Jack Lemmon strained it through a tennis racket in "The Apartment." Walther Matthau threw it against the wall in "The Odd Couple."

Ever since spaghetti arrived in this country when Thomas Jefferson brought a pasta press back from a trip to Italy, the simple flour and water mixture has often been incredibly mishandled. Despite the fact that it usually ends up as a soggy, formless over-cooked mass, it has become enormously popular. And it has undergone some dramatic changes, which is not surprising since Americans are seldom content to leave well enough alone, especially when it comes to food.

So spaghetti, macaroni, lasagna, fettucini, ravioli, tortellini el al have been enriched with vitamins and minerals, made with whole wheat flour, wheat germ and torula yeast, colored and flavored with artichokes, beets and carrots. Jefferson would never recognize the stuff. And sometimes it's equally difficult for a modern-day aficionado of the traditional pasta to recognize its permutations or even begin to guess what's in it.

Government regulations haven't helped much. An ingredient statement for pasta, like a number of other processed foods, is not required as long as the ingredients used fall under the "standard of identity" set for the product by the Food and Drug Administration. So, although, you can be pretty certain an ordinary pasta product contains flour (or as it is often called, semolina), the label need not say so.

Traditional pasta - what devotees consider the best - is made of 100 per cent durum wheat. This hard wheat has most of the nutrients milled out of it. Some of those nutrients are put back in through the process of enrichment. If the pasta product is made with enriched semolina, the name would have to indicate that, as in enriched spaghettini. But the specific vitamins and minerals used to enrich the semolina do not have to be listed on the label, information consumers might find useful.

Vegetable pastas, like spinach noodles, must contain 3 per cent of the vegetable solids, and the only labeling requirement is that the name include the particular vegetable used, as in artichoke macaroni. Again, no further ingredient statement is necessary.

Pasta products sold under the name "green noodles" fall into a category which according to Taylor Quinn, director of FDA's compliance branch in the bureau of foods, "probably aren't regulated."

Health and natural food manufacturers, not content simply to enrich traditional pasta, have created their own variations. One health food company, Erewhon Inc. of Boston, produces 20 pasta products made with whole wheat. Some are further enriched with soy flour, brown rice flour and vegetable solids.

A new form of pasta, made with the addition of Jerusalem artichoke flour by DeBols Nutritional Foods, is becoming increasingly popular, proboably because it is supposed to have a lower carbohydrate content.

And just as manufacturers of fabricated foods are criticized for "fooling around with Mother Nature," aficionados of traditional pasta are critical of any engineering work done on it.

For most pasta lovers there is little question that the flavor and texture of these alternatives are nothing at all like the original.

But even for them, getting a properly cooked pasta dish is a problem. More often than not, the texture is destroyed by overcooking. While 20th century cookbooks do not suggest boiling pasta for three hours, as recommended in an 18th-century American cookbook, most people do not know how to cook it properly. If they do not overcook it, they let it sit in the water while the rest of the meal is prepared, or they reheat it . Pasta is cooked when it is still slightly firm, or as the Italians say al dente , an expression that translates literally "to the tooth." It should be served immediately.

Because the myriad of pasta shapes, estimated at about 600, have different cooking times, it is impossible to give a general rule. To cook pasta to your taste you must test it several times. The minute it is ready the water must be drained off or the pasta will continue cooking - and softening. Allow four quarts of water for a pound of spaghetti.Bring it to a rolling boil and add a teaspoon of salt per quart of water. Put the pasta, in, all at once, and allow the water to return to a boil as quickly as possible. Then keep checking. When the pasta is done to your liking, drain it in a colander, shaking it a bit to remove excess water. Do not rinse it unless the pasta will be cooked again such as in a lasagna recipe. Serve it with the sauce at once.

Fanatics insist that certain varieties must be served with particular sauces. But the only really useful rule is that sauces should be served over tubular pasta, ie. spaghettis and macaronis, in order for the sauce to be absorbed. On the other hand, if all you have in the house are egg noodles and a marvelous pot of clam sauce, there are worse ways to dine.

Americans have come a long way from their first taste of spaghetti and meat balls. When they became a little more adventuresome they moved on to ravioli and lasagna, up through fettucine Alfredo to linguine and clam sauce and finally on to tortellini in Washington. Now the rage in New York appears to be a pasta dish known by its slang name in Italian, paglia e fieno , which translates to "straw and hay." It refers to the colors of the two kinds of pasta in the dish - the pale green of the spinach noodles and the yellow of the fettucine.

Waverly Root, in his overwhelming compendium "The Food of Italy," considers the name "a disrespectful description of a delicious combination, egg tagliolini and tagliolini verdi , in a cream sauce containing shredded prosciutto and peas."

Marcella Hazan describes the dish in "The Classic Italian Cookbook" as "one of the most exquisite." But she and Root do not agree on the proper sort of ham. Hazan says: You may substitute prosciutto for the ham, but it will give you a somewhat sharper flavor and coarser texture." She acknowledges that the dish should contain "sauteed tiny fresh peas," but gives the recipe with mushrooms, because, she notes, very young freshly picked peas are so seldom available.

This is an adaptation of her version. STRAW AND HAY (6 to 8 servings) 3/4 pound crisp, white mushrooms 2 tablespoons finely chopped shallots or yellow onions 6 tablespoons butter Salt Freshly ground black pepper, about 6 twists of the mill 6 ounces unsmoked ham, shreded 3/4 cup heavy cream Enough fettucine for 3 or 4 persous, preferably homemade Enough spinach pasta for 3 or 4 persons, preferably home-made 1/2 cup freshly grated Parmesan cheese

Slice off the ends of the mushrooms stems. Clean and dice into 1/4 inch cubes, set aside. Choose a skillet that can later accommodate the mushrooms loosely. Put in the chopped shallots and half the butter and saute over medium heat until the shallots have turned pale gold in color. Turn the heat up high and add the diced mushrooms. When the mushrooms have absorbed all the butter, briefly turn the heat down to low; add 1 teaspoon salt and the pepper, and shake the pan, moving and tossing the mushroons. As soon as the mushrooms juices come to the surface, turn the heat to high and cook the mushrooms for about 3 minutes, stirring frequently. Turn the heat down to medium, add the ham, and cook for less than a minute, stirring as it cooks. Add half the heavy cream and cook just long enough for the cream to thicken slightly. Taste for salt. Set aside.

Choose an enameled, iron pan or other flameproof serving dish that can later accommodate all the noodlels without piling them too high. Put in the rest of the butter and the cream and turn heat to low. When the butter is incorporated into the cream, turn off heat.

Bring two pots, each containing four quarts of water, to a boil. Add a tablespoon of salt to each. First drop fettucine in one pot and stir. Then drop spinach noodles in the other pot and stir.

Taste spinach noodles for doneness soon after water returns to boil. They should be quite firm because they will continue to soften up while cooking with the sauce. Drain well and transfer to the waiting pan. Immediately after, drain the fettucine and transfer them to the same pan.

Turn heat to low and toss the noodles, coating with butter and cream. Add the cheese and mix into noodles. (The entire step should not take more than a minute.) Turn off heat. Make a slight depression in the center of the mound of noodels and pour in the rest of the mushroom sauce. Serve immediately, with a bowl of additional grated cheese on the side.