that makes sense. I was just looking for the reason why so I can sleep well at night
You are overthinking the problem and treating it like black magic when the problem is one of basic biological science; namely, how do we ensure that our culture owns the wort? We do so by pitching enough cells to ensure that we reach maximum cell density before any invaders have a chance to take control of the wort. A yeast culture owns a batch of wort by lowering the pH below 4.6, which prevents a whole host of pH sensitive bacteria, including pathogens, from gaining a foothold in the wort. It also consumes all of the dissolved O2
, which prevents wild aerobic microflora from gaining a foothold in the wort. Finally, a yeast culture produces ethanol, which is toxic to all carbon-based lifeforms at a given level. Brewing yeast cultures have been domesticated via selective pressure to be able to withstand the ethanol levels encountered in brewing. Most wild microflora cannot withstand the ethanol levels encountered in fermentation (which is why rinsing yeast with and storing it under boiled water is a bad idea).
A concept that brewer's need to burn into their minds is that fermentation is controlled spoilage. The key word here is "controlled." We want the pitched microflora to own the wort. The problem that we face is that the bacteria cell count doubles three times in the same amount of time that it takes for the yeast cell count to double, and no brewery is sterile. The difference in replication rates means that the bacteria cell count grows by a factor of 8 (2 * 2 * 2) every time the yeast cell count doubles; hence, we are looking at 8n
growth models for bacteria and yeast respectively when we normalize the growth rates to the time it takes for the yeast cell count to double.
Here’s the reason why we make a starter:
yeast_cell_count_at_time_t = initial_cell_count * 2(t / replication_period_in_minutes)
, where t is the amount of minutes that have elapsed since the culture transitioned from the lag phase to the logarithmic phase
bacteria_cell_count_at_time_t = initial_cell_count * 8(t / replication_period_in_minutes)
, where the t is the amount of minutes that have elapsed since the culture transitioned from the lag phase to the logarithmic phase
Let's look at a situation that we never want to have occur; namely, pitching so little yeast that the culture has to spend 24 hours in logarithmic growth in order to reach maximum cell density (this situation was common in the bad old days).
t = 1440 (24 hours into the logarithmic phase, or 16 replication periods because the average replication period for a yeast cell is 90 minutes)
2 raised to the power of 16 is 65,536
yeast_cell_count_at_time_t = initial_cell_count * 65,536
8 raised to the power of 16 is 281,474,976,710,656
bacteria_cell_count_at_time_t = initial_cell_count * 281,474,976,710,656
If the yeast cells do not own the media long before twenty-four hours of exiting the lag phase and our brewery is not clean enough to eat off of the floor, it’s all over; therefore, we want to ensure that the yeast culture never needs go through more than six to seven replication periods in order to reach maximum cell density (signaled by high krausen). The reason why we pitch at high krausen is because it shortens the lag phase, which starts replication earlier.
maximum_cell_density_for_1L = 200 billion
Most brewers who use 5-gallon soda kegs start 5.5 gallons of wort in the primary.
maximum_cell_density_for_5.5_gallons = 21 * 200 billion = 4.2 trillion (5.5 gallons is roughly 21 liters)
our_culture_cell_count_low = 50 billion
number_of_replication_periods = log(4.2 trillion / 50 billion) / log(2) = 6.4 replication periods (or 9.6 hours spent in the logarithmic phase)
our_culture_cell_count_high = 200 billion
number_of_replication_periods = log(4.2 trillion / 200 billion) / log(2) = 4.4 replication periods (6.6 hours spent in the logarithmic phase)
If a replication period is 90 minutes on average at ale fermentation temperature (64-68F), then our yeast cultures will saturate the wort within 6.6 to 9.6 hours after leaving the lag phase, making the time spent in the lag phase the hold up. When we pitch a culture that has sedimented (i.e., quiescent cells), we are pitching cells that underwent survival-related morphological (cellular) changes. They stored glycogen and the disaccharide trehalose. The also thickened their cell walls. It takes time after pitching for these changes to be reversed. Additionally, all replication past high krausen is for replacement only
, and mother cells share their ergosterol and unsaturated fatty acid (UFA) reserves with their daughters and their daughters share their reserves with their daughters and so forth; therefore, allowing a starter to proceed past high krausen wastes ergosterol and UFAs
. Ergosterol and UFAs are synthesized by the pitched yeast cells during the lag phase. The more ergosterol and UFAs a cell needs to replenish, the longer it remains in the lag phase and the higher the O2
load placed on the wort because these compounds are synthesized in the respirative metabolic pathway using O2