[S. cerevisiae]

http://braukaiser.com/blog/blog/2013/05/28/starter-wort-gravity-and-yeast-growth/

DeClerck stated that 1 gram of extract contains enough carbon to produce 1 billion cells.  Other scientists have managed to collect data that cells counts as high as 1.5 billion cells per gram are possible, but 3 billion cells per gram does not pass the sniff test.  The total cell count is bounded by the maximum cell density for the culture.

[kramerog]

Yeast growth is actually limited by volume, dissolved O₂, and the amount of carbon available to the cells (for those who do not known, sugar is carbon bound to water; hence, the name carbohydrate).  Maximum cell density is maximum cell density.   If volume had no bearing on cell count, then a culture would remain in the exponential phase for the length of a fermentation.  Conversely, if a culture runs out of carbon before it reaches maximum cell density, it will never reach maximum cell density.

The interesting thing about dissolved O₂ is that the load placed upon the medium is not solely dependent on the health of the cells when they are pitched.  Different strains have different O₂ requirements.  A scientist named of Brian H. Kirsop outlined four classes of O2 demands by yeast strains.

http://onlinelibrary.wiley.com/doi/10.1002/j.2050-0416.1974.tb03614.x/pdf

Class O1: Yeasts whose requirement is satisfied if wort is half saturated with air (4ppm dissolved O₂)

Class O2: Yeasts whose requirement is satisfied by air-saturated wort (8ppm is the maximum dissolved O₂ from air at sea level)

Class O3: Yeasts whose requirement is satisfied by oxygen-saturated wort (40ppm dissolved O₂)

Class O4: Yeast whose need is not satisfied by oxygen-saturated wort (> 40ppm dissolved O₂)

I am currently working with an O3/04 yeast strain.  I am almost certain that the strain is the John Smith culture, which is related to the Samuel Smith culture.  I am willing to bet that most of the commercial yeast strains sold by Wyeast and White Labs are class O1 and class O2 strains, with easy to use strains such as 1056/WLP001 and 1098/WLP007 being class O1 strains.

Any idea if the Dupont strain is class O3 or O4?

[narcout]

http://onlinelibrary.wiley.com/doi/10.1002/j.2050-0416.1974.tb03614.x/pdf

There is some interesting information in that article.

I didn't realize the olive oil trick had already been studied back in the 70's (well, oleic acid anyways).

[S. cerevisiae]

Any idea if the Dupont strain is class O3 or O4?

No, I do not have any data on the strain. 

[klickitat jim]

You want the yeast to be as healthy as possible and the lower the gravity (to a point) the better in that regard.  There is no benefit to increasing the gravity and it might even be detrimental.

I don't know if I agree with that. If you're starting with healthy yeast from a fresh starter, then my understanding is that you can build up their tolerance to gravity and alcohol in successive generations. That certainly matches my experience, at least. For really big beers I have had great results by pitching from a yeast cake from a batch that was in the 1.060's, which was previously stepped up from a normal-gravity starter or a session beer.

AFAIK, there is no such thing as building up "tolerance".  You simply want more healthy cells, and a lower gravity will make sure they're healthy.

It seems like that would take mutation to a more tolerant strain. In my pea brain understanding, maybe what seems like building up tolerance is actually just slowly reaching tolerance limits. I could be totally wrong

[S. cerevisiae]

I've just set off a 750ml shaken starter of WLP550 ('Chouffe') planning to step that up x5 by volume in the next step, ready for a 20L 1.075 tripel.  That volume is all I can get in a glass vessel, which is where I'd prefer to keep it.

Who in the world told you that you need that much yeast for 20L of 1.075 wort?    A gravity of 1.075 is higher than normal, but it is not very high.  That's a 2L starter beer at most.

[69franx]

From what I have read here, yeast growth is limited by stress and volume of starter. Only so much can grow in a specific size starter, the only difference the gravity makes is whether or not the yeast are stressed or not

It is actually limited by the amount of extract (i.e., sugar) available in solution. Here's an experiment Kai ran a few years ago. Essentially, the same amount of yeast grew in 400mL of a 5 Plato wort as it did in 200mL of a 10 Plato wort. Yeast growth and viability didn't start to decrease until he got to 20 Plato (~1.083).

http://braukaiser.com/blog/blog/2013/05/28/starter-wort-gravity-and-yeast-growth/

Just when you think you are starting to understand something... And then you realize you were ignoring part of the equation. Learn something everyday

[S. cerevisiae]

Just when you think you are starting to understand something... And then you realize you were ignoring part of the equation. Learn something everyday

Frank, I would take everything on Kai's site with a grain of salt.  Kai is smart guy, but a lot of his work does not pass the sniff test.  Let's take how he makes plates (http://braukaiser.com/wiki/index.php?title=Making_Plates_and_Slants).  No one who knows what he/she is doing makes plates that way.  The proper way to make glass plates is to dry sterilize the glassware in an oven and pressure cook the media in a separate covered container.  The media is allowed to cool to between 120F and 140F before the plates are poured in the presence of a flame source. 

Here's a photo of Kai's plates:



Did you notice the large droplets of water on the covers?

