If you want to store your cooked jam or jelly on the shelf, at room temperature, for up to a year, YES, you need to do a water bath.
The USDA/National Center for Home Food Preservation now recommends water-bath processing for jams and jellies that are going to be stored at room temperature, whether made with high sugar or low sugar. This is a change from the past.
What the water-bath process does is super-heat the air space at the top of the jar and kill any mold spores or yeast that may be in the air. You don’t want to seal in any live mold spores or yeast as they will grow in the low-sugar jam or jelly if they are not killed. The extra heat of the water bath also creates a stronger seal than if you just let the jars cool and pop.
However, if you don’t want to do a water bath and have freezer space, cooked jam and jelly can be stored in the freezer. Follow the directions as if you were going to do a water bath, but leave ½” of headspace in the jars (instead of ¼”). This extra space allows for expansion when the jam freezes. Allow jars to cool on the counter, then store in the freezer. Defrost in the refrigerator before eating. Best eaten within 1 year of freezing.
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Posted in: 6. Questions About Jam-Making Equipment and Water Bath Canning