If you are new to food preservation, you might ask why there are two different processes for canning food: boiling water bath and pressure canning. What is the difference between them? This question often arises as individuals begin growing and preserving their own food. The decision process can seem intimidating, especially with the many differing viewpoints found on social media or stories of traditional methods passed down through generations. Fortunately, science can help us understand which methods are most effective to ensure our food is safely preserved.
The National Center for Home Food Preservation and the USDA can help take the guesswork out of your project. Their recipes have all been rigorously tested, taking into account factors such as food density, pH, and heat penetration in the jar. That’s why their guidelines are considered reliable and “approved”.
Returning to the original question: Why are some foods preserved using a water bath while others require pressure canning? The answer lies in the acidity, or pH, of the food. A basic understanding of the pH scale, often introduced in high school science classes, is helpful here. This scale measures the acidity or alkalinity of a substance, with 1 being highly acidic and 14 being highly alkaline. Water is at the center of the range with a neutral pH of approximately 7.
Understanding pH is crucial in food preservation because it directly affects food safety. The pH level determines how well spoilage microorganisms can grow and multiply in food. These microorganisms thrive in foods with a neutral pH. A higher acidity level creates an environment that inhibits their survival. The critical threshold for microbial growth in food is a pH of 4.6. Foods with a pH below 4.6 are classified as high-acid foods, while those with a pH of 4.6 or higher are considered low-acid foods. This distinction determines which canning method can be used.
From the USDA Guide to Home CanningHigh-acid foods and acidified foods are safely preserved using the boiling water bath method. High-acid foods, like berries, are naturally well below pH 4.6, so need no additional acid when combined with boiling water processing to safely prevent microbial growth. Acidified foods are naturally low-acid foods that have been preserved using a solution such as lemon or vinegar to reduce their pH below 4.6. A common example is pickling. This process involves submerging low acid foods, such as cucumbers or beans, in an acidic solution before canning. The combination of acidity and the boiling water bath process, which heats food to 212°F, prevents the growth of spoilage microorganisms.
Low-acid foods, on the other hand, must be processed using a pressure canner. This method allows for higher temperatures to be reached inside the jar. Since low-acid foods such as meats, beans, fish, and many vegetables lack the natural acidity needed to inhibit microorganisms, they require higher temperatures to ensure safety. Boiling water in an open pot will never reach higher than 212F no matter how long it boils. This temperature is insufficient to destroy the microorganisms of low-acid food in sealed jars. Using a pressure canner allows the food to be heated to an internal temperature of 240°F and 250°F. Maintaining these temperatures for a specified time ensures that spoilage microorganisms, such as the dreaded Clostridium botulinum, are eliminated.
If you find it hard to keep track of these rules – or prefer to experiment with your ingredients – consider other preservation methods like freezing, which allow for more flexibility. For example, you could make and freeze a custom hot sauce with any ratio of peppers, tomatoes and vinegar without worrying about the pH.
For further questions about food preservation, contact the Local Foods Teams in St. Lawrence or Jefferson Counties. We love hearing from you!
The Master Food Preserver volunteers will be at many farmers markets this summer in St. Lawrence County, answering community questions and sharing resources. Find dates on our event calendar here.
National Center for Home Food Preservation:
- Has a searchable database of recipes as well as overviews of each food preservation method: nchfp.uga.edu
- Houses the USDA Complete Guides to Home Canning: https://nchfp.uga.edu/resources/category/usda-guide
Harvest Kitchen at the Extension Learning Farm in Canton, NY
- Some of our favorite recipes, that we’ve used in past hands-on workshops can be found here: https://stlawrence.cce.cornell.edu/harvest-kitchen/seasonal-recipes
Mike Myers is the Taste NY Store Manager for the North Country Welcome Center in Alexandria Bay. He took the Master Food Preserver training in 2022 and fields preservation related questions for home cooks in Jefferson County. He also works with food entrepreneurs who want to sell their goods through the Taste NY stores. At home in Antwerp, he enjoys making pepper relish, keeping bees, and raising meat rabbits.
Flip Filippi coordinates the Master Food Preserver volunteers as part of her role as Local Food Program Leader at CCE- St. Lawrence County. She offers hands-on food preservation and cooking workshops incorporating ingredients grown or raised in northern New York. Some of her favorite items to preserve are pressure canned dry beans and water bath canned peaches and pears.