Originally Posted by Aunt_Ester
Hum. I've always decanted off the old starter. Thanks for the tip, Revvy
Why would you not decant of the spent wort unless the yeast are still in suspension which they should not be if you have turned off the stir plate and they have finished feeding.
It really goes back to what you are trying to achieve with this starter. How many cells are you aiming for?
If you leave spent wort in the container that limits the amount of fresh wort you can add and hence the number of new cells that you will achieve with the 'step up' .
If you have a 1L starter and when it is finished just add another 1L of wort without decanting, then you do not really have a new 2L starter. You really are starting again with a 1L starter to which you have added a greater number of cells.
If you were to decant and add 2L of fresh wort you could expect to get a significant increase in total cells over what you would have got with just adding 1L of fresh wort.
I always pour off and then add the fresh wort.