Bit unsure with concept of 'safe' FG

[jezo]

At the risk of causing 'Have you REALLY tried to search the forum' cries, can I post my worry here?

I was making ciders for many years and the concept of remaining sugars was simple: unless you sulfited or by sheer luck the yeasts would gobble up all sugars in the fermenter and you ended up with perfectly dry cider. With beer recipes I see different final gravities from a not so diverse range of malts in the wort. If the recipe says FG 1.015 - what explains that the yeast will stop at that point and refrain from mopping up the rest of the sugars? Is this a known quantity of unfermentables in the combination of malts? A yeast that will give up precisely at that point? How do I know that it's not going to drop massively in the bottle?

This is not theoretical: I have 20l of weizen racked into the carboy 10 days ago. It's at about 1.010 right now, but it still burps from time to time. The recipe indicated 1.012 as FG. If I bottle now, adding some priming to bottles and the beer still fermenting - am I going to end with a fire extinguisher instead of a drink? Is there such thing as a 'safe' FG for bottling IF your airlock still gives an occasional bubble?

 

[Yooper]

Malt is mashed to change the starches to fermentable sugars- but the sugars from malt (usually maltose and maltotriose) are not 100% fermentable like monosaccharides and disaccharides like fructose and glucose.

There ARE simple sugars in beer wort, but wort has far more less fermentable sugars.

Brewing yeast will ferment the sugars it can, and then it is finished.

If the FG is the same after three days or more, especially if it’s starting to clear it is finished and won’t ferment more.

 

[jezo]

So if my beer went below designated 1.012 I must have mashed more fermentables and less unfermentables from the recipe? Could I have managed to skew it so much that it is able to ferment to near-dryness and so am I not advised to bottle at all until all activity in the airlock has ceased?

 

[Craigerrr]

Summed up very well by @Yooper
I would add that the temperature you mash at will affect the sugars that are produced. A lower mash temperature will produce a more fermentable wort, a higher temperature a less fermentable wort as long chain sugars are produced, which are not fermentable. There is also the variable of different yeast strains and to what degree they will attenuate to.

There is a free online version of John Palmer's "How to Brew" which goes into great detail on this and everywhere aspect to brewing. Which is very, very helpful. This is not to suggest that you don't ask questions here, we are always happy to help. It is just a great resource.

 

[Craigerrr]

So if my beer went below designated 1.012 I must have mashed more fermentables and less unfermentables from the recipe? Could I have managed to skew it so much that it is able to ferment to near-dryness and so am I not advised to bottle at all until all activity in the airlock has ceased?

I would not make any decisions based on air lock activity. If the gravity is stable over 3 days you are then good to bottle. I have had yeast over attenuate numerous times, the little devils will surprise you sometimes!

 

[Hawkbox]

Yeah I think what you're looking for is when is it safe to bottle? If it's around 1.010 (in that general range) and hasn't changed in 3+ days you can safely proceed. The final gravity can vary depending on things like Yooper said but the safety part is just making sure the yeast have tapped out.

 

[BarbarianBrewer]

At the risk of causing 'Have you REALLY tried to search the forum' cries, can I post my worry here?

You guys are all missing the obvious response.... @jezo, Have you REALLY tried to search the forum?

Welcome to BF!

 

[soccerdad]

Remember that measurements in a recipe are accurate, but not precise. Your roasted barley may act differently than mine. Your 2 row as well. I might throw in 4 ounces more for one reason or another. My yeast may have a few million fewer cells than yours. So 1212 may be a very accurate recipe based estimate, but there are a dozen (or more) factors that may send it to 1009 or stop at 1013. When it hasn't changed in 3 days, package it. Chances are it will taste good.

 

[Josh Hughes]

@jezo all questions are good. Every thread someone can something and I usually do.

 

[Craigerrr]

You guys are all missing the obvious response.... @jezo, Have you REALLY tried to search the forum?

Welcome to BF!

That is funny!
@jezo , @BarbarianBrewer is totally messing with you, don't be shy with your questions, we have all benefitted from plenty of mentoring here on this forum

 

[Hawkbox]

If I ever tell someone to search the forum instead of helping them, somebody slap me.

 

[Craigerrr]

somebody slap me.

Is that with a Jim Carrey voice?

 

[Josh Hughes]

Yeah this is a great place

 

[Hawkbox]

Is that with a Jim Carrey voice?

No....

 

[Craigerrr]

No....

Maybe just a little...

 

[Donoroto]

At the risk of causing 'Have you REALLY tried to search the forum' cries, can I post my worry here?

Yes. Anytime and always.

Unlike most forums, we're not like that.

 

[Nosybear]

At the risk of causing 'Have you REALLY tried to search the forum' cries, can I post my worry here?

