What Happens In A Yeast Starter?

I had to post this wonderful piece of information about yeast and yeast starters.  A great great brewer friend of mine, Wade, over at the BKB forums posted a reply to a discussion we were having about yeast starters and cell #s provided in the yeast packages we all typically buy from WhiteLabs and Wyeast.

Wade, a.k.a. 1n1m3g in cyber-land, is currently working on his PhD in the biological sciences arena and works exclusively with yeast for his research.  He and I have talked in the past about yeast; and he really knows his stuff.  When Wade is not working in the lab or being a great father and family man he can be found bumming around with the Boston Wort Processors, a homebrew club here in the Boston area. (Please consider checking out their upcoming Homebrew Competition)

So please take a minute to read and digest the post I have copied below.  Its a bit long, but it’s amazingly informative.

From Wade:
“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?  smile  Cheers!”

(Standing) BREW ON!

Comments

  1. Thanks for the repost. Two questions come to mind (in the chance that Wade is reading the repost):

    1. What does Wade think about Jamil’s Pitching Rate Models (www.mrmalty.com), as its mostly based on viability and only infers vitality?

    2. Any suggestions for good references for growing your own yeast up from a few cells (i.e. approximate cell growth rates in each step-up? How do you know when you get the the appropriate pitching rate when growing from single cells)? How do you (relatively simply) model the growth up?

    Great contribution!

    -JW

  2. I have a question also about using a stir plate. I do use one, but I wonder if it actually introduces a significant amount of oxygen once carbon dioxide starts being produced. Doesn’t the CO2 displace and/or dilute the oxygen. OR is there enough oxygen present?

  3. chemgeek: Keep in mind that during the growth phase when O2 is present fermentation isn’t happening. So in theory a well oxygenated/aerated starter wort (i.e. stir plate) there is no fermentation. Fermentation is an anaerobic process. How much Co2 is being produced during aerobic yeast metabolism for growth…I need to look into that, but it would definately be less than.

    JW- great questions, I’ll see if I can get an answer. Maybe Wade is lurking, but I don’t know. Stay tuned.

  4. chemgeek, all I know is that I experienced a much better fermentation, and yeast characteristics with my newly purchased stirplate. I’d say it’s the second best thing I’ve bought for brewing next to the plate chiller.

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