Mechanisms for Pitching Rate Guidelines
 

So the standard pitching rate that gets quoted all the time is 1 million cells/mL/(degree Plato) plus or minus 25% or so depending on what reference you read and if you're brewing an ale or lager.

Qualitatively I've usually heard this explained as 'high gravity wort is more stressful to the yeast so you need more cells for a healthy ferment', but does any know (or care to speculate about) the actual mechanisms that stress yeast in higher gravity wort? Also, any other reasons for needing more yeast in high gravity wort besides stressed out yeast?

Off the top of my head I could think of osmotic stress, less dissolved oxygen in the wort, higher ABV towards the end of the ferment, maybe a larger buildup of waste products due to more fermentation..... It would seem like some big breweries or brewing scientists would have looked at this and that's what guided the pitching rate guidelines.

Basically, yeast multiplies based on the amount of sugar in solution and tries to come to a comfortable equilibrium so each cell ferments a certain amount of sugar. When they multiply, they kick out flavor compounds. Too much growth and they get stressed and produce possibly unwanted flavor compounds. If they can't multiply enough to ferment the sugar comfortably, they can kick out possibly unwanted flavor compounds. Temperatures will effect growth, colder temps will retard it, higher temps can let the yeast get out of control and produce unwanted flavor compounds.

Some yeast strains produce good flavors at .5 Million cells/mL/P. Lager strains when pitched cold work better at 1.5 Million cells/mL/P typically. Use too much yeast of any strain and you don't get the growth that produces the good flavor compounds.

Is it really as simple as more sugar = faster yeast multiplication? What made me start wondering about this was on another thread about the gravity used to make starters it came up that (for starters at least) you get more yeast multiplication in a 1.020 SG starter versus a 1.040 SG starter (but the 1.040 is preferred for starters since the yeast don't have to make as much of an adjustment when you pitch them into the wort).

This is pure speculation, but it seems that something more along the lines of "when yeast multiply in higher gravity wort they're more likely to produce off-flavors (because they're stressed out by x, y, z) so you want to limit the amount of multiplication that has to take place by pitching more yeast" could be going on.

I didn't and wouldn't say a "faster yeast multiplication". More sugar means you need more yeast so they grow the same amount (produce the same amount of esters) as they would in a lower gravity wort. Osmotic pressure is a factor in higher gravity wort so you want to make sure you supply lots of oxygen so the yeast can produce the sterols they need for a healthy and pliable cell wall. A healthy cell wall allows healthy budding, regulates fermentation rate and keep things out that harm the cells later on like alcohol. Oxygenate the starter (even when using a stir plate), the wort before pitching and again after a few hours.

Pitching more yeast than the recommended pitching rate is not a fix for prior sluggish or incomplete fermentations at the proper pitching rate. Generally, you want 4-5X growth for a good ester profile in Ales. When you over pitch, you don't get the growth and hence you don't get the good tasting/smelling esters and are left with a blandish beer.

I've had to diagnose incomplete fermentations before. The beer wouldn't complete fermentation compared to a forced ferment test. Getting a microscope has revealed that my process or stir plate does not grow the same number of cells as MrMalty suggests. There would be 75% of what the calculator shows and with certain strains, fermentation would not finish. WLP007 being one of them that gave me grief a few times. California Ale yeast (from either liquid supplier) didn't seem to care at all.

Another cause of incomplete fermentation I've found is temperature shock. I ferment in upright freezers with temp controllers. If fermentation is slowing down and the controller kicks on, it can put the yeast to sleep prematurely. When I see that fermentation is slowing, the controller gets turned off and the spunding valve turned up. The ester profile has been locked in by then anyway so a rise of a few degrees isn't going to hurt anything, especially if the pressure rises too.

Well, so maybe not faster yeast multiplication, but it seems like you're arguing that more sugar content in the wort leads to more yeast growth (a higher final cell count to starting cell count ratio). Is that a fair summary? If that's what you're saying, this is the part I have trouble with due to the comment above about 1.020 SG starters giving you higher cell counts than 1.040 starters.

Can you post the link regarding 1.020 vs. 1.040 starters please? I have a hard time believing it too. There is an exponential-ish relationship between inoculation rate and doublings (click the graph I made). I've never seen data on gravity vs. doublings at a fixed inoculation rate.

http://cdn.homebrewtalk.com/images/1...ings-56367.bmp

Yeah, the gravity versus final cell count (or doublings) graph is one I really want to see, but I haven't found one anywhere. As for the 1.020 vs 1.040 starter, I've seen it popup multiple places but none of the sources are terribly satisfying. Most of these sources also state that yeast companies use 1.020 SG culture media for growing their yeast.

http://morebeer.com/brewingtechniques/library/backissues/issue2.3/kingtable.html
http://www.franklinbrew.org/wp/?page_id=124
http://www.homebrewersassociation.org/forum/index.php?topic=4839.5;wap2

If you assume that for a given wort the yeast always grow to the same final cell count, then you'd expect an exponential (actually negative logarithmic) relationship between doublings and inoculation rate.

Final cell count = Initial cell count * 2^(Doublings)
Doublings = log2(Final cell count)-log2(Initial cell count)

That equation qualitatively matches your curve pretty well.

First link:
"I believe that yeast pitched into the wort will start faster if it does not have to adjust to a significantly different gravity."
I don't agree with that. In fact, I believe that making a starter at a higher than 1.040 gravity is worse for the yeast but this is based on more recent information (Jamil?), not my own studies. You should be weary of information that suggests you should add any amount of hops to a starter. I suppose this is before the days of fermcap-s.

Second link: My opinion is that information as it relates to homebrewing from 2001 should be closely scrutinized.

Regarding the 3rd link where Martin talks about keeping the headspace full of sterile air, I have not seen an increase in yeast growth using that procedure (I didn't know anyone else had tried that! click the pic). There is no real quantitative information in those links for me to infer that a 1.020 wort will make more yeast than a 1.040. 1.020 is gentler on sleepy yeast I imagine and maybe that's why it's standard for yeast companies to propagate their stored yeast that way.

double drilled stopper (with foil around the lip to keep dust off) with blow off tube and sterile air injection into starter on stir plate.
http://cdn.homebrewtalk.com/images/1...hoto-56374.jpg

Yeah, I agree none of those links are terribly convincing but I'm sure this has to have been looked at more scientifically somewhere. Googleing around a bit...

http://www.ajevonline.org/content/49/3/283.short

Can't access the actual article and the experiments were done with grape juice, but the abstract says that going from a sugar concentration of 200g/L (about 1.067 SG I think) to 300g/L (1.096) lowered the final yeast cell biomass for all the yeast strains they studied.