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I love recycling yeast, see it as a great way to save money, and if you find a good one, they are great to use over and over again.

I was chatting with a guy in my homebrew club who is a distiller (for a commercial operation), a very talented brewer, and just a general science nerd.

He stated that an acid wash (pure phosphoric - not Starsan) would GREATLY improve the attenuation rates and general viability of multiple-generation yeast. Essentially, if the upper case "O" is the mother yeast cell, and the lower case is the new/baby yeast cell

Oo

When the baby splits off from the mother

O --> o

it leaves a scar (x) on the cell wall.

Ox o

If left alive, those mother cells will eventually need to work way harder even in optimum conditions for the strain. Eventually, those scarred cells will outnumber the viable cells if you keep harvesting and regenerating. The result for your beer is you will get unwanted esters, phenols, and other byproducts because these cells are working harder than they should be. The phosphoric acid kills the cells with the scars, but not the new ones.

Does anyone have any experience with this practice or this phenomenon of reproduction?

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    As a biochemist and someone that has worked with yeast, bacteria and many eukaryotic cells... I don't see how phosphoric acid washing eliminates scarred yeast cells. Budding does indeed leave scars, but I have never heard of that making them more susceptible to acids. I think the real reason that attenuation increases is because the acid wash helps release hop resins from the cell walls of all the yeast (new and old), which in turn greatly improves there ability to take up maltose from the wort.
    – brewchez
    Commented Jan 7, 2012 at 17:42

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I'd not heard of yeast scarring before, so I researched a bit.

From Replicative ageing and senescence in Saccharomyces cerevisiae and the impact on brewing fermentations

Ageing is the predetermined progressive transition of an individual cell from youth to old age that finally culminates in death. Yeast replicative ageing is a function of the number of divisions undertaken by an individual cell, and may be measured by enumerating the number of bud scars on the cell surface.

So, scarring is used as a direct measure of the yeast's age. But it's not thought that the scars themselves cause deterioration of the yeast:

[...] increasing the deposition of chitin, the major component of bud scars [...] has little effect on longevity.

but rather other factors, such as genetics, play a key role in the vitality of the yeast:

It has been postulated that ageing occurs either by a genetic inhibition of metabolically essential proteins and enzymes, such as DNA repair enzymes or antioxidants

An aged yeast cell exhibits a distinct morphology and physiology from that of younger cells.

It's known that acid washing kills off weaker yeast cells and bacteria, leaving the stronger yeast behind. Although I can't find a direct reference, presumably the morphological and physiological changes make the older cells weaker, and so they are killed off during the acid wash, leaving only young/middle generation cells behind.

If you have a conical fermentor, another method is to crop and repitch yeast from the top of the cone. This article - Yeast Storage and Fermentation: Effects on Viability and Flavor Production offers a lot of information about yeast handling and touches on yeast scarring and acid washing.

Benefits Claimed for Acid Washing

  • De-flocculation and ease of handling

  • Reduced lag period in fermentation

  • Cleaning the cell surface

  • Reduced time to achieve desired diacetyl

(Emphasis added.)

It doesn't specifically mention removal of old cells, however, the article does include a graph showing the number of scars observed from yeast cropped from a range of 12 regions from the bottom to the top of the cone. Those nearest the top were newest and consequently had the fewest scars.

Both articles mention that the bottom of the cone is usually dumped, and only yeast from the top of the cone is repitched. They both mention that pitching younger cells reduces the off-flavors associated with stressed yeast.

To sum up - you want to pitch only young/middle generation cells from the slurry to avoid off-flavors, and these can be obtained either by acid washing or by cropping from the top of the cone.

Links

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Keep in mind that what applies to a commercial operation often has no applicability to homebrewing. Most homebrewers I know who have tried acid washing their yeast end up deciding that it's a PITA for little to no payback. In addition, unless you have a lab, the more you mess with your yeast the greater the chance of contamination. While the theory you cite is not doubt valid, on our scale it just doesn't seem to be worth the effort.

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    is this really an answer or just conjecture and hand-waving? if the theory is "no doubt valid", I'm sure there are others besides me that would appreciate additional details or links.
    – mdma
    Commented Jan 6, 2012 at 23:54
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    He asked about experience with the method and I posted the experience that you can do it but it's not worth the hassle.
    – Denny Conn
    Commented Jan 7, 2012 at 16:34

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