In the past my harvested yeasts were always a light peanut-butter color. I have noticed that my W-34/70 yeast (8th gen) has turned a grayish color while in storage.
My Kolsch yeast (Wyeast - 3rd gen) has a slight purple tint to it.
Are these colors telling me the yeast has problems? The aroma from the storage bottle is still Ok.
Purplish is weird. But I wouldn’t entirely rely on color but, that said, creamy, pale colored slurries are always the best. I always use my slurries within a couple of weeks or start fresh.
Not sure if you used wheat in the kölsch yeast but Best Wheat malt always left behind a weird purple color when draining the mashtun
Yes, creamy pale color is what I am accustomed to.
Just wondering about the grayish tint that is now prevalent in the yeast?
Might just start fresh and not take any chances.
The color change is due to your practice of serial overpitching. Yeast cells turn gray as they become non-viable. Overpitching causes a culture to become progressively older with each successive re-pitch due to suppression of new growth. If you want your cultures to last more than a couple of repitches, you may want to start pitching between 150 and 175ml of thick slurry per 5 gallons in the future. Garrett Oliver was the first pro-brewer I heard mention that overpitching is a bad practice. He also mentioned that he frequently underpitches to reduce the average age of the cells in his pitching yeast. That was back in the 90s.
Over pitching? Yes, probably so. But the beers have always been extremely good, with no signs of negative issues resulting from the quantity of yeast. I’ll cut back on the yeast amount going forward, and see what effect this has on the beer, and the yeast that is harvested.
That practice is not about beer flavor. It is about maintaining culture viability. People routinely dump new wort onto an old yeast cake. Does it work? Absolutley! Is is a bad practice if one serially repitches? Absolutely! A yeast culture needs to go through four to five replication periods per fermentation in order to keep the culture from becoming mostly non-viable cells. You do not have to take my word in it. Here is an answer on White Labs’ website : Yeast and Fermentation | White Labs
Another well written article on the subject:
Without reading the links- is the take-home message that pitching the correct number of cells about having those pitched cell go through a population growth phase first (producing new cells)? Over pitching eliminates the growth phase and so the pitched cells go directly to fermentation?
Do I have that right?
Yes, you are correct. I discovered years ago through experimentation that I preferred the flavor of beer fermented with part of a slurry rather than the entire thing.
Does yeast not always go through an initial acclimation stage, where O2 is taken up, followed by a growth stage? Whether you under pitch, or over pitch? I always over pitch my lagers.
It is my understanding that if you overpitch yeas5 will go through fewer growth cycles, which can have a negative effect on flavor. That has also been my observation based on experience.
Please define growth cycles. Is this a single cycle, or growth phase, or are there multiple cycles / growth phases involved?
How many cycles will a standard pitching rate experience, and how many fewer cycles will an over pitched yeast experience?
Has anyone done A-B taste tests to confirm the results and flavor impact?
Here is what I found on Brulosophy…
I’m talking about 4-5 cycles of cell replication. Now, I’m now watching with a microscope to see what they’re doing. I just know that my experiwnce is that fewer is better, as long as it’s not too few. Close enough for me. Mark might have numbers, but as Chris White said to Drew and me “homebrewers are too hung up on numbers.” For me, the beer tells the story. In terms of Brulosophy experiments, theyre interesting data points but not conclusions. Data points from you or me are equally valid.
A yeast culture will almost always experience a lag phase where it consumes O2 and builds ergosterol and unsaturated fatty acid reserves. What differs is the number of replication periods it takes to reach maximum cell density when one overpitches.
Let’s start with the math. Nineteen liters (five gallons) of wort has a maximum cell density of approximately 200,000,000,000 * 19 = 3.8 trillion cells.
Batch size = 19L
Batch_Maximum_Cell_Density = 19 * 200,000,000,000 = 3.8 trillion
Pitched_Cell_Count = 200 billion
Replication_Periods = log(3800 / 200) / log(2) = log(19) / log(2) = 4.25 replication periods
Batch size = 19L
Batch_Maximum_Cell_Density = 19 * 200,000,000,000 = 3.8 trillion
Pitched_Cell_Count = 400 billion
Replication_Periods = log(3800 / 400) / log(2) = log(9.5) / log(2) = 3.25 replication periods
Batch size = 19L
Batch_Maximum_Cell_Density = 19 * 200,000,000,000 = 3.8 trillion
Pitched_Cell_Count = 800 billion
Replication_Periods = log(3800 / 800) / log(2) = log(4.75) / log(2) = 2.25 replication periods
Batch size = 19L
Batch_Maximum_Cell_Density = 19 * 200,000,000,000 = 3.8 trillion
Pitched_Cell_Count = 1.6 billion
Replication_Periods = log(3.8 / 1.6) / log(2) = log(2.375) / log(2) = 1.25 replication periods
As one can see, doubling the pitch rate reduces the replication period count by one.
How do these pitched cell counts translate to thick slurry? The generally accepted cell count per milliliter of thick slurry is 1.2 billion cells.
Pitch_200_Billion_Cells = 200 / 1.2 = 167ml
Pitch_400_Billion_Cells = 400 / 1.2 = 334ml
Pitch_800_Billion_Cells = 800 / 1.2 = 667ml
Pitch_1600_Billion_Cells = 1600 / 1.2 = 1334ml
If we follow the standard practice of cropping from the middle of the cake, we have mostly new cells with a pitch rate of 200B cells. The new cell count is reduced slightly with a pitching rate of 400B cells. However, after we get to 800B cells, we are looking at a minimum of 21% old cells even if we crop perfectly. By the time we get to 1.6T cells, half of the cropped cells are old. Old yeast cells are susceptible to the build up of something known as reactive oxygen species (ROS). ROS can cause damage to DNA and RNA, not to mention cell death.
I typically will pitch one qt of thick yeast slurry into a 10 gallon batch. Or, a pint per 5 gallons.
This is always the harvested yeast from a previous brew. How many cells are present?
Is this over pitching, and if so, by how much?
Earlier this year or late last year, I went to a slurry repitch using a stainless ladle - it happened to be just shy of 200 ml. My results with one ladle or a ladle and a bit more have been good. I just got lucky, I guess, that I stumbled upon a reasonable re-pitch rate for 5 gallons. I use shy of 3 ladles for 10 gallon batch re-pitches. Just my one data point, I know.
Omega Yeast did a seminar for my homebrew club recently. Mark highlighted an experiment they did where they scraped a plate and used that to prop a 100 mL starter. They did this with 4 strains, a hefe, lager, kveik, and west coast ale (I believe, I’d have to check my notes). They let it ferment out completely and performed cell counts, which ranged from ~100-900m/mL. The volume was also visually different. They used filter paper to separate the solid from the liquid, and all had the same mass (approximately). Their conclusion was that cell count wasn’t a good measure as cell size and other factors came into play. They also mentioned some issues with different yeasts reacting and metabolizing dyes different, which can lead to viability% not matching practical results. Yeast is weird.
I think I remember someone saying, you’re trading carbon for biomass when it comes to building yeast. So it stands to reason that there would be this much variance.
A pint is 473ml. If you are pitching 473ml per 5-gallons, you are overpitching by 473 / 167 = 2.8 times. That is a significant overpitch. When the Siebel Institute was in the game of propagating their cultures for microbreweries, their suggested pitch rate was one quart of slurry per barrel. A barrel is 3.1 times the volume into which you are pitching a quart of slurry. Unless you guys are total pigs with respect to brewery hygiene, there is nothing to be gained by pitching that much yeast.