Although it’s surprisingly easy to can tomatoes, it’s also a kitchen task with a few hard-and-fast safety rules. I grew up canning, learning from the women in my family, and have taught canning classes for five years. Here are questions that my students ask most often:

What varieties of tomato are best for canning? For paste? For jams or chutneys? What is the expected yield?

Although just about any tomato can be canned, Brandywines are consistently exceptional. They are meaty, large, not too seedy or full of watery pulp, and a very pretty red. They are also widely available and reasonably priced compared with many heirloom varieties.

Always choose disease-free tomatoes. If you cut them open and they are black inside, that is a sign of blight, and those tomatoes should not be processed. Overripe tomatoes, often marked as “seconds” at farm markets or farm stands, are wonderful for canning. Just make sure to cut out any bad spots during prep.

Cathy Barrow's water-bath canned tomatoes, processed in a pot on the outdoor grill. (Stephen A. Behrens/PHOTO BY STEPHEN A. BEHRENS)

Romas, San Marzanos or other “paste type” tomatoes are best for, well, paste. Frankly, paste may be the least rewarding tomato product to put up, because 20 pounds of tomatoes yields only two or three cups of paste.

Any tomato can make a superb jam or chutney. The more watery the tomato, the longer the cooking time. Consider the color of the tomato when making these long-cooked products. Yellow tomatoes or green (ripe) tomatoes do not cook down to an attractive color; try the dark ruby, carmine or purplish types for a gorgeous end product.

Expect two to three pounds of raw tomatoes to convert to one quart of crushed tomatoes. For tomato sauce, plan on four to five pounds per quart. For jam, chutney, ketchup and other long-cooking, thick preparations, it will take about eight pounds to make a quart.

Why do tomatoes have to be peeled for canning? What’s the best way to peel, seed and make sauce?

Tomato skins can be tough and bitter, so it’s nice — but not necessary — to remove them from tomatoes to be canned.

A ridiculously easy and satisfying method, presuming you have the freezer space, is to seal tomatoes inside food-safe plastic bags in the freezer. After a few hours, the skins will split and slip right off. This is particularly useful if you are growing tomatoes, as gathering sufficient weight to make canning efficient may take a few days. Holding them in the freezer will not change their taste, texture or appearance.

The more widely used method for peeling tomatoes is the one you’ll find within many Washington Post recipes: Use a sharp knife to score an “X” at the bottom of each tomato and remove the stem. Drop a few at a time into a pot of boiling water and remove them as soon as they bob to the surface. Peel off the skins as soon as the tomatoes are cool enough to handle and discard the skin. (I use a cooler filled with ice to stop the cooking and hold the tomatoes as they come out of the boiling water.)

If you are planning to make a sauce, a food mill is indispensable. There are classic stainless-steel food mills that clamp on the edge of a counter or table. Simply feed cut up tomatoes into the top, skin and seeds are ejected out the side and the sauce pours from the spout. These mills are generally available at hardware stores; used ones are often on eBay, in secondhand shops and garage sales. They make fast work of a big box of tomatoes. Even easier: the food-mill attachment for the KitchenAid stand mixer.

Hand-cranked food mills like Oxo’s version fit over a pot and require no table or counter for clamping.

What is the difference between water-bath canning and pressure canning? When do tomato products need to be pressure-canned?

The natural acidity of tomatoes varies. Acidity generally keeps foods safe (free of botulism). The National Center for Home Food Preservation recommends adding two tablespoons of lemon juice or 1 / 2 teaspoon of citric acid per quart of tomatoes to maintain safe acid levels.

The water bath process will bring the contents of the jar to 212 degrees Fahrenheit, the temperature of boiling water, sufficiently safe for the most useful canned tomatoes in the pantry: crushed tomatoes and tomato sauce (see related story). Start with a pot that’s deep enough to hold the jars and enough water to cover them by an inch or two. Place a rack or a folded dish towel in the bottom of the pot; this will keep the jars from rattling (and possibly breaking) once the water starts to boil.

