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crimsondrac
08-24-2010, 02:24 PM
Purely just an informational question, but I was wondering about the amount of yeast to use per gallon. I know most packets of wine yeast you buy, says it can be used in 1-5 gallons batches. However, if the yeast grows on it's own, could you not, in theory use the same packet on a 10 gallon batch, knowing it may take longer for the yeast to start? Is that the only purpose of using X amount of yeast per gallon is to start the fermentation faster or is there some other reason? Does the yeast get too diluted and can not grow at all?

akueck
08-24-2010, 02:40 PM
You can start from one cell and go from there, but the reason you use a certain amount of yeast is cell density. For one cell, you'd be using a few ml to grow it into lots of cells, moving up the volume slowly, etc. The yeast in one packet is good for 5 gallons (of moderate gravity) because the cell density is appropriate. Starting with not enough yeast cells, the yeast will have to work harder & longer to reach optimum density. This does a couple things: less sugar is turned into alcohol (more is turned into yeast), more byproducts of yeast growth are produced (possible off flavors), the possibility of cell mutation is high (due to more generations of yeast, predictable fermentation behavior is lost), the possibility of stuck fermentation is higher (cells which bud a lot are less able to ferment fully, mutant daughters might ferment less too), and with a sufficiently low pitch rate the chances of infection are higher (you'd have to seriously underpitch for this to be a concern). You would also have to feed the yeast more in order to give them the required compounds for extra cell division.

For moderate gravity (up to about 1.090-1.100), about 1 g/gal is a good amount of yeast to pitch assuming you properly rehydrate for maximum cell count. Higher gravity and you should add a little more yeast (~1.2 g/gal for about SG 1.115 for example). Adding too much yeast is also a problem: certain flavor compounds are produced during the "lag phase" which you can essentially skip by pitching tons of yeast. This is not a huge problem, and honestly you'd have to overpitch by a factor of about 10 to run into it. Even then, you might not notice unless you're using a very particular strain known for yeast-derived flavors (some Belgian ale strains, for example).

crimsondrac
08-25-2010, 12:44 PM
Wow, that is a great reply. Thanks. I did not realize that much depended on the proper amount of yeast to add. I am sure that amount is somewhat flexible as you examples state you have to be pretty extreme at one end or the other to mess it up bad, but still, you do have to be careful to add close to the right amount.

akueck
08-25-2010, 11:47 PM
Yeah, I try to add within a factor of 2 of the "ideal" amount. You can probably get away with anywhere from 0.5 to 4 times ideal and barely notice.

From what I've read/heard, a good fermentation sees the population of yeast go up about 5-fold from pitch to maximum cell count. So that's between 2 and 3 doublings, giving you roughly the factor of 2 to work with. You can work outside that range, just be cognizant of the challenges/limitations. It's easy to give yourself a hard time by pitching too little yeast, but too much is usually harder to accomplish.

I will add one thing about overpitching. For mead and wine this doesn't really happen, but for beer it is common to pitch onto a yeast cake from previous fermentations. Here you run the risk of severe overpitching (depending on what gets pitched onto what) and can pick up off-flavors from autolyzed yeast. Best practice is to rinse the yeast to separate the dead cells and other solids (trub), and pitch an appropriate amount of viable yeast (this is closer to what commercial operations do).