[richardt]

Enzymes are proteins that help catalyze chemical reactions (e.g., breaking down long chain starches and sugars into smaller units) and each enzyme has different temperature and pH optima for ideal function.

Most of us are well-versed in the effects of mash temperature on mash enzyme activity:
Elevated mash temps (and adequate exposure times) can irreversibly denature enzymes (e.g., 170F x 20 min).
Very low mash temps means sub-optimal, or no, conversion.

I was wondering, though:  What are the effect(s) of an excessively high (or low) mash pH?
Are those effects exposure time-dependent? 
Are the effects "irreversible?"

[denny]

This will answer some of those questions.  From How to Brew...

[richardt]

Yes, I've seen that figure before. 
Still wondering if is possible to have deleterious effects on enzymes (or the wort, or the beer) if the mash pH is too high or too low for too long.  Sometimes I forget to check right after stirring.

[hopfenundmalz]

Saw a talk given by a pro on mashing.  He had a graph that showed the efficiency dropping like a rock at 5.8 ph and above.  On the Palmer graph that Denny posted, you see that you get out of the range where the enzymes work, and not much happens to give you sugars.

[tschmidlin]

Saw a talk given by a pro on mashing.  He had a graph that showed the efficiency dropping like a rock at 5.8 ph and above.

This is interesting.  Changes in pH will affect the protonation states in the amino acids in the enzyme, which will change the structure.  I would guess that there is a catalytically important residue in the active site that changes protonation state at pH ~5.8.

pH effects should be mostly reversible, although less so at the extreme ranges of the pH scale.  Strong acids can denature proteins and cause aggregation, and much faster than a temp a few degrees too high.

If the effect is due to changes in protonation, that would happen on a very rapid time scale.  How fast depends on a number of things, but a ms time scale (or faster) is not unheard of.

[a10t2]

Edit: Tom beat me to it.

[hopfenundmalz]

To say more about the aforementioned talk.  We worked as stewards at the WBC in Chicago in 2010.  We got to attend one talk of our choosing at the CBC as a "reward".  The talk was by the brewer from Trumer in Berkley. I remember the graph, and had sketched it in some notes that I no longer have.  Think that he was stressing the importance of pH to the mash, if it is detrimental to be at higher pH for a short period, can't remember that.

[tschmidlin]

Think that he was stressing the importance of pH to the mash, if it is detrimental to be at higher pH for a short period, can't remember that.

It really shouldn't be, but it needs to be at pH and temp for enough time for conversion, so if you aren't sure how long your pH was off you should mash longer.