Canning tomatoes

This is an overview of the general process and process choices for canning tomatoes as basic tomato products: whole, crushed, juice or sauce.

It’s an advanced, background knowledge page.

Follow this link if you are just after actual tomato recipes right now.

If you have even more questions, such as can I can yellow tomatoes, cherry tomatoes, how ripe should they be, etc, see ‘Tomato home canning considerations: colour, variety, ripeness‘.

See also: Drying tomatoes.

How to home can tomatoes

You can easily and safely can tomatoes at home. When you follow USDA directions, you get a quality product superior in every way to the tinned tomatoes you purchase at the grocery store, without all the added sugar and salt, and, if you get your tomatoes at harvest-time prices, at a fraction of the cost.

How to can tomatoes depends on what form you are canning them in.

There are many different methods of processing tomatoes and tomato variations, and they can all require very different processing times, so please be sure to always double-check your times even if you think you remember.

Methods of home canning tomatoes

Tomatoes are one of the few food items which can be either pressure-canned, or water-bathed (or steam-canned.)

Regardless, they must have a small amount of an acidic substance added to them to make them safe for home canning, because tomatoes are not reliably and consistently acidic enough to prevent botulism on their own. (See section below on Acidification.) (And no, it’s not just newer varieties: it’s always been that way. See Heirloom Tomatoes.)

When you are pressure canning tomatoes following USDA guidelines, the higher heat effect of the pressure canning is not used as a element in itself to kill Clostridium botulinum spores: it’s used only to decrease the processing time. ^[1]https://nchfp.uga.edu/tips/summer/canned_tomato_types.html

The National Center for Home Food Preservation explains why:

Tomatoes … can have a natural pH above 4.6 (at least up to 4.8). But rather than develop a pressure-only process as if they were all low-acid, since they are so close to 4.6, USDA decided instead to recommend a small amount of acid be added so they can be treated as a food with a pH less than 4.6 for home canning. Therefore they are suitable for boiling water canning when the acid is added. (The commercial industry often also adds citric acid to tomatoes to be able to give them a less severe heat treatment than would be needed for botulism and other bacterial controls.)” ^[2] Burning Issue: Acidifying Tomatoes When Canning. National Center for Home Food Preservation. September 4, 2013. Accessed March 2015.

To be clear: even when processing in the pressure canner you must still acidify the tomatoes. The pressure canning process developed is not a heat substitute for acidification; it’s just a somewhat faster equivalent that still requires acidification. The equivalent pressure canner times were developed by Penn State. ^[3] Andress, Elizabeth. “History, Science and Current Practice in Home Food Preservation.” Webinar. 27 February 2013. Accessed January 2015.

Dr Elizabeth Andress of the National Center for Home Food Preservation says,

The USDA process for home canning of tomatoes is only based on it being below a pH of 4.6 or treated as an acid food. Even when we provide a pressure canning alternative in the USDA book it is only the equivalent of a boiling water process. In other words you can use a higher temperature in the canner so you can have a shorter time, it was never developed as a process for pressure canning for elimination of concerns about botulism as if it were a low acid tomato. So that’s why we say you still have to acidify if you use the pressure canning when both are offered, they are only equivalent processes, there is not a separate pressure canning process for controlling botulism in tomatoes that anyone has researched and properly tested.”

Pressure canner times are presented as an equivalent of boiling water treating tomatoes as a low pH, high acid food. The pressure canner recommendations were not developed for treating tomatoes as low acid foods, thus acidification still needed in pressure canning tomatoes. There is no [pure] pressure canner process per se that anyone has researched and tested as of yet. These equivalents were developed by USDA/ Penn State Center for Excellence in the 1980s and are still the current recommendations.” ^[4] Andress, Elizabeth. “History, Science and Current Practice in Home Food Preservation.” Webinar. 17:00. 27 February 2013. Accessed January 2015.

While Bernardin provides water-bath canning instructions for tomatoes, they seem to prefer that you pressure can them instead — for product quality:

Processing tomatoes in a pressure canner may produce a more nutritious and higher-quality product because the tomatoes are heated to a higher temperature but are processed for a shorter period of time. The longer a food product is exposed to heat, the greater the lose of nutrients and overall product quality. If you have a pressure canner, it may be the best choice when processing tomatoes.” ^[5] Kingry, Judi and Lauren Devine. Bernardin Complete Book of Home Preserving. Toronto, Canada: Robert Rose Inc. 2015. Page 370.

