Oven canning is a method of preserving food by heating jars of food in the oven. It gained popularity during World War II when pressure canners were scarce. However, oven canning does not reliably kill bacteria that cause botulism, so it is not recommended as a safe canning method by the USDA.
What is oven canning?
Oven canning involves filling jars with food, capping them, and heating them in the oven. The goal is to create a vacuum seal as the contents heat up and the air escapes from the jar. This vacuum seal prevents microorganisms from entering and spoiling the food.
The processing time and temperature for oven canning varies based on the type of food being preserved. For example, tomatoes may be processed for 45 minutes at 350°F.
Why is oven canning potentially unsafe?
Oven canning does not heat foods evenly enough throughout the jar to kill all bacteria and toxins. It’s difficult to maintain a constant heat in an oven, and air pockets within jars prevent even heating.
The major safety concern with oven canning is Clostridium botulinum. This bacteria produces a deadly toxin that causes botulism poisoning. It can survive traditional oven canning temperatures.
Botulism spores are commonly found in soil and dust. So they could easily be transferred to foods like tomatoes. Proper processing above 212°F is required to deactivate the botulinum toxin. Oven canning often does not meet this temperature requirement.
Expert recommendations against oven canning
The USDA and extension services strongly advise against oven canning all low-acid foods, including tomatoes. The National Center for Home Food Preservation states:
“Oven canning is extremely dangerous because low acid foods…support the growth of the bacteria that cause botulism. Oven temperatures are too low to destroy botulism spores and the jars cool too slowly to prevent toxin formation.”
They warn that many outbreaks of botulism poisoning have been caused by oven canned foods like tomatoes and meats.
University extension services, including Penn State, Purdue, and Ohio State all warn against oven canning as well. They say oven temperatures vary significantly, air pockets form in jars, and it is unsafe for preserving low acid foods.
Safe options for canning tomatoes
To safely can tomatoes at home, it is recommended to use a pressure canner or boil water canner.
Pressure canning
Pressure canners allow you to achieve temperatures above 212°F by pressurizing the canner. The USDA recommends pressure canning all low acid foods like tomatoes, vegetables, meats, and seafood. The high heat kills botulism spores and ensures safety.
Follow validated pressure canner instructions for the specific food type and jar size. Process pint jars of tomatoes at 11 lbs pressure for 25 minutes.
Boiling water canning
While not recommended for all vegetables, you can safely boiling water can tomatoes because they are considered a high acid food. The USDA approves boiling water canning methods for foods with a pH below 4.6. Ripe tomatoes generally fall between 4.0-4.5 pH.
To boil water can tomatoes:
- Wash and peel tomatoes.
- Fill jars leaving 1⁄2” headspace.
- Adjust lids and process pint jars in boiling water for 40 minutes.
Always use up-to-date boiling water canning instructions for each food type. Only can high acid foods like fruit, pickles, and jams this way.
Conclusion
Oven canning does not achieve adequate temperatures to kill bacteria like Clostridium botulinum that causes deadly botulism poisoning. Well-researched recommendations are clear that oven canning tomatoes and other low acid foods is extremely unsafe. Always use a pressure canner for these foods. Boiling water canning is a safe option for tomatoes, since they are a high acid food when ripe. Follow validated canning instructions carefully to ensure safety and quality. Avoid oven canning to eliminate the risk of botulism.