THE answer, as usual, was easy. It was just a matter of having the scientific evidence to prove what common sense made clear: all it takes to rid canned foods of excess sodium is a strainer and old-fashioned tap water.

According to research at the Duke University Medical Center in Durham, N.C., a one-minute rinse under tap water washes away 76 to 79 percent of the sodium present in canned tuna fish. Two more minutes washes away an additional five percent, leaving a total of 65 milligrams.

Ditto for green beans, says researcher Rita T. Vermeulen, a registered dietician who has published her data in the April issue of the Journal of the American Dietetic Association. A one-minute rinse in tap water reduced the sodium in canned green beans from 308 milligrams to 184 (although some nutrients are rinsed away with that cooking liquid, unforturnately).

The procedure, says Vermeulen, is an inexpensive option for people interested in cutting down on dietary sodium who don't want to pay high prices for specialty sodium-reduced products.

The tuna tastes like sodium-reduced tuna that's available commercially, says Vermeulen; in general, most foods resemble their commercial lower sodium counterparts. Recently, more companies are producing lower sodium foods, says Vermeulen, who adds that she's happy the problem of finding these foods "isn't as great as it used to be."

"I think the results are good to hear," says Bonnie Liebman, nutritionist with the Center for Science in the Public Interest. It "provides another avenue for people who want to cut back on sodium in their foods. It's particularly useful for people who don't need to go on a no-salt-added diet."

A quick dinner for the sodium-conscious no longer takes a trip to specialty stores. This one can be put together with just a trip through the express lane of the average supermarket, provided you have certain staples on the shelf, including olive oil and black pepper.

EXPRESS LANE LIST: canned tuna fish, green beans, scallion, garlic, lemon juice, tomatoes, basil, pasta spirals. TUNA SALAD VINAIGRETTE (4 servings)

1/2 cup olive oil, plus a little more for tossing pasta

3 tablespoons freshly squeezed lemon juice

2 cloves garlic, mashed and minced

Freshly ground black pepper

1 teaspoon dried basil or 1 tablespoon fresh

8 ounces pasta spirals

2 7 1/2-ounce cans tuna fish, drained

1 pound green beans

1 bunch scallions

4 large, ripe tomatoes

In a jar with a tight-fitting lid, combine olive oil, lemon juice, garlic, black pepper and basil. Set aside. Boil pasta until tender and drain. Toss it with a little olive oil to keep it from sticking. Set aside. Place canned tuna in a strainer and rinse for 1 minute under running tap water to remove excess sodium (if desired). Set aside. Trim green beans and steam them until tender, about 10 minutes. Meanwhile, mince scallions, including tops. Combine pasta, tuna, green beans, and onions in a large bowl. Toss with dressing. Place on a salad plate and garnish with a semi-circle of overlapping tomato slices, flavored with a generous supply of freshly ground pepper.