How to export image with caption

Friends,

I'm trying to export some images from Aperture 3.2.2 with their captions. I am unable to do so. When I go to presets>file naming in the Aperture menu, there is a "caption" option. However, none of the metadata options to be included include "caption." There's custom name, etc., but no "caption." What am I missing?"

Thanks!

Steve