Acidity Complications for Canning Tomatoes
 | Author:
Allison Kosto, MSU Broadwater County Extension Agent
MSU Broadwater County Extension Agent |
Acidity Complications for Canning Tomatoes
Allison Kosto
MSU Broadwater County Extension Agent
It’s a wonderful feeling when the garden is overflowing with produce. Then the challenge becomes preserving the bounty for enjoyment throughout the winter. If you have been reading my column for a while, then you might recall my journey into food preservation. While it’s not my area of expertise, I am grateful that MSU Extension has a wealth of resources on the subject and colleagues that I can call for assistance. My latest adventure in food preservation has been tomatoes. Last year, I successfully made and canned salsa which was a huge hit with my family. This year, I’d like to make tomato or spaghetti sauce. However, I’m discovering there are some intricacies that make canning tomatoes unique. Here are a few important details and tips that I’ve learned along the way.
Safety is the top priority in food preservation. It is critical to follow safety guidelines to prevent foodborne illness by bacteria, mold and yeast. The worst case scenario is a toxin called botulism. This may seem farfetched but there was a situation, just a few years ago, in Ohio where home-canned potatoes in a potato salad contained the botulism bacteria at a potluck. This caused a large botulism outbreak. MSU Extension uses USDA home food preservation guidelines to help prevent these types of outbreaks.
There are two primary types of canning, pressure canning and water bath. To determine which method you need to do depends on the type of produce. Low-acid foods including vegetables, meats, poultry and fish require a pressure canner. Acidity is essential to prevent botulism. These foods have little natural acid, so they must be processed at higher temperatures and pressures that can only be achieved by pressure canners. High acid foods which include fruits, properly acidified tomatoes and properly acidified pickled products can be processed with either a water bath or pressure canner. Open kettle canning and the use of conventional ovens, steam canners, microwaves and dishwaters for canning are not recommended, because they do not prevent the botulism toxin.
For many years, the standard for canning tomatoes was that it was safe to use the water bath method. However, recent research has shown that that isn’t necessarily the case. This is because tomato varieties have changed over the years. In general, tomatoes grown today have less acidity than ancestral tomatoes. Even heirloom varieties have been shown to have low acid. Additionally, the acidity level changes as the plant grows. It’s highest in green tomatoes and decreases as they mature. Do not use tomatoes from dead or frost-killed vines which are even lower in acidity and can contain harmful pathogens.
Tomatoes need to be acidified before canning by adding 2 tablespoons of bottled lemon juice or ½ teaspoon of citric acid per quart. If you are canning products such as salsa, catsup or barbecue sauce make sure to follow a USDA or university recipe that is research-tested to ensure it has enough acidity. Some recipes, for example, spaghetti sauce, require a pressure canner because they do not add enough acid.
The length of canning time has also changed over the years and is often longer than historically done. There are also adjustments that need to be made for elevation. While it's tempting to follow a recipe that has been handed down for generations or found through a Google search, find one that has been tested. Reliable sources recommended by MSU Extension include the National Center for Home Food Preservation with the University of Georgia and the Ball Blue Book Guide to Preserving. When in doubt, pressure can tomato products to be safe.
If canning tomatoes isn’t up your alley, another great option is freezing. Tomatoes can be frozen raw either whole or in pieces. Frozen tomatoes work great in sauces. They can also be juiced, pureed or stewed to be frozen.
The verdict is still out on whether I’ll try to make tomato sauce this year. However, several batches of salsa are in the works! For more resources about food preservation, you can find MSU Extension in Broadwater County at 416 Broadway in Townsend. Check out my blog at https://www.montana.edu/extension/broadwater for the salsa recipe and links to more resources.
