There's something about marinades that brings out the dabbler in most cooks. You start with some oil and vinegar or lemon juice and then begin throwing in things almost willy nilly, based on what feels like the right thing to do.
Think of it as getting in touch with your inner pork chop: "If I were lounging in a warm olive oil bath, how would I like to smell?"
Almost invariably, the answer turns out to be something like: "A bunch of minced garlic, some salt and pepper, a couple handfuls of rosemary. . . ." And so it goes until either the whole concoction smells just right or you run out of ingredients.
Sometimes it seems that marinades are the modern equivalent of a previous generation of cooks' "secret sauces." Throw in a little bit of this, a little bit of that . . . nothing can be too outlandish and, in most cases, the more of an ingredient you use, the better.
All in all, it's probably a good thing that none of it makes much of a difference.
That may seem heresy to dedicated marinaters, but that's how it is. To understand why, you have to look at how a marinade works.
For one thing, when you talk about marinating, you're talking about two very different goals. The first--and the most controversial--is tenderizing. It is pretty well established in the scientific literature that altering the pH of meat to either end of the scale will increase tenderness. At least, tenderness as scientists measure it--with something called a Warner-Bratzler shear device. In addition, lowering the pH means an increase in the water holding capacity--meaning moister, plumper meat after cooking. All this will happen if you wait long enough.
In practice, this means soaking the meat in a mild acid. You would not, for example, soak a chuck roast in ammonia (a base), or in hydrochloric acid (a strong acid)--unless you were a scientist studying marinades. Lemon juice is the strongest of the household acids, with a pH of 2.1 to 3 (the lower the pH, the stronger the acidity). Vinegar (pH of 3 to 4) and wine (3.5 to 4.5) are also commonly used. So is yogurt (4 to 4.4). Even onions have something of a tenderizing effect, thanks to the sulfuric acid compounds that are created when they are sliced.
Vinegar, in addition to its acidic properties, has the added advantage of dissolving the connective tissue found in meat. Studies have found that acetic acid is as much as twice as effective as other acids.
(There is also a class of enzymes called proteases that will tenderize. They are present in several fruits, including kiwi, pineapple, figs and, most significantly, papaya--the enzyme papain is the active ingredient in most commercial meat tenderizers.)
The problem is, all of these things take time to work.
"Can you get a tenderizing effect with vinegar? The answer is yes, it'll just take a little while," says P.J. Bechtel, head of the Food Science and Human Nutrition Department at Colorado State University. Bechtel was co-author of a 1992 paper on how marinades affect the texture of beef; he found that meat needed to reach a pH of about 3.25 for the tenderizing/water holding effects to begin.
"In most cooking situations," Bechtel says, "there's just not enough time to let the marinade really penetrate a large piece of meat so that you get that pH effect all the way through. Unless you make little holes in the meat--mechanically get the marinade in--it will take a while for it to happen. Days, in fact."
So, if you soak a chuck roast in acid long enough, you'll wind up with a more tender piece of beef. But what does that mean, really? "Tender" is a difficult concept. It can mean "soft" (and that's how the Warner-Bratzler shear device would define it), or it can mean something more along the lines of al dente --"offering resistance but still yielding" (that is how you and I would define it).
Unfortunately, if you soak a piece of beef in a mild acidic solution long enough for the center to become tender, the outside is highly likely to become soft and mealy. (The same holds for enzymatic softening--such as that from papaya and kiwi.)
"We haven't got a good way of measuring the difference between the two," says Jan Novakofski, professor of meat science at the University of Illinois at Urbana and a co-author of the beef paper. "My subjective impression is that [the surface will get mealy]. If you get enough swelling of connective tissues and muscle tissues, they fall apart."
If you want to simply tenderize tough pieces of meat, you're probably a lot better off using other techniques: cooking with moist heat methods (braising or stewing) that replace lost moisture as the meat squeezes it out, or by mechanical means (pounding the meat, or slicing it thin on the bias) that break the tough meat fibers.
The other reason we marinate, of course, is taste--simply getting the flavor of a liquid (vinegar, wine, lemon juice, yogurt, etc.) onto meat.