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Old 03-03-2010, 07:12 PM   #1
Bob
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Default Why Not to Pitch On Your Yeast Cake

Knocking out onto a yeast cake is never a good idea. It is - outside of a very, very few specific instances listed below - always over-pitching.

Many amateur brewers report acceptable results by this practice. That's fine, and I applaud their success. Their success does not excuse that this is bad brewing practice.
[Author's note: I categorically refuse to argue with anyone about this. If you practice this and have success, I wish you all the success and enjoyment in the world. Better and more knowledgeable brewers and brewing scientists than you or I could ever be support what follows. If you can bring to bear some significant data from the field of brewing science which indicates otherwise, please send it to me; I like the taste of crow when I deserve it. Otherwise, please don't cloud the issue.]

Over-pitching is always detrimental to the beer. This does not say the beer will taste awful. Rest assured, however, that were one to place samples of the exact same beer - one fermented by overpitching and one by properly inoculating the wort - the properly pitched example will taste better. Blind taste tests prove it.

The two most obvious effects of over-pitching are off-flavors. First, yeast material in excess quickly leads to autolysis, which has flavor by-products with very low flavor thresholds. In properly-pitched beers, this effect can take months to show itself. In beers with an excess of yeast solids, it can manifest in a matter of weeks - in fact, the time spent in a home-brewer's primary.

Second, tasters have observed thin beer, beer lacking in body and mouthfeel. To be perfectly honest, the exact cause of this effect is unknown, but it is strongly correlated with over-pitched yeast, so a connection is highly likely.

Third - and most important for the home-brewer - is suppression of esters. Yeast rely on the growth phase to reproduce enough cells to fully colonize the wort. In that phase, they use malt-based nutrients and the oxygen you provide during aeration to synthesize the components needed to build new cell walls during reproduction. While they're reproducing they're producing esters. All yeast produce esters, even lager yeast, and all beers benefit from ester production (yes, even lagers). Just because you can't taste as much ester from WLP840 as you can from Ringwood doesn't mean that WLP840 doesn't throw esters! Esters are absolutely necessary to beer, theory about "clean yeast" be damned.

When you over-pitch the colony doesn't need to reproduce. Thus measurably fewer esters are produced. This, while always detrimental to beer flavor, is noticeable in American and English strains and very pronounced with certain more flavorful strains, like Belgians.

The brewery where I served my apprenticeship was a Ringwood brewery. If you know yeast, you know Ringwood is very flavorful; most would say too flavorful. We fermented
more than a dozen styles with Ringwood, including such "clean" (meaning non-estery) styles as American Pale Ale and Wee Heavy. In order to brew those styles, we deliberately pitched more yeast than the following technique said we needed. This relative over-pitching, combined with temperature manipulation, deliberately suppressed ester formation. By manipulating the cell counts at inoculation we could produce the same flavor effect as switching strains entirely. Importantly, we did not simply dump in more yeast; instead, we pitched at a carefully-determined higher rate than normal.

Okay, Bob, what's the d*mn rate? You've had your rant, now get to the Straight Dope.

The standard, according to Fix, Daniels, Bamforth et al., is one million active cells per milliliter of wort per degree Plato. Or:

1,000,000*1ml*1°P

Constants:

  • There are 3785 ml per US gallon, or 18,925 ml in 5 US gallons. Round this up to 19,000 (19L) for ease of calculation if you do predominantly 5 gallon batches.
  • One °P is roughly equivalent to 4 points of gravity. For an approximation, 1.048 is thus 12°P. This goes askew above around 1.060; in fact, you may find it simpler to consult a chart like this: http://plato.montanahomebrewers.org/ - or use the formula:
{Plato/(258.6-([Plato/258.2]*227.1)}+1 = Specific gravity

Thus, for 5 gallons of wort at 12°P, we need

1,000,000 * 1 ml * 12°P, or 12,000,000 cells per ml.

12,000,000 * 19,000 = 228,000,000,000 - that's 228 billion cells.

You can expect around 1 billion active cells in a ml of harvested slurry, depending on how much trub and break material makes it into the fermenter. My counts ranged from 0.5 billion to 2 billion, depending on style brewed and brewery practice. 1 billion is a solid average across a dozen different breweries, professional and amateur.

