http://www.beersmith.com/forum/index.php?topic=1883.0;topicseenFrom what I've read about yeast, there is definitely a "right" or "adequate" amount of yeast required for each batch, and that amount is determined by gravity and total wort or batch size. Palmer talks about it some, and Zainasheff offers a calculator at mrmalty.com. Yeast desire simply to reproduce and eat; yeast reproduce by budding and splitting into more cells, and their critical requirements for splitting are oxygen and adequate sterols and FAN (usually provided by malt). When put into a new and sugar-rich environment of wort, they quickly take a reading on the oxygen levels and the types and amounts of sugars. The lag phase (before airlock activity) is when they consume the oxygen and reproduce. Once the oxygen (and/or sterols) is gone, they shift to eating and the total amount of yeast cells is largely fixed. A White tube or Wyeast pack of 100 billion cells is supposed to do five gallons of ~1.050 wort. (A 11-gram package of dry yeast might have 200 billion cells, which may explain some brewers strong loyalty to their favorite dry yeast - it has twice the yeast and always works.)
This fixed amount of yeast cells eat until they either exhaust the wort of sugars, or they get exhausted and/or drunk and fall out of suspension (that is why yeast's 'alcohol tolerance' is important for bigger beers). So the perfect amount to pitch varies wildly with the gravity, batch size, and the desired yeast effects of the style. To overpitch with too much yeast might mean the yeast quickly chew thru the wort and NOT generate the desired phenols and esters of say a Belgian tripel. But overpitching might be fine on an APA or stout for which you want zero yeast effects. Overpitching might be great on a 1.040 beer on which you plan to re-use the yeast cake for a 1.094 Imperial Stout. The 1.040 beer acts like a huge starter and the yeast are not stressed into mutated little freakyzoids that no longer eat maltose.
To greatly underpitch with too little yeast would mean that wort sugars go uneaten because the insufficient amount of yeast could not finish the job. Residual sweetness or even a syrupy under-attenuated wort might result. To
slightly underpitch might moderately stress the yeast to generate more of the desired phenols and esters in a hefeweizen. It's all a question of degrees.
The two helpful acts a brewer can perform are proper aeration or saturation with oxygen of the wort, and pitching the
proper amount of yeast for the gravity, batch size, and desired yeast effects for that specific beer. One size may not fit all. Hope that helps. Definitely check out howtobrew.com and mrmalty.com for the details.
