However, allowing fermentation to proceed to quiescence pretty much ensures that the cells have depleted their ergosterol and unsaturated fatty acid (UFA) reserves and have undergone morphological changes. The storing of glycogen is followed by thickening of the cell wall, both of which are survival mechanisms. Cell wall thickening has to be reversed, and the ergosterol and UFA reserves that were depleted after high krausen was reached have to be replenished (all reproduction after the end of the log phase is for replacement only, and mother cells share 50% of their ergosterol and UFA reserves with each daughter). These processes increase dissolved oxygen (O2) requirements and lag time upon pitching. Pitching at high krausen ensures that one is pitching cells that at the peak of health with ergosterol and UFA reserves that merely need to be topped off. Lower initial O2 requirements means that more dissolved O2 is available for the cells that are created during the log phase.
Yeast cells do not need glycogen reserves to reproduce or convert glucose (C6H12O6) to ethanol (CH3CH3OH), carbon dioxide (CO2), and other metabolites. They need pliable plasma membranes. A pliable plasma membrane allows nutrients to enter and waste product to exit the cell. That's where ergosterol and UFAs enter the picture. Ergosterol and UFAs make a yeast cell's plasma membrane more pliable. Ergosterol and UFAs are synthesized by shunting oxygen and a small amount of carbon (sugar is carbon bound to water; hence, the name carbohydrate) to the respirative metabolic pathway. All reproduction in batch propagation and fermentation is fermentative due to the glucose level of the solution being above the Crabtree threshold of 0.3%. To put things into context, pale extract contains approximately 14% glucose. In order to remain below the Crabtree threshold, we would have to hold the wort gravity to around 1.008, which is a 2% weight by volume (w/v) solution due to the fact that 1ml of water equals 1g.
0.02 x 14 = 0.28% glucose w/v
A glucose level of 0.28% prevents the Crabtree effect from kicking in, allowing the Pasteur effect to take over. Reproduction becomes respirative, resulting pyruvate being converted mainly to energy, water (H20), and CO2 via the Krebs cycle. Respirative reproduction is at least 16 times more efficient than fermentative reproduction. That's why the large dry yeast manufacturers propagate aerobically in a bioreactor.
To put thing into layman's terms, one should never allow a starter to ferment out because doing so places the culture in the yeast equivalent of hibernation. The storing of glycogen is the yeast equivalent of a bear putting on fat for the winter. Yeast cells store glycogen because they do not know how long it will be before a carbon source (e.g., sugar) becomes available. The glycogen that is stored at the end of fermentation is slowly consumed while the yeast cells are in storage. The cells die after they consume all of their glycogen reserves, resulting autolysis. The word "autolysisâ is composed of the Latin "auto," which means self (e.g., automobile means self-mobile) and the Latin "lysis," which means to break apart. When put together, autolysis translates to "self-break apart" or "self-destruct." The reason why we refrigerate crops is to slow metabolism; thus, lengthen the amount of time that it takes for the cells to deplete their glycogen stores. The thickening of the cell wall is in part to prevent premature autolysis.
If one has ever watched a brewery top crop, one has noticed that they skim the first head (known as the brown head) and crop the head that appears after the first head is removed. This first head is removed because it contains organic matter that was scrubbed from the wort as it came to high krausen (it also contains cells that exhibit early flocculation). The second head is taken at roughly 50% attenuation, which is still high krausen. We want to pitch a starter at high krausen for the same reason why brewers skim at high krausen; namely, the cells are in the peak of health. Healthy cells have lower O2 needs upon pitching, and they do not need to reverse the morphological changes that occur at the end of fermentation; hence, lag time is reduced and more dissolved O2 is available for the yeast cells that are created during the lag phase, resulting in healthier fermentations and resulting crops.
