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My handy-dandy "Beer Craft" book has a handful of recipes with useful information, including expected targeted pre-boil gravity, original gravity, and final gravity. Their pale ale recipe, for example, calls out a pre-boil gravity of 1.035, original gravity of 1.055, and final of 1.012.

What I'm wondering is... why does gravity increase from pre-boil to original?

There's two things that happen here; you boil off some liquid, and you add hops. You already have all your sugars from the malt. My first thought was that it might be because you're increasing the density of sugars by losing water in the boil, but the ratio of liquid retention changes depending on batch size; I expect to lose about a gallon in an hour, regardless of whether I start with 2 gallons or 6; but the gravities stay the same. My next thought was that the hops must affect the density, but what we keep from the hops after the boil doesn't go away as the beer ferments... or does it?

What's the science behind this?

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The recipe is making some assumptions about the rate of boil-off, which may or may not be accurate for your setup. My system boils off around 1 gallon of water each hour. If I'm boiling 6 gallons for one hour this equates to a 1/6 = 17% increase in gravity. If I boil 2 gallons for one hour, it's a 1/2 = 50% increase in gravity. –  Tobias Patton Jul 18 '13 at 20:03

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The liquid boils off but the sugars remain. Since specific gravity is a measure of density, the wort becomes more dense when there's less liquid but the amount of sugar remains the same.

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Asking the same thing I asked of Tobias -- I should expect the gravity numbers to be different if I move from a 1 gallon batch to 5 gallons, then? –  object88 Jul 18 '13 at 20:46
    
Not if you account for boiloff correctly. assuming you mean the gravity numbers and not the boiloff numbers. –  Denny Conn Jul 18 '13 at 21:33
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OK, I just made a realization by playing with Beer Recipe Calculator. When you scale, the original gravity remains constant, but the preboil gravity DOES change. I was missing that important bit, and it aligns with boil-off. –  object88 Jul 18 '13 at 22:22
    
Good on ya! Glad you realized that. –  Denny Conn Jul 18 '13 at 22:42
    
An accepted answer without an upvote? –  Denny Conn Jul 18 '13 at 23:12

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