Yeast: phases, and pitching practices.

[Lithobid]

As a fairly new all grain brewer, have some fundamental questions concerning life cycles and growth stages of brewing yeasts. I've found there are some gaps in most of the online, and published literature that leave me with some questions and concerns about when to pitch, and when to re-start yeasts that I've been culturing.

I've read extensively through Kai's website (braukaiser.com) and also a wonderful article Yeast Propagation and Maintenance Practices published by R.M. Raines on the Maltose Falcons website. They however leave me with some fundamental curiosities.

I understand as outlined in several articles I've come across, the different phases of yeast propagation, ie. lag phase, accelerated growth phase, exponential phase, decelerated growth phase, and stationary phase. Where my questions arise, is at what phase should we be pitching the yeast? I've read from several microbrews and podcasts, that pitching at high Krausen is optimum. This however conflicts with what a lot of home brewers seem to do with their starters, in which they crash cool them decant the soft beer and then swirl and pitch the yeast after a period 24-36 hours. This seems to be more of pitching while the yeast is in the first phase.

If pitching at high Krausen is optimum, then why is there so much emphasis on oxygenating your wort, at this point the yeast cell walls have fortified, and there should be little need for excess oxygen, correct?

If pitching at the initial phase is optimum, then during the creation of the starter you'd want to oxygenate, allow to Krausen, and then crash cool and decant, what percentage of the yeast cells would no longer be viable at this stage?

Im also not sure if either will make a difference. Any input would be gratefully appreciated.

 

[mkeckjr]

I'm not much of a microbiologist, but I will chime in only because I've recently been re-reading Chris White's book, and I think the first couple of chapters in his book cover a bit of this. I will re-hash what I've been reading (probably incorrectly since the book is not in front of me) and hopefully anything I'm wrong about others can correct.

First, a tiny bit of what I remember: at the high level there are two pathways that sugar can be broken down by yeast. The primary pathway is through aerobic respiration, which uses oxygen from the wort and a co-enzyme called nicotinamide adenine dinucleotide (NAD+) to facilitate the production of adenine triphosphate (ATP, which can be though of as "cell energy") and excess pyruvate. The second pathway breaks sugar down into ethanol and CO2 using the excess pyruvate from the primary in the reaction and freeing up more NAD+. The yeast very much prefer the primary pathway because it produces energy and does not produce ethanol, which is poisonous to yeast. However, eventually the oxygen in the wort is depleted and/or the supply of NAD+ has been used, and the yeast decide to use this secondary pathway which FREES NAD+ for use in the primary pathway. (Sidebar: it turns out even if there is infinite NAD+ and O2 that the yeast will take the secondary pathway sometimes anyway; I think this is called the Crabtree effect).

I cannot comment at all on your first question; I don't know if it matters when you pitch yeast (at high krausen or not), but I would imagine that as long as the cake is relative fresh (a day or two), both will perform very similarly, and what I think you're going for is high viability. At a couple of days you should be at something like > 90%. However, I would imagine in either case (top cropping or making a starter and then cooling) you want to oxygenate the wort to promote healthy aerobic respiration in this new medium.

The approach you describe in your final comment is what I do: starter on a stir plate to oxygenate, crash cool, and then decant/pitch within a day or two. I have good results with this approach, and it's important to me because I brew a lot of lagers, so I don't want that extra starter wort in there, as I ferment that at room temp and it probably has off-flavors.

For what I've stated incorrectly, hopefully others more experienced will chime in and address that. My suggestion is to read chapters 2-4 of Chris White and Jamil Zainasheff's "Yeast" book, and that should cover all that stuff.

 

[Lithobid]

Appreciate the info, I've gotten from several sources that it's a 6 of one 1/2 a dozen of another type of situation, though if you are not going to pitch a starter at high Krausen, then give it time to ferment out and settle before pitching in the wort.

 

[hannibalmdq]

Yeast in wort never respire. This information continues to be repeated though. Oxygen is not necessary for fermentation. The crabtree effect is always active when yeast are added to wort because of the levels of sugar in the solution.

