What happens inside your fermentation fridge?

September 30, 2019

On the 13th December 1900 Professor Adrian Brown, Head of the British School of Malting and Brewing at The University of Birmingham delivered a paper to the Midland Counties Institute of Brewing at The Grand Hotel Birmingham.


Clearly, beer was taken very seriously in those days!




The subject of his paper was the research he had been doing to measure the heat given off during fermentation.


Basically, the measurement of the exothermic heat given off by yeast as it breaks down sugar molecules into alcohol and C02.


I won’t go into the thermo-chemistry here (you can read it if you are interested in the paper itself, which you can download here) just the main points of his research and how we have revisited it.





Professor Brown figured that one of the principal difficulties in carrying out his research was reducing the influence of external temperatures on his measurements. So he chose to do it in a brewery, in an enclosed space, with a relatively stable temperature and a large volume of fermenting liquid.


The fermenting vessel used was 9ft by 4ft by 6ft, which equates to roughly 1500 gallons. A lot of beer!


What he found in his research was that the heat produced by the fermentation raised the external temperature in the room by 2 degrees C.


This was the first scientific experiment to look at the correlation between fermentation and heat and leveraged earlier work by Berthelot and Rechenberg.


Professor Brown’s work was hampered by a number of things.


Although he’d tried to minimise the effects of external temperature it was still there. Further, he was taking manual readings for temperature and specific gravity.


We are more fortunate. A fermentation fridge effectively isolates the liquid from external temperatures and, with the use of technology we can measure data in real time.


It was this paper that led us to repeat Professor Brown’s  experiment.


Using two temperature probes, one in a thermowell measuring the beer’s temperature and the second probe in a 1 litre bottle of water (since we know how much energy is required to raise the temperature of 1 litre of water by 1 degree C) to measure the ambient temperature, we electronically monitored the temperature during the fermentation of a beer (it was Grainfather’s recipe for ‘Grapefruit Juice’ – Galaxy and Ella hops, delicious!).


We also used a Tilt hydrometer to get a reading of the Specific Gravity of the fermenting beer over time.


The results are shown below.




As you can see from the chart, once the yeast start to ferment the beer the temperature differential between the ambient and beer probe widens.


At peak ferment that difference is greatest.


As fermentation tails off so the differential narrows and eventually, as the fermentation comes toward its end once the yeast have consumed all the available sugars, so the ambient temperature starts to oscillate more wildly as the dampening effect of the heat generated by the yeast tails away and the heating and cooling elements in the fermentation fridge have to work hard to keep the beer at its set temperature.


We repeated the experiment with a number of different beers and found the results were the same. Indeed one of them (a high alcohol stout) had such a large temperature differential  we predicted a ‘blow off’. Two hours later the fermentation fridge was covered in brown, sticky half fermented beer!


Our next step was to determine whether there was a correlation between the temperature differential (amount of energy given off by the yeast as they consumed the sugar)  and the specific gravity.


By using Professor Brown’s calculations we found that we could estimate the specific gravity at any time. The chart below illustrates this.


For the experiment we used both a Tilt and calculus to measure/estimate Specific Gravity.



As you can see from the data, properly positioned an ambient temperature probe in 1 litre of water placed in the fermentation fridge so that it doesn’t pick up too much ambient radiation from the beer or the heating/cooling source gives a reliable indication of SG progress throughout the ferment.


Nearly 120 years after Professor  Brown’s experiments we can utilise his work to monitor fermentation in a fermentation fridge.




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