Image: University of Wisconsin Extension Service
As most avid canners know, 4.6 is the pH dividing line between acid foods that can be safely water bath canned and less acidic foods that need to be canned in a pressure canner. Most fruits have a lower, i.e. more acidic pH and can be water bath canned.
Tomatoes, on the other hand, are often near the 4.6 pH level and USDA tested recipes will call for adding either bottled lemon juice or citric acid (I prefer citric acid as the taste is more neutral).
I used to think that this issue was because different tomato varieties vary in their acid content. It turns out that it’s more about when tomatoes are harvested, not to mention what the weather was like during the growing season. Add this variability to other factors, such as how many cans you put in your canner, the material your pot is made out of and the type of heat source and you end up with a tricky question for the food scientists who test home canning recipes. All of these factors are why the recommended hot water bath canning time for raw packed tomatoes is 85 minutes.
I’ve hot water bath canned tomatoes and got great results (especially with San Marzano tomatoes). But 85 minutes is a long time. You can cut the processing down considerably and get better results by pressure canning tomatoes. Here’s a raw pack recipe that includes both hot water and pressure canning instructions. Note that you still need to acidify.
Thanks to Linda Harris, UC Cooperative Extension Specialist, Food Safety and Microbiology who gave a lecture at the Master Food Preserver conference where I gleaned these factoids.
I’ve pressured canned my tomatoes for many years and have never had a single upset tummy. But when I freeze tomatoes, I get inevitable tummy aches. So in the end, doing it right with pressure canning pays off since the bottles will last well for several years if you can’t use them up fast enough. Nothing like some ‘insurance’ for all the hard work.
Once upon a time I used to raw pack tomatoes the way my grandmother and great-grandmother did. I gave them a water bath but nothing like 85 minutes. Sometimes I put in a couple basil leaves. I never added any acid.
I’m here to tell the story and I gotta say, those tomatoes were great. Next best thing to fresh!
Then I started reading Ball’s book on canning and USDA recommendations. Now I just can’t do it anymore. But those tomatoes were not only incredibly easy, they were incredibly good.
I have done mine for years with 45 minutes of time in the water bath with no problems. But I always add a bit more than the required lemon juice and leave the tomatoes in the boiling pot during blanching a bit longer than normal so that they have actually had some extra cooking time there, too.
Wow, the booklet that came with my pressure canner recommends 10 minutes at 5 pounds of pressure. I’ve been processing for 15 minutes, just because I’m weird, but even that is below current standards. Also, I’ve never acidified. Does anyone know why the lemon juice is necessary even with pressure canning?
I believe it’s because the current generation of hybridized tomatoes bred for shipping not flavor have lower natural acidity. But if you’re growing your own heirlooms you’ll peel a dozen or two and your hands can feel the acid!
But, SERIOUSLY, don’t take my word for anything ’cause I do everything wrong according to the USDA.
I’m wondering too. Why would the acid matter when pressure canning?
I don’t add any extra ‘acid’ when I pressure can, mostly because I follow the recommended timing.
One reason for the added acid is that often people add something quite alkaline like onions or garlic to their tomatoes and this would increase the pressure time but they don’t realize this. Extra acid will drop the time to ‘usual’ again. I do a simple tomato soup, quarts for 25 min or a full spaghetti sauce for 35 min.
Most heirloom tomatoes don’t fall into the category of ‘sweet’ so there’s no real need to add extra ‘acid’, it’s just a general precaution.
THANK YOU for this. I’ve long explained why I pressure can my tomatoes and sauces to friends, but I seem to be in the minority with this caution. THANK YOU!!!