Microscope Cell Count Viability?

[mpcluever]

So I'm finally getting the last couple things to use the microscope I've had for years intending to do yeast counts. I've got the methylene blue to stain cells. What I don't understand is, all the instructions I read say to count ALL cells, then go back and count living cells so that you can calculate viability. Is there a reason I wouldn't just count viable cells? Why do I care that some cells are dead? Obviously a mostly dead sample is a problem, but I would see that in the microscope without counting double.

I've got the Brewing Elements Yeast book and it says to count all and viable
Brewing Viking on Youtube has a lot of great videos on the subject https://www.youtube.com/channel/UC8_kozGERzHnjVIizhnh43g/videos

 

[Oginme]

The way I was trained was to count all cells and then to count the dead ones. The idea is that viability is an indicator of cell health for those cells remaining and can also give the counter a clue that something may be off in that so many of the cells are dead. I track both and proportionally increase my pitch rate if the % viability is below 90%, which happens when I am looking at a harvested slurry of yeast.

Another thing that I did at the start was to perform cell counts on packages of liquid yeast. This gives you a read on your cell counting prowess versus something that is a 'known' quantity. It is also very enlightening in terms of tracking cell viability versus package age. I very quickly determined that the % viability vs age for most yeast starter calculators is very pessimistic, or maybe I take better care of the packages versus those people who developed the calculators.