Filled jars of acidic preserves such as jam and marmalade are boiled fully submerged in a pot of boiling water for 10–15 minutes. This is known as water-bath canning, and the time may be longer depending on jar size. This method is not sufficient for low-acid canned vegetables such as beans and peas. Those foods require a temperature that boiling water cannot reach; otherwise, they carry a risk of serious food poisoning called botulism.
Which Foods Can Be Safely Processed in a Water Bath?
Water-bath canning is safe only for naturally acidic foods or foods with vinegar added. These include jam, marmalade, fruit compote, and vinegar pickles. The acidic environment in these foods prevents botulism spores, which cannot be destroyed even at 100°C / 212°F, from growing, so a shorter boiling time is sufficient. If fruit is too sweet to provide enough acidity for jam, adding some lemon juice to the recipe brings the acidity within the safe range for this method.
Why Do Low-Acid Foods Need a Different Process?
Botulism spores in low-acid vegetables such as beans, corn, and carrots do not die at the 100°C / 212°F temperature of boiling water. They must reach 116–121°C / 240–250°F in a pressure canner to be destroyed. A processing time of 20–90 minutes is given, depending on the vegetable and jar size. A household pressure cooker may not reach the required pressure and time reliably, so it is not considered a dependable alternative for this process.
Is Sterilizing Empty Jars the Same as Processing the Food?
No. Boiling empty jars for 10 minutes beforehand and drying them only sanitizes the containers. To make the food inside safe, the filled jar must also be boiled for the time described above or processed under pressure; sterilizing empty jars does not replace this step.
How Can You Check Whether a Lid Is Sealed?
After processing, let the jars cool to room temperature. As they cool, the metal center of the lid should dip inward and make a slight “pop” sound, which indicates that a vacuum has formed and the lid has sealed properly. If the lid still moves up and down when pressed, a vacuum has not formed; store that jar in the refrigerator and use it soon. Covering jars with a cloth while they cool can soften rapid temperature changes caused by drafts and reduce the risk of the glass cracking.
How Does Jar Size Change the Processing Time?
The times above are generally for medium jars of about half a liter. Do not use jars larger than the sizes given in the recipe; heat takes disproportionately longer to reach the center, and no safe time is known for larger jars. Do not shorten the time for smaller jars; use the time for the next larger size. Above about 300 meters (1,000 feet), water boils at a lower temperature, so extend the processing time according to the recipe's altitude chart. If a jar contains a low-acid ingredient such as meat or dairy, a pressure canner is still required regardless of jar size; a water bath is not sufficient for such contents at any size. Before placing jars in boiling water, arrange them so they do not touch and leave a few centimeters of space between them, allowing water to circulate evenly around each jar.
This content is for informational purposes only and is not a substitute for medical advice. Consult your doctor before making a decision about your health.