Stopping SO4

[Chalkyt]

Just been looking at CvilleKevins sticky again about yeast, sugar etc, and noticed that it was possible to stop a SO4 fermentation by racking. The suggestion is that because SO4 compacts a lot, you can rack a couple of times and leave all of the yeast behind.
Has anyone had experience with this? The reason I ask is that I have a couple of quite slow secondary fermentations going. Primary with SO4 was a bit slow but I put this down to old yeast and quite low temperature (below 10C). I racked slowly and carefully at just over 1.020 and it took a few days for anything to happen. It is now moving slowly at 15C and I wondered if I have actually racked without bringing any (or not much) yeast across and therefore almost stopped the fermentation. I haven't had this with SO4 before but this juice is very clear (no sediment was stirred up) and from an orchard whereas mine from my own apples is usually still a bit cloudy at this stage. Any experience or opinions?

 

[wasully]

There is going to be a lot of yeast in suspension, beyond what is gathered at the bottom. Several rackings might give a stuck fermentation, especially combined with low temperatures. But simply seeing compacted matter isn't enough and a single racking.

 

[Chalkyt]

Yep, that was about my understanding, but progress had been very slow (not a bad thing!) and I wondered if I had racked to the point where fermentation wouldn't continue since this is a technique suggested by Jolicoeur for stopping fermentation.

Yesterday the fresh yeast turned up so I added 1/4 tsp of yeast and 1/4 tsp of DAP to a small amount of juice from each each 1 gallon carboy, let it sit for an hour then added it back. Wow, I don't know what caused the reaction but the carboys instantly went crazy for a few hours... bubble, bubble, bubble. They have settled today at around 1.015 so there is still a way to go, and had dropped by a point overnight. But after a few warmer days the temperature was back to 1C outside and 12C inside overnight, so slowing down would be expected. I don't know if the old yeast was indeed struggling a bit but today everything seems to be back on track.