You are acting like there is a ton of evidence that supports the use of stir plates when propagating brewing yeast cells. The little evidence that does exist is the work of home brewers, and it is usually based on a sample size of less than ten. Stir plate usage in the home brewing community is based on a misinterpretation of research performed by Maribeth Raines in the nineties. I work for a life science research organization, and there is not a single stir plate being used for anything other than mixing. Orbital shakers, wrist-action shakers, and roller tables are used for suspension cell culturing in our labs.McMullan wrote:[
It's not acceptable, scientifically, to simply extrapolate from basics about the physical and chemical nature of a foam, especially when there's a biological (yeast) component involved. This is not sound science. It's quite the opposite, actually. It's crystal ball gazing. If you want to prescribe a different way for preparing starters, you first need to provide convincing evidence that the current method of choice (a stir plate) is inferior. Assuming any relevance between a patent associated with reducing theoretical cell membrane 'shear stress' in bioreactors and a yeast starter prepared on a stir plate is crystal ball gazing for sure. You need to design experiments (or at least find some published ones) that specifically test your assumptions about home brew yeast starters. That would be more scientific.
Yeast cells need carbon and O2 to replicate
Brewing yeast cultures do not need to be stirred to remain in suspension in batch cultures because most brewing yeast strains exhibit NewFlo flocculation. NewFlo strains do not floc until glucose, mannose, sucrose, maltose, and maltotriose have reached genetically set levels.
Slowly stirring a culture whose volume displaces 50% of the volume of the Erlenmeyer flask in which it is being propagated does not provide much in the way of O2 absorption over a non-stirred culture of the same volume in the same flask because it does not increase the specific surface area of the gas-liquid interface. Spinning the medium fast enough to create a vortex does in fact increase O2 absorption, but it does so by a) increasing the gas-liquid specific surface area, and b) creating a vacuum that draws air into the vortex. Spinning a culture fast enough to create a vortex also increases the amount of shear stress placed on the cells. You can attempt to refute the shear stress claim, but the evidence does not support your thesis.
Truth be told, I did not set out to create a new method because an old method did not work. My method was a purely serendipitous event. I was propagating one U.S. quart starters in a forty-eight U.S. fluid ounce glass juice bottle. I went to make a starter and discovered that my juice bottle was cracked. I had a one U.S. gallon glass jug (demijohn in UK speak) on hand that I used to make small batches of mead that I decided to use as a quick fix to my problem. Shaking the culture until it was almost all foam was the result of being incredibly strong and in good cardiovascular shape from spending my teens through my mid-thirties in the gym. The difference in fermentation was immediately noticeable, so I kept using the method. I did not realize that shaking the starter until it was almost all foam resulted in a huge increase in specific surface area with respect to the gas-liquid interface until I revisited my university physics text a few years later.
Even with this knowledge, I switched to using a stir plate many years later based on the claims being published by other home brewers. The difference that I noticed right away was that my stirred cultures smelled off, which resulted in the requirement to decant the supernatant. The second thing that I noticed was an increase in fermentation metabolites that signaled stress. At first, I just wrote the differences in performance off as something that would take time to perfect. However, after a year of using a stir plate religiously, I decided to brew a double size batch for experimentation purposes. I wanted to see if the problem was real or if I had fallen victim to nostalgia. Well, that test confirmed that I was not being nostalgic. Wanting to see if my results were reproducible, I posted my method on a U.S.-based home brewing forum where it was met with the same level of doubting Thomasism that you have expressed; however, over time, more and more home brewers were reporting an improvement over using a stir plate. While some have switched from shaking to pure O2 injection in order to avoid the laborious task of shaking a starter extremely vigorously for a couple of minutes, I do not know of a single brewer who has tried the method that has gone back to using a stir plate. In fact, many have sold their stir plates. From reading this thread, it appears that several brewers in the UK have been able to repeat my results.