By Diane Wright Hirsch, MPH, RD
UConn Extension Educator – Food Safety
In the recent past, the tomato crop has not been so great, but thankfully, this year looks like a winner. The tomato plants in my garden are pulling the stakes over; they are so laden with fruit. Next year, it will be back to cages, I think!
An abundant harvest may mean that you want to can some of your tomatoes this year. If you are new to the task (or maybe even more importantly, if you are a long time tomato canner), it is essential that you learn the safest techniques for canning. Over the thirty plus years that I have been canning, recommendations for the safe home canning of tomatoes have evolved. Here is what you should NOT be doing anymore:
- Canning tomatoes without processing them in a water bath or pressure canner
- Processing tomatoes without adding lemon juice to each jar as added “safety insurance”
- Processing quarts of tomatoes in a water bath canner for 30 minutes
So, what are the safety concerns related to home preservation of tomatoes? They include the possibility of spoilage and even botulism poisoning. “But aren’t tomatoes acidic?” you might ask. “Doesn’t that mean that you do not have to worry about botulism?”
Well, the answer is yes and no. It depends on the tomatoes. The pH of tomato varieties can range from an acidic low of 3.8 to a much less acidic pH of 4.7. The story goes that as the consumer has demanded a less acidic tomato, hybrids have been developed that taste less acidic—but some argue that this is due to more sugar in the tomato, not less acid. Would that mean that older heirloom varieties are likely to be more acidic? Not necessarily.
A study by Heflebower and Washburn at Utah State in 2010 looked at hybrid varieties, open pollinated and heirloom varieties. The results indicated that the average pH was 3.92 for the hybrids they tested, 4.03 for open pollinated and 4.16 for the heirloom. Of course all of these are below the 4.6 borderline. The authors still recommend the addition of acid to ensure that the final product remains below 4.6. In a bulletin from the North Dakota Extension Service regarding the use of lemon juice in canned tomatoes and salsa (Garden-Robinson, Houge, & Smith, 2004), the pH of 15 different tomato varieties were tested. The pH readings were taken from the pure tomato pulp prior to canning and again after lemon juice was added and the tomatoes were made into salsa. In this particular study, all of the raw tomatoes tested had a pH from 4.8 to 5.2 prior to canning.
In your home garden, a number of factors can influence the pH of even the more acidic tomatoes: these include soil, vine health, ripeness.
When using a water bath canner we caution home canners to be mindful of the pH of the food you are canning. As a general rule of thumb, if the pH is 4.6 or higher, the food is considered to be “low acid” and should be canned in a pressure canner. If the pH of a tomato product is below 4.6, there is the chance that if clostridium botulinum is present when processed in a water bath canner, that the bacteria will develop the spore form during the heating process, protecting it from destruction. Once the tomatoes are stored at room temperature, the spore can germinate and allow the production of toxin by the bacteria. This generally happens at room temperatures when the environment is moist, low in acid and free of oxygen. A pressure canner can reach temperatures well above the boiling point of water achieved in a boiling water bath canner. These temperatures (240 degrees F) will destroy botulism spores.
So rather than risk the chance of botulism at worst or spoilage, a less ominous outcome, but still one that results in the waste of food and a lot of hard work, why not just learn to can tomatoes using tested, safe methods? The go-to source for up to date information on canning tomatoes is the National Center for Home Food Preservation (www.uga.edu/nchfp). You will find directions for canning tomatoes and tomato products in a water bath canner and a pressure canner. For any tomato products, be sure to add a tablespoon of lemon juice or ¼ teaspoon of citric acid to pints and two tablespoons of lemon juice or ½ teaspoon of citric acid to quart jars.
Some folks are nervous about using the pressure canner. New pressure canners are easy to use and have many built in safety features. And using one makes sense for canning tomatoes. If you want to can tomatoes using no added water, you will need to process quarts in a water bath canner for 85 minutes. If you have lots of tomatoes, that’s a lot of minutes! And a lot of electricity or gas is used in the process. The same tomatoes will only take 25 minutes in a pressure canner at 11 pounds of pressure. It just makes sense to consider the pressure canner in this case.
In addition, if you want to make tomato products that contain significant amounts of low-acid ingredients such as meat, garlic, onions, peppers, or mushrooms, the pressure canner is essential for producing a safe product. All of these low-acid ingredients will surely bring the pH of your sauce or other tomato mixture well above 4.6 even if you start with acidic tomatoes.
The National Center also has tested recipes for canning some tomato products that have a lesser amount of low-acid ingredients. There are recipes for salsa, a tomato-vegetable juice, ketchup, and barbecue sauce. For each of these products, vinegar or lemon juice are important ingredients, adding the acidity needed to safely can in a water bath canner. Never add any additional ingredients or an extra pepper or two as this will likely make these products unsafe to can in the water bath canner.