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.