The Road Home

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Friday, October 11, 2013

Why Acidify Tomatoes?

Since tomatoes are naturally acidic, why do we have to acidify them? If you grow an old heirloom tomato that has all of it's old fashioned acidity and has not been tampered with like some of the newer hybrid low-acid varieties, do you still need to acidify them? Why?

All of my canning books and several places online all agree that tomatoes should be acidified before being canned. It is the pH of the tomatoes that determines whether they are acidic enough alone, or require additional acidity to be safely preserved through the canning process. As I have learned to can more things, I have come across some techniques that I have never heard of before. The question "Why...." tends to come to mind often. This question of acidifying warranted some research on my part to understand why.

To this end, I read through my canning books. Stocking Up and the Ball Complete Book of Home Preserving both recommend always acidifying any tomato product that is canned. The Penn State Extension website explains that the onset of most acidification recommendations came about because of a couple of very unfortunate botulism cases in 1974, which were caused by home canned tomato products. None of us ever want that to occur because of the techniques we used in canning. Jackie Clay's Growing and Canning Your Own Food also points out the need to add lemon juice to any canned tomato products to insure adequate acidity. Putting Food By points out that "Nowadays home-grown tomatoes often nudge the 4.6 pH cutoff, beyond which they are considered low-acid...", thus, they would need to be acidified for safe canning.

Tomato Sauce

So, this body of research definitely points to the need to acidify tomatoes. But, back to my original question. If you grow known old-time tomatoes that have been around for 100 years, like the
Arkansas Traveler, do you still need to acidify? There have been many new tomatoes developed with varying levels of acidity. Another thing to consider is the conditions under which the tomatoes are grown and how that may affect acidity. has an interesting article about heirloom seeds. In part, it says, "How are heirlooms defined? A tomato becomes an heirloom after seed savers have grown the same open pollinated (non hybrid) variety over time. A more definitive, but still arbitrary guideline defines an heirloom as being in cultivation for 50 years or more. There are some old commercial varieties that meet the open pollination definition, some of them cross bred from family heirlooms, such as Arkansas Travelers." Does that mean my Arkansas Traveler tomatoes should be fine for canning without added acidity? It is still a question I have.

While I was pondering and researching I thought I would ask some of my fellow bloggers their opinions on this topic. Here is the question I posed to them and their responses.


Question: "If we grow old heirloom tomatoes, why do we need to acidify them when we can? I have read all of my canning books and some online sources and everyone says that since we can't determine the acidity of our tomatoes because of the difference in growing conditions, we always need to acidify them. Why? I know there were a few deaths back in the 70's, and there have been many tomatoes developed for their low acid flavor, but what if we don't grow those? I guess I feel like some things are just overboard. I thought I would do a post on it and wanted to see if you would contribute your opinion."

Patrice Lewis: "In theory, heirloom varieties of tomatoes don't need extra acidifiers.  In reality, however, it's **always** wise to add them.  The reason for this is, many new heirloom varieties have been developed in the last thirty years.  An heirloom variety simply means it breeds true, not that it's an ancient line.  A low-acid hybrid can become open-pollinated ("heirloom") after only seven generations, or seven years.  That's why you always want to add acidifiers to tomatoes, regardless of whether they're heirloom/open-pollinated, since you never know if it's a newer "heirloom" with a low acid content.
Hope that helps...

The Canned Quilter: "Ya know, I canned tomatoes for years and years and NEVER added lemon juice. My mother never canned her tomatoes with lemon juice. Lemon juice is usually pretty cheap and I add it when I have it. I have been known to can a jar here and there without it though when I just do not feel like running to the store when I run out, etc... I grow, like you, all heirlooms tomatoes with presumably high acid content though, so I do not really sweat it too much. I am sure if you ask the powers that be that they will come up with a plausible reason and so I humor them most days for the sake of argument. Otherwise it is like Vegas, "What goes on in my kitchen, stays in my kitchen!"
Hugs, CQ"

Leigh: "That's a good question. I've pretty much blindly followed the rule to add lemon juice or citric acid to my canned tomato products, but have always wondered if I really needed to. Looking at Putting Foods By (mine is a 1991 edition), tomatoes seem to be on the borderline of "safe" for water bath canning, with too many variables to not add additional acid.
I agree that heirloom tomatoes ought to be more acidic by nature but canning directions seem to assume that folks will be canning hybrids. Perhaps testing various hybrids with litmus paper might be an interesting experiment.
I agree that the increasingly strict food guidelines are an overreaction to unfortunate, but rare or isolated incidents. Sadly, I think our culture is becoming afraid of food, real food. We're afraid of dirt, we're afraid of "germs," we're afraid of raw food. I think, too, that the "experts" assume that we ordinary folks are dumb and unable to reason things out. I don't know of anyone who deliberately wants to botch a canning job and potentially poison themselves. Yet it seems to me, that industrial, commercial food preservation isn't actually any safer. Unfortunately our public food supply is sterile and dead. The sad consequence seen in the general deterioration of health.
Out of curiosity, I took a garden pH meter I'd purchased but never used, and used it to check the pH of the tomatoes I'm working on. These are mostly Amish Paste that I popped into the freezer to process at a future date. There are a few hybrids in there as well. I don't know how accurate those meters are, but the cooked down tomatoes (no water except possible ice crystals from being frozen) registered a pH of 6.3. My applesauce (water added to cook down the apples) was 5.0. I reckon I'll definitely be adding that citric acid to this batch.
Very best wishes, Leigh"

