Can you please discuss the advantages/disadvantages of pitching onto an existing yeast cake (assuming the cake, and previous beer, smelled and tasted clean). Specifically, I have seen it argued that while there may be obvious advantages such as no starter prep, less cleaning, and perhaps a shorter lag time, there may be negative effects created due to too many old or dead yeast cells, or perhaps maybe even problems with over-pitching? How might using an existing yeast cake affect the taste and quality of the resulting beer.
Tobias may have a better answer, since he works with pitching almost on a daily basis. My feeling is that pitching into an existing yeast defeats the assurance of uniform quality and fermentation kinetics that the brewmaster has when starting each time with a batch of new yeast.
You never know how many dead cells, stressed cells, mutated cells and damaged cells that you have in the existing yeast. You would never know at which repitching cycle would be best to pitch into unless you have extensive laboratory data.
Dr. Clayton Cone
I believe that in some instances there may be a benefit in placing fresh wort on top of an existing yeast slurry. You are correct in stating that there may be an advantage in terms of lag phase, particularly if the beer has not been sitting on top of the yeast for a long period of time. However, the effects that you see are likely to be dependent on the yeast strain and the characteristics of the previous fermentation.
For example, high gravity or high alcoholic fermentations can leave the yeast severely stressed. Such yeast will invariable perform poorly as there is likely to be a number of dead cells which could influence the clarity, foam and pH of the subsequent fermentation (due to products of autolysis). The wort utilized in the subsequent fermentation may also need to be adjusted to accommodate the differing population.
This may be hard to achieve without having a detailed knowledge of the yeast strain. For a dried yeast inoculum the yeast is in perfect condition, highly vital and well aerated and contains enough lipids within the cell wall to achieve the required number of cell divisions. Yeast which has been through a fermentation will generally be depleted in sterols (as well as other compounds such as glycogen) and may not be able to produce daughter cells, or if they do, these cells may be fermentatively 'weaker' than normal. The wort may therefore require a higher degree of aeration to provide enough oxygen for sterol synthesis.
The subject of cell division also raises a serious issue as yeast generally divide approximately 3 times during a fermentation (again this is strain specific and dependent on extrinsic factors). Consequently, you are correct in stating that by simply adding beer onto the yeast you are effectively overpitching. For example instead of starting with 15million (1.5E+07) cells/ml wort you would be adding around 120million (1.2E+08) cells/ml. Couple this with an additional 3 fold increase in the new fermentation and theoretically you could end up with almost 1000 million (1E+09) cells/ml ! In practice the population is not likely to get quite this high, but it would still lead to major flavor variations.
One other issue with reusing yeast is the possibility of transferring contaminants. Bacteria present in low numbers at the end of one fermentation will rapidly overtake the yeast if pitched into fresh wort, due simply to shorter division times.
To summarize, it is possible to achieve good, clean fermentations by reusing yeast. However, given the number of variables involved (not least the strain and type of fermentation performed), it's difficult to predict the characteristics of a beer produced by repitching a yeast culture. Suffice to say that without adjustments to a number of fermentation conditions the subsequent fermentation may not be as predictable or reliable as when using fresh yeast.