19 Aug 2009 @ 10:06 AM 
 

Those wacky numbers and what they mean about your beer.

 

A couple of weeks ago when I wrote my little column about Clarity in Labeling, a buddy of mine asked me afterwards, “Why would anybody need to know the original and final gravity of the beer they’re going to be drinking if the alcohol percentage is listed?” And it’s a fair question. Why indeed? He followed it up thereafter with, “And what does SRM refer to?” So for him and for many, many more people who probably have these questions, I write this post.

Hang with me. It might get a smidge technical and mathy, but by the end you’re going to get some good drinking advice.

I’ll start with SRM, since that’s easy and ends with itself.

SRM = Standard Reference Measure. It’s a standard method of referring to the color of beer.
Beer colors
Without getting too technical: 0 = clear. 40 = black.

How it’s measured (with a spectrophotometer, measuring the attenuation of light through a specifically sized sample of beer) is not important. What’s important is that it’s predictable. A brewer can guess pretty closely, when they are formulating the recipe, what the final SRM is going to be based on the grain bill. You, as a drinker, can know exactly what your beer is going to look like ahead of time if you know the SRM.

You may run into measures of beer color called Degrees Lovibond (°L) and EBC.

Joseph Lovibond used samples of colored glass to match beer colors in a standard manner. When the Standard Reference Measure was developed, it was done so at a particular wavelength of light to match °L as closely as possible, so °L and SRM are essentially interchangeable.

EBC is the European Brewing Convention. It is a color scale developed independently of SRM (which was developed by the American Society of Brewing Chemists). As it happens, EBC to SRM conversion is easy. It is almost twice SRM.

SRM * 1.97 = EBC

Further reading: Check out the Wikipedia Article about SRM and beercolor.com. In addition, Randy Mosher’s Tasting Beer also has an excellent section about beer color.

IBU = International Bittering Units. Sometimes referred to just as BU’s. (Remember that.)

IBUs refer to how bitter your beer is.

If you’re a homebrewer, you may be familiar with AAU (Alpha Acid Units) and HBU (Homebrew Bitterness Units). They are different. AAUs and HBUs refer to the bittering potential of hops based on the percent of alpha acids in the hops. IBUs are a metric measurement of one milligram of iso-alpha acid per liter of beer.

Okay, that’s technical.

The quick and dirty is that the higher the IBU the more bitter your beer is. There’s a fantastic article about it at BYO.

Hold on to BUs, we’ll get back to them.

OG = Original gravity, FG = Final gravity.

These both refer to the specific gravity of liquid. It’s a measure of density. The specific gravity of water at 60°F is 1.000. If you add sugar to water, say – for example – in the form of malt, the specific gravity rises because the liquid is more dense. Original gravity refers to the specific gravity of the wort pre-fermentation. The higher the number, the more sugar is present. The original gravity will look something like this: 1.054. Sometimes, you’ll drop the first two number and refer to the last two as GUs, or Gravity Units. An OG of 1.054 would be 54 GUs. Hang onto that. We’ll get back to it.

Final gravity refers to the specific gravity of the liquid post-fermentation. This is how brewers can calculate the Alcohol Percentage by Volume of the beer.

If your OG = 1.054 and your FG = 1.008, your ABV = 6.25% alcohol.

The formula is (OG – FG)*1000/7.36.

It’s easier if you just drop the “1.” (which I’m going to do for the rest of the article):

54-8 = 46
46/7.36 = 6.25

But! That’s not the most important thing that OG and FG tell you. If you know your OG (GUs!), FG, and BUs ahead of time, you as a drinker, can make a fairly accurate prediction about how your beer is going to taste.

So, if you know the OG and FG, here’s what you can tell about your beer – at least most of the time:

Low OG, low FG: Usually a dry, light-bodied beer. Think a lot of your crisp lagers, wheat beers, etc.

High OG, low FG: Usually a dry, full-bodied beer – probably also incredibly alcoholic. Think Abbey ales.

High OG, high FG: Usually a sweeter, full-bodied beer.

Low OG, high FG: Usually undrinkable. Wha? Yeah. If there’s not a lot of sugar (low OG), the yeast will almost definitely be able to work most of it out of the solution. If it’s a fairly low OG and a fairly high FG, there’s probably something wrong. You will probably never see this in a commercial product.

So what’s high and low? A normal beer – say, a medium-bodied Pale Ale – at 5.5% ABV will probably have an OG of ~50 and an FG of ~10. There’s your mid-range average. 90% of the beer you’ll run into the market will be mid-rangeish, but the numbers will trend in directions. Look for them (if they’re published). If you have homebrewing friends, ask. They should know.

