The Curious Case of Canning Tomatoes | High or Low Acid?

Preserving Food

The history of the tomato is not as mundane as you would expect from a simple fruit. In fact, there’s absolutely nothing simple about the tomato or its tumultuous history at all. If all of the tomatoes ever produced could talk, they would captivate you with a harrowing story of persecution and eventual redemption. The truth is that throughout history, the “apple of love” has been revered, feared, hated and only relatively recently accepted. However, this is a story for another blog article (and rest assured, it’s coming).

Today, while tomatoes and their virtues have been widely embraced, the truth is that they are still steeped in controversy. I’m of course referring to that age old debate- Are they fruits or vegetables?

In canning, a food’s acidity is incredibly important. Knowledge of your food’s pH will guide how it is processed and canned. Fruits are typically acidic, having a pH below 4.6 while vegetables are generally low acid, with a higher pH. True to their controversial nature, tomatoes reside smack-dab in the middle. Most are acidic, but many varieties exist at a pH of 4.6 or higher. So, the debate continues- what the heck are tomatoes and how does this affect your canning?

Low Acid Tomato Varieties

Not all tomatoes are high acid, and not all tomatoes are low acid, a somewhat annoying division that has frustrated many a home canning enthusiast. Luckily, the USDA and University of Minnesota Extension have done us a favor by listing the tested tomato varieties that have a pH of 4.6 or higher. They’re listed below.

Ace
Ace 55VF
Beefmaster Hybrid
Big Early Hybrid
Big Girl
Burpee VF Hybrid
Big Set
Cal Ace
Delicious
Fireball
Garden State
Royal Chico
San Marzano

It’s important to note that these are only the tomato varieties that have been tested. There are likely more that should be considered low acid. Figs can also be considered borderline acidic, so the following input regarding canning tomatoes applies to them as well.

Canning Tomatoes

Canning tomatoes is one of the primary joys associated with home canning. There is so much that can be done with them. Whether canning them whole, crushed, peeled, in a sauce or paste it’s important to know that they require an extra step. Acidifying your tomatoes with lemon juice or citric acid is currently considered best practice, no matter what variety you’re using and no matter how you process them.

Acidification of Tomatoes

The process of acidifying tomatoes is relatively simple and the USDA has developed a well-researched method to do so for all varieties. The acidification protocols for canning whole, crushed or juiced tomatoes are shown below:

Add 2 tablespoons of bottled lemon juice or 1/2 teaspoon of citric acid per quart of tomatoes.
Add 1 tablespoon of bottled lemon juice or 1/4 teaspoon of citric acid per pint of tomatoes.
The lemon juice can be added directly to the jar before OR after filling it with the tomato product.
The lemon juice must be bottled as it has a standardized acidity, the acidity of fresh-squeezed lemon juice may vary.
Sugar can be added to offset the acidic taste that accompanies lemon juice and citric acid.
The acid can NOT be decreased after canning has occurred.

It is highly recommended that you follow these steps while canning tomatoes, no matter what variety you’re using. After acidifying your tomatoes, you can safely process them in a water bath canner and rest-assured that your final product will be safe to consume.

Acidification of Tomato Products

You should take special care when canning tomato products like sauces or pastes. Most sauces require the addition of low acid foods like onions, carrots, celery etc. for flavor. The addition of these extra low acid components may alter the acidity of your mixture enough that the above acidification methods are not sufficient. To be safe, tomato products should always be pressure canned at higher temperatures, especially when meat and mushrooms have been added to the mix. Please be aware that acidification of tomatoes is still required when pressure canning.

Processing Times

Processing times are also very important for safe canning of tomatoes. The following standard processing times were developed at Penn. State and taken from the PSU Extension website.

Raw-packed acidified tomatoes in pint or quart jars should be processed in a boiling-water bath canner for 85 minutes or a pressure canner for 25 minutes (10 PSI for a weighted gauge canner, 11 PSI for a dial gauge canner at an altitude of under 1000 ft).
Hot-packed crushed tomatoes in pint jars should be processed in a boiling-water bath canner for 35 minutes or a pressure canner for 15 minutes.
Hot-packed crushed tomatoes in quart jars should be processed in a boiling-water bath canner for 45 minutes or a pressure canner for 15 minutes.
Hot-packed tomato juice in pint jars should be processed in a boiling-water bath canner for 35 minutes or a pressure canner for 15 minutes.
Hot-packed tomato juice in quart jars should be processed in a boiling-water bath canner for 40 minutes or a pressure canner for 15 minutes.

Use these standard processing times to assess recipes that you come across while planning your preserve. As a general rule, it’s suggested that you only follow recipes from the USDA and cooperative extension or from a Ball Blue Book which have been scientifically tested.

A word of Caution

Note that tomatoes must be acidified in order to prevent germination and growth of C. Botulinum spores. Growth of this bacteria in low acid food increases your risk of botulism.

While acidification theoretically mitigates this risk, it’s still possible for the Botulinum bacteria to grow in high-acid food. Improper acidification of foods may result in internal pockets of low acidity. Further, if acidified food is improperly processed, some strains of fungi can survive and produce local pockets of non-acidity, facilitating bacterial growth and formation of the toxin. Proper processing and acidification should eliminate this risk, so please follow approved USDA or Ball Blue Book recipes.

Respect Canned Tomatoes

Canning tomatoes is not something to be feared, but it should be respected. They are a unique fruit that requires extra care to can safely. Make sure that you’re educated and have all the right equipment and ingredients beforehand and you shouldn’t have any problems.

Happy Canning.