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Why does pitch count matter

Wildrover

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We've all read a number of times and have all been conditioned to believe that enough healthy yeast is vital to producing successful beer.  I'm not here to challenge that but I am here to ask why that is.  Please understand that I am not a chemist or biologist or any ologist that would know the answer or have a background that would inform the answer to this question.  My understanding of yeast is that it multiplies as it is feeding so, at least in my mind, even if you start with a single yeast cell, that should be enough to ferment the whole batch. 

Of course, we know that's not true but what I don't know is why its not true?  Why would keep a single cell from being the catalyst that ferments the whole batch of beer? 

I know Jorge, the owner of brewbeeranddrinkit.com has said that he doesn't worry about pitching rates.  Speaking anecdotally I someone who has a group of friends who have recently discovered the hobby.  These guys use one smack pack between three 5 gallon batches.  They haven't been indoctrinated into the settled processes of homebrewing.  However, I've tasted their beer and although not great, it's not bad.  More importantly, that one smack pack seems to be fermenting all three batches pretty much all the way down.     

Just curious,

thanks

wr
 
I imagine that you would have to keep track of final gravity of your batches to see if there was any difference. I would expect a variance here when you try to be repeatable. You probably couldn't taste the difference between 1.012 and 1.017, but then again maybe you could.

There is also a possibility that underpitching could create off-flavors. Not huge because I have underpitched most of my batches until this point and haven't noticed much yeast off-flavor.


Obviously 1 yeast cell would take days and maybe even weeks to show signs of fermentation. There is probably more than 1 cell of bacteria in the wort which will also be multiplying as well to see who gets to eat the wort first. A book on yeast I read recently suggests a growth factor of 3? SO the orginal cell makes 3. Those 3 make 9. etc.. I imagine by the time you get to 100 billion the original yeast cells have eat a lot of the wort and pissed out alcohol making it a hostile environment to be born in and subsequently die without much production.
 
Wort is a paradise for bacteria. That's why malt extract is often used in the bottom of petri dishes. You know, those little round glass dishes where they culture bacteria and mold?  There's a good chance that it's growing in malt extract.  So the quicker the yeast get going, the more likely it will crowd out anything that could contaminate it.  Sure you've boiled the liquid and sanitized the equipment, but there's still a chance that something made it in there.  Good pitching helps to ensure that any bacteria or mold that made it in there will never have a chance to grow.
 
I'm not a chemist, biologist or any ologist but have readied Yeast The Practical Guide to Beer Fermentation, by Chris White with Jamil Zainasheff. According to them underpitching affects the flavor, overpitching  negative affect on yeast health. Every time a yeast cell reproduces it damages it's cell wall, eventally killing the cell. Starting with a single cell or colony involves a lot of time to grow to the right pitch rate. ( sterilizing, washing, Propagation, Testing , Etc) 

I would recommend the book goes into great detail on Yeast.
 
I've tasted enough under-attenuated beers at competitions to know that issue happens frequently, especially in lagers such as Marzen.  I think some ale brewers try a lager and pitch the package, as they likely do and get away with in ales, and it catches up with them in a lager where the cell count needed is double.  So knowing that happens even with a full-package pitch, I'd imagine a single-cell pitch, or even an expired 8-month old package pitch would yield similar results. 

And, much of the beer's positive flavors come from esters and phenols created by the yeast.  Proper pitching rates gets the brewer in the proper range to create the desired beer flavor profile. 



 
Based on my literature research and personal “research” I will add my opinion.  Take all of this with at least one beer.  I haven’t provided any references and I am writing this from a defective memory.

Yeast cells multiple by division, 1 cell becomes 2, 2 cells become 4 and so on.  Each cell can divide about 10 times before it dies.  In an aerobic environment (with oxygen) yeast will mostly consume sugar, and reproduce, creating Carbon Dioxide and WATER.  In the anaerobic environment (without oxygen) yeast will consume sugar, and create Carbon Dioxide and ALCOHOL.  This is goodness.  There are other by-products of the reproduction that don’t taste so good which is why you want an adequate pitching rate.

[Note: The function of a stir plate in the development of a Starter is to continually stir in new oxygen and allow the CO2 to out gas.  This maintains the best conditions for growth.  To help you understand the off flavors drink some of the starter liquid the next time you make a starter!]

