How to Sterilize Jam Jars

Before sterilizing jars, remember that not every food can be canned using boiling water. High-acid foods — jam, marmalade, canned fruit, and vinegar pickles — can be safely preserved at home in a boiling-water bath. Low-acid foods — plain fresh green beans, corn, peas, mushrooms, carrots, potatoes, meat, and broth — cannot be safely preserved in a boiling-water bath, because Clostridium botulinum spores do not die at 100°C / 212°F and can survive and produce toxin inside an oxygen-free jar. These foods must be processed in a pressure canner that reaches about 116°C / 240°F.

First wash the jars and lids in hot, soapy water and rinse them thoroughly. Then arrange the jars in a pot, cover them with water to 2–3 cm above the tops, and boil for 10 minutes after the water comes to a boil; if you live above sea level, add 1 minute for every 300 meters. Keep the jars in hot water until you are ready to fill them, since a cooled jar can be recontaminated and may crack from thermal shock when filled with hot jam. Do not place the jars directly on the bottom of the pot; put a wire rack or clean cloth underneath, since direct contact with the glass bottom can cause localized heating and breakage.

Jars can also be sterilized by heating them in a 120°C / 250°F oven for 15–20 minutes, but lids must not go in the oven: dry heat can harden the gasket on the inside of a metal lid so it no longer seals properly. Prepare lids by leaving them in boiling water for a few minutes. Do not use lids with cracked, dented, or swollen seals. One lesser-known shortcut is that if filled jars will be processed in a boiling-water bath for 10 minutes or longer, pre-sterilizing the jars is unnecessary; the processing time already does that job.

Fill the jam into a hot jar while the jam is hot, leaving 0.5–1 cm of headspace at the top; a completely full jar cannot form a vacuum inside as it cools. After filling, be sure to wipe the rim with a clean, damp cloth, since even a single grain of sugar left on the rim can prevent the seal from seating and cause the jar to spoil silently months later. Turning the jar upside down for a few minutes is common, but it does not replace a water bath; treat it as an extra precaution, not the only method.

Jam can be stored in a pantry because sugar binds water and reduces the free water available to microorganisms; for this, sugar should make up roughly 55–65% of the total weight of the cooked jam. Jam made with less sugar is not a shelf-stable preserve and should be refrigerated. Discard a jar with mold in its entirety: the visible part is only the mold’s reproductive growth on the surface, while its roots extend far below the spot. Once cooled, the jar lid should be slightly indented inward; discard a bulging lid or one that pops up and down without tasting the contents.