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Old 01-14-2012, 02:39 AM   #1
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Default Relative Bitterness Ratio (RBR)

I have a new calculation that I've been using when determining the expected balance of my beer recipes. It is a child of the commonly-used Bitterness Ratio (BU:GU), and the numbers output can be read in the same way as BU:GU. However, the one thing that is taken into account with the Relative Bitterness Ratio (RBR) that BU:GU does not account for is Apparent Attenuation (ADF).

The higher the degree of Apparent Attenuation (ADF), the more fermentable sugars are consumed and the less residual sweetness is left behind. That means that as ADF gets higher, beer balance tends more toward the bitter end of the scale. As ADF gets lower, beer balance tends more toward the sweet end of the scale.

For example: A beer that starts out at an OG of 1.050 at 25 IBU would be said to have a Bitterness Ratio of 0.5. If it were split into two batches and one had an apparent attenuation of 80% (Beer A), while another had an apparent attenuation of 60% (Beer B), Beer A would be perceived to be more bitter than Beer B, as the latter has considerably more residual sweetness.

Beer A would have an RBR of 0.517 while Beer B would have an RBR of 0.417--a fairly big gap in perceived balance.

In the pages linked below, you can find a plethora of information explaining details about the Relative Bitterness Ratio (RBR). If you aren't interested in the details, I've included the formula for figuring out RBR as well as a simple RBR calculator if you just want to input some numbers and get the results.

To quickly explain the formula:

RBR = Relative Bitterness Ratio. ADF = Apparent Attenuation. 0.7655 is the average ADF of all beer styles (according to the BJCP style guidelines). 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.

Code:
RBR = (BU:GU) x (1 + (ADF - 0.7655))
You can see the full original post (basically the same as this one) over at Mad Alchemist, but I'll be happy to read and respond to posts here.
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Old 01-14-2012, 02:50 AM   #2
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I see what you are doing there. Makes sense. But who only gets 60% ADF? I get about 75% every time. Standard mashes I guess.

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Old 01-14-2012, 03:25 AM   #3
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I used 60% and 80% to illustrate a point (and I do get ADF in those ranges depending on the beer). I specifically tailor my mash schedule, yeast selection, and fermentation temperature in part to determine my intended attenuation.

I'm not sure if ADF is what you're talking about here, though--ADF is Apparent Attenuation. I think you are talking about Brewhouse or Mash Efficiency since you mentioned "standard mashes."

Apparent Attenuation (ADF) is the percentage of fermentable sugars converted by yeast to ethanol and CO2.

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Old 01-14-2012, 03:27 AM   #4
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I think this has been kicked around and it is also tied to mash temps. Getting a good number on how that will affect any given brew session seemed to be the sticking point.

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Old 01-14-2012, 03:35 AM   #5
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I've found that I can fairly accurately predict my apparent attenuation before brewing based on a combination of knowing my equipment, changing my mash temperature (higher temperature means less attenuation, lower temperature means more attenuation), and choosing yeast with attenuation in mind.

I've found that BeerSmith is quite accurate at predicting my likely ADF now that I know what to expect. I would recommend using that or similar software to aid in the calculations--it'll make your life easier.

That said, you can't be absolutely certain what your Relative Bitterness Ratio is until after fermentation is complete, but I've found it useful in planning recipes.

RBR isn't something you should bother trying to calculate until you know your system well and are fairly advanced at homebrewing. Basically, BU:GU wasn't quite cutting it alone, so I decided to tackle the problem head-on and have found it to be quite useful in the past several batches I've brewed.

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Old 01-14-2012, 03:47 AM   #6
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Quote:
Originally Posted by Hop View Post
Apparent Attenuation (ADF) is the percentage of fermentable sugars converted by yeast to ethanol and CO2.
So are you proposing that apparent attenuation (wort fermentability) is not related to mash temps? I think you'd have a hard time with that one in these parts.

I wasn't thinking efficiency.
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Old 01-14-2012, 03:52 AM   #7
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Oh, no not at all. Apparent Attenuation is highly dependent on mash temps. I must have misunderstood your comment entirely if I led you to believe that.

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Old 01-14-2012, 03:57 AM   #8
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What number represents a 'noticeable' difference on your scale? Given the numbers above you might have to multiply by 100 because peoples bias towards thinking decimals aren't very meaningful. Think percentage.

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Old 01-14-2012, 04:06 AM   #9
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Ha! Well, you're looking at a 20%+ gap there.

Relative Bitterness Ratio (RBR) numbers are very similar to BU:GU in that 0.5 is roughly balanced between sweet and bitter.

(On average) The sweetest-balanced beer style is Fruit Lambic at 0.113 RBR. The most bitter is the Imperial IPA at 1.135 RBR. An American Amber Ale is 0.611 RBR and an Irish Red Ale is 0.430 RBR.

Hopefully that gives a little clearer picture on the numbers. If you use BU:GU already, the numbers will be familiar. I did consider multiplying the values by 100 because they are easier to understand that way, but I decided to leave them as-is because many of us are used to the BU:GU numbers already, and RBR is simply BU:GU corrected for attenuation relative to other beers.

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Old 01-14-2012, 12:47 PM   #10
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I do pay attention to bitterness ratio when I design my recipes. Since your RBR requires ADF, it's really not possible calculate it until after fermentation. So, while it's an interesting and accurate way to characterize the beer after it's made, it's not a good tool for recipe creation (unless your ADF is well outside the norm, and you know for certain what it till be).

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