I have a related question, Yooper... if you're adding (for example) two packets for a brew, can't you just do a starter of wort, and let it work for a few hours (esp while you are cooling your wort)? I thought it was a matter of number of cells, rather than the number of vials / packages of yeast.
Well, I am a big believer in either pitching at high krausen, or waiting until the starter is fermented out completely before pitching, so it usually requires a day or two in advance.
Yeast works in two stages. I don't know the details about all the chemistry and biology, but from what I understand, yeast uses oxygen in the first stage to multiply and increase cell count. That's why aeration is important. Once it runs out of oxygen, it starts chewing on sugars and that's the second stage. When you do a starter, the idea is that you want to give the yeast oxygen so that it enters the first stage that is multiplication. You want to time your starter so that your yeast is about to enter the second stage when you pitch it. Then you aerate your wort and the yeast continues to multiply again. So a starter is the poor man's way of pitching more yeast. So pitching two vials or packs of yeast achieves the same goal of pitching a higher cell count.
Sometimes, you need a pretty big starter. Say you're doing a 1.090 OG beer with a package of yeast that is 3 months old. You'll need 305 billion cells to meet this need. Some of the options might be- make a 5L starter with 2 yeast packages, pitch 9.2 packages of yeast (no starter), or use 4 packages of yeast and a 2L starter. Those options should all give you the same cell count, but a couple of of different ways to get there.
The yeast packages are labelled "pitchable" but there are always the
buts. Even White Labs says on their website: White Labs recommends on their label to make a starter "if the gravity is over 1.070, if the yeast is past its "best before" date, or if a faster start is desired" and: What is "Pitchable"?
White Labs Pitchable yeast is designed to ferment a 5 gallon (20 liter) batch of homebrew beer. We package each vial with 70 to 140 billion yeast cells, which corresponds approximately to a 1-2 liter size starter. Some of the liquid yeast on the market is packaged in small quantities, and should not be used directly in 5 gallons. Lag times are typically between 5-15 hours.
So, you can see that even if the vial has 100 billion cells (the middle of what WL claims), that you may need 300 billion cells for some ales and even more for lagers.
A starter will ensure that you have viable yeast and that they are ready to get to work, whenever you use liquid yeast of any brand.