During the stationary phase, yeast growth ceases.
Despite the rapid rate of yeast growth, a relatively large yeast starter or slurry of yeast is required for optimal beer production. Siebel Institute recommends one-sixth of the batch-size, one-tenth if you continuously aerate or agitate your starter. The scientific basis for this is currently unknown.
Use of lower pitching rates such as those typically used by home brewers - 1/20 to 1/100 of the batch size - yield slightly higher terminal gravities because of an alteration in the metabolism of sugars in wort. Generally the order of sugar consumption is glucose (10% wort sugar), sucrose (2% wort sugar), maltose (40% sugar), then maltotriose (10% sugar). When underpitched, all of the later fermented sugars are not metabolized. The end result is a sweeter beer with terminal gravities above 1.010. Underpitching along with underaeration of the wort (oxygen levels should be about 8 ppm) can yield gravities above 1.015.
As you increase the overall amount of yeast growth (such as occurs during underpitching), ester formation supposedly increases (according to the Siebel Institute). Theoretically, then, optimal fermentation does not involve significant yeast growth but rather more yeast metabolism of the wort and fermentation to alcohol. Optimal pitching rates should produce a balance between yeast growth and metabolism.
The only practical way for home brewers to achieve these types of pitching rates is by repitching their yeasts. Otherwise you would be pitching a 1-gal starter (or its sediment) into a 5-gal batch! I recommend a compromise that is practical and routinely yields good results. Start with about 10 mL of wort, step it up at least once, and use 0.5-1 qt of starter/5 gal of wort. This is 1/20 to 1/40 of the total wort volume. Each of these steps is about 1-1.5 log dilutions and, according to the growth curves, should take less than 24 h to reach maximum growth. In general, strong ales and lagers require higher pitching rates (at least 1 qt/5 gal). Also, beware that using the standard starter conditions (1.040 O.G. hopped wort grown at room temperature) in these styles of beer may "shock" the yeast and increase the overall lag time. In this case, you may want to adjust your starter so that it more closely resembles the wort it is going to be pitched into.-Maribeth Raines
Maribeth Raines received her Ph.D. in biochemistry and is currently assistant professor of radiation oncology at the UCLA School of Medicine. She also consults with Brewers Resource (Woodland Hills, California), for whom she developed a yeast culturing kit for home brewers.