What's more, it's a pretty reliable source.
"Comparison of Lactic Acid ContentsOne observation: I think he's talking about pre-Prohibition. The numbers he gives for English styles are a lot higher than what I see in the Whitbread Gravity Book. Typical values there are 0.05 to 0.10. Of course, by the 1930's most British beers contained a fair percentage of sugar and unmalted grains.
Lager beers and mild English beers contain considerably less acidity than the English stock beers, which on account of some additional acidity developed during the storage period from ferments other than culture yeast.
The amounts of free lactic acid in the different beers, which may be considered characteristic of the beers, are as follows:
Weiss beer, about 0.50 per cent Stock ales and stouts, about 0.20 to 0.30 per cent Mild ales, about 0.15 to 0.18 per cent German lager beers, about 0.10 to 0.15 per cent American lager beers, about 0.05 to 0.10 per cent
The acidity in lager beer is not due to the development of lactic acid bacteria during its production, but to the development of this organism during the growth of the malt and by the action off the yeast on the neutral phosphates during fermentation. Unmalted cereals, which are generally employed during the production of American lager beers, contain no acidity, hence the low quantity of acid in those beers brewed with high percentages of sugar, refined grits, etc."
"Beer from the Expert's Viewpoint" by Arnold Spencer Wahl and Robert Wahl, 1937, page 161.
Robert Wahl was half of Wahl and Henius, whose "American Handy-book of the Brewing, Malting and Auxiliary Trades" is a wonderful resource. Arnold Spencer Wahl was his son.
That the acidity in Stock Ales came from a secondary fermentation, presumably by Brettanomyces, is no surprise. In Lager did it really come from the malt? I know they have something called acid malt in Germany, where the lactic acid bacteria naturally present on the grain is allowed to partially acidify it. Is that what he means?