Brewing a true lager takes a little bit more work and specialized equipment than brewing an ale. The most important factors in brewing a good lager outside of recipe formulation are: yeast strain selection, pitch rate, and fermentation temperature.
Yeast Strain Selection:
Once the recipe has been chosen, the next task that will have the biggest impact on the final beer is yeast strain choice. Home brewers have a wide selection of lager strains to choose from. The strain should be chosen based on the style of beer, the fermentation temperature, original gravity, and time available for conditioning or lagering.
Lagers typically have a reduced ester profile and are characterized as clean with discernable malt character. It is very important to recognize that pitch rate is directly related to ester production. Increasing the quantity of yeast pitched is the most effective method of reducing the ester profile in the finished beer. A minimum of 12 million cells per milliliter is recommended to keep esters at a minimum.
One Wyeast Activator pack will deliver about 6 million cells per milliliter to 5 gallons of wort. In order to increase this rate to 12 million cells per milliliter it is necessary to either pitch two Activators or to make a 0.5 gallon (2 liter) starter with an Activator.
One of the most common questions we field is, “should I start my lager warm or cold?” The answer depends on how much time you have for your primary fermentation and how clean you want your final beer. The best results will be achieved by pitching at least 12 million cells per milliliter into cold and well aerated wort (48 to 58°F, 9 to 15°C).
If a faster primary fermentation is desired or you are pitching less yeast, then it is best to start a little bit warmer and then cool to the desired fermentation temperature once signs of fermentation are evident.
A secondary fermentation at a temperature below the primary fermentation allows for a slow reduction of any remaining fermentable sugars. This secondary fermentation can take from one to three weeks at temperatures starting between 39-41°F (4-5°C) and slowly falling to as low as 33°F (1°C). The length of the secondary depends on the amount of fermentable sugars remaining.
Conditioning or Lagering:
Lagering is a time when harsh flavors from fermentation are mellowed. Yeast re-absorb some of the ester compounds from fermentation as well as some of the sulfur compounds. Malt tannins coagulate with haze-forming proteins and precipitate out along with some sulfurous compounds.
Temperatures should remain very stable during lagering, generally in the range of 33-34°F (1-2°C). Contact with oxygen at this point is very detrimental to beer flavor and should be avoided at all costs. Lagering time depends on many factors. If a cold secondary fermentation was employed, then the length of the lagering period can generally be decreased. A lagering period of one to four weeks is typical.