"SO WHAT'S a canning jar look like, anyway? How does it work?" asks a sheepish inquisitor of Karen Cozza, Northern Virginia's food preservation hot line technician. "Well, a canning jar," she replies with an easy telephone voice, "is a heat-tempered jar. It has a lid with a sealing compound and a ring or screw band." Cozza may have said it a dozen times already this day, but the caller will never know.

Cozza's friendly manner is just one indication that she enjoys answering preservation questions. The other indication is that she takes the time to give frantic callers the right answers, with reasons. Tomatoes, for example, need only be water bathed in a hot pack for 35 minutes because of their high acid content; pints of green beans, however, must be pressure-canned at 10 pounds pressure for 20 minutes because they are low in acid and attract bacterial growth.

As for the jars, "we only recommend using Mason jars specifically manufactured for home canning because they are tried and true," she continues. "Old mayonnaise jars were made for one-time use only and could have weak spots." She is calm, and carefully explains how to properly use the jar. Once the mystery is cleared up, she offers recipes and timetables to help the caller get started.

While the hot line is funded by the county extension agencies in Alexandria, Fairfax and Arlington for residents of those jurisdictions, Cozza says she receives an average of 20 calls each day from all over the metropolitan area. The average call lasts 10 or 15 minutes, she says. "People don't call with just one question--they've got a whole list."

Late August, the height of the fruit and vegetable season, is also the height of the preserving season. Home economists at all the county extension service offices are ready with answers to home canning problems. What they've noticed is that many preserving questions are repetitive, like the question Cozza regularly receives about canning jars. What is pressure canning? Can fruit be canned in water? Can pickles be canned without salt? Can you leave the sugar out of jams and jellies?

Before you make a call, though, here is some of what the extension agents will have to say. First, you'll need to understand the types of preserving and what's recommended. Then you'll want to know the procedure for putting it all together, plus some of the potential snags. Finally, some recipes for stocking your winter pantry. Types of Preserving

Pressure canning: The preservation of low-acid foods, such as meats and vegetables, in a 240-degree steam bath at 5, 10 or 15 pounds pressure for a specific period of time. The high heat of a pressure canner prevents the germination of Clostridium botulinum spores, which create a potentially fatal toxin leading to botulism poisoning. The toxin only grows at room temperature in low-acid, oxygen-free environments, Cozza says. In addition, home economists recommend boiling home-canned vegetables and meats for 10 minutes before serving to kill off any botulism spores that may have germinated.

Water bath canning: A means of preserving fruits (high-acid foods) by submerging the filled jars in up to 2 inches of boiling water (at a constant 212 degrees) for specified periods of time. It requires the use of a rack, to hold the jars, and a large kettle with a lid.

Steam canning: Steam canners work just like the stove-top vegetable steamer in which the jar sits on a rack above boiling water. Manufacturers tout them as being more energy efficient and time saving, says Cozza, but "we think they are unsafe." Steam heat fluctuates above and below 212 degrees and could result in underprocessing or overprocessing.

Microwave preserving: Setting the jar, without its lid (or it would explode), in the microwave oven for a specified period of time. This is not recommended because leaving the lid off exposes food to bacteria in the air and the heat waves may not penetrate the contents evenly. Water bathing in a microwave is impractical because of limited space. Procedure

Before you get started, first and foremost, remember to use top-quality fruits and vegetables. Canning, after all, does take time to do it right, so why not use the best?

Clean, slice and skin the produce and decide whether to hot pack or raw (cold) pack the produce. These terms apply to the temperature of the produce before it goes into the jar. Hot pack refers to the boiling of produce in water or syrup (times vary according to each fruit or vegetable, see recipes below); raw pack is unheated produce. Hot pack is somewhat better as it wilts produce, making more storage room in the jar. It also eliminates the problem of floating fruit. Only tomatoes, among fruits, must be hot packed to kill off bacteria. Processing times are generally longer in a raw pack.

