I have a few Mr Beer extract kits left over from when I started this hobby. Can I ferment a 3 gallon recipe in a 5 gallon bucket? Will there be too much head space?
You should be fine using a 5 gallon bucket, especially during primary fermentation. The CO2 produced should push out all the excess oxygen. In secondary, you might have a problem, but probably not unless you're leaving the beer in secondary for months.
Why only a 3 gallon recipe though? Go for broke with 5! :)
Agreed - I do 3 gallon batches in 5 or 6 gallon carboys all the time.
As PJ notes, the only trick is going into secondary, since you have so much more room for oxygen in there. If you can't put a blanket of CO2 into your fermenter to help keep the oxygen away from your wort, then at least make sure you're careful on transfer. Don't splash it too much, because then you will be oxygenating your wort, as well.
I almost always do 3 gallon batches, so cheers! Don't give into pressure to do enormous batches!
This is a great trick for those without a co2 system. attach a small tube over your airlock so any escaping gas has to go through it. put the other end in the fermenter. an active batch of beer produces enough co2 to purge most of the air out of the secondary.
a second thing I do, is transfer a bit early. I wait for signs of fermentation to diminish and the yeast to flocculate but transfer before the airlock stops bubbling. There is enough dissolved co2 in the young beer that it will bubble slowly for a while after primary fermentation has subsided. while transferrring a lot of this co2 will come out of solution and help purge the secondary of air.