Question about over and under pitching yeast.

I just can't wrap my head around how you can pitch too much yeast. Now I can understand if it is washed and too much trub or thrown on trub it could cause off flavors but it seems like pitching a pound should thoretically be OK.

I understand that under pitching could stress the yeast or if it is too under pitched it may not start at all but I keep seeing that pitching the proper amount helps get the beer to the correct FG and that doesn't make sense to me because it seems that once it starts it will reach a good FG because yeast is multiplying.

Just off the top of my head,

Yeast can produce flavors that are desirable in certain cases, over pitching can subdue this.

Over pitching results in lazy yeast, so later generations are weaker.

Over pitching may be a waste of yeast / effort (depending on how you got the yeast).

Greatly over pitching can result in lots of autolysis and can affect flavor.

Reaching FG is one part of brewing your desired beer, not all parts.

Overpitching can also raise the alcohol content too fast and stress the yeast causing them to release off flavors.

I think I get the overpitching now but still not sure on underpitching causing you to not reach your FG. It seems that if fermentation starts and doesn't get stuck it shouldn't matter if it is 5 grams, 11 grams or 20 grams. As long as it starts it seems it should reach whatever FG it should reach.

CatHead said:

I think I get the overpitching now but still not sure on underpitching causing you to not reach your FG. It seems that if fermentation starts and doesn't get stuck it shouldn't matter if it is 5 grams, 11 grams or 20 grams. As long as it starts it seems it should reach whatever FG it should reach.

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if you have a big beer the yeast can get worn out over time and stop replicating enough to get to the final gravity. even if it's not a big beer the struggle to ferment without enough yeast will stress the yeast possibly causing off flavors, apathy and quitting.

eastoak said:

if you have a big beer the yeast can get worn out over time and stop replicating enough to get to the final gravity. even if it's not a big beer the struggle to ferment without enough yeast will stress the yeast possibly causing off flavors, apathy and quitting.

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In this economy those lazy bastards should be happy to have a job. They need to get up off the bucket and back to eating my wort and peeing some alcohol and farting some CO2.

Overpitching IMHO is bad and the first thought to cross my mind is how are you providing O2 to all of the yeasties? They all need specific conditions met, including elements like oxygen, zinc, etc. its like throwing a overpopulated 3rd world country in a small lake. Not everyone will get a drink of water, a bite to eat, therefore some will be malnourished and will mutate and do funky things not to mention DIE!

Likewise, certain beers need specific esters created during the budding process. Why would cells multiply if there are already too many homies at the party?

I could take a shot of bland vodka, and take a bite of a bannana with cloves sprinkled on it, and chase it with a glass of water, but that is a poor substitute for a hefeweizen

CatHead said:

I think I get the overpitching now but still not sure on underpitching causing you to not reach your FG. It seems that if fermentation starts and doesn't get stuck it shouldn't matter if it is 5 grams, 11 grams or 20 grams. As long as it starts it seems it should reach whatever FG it should reach.

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If there is not enough yeast, it will take longer to set up the conditions that the yeast need to grow properly. This will cause them to become stressed and potentially produce off flavors, lower growth/consumption rate, lower viability, and significantly shorten lifespan (among many other issues). They need to first produce a certain concentration of protiens, enzymes, etc. to be able to healthily grow and carry out typical cell functions. The lower the number of cells, the harder it is to reach these concentrations and the cells begin to "stretch" their resources. There is always a critical minimum concentration for a successful fermentation. I've seen many fermentations fail due to underpitching.