When canning tomatoes safely at home, be sure to use tested canning recipes that add citric acid, vinegar or lemon juice. Tomatoes are a medium-acid food with a pH in the range of 4.5–5.2. You must use a tested canning recipe that has determined the amount of acid to add in order to increase the acidity by lowering the pH to a safe level below 4.6.
The Science behind pH
When ingredients dissolve in water (H20), they produce ions H+ and OH–. If there are more hydrogen (H+) ions, the solution is acidic. If there are more hydroxyl (OH–) ions, it is alkaline or basic. Therefore, acidity is determined by measuring the potential hydrogen, abbreviated pH. The pH can range from 0 to 14. A pH of 0 to 6 is acid, 7 is neutral, and 8 to 14 is alkaline. Lemon juice has a pH of 2 to 3. Tap water has a pH near 7 (neutral), while rainwater is around 6 (slightly acidic) and seawater is around 8 (slightly alkaline). Baking soda solutions have a pH around 9, and household ammonia has a pH of 11 to 12. High-acid foods have a relatively low pH of 4.6 or less.
You cannot determine a safe acidity level by simply testing the pH when canning tomatoes or any other food; you must use tested canning recipes. For one thing, the pH of a food fluctuates from one variety to the next (plum tomatoes versus beefsteak tomatoes, for example) and within a specific type (green, mature, ripe, or overripe tomatoes).
In addition, something called water activity (aw) influences the canning process. Water activity is not the same as water content. Aw is a measurement of the free water on a molecular level, or the water that is available for food-borne microorganisms to grow. Other ingredients in canning recipes such as sugar, salt, liquids other than water (tomato liquid), and the ratio of solids to liquid also influence water activity. This is why tested canning recipes are the only safe way to can foods at home.
Citric acid, vinegar, and lemon juice
The following acids are available to increase the acidity (lower the pH) in tested canning recipes when canning tomatoes safely at home:
- Citric acid powder (food grade) is available from suppliers selling natural foods, nutritional supplements, and candy-making supplies. Vinegar or lemon juice may be more readily available.
- Commercial vinegar (5 percent or higher acidity) is available in any grocery store. You must use commercially bottled vinegar with known acidity strength of at least 5 percent (50 grain).
- Bottled lemon juice (a type of citric acid) is readily available in grocery stores. You must use commercially bottled lemon juice because the pH of fresh lemon juice fluctuates.
Which acid is best for canning tomatoes?
Last year, I canned pint jars using each of these three types of acid, with and without sugar. In all jars, I used the same batch of locally grown, field-ripened San Marzano tomatoes. This paste-type tomato is very sweet when fully ripe. For each type of acid, I found that the canned tomatoes without sugar tasted slightly better. Added sugar tended to overwhelm the tomato flavor making the canned tomatoes a less than optimal ingredient in some recipes.
Canning tomatoes with citric acid powder produced very good tomatoes with no competing flavors. As expected, tomatoes canned with commercial cider vinegar and bottled lemon juice had altered flavors. Note that the flavors mellow (like most pickled or acidified products) after 4 to 6 weeks, so it’s best to test your canned products after several weeks. After a few months, the tomatoes acidifed with cider vinegar had rich tomato flavor and pleasing acid balance. Tomatoes canned with bottled lemon juice had a lemony undertone that seemed to compete with, rather than balance the tomato flavors.
When using canned tomatoes in cooking, I found that lemon juice didn’t work as well as cider vinegar with many of the dishes I like to prepare, such as marinara sauce, chicken cacciatore, lamb moussaka and other dishes from central and east Europe. If you prepare more Latin American dishes or tomato sauces to serve with seafood, you may find lemon juice more acceptable. However, the flavor of bottled lemon juice is not comparable to fresh. Overall, many traditional canners with whom I have spoken find the flavor of bottled lemon juice objectionable and the last choice for increasing acidity when canning tomatoes. Everyone seems to agree that citric acid powder is the preferred choice because it contributes no off flavors.
If you are unsure whether to use lemon juice or vinegar, can a few jars with each type of acid. To counteract lemon or vinegar flavors, or simply as a matter of preference for your style of cooking, you may also add sugar. Since the acid and sugar are added to the jar before filling with tomatoes, it is easy to experiment when canning tomatoes at home to determine the type of acid that works best for you. Just remember to mark the lid clearly so you remember which jar is which!
You might also like:
- How to get started with the canning preservation method
- Methods and recipes for oven-dried tomatoes, dried fruits and vegetables
- An overview of 10 home food preservation methods from ancient to modern
For information about eight different food preservation methods, including over 300 delicious recipes, get the book The Home Preserving Bible by Carole Cancler, available from booksellers everywhere.
