Changes

Reusing a sour yeast cake can often provide great results. Brewers have reported success re-pitching on very old yeast cakes (2+ years) without getting off flavors from yeast autolysis. After several months, ''[[Saccharomyces]]'' tends to die off due to the low pH in a sour beer. The bacteria and ''[[Brettanomyces]]'' tend to survive the lower pH, and their cell counts can be high in even an depending on how old the yeast cake is (interestingly, ''Brettanomyces'' remains more viable over time if it was co-fermented with ''S. cerevisiae'' than if it was fermented by itself; i.e. [[100%25_Brettanomyces_Fermentation|100% ''Brettanomyces'' beers]] <ref name="Hubbe"></ref>). 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>. Using a young yeast cake is also a viable option, and may carry over more surviving ''Saccharomyces'' cells as well as more viable cells of the other various microbes. In general, washing the yeast cake is not necessary. The beer itself can also be used as an inoculate and might be more desirable so as to avoid trub. If the beer has sat in a barrel ''Acetobacter'' and other unwanted microbes might be more present on the surface of the pellicle, and would remain after racking the beer out of the barrel, so some professional brewers advise using beer as an inoculate for this reason <ref>[https://youtu.be/IGzoh4brILA?t=52m30s Yakobson, Chad. Interview on Craft Commander. 12/20/2016. Retrieved 12/22/2016.] (~52 mins in)</ref>. If the yeast cake is particularly old, perhaps say older than 1 year, then making a starter with the slurry will help guarantee the viability of the microbes. Such a starter can be treated the same as a [[Mixed_Cultures#Starters_and_Other_Manufacturer_Tips|mixed culture starter]] that can be assumed to not have any viable ''Saccharomyces''.

In general, it is advised to pitch a fresh culture of ''Saccharomyces'' to ferment the bulk of the wort sugars. This can be done before adding the sour yeast cake, or at the same time. 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 1-4 daysbefore pitching a fresh culture of ''Saccharomyces''. After 1-4 days, a fresh culture of ''Saccharomyces'' is then pitched to finish the fermentation. The 1-4 day head start gives the souring bacteria a head start and results in a low pH beer. The harvested yeast cake can also be pitched at the same time as a fresh culture of Saccharomyces, or afterward. The decided timing on when to pitch the harvest sour yeast cake will affect the acidity of the sour beer: early pitching of the sour yeast cake generally produces a more acidic beer, and later pitching generally produces a less acidic beer. It might also be possible to not pitch any fresh yeast and rely completely on the sour yeast cake to fully attenuate the wort. In this case, it might take 3-10 days for the fermentation to begin because this approach would be relying on the ''Brettanomyces'' that is alive in the yeast cake, and the growth phase of ''Brettanomyces'' can take a week or so. If relying only on the sour yeast cake to fully attenuate the wort, making a starter for the yeast cake is preferable to ensure that the microbes in the yeast cake are viable. A [[Pellicle|pellicle]] might also develope early, depending on what types of microbes are in the yeast cake and if they have a tendency to develop pellicles or not.