The reason why we want to avoid dumping yeast cells from a White Labs vial into 1.060 wort is because we are dealing with a unknown number of viable cells that may not be in the best of health. A large deferential between the amount of solute inside of the cell and the amount of solute outside of the cell will cause water to migrate through the cell membrane to outside of the cell, resulting in dehydration, which, in turn, can result in cytorrhysis (the complete collapse of the cell wall). It also helps that it is easier to dissolve oxygen in 1.030 wort than it is 1.060 wort.
Do you why pitch rate increases with respect to gravity? It’s because the osmotic pressures encountered in high gravity wort place a lot of stress on a yeast cell’s plasma membrane (i.e., the high solute differential thing mentioned above). If we couple the osmotic pressure problem with the fact that it is more difficult to dissolve oxygen in high gravity wort than it is low gravity wort, we quickly realize that we have to limit the number cell divisions that need to occur before the stationary phase is reached, as each division results in a mother cell sharing its ergosterol and unsaturated fatty acid (UFA) reserves with its daughter cell (replacement divisions will need to occur during the stationary phase; hence, we need reserves going into the stationary phase). Ergosterol and UFAs are critical cell plasma membrane health because they make it more pliable, which, in turn, is critical yeast cell metabolism.
An important thing to remember is that a yeast cell loses turgor pressure when it shrinks due hypertonic situations (high levels of external solutes). Turgor pressure is the pressure that pushes a yeast cell’s plasma membrane against its cell wall.
Another problem that we encounter in high gravity fermentation are high ethanol levels. Ethanol is hygroscopic, which results in water being drawn out of the cells through their plasma membranes, which, in turn, results in shrinkage and loss of turgor pressure. In effect, yeast cells quit fermenting at a point because they become too dehydrated to pass nutrients and waste products through their plasma membranes, eventually resulting in cell death.
With that said, we encounter two problems when growing yeast cells for use in high gravity fermentations. The first problem is basic biomass growth, that is, we need to increase the overall yeast cell count, so that our pitch rate is closer to maximum cell density. The second problem is that we need to grow cells that do not go into the yeast equivalent of cardiac arrest when pitched into high gravity wort. What we are doing by increasing the gravity with each step is progressively selecting cells that can endure higher and higher osmotic pressures. It’s basically survival of the fittest. The cells that do not have the right stuff do not replicate and are replaced by the offspring of cells that do have the right stuff. This selection process is basically what happens in the 6 -> 8 -> 10 progression that narvin mentioned. There is nothing magical about using a larger batch. It’s all fermentative growth.
Topic: HELP: WLP540 Abbey IV Attenuation (Read 10863 times)