Hot or raw pack the produce into the jars leaving recommended "head" space (see specific recipes below) at the top for expansion. Wipe the edge clean; take a plastic or wooden utensil and run it along the inside edge to eliminate any water bubbles. Place the lid on the top edge. Place the screw band on the top edge of the jar and screw down with the full strength of the hand. This should be a nice firm turn, without wrenching it on as tightly as possible. Process (see times below) in a boiling water bath or pressure canner. Immediately remove jars from the water-bath canner or let pressure go down to zero on the pressure canner. Place the jars on a rack to cool for 24 hours before taking the rings off.

Check the jars for proper seals after the rings have been removed. The tops should be dented in and should have a clear ring like a bell when tapped. (A dull thud is a sign of improper sealing.) If the lid has a wrinkle in it, then it was screwed on too tight. If you can lift the lid with your finger then you know the lid wasn't screwed on tight enough. You should have to remove the lid with a can opener. Store the canned goods in a cool, dry place. Potential Snags

Misuse of salt: Salt is not necessary to can vegetables; it only acts as a flavor enhancer. Salt, however, is extremely important for pickle brine. In fermented pickles salt encourages the growth of lactic acid bacteria which, in turn, gives them their sour acid taste. Salt also firms cucumbers and draws out water. In addition, there's a certain balance between salt and vinegar that helps raise the acid level in pickles necessary to kill off bacterial growth.

Eliminating sugar: Leaving the sugar out of jams and jellies will change their consistency. There is a critical sugar-to-acid-to-pectin ratio that causes them to gel. However, a new calcium-solution product, low methoxyl pectin, works with the acid causing gelling without sugar. It is available in bulk from Kennedy's Natural Foods in Maryland and Virginia, eight ounces for $7.24. It will not create a firm gel, but it will be thick enough to use on toast and rolls, Cozza says. Jams and jellies made with low methoxyl should be stored in the refrigerator.

Fruits, on the other hand, do not require sugar syrups when canned. They can be packed in their own juices or other fruit juices such as apple juice (most common) or white grape juice. Processing is the same as with a sugar syrup.

Loss of liquid in jars during pressure canning: This has a number of causes, the most common being that the pressure has not been maintained at a constant point during processing. Other reasons are hurrying the cooling process by releasing the pressure valve before the pressure is down, not fully tightening the jar lids or packing vegetables in the jar too tightly. While the problem may result in some discoloration and loss of quality, the liquid loss is harmless to the consumer as long as the jar is perfectly sealed. Canning Fruits and Vegetables

Corn: Freezing corn on the cob is tricky, since blanching is necessary to deactivate enzymes that destroy nutrients, flavor, texture and color.In order to kill the enzymes through to the center, corn on the cob must be blanched for 11 minutes, which is longer than normal cooking time. For kernels, however, blanch corn on the cob only 4 minutes. Cool in ice water. Cut kernels at about 2/3 their depth to the cob. Pack into freezer containers leaving 1/2 inch at the top or into freezer-weight plastic bags.

Corn can also be canned in a hot pack. Cut the corn kernels from the cob at 2/3 their depth, without prior blanching. Add 2 cups boiling water for each 4 cups of corn. Bring to a boil and pack immediately into jars and add liquid to within 1 inch of the top. Salt is optional. Run a plastic knife or wooden handle along the inside edge to eliminate water bubbles. Process in a pressure canner in pint jars for 55 minutes at 10 pounds pressure.

Eggplant: Eggplant is best frozen. The biggest problem in preserving it is discoloration. Prepare it in small batches. Cut the slices 1/3 of an inch thick and blanch 4 minutes in a gallon of boiling water mixed with 1/2 cup lemon juice or 4 teaspoons citric acid. Cool, drain and pack into freezer containers 1/2 inch from the top or into freezer-weight plastic bags.

