When we have a canning and food preservation class, one question we often get asked is how do we know when to use the water bath method and when to use the pressure canning method.  As it turns out, the pH is the answer to all of our preservation questions. While 42 may be the answer to the Ultimate Question of Life, the Universe, and Everything (read A Hitchhiker’s Guide to the Galaxy), it turns out that 4.6 is the ultimate answer to how do I can this safely?  What does that mean?  Read on to find out.

First of all, what is pH?  pH refers to the acidity of a food and is measured on a scale from 0 to 14.  A seven (7) on the scale is neutral with 0 being highly acidic and 14 being extremely basic.  So why does this matter?  According to https://extension.okstate.edu/programs/oklahoma-gardening/recipes/ph-and-home-canning.html, the pH determines which microorganisms are capable of growing in your canned product and therefore which method of processing is needed.  Microorganisms are able to survive and grow in pH environments between 4.6 and 9.

Luckily, most food items are naturally acidic, meaning their pH values are less than 7. As the pH values decreases and food becomes more acidic, the microorganisms decrease as well.  There is a magic number when it comes to safe canning: 4.6.  At a pH of 4.6, Clostridium botulinum, the bacteria that causes botulism–a potentially deadly condition–cannot grow and reproduce.

This means that the pH value 4.6 is the line between foods that are safely processed in a boiling water canner versus those which will have to be processed in a pressure canner. Foods with a pH greater than 4.6 require pressure canning. Remember, lower is better if you are planning to use a water bath method of preservation.

If a food item is made up of ingredients with a pH of 4.6 or lower, these are called acid food. Acid foods include apples, peaches, strawberries, lemons and most other fruits.  When some ingredients are a higher pH than 4.6 but we bring the overall value of the food down to 4.6 or less, this is referred to as an acidified food.  Examples of acidified foods are “pickled” products such as dill pickles, sauerkraut, and pickled beets.  If the pH is higher than 4.6, these are called low acid foods.  Low acid foods include most vegetables and meats. Acid foods and acidified foods are safe to water bath can.  Low acid foods, on the other hand are not safe to water bath can and must be pressure canned to be safe.

An important rule of thumb is to always use a science based, thoroughly tested recipe when home canning foods. As good as a recipe on the internet may look, it just isn’t worth it to use an untested recipe that could make you or your family sick.  An excellent resource for safe canning recipes is the USDA National Center for Home Food Preservation at http://nchfp.uga.edu/.  However, any recipe from any extension program across the country should be safe to use.

There is an excellent fact sheet available at https://hgic.clemson.edu/factsheet/canning-foods-the-ph-factor/ that lists a table with pH ranges of many common foods. If after all this you still have questions, I would point you in the direction of your local extension office.  They have access to many safe food preservation recipes.  They are also a wonderful resource for all things agriculture, including gardening and food preservation.