauce comes together; if you have trouble, add a tablespoon or so of cream and continue stirring until the sauce is holds. Return the pan to the stove and continue heating.