Really negative RA

[chumpsteak]

Hi all,

I've recently received my water report from Ward Labs, and while I thought my water looked pretty good I'm having trouble figuring what all to add. I've done a lot of reading on this forum but I'm still confused about alkalinity in general and why my RA goes negative when I try to figure additions for lighter colored beers.

Here's a EZ water spreadsheet I did for an American Wheat I want to brew this weekend. I did the additions to get the profile I want, but then when I add sauermalz to get the pH right the RA goes way negative.

If someone could take a look at this and provide some feedback I would really appreciate it.

 

[ajdelange]

The definition of RA is
RA = alkalinity - (Calcium_hardness + Magnesium_hardness/2)/3.5

Pure water (DI) has very low alkalinity (2.5 ppm as CaCO3). This if you add enough calcium chloride to increase the water (permanent) hardness to 350 you would have RA of -97.5. This is one of 2 ways to get negative RA. The other is to add enough acid to the water to bring it's pH below 4.3. Alkalinity is defined as the amount of 0.1N acid which must be added to 100 mL of the water to lower its pH to that value (different labs use different values). If the pH is already below 4.3 then acid must be removed to reach 4.3 or, equivalently, base must be added so the alkalinity is negative.

Clearly RA was never intended to be used in this way but in this case gives the right answer. 2% sauermalz with the water you have and the grain bill you propose should give you a pH of close to 5.5.

 

[anicola]

RA needs to be negative to balance the relative alkalinity of the lightly kilned malts

 

[anicola]

I think of negative residual alkalinity as 'residual acidity' which will neutralize the relative alkalinity of the light malts and bring mash pH down to the target range

 

[chumpsteak]

Thanks for the very helpful responses guys. So if i understand correctly then i should always just shoot for keeping the mash pH in the desired range and not worry about the RA number too much?

 

[mabrungard]

I think of negative residual alkalinity as 'residual acidity' which will neutralize the relative alkalinity of the light malts and bring mash pH down to the target range

Uh...no Light malts and all other malts have acidity. They have no alkalinity to speak of. A negative RA just means that there may be little alkalinity left in the wort solution to buffer the mash pH to an appropriate pH. The mash pH may be lower than desired with a negative RA, but that is dependent upon the grist. It could also be OK if the grist is light colored and has little crystal malt.

 

[ajdelange]

Actually malts do have alkalinity. Any time you add acid to something to lower the pH you are overcoming alkalinity - you are supplying protons to some conjugate base. Suppose you have a base malt that exhibits distilled water mash pH of 5.7 and has a titratable acidity of 25 mEq/kg-pH which seems typical. To bring a kg of that malt to pH 4.3 would require 1.4*25 mEq of acid per kg and the malt would, thus, have an alkalinity of 35 mEq/kg if the same definition of alkalinity that we use for water defines the end point. Obviously the alkalinity depends on the pH chosen for the end point of the titration. What is important is that the effect of the buffering capacity of the malt be recognized and you can call it alkalinity or acidity as you choose with alkalinity being minus acidity and conversely. Actual numbers depend on the pH values between which you wish to move pH. If you wanted to mash this example malt at pH 5.5 you would have to add 0.2*25 = 5 mEq per kg acid in order to overcome the alkalinity of the malt. That means that 5 mEq alkalinity has been destroyed or that
-5 mEq acidity has been created.

 

[mabrungard]

AJ is technically correct. I still have to say that in a practical sense, malts do only have acidity when you revise your reference standard from 4.3 to a more indicative reference of pH 7.

But since our definition of alkalinity used in RA is 4.3, he is correct that malt is an alkaline material. But this is not to say that malt does not have acidity. As noted above, adding malt to very low alkalinity water results in a pH drop into the mid 5 range. The malt neutralized some alkalinity from the water.

 

[ajdelange]

It seems a sensible reference pH would be 5.4. Any malt with a DI mash pH of less than that would require that base be added to bring pH up to 5.4 and thus would be considered an acidic malt. Conversely any malt with a DI mash pH of more than 5.4 would be considered an alkaline malt as acid would have to be added to get the pH down to 5.4. This implies that 5.4 is the ideal pH which of course it isn't. Any value between 5.3 and 5.6 could be picked from that POV and this emphasizes the arbitrariness of it. Nevertheless I'm sure we all think of patent malt, for example as acidic. I'm not sure so many would think of Maris Otter, for example, as alkaline though, of course, if Patent is acidic, MO is alkaline.

 

[mabrungard]

Good points!

 

[chumpsteak]

Thanks guys, very helpful information here. You've both given me a better understanding of alkalinity and mash pH. Thanks again for the help.