BU:GU – Finally, this is the thing that can tell you what, in my mind, is the most important piece of information: How balanced your beer is going to be. Bittering Unit to Gravity Unit ratio. The only real guide I’ve seen published on this is (again) in Randy Mosher’s Tasting Beer. (Maybe I’ll throw a reference guide out on the web later.)

To give you a quick and dirty idea here are some values based on average numbers from BJCP guidelines.

Light lager: 10 BUs to 30 GUs (1:3) (little-to-no hop flavor)
Bohemian Pilsner (like Pilsner Urquell): 40 BUs to 50 GUs (1:1.25) (fairly well-balanced, hops evident)
English IPA: 50 BUs to 62 GUs (1:1.25) (fairly well-balanced, hops evident)
American IPA: 65 BUs to 65 GUs (1:1) (hoppy)
Imperial IPA: 90 BUs to 80 GUs (1.125:1) (very hoppy)
English Barleywine: 52 BUs to 100 GUs (1:2) (sweet, low hop flavor)
American Barleywine: 85 BUs to 100 GUs (1:1.2) (sweet, hops evident)

Of course, these are average values, each individual beer that you have will be different, but knowing the values up front can tell you what to expect.

To tie this all together, let’s go back to the example that I posted in my “Clarity in Labeling” column, which happens to be the Irish Red ale that I made for a friend’s wedding.

What makes my Irish Red great.

You can see from the SRM of 17 that it’s fairly dark, but not opaque. The Roasted Barley listed up in the grain list is what I was depending on to give the beer a reddish color. Indeed, I came out with a ruby-red kind of color that I think was quite lovely.

You can see from the OG of 50 and the FG if 13 that I listed Alcohol by Weight when I meant to list Alcohol by Volume (whoops). The ABV should read 5%. Heh heh.

Mid-range OG and FG tells you that this is a fairly medium-bodied beer. Maybe a tinge sweet because of the FG over 10.

The IBU listing of 25 tells you that this is not a very hoppy beer and looking at the BU:GU ratio (1:2) tells you that this is balanced toward being a malty beer with low hop character which, I hope, is exactly what we’re looking for out of an Irish Red.

This is just a guide. They’re not rules. Overall, nothing’s going to teach you this stuff better than experience. Until commercial labels carry this kind of information, the best way to do it is to make your own beer or find someone who does. Every time you can drink a beer with these types of descriptive numbers, do so. Think about what they mean to the beer that you’re tasting.

And if you have any questions, by all means, post ’em here.

Tags Tags: , , , , , ,
Categories: appreciation, homebrew
Posted By: erik
Last Edit: 19 Aug 2009 @ 10 06 AM

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Responses to this post » (29 Total)

 
  1. Russ Carr says:

    I could go on and on, but “is no time….Let me sum up.” Brillliant piece. Obviously it’s not the absolute solution, because nuances don’t check their math, and many brewers don’t supply that kind of info on the bottle or box (which is where you’re most likely to check when you’re out on a beer run) but it still sets up some good rules of thumb when it comes to making picks based on your preferences.

    Also the math geek in me loves quantifying stuff like this.

  2. ingrate says:

    That was the single most informative column on decrypting my beer I have ever read. Thank you for breaking it down. My local alehouse just because much much more interesting.

  3. ingrate says:

    became*, not because. No…I haven’t started early today. >P

  4. Russ Carr says:

    As soon as I posted my initial comment, I was inspired with an idea. Someone could create the equivalent of a Proportion Wheel* (commonly used by photographers and graphic designers) only with the relevant numbers above, and offer it to brewers and consumers alike as a convenient way to ascertain the qualities of particular brews. What’s more you could add an additional table to the wheel which would then suggest appropriate food pairings based on the beer’s qualities.

    *Or, if I could drag myself into the current century, it’d be a killer iPhone App.

    • erik says:

      That is a really killer idea, but seriously hard to get right – at least on a wheel, I think.

      There are so many variables!

      • Russ Carr says:

        On a wheel, yes; that just happened to be the first analogue that popped into my head.

        But ultimately it just comes down to mathematics. It would be akin to any other data management system that worked with set rankings to calculate results which fell into a set-sum range. I remember writing a program back in HS that let us do a “Who’s your perfect match?” for the Valentine’s Day issue of our newspaper. Students answered a questionnaire that averaged their answers (on a 10 pt continuum) to a series of questions. When all of the students’ data was entered, you could then check any one student’s file against the rest of the student body, to see which other student(s) was (were) most compatible.

