I was wondering what the best way to stop fermentation is. I've read that heating up the cider would be a good way, and also cold crashing works as well. Will adding some potassium sorbate help? Thanks!
Sorbate + metabisulfite can be used to halt fermentation, but you need both of them. It does tend to leave a sulfur odor behind sometimes, though, so maybe it's not the best way if you have a good nose.
Cold crashing is the easiest. Pasteurization is probably the best overall for cider, I think.
The only reason for the switch was because of dietary issues with the salt. I have used both but prefer the campden (sodium). However the recommended 1 tablet per gal of campden is 150ppm of SO2 and the recommended potassium metabisulfite of 1/4 teaspoon per 6 gal is 75ppm of SO2, so my research on it is considered incomplete since it wasn't an equal amount of each.
You can't really halt fermentation with campden and sorbate. Neither kill yeast, but the sorbate inhibits yeast reproduction, and works better in the presence of sulfite.
What that means is if you still have tons of yeast in suspension, adding the sorbate and campden won't stop fermentation. That's because the yeast don't need to reproduce any more- there are plenty to ferment.
Stopping an active fermentation is like stopping a freight train. You may be able to do it, but if you just dump sorbate and campden into the cider you'll probably just get some off-flavors and not a stopped fermentation.
It's more likely to work well if you chill the cider at the desired level of fermentation. Wait at least several days, and it should start to clear. After it's clear, you can rack off of the lees (where much of the yeast is) and into a solution of campden (potassium metabisulfite) and sorbate. I'd go with 1 campden tablet per gallon (50 ppm free SO2) and 1/2 to 2/3 teaspoon of sorbate per gallon. Wait and see if fermentation did indeed stop, and then bottle in about a week if there are NO signs that fermentation will restart.
A couple of things- sorbate does have a taste. Some people find it unpleasant. Instead of using the campden/sorbate, some people like to bottle at a specific SG and then bottle pasteurize. I haven't tried it, and I'd be afraid to do it, but Pappers has a great sticky on how he does it.
Papper's method is probably more reliable, but a bit more work
I've used campden and sorbate to stop an active fermentation. Its easy and effective with ale yeast but tastes like ass (unless you like the taste of vanilla and baby aspirin in your cider, in which case it might be worth a shot)
The only reliable way to stop fermentation is to pasteurise. Safer to pasteurise with the caps off, then cap the bottles. You can pasteurise with caps on but don't handle them until cool, they may explode as the heat makes the contents expand.