Q: After fermenting my ale at 70 °F (21 °C), I racked it into the secondary and slowly cooled it down to 45 °F (7 °C) in a refrigerator. I am concerned that the yeast may be too dormant to carbonate when it is time to prime. Am I worrying for no reason, or should I look to fresh yeast for carbonation? If I propagate a new starter solution of yeast, how much should I use? At what stage should it be pitched? (I’m concerned about adding more fermentable sugars with the starter and giving the yeast alcohol shock.) What part(s) of the starter solution should be pitched (just the slurry, or the slurry and all liquid for suspended yeast)? Any help would be appreciated — it’s almost time to bottle.
A: Yeast that has been cooled after primary fermentation will stay in good shape longer than yeast that has been allowed to sit at room temperature. Cool temperatures slow down yeast metabolism; because it does not run through its glycogen reserves nearly as fast, it can remain viable longer.
As long as the beer has not been more than four weeks in the secondary you will probably have no trouble getting a bottle fermentation from your yeast. The only situation where cold temperatures might cause a problem is where it induces so much dropout that there simply are not enough yeast cells to do the bottle fermentation. You can usually judge the size of the remaining yeast population by evaluating the clarity of the beer. The need for suspended yeast is why I recommend adding isinglass finings at bottling rather than in the secondary.*
Of course, you must leave the primed beer at room temperature for a few days to allow the bottle fermentation to go to completion. After that, cooling it down would help the absorption of CO2 from the headspace into the beer.
If for any reason you feel unsure of your fermentation yeast, it is always possible to pitch some fresh yeast at bottling time, as you say. I would make up a one-pint starter from a smack pack and let it pretty well ferment out. This should give plenty of yeast for a bottle fermentation. If you let it ferment out before pitching, alcohol shock and sugar content will be non-issues. Make up the starter wort to a gravity similar to that of your beer. I would pitch the whole starter, but if most of the yeast has settled to the bottom of the jug, you could decant it and just add the slurry at the bottom. You could use your normal priming rate.
If you still have problems with bottle fermentation, it probably means (as a rule) that the yeast was old and worn out, most likely because of inadequate growth in the primary fermentor.
*Isinglass is safe to ingest, as is gelatin and gelatin-polysaccharide products such as Clearfine (made by Siebel and sold through Crosby & Baker). However, all of the “yeast droppers” I know of are animal based — isinglass is often referred to disparagingly as “fish bladders” — so strict vegetarians will want to avoid beers made with them.
Other clarifiers, such as Polyclar and silica gel, I am not so sure about. I would not recommend adding them to the bottle in any case. You need to mix them into the beer in the secondary fermentor and let them settle out before racking.
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