Now for the math.  And some of the guess work

This is all based on about a 1.050 gravity.  A typical pitch rate is 75,000-150,000 cells per ml per point of gravity or about 100-200 billion cells per 5 gallons.  (The numbers are all rounded off so don’t get too picky).  The maximum number of cells that can be supported per liter is about 200 billion [Yeast, Chris White].  Based on this at the end of fermentation of 5 gallons there are about 4,000 billion yeast cells.  That’s roughly 42 divisions from a single cell.  If you pitch 100 billion cells per 5 gallons (approximately the general recommended pitch rate) there are only 4-5 (probably less) reproductive cycles from pitch to ferment out.  If you pitch at half the recommend rate that’s only one additional reproduction cycle, probably not a problem but as you get smaller and smaller you will start to have problems. 
1) your wort might run out of oxygen before the yeast can properly reproduce resulting in under attenuation and
2) each reproduction cycle will produce additional off flavors. 
Also remember, the commercial “liquid” yeast contain about 100 billion cells at packaging.  These cells will die off at about 10% per week (I’m guessing but I’m not off by much), therefore a 2 week old package will only have about 80 billion cells. 

The “art” of beer brewing has been centuries in the making.  This has all been by trial and error and the general wisdom is just that, wisdom.  I am sure there are a number of “old wives tales” built into that tribal knowledge and you are certainly welcome to experiment on your own.  If you do, please keep good records and let us know the results. 
 
Great input from everyone! This is all really a hypothetical thought experiment for me so please bare with my assumptions.  If we assume a perfectly  sanitized environment with no bacteria or other forms of contamination and if you were able to continuously aerate the wort to the levels needed and only as needed then, in theory, one cell would indeed ferment the whole batch?

The problem though would come from the effects of dealing with a race of mutant yeast cells fermenting the batch and not the healthy cells that produce the flavors you want?  In my mind I'm remembering the old days when we used tapes instead of mp3s or cds.  I remember as a kid if you wanted an album you could copy someone's tape instead of buying the original.  Of course, your copy wasn't going to sound as good as the original.  If you let someone borrow your copy of that copy then their copy would be even more degraded than yours and thus, even more worse than the original.  You can get away with this for a little while but the more removed you are from the original the more degraded each copy is going to be.  Or, given the subject of some sci-fi/horror movies/books, its like making a clone from a clone, not the original.  The more the clone is made from a copy the more distorted and mutated they become (I'm thinking of a specific movie but can't remember the title?). 

In yeast terms, this means that the more cells you are pitching the less splitting and reproductive cycles and thus, fewer mutations and so healthier yeast are fermenting your beer and not the overly mutated cells that came from asking one cell to do the whole job?  If this is a good understanding, it begs my next question, is over pitching the result of too many cells that there isn't enough for them all to eat and thus...well what?  I was of the impression that when the cell runs out of food they simply go doormat.    That's obviously not the case but what does happen when there is too much yeast? 

thanks again for all the input

wr
 
I don't know the problem with over pitching.  But the reality is the commercial yeast is grown from only one cell.  But most of the reproduction is conducted in very well controlled conditions. 

The start of the yeast growing process is to take a very small sample of yeast from the master culture (number in the hundreds) and spread them out on a petri dish (Streaking).  The yeast are streaked (ie spread) over the dish to isolate several single cells.  These single cells will grow into single cell colonies of thousands of cells.  In any sampling of yeast cells there will be mutant yeast, wild yeast and other bateria.  By streaking out to single cells it is easier to extract a colony of healthy cells of the correct strain.  The single cell colony is then processed (grown) by repeated stepped starters until the desired quantity of yeast is achieved.  Once an original pitching quantity is grown (tens of ml for home brewers to gallons for big batches), the breweries will harvest and repitch somewhere between 4 - 10 times before going back to the original culture to restart the process.  The profesionals can do this because there is little or no delay between batches and they are better able to harvest the yeast under controlled conditions. 

With 4-10 reuses there are probably about 100 reproduction cycles involved.  The risk of mutating is exacerbated by the trub from brewing.  Again the commerical brewers are better able to harvest just the yeast with little of the brewing detritous.  The general number I have seen is no more the 4 repitching for the home brewer and still good sanitation control is required.
 
MaltLicker said:
I've tasted enough under-attenuated beers at competitions to know that issue happens frequently, especially in lagers such as Marzen.  I think some ale brewers try a lager and pitch the package, as they likely do and get away with in ales, and it catches up with them in a lager where the cell count needed is double.  So knowing that happens even with a full-package pitch, I'd imagine a single-cell pitch, or even an expired 8-month old package pitch would yield similar results. 

And, much of the beer's positive flavors come from esters and phenols created by the yeast.  Proper pitching rates gets the brewer in the proper range to create the desired beer flavor profile.

Interesting. Can you also taste the difference between a fully fermented low attenuation yeast strain and an under-attenuated high attenuating yeast strain?
 
grathan said:
Interesting. Can you also taste the difference between a fully fermented low attenuation yeast strain and an under-attenuated high attenuating yeast strain?

