And now, Crab apple jelly.

In the recipe 'they' write "Just skim off the scum that forms on the top".  'They' don't say as fast as you skim  more appears!  What is the cause of this scumming anyway?  Still we've got four jars of rosy crab apple jelly.

We have an immense/ imense Rosemary bush in the back garden,  I keep meaning to severely cut it back,  I made rosemary jelly once before, I'll try and make more...
Cheers,  Tony.