Produce News
Extending the Local Season
Canning Tomatoes

I am passionate about eating locally across the seasons in a Northern state such as Wisconsin. Though this endeavor presents a series of challenges, all of them are surmountable as long as you have a strategy. Granted, a strategy takes some amount of planning and education; however, incentives to eat locally are bountiful, and extend far beyond the (legitimate) logic of supporting a true local economy and the reduction of our dependence upon difficult-to-source foreign oils. These include, but are not limited to, environmental concerns and desires to strengthen community, both through the preservation of local jobs and the accountability resulting from the creation and maintenance of long-term local business relationships.

Most any fruit and vegetable may be canned for later consumption, though for our purposes here I have chosen to discuss canning tomatoes because of their popularity and versatility. If you are a gardener, tomatoes are a great choice to can because of their abundance in the late summer months. If gardening isn’t your thing, Owners of the Co-op may preorder local organic and local conventional tomatoes by the case from the Produce Department for canning, and for doing so you will receive a 10% discount on the price unless the item is already on sale.

Though many of our customers over the years have expressed deep intimidation about embarking on the journey of learning to can, I would like to reassure you that this is an American tradition that “regular folks” have participated in for well over a century. There are some initial terms that must be learned and it is important to follow the instructions very closely for safety purposes, but once you get the gist of it, it is not intimidating at all, provided you choose to can in small batches. When I first began canning it would have been helpful to have these steps demystified, so I will outline the process in the way that I wish it had been explained to me. Before I go into any specific detail, the basic steps in canning are: washing and sterilizing jars and new lids, washing and prepping the tomatoes, then transferring the prepared tomatoes and some form of acid like lemon juice into the sterilized jars which you will then boil in water for a specified length of time in order to kill harmful bacteria. As long as you follow each of these steps closely, you are in great shape.


In order to get started you must have these tools: a canning kettle and rack, a funnel, wide tongs that will pick up a Mason jar, sufficient Mason jars with brand new lids and bands, and a good resource for safety information. (Reusing canning lids is not an option. Only new lids made for canning jars are safe to use for food preservation. Reused lids will not seal, allowing potentially deadly bacteria to grow in the canned goods.) Canning kettles are typically sold with a rack, and the other tools are often sold together in what is called a “canning kit.” Many of these items are available here at the Co-op; otherwise you can purchase them at our local Ace Hardware across the street. Want a peek at these items so you’ll know what to look for? I often do research online first, then purchase the items from a locally owned store or producer when possible.

As for resources and instruction on safety, I highly recommend two tools, the first of which is the UW-Extension’s website. There they offer free online safety tips for home canning, available in a printable PDF file. I go to this address: and enter the words “canning food” into their search engine. Instantly you will have pages and pages of printable articles at your disposal. The second useful tool I suggest is a good book. My favorite is called Putting Food By. I bought my copy here many years ago, and it has gotten a lot of use. Though it lacks photographs, I find that I rely on it far more for good advice and dependable recipes than books with fancier appearances. Another great option that has less information but plenty of stunning photographs and quality recipes is the Ball Blue Book Guide to Preserving, which is published by the Ball Corporation, makers of Ball glass Mason jars.


What I am outlining here is the method known as “water-bath canning.” There are other safe methods of canning, but this is the one with which I am most familiar.

