YEAST: the power and the glory
OK, OK, i guess I can try to make some sort of contribution to this discussion here. I think there is a bit of confusion here about the yeast that come in the liquid from the homebrew stores, either smack packs or vials. There are two key terms everyone should be aware of and the differences between the two. One is yeast viability and the other is yeast vitality. Viability is easy to define as it basically describes the overall number of yeast cells that are alive. If you were able to count the cells under a microscope and then plate out a small, countable number of cells onto an agar plate the viability of the yeast would be the number of cells that actually form a colony as compared to the number of cells you plated. Vitality on the other hand is a more ambiguous term that describes the overall health of those viable cells. In brewing terms it could be described as how fast the cells could divide and as well as how efficient they are at fermenting sugars. Cells will have high vitality if they have sufficient fermentation precursors stored up. For example, oxygen is required to build up sufficient cell wall components prior to fermentation because during fermentation (in the absence of oxygen) these cell wall components are not synthesized and are depleted upon every cell division until a lower limit threshold is reached and the cells can no longer divide thereby decreasing fermentation efficiencies. So, even before you pitch your yeast, you can easily have a population of cells with high viability but low vitality if not properly prepared. For example, the older a vial or smack pack is the lower the viability is as well as the vitality, but I think that vitality drops off much faster than viability over time.
So back to the question of starters. Creating a population of cells with high vitality requires that you give the yeast the proper nutrients that prime them for fermentation. I think this is where Wyeast smack packs are superior to White labs yeast vials. When you pop the smack pack you release vital nutrients to the yeast so the somewhat dormant yeast greatly increase their vitality. There is no way to do this for White labs unless you use a starter to wake them up. A starter is useful for both because depending on the starter technique you use you can greatly increase the total number of viable cells while at the same time increase the overall vitality of the entire population. The most important component for the starter is oxygen. Why force the cells to start fermentation in a starter when you are just going to pitch them into an oxygen rich wort, which inhibits fermentation, only to have that oxygen quickly depleted requiring a switch back to fermentation? If you use a stir plate to add oxygen continuously to the starter you can greatly increase the overall numbers of cells in a smaller volume of starter wort. These cells will have built a nice ample store of the cell wall components required for proper attenuation of your beer. If you prefer the more traditional method of a still starter you will be better off if you give the starter a stir twice a day or so to scrub out the built up CO2 and introduce more O2.
I think a better place to add things like yeast nutrient (i.e. Servomyces) would be at pitching or a day or two into the fermentation. The yeast nutrients add things like metal ions required for enzyme function as well as free nitrogen required to synthesize these enzymes. Adding this to the fermenting wort of your beer will give the yeast a boost as they use up the limited nutrients that come from the malt.
I use a stir plate for my starters because I don't use smack packs or vials, but am instead building up my population step by step from literally a single yeast cell. I need the extra oxygen to get to the proper number of viable cells with the added benefit of also getting high vitality. Either way, though, if you are using store bought liquid yeast, make sure the package is as close to the manufactured date as possible and if you have the capability, use a starter of any technique (stir plate of otherwise) to increase cell vitality.
Wow, is that enough of an explanation? 
Cheers!
Probably the most valuable post on the entire website.
What type of increase in population size do you really get when pitching a vial of White Labs into a typical 1L starter?
Or what may be more valuable an answer, how big of a starter to you need to actually double the cell count of a White Labs vial?
There is confusing information out there about available nutrients and how it limits the amount of growth you can get. I would assume then that if you don't get more than one doubling, or maybe not even on doubling, that the start is at least increasing vitality through oxygen introduction and metablism with fresh nutrients (starter wort).
Thanks 1n1m3g!
Great post. My Thanks as well 1n1m3g!
About 2 years ago I would have read about a third of that post and woke up drooling on my arm with visions of flying yeast cells around my head. Great post that really breaks it down for someone looking for a lot of information in a small concise format. One of the things I love about this hobby is the ability to always take it to the next level and yeast management is vital for many of thes "next levels."
Great post 1n1m3g. I'm sure that you told me that before (actually I don't think you ever broke it down like that). This post would have been really helpful about 3 years ago. Ha ha.
Seriously thanks for the breakdown of what the hell actually goes on during a starter fermentation. This explains why my stir plate starter took off so quickly during actual fermentation.
Now what about actually putting oxygen into the wort? I don't think I saw you do that during the brewday 1n1m3g, I know Thirsty does, but I don't believe brewchez does? So what gives guys? do you not think the wort actually needs added oxygen? Thirsty has noticed significantly better fermentation adding oxygen or he wouldn't be doing it. Although his beers usually have an og of 1.080 or better.
I always give my wort and yeast starters a shot of O2 before pitching my yeast. Keep in mind that the boil drives off O2. I think it was BYO who had an article a few months ago comparing levels of dissolved oxygen using various methods: pouring directly into fermenter, using an aquarium pump, injecting pure O2, etc. I'll dig it up so I can post the numbers but the bottom line was that injecting pure O2 resulted in the highest levels of DO.
So I went with the step up plan. Pitched a vial each into 2 jugs with 1/2 gallon each yesterday, then split that into 4 jugs today, and added another 1/2 gallon to each of the 4. I have them waiting in line to make ther way onto the stirplate. 
If i'm reusing yeast, and have it in the fridge for about 2 weeks, would it be benificial to throw it on the plate stirrer with another starter, or should I just let it warm up and pitch?
bruguru wrote:
If i'm reusing yeast, and have it in the fridge for about 2 weeks, would it be benificial to throw it on the plate stirrer with another starter, or should I just let it warm up and pitch?
2 weeks, pitch it.
3 weeks, pitch it
4 weeks, eehhh
thirsty wrote:
bruguru wrote:
If i'm reusing yeast, and have it in the fridge for about 2 weeks, would it be benificial to throw it on the plate stirrer with another starter, or should I just let it warm up and pitch?
2 weeks, pitch it.
3 weeks, pitch it
4 weeks, eehhh
5 weeks, drink it
6 weeks, toss it
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