When the Cell Density Meter is released we want to make sure we have worked out all the bugs, smoothed all the wrinkles, and that it works seamlessly. Our goal is straight forward operation with complex algorithms underneath to provide accurate measurements in the strenuous brewing environment.
At this point we have several working algorithms and are evaluating how they can best be implemented. The enclosure is in its final stages of fit checks and adjustments.
To put the Cell Density Meter though it's paces I conducted a comparative study of starter growth with and without a stir plate. In summary the yield is about the same, but the cell growth is much faster with a stir plate. Download the full report below.
Agitation Effects on Growth Rate and Yield of Brewer’s Yeast
Very interesting stuff! I suppose this would make it easier to better know just the amount of cells we're harvesting when stealing slurry from a starter. Cheers!ReplyDelete
Thanks, that's how I hope people will use it. It takes out the guess work, and eliminates error do to variance from starter calculators. I use it to monitor my starters and then put them in the fridge when my desired cell count is reached.Delete
I'm a little confused by this. All of the starter calculators I've seen that give you the option to choose between "stir plate" and "no agitation" will calculate drastically different final cell counts based on the type of agitation. Are they assuming the growth is arrested after a certain period of time, or is there something else I'm not understanding?ReplyDelete
Excellent question. I've seen the same thing. From a biological stand point the cell density shouldn't effect growth until you get to the point where the cells are interacting with each other instead of the wort around them. This occurs at very high cell densities. Yeast growth is much like bacteria or any other microorganism: If there is food, they grow and divide.Delete
You're idea is similar to what I have been thinking as well. That the single set of experiments done by Chris White don't accurately represent the way most homebrewers make a still starter.
Starters are simple batch cultures. The microorganism will continue to divide until the nutrient is depleted. For a still starter the limiting nutrient can be oxygen. Aerating once at inoculation will only provide enough oxygen for about 4 doublings of cells (See Greg Doss "Practical Pitching" on the MBAA site)
Chris White explains in his book "Yeast" that he didn't aerate his starters. My hunch is that oxygen was a limiting nutrient in his experiments.
48 hours if a very typical culture time used in laboratory experiments. I've seen it in nearly every paper I have read on the subject. I also suspect that Chris only allowed the propagation to occur for 48 hours which would make his numbers align with my findings.
I do have a calculator in the works that is almost ready to roll out that shows the growth over time for both still and stirred starters.
Interesting. So in your experiment, was the unagitated wort aerated prior to pitching the yeast? Also, I'm curious why the unagitated starter was only 3ml, while the agitated starter was 500ml. I guess this wouldn't have any effect on the results?Delete
Your findings actually help me with something I've had trouble understanding in my own experience. I used an online calculator to find out how to grow a vial of Brett (50B cells) to a pitchable rate for a 10 gallon batch of 1.070 wort, using an unagitated starter. The calculator said I needed to do two steps of 1 gallon each, and that would get me close to the almost 500B cells needed for the batch. When I did finally pitch the yeast, fermentation was very vigorous in less than an hour, and I knew I had massively overpitched.
I guess what I'm taking away is that the calculators I've been using aren't as accurate as I had thought, and that stir plates are not at all necessary for the homebrewer, except as a means to cutting the time it takes to grow a starter.
Can't wait to see your calculator, and thanks for the work you're doing!
Yes, The still starter was shaken after inoculation to aerate. Volume of the fermentation vessel doesn't play a large role in propagation.Delete
There is also variation from one strain of yeast to another. This is something in trying to capture in the calculator as well.
Sorry Steven. But these findings seem to contradict Kais findings herehttp://braukaiser.com/blog/blog/2013/03/25/stir-speed-and-yeast-growth/ReplyDelete
What's your take on this?
Nothing to be sorry about, this is part of peer review. Kai does very good work. The relationship between stir speed and growth rate has been explored by others as well. (See Factors Affecting the Growth of... By Camacho-Ruiz). I have no doubt that the growth rate is higher on a stir plate and that higher RPM produce even higher growth rates, but the yield is about the same. Because of the crab tree effect yeast primarily use the fermentation pathway. At very low gravity (0.5% w/w) higher yield can be achieved by use of the respiratory pathway. This is what is used in chemostat cultures. (See An Interlaboratory Comparison of ... Van Dijken)Delete
I see. So your last paper was on anearobic batch cultures? I see no reference in the paper to the cultures being sealed, air locked or aerated.Delete
It must not have been clear in the Materials and Methods section. The "still starter" was inoculated and the aerated by shaking for 30 seconds. The lid was loosely attached for the remainder of the fermentation. The "stirred starter" was on a stir plate with a foil cover. Yeast prefer anaerobic fermentation when the sugar concentration is about about 0.5% w/w even in the presence of oxygen (See Crab Tree Effect) The Inter-laboratory Study mentioned above actually characterizes this point using several strains of yeast.Delete
Thanks for clearing that up Steve.ReplyDelete
BTW I forgot to thank you for your contribution. Invaluable work!
Thanks! I appreciate the dialog with you to flesh out some of the details!Delete
What is the density of the reference standard you are using to ensure that your algorithm is calculating correctly? Also what is the margin of error you are hoping for?ReplyDelete
We are calibrating using a matrix suspensions of yeast in wort to allow the algorithm to accurately estimate cell density independent of the color of the wort. We expect to have better accuracy than using a hemocytometer.Delete