Yesterday was brew day. I had already prepared my starter 2-3 days before. Most of the yeast had settled in the bottom, but I only pitched the wort of the starter, not the slurry. Needless to say that no activity was present this morning. Luckily I have another mason jar of s-04 and i'm making a new starter at the moment.
My question is, how come if the beer ferments for a few weeks that there is not enough yeast in suspension to start fermentation?
You need about 1 million cells per degree plato per milliliter for healthy fermentation. That works out to about 250 billion cells for 5 gallons of 1.052 wort. That's about 100-200 grams of yeast cake in most cases.
Normal healthy pitches have about a 6-12 hour lag phase where the yeast cells are acclimating to the wort and preparing for cell growth. After that point, they will get to about 100 million cells/mL at a doubling rate of about 2 hours, so to go from 10 million cells/mL to 100 mL cells/mL takes approximately 6-8 hours after lag phase. In properly pitched beer, growth should be at near maximum rate within 18 hours.
If you're just pitching almost fermented out beer, there is only going to be approximately 0.5-1 million cells per milliliter. To get the 250 billion cells, you would need 250-500 liters of beer.
If you were to pitch, say, 2 L of beer, it would contain approximately 1 - 2 billion cells instead of the proper 250 billion. Assuming the same lag phase and doubling rate, to get to full fermentation will take about 16 hours longer.
If you notice, this does not really make sense. It would seem to suggest that radically underpitching would add at the most a day to your fermentation. In reality this is not what happens, because the yeast start to consume oxygen immediately and change the nature of the wort BEFORE the cell population reaches maximum concentration. The yeast will respirate every available scrap of oxygen, and when they do, growth slows down dramatically. Those extra 16 hours turn into days and days of extra time as the cells slowly grow instead of reaching maximum concentration in a burst as they do in healthy fermentation with plenty of oxygen and plenty of cells.
Compare this to bottle conditioning. Very low pitch rates can take months to finish fermenting even the most minute quantities of sugar, because there is no oxygen, and thus no cell growth.
