Tomatoes seem like they should be one of the easiest foods to preserve.
They grow abundantly in late summer, they’re naturally acidic, and generations of home cooks have been putting tomatoes into jars for years.
So why do tomatoes need special attention when you’re canning them?
Because “acidic” doesn’t always mean acidic enough.
And understanding that little distinction is one of the most important things a beginning home canner can learn.
What Does Acidity Have to Do With Canning?
When we’re talking about home canning, foods are generally divided into two broad groups:
Acid foods have a pH of 4.6 or lower.
Low-acid foods have a pH above 4.6.
That dividing line matters because the microorganisms we’re concerned about in home-canned foods behave differently depending on the acidity of the food.
Low-acid foods require pressure canning because a pressure canner reaches temperatures considerably higher than the boiling point of water. Those higher temperatures are necessary to safely process foods such as meat, poultry, and many vegetables.
Acid foods can generally be processed using a boiling-water canner.
That’s the basic idea.
But then we get to tomatoes.
Aren’t Tomatoes Acidic?
Yes.
Usually.
And that’s where things get interesting.
Tomatoes have traditionally been treated as an acid food for home-canning purposes. But some tomatoes can have a pH above 4.6, which means they aren’t reliably acidic enough to be treated as a safe acid food without additional acidification.
That’s why current USDA/NCHFP recommendations call for adding bottled lemon juice, citric acid, or 5% acidity vinegar when canning plain tomatoes and certain tomato products.
The National Center for Home Food Preservation explains the reasoning and current recommendations here:
https://nchfp.uga.edu/how/can/how-do-i-can-tomatoes/canning-tomatoes-introduction
In other words:
You don’t get to skip the acid just because the tomatoes taste acidic.
Your taste buds aren’t a pH meter.
Want the Exact Acidification Amounts?
NCHFP has a separate page with the current amounts for bottled lemon juice, citric acid, and 5% acidity vinegar for tomatoes.
https://nchfp.uga.edu/how/can/how-do-i-can-tomatoes/tomato-acidification-directions
That’s a page worth bookmarking if you’re going to be canning tomatoes.
Why Does This Matter?
Because we’re not simply trying to make tomatoes taste good in a jar.
We’re trying to make them safe to store at room temperature.
That’s why home-canning recipes aren’t something to casually modify.
The amount of acid, the ingredients, the jar size, the headspace, and the processing method all work together.
A tested canning recipe has already taken those factors into account.
That’s also why you’ll sometimes find older tomato-canning recipes that don’t look quite like today’s recommendations.
Home-canning guidance has changed as researchers have learned more about the natural variation in tomato acidity and about the conditions necessary for safe preservation.
So if you have an old family recipe tucked into a cookbook or handwritten on an index card, treasure it for the memories — but check current tested guidance before using it for shelf-stable canning.
What Is a Water-Bath Canner?
A boiling-water canner is essentially a large pot designed to hold jars while they are covered with boiling water.
The jars sit on a rack rather than directly on the bottom of the pot, allowing hot water to circulate around them.
For properly acidified tomatoes and other appropriate acid foods, boiling-water processing can provide the heat treatment called for by the tested recipe.
NCHFP’s general food-safety information explains the principles behind safe home canning here:
https://nchfp.uga.edu/how/can/general-information/ensuring-safe-canned-foods
And this is where water-bath canning becomes a very approachable place to start.
You don’t need a complicated piece of equipment.
You need the right equipment, the right recipe, and a little patience.
What About a Pressure Canner?
A pressure canner is a different tool for a different job.
Pressure canning is necessary for low-acid foods because it allows the food to reach temperatures higher than boiling water can provide.
That’s why we’ll be talking about pressure canning later this week when we move from tomatoes to chicken.
And here’s an important point:
Pressure canning doesn’t automatically make every canning recipe interchangeable with every other recipe.
A tested recipe tells you which processing method to use, how long to process, and at what pressure when pressure canning is called for.
Follow the method that goes with the recipe.
One More Reason Tomatoes Deserve Respect
There’s another thing I want beginning canners to understand.
Tomatoes are forgiving in the kitchen.
Canning tomatoes is not the place to be casual about food-safety rules.
You can change a fresh tomato recipe because you don’t like onions.
You can add a little more garlic to tonight’s tomato sauce.
You can decide you want extra basil in the pot.
But when you’re making a shelf-stable canned product, don’t assume that a change that works perfectly well for cooking will also work for canning.
That’s where tested recipes matter.
The Good News
None of this means tomato canning is difficult.
Quite the opposite.
Once you understand why the acid matters, the process starts making a lot more sense.
And tomatoes are still a wonderful place for a beginning canner to learn about boiling-water canning.
Tomorrow, we’ll move from the why to the how.
I’ll walk you through a tested tomato-canning method, including the preparation, acidification, jar filling, processing, and the little details that make the difference between simply putting tomatoes in a jar and actually preserving them safely.
Because that’s really what Putting Things Back is about.
We’re not just filling jars.
We’re putting food away for another day — safely.
A Few Reliable Canning Resources
If you’re learning to can, these are the sources I’d bookmark rather than relying on a random recipe you happen to find online.
National Center for Home Food Preservation — Canning Tomatoes
https://nchfp.uga.edu/how/can/how-do-i-can-tomatoes/canning-tomatoes-introduction
This is the NCHFP’s overview of tomato canning, including current guidance on tomato acidity and acidification.
NCHFP — Tomato Acidification Directions
https://nchfp.uga.edu/how/can/how-do-i-can-tomatoes/tomato-acidification-directions
This page gives the specific acidification directions for tomatoes.
NCHFP — Ensuring Safe Canned Foods
https://nchfp.uga.edu/how/can/general-information/ensuring-safe-canned-foods
This explains the principles behind safe home canning and why the appropriate processing method matters.
USDA Complete Guide to Home Canning
https://nchfp.uga.edu/resources/category/usda-guide
The NCHFP hosts the USDA’s Complete Guide to Home Canning, including information on fruits, vegetables, tomatoes, meats, poultry, jams, jellies, and other home-preserved foods.
For this week’s canning posts, these are the sources I’m using as the safety foundation. When it comes to home canning, tested recipes and current recommendations matter.
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