Can I take raw chicken out of a mainade that consist of soy sauce, vinegar and other things and put ribs into it to marinade as well? Its a couple days old and been in the fridge the whole time.
It's a bad idea to re-use a marinade, especially to marinade a different meat or poultry:
- Chicken may contain salmonella, if yours does and you re-use the marinade you'll then contaminate the next thing
- By the time you re-use it is old, the older it is the less safe it is to use
- It will have lost most of its effectiveness: the flavors and acidity will have acted on the chicken, so the marinade will have lost much of its flavor and ability to tenderize. Even if the marinade was safe to use again it wouldn't have the kind of kick to give you a good result on the next thing
It's a good thing to want to re-use things where possible and avoid waste. In this case it's not worth the risk, and unlikely to impart any flavor anyway.
You can re-use marinade if you prepare it for reuse.
You can put the left over marinade into a sauce pan and bring it to a boil. Do this slowly over a medium to low temperature, stirring frequently with soy-based or sugary marinades to prevent burning. Cool it down quickly and refrigerate. I put some ice and water in the sink around the hot pan and stir the marinade to cool it. I have added a few additional herbs and spices to the hot marinade to perk it up. When it is cool, I add a little more vinegar, mix, and then pop it into a jar and into the refrigerator. Sometimes, this has even yielded a better marinade, since a little heat, reduction, and "sit time" will really bring out the flavors, especially for soy sauce. I have kept this kind of marinade for a week with no ill effects -- used it first on a Saturday, boiled and refrigerated it, then re-used the boiled version the next Saturday.
Finally and foremost, you have to make sure that you cook food to the safe minimum temperature. Salmonella and other biological contaminates are not that uncommon. Cooking to the proper temperature ensures that the food is safe to serve. See the USDA Site for more information.