I agree that not being able to see the clearing and the build-up of lees is a negative - this is my one big complaint with using the corny kegs. When I take samples for tasting I can check the clarity, but it just isn't the same as being able to look at a carboy.
The lack of intermediate sizes doesn't bother me much. When I have done larger batches, I just split it into two kegs - since I flush them with CO2 or inert gas it doesn't really matter how much head space is in there. If the total volume in the two gets down to 5 gallons, I can combine them again.
Fermentation in the vessel really doesn't present a problem. Sometimes folks who brew beer will prime the beer in the keg so that it ferments to carbonate the beer naturally and it seems to work just fine. If you do have some refermentation that occurs, you can release the pressure every once in a while as you check on it. I do this as my meads de-gas while aging after fermentation is complete (typically every 2-3 weeks). Alternatively you can keep an airlock on (or a fancy pressure release valve if you prefer).
As for an exploding keg, I'd like to know what someone did to cause that - it can't have been easy. I would love to see that (at a safe distance, mind you). I may get my chance, as I am planning to cook a batch in my "attic estufa" this summer and plan on keeping it in the keg. In 120 F heat, if a keg can go, that will probably do it. You've got me thinking that I may want to keep an airlock on it (maybe with glycerin so it doesn't dry out).
On a different note, when I use a bucket, I hate fussing with bucket lids that are hard to get on and off (if they seal well). I saw the
Tuff Tank fementers with a screw top advertised and I was wondering if they might be a lot easier. Is anyone using these (or something similar)?
Also, Butterlily you said your husband and mother-in-law can taste the difference between the carboy and bucket. How's that possible? Is your bucket old? Does it have some odors that remain in it between batches? I'm just wondering because I've not heard that before.
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