Starsans, when properly prepared is a 0.08% phosphoric acid solution. (Think about the concentration. Even with just a trace of acid it is strong enough to kill nearly all bacteria) The pH of this solution is between 2 and 3 which is just about the recommended pH for yeast washing. If a small portion of yeast is combined with a much larger portion of star sans the pH will stay in the yeast washing pH range. If this yeast slurry with acid was added to the beer it would likely be noticeable sour. To prevent this, the acid can be neutralized.
Phosphoric acid not technical a strong acid although it is quite powerful with a pKa of 2.1(1). If this is combined with a small amount on a weak base the acid will pull all of the needed OH ions from the weak base to the point when all of the hydrogen ions have been cancelled. Because the ions to not disassociate easily from the weak base creating a solution that is too alkaline is not a concern.
Another advantage of using a stronger acid with a low concentration, That means the highest concentration of salts in also very low. One weak base that nearly everyone already has is baking soda, or sodium bicarbonate. This has a pKa of 10.3which is much closer to the neutral pH of 7. When sodium bicarbonate reacts, it breaks down into carbon dioxide and sodium hydroxide. Three salts can be formed from the combination of sodium hydroxide and phosphoric acid.(1) These are monosodium phosphate, disodium phosphate, and trisodium phosphate. These salts are fairly soluble in water, and will also be in low concentration. Crashing and decanting the yeast will remove a large percentage of this salt.
Recently I put this to the test. A slurry was selected that had low viability of 50%. It also had easily distinguishable bacteria. These were 15-20um long rods that stain with Methylene Blue. Two starters were prepared, one washed with Star San and the other was washed with water.
In summary, it looks like this is an easy way to truly wash yeast.
- Add equal parts slurry and prepared Star San to a container and allow to rest for one hour.
- Add a pinch of baking soda to the container to cancel the acid.
- Add DME and water to create a starter and ferment to completion.