Although it’s always best to use fresh berries when possible, jam can easily be made from frozen strawberries as well. It's best to start the jam-making process with strawberries that have already been defrosted.
Add strawberries, cup lemon juice and granulated sugar to a deep, heavy pot. For every quart of frozen berries, use 1/8th cup of lemon juice and about 4 cups of granulated sugar. Stir together until the mixture begins to come together.
Turn the burner up to the highest level of heat that it can reach. Continue stirring.
Continue stirring the mixture occasionally. The strawberries should start to soften and take on a liquid form. At this stage, add the pectin. Use the amount recommended on the package for the amount of berries you've cooked.
Bring jam to a rolling boil and cook for one full minute. Add a pat or two of butter and stir into jam. Turn the heat off.
Funnel jam into prepared home canning jars and can in your usual manner.
It is possible to make strawberry jam without pectin. In fact, that’s probably how your great-grandmother used to make it before pectin was commercially available. A pectin-free process involves reducing a lot of the natural juice out of the strawberries, which may reduce their nutritional value. If you wish to try making strawberry jam without pectin, omit all ingredients except the strawberries and the sugar. Use about 50 percent more sugar and boil until the jam starts to thicken. Ladle into your prepared home canning jars as above. Pectin-free jam may be runnier than jam with pectin.
It is also possible to make strawberry jam without added sugar, for those with dietary concerns. Alternative sweeteners such as stevia or sucralose replace the sugar. Consult product packaging to determine the proper amount for your recipe.
Do not underboil the jam, or else the pectin will fail to properly set. Instead of strawberry jam, you’ll have strawberry sauce.
Do not overboil the jam, or else the pectin will start to break down, which will also result in a failed jam set.
Keep your batches of jam small. Making too much will cause the jam mixture to heat unevenly and could effect the pectin's ability to thicken.