Hydroxyl apatite precipitation

[abrix]

I have a situation where I am looking at using phosphoric acid to reduce the alkalinity of some pretty hard and alkaline brewing liquor (256 total alkalinity and 410 total hardness, both as CaCO3). A quick run of Mr. Brungard's Bru'n Water spreadsheet gives me the quantities I need to go from ph 7.2 to 5.7 for both strike and sparge water.

And then I make the mistake of turning to Appendix B of Water, where I learn about the high likelihood of precipitating hydroxyl apatite and reducing the Ca in solution to something like 35-ish ppm.

The appendix notes that this would result in a further pH reduction, which I don't see as an immediate issue -- this is for a very pale beer -- unless that pH reduction is really significant.

Are there any other significant consequences to precipitating apatite (in the HLT, pre-mash and pre-sparge)? Could it reduce the effectiveness of a gypsum addition to the mash or boil? Any other potential downstream negative consequences? Or can additional calcium additions to the mash make up for losses as apatite?

Thanks in advance!

 

[ajdelange]

You would need 4.6 mEq/L protons to get this water to pH 5.7. If you got that from phosphoric acid you will be saturated at any level of calcium above 36 mg/L. This I get from using the same spreadsheet that produced the graphs in the book so its hardly surprising that I obtain the same result you got from the chart. If you add 4.6 mEq/L phosphoric acid (85% is 14.14N and 10% is 1.04N WRT pH 5.7) you will get precipitation which
1)removes calcium from the water
2)removes phosphate from the water
3)releases additional hydrogen ions so that the pH will go below 5.7

How much is hard to calculate so I think the best approach would be to take a liter of the water and gradually add 10% phosphoric until you reach pH 5.7. Some apatite will probably precipitate and then redissolve as you add additional acid. Once reaching 5.7 you could measure the alkalinity of the water, subtract the original alkalinity as adjusted to 5.7 and figure out how much total phospo is in solution. Deducting that from the amount you added tells you how much precipitated and how much calcium went with it. This seems like a lot of trouble. It is probably better to use another acid (lactic - which might get you to the point where you can taste it) or, better still, use boiling, lime treatment or dilution with RO to get rid of that alkalinity. Your water is very hard and carbonaceous. People seem to be evolving the rather cavalier attitude that this isn't a problem as all you have to do is add phosphoric acid. Things can be done that way but it adds lot of complexity to the problem if you try to figure out what will actually happen when calcium is present.

 

[abrix]

Is it crazy to use a combination of lactic and phosphoric acid (adding each separately and slowly)? My gut feeling is that (a) I would get some flavor impact from straight lactic acid and (b) a combination of lactic and phosphoric (adding lactic first and then phosphoric) might be an acceptable compromise.

At this point, I can clearly see the appeal of starting with RO water and building it up as needed, especially for paler beers.

 

[ajdelange]

Yes, crazy like a fox. IOW I think that is an OK approach. But remember that you are going to need acid not only to overcome the water's alkalinity but the alkalinity of the malts you are using. Depending on the malt that can be quite a bit and you will wind up with a lot of acid anions (lactic and phosporic) in your beer. If that's OK then no problem.

 

[abrix]

Just wanted to say thank you for your feedback and suggestions!