Note that tomato canning processing recommendations apply to pure, plain and simple tomato products, with nothing added to them (aside from dried herbs, salt and pepper.)

When you add stuff to your tomato products, you are canning a different product and different recommendations will apply:

Never process tomato sauces with meat or mushrooms in a boiling water bath. Use a pressure canner.” ^[6] Penn State Extension. Acidify Tomatoes before Canning. 10 August 2012. Accessed January 2015.

Tomatoes must always be processed

Jars of tomatoes must always be processed.

Bottling tomatoes without processing the sealed jars killed two people in 1974.

…early in 1974 there were two deaths from botulism poisoning traced directly to home-canned tomatoes and tomato juice…. Meanwhile the public health officers discovered what actually allowed the spores of C. botulinum to make the toxin that killed the victims. Common bacteria or molds grew in the food in the jars and thereby reduced the acidity because the natural acid in the tomatoes was metabolized by the micro-organisms as they grew and developed. It was established after compassionate, but thorough, investigation that these bacteria or molds survived either because the tomatoes were canned by the discredited open-kettle method, or entered under the lid of a jar that wasn’t adequately sealed. Inadequate processing is virtually always the cause of food poisoning that develops during shelf life.” ^[7] Hertzberg, Ruth; Greene, Janet; Vaughan, Beatrice (2010-05-25). Putting Food By: Fifth Edition (p. 119-120). Penguin Publishing Group. Kindle Edition.

In 1999, an unprocessed jar of tomatoes sent six people in Toronto, Canada to the hospital with botulism. ^[8] BC Centre for Disease Control. Botulism in British Columbia: The RISK of Home-Canned Products. January 2015.

Tomatoes that are just “bottled” (sealed in a jar, with no processing) are prone to developing mould. Besides the disappointing food loss, this renders the entire jar extremely unsafe:

The growth of mold on the surface of improperly canned tomatoes can result in a potentially dangerous situation. Metabolites produced by the mold can lower the acidity of the tomatoes. The pH may be raised to a point where C. botulinum spores, if present, can germinate, multiply and produce toxin. Therefore, any moldy tomato product should be discarded in its entirety.” ^[9] Penfield, Marjorie P. and Ada Marie Campbell. Experimental Food Science. San Diego, California: Academic Press. 1990. Page 273

The tomatoes could also contain salmonella, listeria, Campylobacter jejuni, etc. Many people got sick after eating unprocessed jars of tomatoes and it was just credited to a “24-hour flu.” There’s no such thing; it’s food poisoning. ^[10] Brayer, Toni. (Chief medical officer for Sutter Health West Bay Region.) Stomach flu is really food poisoning. SFgate.com 2 October 2012. Accessed July 2015.

Don’t trust anything called ‘tomato canning tablets’: “Tomato canning tablets should not be used as they are ineffective.” ^[11] Barbara Willenberg, Nutritional Sciences, University of Missouri-Columbia. http://missourifamilies.org/quick/foodsafetyqa/qafs272.htm

Comparison of processing times

Here’s a comparison of USDA research-tested processing times for water-bath / steam canning for six standard tomato products (times given are for sea level 0 to 300 m / 0 – 1000 feet).

(You don’t need to memorize this; processing times will always be given by individual recipes. This is just for ‘intellectual’ comparison purposes here, to give you a general idea of times involved.)

	½ litre (1 US pint)	1 litre (1 US quart)
Tomato Juice	35 mins	40 mins
Crushed Tomatoes	35 mins	45 mins
Standard Tomato Sauce	35 mins	40 mins
Whole/Half Tomatoes in water	40 mins	45 mins
Whole/Half Tomatoes in tomato juice	85 mins	85 mins
Whole/Half Tomatoes no liquid	85 mins	85 mins

Home canned tomatoes must have acid added to them to be safe

Tomatoes, even heritage tomatoes, can’t be relied on to be acidic enough to be safely home canned without a helpful nudge from you.

