Although it's always best to use fresh berries when possible, wonderful jam can easily be made from frozen strawberries as well. Defrosting your strawberries beforehand will speed the process along, but you can use them straight from the freezer as well---it will just take a little longer to reach jam stage.
Place strawberries, lemon juice, sugar and pectin in the pot. Stir together; if berries are frozen, you may have to wait until they defrost.
Turn burner up to the highest level of heat that it can reach. Continue stirring.
Observe your jam as the strawberries start to turn to mush and the mixture begins to come together. Stir occasionally; stirring continuously is not necessary. If you are comfortable, it is even possible to be doing something else when you've reached this stage in your jam-making. Just make sure you're close enough to your pot that you'll notice when it starts boiling.
Bring jam to a rolling (or hard) boil and leave it there for one full minute. Add butter and stir into jam. After jam has been at a rolling boil for one full minute, turn the heat off.
Funnel jam into prepared home canning jars and can in your usual manner.
- Ball Blue Book Guide to Preserving (100th Anniversary Edition), Hearthmark, 2009
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