Dear Mr. Wizard,
When racking to a secondary, I find that a lot of active yeast can be left behind, especially if it is done too soon. I don’t see much risk leaving beer on a small amount of sediment. It seems to me that racking to the secondary should take place when fermentation has reached a particular level, say 80% complete or when a hydrometer reads 1.01. What is your opinion on this?
Mr. Wizard replies:
Personally, I like to minimize the number of times wort and beer are transferred because with each transfer there is a risk of damaging the beer either by contamination or oxidation. I, like most brewers these days, use cylindroconical fermenters and the only time the beer is typically moved is after fermentation — either en route to the filter or directly to the serving tank for unfiltered beers. We do rack some of our beers to a secondary fermenter when we dry hop using whole hops or when we are making some beers that are aged in oak.
When racking into a secondary is deemed appropriate, I like to do the racking before fermentation is complete to help minimize oxidation since active yeast will quickly reduce the level of any oxygen introduced during transfer. If you wait until fermentation is complete and then rack, the likelihood of oxidation increases since yeast activity wanes after fermentation is complete. This can be especially problematic when dry hopping since whole hops have entrained air.
I agree with your rule of thumb of racking when the fermentation is about 80% complete and also agree that a small amount of yeast carry-over is not detrimental. In fact, when beer is transferred with very little yeast, I get concerned about oxidation and will use methods to remove oxygen from the vessel I am going into. At home when kegs are used, the easiest way to do this is to fill the keg with water and displace the water with carbon dioxide prior to filling.
I recently learned that many winemakers use pelletized dry ice to do the same thing. They place pellets of dry ice in a tank and allow the dry ice to sublime. This forms a nice blanket of carbon dioxide in the bottom of the tank and the wine is filled under the carbon dioxide blanket. This method is easy to use if you have access to small chunks of dry ice. This method requires attention to detail since dry ice in a closed container is a great way to make a little gas bomb. If you choose to try this method, do not place the dry ice in a closed vessel, rather leave the vessel vented to the atmosphere to ensure that pressure is not built up in the carboy, keg or whatever you are using.