Scaling recipes - does it affect IBU?

[MagnusTS]

I hope this isn't a daft question, but I can't work out what's happening...

I am going to do a few small batch 5L brews, and I thought I could just use the scale recipe feature in the Grainfather Recipe Builder to convert 23L or 20L recipes into 5L format.

But when I do this, the predicted bitterness and colour drop.
Is this a real thing or a software glitch?
I can't get my head around it - if I reduce the amount of hops to a quarter, but also reduce the brew volume to a quarter, shouldn't the bitterness stay the same?

For example below is the 20L recipe for James Morton's Extra, Extra Special Bitter, which works out at 32.7 IBU and colour 17.5 EBC. But when scaled to 5L, the bitterness drops to 25.7 IBU and colour 14.8.

 

[Bigcol49]

Hi!
Some information is cut off from the bottom of one screenshot - EKG boil time.
Would you be boiling a 5 litre batch for the full 60 minutes? If not, this will change the IBU calculation.
Try this - some nice bedtime reading!
http://howtobrew.com/book/section-1/hops/hop-bittering-calculations

 

[MagnusTS]

Hi!
Some information is cut off from the bottom of one screenshot - EKG boil time.
Would you be boiling a 5 litre batch for the full 60 minutes? If not, this will change the IBU calculation.
Try this - some nice bedtime reading!
http://howtobrew.com/book/section-1/hops/hop-bittering-calculations

Yes, sorry, couldn't fit everything in on the screenshot.
Boil time and hop addition times were the same in both 20L and 5L version.
Just quantities reduced to one quarter.

I entered the same 20L and 5L recipes into the BrewUK recipe builder, and in that recipe builder the bitterness and colour came out almost exactly the same in the 5L version as in the 20l version.

 

[Fil]

afaik hop utilisation like most things brewing will increase with volume, BUT, the difference this will have between 5 and 25l is probably negligable as i understand it, when scaling down from the industrial volume to the homebrew standard 5 gallon i think there is a nominal few% nudge up on the hop bill to account for this..

How old and how had the hops been stored, perhaps your pack sat on top of the batch and took a lot of direct sunlight @ the shop??

Les Howarths Homebrew database has a succinct description of hop utilisation and its calculation and pitfalls which you may catch in the amazon look inside feature https://www.amazon.co.uk/dp/059529720X/?tag=skimlinks_replacement-20
(you need to get your head wrapped around it to an extent to decode his uber compact recipe notation

 

[MagnusTS]

afaik hop utilisation like most things brewing will increase with volume, BUT, the difference this will have between 5 and 25l is probably negligable as i understand it, when scaling down from the industrial volume to the homebrew standard 5 gallon i think there is a nominal few% nudge up on the hop bill to account for this..

How old and how had the hops been stored, perhaps your pack sat on top of the batch and took a lot of direct sunlight @ the shop??

Les Howarths Homebrew database has a succinct description of hop utilisation and its calculation and pitfalls which you may catch in the amazon look inside feature https://www.amazon.co.uk/dp/059529720X/?tag=skimlinks_replacement-20
(you need to get your head wrapped around it to an extent to decode his uber compact recipe notation

Thanks for the info. I'll have a look at that link. I might need a clearer head to understand it though...

I've not brewed this yet, so I don't know if the 5L version is actually lower IBU. It is just the Grainfather recipe generator is giving me a predicted lower IBU (and colour) for a 5L batch that is directly scaled down from 20L.

I don't know why this would be. With boil size and hop addition all equally reduced I thought concentrations and hop to boil volume ratios would all be the same and so utilisation would be the same.

Maybe it is something to do with the amount of boil off from a smaller boil volume? Is there proportionally less boil off from a 5L boil. But that would affect the final abv as well wouldn't it? And the 20L and 5L are both giving me the same predicted abv.

Making my head hurt. :sad1:

I think I am going to ignore it and just do the scaled recipe anyway.

 

[Fil]

ahh Perhaps the G/f recipe builder has been 'tuned' to provide accurate results within the volumes that the g/f is expected to brew.. 11-25l ?? and beyond those volumes the results could vary???? tho thats just a big guess..