mabrungard wrote:
Adding phosphoric acid to the mash would be preferable to adding it directly to the water before mashing?
I don't think it matters. To get to a particular mash pH you must 1. Move the water to that pH and 2. Move the grist to that pH. The total amount of H+ required to do that should be the sum of the H+ required by both 1 and 2.
mabrungard wrote:In the case of sparge water acidification, it appears that phosphoric acid may not be an ideal choice if the brewer expects to carry over the same calcium concentration into the wort. That effect would still be minor if I understand your analysis result.
If the water is devoid of calcium the reaction would be (for alkalinity of 100 and acidification to pH 5):
.945H3PO4 + HCO3- --> 0.96CO2 + .04HCO3- + 0.992H2PO4- +0.006HPO4--
(coefficients are millimoles)
Thus (per liter) it takes about 1 mmol of the acid to convert enough bicarb to CO2 to get the pH to 5 but about a mmol of the monobasic phosphate ion remains. This will strip remaining calcium from the mash/wort.
OTOH if the water is also hard to the extent of 100 ppm as CaCO3 we should get something like:
.5Ca++ + HCO3- + (0.3+.01)H3PO4 + 0.1H2O --> .05*Ca10(PO4)6(OH)2 + 0.96CO2 + .04HCO3- +.01H2PO4-
Considerably less phosphate is required here (0.3 to precipitate the calcium and get rid of the alkalinity and the rest to acidfy this now 0 alkalinity water to pH 5) and much less monobasic phosphate ion gets carried over.
So it appears that you must pay the piper one way or the other. In fact, the actuality will lie somewhere between the two extremes because in the case of the example the system will go below saturation well before pH 5 is reached and the mechanism for proton release switches to the first of the two at that point.
mabrungard wrote:The quantity of phosphate in the mash is larger than the quantity of calcium supplied by the water, but I take it that the precipitation reaction cannot go to completion in the mash because the phosphate in the malt is not in an acidic form. Would that be the case?
At mash pH most of the phosphate will be in the monobasic form so the reaction is
10Ca++ + 6H2PO4- + 2H2O --> Ca10(PO4)5(OH)2 + 14H+
as opposed to
10C1++ + 6H3PO4 + 2H2O --> Ca10(PO4)5(OH)2 + 20H+
for the acid. IOW H2PO4- is an acid (it has 2 protons to give) while it is simultaneously a base (it's amphoteric) because it can take up a proton.
mabrungard wrote:The calcium phosphate precipitation is not a huge factor in the overall carryover of calcium into the wort. Looking at Tables 7.5 and 7.6 of Malting and Brewing Science confirms that although there may be a reduction in the calcium concentration, it appears to be minor. Is that in line with your findings?
I don't think we can draw too many conclusions from the table is BHS&Y. If no phosphoric were used in pH adjustment of water or mash then there would be no reduction in calcium levels. This would also be the case if mash pH reduction were effected by the use of other acids. You'll note that both chloride and sulfate levels are higher in the wort and beer than they are in the liquor and this suggests that for ale being described a mixture of sufuric and hydrochloric acids may have been employed. This is consistent with my understanding of commercial brewing practice in the UK. CRS (Carbonate Reducing Solution) is a blend of sufuric and hydrochloric acids. In addition to this remember that malt itself contains about 0.14% Ca w/w.