A commonly accepted optimal range for mash pH is 5.2 - 5.7 with 5.5 being optimal for starch conversion activity but many authors report wort and beer quality benefits if the pH is lowered into the 5.2 - 5.4 range [Kunze, 2007][Narziss, 2005]. Kunze in particular lists the following benefits for a mash pH as low as 5.2. Since it is a good and fairly comprehensive list I cited it here. Some of these benefits listed will be explained in the following sections [Kunze, 2007]:
The enzymatic activity in the mash is increased as all important enzymes get activated. (except for alpha amylase which starts to suffer at a pH below 5.6)
More zinc, an essential yeast nutrient, goes into solution
The extract yield (efficiency) is improved
The protein coagulation and precipitation is improved (improved break formation)
The redox potential is improved which results in a lower susceptibility to oxygen.
The run-off speed is improved
The color increase during the wort boil is reduced
Better trub precipitation and faster pH drop lead to faster fermentation and greater attenuation of the beer.
Lover viscosity improves filterability
The taste of the beer is more rounded, fuller and softer. The beer is crisper, more fresh and shows more character.
The hop bitterness is more pleasant and doesn't linger
The foam is more stable and denser
The color of the beer is lighter
Mash oxidation is reduced since the main culprit, the lipoxigenase enzyme, doesn't work well at low mash pH conditions
Haze stability is improved
Beer digestion is stimulated. This is a positive effect of the lactic acid
Susceptibility to microbial spoilage is reduced through
Lower beer pH: beer spoilage organism don't grow below a pH of 4.4
Higher attenuation