Sorry, spell checker erased an L in my post. I wanted to say that I want to make two 2L starters (to pitch in two six gal. lager batches). I don't have four 1 gal. jugs.
Headspace is critical to achieving a proper shake. One is attempting to turn the wort into as much foam as is humanly possible. This method is designed to be low-tech and low-cost.
With that said, another thing that I am attempting to dispel is the insane notion that people have to hit the cell counts provided by yeast calculators in order to have a healthy fermentation. It is better to have 60 billion healthy, ready to go to war with the wort cells than it is to have 200 billion stressed cells that are barely clinging to life. The difference between 60 billion cells and 200 billion cells is approximately 180 minutes of propagation time.
The maximum cell density for 2 liters of wort is roughly 400 billion cells. The maximum cell density for 5 gallons of wort is roughly 3.8 trillion cells; hence, pitching 400 billion cells requires log(3,800 / 400) / log(2) = ~3.25 replication periods. Pitching 200 billion cells requires log(3,800 / 200) / log(2) = ~4.25 replication periods. The notion that one has to pitch a 2 liter starter into 5 gallons of normal gravity lager wort in order to have healthy fermentation is ludicrous. As there is more than enough carbon (sugar is carbon bound to water) in the average batch of wort to support the growth of 3.8 trillion cells, the limiting factor is dissolved O2 because the ergosterol and unsaturated fatty acids (UFA) that are synthesized by mother cells while O2 is still in solution is shared with all of the daughter cells that are created after O2 is depleted. Cell health declines as ergosterol and UFA reserves decline.
By pitching cells at high krausen instead of waiting until they have reached the stationary phase and prepared for starvation, we are pitching cells with non-depleted ergosterol and UFA reserves. The difference in ergosterol and UFA reserves from pitching a yeast culture at high krausen instead of waiting until the cells enter the stationary phase results in a lower initial load being placed on dissolved O2, preserving more O2 for future generations of yeast cells. Ergosterol and UFAs make yeast cells more pliable, which, in turn, allows for the passage of nutrient and waste products through their cell walls.
Cells with healthy ergosterol and UFA reserves are more ethanol tolerant than cells with depleted ergosterol reserves.http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3274781/
"Although most organisms cannot tolerate high levels of alcohol, certain yeasts (e.g., Saccharomyces cerevisiae) are able to maintain viability in the presence of up to 15–20 vol % ethanol. Through natural and directed evolution, yeasts have developed many strategies, also known as survival factors, to deal with ethanol toxicity (1). One important survival factor is to modify plasma membrane composition by increasing the content of unsaturated lipids and ergosterol
The reason why yeast cells in large part stop fermenting above a certain alcohol level is because they become dehydrated causing a reduction in cell size and a loss of turgor pressure. Ethanol is hygroscopic; hence, it draws water out of yeast cells. A similar thing happens when we pitch yeast into high gravity wort; however, it is phenomenon known as osmotic pressure. Osmotic pressure is the tendency of water to be drawn to the side of a semi-permeable membrane that contains the highest solute concentration. What happens when we pitch yeast cells into high gravity wort (hypertonic environment) is that water is drawn out of the cells into the wort, which, in turn, causes cell size to decrease resulting in reduced, if not outright cessation of cellular metabolism or even cell death.
The following paper contains images that show what happens to yeast cells when they are exposed to high gravity solutions and high alcohol levels: http://onlinelibrary.wiley.com/doi/10.1002/j.2050-0416.2003.tb00162.x/pdf