The Art of the Marinade
A butcher who hangs his meat can, to some extent, reverse this effect. If you ever find one, hang on to him for dear life. You have found a rapidly disappearing treasure. The alternative is to use marinades.
Marinades can be wet or dry and you can add just about any flavor you desire to them. When a marinade is liquid, the base is usually acidic. As I said earlier, this may be fruit juice, but wine, vinegar or even beer may also be used. As a matter of fact I know of one cook who uses Coca-Cola.
If it marinades meat, just think what it's doing to your insides. Steeping in the marinade may take anything from a few minutes to several hours, and even overnight in the refrigerator. The more delicate the proteins, though, the more chance there is of overdoing things. Fish, for example, should rarely be in a marinade for more than 30 minutes, particularly if lemon juice is involved (which it usually is).
Red meat, on the other hand, can sit in the refrigerator all night without coming to any harm. So can pork and chicken, providing there are no enzymes present of the type described above.
In other words, if you are using fresh fruit juice, keep the marinating time to a minimum. That is to say, no more than an hour at the outside. Come to think of it, if either of those meats needs longer than that to tenderize it, it's definitely time to change your supplier.