Leena Cans: Buying a pH meter and testing my onion jam

By on Aug 25, 2011 in Canning, Leena Cooks, pH of food | 8 comments

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Time to test the pH of my water-bath canned onion jam

For me, it all started last year around March. I was preparing for Tigress’s March Can Jam focusing on alliums, and was dying to can some caramelized onion jam. I must have read damn near ten canning cookbooks, and not one had an onion jam recipe. And with no tested, from a cookbook recipe to base my recipe off of, I had to resort to making a delicious onion relish.

During the process, I learned that onions are a low-acid food and require a high amount of acid to be able to can with a water bath canner. This means it is trickier than usual to create a water bath onion jam recipe, because all of the ones I found were for pressure canning (and I don’t own a pressure cooker). Tigress even warned us that onions were prime subjects for botulism. Seeing that I was also seven months pregnant at the time and slower than a pig in molasses, I gave up on my water bath onion jam search.

Cut to the beginning of August. I finally coming out of my “I had a baby” fog and wanted to can something delicious, so I went back to the Tigress Can Jam March round-up, and found this onion jam recipe by Rebecca of Market Life SF blog. It looked delicious, and in my rush to can the recipe, my baby-addled brain completely forgot about the great onion jam search of ’11. I tweaked the recipe to my liking and canned away! I forgot until I tried to post the recipe on Punk Domestics, and Sean kindly pointed out that my acid might be off.

I’m not a big fan of botulism, so I first contacted Rebecca from Market Life SF to find out where she got her original jam recipe (a Martha Stewart magazine). I assume Martha tests her canning recipes, but I wanted to be sure. I read up on the subject of water bath canning a non-pickled onion product. This Doris and Jilly Cook post was a great refresher on the debate, and inspired me to want to test the pH of my recipe. After some more research (which I did in between feeding and entertaining a crying baby because I’m a rock star), I discovered that pH strips, which Doris and Jilly suggest using, are only safe for food products with a pH of 4.0 or less. I was fairly certain my onion jam would be slightly over 4.0, so I would need a pH meter to properly test it. Hot damn, it took a lot of work to get to this point!

More photos, and tips on selecting a meter and using it test your canned foods after the jump.

My Hanna pH meter with buffer and cleaning solutions.

I relied on this article by William McGlynn, Extension Horticultural Food Scientist from Oklahoma State University for tips on buying and using a pH meter to test canned food products. Here’s a summary of what I learned:

Tips for buying a pH meter:

  1. Needs a resolution and accuracy of 0.1 pH or higher
  2. Two types of probes- detachable (more expensive, probe replaceable) and all-in-one (more affordable, probe permanent). Probes are the most sensitive part of the meter, so type of probe depends on your usage of the meter.
  3. Look for automatic temperature compensation and calibration buffer if possible
Close-up of the probe and the thermometer.

I ended up purchasing this meter by Hanna. It was on the more expensive side, but I was also considering this option, which is more affordable. I also bought extra buffer solutions and a storage solution suggested by Amazon.

Once you buy your pH meter, these are the steps you should follow to test your canned food:

    1. Determine food type and prep food accordingly: homogenous (uniform consistency like salad dressing), liquid/solid mixtures (salsa, pickles), semi-solid foods (Puddings, jams, etc.), oily foods.
    2. My food was considered semi-solid, so I pureed it to a uniform paste. If liquid is needed to blend, you are supposed to use 20 parts distilled water to 100 parts food. I decided that 100 parts food = 50 ml jam, 20 parts liquid= 10 ml distilled water. I ended up using 200 ml jam and 40 ml distilled water for the finished uniform paste. Note if your food is not semi-solid, you will need to consult this article to determine how to prep your food.
    3. Calibrate meter with buffer solutions according to instructions provided.
    4. Rinse sensing probe with distilled water and blot (do not rub) with lint-free tissue paper (I used paper towels).
    5. Submerge probe in sample, and after allowing one minute for meter to stabilize, record pH. (Note- my meter has a stability indicator, which helps promote a properly taken sample.)
    6. Rinse probe, blot dry (do not rub!), and test again. Two pHs should agree within the accuracy limits of the meter.
    7. Rinse probe in distilled water, blot dry, apply a few drops of storage solution, and store meter.

