As I have never brewed beer (and as I am still in the early stages of my beer journey), I don't purport to be an expert on the ideas in this post, but I offer these rudimentary thoughts to help clarify some things for those readers who like beer but asked me what IBU is and some other such questions. Some enjoy my blog and beer yet do not understand the difference between beer styles, but understanding what differentiates styles can help you anticipate what you might get so as to help you order something new you might enjoy!
For beginners: try to stick with me until the end and I think you will find this most enlightening and useful! Coming soon in follow-up posts: how to pour a beer and how to assess a beer!
Note that if you are very well-versed in these concepts I am sometimes both insufficiently versed (feel free to comment) and sometimes trying to intentionally keep things simple enough for others to follow along. I am also not citing things here, but rather going from 'as I understand it' though there is certainly more available for those who wish to seek out more in more elaborate and precise detail.
I will begin by considering International Bittering Units (IBU) which I have been asked about, before briefly explaining gravity for the purpose of concluding with an assessment of how this allows us to consider a beer's (im)balance and likely flavour elements (without consideration of the specific malt, hops, and yeast used that differentiate flavours more precisely).
I have been asked explicitly about International Bitterness Units (and which I comment on from time-to-time). IBU is actually a replicable, precise scientific measurement of the bittering acids from hops that remain in a beer (usually measured precisely but sometimes estimated for cost reasons). IBU is represented by a number from 0 to well over 100 and some brewers have even taken to putting these numbers on their labels or their websites. An excellent chart showing typical IBUs by beer style is available here and others abound on the web. Generally, the higher the number, the more bitter the beer will taste, but with the following caveats:
1) Since beer is characterized by some balance between the sweetness of the malt and the bitterness of the hops, a higher number matched by a substantial amount of malted barley means that the beer will not taste as bitter as an actually less bitter (in IBU terms) beer that also has substantially less (and less sweet) malt. That is, IBU tells us something but not everything: it tells us precisely how much 'hoppy' acid remains in the final product, but not how balanced that beer will taste - and I have tasted fairly well balanced beers of around 100 IBU and quite imbalanced, bitter beers of around 40 IBU.
2) The threshold for human discernment of IBU is 6, which means that even expert palates cannot discern between a 40 and 45 IBU with all other factors being equal; it takes a 6 or greater difference for us to even be able to note the difference.
One crucial point coming from these two disclaimers above is that while this number is precise (when measured and not estimated) it does not tell us anything precisely but rather provides a guide for what we might expect in a given beer (or beer style).
HOWEVER, a newer and perhaps more precise way of considering the bitter-sweet balance examines the relative amount of bitterness compared to the malt that provides sweetness. How does this work?
Well, brewers can and do measure the Original Gravity (OG) of the wort (boiled water and malted barley that will ferment and become beer). Original gravity compares the density of the wort to water, meaning it measures the amount of dissolved solids (read: sweet malt) in the wort (as a ratio of density in contrast to water).
Since it is these dissolved, sugared grains that get consumed by the yeast creating alcohol as a by-product, there is a direct relationship between a beer's gravity and its alcohol percentage. The higher the gravity, the higher the alcohol.
Gravity is measured on a scale where 1 = water, or identical to water, while beer ranges from 1.020 to 1.100 and above. A 1.100 OG will make for a much stronger (and typically much sweeter) beer than something with a lower gravity.
Though not always exact (and thrown greatly askew by beers with living yeast fed by sugar such as Trappist and Abbey ales) there is a rough estimation that is fairly accurate as a way to see the relationship between OG and ABV (alcohol by volume): if you drop the one and the decimal point and express it as a percentage, you are usually pretty close. That is, barring other factors, a 1.080 gravity beer will usually be around 8% ABV. A 1.100 will be around 10%, a 1.045 will be around 4.5% and so on. Conversely, a (non bottle conditioned) 5% ABV beer probably had an original gravity of around 1.050. Yes, this is rough and not always accurate, but the converse also holds for the next (and final) explanation in how to estimate a beer's flavour if you know the IBU and ABV.
The final factor of pertinence here is BU/GU ratio - that is, bitterness units to gravity units. Since gravity measures dissolved malt that provides both alcohol and sweetness, it also provides a rough estimation of the amount of sweetness in a beer (to be balanced by the bitterness).
To calculate this, you must also drop the one and the decimal place from the gravity units and make the division. That is, a beer with 30 IBU and an Original Gravity of 1.060 (perhaps estimated by a 6% ABV) would result in a 30/60 ratio resulting in 0.5 when divided. A .5 BU/GU is considered perfectly balanced while anything higher is more bitter (and anything approaching or surpassing 1 is extremely bitter) while anything below is sweeter! For another example, an IPA with 70 IBU and an OG of 1.060 will be quite bitter indeed (at 70/60 or 7/6 BU/GU) whereas a milk stout with just 20 IBU and an OG of 1.080 would be quite sweet indeed (at .25 BU/GU).
This doesn't tell you the specifics of the types of malt and hops used, nor the roasting of the malts, nor of any adjuncts and - hence - not the specifics of how the beer tastes, but it will give you a general idea of one of the primary things we all ask (consciously or subconsciously) when we drink a beer: how balanced or imbalanced (in which direction) is this beer (likely to be)?
When you can understand the specifics of beer characteristics, you can have a good idea of what to expect by just these numbers even if you aren't familiar with too many beer styles.
Some relevant links:
BU/GU Chart
BU/GU ratios of certain styles are discussed on this homebrewing blog
Beer Judge Certification Program Style Guidelines (Industry standard and VERY detailed/precise)
Beer Advocate Style Guidelines (well-respected beer web resource, but less detailed and more general/colloquial style explanations which may be preferable for some)
Beer Advocate's Beer 101
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