One question I often get asked is how to marinate meats when cooking them sous vide. The question is usually whether or not you can marinate meat while it is cooking in the sous vide machine. I figured I'd answer it here so other people can weigh in as well.
To clarify, this question just addresses whether or not you should cook a meat in it's marinade. Sous vide works great with the traditional method of marinating the meat, draining it, then cooking it. The only question is can you do the steps at the same time.
I think the first step to answer this question is to define what a marinade is in terms of this conversation. I'm using the term "marinade" loosely here, as anything you let meat sit in before cooking it, instead of a stricter definition.
There are basically two types of marinades people are referring to: ones that add flavor and ones that are designed to tenderize or change the meat. Flavor "marinades" can contain almost any types of flavored ingredients from rubs and pastes to flavored liquids and herbs. Tenderizing marinades typically contain acid or enzymes, or are a brine, and have some flavor compounds added.
The short, generalized answer to the question is: marinades that only add flavor and aren't meant to change the meat (through tenderizing or brining) tend to work fine at the same time as sous viding while tenderizing marinades don't work as well.
There are several issues you can run into if you try to cook a meat using sous vide while it is in a tenderizing marinade.
The first issue I have is that most tenderizing marinades aren't needed. The sous vide process should be able to tenderize the meat enough that the marinade becomes irrelevant. Changing the cooking time should give you the tenderness you want without needing the marinade.
Another issue is the question of timing. Many tenderizing marinades or brining recipes call for specific amounts of time that meat should be in them. These times rarely line up with the amount of time the meat should cook for. This means something has to give time wise on either the marinade or the sous viding, and the end result won't be as good.That means something has to give time-wise and the end result won't be as good as it could be.
To further complicate the matter, most marinades and brines are designed to work on raw meat. Once the food goes into the sous vide bath it quickly comes up to temperature and is fully cooked. A 1" / 25mm steak will be cooked through in about 50 minutes, meaning the marinade will be trying to work on cooked meat for most of the marinating time. The outside .4" / 10mm will be cooked in only 8 to 10 minutes. Once the meat is cooked, the protein has been changed and will be affected differently by the marinade or brine.
A final issue is that due to the sealed environment there is no evaporation, as opposed to braising. This means alcohol based marinades, or marinades with high acidity don't reduce during cooking and can take on bad flavors.
In general, flavoring marinades are fine to use at the same time you sous vide the meat, or to leave on during the sous vide process. You might lose some penetration if you don't let the meat sit in the marinade first but cooking it won't hurt the flavor of the food. Just make sure the marinade is non-alcoholic, has a low acidity, and isn't meant to be used as a brine.
And remember, if you just have to use your favorite acidic marinade, just marinate the meat like usual, remove it from the marinade and then sous vide it. It should turn out great!