Sterilize jars
Sterilizing jars means heating clean glass jars to kill microbes before you fill them with jam or pickles. Boiling them for 10 minutes is the tested way.
Also called Sterilise jars, Sterilizing jars, Sterilising jars.
Sterilizing jars means heating clean glass jars to kill the yeasts, molds and bacteria on them before you fill them with jam, pickles or other preserves. It matters most for preserves that get little or no further heat after the jar is sealed. Sterilizing is not the same as preserving: whether a filled jar is safe to keep at room temperature depends on the recipe and on how the filled jar is processed.
How to do it
This is the US home-canning method:
- Check the jars for chips and cracks, and use new flat lids.
- Wash jars, lids and bands in hot, soapy water, or in a dishwasher, and rinse them.
- Stand the jars upright on a rack in a large pot or canner, and fill the pot with hot water to 2.5 cm (1 inch) above the tops of the jars.
- Bring to a boil and boil for 10 minutes. Above 300 m (1,000 feet), add 1 minute for each extra 300 m (1,000 feet).
- Leave the jars in the hot water until you need them. Lift them out with a jar lifter, drain them and fill them while they are still hot.
- Prepare the lids as the maker says. Many current two-piece lids do not need heating.
Why it works
Ten minutes in boiling water kills the yeasts, molds and most bacteria that spoil jam and pickles. Hot jars also do not crack when you pour in hot food.
US guidance says you only need to sterilize jars when the filled jars will be processed in a water bath for less than 10 minutes. If the processing time is 10 minutes or longer, or you are pressure canning, clean, hot jars are enough, because the processing sterilizes them.
Common mistakes
- Pouring hot jam into cold jars, which can crack them.
- Touching the inside of a sterilized jar or lid.
- Thinking sterile jars make any recipe safe for the shelf. Low-acid food needs pressure canning to prevent botulism.
- Reusing old flat lids, which may not seal.
In recipes
Many British recipes heat clean jars in a 140°C (275°F, gas mark 1) oven for about 15 minutes, then fill them with hot jam and seal them with no further processing. US home-canning guidance does not recommend heating jars in the oven, because dry heat is uneven and can crack them, and it advises processing filled jars in a boiling water bath. If you want jars to keep for months at room temperature, follow a tested recipe that includes processing and leave the right headspace.
Related terms
- Water bath canningWater bath canning preserves high-acid foods like jam, pickles and acidified tomatoes by boiling sealed jars. It is not safe for low-acid foods.
- Pressure canningPressure canning heats sealed jars to about 116°C (240°F) under steam pressure. It is the only safe home method for low-acid foods such as vegetables.
- HeadspaceHeadspace is the empty gap between the food and the lid of a jar or container. The right amount helps canning jars seal and stops frozen jars cracking.
- PickleTo pickle is to preserve food in an acidic liquid, either vinegar or a salty brine that ferments. Quick fridge pickles are ready in an hour or less.
- BotulismBotulism is a rare but serious illness caused by a toxin that grows in low-acid, airless food, such as badly home-canned vegetables or garlic in oil.
- Shelf-stableShelf-stable food can be stored unopened at room temperature, like canned goods, dried pasta and UHT milk. Many of them need the fridge once opened.