Those of you who are leaving next in the fermenter for long periods in order for the yeast to "clean up" might be interested in this conversation I had with John Palmer.....
There is so much erroneous information in what John wrote that about the only thing that is correct is the level of contamination 150 years ago. I like and respect John, but he needs to stick with what he knows best and that is the brewing side of beer production, not the biological side of beer production.
He kind of got a few things correct, but he is absolutely wrong about what causes the shift from the log (exponential growth) phase to the stationary phase. What causes yeast cells to switch from the log phase to the stationary phase is achieving maximum cell density (i.e., the yeast cell population is self-regulating). That is why I urge people to pitch their starters at high krausen because all reproduction after a fermentation enters the stationary phase is for replacement only (i.e., high krausen signals the switch from the log phase to the stationary phase). When a mother cell buds a new daughter cell, she shares her ergosterol and unsaturated fatty acid (UFA) reserves with the daughter cell, which means that mother cells will exhaust more of their reserves if a starter is allowed to ferment beyond high krausen. Wasting ergosterol and UFAs results in a higher O2 requirement and a longer lag phase when the starter is pitched. Allowing a starter to ferment out places the cells in a quiescent state where they have undergone morphological changes that have to be undone when the culture is pitched, resulting an even longer lag phase.
What causes yeast cells to stop fermenting is the exhaustion of the carbon sources they can convert to energy. I cover carbon sources in my blog entry entitled "Carbon Credits" (see https://www.experimentalbrew.com/blogs/saccharomyces/carbon-credits
). Most brewing yeasts fall into the NewFlo genotype, which means that flocculation will not occur until mannose, glucose, maltose, sucrose and the more complex saccharides that a yeast strain can reduce to one of these sugars are exhausted. A very visible example of NewFlo flocculation occurs with Lallemand Windsor. The reason why that yeast flocs so early is because it cannot break maltotriose down into three glucose molecules via the two-step process of breaking maltotriose into one molecule of maltose and one molecule of glucose followed by splitting the maltose molecule into two glucose molecules. In essence, Windsor does not stop fermenting because it has exhausted its ergosterol and UFA reserves. It stops fermenting because it has exhausted the carbon sources that it can metabolize, which triggers flocculation.
John is also absolutely incorrect about the yeast not cleaning up things during the stationary phase. The main thing he got right is that slowing down exponential growth leads to cleaner beer because that is were most of the metabolic trash production occurs. I covered this information in detail in my blog post entitled "Have You Seen Ester?" (see https://www.experimentalbrew.com/blogs/saccharomyces/have-you-seen-ester
). The main reason why we do not want to introduce O2 into fermented wort is because it can cause diauxic shift, which results in yeast cells using ethanol as their carbon source. Ethanol is the result of a yeast cell's anaerobic (fermentative) metabolic pathway being inefficient. If one introduces enough O2 after fermentation is complete, the cells will switch to using ethanol as a carbon source via their aerobic (respiratory) metabolic pathway, which is 100% efficient. The aerobic metabolic pathway converts carbon to energy, water, and carbon dioxide gas. The big dry yeast companies take advantage of the respiratory metabolic pathway's higher efficiency by propogating in devices known as bioreactors. All brewing yeast strains are Crabtree positive, which means that they will follow the lag, log, stationary, quiescence pattern when the gravity of medium (wort) is above the Crabtree threshold. What happens in a bioreactor is that medium (molasses) is kept below the Crabtree threshold and continuously refreshed at rate where it never exceeds it. O2 is added and the medium is stirred to make it uniform. By propagating yeast cells in a bioreactor, Lallemand and Fermentis are able to produce more yeast using less carbon. We need to remember that sugar is a carbohydrate and carbohydrates are built as multiplies of a carbon atom bound to a water molecule. Glucose is C6H12O6, which is six times CH20.
One last thing, he also kind of got the number of times a mother cell buds in a fermentation correct. As I covered in my blog entry entitled "Yeast Cultures Are Like Nuclear Weapons" (see https://www.experimentalbrew.com/blogs/saccharomyces/yeast-cultures-are-nuclear-weapons
), the yeast biomass grows at a rate of 2^n, where the symbol "^" denotes raised to the power of. The variable "n" is the number of replication periods that have elapsed. The number of replication periods required for a yeast culture to achieve maximum cell density after being pitch is calculated as the log base 2 of the number cells needed to reach maximum cell density divided by the number of cells pitched. Five gallons is basically 19 liters. If we pitch a 1L starter at high krausen then we need 19 times the number of cells pitched to reach maximum cell density. However, the yeast cells will not require 19 replication periods to reach maximum cell density because growth is exponential, not multiplicative; therefore, we need to take the log base 2 of 19. Most calculators due not have a log2 function, but can compute the log base 2 of 19 by dividing log(10) by log(2) using the base 10 log function.
number_of_replication_periods_needed = log(19) / log(2) = 4.24792751344359 replication periods
We can verify this result by raising 2 to the 4.24792751344359 power.
times_larger = 2^4.24792751344359 = 19
What determines how long it takes to reach maximum cell density time-wise is the length of the lag period plus the number of replication periods needed times the length of the replication period. At normal ale fermentation temperatures, the replication period is approxminately 90 minutes long. As we lower the temperature, the length of the replication period grows, which is why it takes longer to see visible signs of fermentation.