There are several pieces of information I want to highlight:
  • Some 'new' heirloom tomato varieties may only be seven years old. One of the things that struck Frank was Patrice's statement. Just because a seed variety is called an heirloom, doesn't mean it has been around for a long time.
  • Keeping that in mind, it doesn't mean that my heirloom tomatoes are really the old, high acid varieties I thought they were. So, unless I can find some definitive research that supports a specific variety as being high acid, then I cannot make the assumption that my heirlooms are indeed the old, high acid varieties that I would like to grow.
  • Not everyone uses additional ingredients to acidify their tomato products when canning as CQ pointed out. I think Leigh's technique to measure the acidity of her tomatoes is very interesting and could be another way to determine whether any additives are needed for safe canning.
  • The growing conditions for any plant will affect the amount of minerals contained in the vegetables produced. So, even if I am growing a known high acid variety of tomatoes, my soil and the weather may affect the level of acidity from month to month and from year to year. 

Arkansas Travelers
My conclusions at the end of this research are this. We must all make wise, educated decisions when preserving food for our families. Don't take a shortcut because it makes the task easier for you. Do what is necessary to preserve good, nutritious food that is safe and will last. 

Frank and I try to grow the most natural, healthy foods that we can. We also limit the number of additives in what we cook and preserve. That doesn't mean I can leave out ingredients that will make our canned foods safe, just because I am trying to eliminate as many 'produced' preservatives as possible.

I hope this helps. Be wise. Be healthy. Keep at it. Learn a lot. And thank God for everyday we have.

Until next time - Fern


  1. I had this dilemma today when I was canning some tomatoes out of my brother's garden for him. Since there were several boxes, I wanted to do the job quickly so I decided to pressure can them in my large canner. It occurred to me that maybe I didn't need to add lemon juice since I was processing them in this manner. I filled the first jar and questioning my decision ran to the computer and tried to find a definitive answer. I was not successful. So...I grabbed the lemon juice out of the refrigerator and a tablespoon and made sure I got the 2 tbs in each jar.
    I read in one of the sites I looked at that the processing time, even for pressure canning, was developed using the lemon juice.

  2. I pressure canned all tomato-based foods last year and will continue to do this. This year our harvest wasn't high enough to can due to weather.

    Pressure canning is easier than basic water bath canning, imo. So tomato meat sauce (spaghetti sauce), chili, tomato paste, tomato sauce, tomato juice, pizza sauce, chili sauce, and BBQ sauce are all pressure canned. Salsa is put up using water-bath canning and the pH is increased accordingly.

  3. Interesting post Fern, well thought out and well concluded. I later thought that the reason my pH was so high was because having been frozen all summer, there were certainly ice crystals in the tomatoes which would have made a difference. Still, I wouldn't have thought to do it if you weren't being curious.

    As far as a self-sustaining way to acidify things like tomatoes, it is possible to make one's own vinegar and that could be added. I would still check pH before canning, but since I can't grow lemons or citric acid, it's a possibility if the need arose.

    1. Hi Leigh,

      What kind of pH meter are you using? I would like to look into getting one.


    2. It's just a cheap on I got at Lowe's for about $6. Here's a link to it on the manufacturer's site, Soil Master pH Meter. I can't say how accurate it is, but as a general guide, it's been interesting. Perhaps to do some serious testing a better quality on would be in order.

  4. This spring I picked up an heirloom plant at the local sticks & bricks and fell in love with the tomatoes it produced. This was a plant I wanted to keep around, so I collected the seed from a few nice toms. I pulled up the plant tag to see what the name was and had a good laugh. Like you I thought 'heirloom' were from long ago plants. I don't think this was from that long ago. It was called the 'mortgage lifter'.

    This was a great post! Lots of good info from some wise ladies, including yourself. Thank you

  5. I've never added acid to my tomatos when I canned tomato sauce. I would add some salt, but it never occurred to me to add any acid.

    I also pressure canned instead of using a waterbath. I never had a can go bad on me.

  6. I made a big batch of tomato sauce, I only added salt and water bath canned them. I did this a week ago. Should I unseal them? Are they bad now? Worried now

    1. There are people that don't add acid to their tomato products at all and they are fine, but there is no guarantee of that. Some folks pressure can their tomato products. This method does not require acidification. It is an individual choice. You may want to do some research on your type tomato and decide what you want to do from there.

      Best of luck,


  7. I found your blog and this post due to a link on Leigh's blog. I thought I would add my experience to the conversation. I have been canning tomatoes both pressure canning and water bath canning for over 15 years and I've never added a ingredient other than tomatoes. I've always thought the lemon juice was to maintain a brighter color like when using in fruit canning. When water canning or pressure canning, I thought that getting the tomato juice/sauce hot enough was killing off the bacteria and making it safe for consumption (thus longer times using water baths vs. pressure)? I will also note that although I do can some heirloom tomatoes given to me by others, the majority of the tomatoes I can are modern hybrids which according to what I read above are even less acidic.

    1. Thanks for sharing your experiences. There are just about as many ways to can as there are people and I can learn from them all.