[BrewBama]

The reality might be that yeast isn’t as fussy as we think it is. It doesn’t have to have an exact amount of starter liquid or cell counts, just somewhere in the ball park. I’ve definitely been in a rush and not had time to do starters exactly how I’d like to but the beers still turn out fine

I believe ‘the ballpark’ is spot on.  I live by the ‘close enough is good enough’ mantra in my brewery.

≥ 85% of a recommendation is close enough around here, < 85% ain’t close enough for me.


Sent from my iPad using Tapatalk

[majorvices]

+3 ... it doesn't have to be complicated. But it can be complicated if that's what you obsess about.

[Saccharomyces]

I have been stating that close is more than good enough when it comes to cell counts for years.  I even wrote a blog entry about the amateur brewing community’s preoccupation with cell counts entitled “Yeast Cultures are Like Nuclear Weapons” back in 2015 (somehow the publication date got changed when I performed a few edits on the text recently).  People have been so focused on cell counts since Kai Troester published his experiments with different stir plate protocols that the forest has been lost for trees.

Let’s examine the first misunderstanding when it comes to yeast cell counts.  The difference between 200 billion cells and 300 billion cells is insignificant.  The difference between 200 billion cells and 400 billion cells is also insignificant in the grand scheme of things. Lag times and dissolved O₂ demands are not significantly reduced until one pitches in excess of 800 billion cells per 5 gallons of wort.  While I may be wrong, I believe that the confusion stems from brewers believing that the yeast biomass grows linearly, that is, the number of cells at replication period N is N times the original cell count.  The reality is that the cell count is closer to 2^N times the original cell count, where N is the number of replication periods that have elapsed and the symbol “^” denotes raised to the power of.  That difference means that after replication period 1, the yeast biomass has doubled in size.  After replication period 2, the yeast biomass has quadrupled in size.  After replication period number 4, the yeast cell count is now 16 times the original cell count.  In essence, yeast biomass growth is binary exponential because every cell that is alive during a replication period buds a daughter cell, which results in the cell count doubling during every replication period.

With that said, there are four basic limiting factors when it comes to biomass growth; namely, a yeast culture’s genetically-defined O₂ demand, dissolved O2 level, amount of carbon in the medium, and the volume of the medium.  The main area where brewers encounter problems with yeast growth is not the amount carbon in the medium or the volume of the medium.  It is the amount of dissolved O₂ needed for the yeast cells to consume the carbon source and turn it into enough cell growth to reach maximum cell density.  At a very simple level, the amount of yeast that needs to be pitched is inversely proportional to the amount of foam on the top of the wort when the yeast culture is pitched because aerating wort results in foam building up on its surface.  For example, the shake/rock wort in an almost full carboy method results in inadequate aeration for all but low O₂ demand yeast strains.   The paint stirrer method is marginally better.  However, like stirring a culture in an Erlenmeyer flask, O₂ pickup is limited by the amount of surface area a brewer can create.  The two most effective ways I have personally found to aerate wort is via a venturi in the drain tubing from one’s kettle to one’s fermentation vessel or direct O₂ injection via an O₂ bottle and diffusion stone.  Of the two, direct O₂ injection is the gold standard, which is why professional breweries tend to use it.  Using an air pump, inline filter, and a diffusion stone works too, but it is not much more effective than a well-designed venturi while adding significantly more complexity to the equation.

