Canning is a method to preserve food using boiling water or steam to create an airtight seal inside of a glass jar. It is a simple process that is often made out to be much more complicated than it actually is. That’s because it is often conflated with other aspects of food preservation, such as pickling vegetables or making fruit preserves. Canning is simply the final step in preserving these items – the complicated part is mastering all the ways that food can be processed before it is canned and put on the pantry shelf. There is tremendous variation in the recipes for foods that are canned and it is important to follow them precisely – especially when it comes to ingredients like pectin, sugar, citric acid, salt, and vinegar that play a role in preservation.
When it comes to canning technique, the main variable is which type of canning device will be used. Water bath canners – essentially a large pot with a metal rack on the bottom to support Mason jars – are suitable for fruits (which includes tomatoes). Here’s a version we like. Canning vegetables and animal products require a pressure canner (we recommend this version) to reach the temperature necessary to preserve these foods safely. Pressure canners cost a bit more than water bath canners, but are worthwhile given that botulism, a lethal disease associated with canned foods, can only be eliminated by temperatures above the boiling point of water in canned vegetables and animal products. (Note: The inherent acidity of fruits helps to prevent botulism, so there is no need for the extra heat in their case. Use a pressure canner for recipes where fruits and vegetables are mixed.)
[Canning] is a simple process that is often made out to be much more complicated than it actually is.
Other than that, the canning process varies only slightly with different types of foods, and this information will be included in the canning recipe. For both methods, you’ll need Mason jars (with lids and bands), a jar funnel, jar tongs, oven mitts, and a plastic spatula. The rubber gasket on Mason jar lids is the key component responsible for sealing out air and preventing spoilage – they are only designed to be used once, so pick up an extra pack of lids for each new round of canning. The threaded metal bands that hold the lids in place may be used over and over, however.
Water Bath Canning for Fruits
- Place the jar rack on the bottom of the pot and arrange the mason jars on top of the rack.
- Fill the pot (with the jars inside; no lids), with water at least 1 inch above the top of the jars.
- Put the pot on the stove, cover it with a lid and turn the burner on high. Use the time it takes for the water to boil to prepare the food that will be canned.
- Once the water has boiled, pull the jars out with the tongs, empty the water they contain and place them on a towel on the counter. Retain the water in the pot.
- Fill the jars with the food product using the funnel, if needed. The recipe should indicate how much space to leave at the top of the jar. (A quarter or half inch is typical – smooth-textured jams and preserves require less, while chunky fruits and vegetable suspended in liquid require more).
- Stir the contents of the jar to release any air bubbles that may be trapped in the jar.
- Wipe the rim of the jar clean with a damp cloth to ensure a good seal with the lid.
- Place the lid on top and screw the band snugly into place.
- Lower the jars into the pot of water. Use a glass heat-proof measuring cup if you need to scoop out excess water to prevent the pot from overflowing.
- Cover the pot, bring the water back to boiling, and set a timer for the length of time indicated by the canning recipe you are following.
- Once the timer goes off, remove the jars and put them onto towel-lined countertop where they must remain undisturbed for at least 12 hours.
After the jars have cooled, confirm that they have been sealed by pressing into the lid with your finger. If the lid can be depressed and pops back up, it is not properly sealed. The lid should not move when pushed if the process was successful. You can also take off the bands and lift up the jars by the edge of the lid to test them – they should remain firmly in place. If the seal is not successful on any of the jars, repeat the canning process or just store them in the refrigerator for immediate consumption.
Pressure Canning for Vegetables and Animal Products
Pressure canners vary by manufacturer, but the process for using them is similar to water bath canning. The main difference is that the jars will sit in just a few inches of water, rather than being submerged. There is a vent on top of the canning pot that allows steam to escape, which needs to be left open for the first ten minutes of boiling (to allow the air to escape) and then closed (to keep the steam in).
For foods that need to be preserved with a pressure canner, the canning recipe should indicate the amount of time to leave the jars in the canner, as well as the amount of pressure that is necessary (in PSI, or pounds per square inch). Pressure canners are outfitted with pressure gauges and the heat can be raised or lowered to achieve the correct pressure level.
Check the instructions that come with the device for more details on how to operate it.
A Few Final Details
Elevation above sea level affects the boiling time needed to seal jars in a water bath canner and the pressure required for canning with a pressure canner. Recipes give the time and pressure for sea level, so use a conversion chart to find the appropriate numbers for the altitude where you live.
Successfully canned foods resist spoilage and retain their color, nutrients, and flavor for a year or more when stored in a cool dry place. It is important, however, to start with only the freshest ingredients – avoid using old, soft, diseased, or damaged produce. Always check the lids before using a canned product and discard any that are no longer sealed.