Bread-Bakers v106.n037.5

Here's another way to justify using start just at the same point when 
one would normally refresh it:

When one makes bread using yeast, one measures it out so that you 
consistently have the same number of yeast cells in the dough. By 
using the sourdough starter just at the refreshment point, one can 
have the same confidence that one is putting the same number of yeast 
and bacteria cells in. Here's why:

Mike's post suggests a "steady state" feeding schedule of 300 units 
of old starter to make 780 units of new starter. To simplify the 
math, let's use 200 units to make 800 units.

At the point one is about to about to refresh the old starter, let's 
say that one's old 800 units has a certain amount of yeast bacteria cells.

Since we only use 200 out of 800 units of the old starter to make the 
refreshed starter and the other ingredients in starter have no 
significant amounts of yeast or bacteria, then the refreshed starter 
will have the same mass as the old starter, but only 1/4 the number 
yeast and bacteria cells. So if one uses this newly refreshed starter 
to make bread, the dough will only have 1/4 the leavening and souring power.

My understanding is that feeding schedules such as the one Mike 
provided are designed so that the yeast and bacteria have just enough 
time to multiply back to their original populations before the next 
refreshment. So by the time the starter matures (is ready for another 
refreshment), the yeast and bacteria populations have increased by a 
factor of four (in this example)...right back to the original population level.

And as a side benefit of this tuned, steady state refreshment 
schedule, one can have confidence that mature starter has the right 
population density of yeast and bacteria.

I hope this perspective helps.

Allen
Home baker
San Francisco