Some preserves such as jams call for crushed fruits.
A clever cook, upon reading that, might instantly start reaching for a food processor or blender to crush the fruit with.
However, this is one time you need to go low-tech, and there’s science behind it.
The blades of a blender or food processor can damage the pectin molecules, in the same way that they would damage starch molecules in mashed potatoes, giving you gloop.
The mechanized blades also introduce too much air into the mixture giving you frothy jam.
Consequently, it’s recommended now that we go back to the old way of crushing fruit manually.
Elizabeth Andress of the National Center for Home Food Preservation cautions,
Do not use a blender or food processor to prepare the fruit. Either finely chop fruit as described in the recipe, or crush with potato masher or other method that does not mix air into the fruit.”  Harrison, Judy A. and Elizabeth Andress. University of Georgia Cooperative Extension. Preserving Food: Jams and Jellies. FDNS-E-43-8. March 2013. Page 4. Accessed March 2015.
Bernardin goes one step further, and recommends that fruit be crushed just “one layer at a time.”
This means fill the bottom of a dish or bowl just one layer deep with fruit, crush it, empty that out, and then do another layer, etc. Here is a 30 second video illustrating the procedure:
Crushing fruits manually gives you a leg up on getting the natural pectin in fruit to work for you in addition to any pectin you might add.
Who’d ever say no to an extra boost at making sure jams and jellies set properly?