What is the tipping point of Ca and Mg precipitation in the mash?

[bitteritdown]

How many mg of Ca and Mg are bound and precipitate out in the mash given a malt makeup of up to 1% phosphates (and I'm sure they bind with other ions also)?

Is it an appreciable number such that when adding calcium / magnesium to the mash one could say "I need an extra .5mg/L of Ca and .25mg/L of Mg because that's how much will be bound."?

I'm guessing Fig. 5 and 6 in this paper hold some answers:

http://www.wetnewf.org/pdfs/Brewing_articles/Cerevesia/Final_galley

 

[ajdelange]

Sorry that it's taken me so long to get to this one. We could start with Kolbach's observation that 3.5 mEq of calcium are required to neutralize 1 mEq of alkalinity. But we also know that the removal mechanism is
10Ca++ + 6H2(PO4)- + 2H2O --> Ca10(PO4)6(OH)2 + 14H+
which implies that 14 mEq of alkalinity are removed by 20 mEq of Ca++ and the calcium loss is thus 20/14= 1.48 mEq of calcium per mEq alkalinity reduced. At the same time we note that Kolbach's observation was for knockout so that probably twice as much calcium is required to get the same alkalinity cancellation in the mash tun. We also have to note that there are other reactions that grab calcium and release protons.

The Cerevesia paper tried to put numbers into the 10 simultaneous equations that govern the reaction given above to see if answers anywhere near consistent with what we've seen was possible under reasonable assumptions about phosphate availability in mash. As I recall the conclusion was that yes, solutions that were reasonable were attainable. It's been quite a while since I wrote that paper however but perhaps there are some answers for you buried in it - if you are willing to tackle 10 equations.

 

[bitteritdown]

Thanks AJ, very interesting paper btw. So at least 29.6 ppm Ca++ is removed (due to phytin reaction?) per 50.05005 ppm alkalinity reduced but taking into account Kolbach's observation we then have 70 ppm Ca++ removed per 50.05005 ppm alkalinity reduced and of course that's still not enough because of other reactions. I'll reference Malts and Malting, Briggs, et. al. to determine the quantity malt brings with it.