You might wonder why Barleywines and Imperial Ales range over 80s in IBU (International Bittering Units). They require substantial amount of hops inpite of not being hop forward styles. The reason being, with FG (Final Gravity) being so high, these beers would come across as excessively sweet, without considerable bitterness.
You may have also noticed that higher gravity IPAs use more hops than their lower gravity cousins. Another interesting observation is, if same amount of hops are used in a high gravity and a low gravity beer recipe, you will notice that the high gravity beer tastes distinctly less hoppy and bitter compared to the low gravity one. This is due to the fact that carbohydrates(which includes malt, sugars and other adjuncts) have a diminishing effect on hop oils.
BU:GU Ratio
BU:GU stands for Bitterness Unit:Gravity Unit. It is important to consider this ratio when designing a recipe, so as to create a beer that stands true to it's style. BU is IBU and GU is your OG (Original Gravity). To calculate, take the number of the OG after the decimal, and divide it into the IBUs.
For example if you are brewing a Dortmunder Export and your OG is 1.048, your IBU is 24, that makes it 24/48. Hence your BU:GU ratio will be 0.5. 0.5 is usually considered balanced, values under that tend to be maltier, values above are hoppier. Bellow graph shows how perceived taste of beer change over differences in BU:GU. While the yellow shade in middle shows a balanced taste, the orange and red shades indicate increasing maltiness and light and dark green indicate more and more increase in hoppiness.
According to BJCP style guidelines, bellow is the BU:GU ratio range for each major beer style.
The BU:GU Ratio is an attempt to quantify how balanced a beer is, but dark malts, estery yeasts, spices and sourness will throw off the perceived balance, and you can't factor all that into an equation. And what about different levels of attenuation? Maltodextrin will throw off the sweetness compared to measured gravity. Some styles will always be out of balance according to the basic BU:GU calculation. Does that make them bad? The BU:GU idea has it's merit, but looking at the taste and bitterness of different beers it is easy to get confused.
Not surprisingly, the average BU:GU ratios vary widely by style. Ray Daniels, in his book "Designing Great Beers" lists the average BU:GU ratio for some popular styles. Now things finally come together. Rather than looking at an 'absolute' measure of balance, we can look at the relative BU:GU ratio for the style.
Ta-da! So finally, we can dial in the relative sweetness to style, whether it's on the high side or the low side of the gravity for that style. Or we can do the same thing for a specific recipe: Scale it up to it's standard version and keep the relative BU:GU balance in line.
Happy brewing!
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