"All fermentable sugars, including fructose, maltose, and sucrose (and galactose to a limited extent), induce the Crabtree effect, but glucose exhibits the strongest effect (2). Brewers’ yeast is often said to prefer fermentation only when glucose levels are high, but “high” is a relative term; glucose in excess of about 0.4% (w/v) will bring on the Crabtree effect; most worts (both all-malt and adjunct) contain an excess of 1% glucose (3), which is more than enough glucose to induce the Crabtree effect. The other fermentable sugars present in wort induce the Crabtree effect as well (for example, a wort with an O.G. of 1.040 [10 °P] is 10% sugar). This high concentration of sugar makes it virtually impossible for brewers’ yeast to respire in wort." Source

Yeast require sterols/lipids to keep their cell membrane in good condition (flexible, etc). When these compounds fall below a certain threshold the cell membrane becomes inflexible and it becomes difficult to split during cell division. This will stress the yeast and can halt their reproductive cycle entirely.

Yeast will synthesize these compounds from oxygen, which is why it is said that yeast require oxygenation for fermentation. That is a half truth. If you can provide those compounds from another source (packaged with dry yeast at certain levels, olive oil, etc), you do not need to oxygenate. That being said, oxygenation/aeration is the most common method for providing sterols in the homebrewing community. It's has nothing to do with respiration though.

Yeast will typically absorb all the oxygen in the solution in a very short timeframe (within 30 minutes under ideal conditions), so adding oxygen will rarely hurt a fermentation.

 

[tgmartin000]

Your yeast ate going to reproduce once pitched, so additional O2 is going to benefit the new cells.

As far as the viability of the starter, you're looking at about 98% viability, based on Mr malty. I always over pitch by about 5% to account for this and things like loss, not getting all the slurry into the fermentor, etc.

 

[trentm]

Y
Yeast will typically absorb all the oxygen in the solution in a very short timeframe (within 30 minutes under ideal conditions), so adding oxygen will rarely hurt a fermentation.

Your entire post is spot on but I wanted to add to this last comment. When yeast is exposed to oxygen, as stated, it is quickly absorbed. Also, as stated, the yeast use oxygen to synthesise sterols/lipids. Once the oxygen is gone (30 minutes) there is none left for the new daughters cells. The parent cell shares her sterols/lipids with the daughter. So each new cell has less and the parent has less, creating cells with compromised cell walls that have trouble budding. Toward the end of fermentation all the cells are less healthy, just when they need it the most. So it's our job to ensure we pitch the healthiest cells possible. This can be done with our starters. It's not so much the amount of oxygen in our starters but a continuous supply. Continuous so that each new cell has some oxygen available to synthesize sterols/lipids. That's why a stir plate works so well. But with all the CO2 being produced it is a challenge to get some oxygen in. Stir fast and don't over fill your starter vessel, it will help to remove CO2 and to bring in air.

Now for the OP question: Pitching at high krausen and after the starter has finished both have advantages. First, high krausen is considered the exponential phase. If you pitch your starter at high krausen your main fermentation will skip the lag phase and go straight into the accelerated growth phase. If you do this there is little need for oxygenating the wort if you have ensured the starter culture has been exposed to oxygen throughout the propagation.

If you allow the starter to finish the fermentation the yeast will go through the lag phase after pitched. This is a good time for the yeast to build up there sterol/lipids if they were not able to get a consistent supply during propagation and if you have properly aerated the wort. But to do that it is good if the yeast have a good supply of carbohydrates to supply energy during the lag phase. These carbohydrates are be built up at the end of fermentation (propagation) so as Chris White points out in "Yeast" it's a good idea to let the yeast rest for 8-12 hours after the starter has finished. I take this to mean at fermentation temp. Then you could cold crash in order to decant or pitch the whole starter if you don't worry about off flavors from the propagation.

I've tried both ways and for my system and beer styles, I allow the propagation to finish, rest for ~8 hour and pitch the whole thing. Caveat: I use wort for my propagations that I have mashed from the same grain I use for the beer and I propagate from slants under ideal conditions. So I only have to have (for my Belgian strains) a 1200-1500ml starter for 11 gallons of beer.