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.
I don't think those 15-20 um rods are bacteria. Bacteria is much smaller.
I have seen these rods before and think that they are crystals. Possibly Calcium oxalate monohydrate. If they were bacteria I would have noticed an off flavor given their rather large concentration in which I sometimes see them.
Phosphoric acid is not a strong acid. It's a weak acid. To get all the protons to dissociate you need to have a pH well above 8.
I've seen some crystalline structures that are about the same size. These look rounded on the ends and are sometimes bent. I'll have to use the oil immersion lens and take a picture next time I see them.Delete
The pKa of Phosphoric is 2.1 which is very low. My mistake on identifying it as a strong acid. It has a lot of strength as an acid, but I take it is not qualified as a strong acid.
Another question. How do you plan to asses the kill rate of the low pH? The only reliable way that I can think of is plating on selective growth agar that suppresses yeast growth.ReplyDelete
A plate suppressing yeast growth would be the reliable way I can think of as well. I chose a slurry that appeared to have bacteria that would be visibly countable. My plan was to count the bacteria before and after the wash in both cases.Delete
What if you intend to use the slurry the same day (as when one pitches on a yeast cake), and no starter is needed? Do you just add an appropriate amount of the slurry/Star San mixture, or does the Star San need to be decanted?ReplyDelete
It would be okay not to decant, but may impact the flavor of the beer. If the Star San is not neutralized you might notice a slight sourness. If it is neutralized with backing soda some of the resultant salts may be tasted. Because all of the reaction products are soluble, most of these are removed by decanting.Delete
Very interesting. I recently put 3/4 cup slurry into the same amount of properly diluted starsan. Let it sit for two hours, used baking soda to raise the Ph to 5 and then made a one liter starter on a stir plate, which took off within maybe two hours.ReplyDelete
I'm flying blind here, though. Did you assess yeast viability after the starsan wash?
That's great that the acid washing worked! Yes, viability was good after acid washing when I checked it.Delete
I was under the impression that star san is neutralized by yeast during the ferment as a source of nutrient phosphorus? Hence the saying "don't fear the foam" or is this such a large amount of star san that it will not all be broken down?ReplyDelete
I wish I could credit this site for getting my info from, but I've been doing this similar procedure and can't recall exactly where I got the info from.ReplyDelete
I've done exactly this procedure and had great results. The microbiologists-turned-brewers tend to be ultra-cautious when it comes to cleaning, storing and preparing yeast. In practice, I've found it to be incredibly simple if basic beer sanitation is followed and results in incredible tasting beers. I can definitely tell the difference now between my own prepared yeasts (washing when necessary and using starters) than with using package yeasts (I use a local brand that contains enough yeast for accurate pitch rates, not like the main brands)... and my method always produces superior results.
I like to think that yeast, to do well, always needs to be prepared and put "on-deck." I think it's a mistake to take dormant refrigerator yeast and let it go straight at a work. They need a little warm-up!
I've worked this method into my yeast system to great success. While I don't get as ultra-cautious as many microbiologists-turned-brewers do, bacterial contaminants are real and working in some sort of acid-wash is necessary if you're tending your own yeasts. The professional brewer's text "The Practical Brewer" by John McCabe lists acid-washing as an industry protocol.ReplyDelete
My brewing is based off of two seasons (an academic calendar) so brewing for a Fall and Spring season. In each season I brew somewhere between 10-15 5 gal batches. I have a favorite all-purpose ale yeast from a local yeast-ery. At the beginning of the season I buy one package and through harvesting and cleaning from the first batch(es) I prepare my yeast for the entire "season."
As a very important aside, I've found it incredibly beneficial to ALWAYS do starters. The previously mentioned local yeast-ery that I use guarantees accurate cell count / pitching rates for the average 5 gal beer, and I can definitely tell the difference with a beer made from a fridge pack w/o starter, than one of my own prepared starters from my harvesting, cleaning, and starting procedures. I don't believe that yeast (even with adequate cell counts) can just come straight out of the fridge from being dormant, thrown into a 5 gal wort and expect to excel. There is a huge and noticeable benefit from getting your yeast warmed up and "on deck" before giving them such a huge challenge as 5gal of wort!
Thank you very much for this method, and good luck to all of you who are forging your own paths, methods and protocols out there for working with your yeasts!