I am a cooking teacher and cookbook author, but I have a question: Why do some recipes say to marinate meat for only one or two hours? For example, I have a pork in orange juice, a chicken in a rum sauce and a lamb in beer. It is easier for me (being on the run) to marinate them overnight or all day.

Marinating, from the Latin mare, meaning sea, is the soaking of meat, poultry or fish in a liquid concoction prior to cooking it, in an effort to improve either its flavor or its tenderness. For a number of reasons, there can be no set rules for marinating. The amount of soaking time depends on the type and acidity of the marinade and the size, shape and texture of the meat, among other things.

Ever since a sea nymph named Thetis dipped her son Achilles into the River Styx to make him invulnerable, the concept of a curative or restorative bath has appealed to our human yearning for quick and easy remedies. Innumerable spas and "therapeutic" mineral-bath establishments around the world profit from this yearning by soaking their customers, both literally and figuratively. But what the heck, a good soak in hot water of whatever mineral composition feels good.

Unfortunately, the sources of our illnesses most often lie deep within our bodies, and "curative" elixirs simply don't soak through our skins like ink into a blotter. (Remember blotters? For that matter, remember ink?) But that physical fact of life hasn't kept the manufacturers of countless salves, balms and ointments from claiming that they provide "deep, penetrating pain relief" or some such phrase. (And while we're in the neighborhood of dubious treatments, magnetic fields do indeed penetrate people, but the health claims for wraps and pads containing sewn-in magnets are, to stick to culinary terminology, pure bologna.)

The point of this rant against quackery is that the soaking of meat, if you'll forgive my sacrilegious characterization of the human body, cannot have an effect much deeper than its surface. And the same goes for marinating beef, pork, chicken and fish. Marinades cannot diffuse deeply enough into the meat to deposit flavor throughout its volume. Marinating is therefore primarily a surface phenomenon, the marinade's ingredients penetrating no deeper than several millimeters, depending on such factors as the density and texture of the meat, its cut, age, thickness and temperature. Fibrous meats, however, may offer capillary paths between the fibers for the marinade to travel through, especially from a surface that has been cut across the grain.

Flavoring Marinades

A flavoring marinade, as distinguished from a marinade intended to tenderize, may consist of a wide range of seasonings incorporated into a liquid or a mixture of liquids. Wine-based marinades are quite commonly used, often seasoned with herbs, spices or other flavorings that one expects will remain on or in the meat and contribute to its flavor during cooking.

The larger the surface area of a piece of meat, the more opportunity a marinade has to affect its flavor. Thus, marinating is more effective in meat that has been cut into thin slices or into one-inch cubes for kebabs, for example, than in the form of a thick steak. Beyond a certain optimal point, marinating for a longer period of time will not effect deeper penetration; it will only intensify the flavor on the outer portions of the meat. So don't expect even a 24-hour soak to affect the flavor in the middle of a roast.

Contrary to common belief, stabbing the meat with a fork to produce punctures as entryways for the marinade is not only futile but counterproductive. Puncture wounds close up almost immediately because of the elasticity of the meat, but their latent tracks may expand and offer exit paths for juices during cooking. Slashing or scoring a thick piece of meat before marinating, however, can boost its flavor by exposing more surface area to the marinade.

Tenderizing Marinades

Marinades meant to tenderize tough cuts of meat almost invariably contain an acid such as vinegar, citrus juice, wine or buttermilk, because the proteins in muscle tissue are denatured -- untwisted and reconfigured into a looser structure -- by acids. But it's a slow process. Smaller cuts and shapes of fibrous meats will require less soaking time, but acid marinades in general require longer times to tenderize meat than to flavor it. The more acidic the marinade, the shorter its optimum tenderizing time will be. In order of increasing acidity, a marinade may contain buttermilk, beer, tomato juice, wine, orange juice, vinegar or lemon juice.

Tenderizing marinades usually also contain an oil, because tough meats tend also to be lean and dry. Prepared vinaigrette salad dressings can serve as a double-threat marinade, because their vinegar tenderizes while their oil and condiments improve juiciness and flavor.

How long, O Lord?

With all of these variables, there can be no universal recommendation for marinating times. But in general, most fish flesh is tender and somewhat porous, requiring short marinating periods of only 10 to 15 minutes. Dense-fleshed fish such as tuna and salmon may require twice that amount of time. Chicken may require two to four hours if skinless and six hours or more with the skin on. Various cuts of beef and pork are marinated for anywhere from four to eight hours, the latter period of time often being referred to euphemistically as overnight. Much longer than that and the meat can turn into mush-coated leather, so don't sleep too late. And don't expect miracles. Tenderness is achieved only by selecting tender cuts of meat or by long, slow, moist cooking.

How to Do It

The first law of marinating is Follow the Recipe. Any recipe worth its salt and pepper, taken from a reliable publication, will have been tested and retested many times until it came out just right. The marinating time is simply another ingredient, like an amount of chopped onion or butter, and shouldn't be monkeyed with.

The most convenient way to marinate meat -- and for safety it must always be done in the refrigerator -- is in a zipper-top plastic bag after squeezing out most of the air to maximize liquid-to-surface contact. If you want to use the marinade to baste with during subsequent grilling or to make a sauce with, set aside some of it for that purpose before adding the meat, because even in the best of households raw meat can harbor pathogenic bacteria. Discard the rest of the marinade after it has done its job on the meat.

Labelingo: Perspicacious reader Randy Clemens of Sylmar, Calif., writes that the label on a Tombstone Original 12-Inch Frozen Pizza says, "Do not eat pizza without cooking." He wonders how big a problem that has been to the Tombstone people. But how about "Do not eat pizza named after gravesites"?

(Have you noticed any silly things on food labels? Send your Labelingo contributions, along with your name and town, to Food 101, Food Section, The Washington Post, 1150 15th St. NW, Washington, D.C. 20071 or to the e-mail address below.)

Robert L. Wolke (www.professorscience.com) is professor emeritus of chemistry at the University of Pittsburgh and the author, most recently, of "What Einstein Told His Cook: Kitchen Science Explained" (W.W. Norton, $25.95). He can be reached at wolke@pitt.edu.