When does a beer stop fermenting...?

[dsteel0]

I'm wondering - when will a beer, left to its own devices in the FV, stop fermenting naturally?

I've Googled it and come up with a couple of answers:

1) When all the sugar is used up - I would have thought this would lead to an extremely dry beer, but I've since read that this maybe only refers to fermentable sugars, and perhaps there are other things left in the beer that will sweeten it, but won't ferment?

2) It will stop fermenting at a point where the alcohol becomes too high for the yeast - I'd always thought this was the case for wine, since wine gets up to 13% or so, the yeast dies off. I've since read that specific beer yeasts can only "handle" up to whatever strength the beer is (i.e., 4, 5, 6%, whatever).

I've read that lots of people leave their beer in the FV for weeks, to ensure fermenting is finished, and for a bit of conditioning, so I'm curious to know what dictates the end of fermentation, as none of the books/aticles I've read seem to cover it.

Given that fermentation stops and then begins again if you bottle and add priming sugar, answer 1) seems to make the most sense. I'm all for keeping things as easy as possible, so I wonder if, rather than trying to work out whether fermentation is finisihed by taking gravity readings etc., I'm better just leaving it in the FV for 3 - 4 weeks?

Cheers.

 

[Ciaran12s]

If you get consistent gravity readings over 2/3/4 days then fermentation has stopped. Provided the temperature hasn't suddenly dropped and stayed low or indeed ramped right up and stayed up, the consistent gravity readings indicate that the yeast have finished. It is good to leave it the fv for a while to let everything settle. I'm not 100% on this, but I would say that if it's at the right temperature and everything has dropped to the bottom, this would also indicate fermentation has finished.

 

[Sadfield]

Pretty much point 1. When the yeast have consumed all the sugar they are capable of consuming. Not all sugars are the same and not all yeasts can process the same sugars. Standard brewing yeasts like Saccharomyces Cerevisiae (Ale Yeast) and Saccharomyces uvarum (Lager yeast) struggle to ferment complex sugars, these remaining sugars (usually dextrins, lactose if added) are what gives beer sweetness and body. Alcohol tolerance is unlikely to be a factor in most beers. The pH of beer drops during fermentation, but I'm not sure if it yeast ever hit an acid tolerance limit that terminates fermentation at normal temperatures.

Sent from my E5823 using Tapatalk

 

[MyQul]

As sadfied says, beer has different sugars in it. Have a look at this pie (MMmm pie) chart to see the different sugars in wort https://eurekabrewing.wordpress.com/2015/01/13/sugar-composition-of-wort/ . As the last paragraph states brewing yeast can convert all sugars to some extent or other except maltotetraose, which is what give the beer body

Unless you're fermenting a lager at cool temps most average OG ale worts will be fully fermented out in a week or less. The rest of the time is for the yeast to clean up any byproducts and to then let it settle out of the beer

 

[Sadfield]

Great link @MyQul

 

[dan125]

Interesting pie chart. Explains why most yeasts seem to attenuate to around the 75% mark??
Makes me wonder how mash temp affects the proportions of the diferent sugars

 

[dsteel0]

Interesting stuff, cheers. The question was mainly borne out of curiosity around what happens with priming - i.e., if the second point had been correct, the yeast would be dead and no priming can take place.

Presumably then, you could add more sugar, and leave the beer for longer and end up with a much stronger beer, more like wine. Is there a point where that ceases to be the case? Could you brew something up to 13% or so? I know you can make barley wine - is that what you'd end up with?

 

[Slid]

Interesting question. I am now saving the dregs of beer for slug traps and once again I notice that if you mix beer dregs from different brews, not even keeping them in the fridge will completely stop the fermentation.

Almost as if the yeast enzymes will gradually chop away at the long complex sugar molecules to make them gradually fermentable.

 

[Hoppy]

1) Yes when all fermentable sugar for that particular yeast is used up.
2) All yeast strains of Saccharomyces Cerevisiae, have different alcohol tolerance levels. This is why many brewers use multi-strain cultures, one or two maybe for flavours they produce, another for its' fermentability/alcohol tolerance. Brewers who use strains like these have to re-culture at intervals to keep the proportions correct.
This is where barley wines come in, some ale yeast only tolerate 5-6% ABV, they inoculate with a wine yeast to bring the rest of the gravity down.
pH is not normally a factor, brewers acid wash yeast as it is more tolerant than bacteria to acid.
To find your attenuation limit (ie when fermentation is fully complete), take some wort, and pitch too much yeast, leave in a warm place, final gravity will be defined. If you want to brew (without priming) leave your beer at 1-2 degrees sac above this. This is generally the rule for cask conditions beer.
WBR
Hoppy

 

[Sadfield]

Adding sugar to a beer will not make a Barley Wine, the clue is in the name.

Acetobacer, Lactobacillus and Pediococcus are all pretty tolerant to acid given that they turn sugar into either Acetic or Lactic acid, more so than Sacchromyces that can suffer from TAS (Terminal Acid Shock). Sour beers wouldn't exist without acid tolerant bacteria.

 

[stevey]

...but you would end up with stronger beer, providing the yeast could cope with higher ABV, I think that's what the OP was asking. you can ferment just sugar up to 20% ABV if you use a suitable yeast.