[gss-content-box color=”gray”]Most commercial canners even add acid to their canned tomatoes, to be safe. It also allows them to heat process the tomatoes for less time than they would otherwise, resulting in a better product:

The commercial industry often also adds citric acid to tomatoes to be able to give them a less severe heat treatment than would be needed for botulism and other bacterial controls. ^[12] Burning Issue: Acidifying Tomatoes When Canning. National Center for Home Food Preservation. [/gss-content-box]

Fortunately it’s really easy to nudge tomatoes down into the acidic range to make them safe for home canning.

You can use citric acid powder, lemon (or lime) juice from a bottle, or, vinegar. Citric acid or bottled lemon juice are usually preferred. The University of Alaska Extension says:

The proper amount of acid is ½ teaspoon citric acid per quart or 2 tablespoons lemon juice per quart.” ^[13] Canning Acidic Foods – Fruits and Tomatoes. Dinstel, Roxie Rodgers. Food Preservation. University of Alaska Fairbanks Cooperative Extension Service. Revised March 2013.

Here’s a complete table from the University of Minnesota:

Acid	Effect on tomato products	Amount (metric)	Amount (US)
Citric acid	Little change in flavour	¼ tsp per ½ litre ½ tsp per litre	¼ tsp per US pint ½ tsp per US quart
Lemon or lime juice (bottled, not fresh.)	Easy to procure and use.	1 tbsp per ½ litre 2 tbsp per litre	1 tbsp per US pint 2 tbsp per US quart
Vinegar (5% or higher)	Noticeable change in flavour	2 tbsp per ½ litre 4 tbsp per litre	2 tbsp per US pint 4 tbsp per US quart

Table source: University of Minnesota Extension. New and Old Tomatoes Need Acid and Heirloom Tomatoes. In: Home Food Preservation Newsletter, August 2012. Accessed March 2015 at https://www.extension.umn.edu/food/food-safety/home-food-preservation-newsletter/docs/home-preservation-august-2012.pdf.

For a full discussion on this matter, including the history see: Acidifying tomatoes for safe home canning

What types of tomatoes can you can?

You may can any variety of tomatoes that you wish. The USDA does not put any restriction on the variety of tomato in terms of safety.  So you may use paste tomatoes (such as Roma) or slicing tomatoes or even cherry tomatoes. Obviously some varieties produce a better end product than others, but the same safe acidification procedure applies to all of them.

The only restriction per se is on the quality of the tomato. The authors of So Easy to Preserve advise:

Because tomatoes have pH values that fall close to 4.6, you must take some precautions to can them safely. First, select only disease-free, preferably vine-ripened, firm fruit for canning. Do not can tomatoes from dead or frost-killed vines. (Green tomatoes are more acidic than ripened tomatoes and can be canned safely using any of the directions given for tomatoes….)”  ^[14] Andress, Elizabeth L. and Judy A. Harrison. So Easy to Preserve. University of Georgia Cooperative Extension. Bulletin 989. Sixth Edition. 2014. Page 55

For a full discussion on colour, variety and ripeness, see ‘Tomato home canning considerations: colour, variety, ripeness‘.

Canning tomatoes ripened indoors

It is fine to can tomatoes that have been ripened indoors.

Tomatoes ripened indoors may not be as flavorful as those ripened out-of-doors, but they may be safely canned in recipes calling for ripe, red tomatoes. [Note: It is an error in Tomatoes Tart & Tasty when it states that tomatoes ripened indoors should not be canned.]” ^[15]Ingham, Barb. What to do with green tomatoes? University of Wisconsin Extension. Blog entry 19 September 2013 at https://fyi.uwex.edu/safepreserving/2013/09/19/safe-preserving-what-to-do-with-green-tomatoes.

Pack types for home canning tomatoes

Note that some methods of preparing tomatoes are hot pack only, some are raw pack only. Some methods will present you with a choice.

Hot pack leads to less separation of material in the jar.  ^[16] Canning Tomatoes & Tomato Products. Powerpoint Presentation. Lunch & Learn, 23 July 2012. University of Wisconsin Extension. Accessed March 2015 .