And that’s it! My onion jam came out with an average pH of 4.07, which means it is definitely safe for consumption! Check it out here if you’d like to try it yourself.

And safe canning!


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  1. Martha Stewart is notorious for having errors in cooking – from flawed recipes, to not following safe food preservation standards. She’s great at marketing the “Martha” way of doing things, but they aren’t necessarily the best or safest way to do things. I love the idea of having a meter and checking for yourself. I know onion jam and fig jam recipes are real big right now, and figs are right up there with onions for botulism risk. Maybe you could play with them next? Good for you for taking the time to share your research with us. I’ve got you bookmarked and will follow you for more useful info like this. Awesome!

    • Leena

      1 Sep ’11

      Hey Stacy. Thanks for stopping by. I will definitely think about playing with figs in the future. I just love having the ability to create my own canning recipes from scratch! Such a luxury.

  2. Amy

    31 Aug ’11

    Post a Reply

    My understanding, as a science teacher, of the pH relationship behind canning is that it needs to be 3.9 or more acidic, which would actually be a lower number.  The closer you get to 7, the closer to neutral it is, and thus the botulinum spores can survive.

    • Leena

      1 Sep ’11

      Hi Amy. Thanks for stopping by and participating in the conversation. I am hardly a scientist, so I appreciate your perspective on this post. 

      From my research, both the FDA http://www.fda.gov/Food/FoodSafety/Product-SpecificInformation/AcidifiedLow-AcidCannedFoods/Regulations/ucm046688.htm 
      and the California Dept. of Public Health (my home state) http://www.cdph.ca.gov/pubsforms/Guidelines/Documents/fdbCANgde06.pdf require a pH of 4.6 or less for canned low acid food products. Specifically, the National Center for Home Food Preservation states “A pH below 4.6 must be achieved before a food can be safely boiling water canner.” http://www.uga.edu/nchfp/tips/summer/home_preserv_tomatoes.html, and my jam was both water bath canned and had a pH well under 4.6. The lowest pH I’ve ever seen recommended is 4.2, and that was suggested as a guide for those who want to play it safe and account for human error (can’t remember where I read it, but if you are interested, I can do some digging). With that in mind, my jam’s pH was still lower than 4.2. So either way, I feel pretty safe with my jam.

  3. Meaghan

    31 Aug ’11

    Post a Reply

    It’s my understanding that pH 4.6 is the cut off for what is safe in a water bath.  But I would need to double check my books to be certain.

    Leena, thanks for this explanation!  I’ve been eyeballing pH meters for a while, because I’d like to can my tomato soup recipe.  I’ve spoken several times with my local extension office to see if they would check my recipe, but they just give me the canned answer (ha!) that I should only use “research tested recipes.”  I understand their reasons (liability), but I know there has to be a safe way to experiment.  For some reason when I’ve read that McGlynn article in the past, it seems like the process should be so much more complicated.  But you’ve confirmed that it really isn’t. 

    • Leena

      1 Sep ’11

      Hey Meaghan. Thanks for stopping by. I’m pretty new to this testing your own canning recipes thing, but it sounds exciting to me. I always found the fact that you had to use a tested, publish recipe just to be safe annoying, even though I understood it was for my own safety. Let me know if you give testing your soup recipe a go. I’d love to hear what meter you get and how your test turns out.

    • Leena

      8 Oct ’11

      Hi Magda,

      Sure. As I read in this article (http://www.foodsafety.wisc.edu/assets/pdf_Files/What_is_pH.pdf)
      pH strips are only safe to test food products with a pH of 4.0 or less. I had a feeling my onion jam would be slightly over 4.0, which it was, so the pH strips would not have been able to give me an accurate reading. Check out that article if you have any more questions. Thanks for stopping by!

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