Canning jam in pint jars

Blueberry jam

You may home can jam in pint (500 ml) jars if the recipe you are using indicates that that size of jar is an option and gives a processing time for it.

Otherwise, if the recipe indicates a smaller sized jar, do not assume that a pint jar will be fine, or guess at a processing time for it. The reason is that heat penetration tests have to be done uniquely both for the density of ingredients in a jar, and for each size of jar supported, because as size increases, the “cold centre” of a jar changes, as does the time for sufficient heat to penetrate through the contents of the jar to reach it.

The National Center for Home Food Preservation does not support the blanket extension of USDA or USDA or National Center recipes that were developed exclusively for jams in half-pints (250 ml) to pint jars. And there is no tested, supported number for jam jar processing regardless of jam type such as 5 minutes for half-pints / 10 minutes for pints, even if you see one going around. ^[1] National Center to Randal Oulton. 20 June 2020. Email on file.

There are a few recipes where a choice of either half-pint or pint size jars is given and supported, but that is because testing of the processing time for each size was explicitly done. Two examples are Peach Pineapple Spread and Blueberry Spice Jam.

Boiling water canner processes for jars larger than those indicated with recipes cannot be extended by any formula or assumption to a larger jar. There is no exception for jams, and nothing the National Center has written should be interpreted as such.

If you want to make a certain type of jam — say, blueberry — and the recipe you are looking at doesn’t give pint (500 ml) jars as an option, then look for a tested recipe that does.

Bigger is not always better

Bear in mind that bigger is not always better. Larger jars of jam certainly are faster to fill, and involves using fewer canning lids. And the large size once opened is fine if there are enough people in the household to use it up in a reasonable time, so that you don’t get bored of that flavour of jam. Otherwise, the smaller half-pint (250 ml) size jars can be a more practical size to work through in a reasonable period of time, and let you vary the selection of jams you have open in your fridge.

You’ll almost never see pint (500 ml) jar sizes as an option for low-sugar and sugar-free jams. The reason is practicality of usage: once opened, without the huge quantities of sugar in them that regular jam has to fight off moulds, the jams spoil within 4 to 6 weeks even refrigerated.