Yes, you can absolutely use the cooking liquid or marinade from sous vide to create a flavorful sauce, but it's crucial to consider food safety and potential flavor changes.
The liquid remaining after sous vide cooking is often packed with concentrated flavors from the food that was cooked. This makes it an excellent base for a sauce. However, because the food is cooked at lower temperatures during sous vide, the liquid may not have reached a temperature high enough to kill all bacteria. Therefore, it's essential to bring the liquid to a boil for at least a minute to ensure it's safe to consume. After boiling, you can reduce the liquid to concentrate the flavors further and add other ingredients like butter, cream, herbs, or spices to create a delicious sauce. If you used a marinade, be mindful of the ingredients. Marinades often contain raw meat juices, which require thorough cooking. Also, ingredients like citrus juice or vinegar in the marinade can become overly acidic when reduced, so taste frequently and adjust accordingly.
Before using the sous vide liquid, skim off any excess fat that has rendered out during the cooking process. This will result in a cleaner, more flavorful sauce. You can use a fat separator or simply chill the liquid and remove the solidified fat from the top.