Pressure canning is necessary when you want to add low-acid flavor agents such as garlic, peppers and/or onion to your tomatoes. A pressure canner is not the same as a pressure cooker.

A pressure canner is a tall, stockpot-shaped piece of equipment with a rubber gasket, a gauge and, perhaps most important, a book of instructions.

Why didn’t my jars “ping”? What is siphoning?

The most frustrating part of a day spent canning tomatoes has to be seal failure, which is most commonly caused by food particles on the rim of the jar that interfere with the rubber gasket’s contact with the glass. Ideally, a vacuum forms inside the jar as its water-bath-processed contents cool down. The force will pull tight the metal lid against the gasket. That makes the pinging sound that signals a successful seal.

To avoid seal failure, wipe the rim and glass thread of each jar carefully with a damp, clean towel before placing the lid. If you are canning something that has oils or fats, use distilled white vinegar.

Sometimes, headspace can be the issue. You’ll find it noted in every canning recipe — a necessary safety check. Although it might be tempting to get just a bit more in each jar, transfer any excess to separate, non-canning containers. An overfilled jar will siphon during water-bath processing, which means the contents will boil up from under the lid, making a good seal impossible.

If only one seal fails among a batch of jars, put that jar in the refrigerator and use it first; it should keep for about a month. If several jars fail to seal, transfer the contents of those jars to a pot and bring to a boil for several minutes. Then use it to refill clean, sterilized jars with new lids and clean rings and repeat the water-bath process.

Is it possible to can using a portable propane burner, like my camping stove? Or the burner for the turkey fryer? How about the extra burner on my grill? Will my glass-topped stove work for canning?

In order: Yes, it’s possible. Talk to the old hands and they’ll speak of canning kitchens outside, on the back porch or in a shed, well away from the house. Camping stoves and turkey fryers are good places to put boiling pots of water for processing jars. Make sure you have plenty of propane. Even a gas grill with a side burner is useful for canning outdoors. Processing the tomatoes outdoors is a good way to keep the heat out of the kitchen.

While canning on an electric stove is no different than on gas, ceramic- or glass-topped stoves can be a challenge to canners. Some manufacturers do not recommend canning on their stove tops at all, while others have strict rules to ensure success. Review the manufacturer’s recommendations for your stove before proceeding. The weight of a large canner filled with jars and water might crack the top of a ceramic or glass stove.

If your ceramic- or glass-topped stove is approved for canning, only boiling water-bath canning is appropriate, as the temperatures cannot be modulated sufficiently for pressure canning to be safe. Use flat-bottomed pots, so the entire surface is in contact with the burner. Make sure the pot is no larger than the circumference of the burner. Never slide the pot. Always lift it when placing it on or taking it off the stove.

How do I can salsa and chutney? What about ketchup?

Salsa, chutney and ketchup are three classic condiments prepared from tomatoes. Each contains onions and garlic, which require a good ratio of acid to keep them botulism-free. (See the question on pressure canning.) Most salsa, chutney and ketchup recipes include sugar, vinegar, lemon juice and other preserving agents, but the proper balance is essential for safety.

For preparations you plan to can for long-term storage, it’s best to use time-tested recipes and follow their instructions to the letter. For many salsa recipes, particularly, pressure canning is the safest way to go.

Are the recipes in my antique preserving book safe? My grandmother’s sauce recipe is delicious. How do I make sure it’s safe for canning?

Rebecca Davis, project leader at the University of Maryland Extension Office, recommends using only recipes printed since 1997. Many new rules have come about with the changes in varieties of vegetables available, the return of heirloom crops and new food science information. Above all, be safe when you can.

If you are determined to can your grandmother’s recipe and you want do so safely, you might contact Martin Lo at He’s a food science professor at the University of Maryland who operates a private laboratory that can test your recipe for a fee.

For more information, go to and the National Center for Home Food Preservation. Barrow, who blogs at, will join today’s Free Range chat at noon: