Foolproof Preserving
The Science of Canning
The key to preserving vibrant peak produce starts with staving off bad bacteria.
04-13-2016
America's Test Kitchen

Today we're celebrating Foolproof Preserving: A Guide to Making Jams, Jellies, Pickles, Condiments & More. There is nothing more disappointing than jams and jellies that don’t set up or pickles that are soggy. Make your efforts count with our step-by-step guide, featuring 100 obsessively tested recipes. The test kitchen demystifies the process, explains the science behind it, and tells you exactly which equipment you need.

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.

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

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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)

Apples 

Apricots 

Blackberries 

Blueberries 

Cherries 

Gooseberries 

Grapefruit 

Grapes 

Lemons 

Limes 

Nectarines 

Oranges 

Yellow Peaches 

Pears 

Plums 

Raspberries 

Rhubarb 

Strawberries 

Medium Acid (pH Value: Near or Above 4.6)

Figs 

Mangoes 

Tomatoes 

Low Acid (pH Value: 4.6 - 7.0)

Asparagus 

Green Beans 

Beets 

Cabbage 

Carrots 

Cauliflower 

Corn 

Cucumbers 

Dates 

Eggplant 

Garlic 

Ginger 

Okra 

Yellow Onions 

White Peaches 

Bell Peppers 

Cherry Peppers 

Chile Peppers 

Turnips 

Watermelon 

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.

Bookstore

Eat Those Summer Tomatoes Year-Round Foolproof Preserving

The art of preserving produce has come full circle, from grandmother’s kitchen to a whole new generation now eager to learn how. This detailed, step-by-step guide from the experts at America’s Test Kitchen is perfect for first-time and experienced canners alike. You’ll get 110 foolproof recipes across a wide range of categories, from sweet jams and jellies to savory jams and chutneys, pickles, vegetables, fruit in syrup, condiments, and more.

 

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