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To the untrained eye, a homebrewing recipe can look like a secret code with abbreviations, acronyms, and lists of numbers. Over the years, brewers have condensed the essentials of a brewing recipe into an easy to reference format that can be quickly reviewed during the brewing process. But today we're going to demystify the code so that you can read a brewing recipe like the pros.
At the top of every homebrew recipe, you'll find the technical specs of what the beer is supposed to be. The original gravity (O.G. or sometimes S.G.) is given first, often followed by the final gravity (F.G.).
The bitterness level is listed as International Bittering Units (IBUs), and the color typically is described by the acronym SRM, which stands for Standard Reference Method.
Additionally, some recipes will also provide the alcohol by volume (ABV), but this is often omitted since the O.G. and the F.G. can be used to calculate this value.
You measure the O.G. of your homebrew with your hydrometer after the wort has cooled, but before fermentation begins. This measurement gives the relative density of the sugar dissolved in the wort. Extract brewers should be able to come very close to matching the O.G. on a recipe, since the amount of sugar in malt extract is a fixed value. Sugar doesn't boil off, so the amount of extract you add to the wort determines exactly how much sugar you'll end up with. If you find that you're not getting the O.G. specified by the recipe, the most likely culprit is that you somehow didn't end up with the right amount of liquid. If you ended up with more than 5 gallons of wort, then the relative sugar density, and thus the O.G., will be lower. The opposite is true if you end up with less than 5 gallons of wort.
It's also possible that your O.G. measurement is off is because of the temperature of the wort. Hydrometers are calibrated to read the correct value at 60°F. My rule of thumb is that for every 8°F above 60°F, add 0.001 to your reading. Wort temperatures above 85°F produce inaccurate readings when using any adjustment formula.
All-grain brewers quickly learn that there is a lot of experimentation that goes into getting the correct O.G. of a recipe. For an all-grain brewer, the amount of sugar extracted from the grain is a product of the the way the grain is crushed, the mash temperature, sparge temperature and even the type of mash tun used. An all-grain brewer usually has to brew several batches and take careful notes before they can accurately predict the O.G. of a homebrew.
The final gravity, or F.G., of a recipe is the same type of measurement as the O.G., but the measurement is taken at the end of fermentation. The difference between the O.G. and the F.G. tells you how much sugar was consumed by the yeast. As yeast eat sugar they produce carbon dioxide and alcohol, which are both less dense than sugar or water, so the density of the wort decreases.
The actual F.G. of a beer depends on yeast health, the type of yeast used, and the fermentation temperature. If your fermentation conditions are perfect then you may come close to the F.G. listed on a recipe, but predicting the F.G. is almost more of an art form than a science. There are a lot of variables that go into fermentation, so a beginning brewer shouldn't be surprised if the measured F.G. is off by about 0.005. Even advanced brewers will find that predicting the F.G. within 0.002 takes a lot of experimentation.
Since the actual O.G. and F.G. of a homebrew can be slightly different than what's on the recipe, I interpret the ABV listed on a recipe as a guideline. Your can calculate your actual ABV by taking the O.G. minus the F.G. and multiplying that value by 131.
Next on the recipe list is typically the IBU rating of the homebrew recipe. The abbreviation IBU stands for International Bittering Unit, and is a measure of how bitter the beer should be. The IBU scale ranges from 10 for a very malty Scottish or Belgian ale, to around 35 for a balanced American Amber, to 75 or higher for a palate dominating Imperial IPA. Any IBU rating in a homebrewing recipe will be an estimated value, typically using either the Tinseth formula or the Rager formula. Both formulas use a combination of the acid level of the hops, the O.G. of the wort and the length of the boil to estimate an IBU rating.
The precise IBUs of a beer can be measured by complex methods called spectrophotometry or chromatography, but many commercial brewers calculate their recipes using the same estimation formulas from homebrewing. Perception of bitterness also changes with the malts and water used in brewing, so having a precise measurement doesn't add a lot of benefit. The human tongue doesn't distinguish well between bitterness levels that are higher than 100 IBUs, so you might as well save the hops if you're thinking of brewing a beer that's more extreme than that.
The last acronym you'll find on a recipe is the SRM, which gives you an idea of what color you should expect a beer will be. An SRM of 2 is a very pale beer, 15 is an amber color and 30 is very dark brown or black.