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02-11-2012, 17:25
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What does 10 IBU, 50 IBU or 100 IBU actually mean to the drinker?

Not a lot.


Because it’s all about context. And I’m about to get geeky...

50 IBU (International Bitterness Units (http://beer.wikia.com/wiki/International_Bitterness_Units)) will make a light lager brutal with bitterness. 50 IBU will make a strong stout seem cloyingly sweet.

It’s more relevant to look at the balance between bitterness and sweetness, so let’s throw away IBUs and bring in the BU:GUs (Bitterness Units: Gravity Unit).

BU:GU is the bitterness ratio and it was first introduced by Ray Daniels in Designing Great Beers (http://www.amazon.co.uk/Designing-Great-Beers-Ultimate-Brewing/dp/0937381500) (which is a great book!). It measures hop bitterness against malt sweetness where the higher the number, the higher the bitterness is to the sweetness in the beer. It’s calculated by dividing the IBU by the original gravity of the beer. So, with a beer of 25 IBUs and an original gravity of 1050 (you drop the first two digits of the OG unless you’re starting with a monster beer that’s in the 1100s, in which case you just drop the first digit) you get 0.5. On the BU:GU scale, that’s balanced – a score of 1 would be a beer with a very high perceived bitterness whereas 0.1 would be very low. (There’s a list of BU:GUs here (http://beersmith.com/blog/2009/09/26/balancing-your-beer-with-the-bitterness-ratio/) and there's a colour chart of BU:GUs here (http://www.madalchemist.com/chart_bitterness_ratio.html))

http://www.tc.umn.edu/~rohw0009/homebrewing/calculator/bvchart.jpg.JPG (http://www.tc.umn.edu/~rohw0009/homebrewing/calculator/bvchart.jpg.JPG)

Image from here (http://www.tc.umn.edu/~rohw0009/homebrewing/calculator/bvcalc.htm)

But then, to make it less straight-forward, another factor comes in: final gravity, or the apparent attenuation of the beer (the amount of sugars which are left in the finished beer). It’s fine knowing the original gravity but obviously a beer with a lower final gravity (or higher attenuation) will be drier in the finish with less residual sugars, which will then influence the perception of bitterness.

The Mad Alchemist (http://www.madalchemist.com/) has come up with a formula for figuring out the RelativeBitterness Ratio (http://www.madalchemist.com/relative_bitterness.html) (RBR) against the same numerical scale as BU:GU (here's another colourful chart (http://www.madalchemist.com/chart_bitterness_corrected.html)). Here’s what they say:

“ADF = Apparent Attenuation. 0.7655 is the average ADF of all beer styles. Since the Relative Bitterness Ratio takes into account balance relative to all beer styles, it uses this as a constant. You are comparing your beer's ADF against the average ADF (0.7655), then adjusting the standard Bitterness Ratio accordingly (it goes up if your ADF is higher than average, down if your ADF is lower than average). Just like BU:GU, higher numbers mean more bitter, lower numbers mean less bitter, and 0.5 is roughly average balance.”

Here’s the sum for a beer with 25 IBUs, OG of 1050 and 80% apparent attenuation:

RBR = (BU:GU) x (1 + (ADF - 0.7655))
RBR = (25/50) x (1 + (0.8 - 0.7655))
RBR = 0.5 x (1 + (0.0345))
RBR = 0.5 x 1.0345
RBR = 0.51725

So if you have two beers with the same IBU and same OG but one has a high apparent attenuation (85%, for example) and one with a low apparent attenuation (55%), they perception of bitterness will be different in both. Get it?

We know the IBU scale and understand that 10 is very low and 100 is very high, so it’ll be just as easy for drinkers to take notice of a BU:GU scale where 0.1 is very low and 1.0 is very high. While IBUs are an interesting piece of information, it means little without measuring it against the sweetness in the beer. That’s where a BU:GU score, worked out according to the Relative Bitterness Ratio, is more relevant and revealing.

Wonderful top photo from here (http://www.flickr.com/photos/wenjo13/906837305/)


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