Crushing (aka mashing) fruit - Healthy Canning
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.