Thus, 228 ml of freshly-harvested slurry is the correct pitch for 5 gallons of 1.048 wort. Conveniently, that's only a few ml less than that found in one cup (8 fluid ounces).

Jamil
Zainasheff recommends a modified pitch rate, advocating 0.75 million cells per ml per °P for ale ferments and 1.5 million for lagers. My experience tells me the standard rule of thumb - 1 million - is quite sufficient for both ales and lagers. If it makes you lager brewers feel better, pitch at the higher rate. It won't harm anything.

The professional small-brewery practice is to harvest yeast from the cone of a conical fermenter and re-pitch a measured amount into the next batch (usually by weight: 1 pound of slurry per barrel of wort). This practice can, without washing or other more-advanced care of the yeast, be extended to ten or more generations. In professional practice I can get between ten and fifteen generations, depending on strain.

Harvesting yeast is shockingly easy! It takes about five minutes after racking.

Here's how:

Equipment Needed:

1 scoop - stainless steel or glass
2-6 glass containers w/lids
Sanitizer

Procedure
  1. Thoroughly clean and sanitize all equipment.
  2. Carefully - you don't want to scratch or ding your plastic fermenter - scoop slurry from the fermenter to the jars.
  3. Close jars tightly and immediately refrigerate.

That's it! Honest!

Always harvest more than you've calculated is necessary. For one thing, you might have goofed a calculation; I do that all the time. For another, harvested slurry - unwashed yeast - loses viability rapidly. Tests have shown that approximately 25% viability is lost per every seven days spent in cold storage. If you must store the slurry longer than seven days, wash the yeast.

The same can be done with starters. Rather than pitching a large starter - with all the attendant diluent - have the foresight to chill the starter container for several days before you pitch to precipitate the yeast. Then decant the spent starter wort and pitch the starter slurry. NOTE: This slurry has approximately four times the yeast solids as harvested slurry, so accommodate this factor in what you learned above!

To be fair, let me list a few of the exceptional circumstances in which knocking out onto a yeast cake might make sense from a pitch-rate standpoint. If you're planning to ferment a very big ale - Imperial Stout, Barley Wine, Quadruppel - starting a smaller beer - Dry Irish Stout, Nut Brown Ale, Belgian Blonde - and knocking out onto the dregs of that beer might actually be feasible, cell-wise. Same with a bigger lager, like Bock. Still, it's better to know how much yeast you're pitching than just dump freshly-brewed wort into the old fermenter. The only way to really know is to harvest the slurry and measure it.

Harvesting is easy, and is smarter than just knocking out onto a "yeast cake".

It allows you to pitch your yeast out to many more generations (10 to 15) than simply dumping fresh wort into your fermenter (which works maybe thrice).
It's excellent, professionally-endorsed brewing practice.
It removes an unknown from your brewing process, i.e., how much yeast is being pitched.*

If a thing is worth doing, it's worth doing properly. It's worth doing to the best of your ability, effort, and means. Knocking out onto a yeast cake not only doesn't really save you anything at all, it also is guaranteed to brew mediocre beer in the vast majority of circumstances. You've learned how to harvest yeast quickly and painlessly.

Now go clean your fermenter.

Cheers!

Bob

* It will never cease to amaze me that brewers who will obsess over how many grams of Saazer are added at flameout, or the differences between US and Canadian 2-row malts, will just put any ol' amount of yeast in their fermenter.


SOURCES

Bamforth, Charles. Standards of Brewing. Boulder, CO: Brewers Publications, 2002.

Daniels, Ray.
Designing Great Beers: The Ultimate Guide to Brewing Classic Beer Styles. Boulder, CO: Brewers Publications, 1996.

Fix, George. An Analysis of Brewing Techniques. Boulder, CO: Brewers Publications, 1996.

Noonan, Greg. New Brewing Lager Beer. Boulder, CO: Brewers Publications, 2003.