Here's what properly made plates look like (I poured these plates back in 2013 using the technique outlined above):



Did you notice that the covers only have a thin layer of condensation? This amount of condensation will flash off during the proofing period. 

Condensation is one's enemy when making plates.  Plates are stored/incubated upside down, and are turned over for streaking and inspection. The gap between the dish and the cover is not air tight, and condensation is a mold magnet.  Guess what happens to large amounts of water on the cover when one turns the plate over? It ends up on the plate, dragging any microflora that made its way between the dish and the cover with it.

You do not have to take my word on it.   There are several videos on the YouTube that demonstrate the process using pre-sterilized plates.  Here's one:  https://www.youtube.com/watch?v=q7I5YN571kw

Did you notice that the amount of condensation on the covers on the plates in the video linked above matches my plates?  The flame source creates an updraft.  The updraft prevents microflora from settling on a plate while the cover is off.  If you look closely at the orange GL45 polypropylene cap on top of the media bottle in the video, you will see what looks like masking tape with black stripes.  That tape is called autoclave tape.  The black lines appear when the media has been subjected to heat and pressure levels that can render the media sterile.

Here's a photo of a piece of autoclave tape that was on one the media bottles in which I prepare 40ml absolutely sterile starter media (I stuck it to a piece of wax paper, so that I could shoot the photo):



40ml of absolutely sterile media in 100ML media bottles (first level starters when propagating a culture from slant):

[69franx]

Thanks Mark. A lot to take in thus early, will have to go over this again once fully awake

[narvin]

"Opening them to shake off the moisture risks contamination and the moisture will be absorbed by the slant media during storage"

Even with a flame source, I'd be more concerned about pouring autoclaved wort in a household kitchen than some sterile moisture that will be reabsorbed by the agar.  Remember, what works in a lab may not be appropriate for homebrewing.  If you get one with mold, throw it out before using it.

[S. cerevisiae]

Even with a flame source, I'd be more concerned about pouring autoclaved wort in a household kitchen than some sterile moisture that will be reabsorbed by the agar.  Remember, what works in a lab may not be appropriate for homebrewing.  If you get one with mold, throw it out before using it.

The plates shown above were poured in my kitchen and proofed at room temperature for three days.  If anything was going to grow, it would have in three days.  In practice, it is much easier to produce mold-free plates in a kitchen using the method outlined above than it is to make mold-free plates in a pressure cooker.  A lab is no more sterile than a kitchen.  Most of the labs that I have visited are not dust-free environments. They key is to make one's own dust-free zone via the use of a flame source.   The flame source does not only heat the air directly above it.  It also heats the air out a specific distance on the sides of the flame.  No dust can land on a plate if the air above it is rising. 

Another trick that can be employed is to not completely remove the cover.  Instead, one only lifts one side of the cover high enough to be able to pour enough media to cover the bottom of the dish.  It takes a little practice to be able to lift one side of the dish cover, pour enough media to cover the bottom of the dish, and place the dish cover lid down in one smooth motion without spilling media, but anyone can master it.

The plate shown below was from the batch of plates shown above.  This plate was made in a 60mm x 15mm petri dish, which is on the smallish side for this type of work. That's Southern Tier's strain.  The colonies in the red rectangle were the candidate CFUs (colony-forming units).



Here's Scottish and Newcastle's strain in a 100mm x 10mm plate (my favorite size):



The complete absence of condensation and anything other than yeast in the plates tells us two things.  Our pouring technique is sound and the sources from which these cultures were obtained practice sound quality control.

[S. cerevisiae]

One last thing, learning to pour plates the way that I have outlined is petri dish material agnostic.  It can also be used to pour pre-sterilized plastic petri dishes, which cannot be placed in an autoclave.   I originally learned the technique in high school, but it was the technique championed by Maribeth Raines when she and Jeff Mellem were starting up BrewTek (a lot of home brewers asked her how she poured plates).  It was reassuring to  discover that professionally lab-trained personnel used the same technique that clueless kids used in high school.

[narvin]

All good advice, but sometimes easy is better at home.  I can verify that the agar media will reabsorb the moisture that forms during cooling over a few days.  Obviously if you are going to use them quickly, this isn't ideal, but it's fine for me.

[denny]

I've just set off a 750ml shaken starter of WLP550 ('Chouffe') planning to step that up x5 by volume in the next step, ready for a 20L 1.075 tripel.  That volume is all I can get in a glass vessel, which is where I'd prefer to keep it.

Who in the world told you that you need that much yeast for 20L of 1.075 wort?    A gravity of 1.075 is higher than normal, but it is not very high.  That's a 2L starter beer at most.

THIS^^^^^

[ynotbrusum]

I had the unique opportunity to visit a university lab and watch plate preparation and even streaked some with salmonella under the watchful eyes of microbiology grad students.  One thing for sure was autoclave was used for all medium (agar) and flames were on whenever working with microbes outside of a positive ventilation hood.

I don't go for that level of sanitation at home by any measure, but I try to be mindful about things to minimize bad results.  If I am in the kitchen I will work next to a lit stove burner for good luck.