I will be interested to hear if the pH meter was flaky or if the readings seem to be accurate. As I've pointed out to AJ and others, there is some buffering system in wort that tends to move wort pH closer to a pH of around 5.4 during the mash. In other words, if you have acidified your mash to less than 5.4 early in the mash, the pH tends to rise toward 5.4 during the mash. The same thing tends to happen when the initial pH is above 5.4 and the pH drops toward 5.4 during the mash.
He's talking wort here. I haven't titrated any wort but have no reason to expect it to behave differently from mash. Mash does not buffer in the usual sense of the word. Every mash has its pH and it resists change in pH just as any mix of acids and bases does but when we say something buffers to a particular pH we usually mean that it has higher buffering capacity at that pH than at other pH's. As no malt curve that I or anyone else (that I know of) has measured exhibits that kind of buffering I wouldn't expect a mixture of malts to show it. To illustrate lets consider a mash made up of 84% base malt (Pils), 15% crystal (80L) and enough sauermalz to bring the mash pH to 5.4 when mashed with 2.8 L/kg water with alkalinity 1 mEq/L). The heavy blue curve in the picture below shows the proton deficit for this mash as a function of pH. As the mash pH is 5.4 the proton deficit at pH 5.4 is 0. Note that the curve is steep at pH below 5.4 and less so above. The steepness is the buffering capacity (how many mEq of acid it takes to change 1 pH) which is plotted as the dashed blue curve (right axis) This curve shows that at pH 5.4 it would take about 180*0.1 mEq of acid to change the pH by 0.1 unit whereas at pH 5 it would take about 0.1*270. IOW the buffering is better at 5 than it is at 5.4. Thus we wouldn't say this mash buffers to 5.4.
To further illustrate the plot also shows (red) what a titration curve would have to look like in a mash that we would say buffers to 5.4. The 'mash' here is a mix of a hypothetical triprotic acid with one of the pK's = 5.4 (a buffer exhibits best buffering near one of its pK's) and enough strong base such that the mixture has the same pH (5.4) as the mash and the same buffering capacity as the mash at 5.4). Note that the differences are pretty subtle. All the chemists will see right away is that the difference is that the red curve contains an inflection point at pH = 5.4. This inflection point is responsible for the humped nature of the buffering capacity curve (red dashed line). This mix exhibits peak buffering at pH 5.4.
The phenomena driving this is not known to me yet, but it has happened enough in practice by me and numerous other brewers that it is apparent that is typical. This is a reason that I caution brewers from over-reacting when they measure an early mash pH off of their target and wanting to add more acid or alkali.
I put the bolds in the quotes because this phrase is the clue which explains the phenomenon. We design our beers for a pH near 5.4 and very often use acid to get to it as the base malts have larger proton deficits than even the colored malts can supply. We grind up the malts, add acid to the water and dump the acidified water into the mash tun. We then grab a sample and check the pH. What have we got? Acid water and basic grain that haven't completely reacted yet. If we use sauermalz we have a similar situation. The lactic acid on sauermalz is dry. As soon as we add water it dissolves in much less time than it takes the acids to reach and react with the bases in the base malts. With time the liquids penetrate the grain particles, the starch matrices burst, the acids transfer their protons and the pH rises. In cases where we must add base to the mash the same phenomenon is at work except that here the protons buried in the (dark) malt particles are not at first accessible and the the pH stays high until they find their way into the solution.
So the phenomenon is due to temporal distribution of proton donor and accepter sites rather than the tendency of mash to act like a buffer at any particular pH. This was made very plain when I did the hundreds of malt titrations necessary to develop the malt models that went into the Palmer/Kaminski book.
Nevertheless...
Wait until the mash is complete and assess the pH then. At that point, it might be OK to adjust the kettle wort pH closer to their target.
While Martin seems a little mixed up on mash and kettle here what he is trying to say I'm sure is that one should not over react to a low pH reading early in the mash. The first couple of times I used sauermalz I about jumped out of my skin from this. My readings were so low I thought I'd better check them and then noticed they were rising. In doing titrations to characterize malt I find that 15 minutes is kind of a minimum and that after 25 - 30 minutes one doesn't see much change.
In any case, OP is talking the kettle here. We don't have to wait for added acids or bases to access the interior of grains in the kettle so the addition of an acid or base would allow quick reaction, given that the basic kinematics of the reaction are fast. You're probably thinking I'm thinking of calcium carbonate and I am. Adding sodium bicarbonate, lime or lye should push the pH right up. If it is really too low. My money is still on a bad pH reading.
