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I have a had a question for quite some time that nobody has been able to answer. The question is more academic or theoretical than it is practical. Perhaps the community here is a tad smarter (nerdier?) than other brewing forums. Everywhere I go, people say that higher yeast cell counts are better. Got a high gravity wort? Use two yeast packs. Got a couple days before brew day? Use a yeast starter. My question is why does this matter.

It seems that, theoretically, if you threw one yeast cell in the wort, it would munch on some sugar, and reproduce. Then it would repeat this cycle until all the fermentable sugar was gone and there would be many many many yeast cells at that point. Seemingly, the only difference between this pitching millions of yeast cells is that presumably the single yeast cell situation would take longer to get to the end point.

Obviously, this is somewhat contrived. Perhaps the single cell dies before reproducing? Ignore issues like that for the point of this question. Why is it important to have many many many yeast cells?

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4 Answers

Yeast behavior in wort is more complex than you think it is.

In short, there's two phases of yeast growth: a lag phase where yeast are getting acclimated to their environment, preparing for exponential growth and fermentation, and the exponential growth phase. Over- or under-pitching will primarily affect the lag phase, generally under-preparing the yeast to really rock and roll during fermentation. That one initial cell you pitched can't consume enough oxygen, zinc, magnesium, &c. from the wort in order to reproduce and ferment the whole batch successfully.

White and Zainasheff's /Yeast/ is highly recommended if you're interested.

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Would give extra +1 for the ref to White and Zainasheff's book if I could. I finished reading it recently. It tells you everything you want to know about yeast, and this question is quickly answered within its pages. –  Galapagos Jim Apr 18 '13 at 17:42
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Yeast pitching rate is vitally important to getting the proper flavor out of your beers. Its been said that yeast can produce up to 500 distinct flavors in your beer that are for the most part entirely dependent on pitch rates and fermentation temperature.

Under pitch and you make your yeast work TOO hard resulting in soapy, chemical or possibly even corn flavors.

Over pitch and you will lose some of the esters and flavors that the yeast add to your beer because yeast when given the opportunity are lazy and don't work hard when they don't have to.

Just like Hops, grains and adjuncts should be measured more or less exactly, your yeast additions so should be properly measured. Use yeast calculators and make the appropriate sized starter and pitch the appropriate amount of yeast to do the job and do it right.

If you under pitch your yeast may not even finish fermenting the beer because its been put through so much stress already.

If you don't really care and are happy drinking whatever your fermenter spits out you can ignore pitching rates and get beer, but if you want to perfect your beer and make high quality homebrew you should really monitor your pitching rates.

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In short, you want your yeast to be mostly be making alcohol, rather than making babies, so you get the appropriate flavor profile and attenuation. You can underpitch, but the flavor will not be the same and attenuation lower.

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I once pitched 4ltrs of yeast into a 11.5hl batch of red. I let the temp sit for two days at 25'C to prompt the lag phase. I think a lot of it has to do with the state of the yeast. This was on it's 10th generation of cropping. Still turned out as I expected, just took a little longer. Just for reference we usually pitch 20ltrs of yeast or more.

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Can you scale this down to homebrew levels? That's the primary focus of this site. –  mdma Apr 18 '13 at 22:23
    
10th generation? meaning you've taken that yeast from the yeast cake from 9 previous sequential batches? i thought mutations occur around 4 or so. –  Landon Apr 19 '13 at 0:33
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