Old yeast starter - dispose or keep?

[jeg3]

I would expect it to be absolutely fine. My concern would be what liquid the yeast has been stored in, what kind of container etc, but I would be very optimistic about being able to revive it.

I've recently resurrected some WY1318 that had been stored in the fridge in a sealed plastic jar under beer for over 12 months - it woke straight up once I put it in some fresh wort.

In addition, other forum members have fairly recently woken up 40 year old yeast from the dregs at the bottom of some very old bottles of beer.

I'm on my second crop of WY1318 from a new packet pitched in December, both have been stored under beer. It's good to hear it survives well. I've got a pack of WY1469 I'm planning to crop as well.

 

[saccharomyces]

We can assume that thick slurry contains between 1.2 and 1.6 billion cells per milliliter. If we use the more conservative figure, then 180ml of thick slurry contains 1.2 * 180 = 216 billion cells. If we pitch 180ml of thick slurry into 23L of wort, we have a cell density of 216,000,000,000 / 23,000 = 9,391,304 cells per ml. George Fix’s rule of thumb for cell density is 750,000 cels per ml per degree plato. Using that rule of thumb, let’s calculate the cell density needed for a 1.050 (12.5P) wort.

12.5 * 750,000 = 9,375,000

In practice, 180ml of thick slurry is more than enough to fully attenuate beers up to around 1.060 (17P) without any hiccups, that is, as long as we properly aerate our wort. A 1.080 (20P) or greater beer is an entirely different animal due to osmotic pressure and the increased difficulty of disolving O2 into the wort. Most of the time, we can get away with underpitching by as much is 50% (e.g., 100B cells instead of 200B cells) because the difference is only one replication cycle, which is easily handled by proper aeration (there will be slight increase in ester production with some yeast cultures). That is the reason why I always state that yeast cultures are like nuclear weapons in that close is good enough. However, things start to get scetchy at 1.080 because one is starting to reach the point where dissolved O2 will not support underpitching, especially when coupled with early cell death caused by high osmotic pressure. For those who do not know, osmotic pressure is the tendency for fluid to be drawn to the side of a semi-permable membrane that has the highest solute content. In layman’s terns, the density of the wort is far higher than the density within the a yeast cell wall. What osmostic pressure does is draw water out of yeast cells, resulting in the loss of cell membrane turgor pressure, which, in turn, causes cell membranes to wrinkle and eventually implode. Pitching a greater number of cells in high gravity wort is about accounting for a) the increased difficulty of dissolving O2 in denser wort and b) premature cell death.

By the way, the photo below is a simple aerator design that I have been using since the nineties. In use, the aerator is attached to the outflow side of the tubing that drains one’s brewing kettle into one’s fermenation vessel. For optimum effectivness, it should be held vertically in the fermentation vessel (I use a racking cane clip). This aerator design is based on the Bernoulli principle (i.e., it is a venturi). It is made from section of 3/8” acrylic racking cane with inside dimension of 1/4”. It does not mater if the tubing is acrylic, copper, brass, or stainless steel, that is, as long as it is food grade; however, it is critical that the inside diameter of the aerator is smaller than the inside diameter of the tubing that drains one’s kettle. What happens is that forcing the wort to flow through a smaller diameter results in an increase in fluid speed and a decrease in pressure, which t causes vacuum to form that draws air in through the holes and mixes it with the wort. Preferrably, one wants to drill the holes at a downward angle from the top of the fluid flow (i.e., the part of the aerator that attached to the tubing that drains one’s kettle). If made and used correctly, one will often have to throttle back the fluid flow because the amount foam created can become quite large; however, one will be assured that the wort has reach full-air O2 saturation.

 

[matt76]

Thanks again @saccharomyces , this genuinely is fascinating stuff. Although not a microbiologist I am a man of science, so the mathematics and science of what you're describing is more than clear for me However.....

.....my area of expertise is actually aerodynamics / fluid mechanics - so what you're describing with your Venturi / aerator tube is fundamental stuff to me . It's kinda like the collar / air hole on a bunsen burner, when you open the air hole the Venturi effect draws in more air and the flame burns hotter.

I'm curious to experiment with this and see if it makes much difference to my current aeration practice - put the kettle on the kitchen counter (10L batches aren't that heavy! ), FV on a chair, open the tap and let it rip. I'd estimate the drop from the tap to the bottom of the FV is about 50-60cm (~2ft in your money ). Thinking about it, apart from minor splashes, no reason I couldn't just put the FV on the floor.....

 

[saccharomyces]

One never knows the educational backgrounds of the people who are reading one's posts; therefore, I attempt to make things as clear as possible when I post. My B.Sc. and M.Sc. degrees are actually in the computer engineering side of computer science. I have been involved in hardware and software engineering for four decades. Biochemistry and microbiology have been hobbies for the last 28 years. Learning a new STEM discipline is usually easier for STEM graduates than it is for non-STEM graduates, but there are always exceptions. For example, my grandfather had to leave school after finishing the 8th grade because he had to go to work to help support his family due to the fact that his father passed away. He passed the steam plant operating engineer licensing exam at age 14 and spent most of his career in electrical power generation. I was always astonished by how much grandfather knew about thermodynamics given his lack of formal higher education. I had to attend university to learn thermodynamics. He taught himself the discipline.

 

[johncrobinson]

That foam nozzle is very ingenious reminds me of a protein skimmer venturi for marine fish tanks

 

[timtoos]

Well I made up a starter today and before I added this yeast I had a taste of the wort. Wow, tasted slightly like vinegar was coming through. Chucked it down the sink and put on a new yeast instead.