When a yeast culture is pitched into a medium that is above the Crabtree threshold of 0.2% weight-by-volume (w/v), it will chose fermentation over respiration.  A 0.2% w/v solution has a specific gravity of 1.0008. Unlike humans, yeast cells have two metabolic pathways. One pathway processes carbon sources (sugar is carbon bound to water; hence, the term carbohydrate) aerobically (we will get to this pathway later when we discuss the Crabtree threshold in greater detail).  We can refer to this pathway as the respirative metabolic pathway.  It only plays a minor role in fermentation, which occurs via the anaerobic (fermentative) metabolic pathway.  Yeast cells always consume carbon via the fermentative metabolic pathway in brewing, even in the presence of O₂.  However, there is a little twist during the lag phase. Yeast cells shunt a small amount of carbon along with O₂ to the respirative metabolic pathway for the creation of ergosterol (the plant equivalent of cholesterol) and unsaturated fatty acids (UFA).  These compounds are necessary to keep yeast cell plasma membranes pliable.  The ergosterol and UFA reserves that are built up during the lag phase are shared with every yeast cell that is budded during the exponential growth phase.  That is an important concept to understand.  It is the reason why we want to pitch a starter at high krausen instead of allowing it to ferment out.  We want to do so because all cell production after high krausen is reached is for replacement only and causes these reserves to be further depleted.  There are also morphological changes that occur at the end of fermentation before a culture settles out that have to be undone when we pitch a culture.  The most significant of is thickening of the cell wall.

Getting back to the Crabtree threshold, dry yeast producers take advantage of the Crabtree effect to milk more cell growth out of the same amount of carbon.  They do so by holding the medium in a steady state below the Crabtree threshold in a chemostat, which is a bioreactor.  Holding the medium at a steady state below the Crabtree threshold prevents yeast cells from switching over from respiration to fermentation as well as undergoing flocculation, which is caused by the exhaustion of mannose, glucose, maltose, sucrose, and higher level saccharides that a yeast cell can reduce to one of these sugars.  New medium and O₂ are continuously fed into the process in order to achieve a steady state.  Dry yeast propagation is significantly more complex than liquid yeast propagation.  Liquid yeast is propagated much like brewers propagate yeast when making a starter.  It is just on a much larger scale.

Why do dry yeast manufacturers propagate below the Crabtree threshold even though it is a significantly more hi-tech process? Well, it is because they are taking advantage of the fact that respirative metabolic pathway generates nine times more energy than the fermentative metabolic pathway using the same amount of carbon.  Respiration pretty much results in energy, water, and carbon dioxide gas.  Alcohol, esters, and the VDKs found in beer are the result of the fact that the fermentative metabolic pathway is lossy.  They are basically the result of the yeast cell equivalent of incomplete combustion.

Now, this information brings us around to why dry yeast requires little to no aeration on the first pitch.  It is due to the fact the carbon source is entirely consumed via the respirative metabolic pathway and that is where ergosterol and UFAs are produced.  Unlike fermentation, the generation of these compounds is a not a build in an early phase, consume in later phases situation.  Yeast cells are continuously building/replenishing ergosterol and UFA reserves when they are reproducing via their respirative metabolic pathway below the Crabtree threshold.   The drying process has nothing to do with not having to aerate wort with dry yeast on the first pitch.  It has everything to do with how the yeast biomass is propagated.

Finally, why do we make starters with modern liquid yeast cultures? The amount of cell growth that occurs in a 1L or even a 2L starter is insignificant. The number of viable yeast cells in a modern liquid yeast culture is significantly higher than a first generation Wyeast smack pack.  I would go as far as to state that the average modern liquid yeast culture can be pitched directly into 5 gallons of wort without a starter.  Pitching a first generation Wyeast smack pack without making a starter was a “pitch and pray” event with lag times measured in days. A 1L starter with a modern liquid yeast culture does two things; namely, it brings the cells out of quiescence and gives them time to reverse the morphological changes they underwent in preparation for quiescence.  The second thing making a starter does is afford yeast cells the opportunity to replenish ergosterol and UFA reserves before going to work on a batch of wort.  That is why it is critical to pitch a starter at or as close to high krausen as possible.  Allowing a starter to ferment out, so that the supernatant can be decanted basically a) wastes ergosterol and UFA reserves for replacement cell production during the stationary phase and b) puts the cells back in the same quiescent state they were when the culture was received from the yeast propagator.  Sure, the cell count has been increased slightly, but that is insignificant.  It is definitely not the reason why me make a starter today.

[HighVoltageMan!]

Pitching a yeast starter at high krausen is nothing new. It's been known for quite some time that this is a preferred way to pitch as it lowers lags times and reduces the pitch rate. If you can do it, by all means do it. This method can be utilized both with SNS or stir plate. Both will produce good results.