I was making ciders for many years and the concept of remaining sugars was simple: unless you sulfited or by sheer luck the yeasts would gobble up all sugars in the fermenter and you ended up with perfectly dry cider. With beer recipes I see different final gravities from a not so diverse range of malts in the wort. If the recipe says FG 1.015 - what explains that the yeast will stop at that point and refrain from mopping up the rest of the sugars? Is this a known quantity of unfermentables in the combination of malts? A yeast that will give up precisely at that point? How do I know that it's not going to drop massively in the bottle?

This is not theoretical: I have 20l of weizen racked into the carboy 10 days ago. It's at about 1.010 right now, but it still burps from time to time. The recipe indicated 1.012 as FG. If I bottle now, adding some priming to bottles and the beer still fermenting - am I going to end with a fire extinguisher instead of a drink? Is there such thing as a 'safe' FG for bottling IF your airlock still gives an occasional bubble?

The yeast will keep going until all the fermentable sugar is gone or, rarely, so much alcohol is produced that it poisons the yeast. The FG is a prediction, not an absolute floor. The actual FG may vary due to mashing differences or other factors.

 

[jezo]

Understood, thank you, very grateful for your patience with a newbie, yes, something different from many other forums. One final query here: if my FGs are consistently lower than what calculator/recipe tells me, what does it broadly mean in regards to the way I mash? I use a steel wire basket version of brew-in-a-bag. Too vigorous stirring, vorlauf and lauthering? Too hot (70C)? Appreciate there may not be a single simple explanation, but if there is, please tell.

The yeast will keep going until all the fermentable sugar is gone or, rarely, so much alcohol is produced that it poisons the yeast. The FG is a prediction, not an absolute floor. The actual FG may vary due to mashing differences or other factors.

The yeast will keep going until all the fermentable sugar is gone or, rarely, so much alcohol is produced that it poisons the yeast. The FG is a prediction, not an absolute floor. The actual FG may vary due to mashing differences or other factors.

 

[sbaclimber]

if my FGs are consistently lower than what calculator/recipe tells me, what does it broadly mean in regards to the way I mash?
...
Too hot (70C)?

In the case of lower FG, then "too cold" would normally be a possible cause. 70°C is actually kinda on the warm side...surprised that you are still getting such good attenuation!

I was having pretty good consistent ~77-80% attenuation with a mash temp. around 65-67°C (mostly Vienna Malt grain bill), then I modified the mash thickness a bit and in an attempt (2 brews in a row ) to correct the strike h2o temp. totally screwed it up and ended up mashing at ~70°C. That caused my attenuation to drop by about 5 points...

 

[Mark Farrall]

Understood, thank you, very grateful for your patience with a newbie, yes, something different from many other forums. One final query here: if my FGs are consistently lower than what calculator/recipe tells me, what does it broadly mean in regards to the way I mash? I use a steel wire basket version of brew-in-a-bag. Too vigorous stirring, vorlauf and lauthering? Too hot (70C)? Appreciate there may not be a single simple explanation, but if there is, please tell.

There's so many variables it's hard to pick out a few for a simple explanation, but here goes... The mash has enzymes that break down the starch into smaller and smaller sugars, eventually breaking them down enough so that the yeast can use them in fermentation. One enzyme is very good at breaking medium sized sugars into small sugars that are easily fermented by yeast (beta amylase). Beta amylase prefers the lower end of the mashing range. It can't break down starch or medium sized sugars, so it relies on other enzymes to do this first.

Other enzymes are great at breaking starch and larger sugars into small to medium sized sugars. Some of these can be used by yeast, but others may still be too complex for some yeast to ferment. Alpha amylase is the key one here and operates at the higher end of the mashing temperature range. Beta amylase struggles at the upper end of the mashing temperature range and will eventually denature at these temperatures.

So if you do a mash at the high end, say 70C/158F you'll get activity from alpha amylase but not a lot from beta amylase as it denatures and it's not very efficient at these temps. You'll still get the predicted pre-boil gravity at this temperature, but your FG will be higher than predicted. I do a couple of these mashes each year for my sour beers and they always finish high.

Moving it down to the other end, say 65C/149F, you'll get really efficient activity from beta amylase. Alpha amylase will be less efficient, but won't denature. Down here you may find your mash missing it's pre-boil gravity, especially if it's a short mash. You can extend the mash time to give the alpha amylase more time to provide smaller sugars to beta amylase. If you do come close to your pre-boil gravity at these temperatures you'll find that you'll get a higher than predicted FG, as it's got a higher percentage for small sugars than expected. It's great for saisons, but not so good for many malty beers.

Considering the amount of enzymes there are in modern malts, the common compromise position is something around 68C/154F. It advantages alpha amylase (which does the bulk of the work), but it gives beat amylase enough time to convert a decent amount of sugar before it denatures.

And even this long post is papering over so many other options, like step mashes, grain choices, gelatinisation temperatures, etc.