        So, provided you know the variables for these beers (your Irish Red example, for instance) you could calculate variance between particular styles (eg, two different IPAs from the same brewer) based on the relationships between each of the characteristics. Furthermore, you could allow for non-mathematical variants, such as the flavors imparted by different hops, by assigning each variety a numerical value (eg, Fuggles = 1; Willamette = 2, etc.). It wouldn’t be foolproof, but you could at least provide an approximation.

        Finally, once you establish a baseline, you could then begin suggesting food pairings based on where a given beer falls along that baseline…dunkels with grilled salmon, bitter with fried cod, porter with roast meat…

        Ideally it’s something with a database that could be updated wirelessly. So when you’re at your favorite beer store or microbrewery, and you know you’re making chili for dinner, you can call up your beer finder, and it’ll help you cherry-pick the ideal brew.

        • erik says:

          I see where you’re going. This is really quite brilliant.

          I need at least a weekend to map this kind of thing out and to figure out what other variables need to go in there.

          Of course, this is all predicated on actually knowing this information about any beer you happen to run across.

  5. Ron Extract says:

    “You can see from the OG of 50 and the FG if 13 that I listed Alcohol by Weight when I meant to list Alcohol by Volume (whoops). The ABV should read 5%. Heh heh.”

    That’s actually backwards. Alcohol is less dense than water, and therefore accounts for less weight for the relative volume that it occupies. For beer the rule of thumb is that the percentage alcohol by weight will be roughly 20% lower than the percentage alcohol by volume, so if your beer is 5% by volume, it would actually be 4% by weight, not 6.5%.

    Also, I had always used a slightly different formula for calculating ABV, going back to a brewing class I took about 15 years ago. At that time, we were taught that %ABV was approximately equal to the difference between the starting and ending Plato, multiplied by a factor of 0.525. Since Plato is roughly GU/4 this translates to multiplying the difference between OG and FG by 0.13125, which is the same as dividing by 7.619. That’s not incredibly different from the 7.36 number that you use, but a beer that’s 5% according to my formula would be nearly 5.2% according to yours. I’m wondering now which is more accurate, or if the ABV of beers with the same starting and ending gravities could actually vary that much on the basis of other factors. If so, what would those other factors be?

    Finally, it strikes me that the in evaluating the perceived bitterness level and overall balance of a beer the ratio between IBUs and FG (or more specifically, % residual sugar), would be at least as relevant as the ratio between IBUs and GUs. Of course, alcohol does come into play, as well, but isn’t a beer that’s bitter and dry always going to seem more bitter than one that’s bitter and sweet?

    In any event, thank you for posting this, and for your efforts in educating the public as to the intricacies of the brewing arts!

    Cheers,
    Ron Extract
    Shelton Brothers Importers

  6. erik says:

    Huh. I wonder where I got that 6.25%, then. I’ll have to go back to my records on that beer. The other formula that I have laying around calculates ABW and then converts to ABW:

    OG – FG * 105 = ABW; ABW * 1.25 = ABV

    That actually gives me 4.86% ABV on that beer up there. Mind you, I have no reference on either of these calculations. The 7.36 sticks in my head like pi=3.1415 does, so it’s what I’ve used, and it’s always seemed accurate. I’m not actually sure where I originally got this other formula.

    BrewCalcs comes out with 5% ABV, as well.

    So – yes – I hadn’t really considered the relationship between IBU and FG before, but you’re definitely right. Maybe all the more reason to write up some sort of chart/guide.

    Although it may be more accurate to include apparent attenuation instead of FG, since FG seems less variable than, say, OG.

    Can you envision a day in which brewers list apparent attenuation on their labels?

    Maybe it would make more sense to play with all of these numbers together to make a whole new metric that can be applied to every beer.

    • Russ Carr says:

      Maybe it would make more sense to play with all of these numbers together to make a whole new metric that can be applied to every beer.

      This is what I’m talking about…more or less. Consider it akin to sabermetrics for baseball. It’s an inexact science, because there are too many organic variables (in the case of beer, quite literally) but you can still produce reasonable statistics wherein given specific values for the quantifiables you so eloquently explained, you will get an objective description of that beer.

  7. erik says:

    Sabermetrics is pretty much exactly what I was thinking of. I’ve got a lot of driving ahead of me today, which is usually when I get to think through things like this. Gonna see what I can come up with.

  8. ingrate says:

    A Unified Theory of Beer?

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