What everyone is seeing is the advantage of pitching at high krausen instead of letting the culture ferment out. Pitching at high krausen has the advantage of pitching cells that are still somewhat in the exponential growth phase as well as pitching cells that still have substantial ergosterol and unsaturated fatty acid (UFA) reserves. The synthesis of these compounds is a big part of what happens during the lag phase. Lower ergosterol and UFA reserves generally translate to a longer lag phase. Now, if we add in the fact that quiescent cells (cells that have fermented out) have undergone survival-related morphological changes that have to be reversed before the cells can go back to consuming sugar in addition to having to rebuild much lower ergosterol and UFA reserves, it is easy to see what a starter that is pitched at high krausen results in shorter time period between pitching and the onset of visible fermentation. Yeast pitched at high krausen also places a lower O2
load on wort due to having higher ergosterol and UFA reserves from the start.
I have also reached the conclusion that the reason why that the shaken, not stirred method works so well for being such a low-tech technique is because it makes available a large amount of O2
during the first hour after the culture is pitched (direct O2
injection provides the same benefit, but it adds complexity and a possible infection source to the equation). I have mentioned before that O2 is dissolves into a liquid at the interface between the gas and the liquid. Anyone who has ever looked at gas-liquid foam has seen that the foam is composed of gas encapsulated in thin layers of liquid.
I am currently attempting to determine if there is any advantage to stirring over shaking. As most brewing yeast strains exhibit NewFlo flocculation (i.e., they do not flocculate until glucose, mannose, maltose, sucrose, and maltotriose reach genetically set levels), brewing yeast cells generally do not need to be stirred to remain in suspension during the lag and exponential growth phases. However, I believe that I may have discovered something about stirring and flocculation that I may have overlooked.
A chance observation that I made last night may be the key to the advantage of stirring a culture. I inoculated 40ml of 5% w/v wort and subcultured two slants on Sunday evening using a slant that I received from the NCYC in the spring. I usually leave the lid tightened down on my 40ml starters for 24 hours after doing my normal shaken, not stirred maneuver with the media bottle. I do so because I want to ensure that I have a positive start. I usually hear a very slight hiss when I loosen the cap. This culture let out a loud hiss, which told me that the culture contained an elevated amount of dissolved CO2
. I wound up having to degas the culture via swirling. One thing that I noticed was that the cells had sedimented. However, they went back into suspension after I degassed the culture. I figured that this phenomenon was the result of swirling to degas the culture. However, the cells were still in suspension when I checked on the culture this morning, so dissolved CO2
level may play a part in premature flocculation.
By the way, the culture that I propagated the other night is NCYC 1333. NCYC 1333 is a Yorkshire culture with a unique set of attributes on paper. NCYC 1333 is supposed to be a head forming, flocculent, and attenuative culture, which is a rather unique set of attributes for any yeast strain, let alone a Yorkshire strain.