Mechanisms for Pitching Rate Guidelines

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bdh

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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.
 
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.

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.
 
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.
 
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.

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.

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.

 
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.
 
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.
 
One of the reasons why you want to pitch a lot of yeast into high gravity beers is simply this:

When yeast experiences an increased alcohol level in its environment it wants to build stronger cells walls. That was shown in experiments where the yeast had access to oxygen at that time. In brewing fermentations, however, this oxygen is not available and all the yeast has are its sterol reserves. If the yeast has undergone lots of growth up to this point these sterol reserves will be depleted since they are divided between mother and daugther cell at budding time. This growth can be limited with larger pitching rates and thus the individual yeast cells will be better prepared for the high alcohol environment

dstar, how much variability in yeast growth compared to Jamil's calculator have you seen? I expect that there is actually a lot and the limited data I have supports that. I have not yet had a chance to run a series of controlled yeast growth experiments. All I have done so far is track yeast growth in many of the starters I made and the growth per extract ranges from 0.8 all the way to 1.7 Billion cells per gram for starters on a stir plate covered with foil. There is no apparent correlation with the initial gravity of the starter or the initial cell density.

Kai
 
dstar, how much variability in yeast growth compared to Jamil's calculator have you seen?

Usually get ~75% of what Jamil's calculator predicts. One time I got 51%. I've been plugging in 25% additional cells needed lately and get what I need out of the calculator. I use foil over a flask on a stirplate. I'm going to re-design the stir plate so I can get more agitation without throwing the bar. I tried pumping sterile air into the headspace continuously but that didn't help. I may try what you did with the stone in the starter wort but that is more stuff to clean.
 
Here are links to two quite recent scientific articles (they should be free) regarding high gravity brewing:
http://onlinelibrary.wiley.com/doi/10.1002/j.2050-0416.2007.tb00259.x/abstract
http://onlinelibrary.wiley.com/doi/10.1002/j.2050-0416.2011.tb00472.x/abstract

The first is a study on the influence of pitching rate on flavor compounds and fermentability, while the second one (written by my boss by the way :mug:) is a review on techniques to improve high gravity brewing (you might find something worthwhile in there) :)

EDIT: <-- Why the heck does the forum software think I'm from Seattle, Washington even though I have entered a completely different location :)
 
Tanks suregork.

On the topic of the accuracy of the pitching rate calculators that are out there, I&#8217;m in the middle of an experimental series where I add various amounts of yeast to roughly the same volume of the same wort and determine yeast growth per gram of extract on a stir plate.

So far the results show only a weak correlation with the curve that is used for Jamil&#8217;s and Wyeast&#8217;s pitching rate calculators. At an initial pitching rate of 50-100 M/ml the results are close to the calculators with ~1.4 B/g but below and above that they differ vastly. At very low picthing rates I still get ~1.4 B/g but at very high pitching rates (200-300 M/ml) the measured yeast growth drops to ~0.5 B/g while Mr Malty predicts better growth than at 100 M/ml. This makes sense since more of the sugar is used by the larger number of cells before sugar is used for actual growth.

I&#8217;ll publish some data once I have a few more data points. The inherent error in cell counting makes it necessary to have lots of data points in order to draw meaningful conclusions.

Kai
 
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