Regarding sharpness and twang like Guinness has with their dry stout. Guinness adds an independently soured portion of beer to their finished beer to achieve that flavor. They don't allow their mash pH to drop out of the typically cited range.
There is a rumor to that effect but I can't vouch for its veracity. I question it because I get those effects without blending stale or soured beer.
Unlike AJ, I strongly recommend that anyone using large proportions of RO or distilled water or other really low alkalinity waters to brew any darker styles will have to utliize an alkali such as lime to produce a desirable mash pH.
I have no problem with that recommendation as long as the need for alkali is verified. In my brewing of stouts additional alkali is not needed nor desirable as mash pH would go too high. Others here and in other fora report similar results.
So as AJ mentions, you can produce a stout with straight RO. But, I can assure you that it will pale in flavor and judging comparison to a stout that is brewed with the appropriate level of alkalinity to keep the mash pH from falling to the low end of the mash pH scale. If I brew a stout with my RO water and do not add lime, the pH falls below 5.2. AJ, I'm surprised that you produce a 5.5 with yuur stout brewing (or did I misunderstand?).
I brew stout with water with RA of about 38. With a grist of Maris Otter base, 10% roast barley and a little flaked barley mash pH comes in at 5.55 or so. I'd like it even lower but generally accept 5.55 as OK. As noted others have reported similar results. In experiments with DI water it took 30% roast barley to get a pH of 5.2 i.e. three times the amount which IMO gives a balanced beer. So no, you did not misunderstand.
In my opinion, a desirable mashing pH for a beer that you want to be smooth and full is about 5.4 to 5.5. If you want a sharper and tarter beer, then 5.3 to 5.4 is good.
I agree that 5.4 is a good point to target but to be honest I have never noticed that higher pH beers are fuller and lower pH ones thinner or tarter (until you get really low). That seems to be determined by the mashing temperature and grist composition more than the mash pH. The lagers tend to have higher pH's and are plenty smooth, of course, but that's because of the lagering. They are plenty rough at the outset. OTOH, an ale which tends to have lower pH will be, if allowed to condition long enough, just as smooth. With the stouts the dry, thin body, tart...(this beer probably comes closer to its model than any other I brew) ... flavors come through despite failure to add alkali, souring beer or acid (which is why I'm a bit skeptical about the second brewery behind St. James Gate story). The stout finishes at pH ~ 4.45 (and most of my Pils near 4.47 for comparison). The stout is tart and thin, the Pils rich and sweet despite the beers having very close pH. The Pils is mashed with a lower starting pH than the stout.
Allowing the mash pH to drop below 5.2 produces poor beer for most styles.
Can't say as I've never brewed one with a mash pH that low. What I have noticed is that the lower I go the "brighter" the flavors and conversely. I agree there has to be a limit beyond which quality would drop but I haven't hit that yet.
As it seems Martin's experiences are quite distinct from mine that would suggest that he is using grists with much more acid in them than I am. As someone reading this would have little way of knowing which camp he falls into it only seems sensible to do the confirmatory pH reading. I can say with certainty that if you follow Ashton Lewis's recipe for Irish stout in Michael Lewis's monograph in the BP series you will not bust pH 5.2 (or probably even 5.5) with low RA water. Anything else, you'd better check mash pH.