Using Wee Heavy and Tripel as examples, and assuming OG of 1.090 for each, I would say absolutely yes.  A properly attenuated Wee Heavy would taste very malty and rich, but not overly sweet from under-attenuation of malt sugars.  A poorly attenuated BGS (i.e., Duvel) of same OG would be sweet rather than dry and crisp.  Often that is due to low pitch rate or too much malt and not enough simple cane sugar that fully attenuates out. 

There are few things I've done as a brewer that were as educational as getting into judging.  Tasting a flight of 12 Vienna Lagers and Marzen, for example, really exposes that difference between malty and sweet. 
 
http://beerandwinejournal.com/barley-legal-aeration-experiment/

Wildrover - This is sideways/sorta related to your original post, at least as far as aeration/oxygen affects cell count growth and health and then that translates to fermentation success. 

The results seems to align with "conventional wisdom," especially with lag time to starting ferm activity.  Less oxygen to consume means transition to anaerobic pathways the soonest, and perhaps too soon, and without the cell count to do the job.

I brewed yesterday with a new dry yeast, and Mr. Malty said I was a tad short on cell count, so I oxygenated a little longer, and sure enough, it took longer to get started than usual, presumably b/c the yeast were consuming the additional O2 and (I hope) will be able to bud some more cells and get it done. 
 
There is a pragmatic part to Wildrover's question that relates to the under attenuated result described by Maltlicker.  The issue is nutrients in all malt wort. Specifically Zinc and FAN (Free Amino Nitrogen). Both of these are important to yeast growth and completed fermentation. As soon as either one is depleted, fermentation slows, then stops.

So, in the case of growth from a single cell, the nutrient level is used up long before fermentation is complete. This is the issue with under pitching. Malt extracts are notoriously low in FAN. DAP and Go-Ferm help with FAN level. Servomyces is zinc enriched yeast hulls.
 
Been reading about mead, and since honey lacks most nutrients that fresh all-malt wort provides, nutrients are vital to add, etc. 

In beer, been using Wyeast nutrient at ten minutes for a long time.  Now I'm going to try GoFerm on my next brew with dry yeast rehydration. 

FWIW, Servomyces has always seemed expensive to me given that it's just dead cells enriched with zinc/nutrients, so long ago I started adding some dry yeast cells from an expired package with my Wyeast nutrient.  The thought being why pay someone to enfuse together what I can pitch separately...
 
Yeast cells do not divide.  They bud.  A bud will form on the cell wall and then come off as a new yeast cell when it's mature enough.  A yeast cell can have more than one bud at a time.  Thus, yeast cells multiply more rapidly than simple dividing would permit. 

In the book "yeast" by Chris White with Jamil Zainasheff it states, "In general, underpitching affects flavor more, while overpitching negatively affects yeast health more over generations.  however, both can result in a less than ideal fermentation with high levels of diacetyl, acetaldehyde, and low attenuation."  "Too low a pitching rate can also result in slower fementation and very long lag times, which allow competing bacteria and wild yeast to grow in the wort."
 
Scott Ickes said:
Yeast cells do not divide.  They bud.  A bud will form on the cell wall and then come off as a new yeast cell when it's mature enough.  A yeast cell can have more than one bud at a time.  Thus, yeast cells multiply more rapidly than simple dividing would permit. 

In the book "yeast" by Chris White with Jamil Zainasheff it states, "In general, underpitching affects flavor more, while overpitching negatively affects yeast health more over generations.  however, both can result in a less than ideal fermentation with high levels of diacetyl, acetaldehyde, and low attenuation."  "Too low a pitching rate can also result in slower fementation and very long lag times, which allow competing bacteria and wild yeast to grow in the wort."

I don't remember where it was but I'm sure it was one of the yeast supplier sites ( wyeast or white labs probably ), and it said that some do divide and some do bud. 
Maybe beer yeast buds and some of the others divide?  Can't remember for sure because it was a lengthy write up that went into the many types of yeast and I believe it went into history as well as yeast strains used for wine and spirits.

At any rate reading the book "Yeast" is great advice.  There is a ton of information in there.
Over or under pitching can both yield undesirable results, with under pitching being the worst due to stressing the yeast.
I haven't seen a definitive answer on over pitching but I know that one undesirable result can be the lack of the signature characteristics of the yeast.

I have also intentionally underpitched to achieve an effect. 
I am not a Hefe fan but I had some that I really liked so I did some research and went with Weihenstephan Weizen ( Wyest ).  In reading about the yeast having a low pitch rate and I believe keeping it at the upper end of the temp range for fermenting was suppose to really bring out the characteristics of the yeast.
Being a relatively inexpensive to make wheat beer I figured what the heck, I'll try that.
Hmm... Funky overpowering yeast characteristics.    I will stick to more ideal yeast pitch rates and temps from now on. 
 
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