Chapter 10 - What is Different for Brewing Lager Beer?
10.4 Yeast Starters and Diacetyl Rests
There are two other items that are significant in brewing a good lager beer and I will describe them briefly. These are Yeast Pitching and the Diacetyl Rest. Lager brewing is best described in a book of its own and fortunately someone has done just that. See the Recommended Reading section in the appendices for more information.
Because of the cooler temperatures, the yeast is less active at first. The best way to ensure a strong, healthy lager fermentation is to pitch a much larger yeast starter than you would for an ale. Where you would pitch a one quart starter solution of liquid yeast for an ale, you would use a 2 or 3 quart starter for a lager. This is the equivalent of about 1/2 to 3/4 cup of yeast slurry. In addition, the pitching temperature should be the same as the fermentation temperature to prevent thermally shocking the yeast. In other words, you will need to chill the wort down to 45 - 55 °F before pitching the yeast. The yeast starter should also have been brought down to this temperature range while it was fermenting. A good way to do this is to pitch the yeast packet into a pint of wort at 60 °F, let that ferment for a day, cool it 5 degrees to 55°F and add another pint of aerated, cool wort. Let this also ferment for a day, and cool and pitch a third and even fourth time until you have built up 2 quarts or more of yeast starter that is comfortable at 45 -55 °F. I recommend that you pour off the excess liquid and only pitch the slurry to avoid some off-flavors from that much starter beer.
Some brewers pitch their yeast when the wort is warmer and slowly lower the temperature of the whole fermenter gradually over the course of several days until they have reached the optimum temperature for their yeast strain. This method works, and works well, but tends to produce more diacetyl (a buttery-flavored ketone) than the previous method. As the temperature drops the yeast become less active and are less inclined to consume the diacetyl that they initially produced. The result is a buttery/butterscotch flavor in the lager, which is totally out of style. Some amount of diacetyl is considered good in other styles such as dark ales and stouts, but is considered a flaw in lagers. To remove any diacetyl that may be present after primary fermentation, a diacetyl rest may be used. This rest at the end of primary fermentation consists of raising the temperature of the beer to 55-60 °F for 24 - 48 hours before cooling it down for the lagering period. This makes the yeast more active and allows them to eat up the diacetyl before downshifting into lagering mode. Some yeast strains produce less diacetyl than others; a diacetyl rest is needed only if the pitching or fermentation conditions warrant it.