Originally Posted by saxman1036
The amount of sugar hasn't changed, and neither (in the end) has the water. So I'm still confused on how boil-volume affects the OG.
OG is the gravity of your wort going into the fermenter. Changes through boiling and topping off are what gets you to that OG. What you are measuring is the amount of sugar dissolved in a volume of water. If you change either the water or the amount of sugar you change the gravity. Boiling does many things--sanitizing, blowing off unwanted compounds, isomerizing hops, etc. With gravity, boiling changes the ratio of water to sugar. To raise final gravity you can either increase the amount of sugar through mash efficiency or reduce the amount of water by boiling down. For example:
If I brew a batch in which I want an OG of 1.050 in a 5.5 gallon batch, and I start with boiling 7 gallons from my MLT, then I need my preboil gravity to be 1.039. However, what if I miss this number way low? If my efficiency is off in my mash and I only get 1.025 in that 7 gallons then with the same amount of boiling I will only achieve an OG 1.031 (into fermenter). This will be a very weak beer and way off my target. However if I boil it down to 3.5 gallons my gravity of 1.050 is hit and I would then cool and ferment this amount. I would end up with much less beer, but it would be at my gravity. (I would have traded volume for gravity) If I insist on having a 5.5 gallon batch then I must accept the weaker beer (unless I add sugar in one form or another).
Note: you would have to boil much longer which would caramelize more sugars (making it sweeter) and would also require you to alter your hop schedule.
The only other option, after pulling off from the MLT and starting the boil would be to add sugar into the kettle--such as Brewer's Sugar, honey, extract, etc. Be careful about these because they will thin out a beer.
I had one batch come in very low--it was during a parti-gyle experiment--so I added Brewer's sugar to the kettle and raised the OG from 1.023 to 1.030. This will of course produce a thinner beer, but I wanted the gravity.
The issue is the topping off. If your gravity is not high enough you can either remove water and have less of a stronger beer, add sugar to make the larger beer stronger, or continue to top off to get you target volume of a weaker beer.
If you have the kettle and MLT volume for a full batch and a cooker that will boil 7 to 8 gallons reasonably, then give up the topping off and instead boil the full batch.