Zainasheff, Jamil. "Proper Yeast Pitching Rates." Mr. Malty. 24 May 2007. 03 March 2010. < http://www.mrmalty.com/pitching.php >
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Old 03-03-2010, 07:43 PM   #2
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OK, no argument, but some discussion, OK? You say "When you over-pitch the colony doesn't need to reproduce. Thus measurably fewer esters are produced." Dr. Clayton Cone of Lallemand says pretty much the opposite...."ester production is related to yeast growth but not in the way you might think. The key element to yeast growth and ester production is acyl Co-A. It is necessary for both yeast growth and ester production. When it is busy with yeast growth, during the early part of the fermentation, it is not available for ester production. Ester production is directly related to biomass production. Everything that increases biomass production (intensive aeration, sufficient amount of unsaturated fatty acids, stirring) decreases ester production. The more biomass that is produced the more Co-enzyme A is used and therefore not available for ester production. Anything that inhibits or slows down yeast growth usually causes an increase in ester production: low nutrient, low O2." The full article is at http://www.danstaryeast.com/library/yeast-growth. Care to comment?

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Old 03-03-2010, 07:53 PM   #3
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You are correct sir! . . . . . . . . . . . . . . . I just wanted to use that smilie is all.

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Old 03-03-2010, 08:11 PM   #4
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Denny,

No worries. The "please don't argue with me" comes from constant bickering with people who want to argue with established brewing science with "it works for me". You're not doing that. You've made a good point, for which I thank you!

I wonder, however, how to mesh that data with the other source material and observed phenomena. If what he says is true, under-pitching will lead to suppressed ester production, and widely-reported observation proves that false - all manner of flavor-impacting byproducts become noticeable when the variable is significant under-pitching. Moreover, widely-reported observation proves that over-pitching leads to significantly suppressed ester production; the corollary is that little or no reproduction taking place is the culprit of that phenomenon.

I'm not calling into question his (or your) scholarship, not by any means. It's just the first I've come across such an assertion, so it's incumbent on me to be skeptical, especially as it apparently flies in the fact of well-recorded (professional and amateur) known phenomena.

I have seen this:

Quote:
It has been noted that a drop in available O2 from 8 ppm down to 3 ppm can cause a four fold increase in esters.
observed in other media, but never the discussion of Co-enzyme A.

Interesting.

You've made me think. I'll be careful of you in the future. And I'll dig further into this Co-enzyme A business.

Regards,

Bob
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Old 03-03-2010, 08:21 PM   #5
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So the moral of the story is that overpitching will lead to less stressed out yeast and thus less yeast character. What about it's effects on attenuation?

For a beer like a double IPA, where i want neutral yeast profile and high attenuation, I would assume a yeast cake and cool fermentation that is slowly increased in temperature until it finishes would be perfect.....

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Old 03-03-2010, 08:29 PM   #6
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Hmm... I just pitched a APA onto a yeast cake last night. It is US05, neutral yeast and also fermenting at 61F liquid temp.

We'll see how it turns out

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Old 03-03-2010, 08:43 PM   #7
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Quote:
Originally Posted by permo View Post
So the moral of the story is that overpitching will lead to less stressed out yeast and thus less yeast character. What about it's effects on attenuation?

For a beer like a double IPA, where i want neutral yeast profile and high attenuation, I would assume a yeast cake and cool fermentation that is slowly increased in temperature until it finishes would be perfect.....
Attenuation is, to the best of my knowledge, not affected by over-pitching. In other words, pitching more than the required amount of yeast will not increase attenuation. There are many factors at work in attenuation, more than simply a surplus of yeast will accommodate.

The ability of the particular strain to metabolize different sugars, and the presence of sugars in the wort (accessible to yeast) are more important than how many cells are present. In other words, if you have a surplus of sugars which your yeast cannot metabolize, no increase in colony size will attenuate those sugars. For example, you could put five pounds of slurry in a 5-gallon batch of beer rich with lactose and no real difference in attenuation would be seen.

Attenuation can be adversely affected by underpitching; the literature is clear on that issue.

Make sense?

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Old 03-03-2010, 08:43 PM   #8
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Quote:
Originally Posted by humann_brewing View Post
Hmm... I just pitched a APA onto a yeast cake last night. It is US05, neutral yeast and also fermenting at 61F liquid temp.

We'll see how it turns out
I'm sure it'll turn out fine. I'm just as sure it'd be better if you took the time to pitch an appropriate quantity of yeast.

Cheers!

Bob
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Old 03-03-2010, 08:51 PM   #9
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Just one quick question on harvesting technique. Have you got a scoop that will fit down the neck of my carboy? I've been trying to figure that out- I've "yeast washed" before, but I'd just like to scoop. A bit lazy, I guess.

Dave

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Old 03-03-2010, 08:54 PM   #10
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meh...either way you are making beer and it will probably turn out just fine..

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