And, some sources now advise that tomatoes should not be raw packed at all: “raw packing of tomatoes is no longer recommended. It has been found that a highly heat-resistant mold is able to survive during the processing. When this mold grows, it changes the pH of the tomatoes from a high-acid to a low-acid product. The canned tomatoes then become susceptible to the growth of Clostridium botulinum.”  ^[17] Patricia West and Evelyn F. Crayton. Wise Methods of Canning Vegetables. Alabama Cooperative Extension System. EFNEP-0190. August 2012. Accessed March 2015 at https://www.aces.edu/pubs/docs/E/EFNEP-0190/EFNEP-0190.pdf.

However, both the USDA / NCHFP ( here ) and Ball ( here ) still have recipes for packing raw tomatoes, with the same processing times as for hot pack tomatoes.

For more information, see our dedicated page on Raw vs hot packs for home canned tomato products.

Canning tomatillos

See dedicated page on home canning tomatillos.

Washing, peeling and preparing tomatoes

Putting Food By says,

Wash the [tomatoes] carefully in fresh water of drinking quality. If many are spattered with field dirt, or have not been staked or mulched in your own garden, add a little mild detergent and 4 teaspoons of 5 percent chorine bleach to each 1 gallon of wash water, [then] rinse well in fresh water. (This thoroughness cuts down bacterial load.)”  ^[18] Janet Greene, Ruth Hertzberg, Beatrice Vaughan. Putting Food By. New York: Penguin. 2010. Page 121.

Peeling tomatoes

All canning directions for tomatoes from reputable sources say to peel the tomatoes. Part of this peeling is quality, so that people don’t get bits of paper-like peel in their mouths. But, more importantly, it’s to reduce bacterial load, by getting the skin off where most of the bacteria and botulism spores may be.

The National Center for Home Food Preservation says,

 Peeling tomatoes may seem like an unimportant extra step, but the texture of the skin was determined to be undesirable and product testing did not include considerations of how the skin would alter the final product safety. So, scald, peel, and chop tomatoes as described in the procedure. Our canning recommendations are meant to be followed as written, since that is how they were developed and changing ingredients or steps may influence not only the quality but also the safety of the final product.” ^[19]National Center for Home Food Preservation. Preserving Food at Home Blog. Try It: Tomato Jam. 19 August 2015.

Thankfully, even if you’ve resisted peeling tomatoes all your life, it’s actually really easy to do. Just boil the tomatoes vigorously in hot water for 1 to 3 to 5 minutes (depending on the tomatoes) until the skins crack and start coming off, then plunge the tomatoes into a pan, sink or large bowl of very cold water. The skins will pull off easily.

See separate entry on Peeling Tomatoes.

Coring and seeding tomatoes

Most directions also say to core the tomatoes. This means to cut out the top brown spot where the tomato was attached to the stem, going into the tomato just a bit below that.

It’s not the same as seeding. Seeding is a separate, distinct operation. For instance, in the Ball / Bernardin Complete Book, the “Bruschetta in a Jar recipe” calls for “chopped cored” tomatoes. A side margin note says, “You can seed them if you prefer, but this is not essential.” The two operations are separate (2015, page 223.)

Seeding gets rid of the seeds in the middle of the tomato. For instance, by running the top of a spoon down the centre of a half or cored tomato and scooping the seeds out.

This appears to be a quality step rather than a safety step. Many canners confess to not doing it. A Master Food Preservers Program Assistant says, “I only peel my tomatoes. I leave the rest of the whole tomato in.”  ^[20] “Linda Lou” Master Food Preservers Program Assistant, Washington State University Extension. Garden Web Forum. 28 February 2010. Accessed July 2015/

A lot of watery juice adheres to the seeds, so some people like to get rid of that when they are making tomato sauce as it means less water to boil out. Some people will save that for making tomato juice with.

Seeding can also be done by passing tomatoes through a food mill, which will strain the seeds out.

Canning tomato products without a food mill

Many recipes for tomato-sauce based products will suggest that a food mill or food strainer be used. One such product is the The Ball® HarvestPro™ Sauce Maker.

Not everyone has a food mill, of course.