  1. Select your fruit. Choose tomatoes that are fresh and unblemished with no punctures or damage to the skin. How many tomatoes do you have? If you’ve preordered a case of local slicers, which is what I typically use, they tend to come in 20 lb. cases. Romas, which are good varieties for sauce, may come in 20 or 25 lb. cases, depending on the vendor. You can plan that it will take 2 1/2 to 3 lbs. of tomatoes to fill each quart jar, which means a case of tomatoes will yield you approximately 6 or 7 quart jars of canned tomatoes. This seems like a good quantity to start with if you’ve never canned before. Like I’ve said in previous articles about home food preservation, the key to getting into canning is to avoid feeling overwhelmed. Start small and you will be far more likely to enjoy the process and return to it instead of seeing it as prohibitively difficult or inconvenient.
  2. Prepare the tomatoes. This involves removing the spot at the stem where they were attached to the plant, which is easily done with an apple corer. Then you must remove the tomato skin, which is done by lowering the tomatoes one by one into a pot of boiling water. You need only hold it under the water for a moment or so, then remove and peel. I use a slotted spoon with a bamboo handle that I believe was intended for frying pot stickers. It works great. Then I lower the hot tomato onto a paper towel or a towel to peel it. Some people do this in a bowl of cool water to protect their fingers from the temperature. The skin should slide right off. Compost or discard the skin and collect the peeled tomatoes in a large bowl for later use.
  3. Sanitize jars and new lids that have been pre-washed in warm sudsy water. To sanitize, fill the canning kettle with water on stove. Lower jars into water. This will be much easier if you first fill them with hot, but not boiling, water. Otherwise they will float and clink around as you try to submerge them. Once submerged, make sure that the jars are at least one inch below the surface of the water. Bring to a boil, and let them remain submerged in the boiling water for 10 minutes. Keep the jars warm. You need not keep them in the oven, but do not sterilize the jars one day and can the next. For safety purposes, you must sterilize the jars and new lids immediately prior to processing tomatoes. Some people sterilize jars in a separate pot of water and leave them submerged until they are ready to fill one by one. This can be helpful if you happen to have another large pot. (In all the years I’ve been canning, I have always sterilized jars before canning, regardless of what it is that I am processing. Most books I’ve read and resources I’ve used simply state, “sterilize jars” in the instructions of any recipe. I have found one book, however, that outlines a rule of thumb regarding the sanitization of jars. Carol Costenbader’s text, The Big Book of Preserving the Harvest, states that sterilizing jars prior to canning is unnecessary if you plan to process an item for longer than 8 minutes. Tomatoes are processed for 85 minutes, which, according to her rule, renders them a safe candidate for skipping this, but only this, step. Since this is the only source I have found that alleges it is safe to can without sterilizing jars I will let you use your discretion on the matter.)
  4. Add 2 Tbs. of lemon juice or 1/2 tsp. citric acid to each quart jar. The addition of acid is necessary in order to prevent the growth of potentially deadly bacteria in the jars. This step is non-negotiable. If canning a lower acid variety of tomato like an heirloom, or any variety that is orange or yellow in color, use at least twice the amount of acid per quart jar just to be safe.
  5. Immediately fill sanitized jars with peeled whole or halved tomatoes using a funnel. Use uniformly sized tomatoes in each jar. Leave 1/2 inch of headspace.
  6. Add optional salt, up to 1/2 tsp. per quart jar.
  7. Remove air bubbles using a nonmetallic utensil.
  8. Wipe the rim of each jar with a clean dishtowel to ensure that not a trace of liquid remains on the rim of the jar. This could prevent a proper seal from forming and endanger your health, so take painstaking efforts to be sure that each jar is clean around the rim before you apply lids and screw on bands. Screw bands firmly but not too tight, because they will tighten some in the water bath process.
  9. Lower full rack of full jars into water. Check to be sure water level is at least 2 inches above the tops of the jars.
  10. Heat the water to a full boil. One the water has reached a rolling boil, set timer for 85 minutes. Turn burner down to a temperature that will allow the water to maintain a gentle boil for the duration of the 85 minutes. If you live in a higher altitude location than Madison, check a resource in your area to adjust processing time. This is necessary for your safety.
  11. Check periodically throughout these 85 minutes to be sure the water level remains 2 inches higher than the tops of the jars. Add additional boiling water if necessary. Keeping a teakettle full of water on another burner is handy for this purpose.
  12. When timer goes off, use a jar-lifter to lift each jar out of the water and onto a clean towel on the counter. Remove all jars, leaving at least an inch between them so that air may circulate.
  13. Did you hear a pop or seven? That’s the sound of each of your jars sealing! That sound is such a quintessential sound of summer in my household. It also means that a proper seal has been formed which ensures your food safety. If you are unsure as to whether or not your jars have sealed, press the lid of each jar after they have cooled. There should be no give. If you fear a jar has sealed improperly when you check them immediately after the canning process, you may refrigerate the contents for use immediately. If all seals look safe, store jars in a cool, dark place. When it comes time to use your locally grown, home canned tomatoes, check the seal when you open each jar. Do not eat the contents if the seal looks moldy or imperfect, especially if you see any discoloration or smell something foul. Since not all harmful bacteria may have a foul smell, pay strict attention to other signs of an improper seal or spoilage. I use a motto taught to me by my mother. “When in doubt, throw it out.” In all my years of canning, I have never seen a jar that looks suspect, yet still I look each time I open a jar of my tomatoes, tomato juice, salsa, corn relish, pickles, jam, marmalade, or what have you.

A final note about safety

Canning at home is safe and easy as long as you explicitly follow all instructions. Do not skip any steps, or you or a loved one could become very sick or even die. I don’t mean to use overly dramatic language or frighten you. Deaths by consumption of home canned goods are extremely, extremely rare. Driving a car or riding a bicycle, activities most of us engage in everyday, are far more dangerous than home canning, and we don’t hesitate to participate in such activities because we adhere to safety rules that protect us. Think of canning in similar terms. Have a healthy respect for its dangers, and use that respect to motivate you to follow steps to ensure cleanliness and sanitation.

Uses for canned tomatoes

Canned tomatoes are incredibly versatile and may be used in soups, stews, salsas, pasta sauces, and in casseroles. My boss, Andy Johnston (quite the home canner himself), inspired me with a recipe for canned cherry tomatoes, which he only preserved randomly one year because he had too many in the garden. Can cherry tomatoes as you would any other whole tomato. Remove from jar and spread on baking sheet. Top tomatoes with chopped garlic and drizzle with olive oil. Place in broiler, and allow to warm just until a dark crust begins to form on tops of tomatoes. There is no need to get them piping hot. Once tops of tomatoes look slightly darkened, remove from oven and serve over (preferably locally made) pasta. He describes this dish as “tasting just like summer.” I will have to be sure to try this.

One more thing

I really hope you decide to give canning a try, even if it is just once. In the midst of winter, nothing compares to opening a jar that you’ve canned yourself, the contents of which either came from your backyard or a farmer in your community. Though it does take work in order to can in the busy summer months, in the winter you will be rewarded for this with locally grown convenience foods. Hunting for your favorite canning recipes can be a fun pursuit, and over the years each of my friends has some canned, frozen, or preserved good that has become known as their specialty. We use terms like “Maggie’s Pickles,” “Sarah and Wendy’s Tomato Juice,” “Katie’s Corn Relish,” “Chris’s Peaches,” “Amanda’s Pesto,” “Nancy’s Strawberry Freezer Jam,” “Dan’s Horseradish Sauce,” and “Andy’s Garlic Powder.” We give these items to friends and family as gifts, and even trade with one another, which allows us to make more batches of the things we’re best at while still ensuring that we’ll have a variety of goods put up for the winter. I hope that you will be similarly inspired. Perhaps one day I will even refer to your dilly beans or raspberry jam by name! Happy canning!