Working out attenuation %

My last brews have been overshooting their OG by a significant amount so I'm not sure what the FG should be. For example, I'm fermenting a pilsner which was calculated to have an OG of 1.048 and FG of 1.009, but the actual FG is 1.055, I'm using W-34/70 which has an attenuation of ~83%, how do I work out what the new target FG is?

TIA!

I found the formula online from the White Labs website: [(OG-FG)/(OG-1)] x 100

Or a possibly more straightforward way:

(1 - attenuation as decimal) x gravity points

Eg: (1 - 0.83) x 55 = 9 gravity points

I find those manufacturer claimed attenuation figures to be wildly inaccurate. If I mash a 1.039 - 1.050 beer in the 64-67 range then every beer I brew finishes between 1.006 and 1.009 regardless of what the yeast packet says.

You could dilute your wort before pitching the yeast, to the desired gravity. It's what I've been doing recently, and means my brew is close to the recipe.

Your calculation in gravity points works like this: (OG - FG)/OG = attenuation (decimal), multiply by 100 for %.

(48 - 9)/48 = 39/48 = 0.8125 (82.25%)

So assuming the same attenuation, 55 * 0.8125 = 44.7 (points the yeast will eat), 55 - 44.7 = 10.3, so expected FG is 1.010.

Alternative method for the second calculation is what Steve did above: 55 * (1-0.8125) = 55 * 0.1875 = 10.3, which works out FG directly.

Hope that helps.

An additional point to this is mashing higher = high FG and mashing lower = low FG, and because this is a Czech pilsner I wanted a lot of body and malt character left in the beer so I mashed at 69°C for 90 mins. I bottled it last night at 1.016 which is a bit high for a pilsner, but it had been on 1.016 for a week, even after increasing the temperature and gently rousing the yeast. I figured it must be finished, I tasted some and was tasting great, none of the sulphur smell or taste which comes with this yeast when it hasn't quite finished yet.

Is there another step to add in that equation to take into account mash temperature?

Sounds good, that's still 71% attenuation.

I don't know of a formula to account for mash temp but there must be as software can account for it. I think this is why one of the blogs I read mashes almost everything at 67c so then he learns how different yeasts attenuate at that temp.

The Grainfather online tool takes the average attenuation you give it (it has defaults for every yeast) and assumes the yeast will do that at 67.5c, it then varies up and down depending on mash temp with a calculable range of 63 - 72c, beyond those it reverts to the average.

I either use the default or the mid-point of Wyeast's stated range, and it's been pretty accurate, I go back to the recipe after and adjust the average attenuation so my FG matches, then I'll use that average in my next recipe.

Do you use any brewing software?

Zephyr259 said:

Sounds good, that's still 71% attenuation.

I don't know of a formula to account for mash temp but there must be as software can account for it. I think this is why one of the blogs I read mashes almost everything at 67c so then he learns how different yeasts attenuate at that temp.

The Grainfather online tool takes the average attenuation you give it (it has defaults for every yeast) and assumes the yeast will do that at 67.5c, it then varies up and down depending on mash temp with a calculable range of 63 - 72c, beyond those it reverts to the average.

I either use the default or the mid-point of Wyeast's stated range, and it's been pretty accurate, I go back to the recipe after and adjust the average attenuation so my FG matches, then I'll use that average in my next recipe.

Do you use any brewing software?

Click to expand...

Yes I use the Grainfather Recipe Tools but it does give me questionable FGs sometime, and I wonder whether it collects its ingredients data from user input or somehow speaking to the manufacturer (whether that's directly asking or through some sort of API, both sound unlikely which makes me think it's user input). On top of this I still haven't quite worked out my efficiency as it seems to get higher every brew. What blog did you read where the brewer did that? I might start doing the same. I'm writing my own brewing software and would be useful to create my own formula suited to my system.

Grainfather's software calculated that if I mash at 69°C then I should have an attenuation of ~79%, I'm slightly worried I've just created 60 bottle bombs which is what happened when I first used K-97, but I don't think I've ever had a fresh pack of W-34/70 stall on me before and I've used it a lot previously.

It was the "Make Your Best" series on beer and brewing magazine's website, Josh Weikert was the author. "Was" because it wrapped up about a month ago after a 2 - 3 year run, I found the series a few months ago and read through the ones that appealed. Something that I've picked up is that he's a bit averse to crystal malt so uses less than most recipes would, he also has a thing for Victory malt and chocolate rye.

Most attenuation figures it uses seem to be around the Wyeast mid-point so probably just manually entered when they designed it. Oh, dear, hopefully you're lagers will be fine.

jceg316 said:

An additional point to this is mashing higher = high FG and mashing lower = low FG, and because this is a Czech pilsner I wanted a lot of body and malt character left in the beer so I mashed at 69°C for 90 mins. I bottled it last night at 1.016 which is a bit high for a pilsner, but it had been on 1.016 for a week, even after increasing the temperature and gently rousing the yeast. I figured it must be finished, I tasted some and was tasting great, none of the sulphur smell or taste which comes with this yeast when it hasn't quite finished yet.

Is there another step to add in that equation to take into account mash temperature?

Click to expand...

I think you'll be fine.....

I recently brewed a bitter using Wyeast 1275 mashed at 70degC - after 3 weeks in the FV i checked the gravity over 3 days and it held steady at 1.019 so i bottled it. After carbonating and conditioning it tastes great - not sweet as you might expect if it hadn't finished fermenting.

The gist of the advice i got was that meeting high like this creates unfermentable sugars which keeps the FG higher, but these sugars don't taste sweet. The problem is, brewing software doesn't take this into account and still predicts a lower FG than both you and i have now found.

By the way, I'm lagering a Czech pilsner at the moment and I'm well jel now that i didn't think of your trick to mash higher to get more malt character! Please do let me know how it turns out once it's drinkable

matt76 said:

I think you'll be fine.....

I recently brewed a bitter using Wyeast 1275 mashed at 70degC - after 3 weeks in the FV i checked the gravity over 3 days and it held steady at 1.019 so i bottled it. After carbonating and conditioning it tastes great - not sweet as you might expect if it hadn't finished fermenting.

The gist of the advice i got was that meeting high like this creates unfermentable sugars which keeps the FG higher, but these sugars don't taste sweet. The problem is, brewing software doesn't take this into account and still predicts a lower FG than both you and i have now found.

By the way, I'm lagering a Czech pilsner at the moment and I'm well jel now that i didn't think of your trick to mash higher to get more malt character! Please do let me know how it turns out once it's drinkable

Click to expand...

I'll let you know how it comes out. From the FV it was really good, but I'll wait and see what it's like when it's ready.