Understanding pitching rate

[nigelnorris]

Something that's always confused me. Yeast is just like a bacteria, put it in a growing medium and it reproduces, by division, endlessly until it's used up the growing medium or until its byproducts attain poisonous levels. Some of the cells die along the way and then the rest die when there's nothing left to eat.

So why does it matter how much of the stuff I chuck into a fermenter? Surely if I just use 1 cell, after x minutes there will be two then four and so on. Apart from what dies off from old age along the way the population will keep growing exponentially until all the available nutrients run out or until the level of toxins inhibits further growth. So I'm going to end up with the same millions of cells, it'll just take a bit longer to get started.

I know that the last paragraph is untrue, where's the error in my logic?

 

[Chippy_Tea]

Yeast produce different flavors during the various stages of their lifecycle. Underpitching lengthens their "growth" phase (maybe a better name is "division" or "budding"). The bulk of a beer's esters are produced during this initial stage, so extending this part of the lifecycle increases this sometimes undesirable quality.

Yeast need oxygen to bud. A homebrewer can usually dissolve enough oxygen into wort to facilitate four buddings. Once out of oxygen yeast begin consuming sugars. In this phase the yeast stores energy to sustain it during hibernation. Yeast will begin consuming the simple sugars moving to the more complex molecules as the easy to digest food diminishes. Once it is full of stored energy the cell shuts down and flocculates.

Underpitching can introduce an insufficient amount of yeast to entirely consume the entirety of the beer's fermentables. Your fermentation may not finish, or there could be enough food for an infection to take hold.

Rich Armstrong

 

[nigelnorris]

Ah it's the bit about division being limited to four times that was missing from my understanding. Thanks!