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Old 09-16-2008, 08:26 PM   #1
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Default Monitoring mash progress with a Refractometer

I messed around a bunch this past winter on my lagers doing step mashes that included a post saccharification rest at 158 F to get some more limit dextrins to improve mouthfeel. I think I was fairly succesful, except in one batch were I got distracted and held the Sacch. rest too long so I think the wort was too converted by the time I took it to 158 F so the final step did nothing. So now I want to use my refractometer to monitor conversion so I can improve on my technique to get more consistent results.

Many people do an Iodine test for full conversion, I'd like to use my refractometer to do that - and then some. Basically I want to be able to whip out my refractometer and check the gravity so I can do my temp. step up at say 85% percent conversion. I'm talking mash efficiency, not brewhouse efficiency. I'm assuming I can get to "full" conversion (100%) - that is based on the numbers given in the malt analysis. Typically I see 80% conversion as a pretty common value as determined by the malster. This is then used to determine the SG potential of the grains, for example, pale malt at 1.039 gravity points per pound per gallon (100% is really 80%, if 100% = 100% the the PPG would be even higher)

I want to talk out loud here to make sure I've got things right. Palmer's book didn't quite spell it out enough. So in my mash, say I have 6 lbs in 3 gals (for math ease) what should by SG be at full conversion? (before sparging). Simple math would say 1.068 (2 X 1.039), but that doesn't account for water absorbed by the grain. If water is absorbed by the grain at ~ 1.25 qts/lb, that would be 1.2 gal. lost to absorption. If we exclude this, then the SG at full conversion would be 1.130!!! which seems way to high. So I'm thinking I should include all or most of the strike water in the calculation as even though it is "absorbed" by the grain it is still available to the rest of the wort as far as gravity is concerned.

Does anyone have a better figure for water absorption to use (for this purpose) or have any other recommendations?

I've got ProMash but I don't see a calculator to use to address this question. It is possible I've overlooked it, or there is some trick I'm unaware of.

I could take readings every 5 min. starting at time zero, and then after a few data points I could with some refreshing of my memory on enzyme kinetics I could calculate when to do my temp. step. I really don't want to have to open up and stir my mash every 5 min. to get a uniform sample. Then of course every grain bill is different so I'd have to repeat this each time I do a different recipe, and then accounting for older grain, yada, yada, yada.

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Old 09-16-2008, 08:46 PM   #2
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What do you mash in? I know I'd lose a lot of heat opening my tun every 5 minutes. I lose about 3 degrees over 60 minutes as it is...

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Old 09-16-2008, 08:54 PM   #3
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I've got a direct fired mash tun so I can add heat as needed.

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Old 09-16-2008, 08:55 PM   #4
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I'm having trouble wrapping my brain around why you would need to discount wort absorbed into the grain. The gravity of the wort in the tun, whether freely flowing around or locked inside the grist, is of the same gravity due to dispersion.

Even simpler, consider 1 pound of maris (1.037PPG) in one gallon strike (I know too loose, but nevermind). If you get full conversion, you should read 1.037 in that sample because it's a concentration measurement that cares nothing about how much will flow out during separation. Sure, if you drain that and get approx .8 gallons out, it will still read 1.037 but now you have 29 PPG because the remaining 8 points is locked in the grain which is why sparging is good stuff.

Help me understand if I'm wrong.

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Old 09-16-2008, 09:30 PM   #5
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Well you certainly can't just ignore the amount of water absorbed by the grain - that would be essentially saying that ALL of the sugar went into the free wort, and NONE was left behind in the grains along with the water they absorbed. 100% efficiency without even sparging, wouldn't that be nice

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Originally Posted by Bobby_M View Post
The gravity of the wort in the tun, whether freely flowing around or locked inside the grist, is of the same gravity due to dispersion.
I'm sure you meant diffusion

I think you're right though. There'll probably be a little bit of a difference because of the finite amount of time allowed for diffusion to occur, but still, I doubt it would be at all significant after an hour-or-so mash.

And just for a sanity check, think of it this way: forget for a moment that the sugar in the wort actually came from the grain in the first place. If you had a bucket of wort, and you dropped a sponge into it, and then drained off the free wort leaving the wort-soaked sponge, the gravity of the wort wouldn't have changed, you'd just have less of it.

And as long as diffusion has had time to take place, as bobby said, it should be essentially the same deal for the water absorption by the grain.
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Old 09-16-2008, 09:48 PM   #6
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Right so the absorbed wort is equivalent to a potential loss of gravity overall. If I have five dimes and remove one I still only have $.40 to contribute. They are each worth $.10.

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Old 09-17-2008, 02:30 PM   #7
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Maybe I'm being a little too much of a science geek. I understand that much of the water absorbed by the grain is still available and in equilibrium with the liquid portion. I did say much, not all though, and here is the geeky part. Some of the water is used to hydrate proteins, cellulose etc. This water is actually tightly bound to these molecules and is not readily available to equilibrate with the rest of the wort. I suspect it is not much though and could be ignored. I've got a few more ales planned with simple mashes so I'll monitor these and see what I get. Part of my problem was I'm using up some older grain and it is not converting real well so it hasn't been the best test material.

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Secondary:
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Brewing soon: Saison
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