I'm under the impression that aeration is only needed for liquid yeast (or from starters) or for when repitching. When pitching straight from a dry yeast sachet, no aeration is needed. Is Verdant different in this?
Whether or not one has to aerate with dry yeast appears to be strain and pitch rate dependent. The newer cultures do not appear to take as well to aerobic propagation in a bioreactor followed by fluidized bed drying as the old standbys. What we know is that the onset of fermentation can be sluggish and desired final gravity is not a given. Why are we seeing these differences? Is it because the newer strains are suffering high cell death? Or is it because the newer strains have higher O2
requirements than the old standbys? One thing I do know is that dry yeast is almost always underpitched and aeration is critical when underpitching. If a pack of dry yeast contains 5 billion viable cells per gram, then an 11 gram package of dry yeast contains 55 billion cells. That is one fourth the pitch rate of a 1L starter pitched at high krausen. With 55 billion cells, it takes log(3800 / 55) / log(2) = 6.1 replication periods to reach maximum cell density on 19L (5 gallons) of wort. That is almost two full replication periods longer than a 1L starter. As the dry mother cells will share their ergosterol and unsaturated fatty acid (UFA) reserves with every cell, directly or indirectly, during the growth and stationary phases, having to go two more replication periods to reach maximum cells density places a higher demand on the reserves that were available before the yeast was pitched. Ergosterol and UFAs keep cell plasma membranes pliable, which allows for the intake of nutrients and expulsion of waste products.
I willing to bet that aerating wort before pitching the troublesome dry yeast cultures reduces the lag between pitching and active fermentation coupled with improving the strength of the fermentation. I have never been a big dry yeast user; therefore, this experiment is going to have to wait until my new brew house is up and running. One thing we know is that starting an 11 gram package of dry yeast in 1L of wort followed by pitching the culture into aerated wort results in dry culture behaving like a liquid culture, even the troublesome cultures. Why is that so? It could be that increasing the cell count by almost a factor of four is responsible for the difference, or it could be that the ergosterol and UFA reserves that the cells have coming out of the package are more than enough to rapidly reach maximum cell density in 1L of wort. These cells then get to go through another lag phase where they shunt O2
and carbon to their aerobic metabolic pathways to recharge their ergosterol and UFA reserves before restarting exponential growth when pitched into a batch or aerated wort.