How Canning Works: Science of Home Canning

There are two methods of home canning approved by the USDA: boiling water canning and pressure canning. Which one you use depends on the acidity, or pH, of the food being preserved. Boiling water canning is used for processing high-acid foods, including most fruits and pickled vegetables. Hot filled jars are completely immersed in boiling water for a period of time and then allowed to cool. As the jars and their contents cool, the lids vacuum-seal the jars for long-term storage. 

Pressure canning is recommended for foods that have very little natural acid, including most nonpickled vegetables, meats, poultry, seafood, and dairy. These must be processed to a temperature hotter than the boiling point of water. A pressure canner is a special pot that heats food to 240 degrees Fahrenheit by way of pressurized steam circulating around the jars instead of boiling water.

When developing recipes for Foolproof Preserving: A Guide to Making Jams, Jellies, Pickles, Condiments & More, we focused on boiling water canning because it is easier and is the most accessible technique for the home cook. We did not give processing instructions for really small batches (two jars) because we felt the recipes would be served and enjoyed right away or stored in the fridge.

In order to can foods safely, you need to understand the importance of pH, which is simply a measurement of acidity. The pH scale runs from 0 (most acidic) to 14 (most basic). A lower pH value means higher acidity, and higher pH means lower acidity. Lemon juice is about pH 2 and pure water is neutral at pH 7. For preserving purposes, a pH of 4.6 is the dividing line between high- acid and low-acid foods. The chart below shows the range of pH values for the fruits and vegetables that we used in our recipes.

Most fruits used to make jams and jellies have a pH of 4.6 or lower and can be safely heat processed in boiling water without having to alter their pH. We do, however, often add an acidic ingredient such as bottled lemon or lime juice to most jams because it helps the pectin (both natural and commercial pectin) to gel properly.

Fruits and vegetables with a pH above 4.6 cannot be safely processed until their pH has been lowered. The reason is that certain bacteria commonly found in these low-acid foods will thrive in a sealed jar and make it unsafe to eat. Known as anaerobic bacteria, they prefer a moist, airless environment and can survive the boiling water canning process. Luckily, increasing the pH of a low-acid fruit or vegetable is as easy as adding some bottled lemon or lime juice or vinegar to the recipe. Note that the pH level of fresh lemon and lime juice can vary dramatically from fruit to fruit and that fresh juice is therefore not a good choice when canning. Bottled lemon and lime juice have a consistent and reliable pH level, which is why we use them in all of our recipes.

Though measuring pH is a fairly simple process, we realize that it’s unlikely you have a pH meter in your kitchen. Don’t worry. All of our recipes have been rigorously tested using our own carefully calibrated, lab-grade pH meter. To err on the side of safety, we made sure that every recipe in Foolproof Preserving: A Guide to Making Jams, Jellies, Pickles, Condiments & More with an option for canning (referred to as “long-term storage” in the recipes) registers well below a pH of 4.6.