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I recently got a "Gnarly Oak" craft brew package. Three 22 oz beers and a glass. However, on the beers are some abbreviations and numbers I don't understand. they all have a 5.5% ABV, and I get the IBU and color numbers but what are the rest? The numbers and abbr's are below:

  • Winter Bock - 15 OG, 62.5 RDF - light, tasty, ever so slightly bitter
  • Hazelnut Dark - 15 OG, 60.4 RDF - nice bitter red with slightly sweet aftertaste
  • Chocolate Stout - 14.8 OG, 66.8 RDF - very sweet with hints of chocolate and malt

EDIT In light of the answers so far, I should say I have brewed my own, and if OG is original gravity, it doesn't fit with the scale I know of. if 15 OG ~= 1.015, it doesn't match what that would give in final ABV. Is this OG on a different scale I haven't seen

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It's probably degrees plato, an alternate gravity scale. I've updated my answer with more details. See en.wikipedia.org/wiki/Plato_scale#Strength for more. –  Fishtoaster Feb 3 at 19:05

4 Answers 4

up vote 14 down vote accepted

From the figures you posted,

OG - "Original Gravity". When measured with a hydrometer, it measures the density relative to water. E.g. 1.010 is 1% heavier than water. The units here are degrees Plato (°P), which describe the amount of dissolved sugars. Here, 15 means 15% dissolved sugar. This means the beer (or more correctly, the wort) contained 15% sugar before fermentation started.

RDF - The real degree of fermentation. This is in contrast to the Apparent Degree of Fermentation, which is (OG-FG)/OG. When the final gravity is measured using hydrometer, the alcohol in the beer, being less dense than water (SG. 0.789) causes the gravity to appear lower, so it looks as though the yeast has fermented more sugar than it actually has. The Real degree of Fermentation takes the alcohol into account and computes the actual amount of sugars fermented. This tells you the how much of the original sugar was fermented (as a percentage).

So, what does this mean in practice?

Winter Bock - 15 OG, 62.5 RDF - light, tasty, ever so slightly bitter

This has an average RDF for a beer that size, so it will be somewhat malty, but not cloying.

Hazelnut Dark - 15 OG, 60.4 RDF - nice bitter red with slightly sweet aftertaste

This has more dissolved sugars in it compared to the Bock, so you'd expect it to be a thicker mouthfeel and slightly more sweet, although you may not taste the sweetness if it's offset by roasted malts.

Chocolate Stout - 14.8 OG, 66.8 RDF - very sweet with hints of chocolate and malt

Being the driest of all the beers, yet the taste description is given as very sweet. This shows that knowing the residual sugars doesn't tell you the sweetness, simply because there are different kinds of residual sugar. For example, dextrins create body, but have relatively little taste, so would taste not as sweet as a beer containing a lot of residual maltose or glucose.

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from the Plato wiki 1° Plato equals four “brewer’s points” (4 x .001), which fits much closer to my idea of getting the ABV, +1 –  CDspace Feb 3 at 19:01
    
Yes, it's about that roughly. The plato and SG scale are not linear, but P = SG / 4 is a good approximation. –  mdma Feb 3 at 19:05

I'll answer one half and leave the other half to someone else.

OG is “Original Gravity”

Specific Gravity (often shorted to just ‘gravity’) is a measure of the density of a liquid. Since its is mostly water, you can compare beer’s density to water to find out how much other stuff is in it. Water has a gravity of 1.000, and beers will often have an original gravity of 1.200 to 1.020.

“Original Gravity” is the density of beer before it’s fermented (when it’s called ‘wort’), and “Final Gravity” is the density after it’s fermented. You can determine the alcohol content of a beer by comparing the two gravities and inferring how much fermentable “stuff” disappeared during fermentation (i.e., was converted to alcohol and CO2).

In commercial beer, you can use OG as a shorthand for how “Big” the beer is because it indicates how much malt and other fermentable stuff was in it. So a lighter-flavored beer like a blonde or a pilnser will often have a low original gravity (1.020 to 1.050, for example), whereas a big stout or barelywine might have a higher original gravity (1.080 up to as much as 1.200).

OG is in important measurement for home brewers, so another use of it on a commercial beer is to help people produce clones of said beer at home by reproducing the OG (among other factors).

Definition

To get a little more technical, Specific Gravity is the ratio of the density of one liquid to another. In brewing, we always use water. So, the density of water divided by the density of water is 1.000. The density of beer divided by the density of water is what we refer to as ‘gravity’ when discussing beer. And since it’s a ratio of two identical measures, there are no units. It’s not 1.000 somethings; it’s just 1.000.

Plato

Homebrewers often use "brewer's points" (eg 1.123), but you'll sometimes see the "Plato Scale", which is more popular in central Europe. A good approximation is that 1˚ plato = 4 brewer's points. So 12˚ Plato corresponds to a gravity of 1.048.

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It's strange you use 2/3s of the article to talk about SG units unit that isn't in the original question. :) –  mdma Feb 3 at 19:06
    
Yeah, I'm used to thinking in US homebrewer terms. I saw OG and thought "Oh, I know that! That's like 1.123!" Of course, I had to amend it after I went back and re-read the question. :S Hopefully there's at least some useful information here. –  Fishtoaster Feb 3 at 19:08

OG = Original Gravity
The amount of sugar available for fermentation, measured based on density vs water (1.000)

RDF = Real Degree of Fermentation
sugar in cold wort has been fermented into alcohol in beer with the term degree of fermentation. More residual sugar == more sweetness

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2  
I'm used to OG numbers in the form 1.0XX. Doesn't 15 ~ 1.015 seem a bit low. Even if it fermented down to an even 1.0, wouldn't that make the ABV about 2%, not the listed 5.5%? Is this another unit of gravity? –  CDspace Feb 3 at 18:57
    
I defer to @mdma, that answers both the initial and follow up question –  Mutmatt Feb 3 at 20:42

OG stands for "original gravity", which is the density of the liquid, before fermentation in the case of beer. Another "gravity" you may hear is "final gravity", or FG: the density of the beer after fermentation. Typically, gravity is expressed as thousandths over 1.0, where 1.0 is water. However, Gnarly Oak says that their Winter Bock has an OG of 15, which is likely in degrees Plato. I leave it to other answers to thoroughly explain what that is.

The density, of course, is increased by the sugar and other "good stuff" that makes beer, well, beer. After the yeast has eaten the sugar and produced alcohol, you can measure the final gravity. What makes this interesting is that if you have the OG and the FG, you can calculate your alcohol by volume with a little math.

I am completely unfamiliar with RDF, however.

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The OG bit is great, but this answer is incomplete with an explanation of RDF –  wax eagle Feb 3 at 18:48
1  
Does it matter if it's incomplete? It probably shouldn't be accepted if it's not complete, but it's still worth an upvote if it's useful and helps answer the question. –  Fishtoaster Feb 3 at 18:59
    
Selfishly, I agree with Fishtoaster. ;) –  object88 Feb 3 at 19:00
    
See my comments above, 1.015 OG wouldn't give the ABV listed. It's probably in degrees Plato , which fits more closely to the numbers. Good try, but a bit off on the explanation –  CDspace Feb 3 at 19:03
    
Ah-hah! Thank you, CDspace. I will correct my answer. –  object88 Feb 3 at 19:19

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