To purée a pineapple, trim off the ends, skin, and eyes, cut out the core, and crosscut it into slices, then process with a blender until smoothly puréed. The pineapple can be puréed just before use, or up to 3 days in advance. In the latter case, refrigerate the pineapple purée with the lemon juice in a nonreactive, airtight container until ready to use. If you like, save the leftover pineapple core and lemon rind to make a fresh, no-cook pineapple syrup.