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Old 12-08-2013, 01:27 AM   #11
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The responder is basing his answer on a study that was performed a couple of decades ago. However, that study involved using a centrifuge and autoclaved distilled water to remove all nutrients from the culture, which forced the culture into dormancy. That level of separation is impossible achieve at home. The culture does not go dormant when rinsed with boiled tapped water using the technique that is prescribed on this forum and other amateur brewing sites. It remains in the stationary phase and starts to autolyse unless forced out of the stationary phase via the introduction of a carbon source.

“In the stationary phase, the accumulated yeast biomass remains relatively constant and the specific growth rate (μ) returns to zero. After a prolonged periods in the stationary phase, yeasts may die or autolyse. This in turn may influence the continued growth and survival of the residual yeast cells.”

Walker, Graeme. Yeast Physiology and Biotechnology. Wiley

Furthermore, boiled water is not sterile, tap water is not nutrient free, and the pH of water is significantly higher than that of green beer, which allows spores to germinate. Water has to be autoclaved at 250F/121C for at least 15 minutes to be considered sterile.

Here's a link to an article about yeast storage that was written by Chris White for BrewPub Magazine:

"Yeast is a living organism and is most happy and healthy when feeding on wort sugars. When fermentation is complete, yeast cells flocculate to the bottom of the fermenter. They then go into a resting state. Yeast under beer is fairly stable, and most brewers agree that the best place to store yeast is under beer."

"Storing yeast under water, as opposed to under beer, is becoming more popular. Sterile distilled water storage puts yeast in a resting state, and some reports suggest yeast can be stored in this manner for years, with no refrigeration. Storage under water is generally done with small quantities of yeast, which are then propagated in a lab. But it is possible that this can be applied to storage of yeast slurries. Some brewers are now trying this. The key is to use sterile distilled water and wash the yeast slurry several times in the sterile distilled water to remove any traces of beer. This is best done with a centrifuge, but that is impractical for most craft brewers. White Labs has had mixed success with sterile water storage, so time will tell if this procedure will work for craft breweries."

I have searched several major scientific research databases for publications that support storing yeast under boiled tap water. However, I have yet to find a single publication that supports the process. If storage under boiled tap water was a viable solution for the long term storage of yeast, there would be hundreds of publications in which the process was referenced because Saccharomyces cerevisiae is one of the most researched microorganisms on the planet. The groundwork for the field of genetic engineering was basically laid by Øjvind Winge using Saccharomyces cerevisiae at Carlsberg Laboratory.

In closing, I have maintained a yeast bank on agar slants for most of the time that I have been an amateur brewer. If sterile water storage was truly viable in a home brewery, I would not mess with slanting yeast. As it stands, the best way to store yeast at home is on agar slants.

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