Number of cells in a packet of dried yeast

Question

I'm trying to improve my understanding of pitching rates, and I'm struggling with some apparent inconsistencies.

An often used starting rule is to pitch about 750,000 cells per milliliter per degree Plato for ales, and twice that much for lagers. So for an example batch of 5 gallons (20 liters) of wort at 10°P (OG = 1.040) you'd need about 150 billion cells for an ale and about 300 billion for a lager, in order to get a consistent and reliable fermentation.

Common wisdom also has it that you usually don't need to make a yeast starter when pitching dry yeast into a medium-gravity wort. Practice bears this out: many home brewers (including your's truly) simply chuck one packet of Fermentis or Lallemand/Danstar dry yeast into a 5 or 6 gallon batch. Most of the time we don't even bother pre-hydrating dry yeast if the OG is less than 1.050 or so. Let alone making a yeast starter, because in practice you simply don't need to. Just add one packet of dry yeast to 5-6 gallons of wort, and you'll be fine. And indeed this has always worked for me.

Common wisdom also states that manufacturers of dried yeast pack tons of cells in their products so that after two years of storage there will still be enough viable yeast cells left to do the job.

BUT...

Looking at US-05 as an example (although the figures work out to pretty much the same for all the other popular dried yeasts), Fermentis specifies that it contains at least 6 billion cells per gram of dried yeast when packed, which means that for a standard 11.5 gram packet of US-05 we're looking at about 69 billion cells per packet.

Fermentis' datasheet recommends pitching 50-80 grams of US-05 into a hectoliter. At the specified 6 billion cells per gram, this works out to not pitching the recommended 150 billion cells in the above example batch of wort, but only 60-96 billion cells.

The instructions on the packet are even worse: there it says that one 11.5 gr. packet can be used for 20-30 liters (5-7.5 gal) which works out to a pitching rate as low as 230,000 cells / ml / P rather than the recommended 750,000.

And that's even ignoring those common, real world cases where the OG is higher than 1.040 and/or when the yeast approaches its 'best before' date. From what I understand the loss in viability of properly stored dried yeast over two years should not exceed 10% or so, yet it is still a factor.

So. On the one hand, common wisdom and real world experience tells us that one packet of dried yeast into a 5 gallon batch of wort up to an OG of 1.050 or so works fine. On the other hand, doing the math and checking that against the popular "750.000 cells / ml. / °P" rule states that this shouldn't be enough by half at best.

What's going on here?

Answer 1

I realize this should be a comment, but it was too long....

Here's what Clayton Cone, head scientist at Lallemand and inventor of Fermaid K, has to say....

Ester and other flavor component production or synthesis is a complex subject because there are so many variables taking place at the same time. You are right, 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. 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. Stirring in normal gravity decreases ester production. Stirring in high gravity increases ester production. CO2 pressure in early fermentation decreases ester production. Taller fermenters produce less esters than short fermenters. High temperature early in fermentation decreases ester production. High temperature later in fermentation increases ester production. Low pitching rate can result in less esters. There are other flavor components such as higher alcohol that have there own set of variables. Stirring increases production of higher alcohols. CO2 pressure does not effect the production of alcohol. Amino acid levels in the wort effect the production of higher alcohols. Most of the higher alcohol is produced during the growth phase (exponential phase) of the yeast. I am sure that there are many other variables. I am also sure that there are beer makers that have experienced the very opposite with each of the variables.

Pitching rates depend on several factors: (1) The speed in which you wish the fermentation to take place. Some professional brew master are in more of a hurry than others; desired beer style, shortage of fermenter space. Pitching rates would vary as a means to increase or decrease the total fermentation time. 10 X 10/6th cell population for normal fermentation rates. 20 X 10/6th or more for a quick turn around. (2) Temperature control. If lack of refrigeration is a problem, the fermentation needs to be spread out over a longer period by pitching with less yeast. (3) Health of the pitching yeast. If the pitching yeast has not been stored under ideal conditions (4C for less than one week) then larger pitching rate must be done to compensate for the deteriorate of the yeast. Increased pitching rates has its limits in trying to compensate for poor storage conditions. (4) When all other variables are under control you can use variations in pitching rates to achieve certain flavor profile that are of interest to you. Conventional wisdom regarding pitching rate can lead to problems. During each fermentation cycle the yeast will increase in size about three times, so if you use all the yeast from the previous batch you will soon be pitching with a huge amount of yeast. Professional brewers usually re-pitch with about 25% of the yeast from the previous batch. Proper handling of the yeast during storage (4C and <7 days) will minimize any problem with long lag phase. Start with a fresh culture of yeast after about five recycles for bacteria control and or after 10 - 15 cycles for genetic drift purposes. There are many who will say that they are proud of the fact that they have used the same yeast after over 100 cycles. More power to them. I wish that I could explain their luck. Good practices suggest frequent renewal with a fresh culture is a good policy.

Answer 2

The .75m / ml / °Plato for ales x2 for lagers, George Fix conclusion. Is recommended for pro brewers.

Large volume brewing has its own challenges. Volume mass pressures etc. But mainly if a batch goes sideways you may be out $1000's. So underpitching is a cheap problem to avoid.

Homebrewing. The .35m / ml / up to 1.040 add .35m for each +0.020 gravity Guide. (x2 for lagers) Is trying to make a blanket rule to fit everything. Mainly to keep hobby costs low and prevent over pitching. Over pitching? Yeah, if a yeast maker advertises ester profiles that you get from a yeast customers expect it. If it's over pitched those esters will be reduced significantly.

As a homebrewer you need to look at the "rules" as a saftey net for the novice. Learn about your yeasts you like and learn how to manipulate them to do what you want.

Answer 3

OK. Apart from the (interesting but essentially off-topic) discussion about ester formation as a function of pitching rates, I have since learned the the main cause of my confusion as addressed in the original question stems from the fact (and there is no gentle way to say this) that the Fermentis data sheets are mostly rubbish.

Independent cell counting (Van Den Berg S, Van Landschoot A. Practical use of dried yeasts in the brewing industry. CEREVISIA. 2003) shows that the minimum of 6 billion cells per gram may be a guaranteed minimum but has little to do with actual cell counts. Van den Berg and van Landschoot found 8 to 18 billion cells per gram in various fermentis yeasts, and MrMalty even puts it at 20 billion cells per gram.

Which makes a lot more sense. So my confusion stemmed not from the yeast but from the misleading data sheet.