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==Reusing a Sour Yeast Cake==
Reusing a sour yeast cake can often provide great results. Brewers have reported success repitching on very old yeast cakes (2 years) without getting off flavors from yeast autolysis (DanABA has done so on 2 year old yeast cakes). After several months, [[Saccharomyces]] tends to die off due to the low pH in a sour beer. The bacterias and [[Brettanomyces]] tend to survive the lower pH, and their cell counts can be high in even an old yeast cake. By pitching new wort on an old sour yeast cake, these microbes (particularly the [[Lactobacillus]]) have access to the simple sugars in the wort <ref>[http://www.themadfermentationist.com/2009/11/brewing-sour-beer-at-home.html Tonsmeire, Michael. The Mad Fermentationist. Brewing Sour Beer at Home. Last paragraph in the "Inoculation" section. Retrieved 2/19/2015.]</ref>. In general, washing the yeast cake is not necessary.
Some brewers will harvest a certain amount of trub from their fermenters (500ml for example), and use only this amount to inoculate a new batch of beer. This will allow the brewer to control the amount of dead trub material that goes into the fresh beer. Michael Tonsmeire often advises that the brewer also pitch a fresh culture of Saccharomyces <ref>[http://www.themadfermentationist.com/2009/11/brewing-sour-beer-at-home.html Tonsmeire, Michael. The Mad Fermentationist. Brewing Sour Beer at Home. Comments section. Retrieved 2/19/2015.]</ref>.
Some brewers have good success reusing a yeast cake or a portion of a yeast cake by leaving the wort in contact with the old yeast cake for 3-4 days. After 3-4 days, a fresh culture of Saccharomyces is pitched to finish the fermentation. The 3-4 day head start gives the souring bacterias a head start and results in a low pH beer.
As with all methods, the species and strains of the microbes being used should always be taken into consideration. Experimentation and repeated processes should be carefully employed by the brewer in order to find the best results for their cultures. For example, using different strains of Saccharomyces cerevisiae as the primary fermenter can produce widely different results.
==The "Old School" Way==