## Sunday, November 11, 2012

### How to calculate fermentation time

Most brewers will tell you that it can't be calculated but will yield that for a known recipe and know fermentation conditions it can be approximated.  However, I think with sufficient data the date of completion can be calculated and the final gravity can be calculated within half a gravity point.

Yes, to the day, and within 1.0005 of the final gravity.

Measurements of fermentation of several beers I have brewed show that if fermentation temperature is maintained that the fermentation rate is exponential.  Using data from three measurements and an exponential fit the data matches very well.  The main source of error is the resolution of the hydrometer which is very accurate, but can only be read to about one gravity point.  The second source of error comes from fermentation start.  If the fermentation is slow to start then the original gravity and pitch time cannot accurately be used as the first point of the data.

Putting it to practice.

I brew and bottle on Friday or Saturday.  So best case would be I would brew a nice hoppy pale ale that I want to preserve the hop flavor of, then bottle the following weekend.  The days in between can be used for data collection.  If I measure the specific gravity 24 hours from the pitch, then on the following Tuesday and Thursday there is sufficient data to preform an exponential fit.

The basic steps are as follows:
1) Record the specific gravity, date and time of three days.
2) Enter the data into excel.  Add a column for the number of days from the time of pitching the yeast.  Add a column for the specific gravity less the final gravity.
3) Add an exponential trend line for the new columns that you have created.
4) adjust the final gravity until the best fit R squared value is achieved (as close to 1 as possible)
5) add a column with the equation that has been derived and plot the subsequent days.
6) The day that the new equation drops below 1 is your beer is ready to bottle.
7) check the gravity of the beer on the day that you have found and it will be on the money if you did everything right.

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