But what is overlooked is that the pitch rates we use today have been adopted by the brewing industry long before homebrewers adopted them. The pitch rates of for ales of .75 million cells/ml/degree plato was adopted to compensate for yeast that was basically at rest after going through an active fermentation. The yeast is pitched over and over again. Enough is pitched to insure a complete fermentation, low lag times and yet still provide for a decent amount of growth to keep the esters at the proper level and  keep the yeast vitality high (young cells) for the next pitch. If the yeast is not pitched at high krausen it will most likely be deficient of lipids (fatty acids) to be able to reproduce properly. That is why when the yeast is pitch in a "quiet state" (for lack of a better term) is extremely important to aerate the wort to levels of 7-15ppm depending on strain and style. This method also works very well. Modern breweries do not pitch yeast at high krausen for obvious reasons and fresh pitches from yeast banks are very fresh and can be pitched at lower rates due to better health and vitality.

I guess the way I see it is there are two different ways of pitching yeast and both work well. One being at high krausen and the other as a slurry that has gone through a fermentation. People's situation changes and they should adopt either one of these methods depending on the beer they are brewing and the situation they find themselves in. It's going to be really hard to get a lager starter in high krausen at fermentation temperature and time it so the wort is ready to have yeast pitched into it. I personally don't have the ability to do this, so I use the method of building up a biomass a couple of days prior to brewing and aerating the wort with pure oxygen. It works beautifully, the beers are fantastic. As far as pitch rates and lags times, it does make a difference to pitch more yeast (not in high krausen) to shorten lags times. Lag times with high krausen starters will be shorter. Both SNS and stir plates work for this method.

A few years back, homebrewers were making a starter at the same time they started the mash. The starter was only going 4-5 hours and they reported shorter lag times than starters that were allowed to finish. As I have previously said, I make a 1/2 liter starter with dry yeast just before the mash and pitch it 4-5 hours later (wort is aerated as if it were to have a liquid yeast pitched into it). Lag times are cut in half, pitch rates are .35. The beer turns out wonderful.

You bring up a lot good points about yeast and the knowledge is welcomed, but I believe both methods work well when they are properly executed. The bottom line is the beer quality.

[denny]

I'd have to say for me the bottom line is what I prefer doing and beer quality is,second to  that as long as goal is served.

[Saccharomyces]

Yes, I know that pitching a starter at high krausen is nothing new.  I have been doing it since I started brewing the better part of three decades ago.  Back then, pitching at high krausen was common knowledge.  Somehow, the practice has gotten lost in the noise of perfecting the stir plate.  The common practice today is to spin a starter for a couple of days before cold crashing it and decanting the supernatant.  That approach never made sense to me. 

In reallity, what is happening today in commercial liquid yeast is that the average packaged cell count is reaching upward toward 200B cells.  Imperial already ships cultures that contain 200B cells.   In essence, pitching an Imperial yeast package into a batch of wort is like pitching 167ml of thick cropped slurry that contains 40% viable cells.  Making a starter today is not like making a starter 20 years ago.  White Labs’ selling point when they entered the market in 1995 was that their preform packaged cultures contained 30B cells, which was significantly more than a Wyeast smack pack from that era.  White Labs considered these cultures to be direct pitch.  However, even with a relatively fresh White Labs preform, lag times could be long.  The longer the lag time, the greater the opportunity for bacteria to gain a foothold in the fermentation because bacteria biomass grows by a factor of 8 in the period of time it takes yeast biomass to double.  A starter back then was as much about increasing the culture cell count as it was about waking it up from quiescence and getting it prepared to go to work.  Today, a starter is about waking a culture up from quiescence and getting it prepared to go to work.

