"Racking, or the filling of the beer into barrels from the fermenting vessels happens after the completion of fermentation. The duration of this is very uneven: in particular Winterbier ferments faster than higher-gravity Sommerbier, likewise, the fermentation finishes more quickly when the temperature of the external air or fermenting room is higher than under the opposite conditions. The usual duration is 5 to 6 days for Winterbier, 8 days for Sommerbier. In the case of the latter, as slow a fermentation as possible is desirable because this gives it a longer shelf life, therefore, in Bavaria, it is brewed only in the coldest months. Brewers have been taught by many years of practical experience to recognise when a particular fermentation is complete. Some have, however, to be more certain, the habit of taking a glass of beer from the fermentation vessel and observing it carefully, and when the liquid therein is completely clear and the fermentation appears complete, they assume that this is also the case of the beer in the vats and therefore the beer could be put into barrels.
Schenk or Winterbier is put into small barrels, holding from 1 to 4 Eimer, Lagerbier or Sommerbier into large barrels holding 20 to 40 Eimer. In the former, secondary, barrel or small fermentation begins immediately, with the latter a bit later. The quicker start of this fermentation in the case of Winterbiers is due to the fact that this is put into barrels, as they say, greener. The bungs are only slightly closed, so that the foam raised by the secondary fermentation can escape. After 8 days, the secondary fermentation of Winterbier is in the main complete, and already perfectly clear; now the bungs are tightened, however the beer still needs to lie for four weeks before being served. Winterbiers intended for later consumption, which have to be lagered longer than one month, the bungs are still not closed tightly at the end of 8 days, but are tightened only 8 days before tapping each individual barrel.
http://4.bp.blogspot.com/_CHrKKDU929...kbier_1944.jpgSommerbier is usually put into barrels by dividing a few brews evenly into several barrels, not filling them up completely and only closing the bungs much later, often after a couple months ago when a white foam has formed on the surface of the beer, after they have been previously filled to within a few inches of the top. The quicker or more firmly the bungs are closed, the faster and more powerful the secondary fermentation, but also the less time such a beer can be kept. The delivery of Sommerbiere, which in Bavaria by law cannot be served before the 1st May, from the Lagerkellers happens only at the early morning at 4 or 5 clock, so no warm air can get into them. In large Sommerkellers the individual sections are often, after being filled, bricked up in order to keep them properly cool."
"Deutsche allgemeine Zeitschrift für die technischen Gewerbe, Volume 2", page 86. (My translation)
