Which is better: old yeast from the primary fermentation or reused yeast from primary fermentation?

Question

It's known that reuse yeast from early stages is better. But during the fermentation the reused yeast multiplies itself, producing newer cells. Said that, I'd like to know if it's better to use yeast from the primary fermenter of the first brew or use yeast from the primary fermenter of subsequently brews. For example, suppose I use some pack of yeast to make a beer and then harvest the used yeast (I will call it by 1-yeast) from the primary fermenter and put it in the fridge. After some time, I use the 1-yeast to make another beer and, again, harvest the used yeast (named 2-yeast) from the primary fermenter. Then I use the 2-yeast for another brew and so on. After n brews, which yeast is healthier: the n-yeast or the 1-yeast? Obviously, the 1-yeast is stored for longer time.

Answer 1

In theory you could reuse yeast forever, that's how breweries were supposed work before the first pure yeast strains were isolated by Carlsberg. But...that probably only works in locations favorable to the yeast (especially certain areas of Belgium), and with continuous brewing happening to keep the yeast in log-phase growth.

In reality, most of this beer had other organisms in it, such as Brettanomyces and probably other stuff. Modern breweries are also less clean than a laboratory. Since they want to make a consistent product, they buy yeast from a lab and use it for 8 - 10 brews, and then replace it before yeast health and contamination mess things up. Also, high alcohol beers stress the yeast, so they'll usually dump the yeast after making something strong.

Stressed yeast also tends to mutate, and accumulate 'petite' mutations (various mitochondial defects, I think). Since they grow slower in the presence of oxygen than non-mutants, they should be out-competed a starter culture conditions. But if the yeast goes directly from brew to brew they might accumulate.

So, after several generations of brewing, if you provide oxygen periodically and a chance for log-growth, you'll have healthy yeast that is probably happier than the stuff that's been in the fridge for a few months...

But it will probably also have some healthy bacteria or wild yeast in it too.

Answer 2

The honest answer is: There's no black and white answer to this. Reason being that yeast is a living micro-organism, making it very difficult (especially on the homebrewer's scale/budget) to measure these sorts of things. Oh, and also every yeast is different in so many ways, one of which being alcohol tolerance and it's effects on yeast health.

I've personally reused yeast over several batches. My yeast & bugs harvested from a bottled sour are now on their 9th generation and they are all within 3% apparent attenuation. For all of those, either I split a 5 gallon's worth of slurry into two 5 gallon carboys of wort, or I built up a starter, and they all seem to be doing well.

If I had to choose between reusing yeast from a recent batch or pitching old yeast from the aged first generation, I'm no doubt going with the most recent re-pitch, with an optional starter depending on how fresh it is. If I have to re-pitch from the first generation, depending on how old, I'll either build up a small (2 liters) or a large (1 gallon+) starter before pitching the yeast, if I do at all (6+ months = start over).

Also, if you want a good read on someone who performed a blind taste test between two batches of either fresh yeast vs re-pitched old yeast, check out this experiment. TL;DR: Out of 13 people tested, only 5 people were able to tell a difference in the beer that used the slurry, 8 could not. He used a triangle test with three samples to try, two of the three samples were beers from a fresh pitch of yeast, one was from a re-pitch. 5 could pick out the one, the other 8 could not. Statistically that isn't enough to say that there's a significant, noticeable difference.