We spread it on sandwiches. Stuff it into hollowed tomatoes. Stir it into casseroles. It's the brown-bagger's reliable, the dieter's staple, the penny-pincher's ace-in-the-hole. And when all else fails and the cupboard is almost bare, it's the cook's salvation.

Americans reach for it so often that we down 700 million pounds a year--65 percent alone used for salads and sandwiches. That translates into 3.1 pounds of canned tuna for every man, woman and child in this country.

In some ways, we take tuna for granted--one can appears indistinguishable from the next. But there are small thought significant differences in those cans, and behind them a highly competitive industry which has experienced intermittent prosperity and slumps due to image problems and a fluctuating economy. Here is a look at America's favorite finned fare and the hard times on cannery row.

TUNA IS one of the foods we seem to know well in this country, and yet maybe we know it too well. How many times do we absentmindedly head for the tuna aisle in our supermarket, stocking up on the brand we've always bought, not even seeing the words any more, but buying the shape and color of label with which we are most familiar?

Even food experts seem to lose objectivity when it comes to Resolute Shopper canned tuna. They stick to the brands their mothers bought and disagree on the additions of celery, pickles and eggs to tuna salad. Heaven help the publication that dares to print the definitive recipe for tuna salad in this country.

So it was with some mild trepidation that we set out to analyze the various brands of tuna available on the market. But before the results of the tasting (see chart page 16), a few words on decoding the fine print on that can of tuna.

First, the additives. All commercial brands of tuna have salt added; many also have vegetable broth included as a seasoning. If you are worried about sodium in your diet, it is a good idea to drain the tuna well before using it; this will remove much of the sodium. In addition, check the label for pyrophosphate--a sodium containing chemical added to the tuna to prevent crystal formation. The taste-test chart notes the brands that include this chemical.

The absence of a federal inspection stamp on your can of tuna is not an indication of inferiority; inspection is voluntary and simply indicates that the tuna is acceptable, not necessarily of high quality.

Beyond these two concerns, finding a favorite brand and type of tuna comes down to texture and taste, with some varieties better suited to particular diets or specific dishes. But anyone who has strolled down the tuna aisle only to be confronted with a bewildering display of labels realizes that it's easier to grab a can of the old reliable and shove on. Mindful of this, we have included the following guide for a little can clarification.

* Solid Pack or Fancy: This is tuna packed with large pieces of meat, with no flakes or fragments. It is usually the most desirable, thus the most expensive.

* Chunk or Standard Pack: Three pieces of solid tuna are put into a can, and the remainder of the weight is made up of flakes. The usual breakdown is 15 to 25 percent flakes, and about 75 percent solid tuna.

* Flakes or Grated: This is made entirely of small crumbs of tuna (often remaining from the packing of the first two grades), packed down into a solid cylinder. The meat for all three grades will be of the same quality, so if you are making a salad, croquettes, or a similar dish where the shape of the tuna doesn't matter, the cheaper grades would be the less expensive choice.

* White Meat Tuna: Only tuna which comes from the albacore species can be called white meat. We seem to prefer it in this country, and pay a premium for it.

* Light Meat Tuna: Made from yellowfin, skipjack or bluefin tuna. This tuna has a slightly darker to much darker color than the white meat tuna. In some instances, the darker tuna tends to be less delicately flavored than white meat tuna.

* Packed in Water: The vogue for tuna packed in water these days stems from the substantial calorie savings over tuna packed in oil. Water-packed tuna (drained) has 36 calories per ounce; oil-packed tuna (drained) has 56 calories per ounce. It is interesting to note, however, that our taste panel far preferred the tuna packed in oil over the water-packed variety, which tends to be dry and rubbery. This bias is reflected in the range of scores given for both.

* Packed in Oil: According to the Tuna Research Foundation, tuna which is packed in oil (other than olive oil) is almost always in soybean oil (which is relatively tasteless), a fact which will be stated on the label.