Awesome! Wonderful! Thank you! We’re using citric acid.
How much citric acid powder did you add per pint to make it safe for canning?
Hi Jessica, To acidify tomatoes to make them safe for canning, add 1⁄4 teaspoon to pint jars and 1⁄2 teaspoon to quart jars. You just add the citric acid to the jar, and then fill with tomatoes using the packing method you have chosen (e.g. crushed tomatoes hot pack, whole peeled tomatoes cold pack, or whatever).
Along with citric acid, you may also want to add salt and/or sugar. (Personally, I add some salt, but I don’t like added sugar. Both are a matter of personal preference. Both help told color and flavor, but do not affect safety). If you want to add either one, try 1/2 tsp. salt per pint and/or 1-1/2 tsp. sugar. Keep notes (I simply write it on the lid–no guessing!) and then adjust next year depending on your preferences.
I have always canned a LOT if Roma tomato juice and never had this problem!!! They won’t stay sealed! They seal at the time of canning but within a few days they come unsealed. Their lids at time of canning have all dented downward. After about 48 hours I remove the rings and I am able to pick them up by the flats and they are sealed. It seems like they are fermenting and separating in the jars. Has anyone ever had this happen? I have been canning,for years and have never had this happen..HELP!!!!!!
Hi Angelia (what a pretty name!),
The only thing I can think of is you are boiling or heating your lids and you shouldn’t be heating newer lids at all (both Kerr and Ball) . Newer lids use a plastic material that weakens and can fail during storage. Read the box instructions to be sure. If it says simply wash and set aside, then do that…without simmering as we used to do.
Argh. That’s frustrating, since they’re testing sealed, but failing later. But yeah, the sealing compound on the lids was changed some time ago and neither company said a peep. I’ve had a higher than normal seal failure rate–it used to be a rare thing, and now it happens fairly often. Not every batch, but every couple batches there’s 1 or 2. Hopefully NOT heating the lids will do the trick.
How much vinegar is used in canning tomatoes? The same as lemon juice?
Hi Tori,
No, the amount of vinegar is twice as much as lemon juice:
Quart jars: Add 1⁄4 cup cider vinegar, 2 tablespoons bottled lemon juice, or 1⁄2 teaspoon citric acid powder per quart. To counteract acid flavors or as a matter of preference, add 2 teaspoons granulated sugar and/or 1 teaspoon salt per quart.
Pint jars: Add 2 tablespoons cider vinegar, 1 tablespoon bottled lemon juice, or 1⁄4 teaspoon citric acid powder. To counteract acid flavors or as a matter of preference, add 1 teaspoon granulated sugar and/or 1⁄2 teaspoon salt per pint.
years ago I found directions for canning green pears and mellon balls, ( one of the canning jar manufactures recipe books) they were wonderful with the mellons retaining the flavor and crispness… I have lost the recipe and wondering if anyone out there has ever seen or heard of it It was a lot of steps and work but well worth it, used as Christmas gifts.
Hi Crystal,
I assume it was pickled pears and melon balls. You cannot can melon balls unless they are pickled, because they are too low in acid. Here is a recipe:
Pickled Melon: Select melons that are full size but green and firm to the touch, including stem area. Wash, halve, and scoop out seeds. Cut about 2 pounds washed melon into 1-inch slices, remove peel, and cut into 1-inch cubes; or, use a melon baller to scoop out balls of melon. In a saucepan, combine 2 ¼ cups cider vinegar, 1 cup water, and 2 ounces fresh peeled and sliced ginger slices, and bring to a boil. Reduce heat, and simmer 5 minutes. Pour hot liquid over melon cubes. Cover and refrigerate melon for 12 to 18 hours. Strain vinegar solution into a large saucepan. Stir in 2/3 cup granulated sugar, 2/3 brown sugar, and ½ teaspoon crushed red pepper (optional), and bring to a boil. Add melon and ginger slices, and return to a boil. Reduce heat, and simmer 1 hour, or until melon turns translucent. Fill hot canning jars with the hot melon chunks and pickling liquid; adjust headspace to 1-nch. Process pickled melon pints for 15 minutes (at 0 to 1,000 feet).
Variation with pears: substitute half of the melon with firm, slightly underripe pears that have been peeled and cored, then cut into 1-inch pieces.
My daughter and I recently took a canning class. Wish I had known about citric acid (powdered) before then. I used to can over 30 years ago, and can not remember my recipes, other than that I used a salt-sugar mix, x-parts sugar to x-parts salt. I don’t remember using lemon, and the salsa, green-tomato relish, and tomatoes we canned in class all have that underlying lemon flavor–yuk! So I’ll try the citric acid powder. Any help on the salt/sugar ratio?