The Ball / Bernardin Complete Book (2015 edition) gives this suggestion of how to make a thick tomato purée from plum-type tomatoes without a food mill:

If you do not have a food mill or Victorio strainer, blanch, peel, core, seed and chop tomatoes. Place in a colander and let stand for 15 minutes. Discard liquid and purée tomatoes in a food processor fitted with a metal blade.” ^[21]Kingry, Judi and Lauren Devine. Ball / Bernardin Complete Book of Home Preserving. Toronto: Robert Rose. 2015. Page 366.

The directions may work with other types of tomatoes as well, but in the context, plum tomatoes were being discussed.

Jar sizes for home canning tomatoes

The USDA methods allows for ½ litre (1 US pint) or 1 litre (1 US quart).

The Ball/ Bernardin Complete gives processing times for three water-bathed tomato products in 1.5 litre jars (1 ½ quarts) .

1.5 litre jars are available only in Canada. These jars may be used to process tomatoes in a boiling-water canner, but only in those recipes for which a specific time is stated for this size of jar. 1.5 L jars are not recommended for processing tomatoes or any food in a pressure canner, as suitable heat processing studies to determine safe processing times have not been established.”  ^[22] Kingry, Judi and Lauren Devine. Bernardin Complete Book of Home Preserving. Toronto, Canada: Robert Rose Inc. 2015. Page 355.

The jar size was also mentioned in their web recipe for tomato juice:

Place required number of clean 500 ml, 1 L or 1.5 L mason jars on a rack in a boiling water canner…”  ^[23]Bernardin. Tomato juice. Accessed 22 July 2015.

Healthy Canning asked Bernardin in July 2015 about the commercial availability of these jars, and received this response:

This jar was discontinued several years ago and the correction is being made on the website to remove the reference of using the 1.5 L size jar.”  ^[24] Correspondence on file with Healthy Cannning. 22 July 2015

Separation of tomato products into solids and water

Some pure tomato products (sauce, juice, crushed) can be susceptible to separation of liquid and solids at times.

See here for a discussion of this and how to help prevent it.

List of home-canned tomato product recipes

Click through for a complete list of home-canned tomato products from reputable sources.

What is better? Water-bath (or steam canning) or pressure canning tomato products

Sometimes you are given a choice of whether to water bath (or steam can), or, to pressure can a tomato product.

Steam canning can often be the fastest, for sure, owing to its reduced come-to-speed times compared to water bath or pressure canning.

The USDA says in their Complete 2015 Guide,

Use of a pressure canner will result in higher quality and more nutritious canned tomato products.”  ^[25] United States Department of Agriculture (USDA). Complete guide to home canning. Agriculture information bulletin No. 539. 2015. Page 3-5.

The authors of “So Easy to Preserve” say,

Tomatoes have traditionally been canned in a boiling water bath (212 F). However, recent research shows that for some products, pressure canning will result in high quality and more nutritious product.”  ^[26] Andress, Elizabeth L. and Judy A. Harrison. So Easy to Preserve. University of Georgia Cooperative Extension. Bulletin 989. Sixth Edition. 2014. Page 55.

And Bernardin explains why,

Processing tomatoes in a pressure canner may produce a more nutritious and higher-quality product because the tomatoes are heated to a higher temperature but are processed for a shorter period of time. The longer a food product is exposed to heat, the greater the loss of nutrients and overall product quality.”  ^[27] Kingry, Judi and Lauren Devine. Bernardin Complete Book of Home Preserving. Toronto, Canada: Robert Rose Inc. 2015. Page 370.

This is not to say that water bathing is now “out” or that the recommendation is to pressure can. It’s just to say, they consider it a good choice, if you have a pressure canner, and feel so inclined.

History of home canning tomatoes

Water-bath versus pressure canning history

In 1933, the USDA advised that water-bath canning gave a better quality product than pressure canning:

Do you always process tomatoes in the water bath? There is another question for the specialists. They say, ‘Yes, always.’ The temperature in the pressure cooker is too high for tomatoes. It overcooks them and spoils the flavor.”  ^[28] USDA Radio Service. Canning Tomatoes. Housekeeper’s Chat. Friday, 25 August 1933.

Thinking on this has now gone the other way; see discussion above.

