Peak in beer freezing temperature plot

I left an open beer in the freezer while monitoring its temperature. The first minutes works as expected. But after the plateau, at about −8 °C, a peak appeared:


[![Beer freezing (open can)](https://i.stack.imgur.com/yxcUs.png)](https://i.stack.imgur.com/yxcUs.png)


I thought it was an artefact because the can was open and the sensor was inside.


So I repeated the experiment with an intact, closed can.


[![Beer freezing (closed can)](https://i.stack.imgur.com/qNKhI.png)](https://i.stack.imgur.com/qNKhI.png)


The same spike occurs slightly below −10 °C. Well into the superfreezing phase, which is much wider this time.


Since beer is a liquid solution, I think this must be some component that changes phase before the others. Or maybe something related with solubility of $\ce{CO2}$ or ethanol as temperature changes.


Do you know what it is due to?


**Update**: reproducibility


After defrosting the can and putting it back in the freezer, the peak appeared again. Same result.


[![Beer freezing repeated experiment](https://i.stack.imgur.com/iFXIW.png)](https://i.stack.imgur.com/iFXIW.png)


In order to discard a sensor issue, this is the freezing curve of the sensor itself:


[![freezing curve of the sensor itself](https://i.stack.imgur.com/STjhS.png)](https://i.stack.imgur.com/STjhS.png)


Curve of full freezing process:


[![Full freezing](https://i.stack.imgur.com/5UOXH.png)](https://i.stack.imgur.com/5UOXH.png)




So it finally turned out to be an artefact, sorry.


The probe was protected by a small metallic tube. Somehow water condensed inside. Since this water is isolated from the main liquid, it performed its own phase-transition releasing its latent heat right next to the sensor.


The effect was not appreciated when freezing the probe alone. I don't know why, maybe because of a faster cooling o due to its physical position. But it was very noticeable when measuring the beer can.


I removed the tube and cleaned the sensor. I also ensured the thermosensitive part is physically in contact with the can; and I insulated it better from the environment. Now the measurement is more accurate and no weird peaks.


Thank you all for your time.


[![Beer freezing, new setup](https://i.stack.imgur.com/Su5Vv.png)](https://i.stack.imgur.com/Su5Vv.png)


 7

A nicely done experiment!


Consider [hand-warmers containing sodium acetate trihydrate](http://waterenergynews.com/how-sodium-acetate-reusable-hand-warmers-work/), $\ce{NaCH3COO.3H2O}$. When heated above the melting point and then cooled, the compound does *not* quickly solidify, but can be greatly [supercooled](https://en.wikipedia.org/wiki/Supercooling). Given an impetus to begin crystallization, such as the shock-wave produced by a "clicker" (or nucleation by a speck of dust), the molecules reorganize from liquid to solid, giving off heat to warm your digits. The increased order of the solid frees up some energy.


In a similar way, when *supercooled* water freezes, it gives off heat. Your graph seems to show that effect.


To see it even more vividly, put distilled water in a polypropylene food container with lid on *loosely*. Boil the water in a microwave oven *for just a few seconds* to remove most dissolved air, which otherwise would nucleate freezing too early. *Do not boil a second time*, or the air-free water can [superheat](https://en.wikipedia.org/wiki/Superheating), and then flash into steam all at once, blowing open the microwave and blowing an internal fuse. [Lest you ask, yes, this happened to me - no burns, but I had to disassemble the oven to replace the fuse.]


Put your thermocouple on the outside of the container (or it, too, could nucleate freezing), and place all in a fairly cold freezer. Likely, the freezer will *not* be cold enough to freeze that water, and your cooling graph should be smooth. Now, [take out the container and give it a sharp *rap* on a counter](https://www.youtube.com/watch?v=ph8xusY3GTM)... watch the water, and observe the temperature change.


Of course, freezing *beer* drives out the bubbles, so I hope you don't mind flat beer. ;-)


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