Hi Lauren,
Tomatoes have become less acidic due the changes in the way they are grown. So 30 years ago, we did not need to add acid. But, now we do to be safe.
For quart jars, try 2 teaspoons granulated sugar and/or 1 teaspoon salt per jar. For pints, use half those amount. I didn’t like sugar in mine, but many people do. It’s strictly a personal preference things. It also depends on the types of tomatoes you use. I like to can San Marzano (plum type tomato) when doing whole or diced tomatoes. For sauce, another good tomato is Stupice.
I’m glad you and your daughter are canning. I’m glad the younger generation is getting interested in preserving, so we keep the knowledge going.
Thanks for this. I canned Tomato sauce with lemon juice almost 2 wks ago and the lemon flavor is strong. Next time I will use citric acid. I’ll also wait a few wks to see if the flavor changes.
Christine, The flavor usually develops fully at about 6 weeks. Yes, I’ve had many people say the lemon is very disagreeable. Good thing there is citric acid.
Is it okay to substitute citric acid for vinegar in salsa recipes? Thank you.
Hi JB,
No, you cannot substitute citric acid for vinegar in a salsa recipe. The addition of onions and peppers (low acid vegetaables) requires that you use a carefully tested salsa recipe for canning. PNW0395 is a booklet of salsa recipes specifically formulated to be safe for canning; some use vinegar and some use bottled lemon or lime juice. Download this booklet here: http://oregon.4h.oregonstate.edu/sites/default/files/home_economics/pnw0395.pdf.
Alternatively, you could can tomatoes (whole, quarters, or diced) with citric acid, and use them to make this simple recipe one batch at a time:
Quick and Easy Blender Salsa
Yield: 2 cups
2 cups peeled whole, diced, or crushed fresh or canned tomatoes
1/2 cup coarsely chopped white or green onion
1-2 fresh jalapenos, stemmed, seeded and chopped or 1-2 tablespoons chopped pickled jalapenos
1 clove minced garlic
2 tablespoons coarsely chopped fresh cilantro or parsley or a combination
1 tablespoon fresh lime juice. or to taste
1/2 teaspoon ground cumin, or to taste
1/2 teaspoon salt, or to taste
1/4 teaspoon cayenne, or to taste
1. In a blender or food processor, place all ingredients in the order listed. Cover and pulse the sauce (turn motor on and off) until the texture is chunky or as smooth as desired.
2. Taste and add additional lime juice, cumin, salt, or cayenne. Salsa can be range from mild and sweet to tangy and hot, as preferred.
3. Transfer to a glass or other non-reactive bowl (glazed pottery or wood). Cover and refrigerate until ready to use.
Notes: For best flavor, do not use metal bowls or utensils with salsa as the sauce can take on a “metallic” taste and/or may spoil more quickly. When serving salsa, remove only the amount you intend to use or replenish your dip bowl frequently; keep any unused portion refrigerated.
This salsa recipe may be frozen. It is not tested for canning. For canned salsa, prepare a recipe specifically developed for canning.
I recently canned about 24 quarts of tomato sauce with the advice of friends. I did not put lemon juice citric acid or cider vinegar to the jars. What problems will I have or will the sauce go bad? Since this is only two days old can I add the acid and reseal? Thanks for your help.
JL,
The problem with not enough acid is that the jars could develop botulin toxin. If they do, it is not always immediately apparent. Over time the jar lids will swell and may leak, indicating active organisms. But before it becomes noticeable, an infected jar can seem normal.
Botulism is a serious disease, not often fatal these days–if you know what you’ve got, and it’s correctly diagnosed and treated. If the attack is severe, but doesn’t result in death, the recovery period can take years–basically it attacks your nervous system and you may have to learn to talk again, etc. All of this is intended not to scare the pants off you, but to information you that it is not something you want to play with.
You could open the jars, add the acid, add new lids, and rep-process. However, I would also recommend that you boil the tomatoes for 10 minutes, then put in (clean) jars with acid and new lids, and then re-process. It’s a bit of work, but better than tossing the tomatoes out or risk botulism poisoning.
If the tomatoes were organically field grown and are a traditional canning variety (roma or stupice), they are more likely higher in acid than more modern varieties grown with fertilizers and you may not necessarily need extra acid. But you really have no way of knowing. Since guessing can lead to serious health issues, it’s really best to be safe.
Good luck. I’m processing about 40 lbs of tomatoes today.