Processing time history

Recommended water-bath processing times for tomatoes were inconsistent for the first several decades of the 1900s.

At first, no processing time was called for at all:

The earliest USDA publications with definite instructions for home canning was the Farmers’ Bulletin 359 (Breazeale, 1909), issued in 1909 by the Bureau of Chemistry… it was thought that [tomatoes] did not require sterilization after filling the jars with hot product.”  ^[29] Andress, Elizabeth L and Gerald Kuhn. Critical Review of Home Preservation Literature and Current Research. II. Early History of USDA Home Canning Recommendations. Athens, GA: University of Georgia, Cooperative Extension Service. 1983.

In 1925, the USDA in a printed timetable called for water-bath processing quart (litre) size jars of whole or diced tomatoes for 25 minutes.  ^[30] USDA Bureau of Home Economics. Time-table for canning fruits and vegetables. April 1924. Page 2.

In 1930, the USDA radio service called for water-bath processing pint or quart (½ litre or litre) size jars of whole or diced tomatoes for 45 minutes. “Process the quart and pint glass jars for 45 minutes, in boiling water.”  ^[31]  USDA Radio Service. Housekeepers’ Chat. Canning Beans and Tomatoes. 1 July 1930. 

In 1941, whoever was writing the radio service notes recommended only 5 minutes: “In fact, tomatoes packed hot need only 5 minutes in the boiling water bath… ^[32] USDA Radio Service. Homemakers’ Chat. Conserving tomatoes. Monday, 29 September 1941.

In 1942, the recommendation was again for 5 minutes: “Pack boiling hot into containers, adding 1 teaspoon of salt for every quart. Fill the containers with boiling-hot tomato juice and process immediately in a boiling-water-bath canner. Process tomatoes for 5 minutes, whether you use tin or glass containers.”  ^[33] USDA Press Service. Homemaker News. Tomatoes – an easy vegetable to can. 26 August 1942.

In 1945, the radio service recommendation was increased to 10 minutes: “After the tomatoes have come to a rolling boil in their own juice… pack them in the jars and process the jars in a boiling water bath for 10 minutes.”  ^[34] USDA Radio Service. Homemaker’s Chat. Tomatoes – Put ’em up! 20 July 1945.

In 1970, the University of Illinois Cooperative Extension recommended 10 minutes for hot pack quarts, and 45 minutes for raw-pack quarts.   ^[35] University of Illinois Cooperative Extension. Canning Fruits and Vegetables. Circular 1112. May 1975. Page 15.

By this point in time, tomatoes certainly had a “controversial history regarding recommended process times.”  Researchers trying to make sense of where the processing time recommendations were all coming from only found a “nebulous history of the process determination.”  ^[36] Andress, Elizabeth L and Gerald Kuhn. Critical Review of Home Preservation Literature and Current Research. II. Origins and Assessment of Current USDA Recommendations
BHNHE and Related Research. Athens, GA: University of Georgia, Cooperative Extension Service. 1983.

When all the bulletins were pulled together, and all the research reviewed to make the USDA Complete Guide, first issued in 1988, the Complete Guide made a break from all previous advice by determining to be completely based on facts, evidence and science, rather than guesswork,  hearsay or past recommendations.

In questioning all the canning advice that had been given in the years before, tomato processing times were one of the things given particular attention. One reason was, as mentioned, the uncertain origin of some of the recommended processing times, and that they were all over the map. The second reason was that they weren’t working: spoilage of home-canned tomato products based on those recommendations had been quite commonplace — they just weren’t working in providing a guaranteed product that would keep, and people were losing a lot of their tomato products to spoilage.

They went back to some research documentation from 1945 that had been recorded but then filed away. Jars of tomato products had been inoculated with spoilage bacteria (a butyric anaerobe), and water-bath processed. Here’s what the researchers from 1945 had recorded and documented: “The results indicated 100 percent survival of the organism after a 30 minute process, 35 and 14 percent survival, respectively, following 40 and 45 minute processes and sterility after 50 minutes.”  ^[37] Andress, Elizabeth L and Gerald Kuhn. Critical Review of Home Preservation Literature and Current Research. II. Origins and Assessment of Current USDA Recommendations BHNHE and Related Research. Athens, GA: University of Georgia, Cooperative Extension Service. 1983.