As for what to do with this pantry staple, these three suggestions will help you out of the tuna casserole rut. TUNA SAUCE FOR CHEF'S SALAD (From The Four Seasons Restaurant) (6 servings) 4 egg yolks 1 1/2 tablespoons imported dijon-style mustard 3/4 tablespoon tarragon vinegar 1 1/2 teaspoons kosher salt Freshly ground black pepper 1 cup olive oil 4 ounces tuna packed in olive oil (oil reserved) 1/2 cup sour cream Juice of 1/2 lemon

In a salad bowl, combine the egg yolks with the mustard, vinegar, salt and pepper; beat until thick and creamy. Add the olive oil in a slow stream, beating the mixture until it is well-blended. Stir in the oil from the tuna, the sour cream and the lemon juice. In a small bowl, mash the tuna with a fork and gently fold it into the sauce.

The Four Seasons Restaurant serves this tuna sauce in a salad of julienned cheese, carrots, zucchini, celery, pepper, cabbage, radish, ham and various kinds of lettuce. The dressing would complement a simpler salad of spinach or lettuce and hard-cooked eggs. FIVE-MINUTE TUNA SAUCE FOR PASTA (2 servings)

Sicilian fishermen catch most of the tuna processed in Italy. This sauce is a variation of a very simple Italian sauce; the natives would never add cheese to a fish sauce, but you may add some grated mozzarella, if you prefer. This is also good for a last-minute dinner, as you can keep almost all of the ingredients on your pantry shelf. 1 cup minced onion 1 teaspoon minced garlic 1/2 cup olive oil 16-ounce can whole peeled tomatoes 1 tablespoon chopped fresh basil or parsley Salt and freshly ground pepper to taste 7-ounce can tuna packed in olive oil (reserve the oil) Fresh pasta or spaghetti for serving

In a medium saucepan, saute' the onion and garlic in the olive oil, adding the olive oil which you have drained from the can of tuna. Cook over medium heat until onions are translucent. Place the contents of the can of tomatoes in a blender or food processor and process until smooth. Add to the onions and garlic and cook over medium heat, stirring, until some of the juice has evaporated. Add the basil or parsley and salt and pepper to the sauce. Lastly, add the tuna, breaking it up lightly with a spoon. Heat through and serve on fresh pasta or spaghetti. VITELLO TONNATO (10 servings)

Veal being very expensive, you can substitute an uncooked turkey breast in this recipe. Leave it on the bone, poach it according to the directions for the veal, allow it to cool and then take it off the bone to slice it thinly (this will give it much more flavor). 4-pound leg of veal (substitute equivalent amount of turkey breast) 1 cup sliced onions 1 cup celery, chopped 3 carrots, washed and chopped 5 sprigs each fresh thyme and parsley 2 2-ounce cans anchovies About 30 ounces tuna packed in oil 2 garlic cloves, crushed 1/4 cup olive oil 3 lemons 1 quart dry white wine Salt and freshly ground pepper to taste 2 cups mayonnaise (preferably homemade) 1/2 cup tiny capers

In a large pot, combine the veal (or turkey breast), onions, celery, carrots, thyme, parsley, anchovies, tuna (along with their oil), garlic, olive oil and the juice and rind of one of the lemons. Add wine and simmer, covered, for about 1 1/2 hours, or until the veal is tender. Remove the veal from the heat, season with salt and pepper and allow to cool in the pot. (You can do this in the refrigerator overnight, if you wish.)

Remove the veal from the cooking liquid when it is cold and slice it thinly and evenly, making the slices as attractive as possible. Arrange on a serving platter.

Strain the cooking liquid and pure'e the solids (vegetables, tuna, etc.) in a blender or food processor, adding as much cooking liquid as necessary. Fold in as much of the mayonnaise as needed to make a smooth sauce. Pour sauce over the sliced veal and sprinkle the capers over the top. Slice the remaining two lemons thinly and arrange around the edge of the platter.

Serve with a cold rice salad, if you wish.