Chop up a head of cauliflower for a stir-fry and let it sit while you prepare the egg rolls and it'll lose some of its Vitamin C. Stew a piece of meat and drain away its drippings and you've probably thrown out about 75 percent of the thiamine. Leave that translucent milk container on the sunny kitchen table and there goes some of its riboflavin.

Are home cooks vitamin assassins? Involuntarily, yes.

In the previous examples, the Vitamin C in the cauliflower has been destroyed by exposure to air, the thiamine in meat has been leached by moist heat and the riboflavin in milk is sensitive to light.

Vitamin losses in foods are the stuff of scientific study, discussion and debate. What happens to the vitamins in a can of green beans after 30 years? What is the effect of holding time on raw asparagus? What percentages of the U.S. Recommended Daily Allowances are in a 3-ounce serving of cooked corn, frozen or canned?

Much of these losses occurs before food ever gets to the supermarket. From the harvester to the packer to the shipper to the distributor to the grocery shelf and finally to the consumer, there are natural time and temperature transitions that make the losses inevitable.

While the fresher the food, the greater the retention of nutrients, unless you live on a farm or have a garden, you have little control over how long it takes before it all gets to your plate.

But you can cook foods properly to help retain what vitamins are left. While it is true that the longer the cooking time and the higher the heat the greater the deterioration of nutrients, there's more to it than just not cooking your broccoli to hospital-style mush.

A final word -- assessing the impact of this on individuals is difficult to quantify; it depends on dozens of dietary variables. Although the data on just how much healthier you might be from consuming steamed rather than boiled brussels sprouts is just not available, if you can maximize your vitamin intake through food, why not?

Here are some tips on how to increase vitamin retention in food while preparing and cooking:

*Heat

Vitamins B and C are soluble in water, and that means a variety of things as far as cooking goes. According to Pat Kendall, a nutritionist with the cooperative extension service at Colorado State University, it makes foods containing them extremely sensitive to heat; when you cook cauliflower or spinach in water, the vitamins leach into the cooking water. In fact, says Kendall, Vitamin C is the nutrient most easily destroyed in this manner. (The obvious rectification is to eat the cooking liquid, either in a sauce or stock.)

Also, more of the vitamins will be leached out of a boiled vegetable if the vegetable is placed in the pot before the water has boiled. This allows the vegetable more time to leach out its vitamins, says John Powers, a professor at the department of food science at the University of Georgia. The solution? When boiling vegetables, place them in the pot after the water boils.

By the same token, the less water you use, the better. More water gives a vegetable a larger area in which to leach. Kendall suggests using not more than 1/4 to 1/2 cup of water when boiling vegetables. And for microwaves, Kendall suggests no more than two tablespoons for a 10-ounce package of frozen vegetables or 1 1/2 cups of fresh vegetable.

In fact, Kendall said that if you wash a vegetable and don't dry it, you should be able to cover and cook it without adding any water at all. (Obviously, you would not want to use this method over high heat. Covering vegetables during cooking is an advantage, too, said Kendall, since anytime you speed up the cooking process, you will save nutrients.)

Thiamine, a B vitamin, is easily destroyed by moist heat (it is also destroyed by sulfur dioxide, the compound used to dry fruits). A USDA study of cooking 14 cuts of pork (a rich source of thiamine) by various methods showed that broiling pork chops retained 64 percent of thiamine present in the raw product, roasting a pork roast retained 52 percent of the original thiamine and braising blade chops retained only 42 percent, according to Joanne Holden, a nutritionist with the Nutrient Composition Lab of the USDA.

Holden said that broiling closes in the cellular tissue, sealing in the vitamin. Longer cooking methods such as braising and stewing allow the vitamin to be leached into the drippings, similar to overboiling a vegetable. You can recoup some of this loss by using the pan drippings for a sauce or gravy, but you also add extra fat to the dish, unless you skim it thoroughly.

Ironically, while broiling and roasting will decrease the fat content of meat, frying -- which adds fat to meat -- may be the cooking method that retains the most amount of thiamine, said Kendall. Here, though, the greater benefit is derived from getting less fat.

