Many homebrewers just leave hitting out target starting gravity (OG) to chance. Essentially, we just add our ingredients and boil for 60 minutes and hope for the best. I like to be a little more in control, so here is what I do.
First off, I like to take my first gravity reading right after I collect my mash run off (Actually, as a batch sparger I like to check the gravity of both run-offs just to check the progress of the spraging steps). I read my gravity and I determine the total points of extract I have in the pot by multiplying by volume; i.e., 6 gallons of 1.040 SG is 240 points of extract. Determining the points at this stage also gives me the most accurate measurement of mash efficiency. I don’t care much for “brew-house efficiency” because it’s several steps downstream of the mash and the mash is where most of your starting gravity shortfalls potentiate from.
Now that I know how many points I have total, I start my boil. I know in general, that I like to boil for 90 minutes for most beers (especially when pilsner malt is involved). After the first thirty minutes, I add my 60 minute hops. After another 30 minutes I check the gravity to see where the boil has taken me so far with concentrating the wort towards my desired OG. This is where the art of brewing comes in. Normally, it’s time to add the 30 minute hops, but if the current gravity indicated that I’ll need to boil off more than another 30 minutes worth of water (knowing your evaporation rate is critical here) then I’ll hold off for a bit before adding the flavoring hops. Using a refractometer is very handy here because you only need a few drops rather than a whole cylinder full for a hydrometer reading. The few drops cools very quickly too so there is no need for temperature compensation.
I then boil until I am almost right at my desired OG. Then I add aroma hops for on more minute (which doesn’t drastically result in more evaporation).
Sometimes my boils are longer than 90 minutes, sometimes they are less than 90 minutes. This has to do in part with how the evaporation rate is going and how much sugar I had in the wort to start with post mash (see above regarding mash efficiency).
Lastly, it’s not a sin to add a little water to a wort that boils off to fast for you, but does not give you enough time for your bittering hops. If your boil off goes too fast and you over-concentrate, the wort a little water in the brew kettle will get you back to the desired OG… Of course, you should calculate how much water that is.
In short, I like to monitor the gravity of my wort as it boils and I plan my hop additions and boil times around that evaporation rate and the desired OG. I find it a bit more comforting than boiling the wort for a set time and then waiting to see how close to what I wanted when I transfer to the fermentor.
Are there other strategies for hitting OG???