Monday, November 15, 2010

Well I'll Be a Monkey's Dunkel - The Gravity Of The Situation

Sadly, my Let's Get Crackin' post will be delayed...  One must remember that the process of making beer involves a living creature... Yeast.  Sometimes yeast just takes a little longer than you think it is going too, so you must wait...   I had hoped that the bottle conditioning would only take 7 days... Alas, it seems that it will take 10. 

In the meantime I wanted to talk about WHY the seeming obsession with taking the Specific Gravity.

The Short answer is that, without Gravity readings you have no idea what the alcohol content of your beer is...   Not that it may matter to some, I mean come on, beer is beer, right?  But there are other reasons to monitor the Gravity of your Wort that will assist you in decided when to bottle your effervescent nectar.

When you measure the specific gravity of the wort at the beginning, you are measuring the amount of dissolved substances in the water, mainly malt sugar, dextrins, hop resins and amino proteins.  This changes the density of the water which normally has a specific gravity of 1.000 at 60 degrees F (15.5 C).  Thus, using my own Original Gravity reading of 1.058 means that it is .058 denser than water.   Yeah, that doesn't mean a whole heck of a lot, but it will be necessary later.  Trust me.

The next issue is that I did not measure Specific Gravity at 60 degrees, for the wort would have been too cool for me to pitch my yeast.  I took my reading at 75 degrees, so I need to adjust my reading... For every 10 degrees in temperature need to add .0012 to the Gravity reading.  Since I took mine at 75 degrees I need to add .0018.

1.058 + .0018 = 1.0598
I know, it still doesn't really mean anything, at least not until we convert this to degrees Plato... Plato is the measuring unit of the Balling scale.  It allows us to compute the amount of sugars dissolved in every 100 grams of wort as a percentage.  There is a simple calculation for obtaining the degrees Plato.  1 degree of Plato is equal to 4 "brew points".  The brew points I am talking about are the points behind the decimal in your Specific Gravity reading (.0598).  Thus, 1 degree of Plato is equal to (4 x.001).  

.0598 / (4 x .001) =  14.95 degrees Plato

Now you know that every 100 grams of Wort contains 14.95 grams of Sugars (14.95 %).  The reason to do this is if you intend on checking your Malt Extraction and Attenuation calculations, but I am not going to get into those right now.  (you may now breathe a sigh of relief)  LOL

Getting back to Specific Gravity... When racking, you need to measure the specific gravity again.  This is to see if the fermentation is complete.... Most Ales will be finished when the Gravity reaches 1/4 or 1/5 of the original Specific Gravity reading.  Since the Dunkelweizen had an initial reading (Original Gravity or OG) of 1.0598, I was looking for a reading of between 1.014 and 1.011 so I will know that my fermentation is complete.(Expected Gravity or EG)

.0598 / 4 = .014   or   .0598 / 5 = .011

When I racked the Dunkelweizen, the Specific Gravity had only dropped to 1.016 at 70 degrees, thus being 1.0172 (1.016 + .0012 = 1.0172).  I knew the yeast were not finished converting my wort into viable beer and they needed at least another week to finish the job.

Finally, after 1 more week, the beer reached the Expected Gravity of 1.012 at 70 degrees (1.0132) so I knew it was time to bottle.

Now the fun part...   Alcohol has a lower gravity than water, so when you take the subsequent readings, as the number reduces, there is more alcohol in the beer.  When you take your final reading (mine was 1.0132) and compute the difference between the Original (OG) and the Ending (EG) you can calculate alcohol content by weight.

1.0598 - 1.0132 =  .0466 Difference

.0466 X 105  = 4.893% alcohol by Weight

Now since we know that alcohol is lighter than water, we can compute the alcohol by volume as well with a little more multiplication.  1.25 milliliters of alcohol weighs the same as 1 milliliter of water, so our volume content calculation becomes....

4.893 X 1.25 = 6.11% by Volume

I know, I know, I'm talking about brewing beer and suddenly it became a math lesson.  Sorry....

But now you know why taking Specific Gravity readings are crucial.  Not only for computing the alcohol content of your brew, but can assist you in gauging when your beer has completed it's fermentation AND help you compute your ability to extract Malt sugars from Grains.  If you are finding your sugar contents are too low you may need to change your extraction procedures.

Cin Cin!!

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