frijole

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I don't understand why the amount of yeast pitched matters. Besides the speed that it will multiply. Doesn't it get to the same point no matter what?

ifishsum

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I think there's more to it than that... variables like wort aeration can affect yeast reproduction and final cell count, so by pitching more yeast you're helping ensure a proper sized colony for fermentation even if something else was lacking. I'd think that the composition of the wort would make a difference as well, the amount of nutrients, etc. It also means the yeast don't get stressed as easily and should help reduce ester production and other possibly undesirable flavors. A good healthy quick start to fermentation is a good thing in my book and reduces your possibility of a stuck fermentation or unexpected flavors.

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yemu

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the yeast can't reproduce endlessly, and the amount of yeast that end up in the beer depend on how much you pitch. In some beers underpitching will cause underattenuation (it can happen if you pitch too low in belgian strong beers).

also you want to pitch the right amount of yeast to have the right amount of growth (which is associated with ester production which in many styles gives flavour and aroma to the beer). Overpitching is as wrong as underpitching (that is why it's not good to pour fresh wort on a whole slurry from previous beer - there's too much yeast in there)

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Joe_Stout

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I've been told about 1 cup of slurry per 5 gallon batch. Does that sound about right?

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As in most things in life, 'It Depends'.



There are a wide variety of factors involved which make oversimplification - like 'pitch a cup of slurry per five gallons - dangerous. Different yeasts produce slurries of different density, which affects final cell count; the age of the slurry and the beer in the fermenter from which the slurry was harvested; the gravity of the wort into which the slurry is being pitched; the aeration of the fresh wort; all of these affect how much yeast is appropriate to pitch.

The best method IMO is to use the Mr Malty Calculator. It's tricky with slurry, but it's about the best place to start.

Cheers,

Bob

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Bob

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To the OP:

Yeast management is a very complex subject, one which can be daunting to the new brewer. Kudos to you for wanting to understand it!

Yeast management is more complicated than just knocking in a packet of yeast, sealing up the fermenter and forgetting about it. At least, it should be more complicated if you have the goal of brewing the best possible beer.

Yeast are living organisms and have very specific requirements for optimum, well, living. They are most productive within a fairly narrow set of parameters, and those can be manipulated to provide or enhance a specific result - one need only look at how fermentation temperature affects the final flavor of Bavarian Hefeweizen, for example.

Through centuries of practical experience, brewers have discovered certain Rules of Thumb. One of these is the practical amount of yeast cells needed to properly inoculate a given wort.

Knowing how much yeast to pitch - and how much you really are pitching - is something learned by reading and doing and educating oneself. For example, the Rule of Thumb is one million cells per milliliter of wort per degree Plato. For five gallons of wort at 1.048 OG, that approximates 225 billion cells. However, experience says that 75% of that number provides excellent results for ales, and ~125% of that number gives better results for lager beers.

The whys and wherefores are far beyond the scope of an Internet forum post. There are books out there, like Principles of Brewing Science, that explain far better than my poor power.

If you have more specific questions, though, I'll do my best to bore the crap out of you.

Bob

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Shawn Hargreaves

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Grossly oversimplified, yeast have a couple of different stages in their lifecycle:

- When they have both oxygen and a sugar supply, they reproduce like crazy, while consuming oxygen and giving off various complex byproducts

- When they run out of oxygen but still have sugar, they ferment that sugar and produce alcohol

- When they run out of both oxygen and sugar, they go into hibernation.

If you pitch a small amount of yeast, they must reproduce many times to create enough yeast to ferment the entire volume of beer. But this only works if they also have enough oxygen for this reproduction (which is why it is so important to aerate your wort especially for bigger beers). Also, the reproduction creates byproducts that contribute important flavors to the final beer. If there is very little yeast, there will be a lot of reproduction, and thus maybe too much of such flavors.

If you pitch a huge amount of yeast, less reproduction will be needed, and thus fewer flavors will be created. This is a good thing if you are going for a clean American ale, but probably not so desirable for a funky Belgian style.

frijole

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I know enough microbiology to know that yeast go into an exponential growth phase at a certain point. So, it seems to me that as long as there is enough oxygen etc. that it shouldn't matter whether you start with one organism, or 1 billion. Assuming that the one survives of course.

The food the yeast need is a constant. The only difference between a lot of yeast and a little is how long it takes to consume that food.

reim0027

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If I understand things correctly, that was Shawn Hargreaves's point of underpitching. The time it takes to produce enough yeast to fully ferment the wort. Too long in the exponential growth phase and you get undesired byproducts.

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Shawn Hargreaves

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My understanding is that the difference is in how many generations you are requiring the yeast to go through before it can settle down to fermentation. The more generations, the more funky flavors are produced, and also the more stressed out the yeast will be, which can adversely effect their ability to produce a clean beer and reach good attenuation.