Now, the premise that a package of dry yeast pitched into 500ml of starter wort erases the yeast culture’s ergosterol and UFA reserves is not quite right.   An 11g package of dry yeast contains around 55B cells.  The maximum cell density for 500ml is 100B cells, making the difference in O₂ demand when the rehydrated dry yeast culture is pitched almost insignificant because there will not be much in the way of replication in 500ml of rehydration wort.  That is a completely different thing than repitching cropped slurry from a batch pitched with dry yeast.  The yeast cells in a crop have gone through a mininum of 4 to 5 replications and that’s if there was no replacement cell growth during the stationary phase.  Aerating wort that is pitched using this dry yeast rehydration method is a good safeguard, but it is not an absolute necessity when we are talking about 500ml of rehydration wort.

Finally, why is the generally accepted rule of thumb for pitching 0.75M cells per milliliter per degree Plato?  After all, maximum cell density is based on volume, not wort density; therefore, why are we pitching more cells into denser wort?  The answer is that the saturation point for dissolved O₂ goes down as gravity goes up.  Lower dissolved O₂ equates to reduced ergosterol and UFA production.  Higher gravity beer has higher osmotic pressure and ethanol levels, both of which are hard on cell walls and plasma membranes

[HighVoltageMan!]

 Today, a starter is about waking a culture up from quiescence and getting it prepared to go to work.

Now, the premise that a package of dry yeast pitched into 500ml of starter wort erases the yeast culture’s ergosterol and UFA reserves is not quite right.   An 11g package of dry yeast contains around 55B cells.  The maximum cell density for 500ml is 100B cells  Aerating wort that is pitched using this dry yeast rehydration method is a good safeguard, but it is not an absolute necessity when we are talking about 500ml of rehydration wort.

Finally, why is the generally accepted rule of thumb for pitching 0.75M cells per milliliter per degree Plato?  After all, maximum cell density is based on volume, not wort density; therefore, why are we pitching more cells into denser wort?  The answer is that the saturation point for dissolved O₂ goes down as gravity goes up.  Lower dissolved O₂ equates to reduced ergosterol and UFA production.  Higher gravity beer has higher osmotic pressure and ethanol levels, both of which are hard on cell walls and plasma membranes

A starter used today is more than waking up the cells, although in a lot of cases this may be true. It's about cell density and vitality, which is still very important. Fresh, young yeast pitched into aerated wort have a better chance to perform well even when the pitch rate is low. Typical pitch rates recommended from manufacturers are much lower than the industry standard; .3 and .5 million cells/mL/degree plato, which will work well in aerated wort for a typical gravity ale. The problem I have, is I can't get my yeast overnighted to my house in an insulated shipping box like a brewery. I get my yeast from a homebrew supplier who may not have treated and stored it very well and it's at least a month old, often it's 2 months old. Not only is the density low, the vitality is low as well. I brew mostly lagers, there is no way I can get my cell count up high enough to withstand a lager fermentation without either getting a custom pitch sent to me or going the more realistic route of building a pitch up. Biomass density is still important, especially in lagers where the yeast growth is hindered by temperature and dissolved cO2 (cO2 trapped by lower temperatures). Higher gravity ales are in similar boat, and pitch rates should be increased with increased gravity, again depending on the yeast strain and style of beer.

As far as my method of using a 1/2 starter. The typical accepted yeast density of dry yeast is 10 billion cells per gram. I have in the past doubted the idea that it could be as high as 20 billion per gram, but I have since change my mind after working more with dry yeast. I believe the manufacturer's estimate of 5-6 billion is way too conservative. I think there is some evidence that it is much higher, 15 million billion per gram or higher is plausible. So my 1/2 liter starter will not increase the cell density, I assume the cell density is at least 15 billion cells per gram. The idea is to do something similar to SNS, waking up the cells and get them to a metabolic state prior to pitch. I agree that I could forego the aeration of the wort, but it's a slight under pitch. Aerating the wort will insure the yeast will preform well, assuming the yeast growth is on the high end because of the under pitch.

This is points out an important role to me that forums have played in the short history of modern homebrewing. These ideas are discussed, debated and mulled over and information is shared to the benefit of anyone who wants to learn more about brewing. I have learned so much from them over the years and I hope in some small way I can return the favor.

Brew on.