Boiling water canning is a great way to preserve foods you love, from jams and jellies to tomatoes and fruits to all kinds of pickles. The canning process can seem daunting—since you are dealing with a huge pot of boiling water and multiple glass jars of food—but each step along the way is in fact quite simple.
Many of the smaller batch (two-jar) recipes in Foolproof Preserving and on our websites can be processed for long-term storage if desired. If a recipe can be processed—and it will say so in the recipe headnote—simply follow the step by step instructions below, using the headspace measurement and processing times in this chart below.
When processing just two jars, you can use a smaller pot than a standard-size canning pot. Just make sure to line the bottom of the pot with a rack, or several jar rings, so that the jars are slightly elevated. Also make sure to cover the jars with 1 inch of water.
1. Heat the Jars
Many recipes call for sterilizing the jars and lids before filling them; however, the USDA says that this is only necessary when processing jars for less than 10 minutes. We don’t sterilize jars in this book because all of our recipes are processed for at least 10 minutes. Yet, the jars do need to be heated before being filled with hot jam or the room-temperature glass may shatter. Jars can either be warmed in the canning pot that will be used for processing or placed under hot running tap water. As for the lids, they do not need to be heated before using, and in fact many manufacturers warn against it.
2. Fill the Jars
As soon as the jam has finished cooking, it needs to be portioned into the hot jars. Given that both the jam and the jars are hot, we found it very helpful to use a wide-mouth canning funnel (otherwise you might make a real mess and possibly burn yourself). A canning funnel works better than a traditional kitchen funnel for this task because the large opening makes filling the jars go quickly (which helps to keep the jam hot), and the funnel nestles securely into the jar so it’s less likely to tip over when full. Because the timing is so important here, we like to have the jars warmed and waiting for the jam.
3. Measure the Headspace
It is very important to leave some space between the top of the food and the rim of the jar, known as headspace. If canning larger pieces of fruit or vegetables in liquid, make sure the solids are fully covered by the liquid and then measure the distance between the liquid and the rim of the jar. Each recipe will spell out exactly how much headspace is required (usually between ¼ inch and 1 inch). The headspace allows for the food to expand as it heats up during processing. If you have either too much or too little headspace, it can prevent the lid from sealing properly to the jar or cause siphoning.
4. Release the Air Bubbles
After filling the jars and measuring the headspace, use a wooden skewer to remove any air bubbles trapped in the jar. For thick jams and jellies, draw the skewer upward to release the bubbles. For larger pieces of fruit or vegetables in liquid, press the skewer against the food to press the air bubbles out. If left unchecked, the air bubbles will collect at the top of the jar during processing and alter the headspace, which can prevent the jar from sealing properly. Once the air bubbles have been removed, be sure to add extra jam or liquid as needed so that the headspace measurement is correct.
5. Add the Lids and Rings
Before adding the lids and rings, it is important to wipe the rim of the jar clean of any drips. Once clean, place the lids on top and screw on the rings until just fingertip-tight. Do not overtighten the rings, or you will prevent any air from escaping the jars during processing, which is a key part of the canning process. Note that the lids can only be used once; they cannot be reused. The rings, however, can be used several times as long as they are in good shape.
6. Process the Jars
Using a jar lifter, lower the hot, filled jars into the rack inside the pot of boiling water. Make sure the jars are covered by at least 1 inch of water; if necessary add more water. Bring the water back to a boil and then process (boil) the jars for the amount of time as prescribed in each recipe. Be sure to start the timer only after the water has returned to a boil. Processing times will vary based on the size of the jars, your altitude, and the type of food inside the jars. Smaller jars and lower elevations have shorter processing times than larger jars and higher elevations. The USDA has determined safe processing times for all different types of food, and we follow their guidelines in all of our recipes.
7. Let the Jars Seal Themselves
After the processing time is up, turn off the heat and let the jars sit in the hot water for 5 minutes. This allows the boiling-hot food inside the jars to settle down and starts the lid-sealing process. After 5 minutes, remove the jars from the pot and allow them to cool at room temperature for 24 hours. As the food cools, it contracts, which makes a small vacuum form inside the jar. The pull of this vacuum pops the flexible metal lid inward, an indication that the jar has been hermetically sealed and oxygen can no longer pass through. To test the seal, press on the lid with your finger; a sealed lid will feel firm, while an unsealed lid will flex under the pressure and make a small popping sound.
8. Store the Jars
The combination of the sterilized jam and the hermetically sealed lid is the reason the jar can be stored for at least one year in a dark, cool place. We recommend storing the jars without their rings. This way, you can quickly tell if the seal has been broken before using, indicating that the jam is not safe to eat. If the seal pops at any time during storage, you must discard the jam; it could contain harmful bacteria or toxins that could make you sick.
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.