I'm of the opinion that before home brewers (and the magazine and book industry that evolved from catering to it) got it wrong and then morphed into an explosion of micro-brewers (who merely continue to parrot the trend...) the nominal 'ideal' target of 5.4 mash pH was intended to be 5.4 as measured at mash temperature, which would be roughly 5.6 pH if measured at room temperature. And I'm also of the opinion that a careful read of peer reviewed brewing industry documents from yore (meaning before home brewing was legalized, and soon after sprouted endless micro-breweries) will support my opinion.
I think this is really interesting. In my home and commercial brewing experience and formal brewing education, the optimum has always been 5.2-5.6 as measured at room temp. This is the first I've heard that this pH range is intended to be at mash temp, not room temp. Based on this, I should be targeting a pH of ~5.6 at room temp. (And it's not really an opinion if it's adequately supported in the literature, "adequately" being the operative word, but I digress.)
Silver, if you have access or links to any of those peer-reviewed docs/studies of yore, are you able to share? I did a brief Scholar search but then moved on because my attention span is measured in squirrels.