In another trade-off, the less you cook meat the more thiamine the meat retains. Thus, rare meat eaters get an added thiamine bonus. Kendall cautions, however, that meat that is too undercooked also runs the risks of harmful microorganisms, which may be greater threats than the loss of some of the benefits of more thiamine.

And for all foods, Jim Smith, head of the vitamin and mineral lab at the USDA's Beltsville Agricultural Research Station, said microwave cooking is less destructive than boiling or methods that use high heat. Heat is retained and the quick cooking process saves the nutrients from leaching, added Kendall.

*Oxidation

Since exposure to oxygen destroys Vitamin C, you should refrigerate fruits and vegetables containing the vitamin as soon as possible, said Bonnie Liebman, director of nutrition at the Center for Science in the Public Interest. (The Nutrition Desk Reference, by Robert H. Garrison and Elizabeth Somer Keats, 1985 , notes that Vitamin A is also lost through oxidation.)

For the same reason, Liebman continued, you shouldn't slice up vegetables and let them sit on a counter too long before cooking them. Every slice you make creates another surface by which Vitamin C comes in contact with oxygen; let them sit and you intensify the process.

Liebman cited a study showing that after 24 hours in the refrigerator, the 47 milligrams of Vitamin C in 100 grams of asparagus dropped to 38 milligrams. At room temperature, the same asparagus was already down to 25 milligrams of Vitamin C.

Oxidation occurs in other situations. The vitamins in produce sitting out all lunch hour in a cafeteria line are constantly being exposed to air. Besides that, the constant heating further contributes to deterioration.

Incorporating air into a product also increases oxidation. For instance, if you prepare frozen orange juice concentrate in your blender, you'll lose more Vitamin C than if you just stirred the concentrate and water with a wooden spoon, added Kendall.

*Light

Riboflavin is susceptible to losses via light, said Powers. Thus, leaving bread or milk in translucent containers and in sun or strong light is probably not a wise idea.

*Acidity

Adding baking soda to a vegetable to retain its green color is a nutritional no-no. The thiamine in the vegetables is susceptible to destruction, according to Powers. (Solution: Cook them for a shorter time to retain color as well as vitamins.)

The same goes for Vitamin C; it is destroyed by alkalis. But cook something in an acid medium and you'll better retain the Vitamin C. That means that if you boiled broccoli in orange juice, Vitamin C deterioration would be lessened, according to Kendall.

*Recipes

Here are some recipes that put these vitamin-saving ideas into practice: RED CABBAGE WITH HORSERADISH SAUCE (4 servings)

To lessen the vitamin C loss in cabbage, the vegetable is placed in the water after it has boiled. Some of the leached vitamins are then recouped by using part of the cooking liquid to make the sauce.

1 medium head red cabbage, cut in large wedges

FOR THE SAUCE:

2 tablespoons butter

2 tablespoons flour

1/2 cup milk

1/2 cup cooking liquid from the cabbage

3 tablespoons prepared horseradish

1/2 teaspoon dry mustard

1 tablespoon plain yogurt

Salt and freshly ground black pepper to taste

Bring a pot of water to a boil. (Use as little water as you can to boil the cabbage; this minimizes vitamin leaching.) Add cabbage, allow water to return to a boil and cook for 6 to 9 minutes. Drain, retaining 1/2 cup of the cooking liquid.

To make sauce, melt butter in a saucepan over low heat. Blend in flour, slowly add milk and vegetable cooking liquid and heat, stirring with a wire whisk, until thickened. Remove from heat and stir in horseradish, mustard and yogurt. Reheat but do not boil. Season with salt and pepper and serve with cabbage wedges. STEAMED LEMON-NUT GREEN BEANS (4 servings)

The beans are steamed, rather than boiled, to retain nutrients as well as crispness.

1 pound green beans, washed

3 tablespoons lemon juice

1 tablespoon butter

1 tablespoon chopped nuts such as almonds, pecans or walnuts

Wash green beans and cut off stems. Steam for 10 to 15 minutes, or until crispy-tender.

