Side-effects of 'lower than recommended' fermentation temperatures?

Question

I have a Ginger Beer kit on the go. It's sitting in the kitchen and ambient temps are ranging from 15 to 20ºC (59-68ºF). The beer was started 3.5 days ago and I was away for the weekend, I can see that Krausen did form up to about 5cm, but it's died back now. The beer has plenty of little bubbles rising, and my blowoff tube is bubbling about once a second during the day, and once every 5 or so seconds at night.

So.. it looks like I have a slow fermentation going on. I'm in no rush, but my question really is: are there any negative side-effects to brewing slowly at low temps like this? In the past I've brewed (accidentally) at 30ºC and the beer was horridly bitter!

Other info: The kit was used with 1kg Munton's Beer kit Enhancer. 100g of Fresh Ginger was boiled and added to the fermenter at the start. The included yeast was used, I have no idea what type/spec it is. First SG reading: 1.041

Answer 1

During the start of the fermentation the Yeast reproduces quickly using the oxygen present in the beer and produces diacetyl which imparts a buttery flavor.

Which is why it is usually recommended to start the fermentation at a lower temperature to slow down the diacetyl production (and the reproduction rate, I suppose).

The beer fermentation is then 'finished' at a slightly higher temperature so the yeast consumes the diacetyl.

So in principle you should rise the temperature after a few more days to allow that to happen.

Now, maybe your beer didn't produce much diacetyl to start with due to the cold temperatures (although the beer temperature will be higher due to the heat generated by the fermentation). So if you can't do that, you may still be good.

Ale yeast usually works on the range of 18C to 23C, bellow that the yeast may floculate before all sugars are consumed, resulting in an incomplete fermentation.

So to summarize the risks are buttery taste (diacetyl) and sweet beer (incomplete fermentation).

I wouldn't worry about it though, just follow your fermentation schedule, measure the gravity and give it a taste, if you find some of the flavors above, warm it up a bit to waken up the yeast and leave it longer.