Yeast Requirements for High Gravity and Lager Brewing
Anytime you are pitching yeast into a harsh environment (e.g. high gravity or low temperature) you need additional yeast. A rough rule of thumb is to double pitch rates for original gravities above 1.065 and triple pitch rates if the original gravity is above 1.085. More technically, a million cells per milliliter are needed for a 1.080 specific gravity beer, or 3 Activator packages for a 5 gallon batch. It is also important to remember that it is more difficult to get oxygen into the solution in higher gravity worts.
At the end of fermentation, high alcohol levels, lack of nutrients, poor yeast health and lack of fermentable sugars can effect attenuation.
Do not re-pitch yeast from a high gravity beer. Yeast left from high gravity brews are usually very unhealthy or dead and will not perform well in subsequent brews.
More yeast is required for lager fermentations. This can be accomplished by using two Wyeast Activator packages or by making a 2L starter. To compensate for this, one Activator package may be pitched into 60-68°F wort. Allow fermentation to begin, then cool the wort to the desired fermentation temperature.
There are different views on the topic of pitching temperature for lager brews. The individual brewer ultimately has to weigh the pros and cons. If a brewer is to pitch the lager yeast at fermentation temperature (55°F and below) then the pitch rate needs to be increased and a slower start to fermentation should be expected. The other option is to pitch the yeast into wort (60 to 70°F) and maintain temperature for 24 hours or until signs of active fermentation are evident and then cool to desired fermentation temperature.