When I juice tart apples (cored and peeled, centrifugal planer type juicer), I get cloudy juice and lots of mealy foam on top. The bubbles are tough. They last for days and I can freeze the foam. What's in the foam and what would it bring to the fermentation? Is there more or less sugar in there compared to the juice?
The foam contains residual particles from the juicing process (I use a centrifugal juicer as well when I do my pear and cactus pear ciders), that have a fair amount of pectin in them. The best way that I know how to remove it (and help the final clarity of your brew) is to skim it with a strainer. A hand strainer works, I have used a grease splash screen that fits over the top of my pitching bucket in the past as well. What little sugars that are in the foam are not going to contribute significantly to the final alcohol content (did my my own experiment to test this in early brewing batches). The juice contains most of the fermentable sugars.