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Question: I have a batch of beer I bottled a while ago that never carbonated. What can I do about it?
Answer: Failed carbonation is usually a sign that yeast has gone dormant or has settled out of the wort prior to bottling. If you can get some yeast action going, the beer should carbonate. Sometimes the problem is from insufficient priming sugar or excess alcohol. The best course of action depends on the type of beer and the procedures you used.
The first thing to do is check the bottom of some of the bottles. Do you see any yeast sediment? Maybe just a bit? If you're lucky, you can wake it up and get it to carbonate. This is most often the case if the bottles went into cold storage too soon after bottling. If that's the case, move the bottles back to room temperature and give them another couple of weeks. Check the bottom of the bottles after a week or so and see if the amount of yeast sediment is increasing. If so, the bottles are starting to carbonate. If your beer is a lager, wait until they are fully carbonated before putting them back into the lagering fridge. Once they are carbonated, put them back into the fridge and reset the temperature to about 60oF or so. Drop the temperature a couple of degrees per day until you reach your lagering temperature.
Most likely, the warm-up treatment won't work. If that is the case, pop open a bottle and take a taste. Does it taste sweet? If so, you have plenty of priming sugar. Then what you need to do is get a hold of some active yeast and put it in the bottles. This process is called repitching.
The most effective and convenient source of active yeast is from your next batch of beer. Yeast from either the primary or secondary will work fine. The yeast in the secondary is preferable, since there is typically less trub mixed in. The type of yeast doesn't have to be identical but make sure it is compatible. Generally, you want to match lager to lager and ale to ale. Avoid strongly flavored yeasts. When racking day arrives, get your bottles ready to go. Rinse off any dust, and them dunk them head first into dilute sterilant. Also sterilize a fresh set of bottle caps, a spoon and a cup. After you rack your beer, dump the yeast sludge into the cup. Be careful to keep it clean. Then comes the tedious part. Pop open the caps of your bottles one at a time. Depending on how full you fill your bottles, you may have to dump out a bit. Put in a small spoonful of yeast sludge (about ¼ tsp is plenty) and then recap. Then keep the bottles at room temperature until they carbonate.
If you don't happen to have another batch of beer going, or if the style is incompatible, then you'll have to make up a batch of starter. Generally, this is just like you would do for a fresh batch of beer. (For instructions on how to make a starter, see the Brewmaster article in the June 2001 edition of the Gazette.) Give it enough time to consume all the sugar, though. About a day and a half should be enough. If you use it too soon, you'll be adding extra sugar to the bottles, resulting in overcarbonation. When the starter is ready, decant off about half of the clear liquid. Then swirl the remainder to mix in the yeast. Proceed to pop open the bottles as above and add the slurry. Stir the slurry often to keep it mixed.
If you think that recapping bottles is a lot of work, you're right. A better approach is to repitch the whole batch in the bottling bucket. You should do this for any batch of beer that you think might have carbonation problems. Examples of this would be high gravity beers, or beers that have spent too long in secondary. Lagers will need repitching if they were cooled too fast in secondary. If your beer is an extra high gravity, (over 1.100 or so, you might want to consider repitching with champagne yeast.
If that bottle of uncarbonated beer didn't taste sweet, you have to consider the possibility that you forgot to put it in. (Doh!) If that is the case, you can buy small sugar tablets at the brewshop. You'll need about 2 ½ grams per 12 oz. bottle to get that fizz going.