One of my batches of cider looks a little bit different than I've come to expect-- the foam on top is thicker and a little bit darker in color than other batches I have made, as well as the other batch I have going right now. This cider was particularly active in fermentation, and in fact fouled up the airlock yesterday so that I had to re-sanitize it. The foam had already looked off before that though.
My question is this- is it likely that something is wrong with this brew, or is it just particularly active? Underneath the foam the cider is still bubbling very happily. What are the possible things that could be going on, and what would they mean?
Thanks for all your help!