Marinades

The purpose of a marinade is to bring tenderness and/or flavor to meat. It's useful for turkey, chicken, steak or whatever you're cooking.

Meat prep thoughts

I learned the hard way just how important meat aging is to beef. A processor was trying to meet a particular schedule and didn't let a half cow I purchased hang long enoough. The result? Insanely tough ribeyes and strip steaks. It was sad.
In my quest to save that cow, I discovered some great ways to tenderize meat. I've also started smoking various things and have discovered yet another purpose to the brine/marinade process: additional moisture!

Marinades and Brines

Good steak: tenderloin, etc

A Great steak marinate to add some flavor, but limited tenderizing effect.:
Beer: enough to cover the meat.
Brown sugar: for one steak, maybe a tablespoon.
Red pepper flakes: 1-2Tbsp.
Salt: optional, not really needed.
Let the steak sit in the marinade (refridgerated for about an hour, turn it over and allow another hour.
Rinse the steak off. Now you can leave it in the refridgerator until you're ready to cook. A day or two is fine, but cover it!
Season the steak as normal. I use coarse sea salt, fresh ground black pepper and some olive oil.

Sad steak: tough cuts like messed up processing, roasts or round steaks.

Goat milk!
Place the meat in goat milk overnight. Goat milk has a great balance of enzymes to tenderize the steak, but not turn it into mush during an overnight soak.
I purchase goat milk on clearance for this purpose. It's often $.75 and I can freeze it. I simply thaw it on the stove and use the pan to keep the meat in
This is how I managed to ressurect the poor, super tough cow.

Poultry!

Poutlry responds very well to being soaked in a brine. (sugar and salt water)
This is especially useful when the bird in question is going to be slow roasted or smoked.
Turkey and chicken get incredibly soft. Turkey usually requires a couple of days, but it's ok to short if if you must.
General rules: Thaw the meat first. You'll get uneven results as frozen meat doesn't absorb anything.
Normal ingredients: pickling salt - this dissolves easily in cold water. brown sugar adds some flavor. Red pepper flakes can add some spice. Garlic and Onion powders give good flavor transfer since they'll dissolve in the water. I often use fresh garlic, but crushed allows more soaking.
Chicken can become so soft that it starts to fall apart during cooking. I prefer an hour or two if I'm taking the time to brine. Overnight is far too long for chicken.