Green beans: They can be either frozen or canned, and the difference is a matter of personal texture preference. Frozen green beans tend to toughen, so they must be picked very young and tender and processed immediately. Older beans, however, are better in the pressure canner. To freeze, blanch green beans 3 minutes then pack in freezer container. Add cold water to within 1/2-inch from the top of the container and freeze. Canned green beans are better hot packed because they become more pliable and the jars can be filled more compactly. To hot pack, cover green beans with boiling water and precook for 5 minutes. Pack into jars leaving 1/2-inch head space. Add salt if desired. To raw pack, pack tightly into jars and cover with boiling water. For either hot or raw pack run a plastic knife or wooden handle around the edge to remove air bubbles. Process in a pressure canner 20 minutes for pints at 10 pounds pressure; 25 minutes for quarts.

Greens: Because of their high density, greens would require extensive processing in a pressure canner. Hence it is more efficient to freeze them. Pick tender, young green leaves. Remove the woody stems. Blanch for 2 minutes in boiling water. Cool immediately in ice water. Drain and package in freezer containers to within 1/2 inch from the top or into freezer-weight plastic bags.

Okra: An abundant crop in this area, okra needs to be pressure canned or frozen. Blanch for 1 minute in boiling water. Cut into 1-inch lengths or leave whole. Pack in jars and cover with boiling water. Run a plastic knife or wooden handle around the edge to eliminate air bubbles. Process in a pressure canner 25 minutes for pints, 40 minutes for quarts. To freeze, blanch 3 to 4 minutes in boiling water. Cool in ice water. Pack into freezer containers to within 1/2 inch of the top or in freezer weight plastic bags.

Peaches: Peaches can be canned or frozen. With hot packing more peaches fit in the jar; they have better texture and don't float to the top above the liquid. In raw packs, the liquid is frequently siphoned out of the jar during processing and there is a residue on the jar that has to be cleaned off. In addition, the peaches float to the top of the raw-packed jar. Although this doesn't hurt them, they do lose their visual appeal. Should you still choose to raw pack, however, lay a couple of wine corks, which remain in the container, on top of the peaches just before sealing the jar and this will solve the floating problem. To process in a raw pack, toss peeled and halved peaches with an anti-darkening agent. Pack into jars leaving 1/2-inch head space and cover with boiling syrup, juice or water. Run a plastic knife or wooden handle along the edge to eliminate air bubbles. Process 25 minutes pints and 30 minutes for quarts. To hot pack, drop peaches into boiling liquid and cook just until heated through. Pack into jars, cover with liquid. Run a plastic knife or wooden handle along the edge. Process 20 minutes for pints, 25 minutes for quarts.

Peaches also freeze quite well. Good freezing peaches are the Blake and Redhaven varieties. Blanch for 30 seconds and peel. Slice and remove the pit. Cut into halves or sections, and use an antidarkening agent such as fruit fresh or lemon juice and pack into jars. Add cold syrup, water or juice and freeze. They can also be dry packed into a container, sprinkled with an anti-darkening agent. Before freezing, however, be sure to remove excess air by crumbling waxed paper and laying it on top of the peaches as a weight that remains in the container.

Tomatoes: Recent research indicates that bacterial spores are not always destroyed during cold pack water bathing, as previously recommended by USDA. Accordingly, USDA has changed its recommended processing procedures. In addition to recommending hot packs for tomatoes, the water bath processing time has been lengthened from 10 minutes to 35 minutes. To can, heat tomatoes to boiling, stirring to make sure they do not stick. Pack into jars to within 1/2 inch of the top. Add 1/2 teaspoon of salt for pints; 1 teaspoon of salt for quarts (for flavor only). Run a plastic knife or wooden handle along the edge to eliminate water bubbles. Process in boiling water bath for 35 minutes. Freezing tomatoes is not recommended, as they become mushy when defrosted because of their high water content.

Zucchini: Zucchini's high water content makes it hard to freeze, but it can be done if grated first. Salt can be added to preserve color (mix with 2 teaspoons of salt per quart of grated zucchini), but if it is, you'll want to remember months later not to add any more when cooking. Steam without water, then pack into freezer bags or containers. You can also freeze zucchini in 1/2-inch slices, by blanching for 3 minutes and packing into freezer containers or freezer-weight plastic bags. Use in casseroles and other baked dishes. Another way to freeze zucchini is to cook it into recipes ahead of time. This vegetable is too soft to can. It will cook to pieces.