I try to make a habit of not posting someone else's work but Jorge at Brew Beer and Drink it did such an awesome job of explaining Brewing Efficiency that I had to do the ole cut-&-paste so I could come back and review this when I needed to and so anyone else checking out this site could benefit as well. Hopefully you will find this as helpful as I did.
Jorge posts some excellent stuff and I hope you will help support his efforts at http://brewbeeranddrinkit.com/go/more-beer.html
From Jorge:
First off, if you are brewing extract, it is almost impossible to miss your OG (Original Gravity)... If you add six pounds of sugar to five gallons of water, you have exactly six pounds of sugar mixed in with your water!....
The only way your hydrometer can be wrong is if you spill wort or you have anything other than five gallons of water... or if you scooped up a sample of your wort when your sugar (DME/LME) is not mixed in well...
But you still have 6 pounds of sugar (malt extract) in water! I don't care what the hydrometer says... that's with extract....
Now efficiency is a bit harder... deals more when brewing with grains, but this should help explain malt extract above...
So starting from the top... we are adding sugar to water to make our wort... pure sugar (sucrose) will give you 46 points of sugar if you add one pound to one gallon of water... that reads 1.046 in the hydrometer...
When you look at a grain's typical analysis sheet, you will see a percentage number under "Extract CG/Dry Basis" (CG = Coarse Grind)... for 2-row it's about 80%... That means that if you were to extract every single possible grain of sugar out of it... and I mean to the last drop, you would get 80% of 46 points per pound... 80% times 46 equals 36.8...
That means you would get a reading of 1.037 (rounded up)... again, that's if you get EVERYTHING out of it... if you have 100% efficiency...
Most brewers will get around 75% efficiency... that means they will get 75% of the 36.8 possible points they can get if they were to be 100% efficient at extracting all sugar from one pound of grains...
At 75% efficiency you get: 75% times 36.8 equals 27.6 points... 27.6 points using one pound of grains in one gallon...
That's all that efficiency is...
Say you brew using 10 pounds of grain... at 75% efficency you will extract a total of 276 points of sugar (27.6 as described above times 10 pounds of grain)... in five gallons that would read: 276 divided by 5 gallons equals to 55.2 which the hydrometer reads as 1.055...
So the point is, you gotta think of how many points of sugar TOTAL you have in your water... when you use DME you get about 42 points per pound, which is why I said at the beginning that you can't screw it up... if you add 6 pounds of DME you will have 42 points of sugar times 6 pounds equals 252 points of sugar total... if you divided that in five gallons you will get 50.4 per gallon or a gravity reading of 1.050...
Efficiency is good for planning... I base my recipes on 75% efficiency... but you gotta know how many points of sugar total you need in case something goes wrong...
And let me tell you about my brew gone wrong...
I wanted to brew a beer with OG of ~1.052... that's 52 points of sugar per gallon of water... so if I do the math backwards... Since it's a 5 gallon batch, I know I need 5 gallons times 52 points of sugar equals to 260 points of sugar total... that's 260 points of sugar total not divided into anything... now I collected about 6.5 gallons of wort which means I should have a gravity reading of 1.040... to get that I do divide the total points of sugar by the gallons they are mixed in (260/6.5 = 40)...
Now when I took a sample (after mixing everything well) I got a reading of 1.033 or 33 points per gallon... so I did the math again and realized that 33 times 6.5 gallons equals to 218 points of sugar total... meaning I was about 42 points of sugar below my target of 260 points I needed...
So what did I do...
Well I could have mashed more grains, but that would take too long and since I'm human, lazy and like the path of least resistance, I figured that one pound of DME would give me exactly those 42 points I needed to hit my OG and go on with my brew day... so that's what I did... I added one pound of DME to the boil, brought my OG to where it needed to be and moved on...
That is a real life example of how you go about using math and this whole efficiency topic... and the moral of the story is, keep malt extract handy when you brew all grain in just in case!
Cheers!
