Go Figure – The Wort (pre-boil)

I have spent a little time discussing efficiency, what it means, and how to calculate it.  Which is all good stuff, but probably dreadfully boring for many, many brewers.  The goal of this series of ‘Go Figure’ posts is to explain how to consistently predict brewhouse efficiency and subsequently hit target gravities so that the brewer can makes the beer that was designed.  If you are the type of brewer who never measures the wort gravity or really bothers to check the volume in the fermenter, you may want to move on before you head begins to hurt.  However if you are frustrated because you never seem to it the gravity you are looking for, read on.

The first step in targeted your gravity and volume is… determining the target gravity and volume,  because you cannot hit a target if you do not know what it is. If you are working from a recipe from a book or online, the original gravity and a target volume are usually given.  The original gravity is most important; we can scale the recipe to almost any volume that is required.

As far as the required volume is concerned first decide how much wort you want in the fermenter.  Add to that the amount of wort you expect to leave behind in the boil kettle with the hop residue and trub.   The ‘boil kettle loss’ will vary depending on the recipe.  A very hoppy pale ale with lots of hops will likely result in more kettle loss.  Whole hop cones can absorb a fair amount of wort, and the loss associated with those will likely be different than a brew with just pellets.  I don’t have a hard and fast rule, but 1 to 2 liters of wort over and above the targeted fermenter volume is probably a good place to start.

Since 5 gallons is a common homebrew batch size, and 5 gallons is about 19 liters, targeting 20 or 21 liters at the end of boil is a reasonable number. For the example I am about to give, I will use 20 liters because it is a nice round number.  For this example I will assume a  target original gravity of 15 ºP or 1.061 specific gravity.

Calculate the amount of fermentable sugar and water required.  That means converting volume to mass.  20 liters of wort at a specific gravity of 1.061 should weigh 21.22 kg (yes, I am using mass and weight interchangeably.  I am not that pedantic.).  Next, we know 15 ºP means that 15% of that mass is sugar, so:

( 21.22 kg wort * 0.15 % sugar by weight ) = 3.18 kg of sugar.

And of course the water mass is just the difference:

( 21.22 total kg of wort – 3.18 kg sugar )  = 18.04 kg water

We are almost done with our work here.  We know  how much water and sugar need to be in the kettle at the end of boil, it is time to account for the water that will be boiled off.  Again this will vary with your particular setup. I generally boil off 3.5 to 5.0 liters of water per hour depending on the batch size and how vigorous I set the boil.  For this example we will assume a one hour boil and a 4 liter per hour boil off rate.

So, our pre-boil wort should be:

( 20 liters wort + 4 liters water ) = 24 liters

It should weigh:

21.22 kg final mass + 4 kg water = 25.22 kg pre-boil mass

And finally the gravity should be:

 25.22 kg / 24.0 liters = 1.051 specific gravity, about 12.6 ºP.

So, armed with this information we can begin to calculate the amount of grain and water that will be required for the batch.  The next post in this series will discuss grain bill calculations.

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