A piece of received cooking wisdom about marinating meats is that the acidity in the marinade — whether from wine, citrus juice or vinegar — is what “breaks down the meat’s toughness.” The further claim that that same acidity “makes the meat tender and juicy” is rubbish.

It is the salt in a marinade that performs the yeoman’s service of tenderizing meat’s connective tissues or musculature, especially if delivered in a liquid (for example, soy sauce). And what carries onto and into the meat those desired and designed flavors of a marinade — garlic or onion, herbs, chile heat or other spice — is oil. Fat is a flavor carrier in all respects, in a cooking preparation or on the palate.

Salt and fat do the flavor work; acidity isn’t that important.

Acidity in a marinade is a bit player, really, and affects merely the exterior of any piece of meat. It doesn’t penetrate, as kitchen saws have it. In fact, if the meat is exposed to acidity for too long a time (depending on the cut of meat, anywhere from 30 minutes for a boneless breast of chicken to a few hours for some gnarly ribs), acidity actually “cooks” or firms up the meat by denaturing its protein (see: ceviche), or turns the meat’s surface to mush, markedly.

A bit of science helps understand the interplay of especially the salt, but also the fat and acidity, in a marinade for meat, fowl or fish.

The point of a marinade is to introduce both flavor and tenderness. Marinades, generally liquids though sometimes concocted as “dry rubs,” do this by brining meat. The osmotic pressure created by sodium pulls water from a place of higher moisture (the marinade) into a place with lower moisture (the meat).

Also because of the salt, the marinade restructures the myosin protein molecules of the meat by loosening them and creating gaps in between them that, further, fill with moisture and thereby increase juiciness. As it does that, it also seasons (literally “salts”) the interior of the meat.

Again, beware: that same osmotic pressure does a sort of push notification of the marinade’s acidity into the meat and, to carry the analogy, may spam it into chalky mushiness if left in contact with the meat for too long.

Fat doesn’t penetrate as salt does or acidity can do. Because it emulsifies and thereby thickens the elements of a marinade, it remains mostly on the surface of the meat with its delicious flavoring compounds. Later, it also acts as a sort of buffer between the surface of the meat and, when it is set to flame or heat, its cooking source.

In a marinade, then, keep the ratio of fat and acidity about 1:1, never skewing higher than that in favor of acidity. And err on the side of caution with time so that any acidity doesn’t veer from help to harm.

Finally, salt with abandon, even pumping up the effect of salt by enlisting the ancillary work of a protease (an enzyme that breaks down proteins) with the salt. Name for that? Soy sauce and its kin. “Too much salt” in a marinade or a brine shouldn’t worry you. The sodium seems to stick around at just the right level. It’s true.

Finally, the cuts of meat to benefit most from marinating are, of course, those with the highest concentrations of difficult-to-tenderize tissues. Chicken thighs versus breasts, for example; pork shoulder as opposed to loin, for another.

As for beef cuts, skirt and “flap” meat (both different, both the same, depending on the butcher or the region of the country) and flank meat fill the bill, as do tri-tip and hanger meat, sometimes cut and sold as “steaks” so-named. Most beef rib meat, short or long, also marinate well.

Grilled or Pan-seared Flap Meat with Sauce Gribiche

From Chef Jamey Fader and Bill St. John. Serves 4.

 Ingredients

1 pound “flap” or beef belly meat such as flank or skirt steak

2 large heirloom tomatoes, peeled, sliced thickly beginning at and in the direction of their “equators”

1 white onion, or sweet onion, peeled and sliced 1/4-inch thick beginning at and in the direction of their “equators”

Olive oil, salt and pepper, for grilling or pan-searing

For the flap meat marinade:

1/2 cup dry red wine (or 100% tart cherry juice)

1/2 cup extra-virgin olive oil

3 garlic cloves, peeled and crushed

1 bunch green onions (scallions), trimmed at the root and 3 inches of dark green sliced away, roughly chopped

1 tablespoon Dijon mustard

1 tablespoon fresh rosemary leaves, loose

2 tablespoons fresh flat-leaf parsley leaves

2 naval or blood oranges, or 1 sour orange, juiced, and their peels, cut up

3/4 teaspoon salt

1 teaspoon freshly cracked black pepper

For the Sauce Gribiche:

1 large egg, hard cooked, peeled and halved

1 teaspoon Dijon mustard

1/3 cup extra-virgin olive oil

1 teaspoon sherry or white wine vinegar

1 small sour pickle or cornichon

10 capers, salted or in brine or vinegar, well rinsed and squeezed of their rinse

1/4-1/2 cup lightly packed fresh green herbs such as flat-leaf parsley, summer savory, tarragon, or a mixture, very finely chopped

Salt and freshly ground black pepper

Directions

In a bowl, mix together well all the ingredients for the marinade. Place the beef meat in a large plastic zippered bag and toss in the marinade, sloshing it around the meat, and sealing the bag. Lay the bag over paper toweling in the refrigerator for at least 3 hours, turning the bag once or twice if possible.

Prepare the Sauce Gribiche: Remove the egg’s yolk to a small bowl, add the mustard and smoosh them together into a paste with the back of a fork. Drizzle in the olive oil, stirring well, making an emulsion. Add the sherry or wine vinegar. Chop the egg white, the pickle, and the capers (if the capers are large) until they’re all minced. Add them to the sauce, along with the green herbs. Mix the sauce together well. Adjust for salt and pepper and set aside to serve at room temperature.

Prepare the tomatoes and onions by grilling or searing them, over medium-high or high heat, in a cast iron skillet (or heavy iron grilling pan) until they are slightly cooked through but nicely charred, 2-5 minutes (far less for the tomatoes than the onions), removing them and setting them aside.

To cook the steak: Drain it of its marinade and lightly pat it dry with paper toweling. Over the same heat as for the tomatoes and onions, sear and cook the steak for 2-3 minutes on each side (no longer, perhaps as little as 90 seconds, depending on the thickness of the meat; you want a char on the exterior, but a thin line of pink or red down the center).

Let the meat rest for 5 minutes before slicing it against the grain, into “fingers” that you will lay atop an arrangement of the tomatoes and onions, everything dabbled with the Sauce Gribiche.

Reach Bill St John at [email protected]