What is Different for Brewing Lager Beer?
When a yeast cell dies, it ruptures - releasing several off-flavors into the beer. When you have a large yeast mass on the bottom of the fermentor, you have a large potential for off-flavors due to autolysis. If this ever happens to you, you will know it. The smell is one you will never forget. It happened to me one time when my wife was making paper as a hobby. She used boiled rice as the glue to hold the shredded paper together. After the rice had been boiled until it became a paste, the paper making was called off that weekend and the pot of rice paste was set aside on the counter top. A wild yeast must have got a hold of it during the next couple days ( I remember it bubbling) and the pot was ignored in the days that followed. A busy week went by along with another busy weekend and the unintentional Sake experiment still sat there forgotten. The following weekend, my wife was once again ready to try making paper. I picked up the pot and lifted the lid to see what had happened to it. My knees buckled. My wife turned green and ran to the door coughing and choking. The stench was appalling! It was heinous! The noxious aftermath of a late night of cheap beer and pickled eggs would be refreshing compared to the absolute stench of autolysis. I hope I never have to smell it again.
Luckily, the propensity of yeast to autolyze is decreased by a decrease in activity and a decrease in total yeast mass. What this means to a brewer is that racking to a secondary fermenter to get the beer off the dead yeast and lowering the temperature for the long cold storage allows the beer to condition without much risk of autolysis. At a minimum, a beer that has experienced autolysis will have a burnt rubber taste and smell and will probably be undrinkable. At worst it will be unapproachable.
As a final note on this subject, I should mention that by brewing with healthy yeast in a well-prepared wort, many experienced brewers, myself included, have been able to leave a beer in the primary fermenter for several months without any evidence of autolysis. Autolysis is not inevitable, but it is lurking.