katzke
Brewhouse efficiency may not be the term you are looking for. I hate the term, as it does not say when in the brew house you are checking. I like to use mash or pre-boil efficiency as it tells if you mucked up the most important part. From there each brew can have a different efficiency as hops suck up wort. Different grain bills can also create more trub.
Actually the Brew House Efficiency is the one truly vital number. It is the measure of exactly how much extract you have in the fermenter at the end of brewing / the potential, or what you could have got if you achieved 100 % extraction and had no losses.
This is a copy of a doc I gave out after a talk at a HUB meeting. It uses the more modern EBC method of calculating efficiency and looks at Plato, was written for a mixed audience of Kit, Extract and Mash brewers.
MHB
Gravity
In brewing everything dissolved in a wort is referred to as extract. Gravity is the term we use to define how much is in solution. Over the years there have been several ways to look at gravity the two most commonly used by brewers today are:-
Specific Gravity (SG) Is simply a comparison between a sample and the the weight of the same volume of water, (working in metric makes this all so easy) we know 1 Litre of water weighs 1 Kg, if a Litre of wort weighed 1.050 Kg we can say it is 1.050 times heavier than water or it has a SG of 1.050.
Plato (P
o) - (there is a good overview in Wikipedia)
Plato is a more refined version of Brix which is an even more refined version of the earlier Balling scale, and is simply a comparison to a known sugar solution. If a wort has the same density as a 10% w/v (10% weight/volume i.e. 100 g in a litre) solution of common white sugar it is referred to as being a 10 Plato (10 P
o) wort.
It's important to note that the density is dependent on temperature and that you must know the temperature at which your equipment is calibrated (in Australia it's mostly 20
o C).
In brewing everything that goes into solution is called "Extract" that includes sugars, proteins, minerals and fatty acids.
Density measures everything in solution; it doesn't tell you anything about what is in solution, just how much.
It's very important to note that water included in anything added to a wort doesn't add to the extract, just the volume.
Given that there are two common scales, I suppose the obvious question is why two and how they are used, but first how they relate to each other.
Unfortunately the exact relationship is fairly complex but across the common brewing range (1.006-1.060 ish) there is a pretty good approximation roughly: -
SG = (P
o * 4)/1000 +1
Or if you want be really precise these equations are good to more than 5 decimal places.
P
o = − 616.868 + 1111.14(SG) − 630.272(SG)
2 + 135.997(SG)
3
SG = (668- (668
2 820(463 P
o)
2)/410
Let's say you are making a fairly simple kit beer with just a 1.7 Kg can and 1 Kg of dextrose
What would the O.G. be? (OG being the term for the gravity at the start of fermentation or Original Gravity)
First up the kit is going to be about 80% solids and 20% water so the kit will add 1.36 Kg of extract to the wort.
Dextrose is roughly 10% water (dextrose monohydrate) so 1 Kg will add 0.9 Kg of extract.
In total you are adding 2.26 Kg, if the wort is made to a standard 22.5 Litres the P
o of the wort is going to be (2.26/22.5) * 100 = 10 P
o
Using the quick formula above the O.G. will be:-
OG = (10 * 4)/1000 +1 = 1.040
Frankly that's piss weak, I want an O.G. of 1.055 so I have to add 0.015 to the gravity. By running the equation in reverse we can work out how much to add.
Or just look at the P
o we want
1.055 = (P
o * 4)/1000 +1 = (0.055* 1000)/4 = 13.75 P
o
We have 10 P
o so we need to add 3.75 P
o (or 3.75% w/v) to 22.5 litres 22.5*0.0375 = 0.84375 Kg of dextrose?
Not quite, remember dextrose is 10% water so to get the right amount of extract we add 0.84375/ (90*100) = 0.9375 Kg of dextrose
Having met just a few brewers I reckon most of them would dump in the whole kilo and be done.
It's fairly clear that a basic knowledge of SG and Plato gives you very good idea of what you will get when you are designing a beer.
If you are a mash brewer it goes a step further and in practice you work backwards from what you want to what you need.
Just taking the simplest possible single malt All Grain (AG) recipe say you want an O.G. of 1.050, you want to make 22.5 L and there will be 2.5
Lleft in the bottom of the kettle with the trub.
All the extract that doesn't end up in the fermenter is lost and is accounted for as part of "Brewhouse Efficiency" (BhE) but is important to highlight the fact that it contains extract to, once you have established what your losses are you can just use the Brewhouse Efficiency.
1.050 SG is about 12.5 P
o so we need 12.5% of 22.5 or 2.8125 Kg of extract into the fermenter.
If our BhE is 70% we need to make a total of
2.8125 /70*100 = 4.0178 Kg of extract
If our malt had a potential yield of 79% we would need
3.65625 /79*100 = 5.0859 Kg of malt
So you will be buying 5.1 Kg won't you?
Plato obviously makes a lot of sense and people often ask why it isn't the only scale we use. Well its great before fermentation, it stops being so sensible after the yeast gets to work.
Alcohol has a lower density than water, obviously much lower than Wort, trying to relate the amount of residual extract, alcohol and water really isn't the job for Plato but it's where SG excels!
