I've been making jam out of last summers frozen fruit. I had a batch of strawberry-cranberry that was particularly foamy. That made me start thinking about why we skim the foam off jam. I do it because that is what grandma taught me to do when canning. I have abandoned some of her other canning practices - is this another of her practices that I can abandon?

So - why do we skim the foam? Is it for cosmetic reasons? Or is there some safety reason?