oddbean @Doram have done the brew yesterday and put everything in a bucket. The instructions say ferment for 9 days, but realised I will be away next weekend and could only transfer to bottles on the 10th day. What happens if the beer is over or under fermented?
First thing - chill, you have nothing to worry about! Nothing will happen to the beer if you leave it in the fermenter after fermentation is completed, certainly not one day. I have left beer in the fermenter for two or three weeks after fermentation completed, and nothing happened, so don’t worry about it.
oddbean What happens if the beer is over or under fermented?
While leaving the beer in the fermenter after primary fermentation completed isn’t a problem, you do not want to bottle a beer that hasn’t yet completed primary fermentation. Not only will it not be ready, you will risk the bottles exploding! This will happen because you will put the beer under pressure while ongoing fermentation creates gas that can’t escape.
In secondary fermentation you will do this in the bottle but in a controlled way (by putting in the correct amount of priming sugar), so will have just enough gas to carbonate the beer without over pressurising the the bottles and getting them to explode.
oddbean The instructions say ferment for 9
Treat the “ferment for 9 days” instruction as guideline, not gospel. Fermentation time depends on a lot of things. Temperature is critical to the time it takes: in hotter temperature the beer will ferment faster, and in cold temps the fermentation will slow down. The recipe can’t know all the exact conditions you will have, so can’t give you an exact fermentation time. As leaving the beer in the fermenter after primary fermentation has completed isn’t a problem, I assume the “9 days” figure is an estimate of the longest time it will take to ferment. So I would guess your primary fermentation will take a maximum of 9 days, but it is very likely that it will take less than that. You can look for when bubbling in the airlock stopped, take hydrometer measurements and see if you have reached the target for final gravity early (this is indications that primary fermentation has completed). If you get to the target gravity and hydrometer readings stay the same after a day, you know your primary has completed and you can bottle the beer. This can sometimes happen as early as a week after brew day, or even faster. But again, if this happened after a week and you have left the beer in the fermenter for three weeks - that’s not a problem.
oddbean How important is the fermenting temperature and how important it is to keep it stable?
Fermentation temperature affects the taste of the beer, so it’s important. Ideally you ferment at the perfect temperature for your yeast and beer style, and keep it stable. This is more important for certain yeast and brew styles, and less critical for others.
Having said that, for most of my brewing years I didn’t know that and have (stupidly) not paid any attention to fermentation temperature. I brewed in all seasons and all temperatures, and I always got beer. However, I now know that this wasn’t a good idea and could have had an impact on the quality of my beer. I still don’t have means to control the brew temperature like the serious brewers have, but at least I brew at times of year when ambient temperatures are moderate (so not in the middle of summer or winter). For the beers I make the ideal temperature (from memory) is 18-20 degrees, so for temperature now should be fine.
