Take a look:
“Malzbier, Sußbier, Caramelbier and so forth. Their common characteristic is a relatively sweet, hop-bitter-free taste; they vary considerably from one another in strength, they are made sometimes from just malt, sometimes also with the addition of sugar. The name "Malzbier" may be used only if at least 15 kg of malt has been used per hectolitre of beer. The malt which is used for these beers is highly-dried Munich malt with a considerable addition (10%) of caramel malt and Farbmalz or Farbebier or caramel. The best mashing scheme is one that produces little sugar (the Springmaisch method), the hopping rate is low (1/2 - 3/4 pounds per 50 kg of malt), as yeast, low-attenuating flocculating yeasts are deliberately used, the fermentation must not be too warm, all with the purpose of a low attenuation. The lagering is short. Draught beer is lagered cold, bunged and filtered, and at racking it is often given a sugar additon of 2-4 kg per 1 hl. In bottles the beer is usually filled with yeast, i.e. unfiltered, so that a secondary fermentation can take place and sediment can be preserved, if this is present, the bottles are pasteurized at 55-60°.”
Ullmann, Fritz ed. (1928), Bier in Enzyklopädie der technischen Chemie Band 2, pp 378, Urban & Schwarzenberg, Berlin and Vienna. [My translation.]
So it was a combination of a mashing method which deliberately produced few fermentable sugars, a low-attenuating yeast and a cool fermentation temperature.
I can guess what you’re thinking: what the hell is a Springmaisch? I had to look it up myself. Which is slightly embarrassing, as one of the highest-ranking results was from my blog. In my defence, it was something I wrote almost a decade ago. And there are a crazy number of mashing methods.
The einmaisch temperature is 37º C. The mash is then added to boiling hot water. The temperature of the mash is so raised to 70º C. The temperatures between are skipped. This method is used with over-modified malt which saccharifies too quickly."
"Brauerhandbuch" by Karl Hennies, 1937, pages 124-127.
This explains the purpose in this context:
"Mashing method with which the optimal temperatures of the b-Amylase are passed over by adding boiling hot water to the mash; as consequence a low final fermentation degree results."
Berufsschule Main-Spessart website
Fascinating stuff, I’m sure you’ll agree. Though it would be nice if the Ullmann text had been more specific about OG, FG and ABV.
15 kg of malt per hectolitre works out to around 1040º, or 10º Plato. So almost Vollbier.
Note how, despite this being in the Reinheitsgebot for all of Germany era, these beers quite often contained sugar. Which tells me that they must have been top-fermented, otherwise that wouldn’t have been allowed.