goatherder
Fancyman of Cornwood
- Joined
- 23/1/06
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We talk a lot about pitching rates. The general consensus seems to be that appropriate pitching rates are an important factor in making good beer.
The problem with pitching rates is, as homebrewers, we don't have a simple method of measuring the cell count in our yeast starters. We read plenty and end up taking an educated guess when we make our starters. We don't really know if the pitch rate is what we assume it is.
A few weeks back on the Brewing Network, Dave Logsdon mentioned his microbiologists had done some work on estimating cell counts. The method is to take a sample of a yeast starter, refrigerate the sample and measure the percentage of settled solids after a period of time. They took cell counts of the samples and published the results. Dave was kind enough to email the slidepack to me. The results they got are here:
In this slide, the numbers are the cell count per millilitre.
I received some more info from Greg Doss, the microbiologist who conducted the experiment. He placed the following caveats on the results:
1: The test is not highly accurate, it gives an estimate only.
2: The strain used was a medium flocculent yeast. Results will be more or less depending on the flocculation characteristics of the yeast.
Greg also advised to let the sample settle for 3 days in the fridge to get these results. My own tests showed that the yeast settled to very close to the final value within 3 hours of placing in the fridge.
Examining the numbers, they indicate a linear relationship between settled solids and cell count. Not surprising when you think about it. If you run a line of best fit over the data points, you end up with the following relationship giving an excellent fit:
Percentage solids x 25 = Cell count in million cells per millilitre
I ran a few trial runs when I made my last starter. I used a 10ml syringe to settle the starter in, mainly because it has clearly marked 1% graduations on it. The biggest problem is the conical shaped plunger in the syringe making measurement difficult. I estimated the conincal plunger occupied 3ml, so I subtracted this from my final result. There are probably better solutions to using a syringe, I just haven't figured it out yet.
To take the sample, I fixed a length of aquarium airhose to the end of the syringe and drew 10ml from the starter. Then, I place the syringe in the fridge and read off the height of the solids a few hours later.
As an example, one of my samples read 10ml - adjusted for the conical plunger, this gives 7ml. This gives 7% solids, so multiply 7 x 25, which gives 175 million cells per millilitre. This number was in the same ballpark as figures I had read for a stirred starter - sosman's brewiki page shows a graph with 250 million cells/ml. This gives me some confidence in the theory and method.
As mentioned before, this is an estimate only. But it lets us do the following:
1: Validate our pitching rate estimates
2: Help to validate our starter protocols to confirm we get the cell rates we think we are getting
3: Compare methods of preparing starters - like shaking vs airstone vs stirplate
Now I'm looking for feedback. Do you think this is a worthwhile exercise?
I think that if we get a few people trying this and submitting results we might be able to generate some more (or less) confidence in the actual numbers.
The problem with pitching rates is, as homebrewers, we don't have a simple method of measuring the cell count in our yeast starters. We read plenty and end up taking an educated guess when we make our starters. We don't really know if the pitch rate is what we assume it is.
A few weeks back on the Brewing Network, Dave Logsdon mentioned his microbiologists had done some work on estimating cell counts. The method is to take a sample of a yeast starter, refrigerate the sample and measure the percentage of settled solids after a period of time. They took cell counts of the samples and published the results. Dave was kind enough to email the slidepack to me. The results they got are here:
In this slide, the numbers are the cell count per millilitre.
I received some more info from Greg Doss, the microbiologist who conducted the experiment. He placed the following caveats on the results:
1: The test is not highly accurate, it gives an estimate only.
2: The strain used was a medium flocculent yeast. Results will be more or less depending on the flocculation characteristics of the yeast.
Greg also advised to let the sample settle for 3 days in the fridge to get these results. My own tests showed that the yeast settled to very close to the final value within 3 hours of placing in the fridge.
Examining the numbers, they indicate a linear relationship between settled solids and cell count. Not surprising when you think about it. If you run a line of best fit over the data points, you end up with the following relationship giving an excellent fit:
Percentage solids x 25 = Cell count in million cells per millilitre
I ran a few trial runs when I made my last starter. I used a 10ml syringe to settle the starter in, mainly because it has clearly marked 1% graduations on it. The biggest problem is the conical shaped plunger in the syringe making measurement difficult. I estimated the conincal plunger occupied 3ml, so I subtracted this from my final result. There are probably better solutions to using a syringe, I just haven't figured it out yet.
To take the sample, I fixed a length of aquarium airhose to the end of the syringe and drew 10ml from the starter. Then, I place the syringe in the fridge and read off the height of the solids a few hours later.
As an example, one of my samples read 10ml - adjusted for the conical plunger, this gives 7ml. This gives 7% solids, so multiply 7 x 25, which gives 175 million cells per millilitre. This number was in the same ballpark as figures I had read for a stirred starter - sosman's brewiki page shows a graph with 250 million cells/ml. This gives me some confidence in the theory and method.
As mentioned before, this is an estimate only. But it lets us do the following:
1: Validate our pitching rate estimates
2: Help to validate our starter protocols to confirm we get the cell rates we think we are getting
3: Compare methods of preparing starters - like shaking vs airstone vs stirplate
Now I'm looking for feedback. Do you think this is a worthwhile exercise?
I think that if we get a few people trying this and submitting results we might be able to generate some more (or less) confidence in the actual numbers.
