I've yet to hear someone explain target residual alkalinity, but would really love someon to point me to that comment.
The pH of a mash is the pH at which the sum of the proton deficits of all the mash components equals 0. This simply means that pH shifts until the protons given up by acidic components are all absorbed by the basic components. There is one exception. When calcium in the water reacts with phosphate in the malt we get
10Ca++ + 6H2PO4- + 2H2O ---> Ca10(PO4)6(OH)2 + 14 H+
Thus, apparently, each 20/14 equivalent of calcium should supply 1 equivalent of protons and neutralized 1 equivalent of alkalinity. This would only be true were all the calcium to react and it doesn't. In fact it takes more than 3.5 mEq of calcium to produce enough protons to neutralize 1 mEq of alkalinity. We are speaking up to this point of only up to the mash. The calcium/phosphate reaction continue is the sparge and boil such that Kolbach observed empirically that 3.5 mEq could cancel 1 mEq as measured at knockout. In any event, RA is defined as
RA = alk - [Ca++]/3.5 - [Mg++]/7
Forgetting the Mg term this says that each mEq of calcium produces 1/3.5 mEq of protons which are absorbed by the alkalinity components (bicarbonate) in the water and thus effectively neutralize some of the alkalinity. If alkalinity is > [Ca++]/3.5 then clearly it won't all be neutralized and if alkalinity is < [Ca++]/3.5 there will be extra protons available to 'neutralize' something else and RA will be a negative number. In any case RA is the
proton deficit of the water and thus an important part of the picture. The problem arises when spreadsheet and calculator authors try to combine the proton deficit of the water with the proton surfeits of, for example, added acids. There is really conceptually no reason why this can't be done if the bookkeeping is kept straight but as what has been set forth here isn't always understood and not made clear if it is this practice leads to trouble, misunderstanding and confusion.
Given a set of malts and a desired mash pH then there is indeed a 'target RA' but it needs to be properly and clearly defined. If we define it as the proton deficit of the water plus the proton deficit of any added acid or alkali we add (it is usually something like this that appears to be what the authors are shooting for) then the target RA is simply equal to and opposite in sign to the sum of the proton deficits of the other mash components (malts). So presented it doesn't seem that this would be very helpful to the brewer. I prefer to display the proton deficits of alkalinity, added salts, acids, bases and malts separately so that the brewer can see the effects of tweaking any one of these. It would be a simple matter for me to sum the deficits attributable to alkalinity and the deficit from the calcium phosphate reaction (a negative number as the reaction produces protons) but I don't see any advantage to that.
RA was originally cooked up as a way of comparing water supplies and for that it is good. Adding acid to mash until a spreadsheet reads a particular RA leaves something to be desired.
I know all this is probably confusing as the terminology is not familiar. It appears that the paper I wrote for MBAA TQ is finally going to be published (over a year since I presented it) and the early concepts are in the Palmer book so perhaps more brewers will become acquainted with it.