So the 5, 10, even 25 minute processing recommendations had just been guess-work, were woefully inadequate, and were resulting in people finding a great deal of their hard work spoiling on the shelves. The spoilage bacteria common in tomatoes was finding even a 30 minute boil to be like a spa-trip.

Consequently, the recommended processing times were increased dramatically for the USDA Complete Guide 1988. For instance, hot pack only of crushed (or quartered) tomatoes was recommended, and 45 minutes was required for quarts. And, instead of being treated generically, various tomato products were all given their own separate processing directions and times.   ^[38] USDA Complete Guide. 1994. Page 3-4.

Further reading: Tomatoes: Origins and Assessment of Current USDA Recommendations.

Evolution of the understanding of tomatoes as borderline acidic needing a boost

“…early in 1974 there were two deaths from botulism poisoning traced directly to home-canned tomatoes and tomato juice…. ”   ^[39] Hertzberg, Ruth; Greene, Janet; Vaughan, Beatrice (2010-05-25). Putting Food By: Fifth Edition (p. 119-120). Penguin Publishing Group. Kindle Edition.

This incident caught the canning world by surprise. The assumption had been all along that tomatoes were acidic enough to prevent botulism from developing.

In the 1970s, a controversy developed over the pH of tomatoes. Some researchers … indicated that some tomatoes may have pH levels of greater than 4.5 whereas the USDA (1976c) reported that this is not the case. Some states recommended at the time that citric acid be added to lower the pH (Gould and Gray 1974.) The Food and Drug Administration (FDA) permits the use of organic acids to lower the pH of tomatoes for commercial processing (FDA, 1988) and the USDA accordingly has changed its recommendations for home canning of tomatoes. Changes include the addition of lemon juice, citric acid, or vinegar (5% acidity) to acidify the tomatoes.”  ^[40] Penfield, Marjorie P. and Ada Marie Campbell. Experimental Food Science. San Diego, California: Academic Press. 1990 .Page 273.

Consequently, in the 1970s and 1980s, various universities did research about tomato acidification, because food safety issues were emerging about mixing tomatoes and peppers. They did work on processes with acidification that would make water bathing safe, instead of pressure canning, as people were afraid of pressure canning. A lot of the acidification research was done by the 1970s by USDA Eastern Regional Research Center in Pennsylvania at Penn State University.  ^[41] Andress, Elizabeth. “History, Science and Current Practice in Home Food Preservation.” Webinar. 27 February 2013. Accessed January 2015.

In 1988, tomato acidification was added to the USDA Complete Guide based on Penn State research.  ^[42] Andress, Elizabeth. “History, Science and Current Practice in Home Food Preservation.” Webinar. 27 February 2013. Accessed January 2015.

In 1999, there was a botulism outbreak from unacidified home-canned tomatoes in Toronto, Canada. 6 people sick. No fatalities. Tomatoes boiled for one hour, then placed into glass jars and sealed. Stored in cellar.  ^[43] BC Centre for Disease Control. Botulism in British Columbia: The RISK of Home-Canned Products. January 2015.

Further reading

Browse site on all tomato canning topics

Acidifying tomatoes for safe home canning

Canning crushed tomatoes

Heirloom tomatoes are not more acidic

Peeling Tomatoes

Raw vs hot packs for home canned tomato products

Tomato home canning considerations: colour, variety, ripeness

Why do some tomato products need acidification but not others?

Elsewhere on the web:

Online canning tomatoes tutorial. University of Minnesota.

Acidify Tomatoes before Canning. Penn State Extension. 10 August 2012.

Tomato Do’s and Don’ts. Penn State Extension. Updated 9 July 2024.

Tomatoes. Andress, Elizabeth L and Gerald Kuhn. Critical Review of Home Preservation Literature and Current Research. Athens, GA: University of Georgia, Cooperative Extension Service. 1998.

Tracy L. Parnell et al. Tomatoes: Safe Methods to Store, Preserve, and Enjoy. University of California, Division of Agriculture and Natural Resources. Publication 8116. 2004.