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Jan 5, 2009 08:00 AM

botulism in canning: is it all about pH?

I've researched all the posts on this board and some government sites about the risk of botulism when canning (or to be more accurate, jarring) at home. I want to follow my own tomato pasta sauce recipe, not one from a cookbook or website, which I know is considered a no-no. But if botulism cannot reproduce at a pH below 4.5, why don't I just make my pasta sauce, and test its pH with a small chemistry kit (close friend is a microbiologist, and is willing to lend me the necessary stuff), and if it's below 4.5, go ahead and waterbath it and assume it will be fine?

Or am I missing some other key bit of information here, and oversimplifying by focusing on the pH?

Thoughts from food scientists and other knowledgeable cooks welcome!

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  1. According to the following site, botulism is harder to grow in acidic environments, such as those for tomato sauce:

    Botulism dies in the presence of air, which is the primary reason for not filling the canning jar completely to the top (leaving usually a 1/2-3/4" space). More here:

    If, however, you're planning to inadvertently get rid of someone, there's always 3 (or 5) bean salad with no air in the top of the jar which you can keep a few months first, preferably in the attic. :


    I've read that I shouldn't keep my home-made sterilized canned/jarred pumpkin puree for more than 3 weeks, but will be testing the last jar in a few weeks and will post (or not) if something dreadful happens.

    2 Replies
    1. re: Caralien

      Wrong! Botulism can not tolerate oxygen, the opposite of what you stated. And air in the jar has nothing at all to do with this. Leaving headspace is for sealing the jars, NOT for preventing botulism! It's physics as water is non compressable and a vacuum is difficult to produce without a gas (air) in the jar. However the air in the jar will be depleted of oxygen by aerobic bacteria, then Clostridium botulinum can grow and produce the toxin.

      And 3-5 bean salad is normally quite acidic and could probably be fine without even canning it. The attic would not have much affect either as botulism is active above 50 degrees or so (though it is most active at 95F so it would be faster).

      "The main limiting factors for growth of C. botulinum in foods are:
      (1) temperature, (2) pH, (3) water activity, (4) redox potential, (5) food preservatives, and (6)competing microorganisms." from CDC. This being said, Temp, redox (oxygen availablility), food preservatives, and competing microganisms are a moot point for pasta sauce. Water activity = High salt and sugar can limit botulism too, but would not be appropriate in this sauce. But PH, if properly measured would be perfect. Some salt also helps as PH and salt content work together to prevent spore growth.

      The fact that you are cooking the sauce first also helps somewhat as one of the problems with canned tomatoes is that they are 'fresh' packed and the bacteria (if present) are not killed before being put in the jar. A sauce, where the tomatoes are thouroughly cooked likely will be free of live spores of the offending bacteria, though it's not 100 percent effective as the spores could be in the air or get in the product off your hands, clothing, etc. The bacteria are naturally in the soil nearly worldwide, and therefore these spores can get on nearly everthing from the wind blowing up dust or though gardening outdoors and bringing them inside on your person. So thourough cooking will help, though not assure that spore growth will occur. The PH testing will be a good measure of protection. It's something that most people can not accomplish so that's why pressure canning is often recommended for tomato products.

      Canned pumkin is very difficult to heat to proper temps without pressure canning. I make a wonderful pickled pumpkin - lots of vinegar and sugar. It needs only a water bath.

      1. re: lokidog

        I think some of the confusion about whether C. botulinum can tolerate air comes from the fact that the bacteria can survive in two distinct forms: vegetative cells, which are metabolically active, and endospores, which are not. The endospores can survive in a greater range of conditions (including aerobic conditions), but don't produce botulin toxin -- they're essentially dormant. You could have endospores all over your kitchen, and they wouldn't harm you. When the conditions are right for the organism's metabolism (no oxygen, low acid, etc.), the endospores produce vegetative cells, which in turn produce the toxin that is deadly to humans. Thus, it's necessary to destroy the endospores before they are introduced into conditions that allow the growth of vegetative cells. So if your canned goods are potentially a good growth medium for the vegetative cells, you have to use high heat and pressure to kill the endospores; otherwise, it's not necessary.

    2. I'm not sure where you live, but there are Master Food Preserver hotlines that provide solid, scientific answers to your question. Check with your local extension office - that is usually where they operate from. You could also check at the land grant school in your state. Before getting information from a trained, trusted source, I would freeze your sauce instead. Botulism is really scary stuff.


      1. According to the CDC there are about 145 cases of botulism in the US per year, and only 15% of those are transmitted through food. If you assume every one of those are due to home canning it's about 20 cases per year, which I don't find particularly scary, considering the number of people who can.

        Yes, check the pH, but make sure you follow the guidelines for cooking times. Don't know what you mean by "jarring". If you mean tossing some cooked tomatoes into a jar and tightening the lid, don't invite me to dinner.

        1 Reply
        1. re: Zeldog

          My impression was that gooseberry planned to put the sauce in jars, rather than cans, and then process in a water bath.

        2. I was just reading a recently-published guide to home canning and the author said that yes, it's ALL about the pH. He recommended using test strips on everything you can. And remember that the pH ca rise a little for a few days after cooking so what's safe when it's fresh out of the pan might not still be safe next week - he said you need to make a sacrificial test jar in each batch (don't bother to seal it) and test the pH again 24 hours after canning to make sure it's still safe. I don't know how essential this really is - I am not a food scientist or canning expert, but he claimed that his book was written with the latest research etc etc.

          The pH needs to be below 4.4 at all times for water-bathing, and if you're hot-packing it into sterilised jars and NOT boiling them afterwards, it needs to be below 4.2.