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Food safety when canning tomatoes – Audio

This podcast was originally published in September 2012.

Barb Ingham, Professor
Department of Food Science
UW-Madison College of Agricultural and Life Sciences
(608) 263-7383

3:02 – Total Time

0:17 – What has recently changed
0:42 – Why added acid is now needed
1:05 – Difference between tomatoes
1:20 – What acid does in canning food
1:48 – How to use acid when canning tomatoes
2:12 – Other canning safety tips
2:52 – Lead out


Sevie Kenyon: Best management practices for canning tomatoes. We’re visiting today with Barb Ingham, Department of Food Science, University of Wisconsin Extension in the College of Agricultural and Life Sciences, Madison, WI and I’m Sevie Kenyon. Barb, what has changed when it comes to canning tomatoes?

Barb Ingham: You might not realize that in the late 1990’s some canning recommendations changed, including those for tomatoes. We now pretty much require that acid is added to tomato products when they are canned and this would include both those that are canned in the boiling water canner as well as those that are pressure canned.

Sevie Kenyon: Barb, what caused this change?

Barb Ingham: There are certainly newer varieties of tomatoes and they’re being bred for a little bit less acidity or certainly more sugar. We also know that there are some organisms that are somewhat newer on the scene, the E. coli 0157H7, it’s relatively acid tolerant. So if we take all of those things together we have to be a little careful with tomatoes than we used to be.

Sevie Kenyon: How much difference is there?

Barb Ingham: There can be as much as an entire PH unit. That would mean 10 times difference in acid between those that are high in acid and those varieties that are low in acid.

Sevie Kenyon: Barb, can you describe for us what the acid does in the canning process?

Barb Ingham: Yes. The acid is important because it actually works as a hurdle for microbial growth. So, acid works with us and allows us to process at lower temperatures so it does give us some flexibility. But we add the acid whether we’re pressure canning or boiling water canning. It doesn’t matter. We simply need it. It’s like an insurance policy, the reason we add acid to tomatoes.

Sevie Kenyon: Maybe you can tell us how the acid is added in the process?

Barb Ingham: Yes. We’re actually adding just a tablespoon of bottled lemon juice per pint of tomatoes or two tablespoons per quart. It has to be bottled or we can use citric acid; it’s a power crystalline, it looks a little bit like sugar or salt, that you might add and that you would add only a ¼ teaspoon per pint or a ½ teaspoon per quart.

Sevie Kenyon: Barb, what other tips do you have for people to be safe?

Barb Ingham: You want to process tomatoes that are of good quality, which means that we don’t want those that are practically so rotten they’re liquid. You don’t want those that are heavily damaged to the point that they are supporting a lot of mold or other bacterial growth because those tomatoes might be problems when we come to canning. Follow a tested recipe. The processing times for tomatoes if you’re using a boiling water canner can be up to 85 minutes. So, making sure that you’re checking with your local county extension office, making sure you’re having an up to date recipe, you’re processing fruit (tomatoes are fruits) of really good quality and then you’ll have something to enjoy all year long.

Sevie Kenyon: We’ve been visiting with Barb Ingham, Department of Food Science, University of Wisconsin Extension in the College of Agricultural and Life Sciences, Madison, WI and I’m Sevie Kenyon.