Making jam from fresh blackberries locks in the flavors of the season. Jams consist of crushed fruit in a syrup, while preserves are made from whole fruit, according to the Center for Home Food Preservation. Precise measurements become critical when making jams because incorrect amounts will lead to an imbalance in the recipe, yielding a jam that does not set properly. Always use the correct technique for measuring blackberries to keep a balanced jam recipe.
Consult your recipe for the amount of crushed berries required and double that number. For instance, if your recipe requires 2 cups of crushed berries, use 4 cups whole.
Divide the number of whole berries needed by two to determine the number of pounds of fresh blackberries if purchasing them from a store or farmers market or picking them by the pound. For the example, if you require 4 cups of whole blackberries, you will need to pick or purchase 2 lbs.
Rinse the berries and remove any stones, stems, old berries or leaves from the mixture.
Place a handful of blackberries into the bottom of a sieve to cover the base in a single layer.
Set the sieve or colander over a bowl and push the blackberries with the back of a spatula to crush the berries and strain out any hard pieces.
Repeat crushing the remaining blackberries in the sieve with the spatula.
Spoon the crushed blackberries from the bowl into a 1-cup measuring cup.
Run the flat portion of the spatula over the surface of the cup to level off the cup for an exact 1-cup measure.
Add this to your recipe when directed, and repeat the filling and leveling if you require more cups of crushed blackberries. Continue to make your jam as directed in your recipe.