We all love marinades, because they add moisture and flavor to our favorite meats when we're grilling. But buying them? No way jose, that's money better spent elsewhere, if you ask me. Make your own dang marinade! It's not as hard as you think it is.
Marinades typically comprise three things: a salt or an acid (or both!), something sweet to balance it out, and then some spices and flavorings. I bet you're wondering why. Well, I'll tell you why!
1) Salt or acid. Acids denature proteins. What does that mean when it's at home? Well, proteins are naturally folded up in quite complex patterns. Acids disrupt some of the bonds that keep proteins folded. The end result of this process is a bunch of little tunnels through which flavor can seep in to your meat.
If you go the salt route, you're adding moisture to the meat because of osmosis. Because the fluid inside the cells in the meat is less salty than the fluid outside, water wants to flow out of the cells to dilute the saltiness. Wait wait, you say, I thought you said you're adding moisture, and this sounds like the moisture is flowing out. Ah, but the salt then dissolves some of the proteins in the meat's cell structure and the meat's cell fluids become more concentrated, and the osmotic pressure sucks the water back in. (Brining, a related technique, involves immersing meat in a salt-water bath, and is great for such meats as turkey and pork that tend to dry out when cooked.)
2) Something sweet. Now, the first reason is obvious - you've just put a whole lot of salt or acid on your meat, and if you don't balance it out, it's going to taste like straight-up table salt, or taking a pull from a bottle of vinegar. Yum. But there is another reason to put sugar in a marinade, and that is caramelization. That's when sugars under heat turn brown and gain complex flavors. (How much better does caramel taste than just a handful of sugar?) Putting sugar in your marinade helps to give meats (especially light-colored ones like turkey or chicken) that pretty brown crust when it's grilled.
3) Flavorings. Well, you wanted it to taste like something, didn't you?
In case you're wondering what are some good options for these three marinade components, here you go: Soy sauce is a great place to get salt (especially for marinating beef - the flavors go very well together), vinegar (in all its delicious varieties) and citrus juices are nice acids, and you can take your pick of your entire spice cabinet to flavor it (just make sure everything plays well together).
I'll give you a sample of a marinade I just made. I wish I could give you a better idea of amounts, but I didn't really measure anything.
Soy sauce (Salt)
Rice wine vinegar (Acid)
Orange juice (I squeezed some fresh and also used some orange-tangerine blend.)
Orange zest (about half an orange worth)
Fresh ginger, grated fine
Sesame oil (careful - a little goes a long way! Available at Asian markets and most grocery stores.)
My preferred method for making marinades is to put everything together in one-a-them glass canning jars, because then you can just put a lid on and shake it. Start off with a goodly amount of soy sauce and a goodly amount of vinegar - probably at least half of your regular-size canning jar - because this is the base of your marinade. Then add the other stuff until it smells and tastes right. I went with orange zest because the citrus flavor was getting buried, but I didn't want to wash everything out with too much juice. Orange zest is powerfully orangey, so it worked well.
My favorite way to actually marinate the meat is to stick it in a ziploc bag, pour enough marinade in to cover it (less than you think, because of the next step!) and then squeeze the excess air out. The marinade makes good contact with the meat and it's easy to turn it around for even coverage.
A note on marinating time. If you're marinating pork or beef, you can leave it in there pretty much all day, but if you're marinating chicken or fish, you only need 30 minutes - one hour TOPS! This is because chicken and fish aren't as dense (so they need less time to soak up flavor) and the surface will actually start to be chemically cooked by the acid in the marinade, and the meat will be dry. Go try some ceviche if you don't believe me.
Once you're done marinating, cook and enjoy! See, that wasn't so bad.
ps - I marinated a tri-tip in this, then gave it a post-cook soak in the marinade in a little tinfoil canoe.