Brewing Success: Calculate Boil Length for Specific Gravity

[Ohio Homebrew]

Hello. I am not quite sure this is the correct place for this question so I will apologize now if it isn't. As a home brewer I mash the malt with hot water and collect the runoff (called wort) which contains the sugars needed for fermentation. Most recipes call for a boil length of either 60 or 90 minutes with an anticipated specific gravity at the end of the boil. My question is this, if I know:

The volume of wort collected in gallons
The specific gravity of the wort before the boil
The anticipated specific gravity post boil
The temperature at which the wort boils, 212° F
An evaporation rate of 20% per hour

How long must the boil be to reach the anticipated specific gravity? I'm not too worried about humidity, ambient temperature, elevation, or other external factors, but knowing this could make planning a brew day much easier.

Thank You,

Tom Dible

 

[minger]

I'm fairly a novice brewer myself, but being able to nail down the OG is kind of pointless if the sugars aren't all fermented and you have a FG not as you expected. Is that true?

 

[Ohio Homebrew]

I'm fairly a novice brewer myself, but being able to nail down the OG is kind of pointless if the sugars aren't all fermented and you have a FG not as you expected. Is that true?

Since the yeast feed off the sugars in the wort if you miss your OG on the low side you will end up with a beer that is lower in alcohol than the recipe might have originally yielded. The thing you need to remember is that the yeast need enough time to eat as much of the sugar as they can, leaving too much could cause bottle bombs if you are going to bottle condition the beer.

Being that I'm fairly new at all grain brewing another approach I can take is to increase the amount of grain I use in a recipe to make up for lack of efficiency in my process. By adding more grain I am in effect adding more sugar that would require less boil time to reach the target FG. I had not thought of this previously.

 

[FredGarvin]

It looks like all you need to do is use the SG before and after to calculate the amount of water you have in the mixes before and after. That will give you the change in water volume and that will be the amount you need to boil off. Since you know your evaporation rate at boiling, you should have all you need to know.

Remember:
$$SG=\frac{\rho_{whart}}{\rho_{water@60°F}}$$

where
$$\rho = \frac{mass}{volume}$$

 

[Ohio Homebrew]

It looks like all you need to do is use the SG before and after to calculate the amount of water you have in the mixes before and after. That will give you the change in water volume and that will be the amount you need to boil off. Since you know your evaporation rate at boiling, you should have all you need to know.

Remember:
$$SG=\frac{\rho_{whart}}{\rho_{water@60°F}}$$

where
$$\rho = \frac{mass}{volume}$$

What is Pwhart?

 

[FredGarvin]

$$\rho_{wort}$$ is the density of the wort mixture. Horrible spelling on my part. Sorry about that.

 

[Ohio Homebrew]

Given a pre boil volume of 8.5 gallons at a SG of 1.030 and a target SG of 1.052 post boil with 1.63 gallons per hour evaporation, could you give me an example of what the math would look like. I want to make sure I'm doing this correctly and mimicking your answer would help me best learn it.

 

[FredGarvin]

Sure. Do you have the amounts of what goes into the wort?

Give me a few to get it down.

 

[Ohio Homebrew]

On my last batch I collected 8.5 gallons of wort with a specific gravity or 1.030 pre boil, and the post boil specific gravity target was to be 1.052. I was aiming for a post boil volume of 5.5 gallons and I calculated that I lost roughly 1.625 gallons to evaporation during my 60 minute boil. I believe the primary sugar is maltose. I don't know really what else to add