In a small saucepan, heat lemon juice to boiling and whisk in butter. Add nuts and cook over low heat for about 3 minutes. Toss with green beans and serve. ITALIAN GARDEN SOUP (5 to 6 servings)

This recipe is adapted from "Soup's On" by Nancy Baggett and Ruth Glick (Macmillan, $16.95). A stock has been added, which is made from the discards of the vegetables used as ingredients in the soup, adding to the vitamin content. In addition, the vegetables are cut in larger chunks to minimize oxidation.

FOR THE STOCK:

1/2 cup chopped onion

1 tablespoon oil

Tops and leaves from 2 celery stalks

Peels from 3 medium-sized carrots

Outer ends of yellow squash and zucchini

4 whole black peppercorns

Peels from 2 cloves garlic

Any other vegetable scraps such as potatoes and/or peels, tomato or onion skins, etc.

7 cups water

FOR THE SOUP:

1 tablespoon olive oil

1 tablespoon butter or margarine

1 medium-sized onion, finely chopped

2 large celery stalks, cut into chunks

2 garlic cloves, minced

4 1/2 cups vegetable broth

1 1/2 cups water

1 cup canned white cannellini or great northern beans, well drained

3 medium-sized carrots, cut into chunks

1 bay leaf

3/4 teaspoon dried basil leaves

1/4 teaspoon dried marjoram

1/4 teaspoon dried oregano

1/8 teaspoon thyme leaves

1/4 teaspoon white pepper

1/2 cup zucchini, cut into chunks

1/2 cup yellow squash, cut into chunks

2 medium-sized plum tomatoes, coarsely chopped

2 tablespoons chopped fresh chives or parsley

Grated parmesan cheese for garnish

To make the stock, saute' onion in oil in a large saucepan until tender. Add remaining ingredients and bring to a boil. Refrigerate the unused portion of the vegetables. Reduce heat and cook, covered, for about 45 minutes to an hour. Strain broth through a strainer and set aside.

To make soup, first combine the oil and butter in a very large saucepan or small soup pot over medium-high heat. Heat until the butter melts. Add the onion, celery and garlic to the pan and cook, stirring, for 3 to 4 minutes, or until the onion is limp. Add the stock and enough water to equal a total of 6 cups of liquid, beans, carrots, bay leaf, basil, marjoram, oregano, thyme and pepper. Bring the mixture to a boil; then lower the heat and simmer, covered, for 15 minutes.

Add the zucchini and yellow squash, and simmer 6 to 8 minutes longer, or until all the vegetables are tender. Stir in the tomatoes and chives and heat the soup for 3 to 4 minutes longer. Discard the bay leaf. Serve and pass a small bowl of grated parmesan for sprinkling. CAULIFLOWER IN CURRIED-ORANGE SAUCE (4 to 6 servings)

Cauliflower, high in vitamin C, is boiled in orange juice. According to Kendall, cooking in an acid medium helps to retain the vitamin. In addition, the cooking liquid is then used as the sauce.

1 small head cauliflower, separated into large flowerets

1/2 cup orange juice

2 teaspoons honey

1/2 teaspoon curry powder or to taste

1 tablespoon butter

2 oranges for serving, sliced

Trim, core and remove tough outer leaves from cauliflower. Combine remaining ingredients in a saucepan, bring to a boil and add cauliflower. Lower heat and cook, covered, for about 8 minutes. Serve cauliflower with cooking liquid, surrounded by sliced oranges. 209 1/2 RESTAURANT BROILED GINGER PORK CHOPS (4 servings)

These chops are broiled, which should retain more thiamin in the meat than other cooking methods.

1 teaspoon dried oregano

2 cloves garlic, minced

3 sprigs fresh parsley, minced

2-inch piece fresh ginger, peeled and chopped

2 tablespoons vegetable oil

4 center-cut pork chops

Combine oregano, garlic, parsley and ginger. Mix with oil until well coated. Spread over pork chops and let marinate 24 hours in the refrigerator.

Broil chops, about 15 minutes, turning once.