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.

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

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