How Canning Works: Science of Home Canning

The key to preserving vibrant peak produce starts with staving off bad bacteria.

Published Apr. 13, 2016.

Home preservation is all about maintaining the vibrant qualities of peak produce, whether it’s from the farmers’ market, the supermarket, or your own garden. To do this, you have to understand how the process of canning works. Heating and cooling fresh fruits and vegetables in jars vacuum-seals them and allows them to be stored for a long time at room temperature, to be enjoyed all year long. The key to preservation is staving off the growth of bad bacteria. Fresh fruits and vegetables are not sterile—rather they’re rife with living microbes, which can come from a number of different places: soil, processing plants, grocery stores, and even the air, to name a few. Left to their own devices, bad bacteria will thrive and quickly take over. The easiest way to kill off microorganisms is to heat the food. This destroys the harmful microorganisms and deactivates the food’s enzymes, thus preventing further deterioration.


Foolproof Preserving & Canning

We set out to create 110 recipes from a wide range of categories that will let you preserve with confidence whether you’re a first-time or experienced canner.

Methods 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.

A Focus on Boiling Water Processing

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.

The Importance of pH

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.

High Acid (pH Value: 2.0 - 4.6)Medium Acid (pH Value: Near or Above 4.6)Low Acid (pH Value: 4.6 - 7.0)






Green Beans






















Yellow Peaches 



Yellow Onions


White Peaches


Bell Peppers


Cherry Peppers


Chile Peppers



High-Acid Foods

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.

Medium-Acid and Low-Acid Foods

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.

Measuring the pH

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.

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