Peak in beer freezing temperature plot

Question

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:

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.

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.

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

Curve of full freezing process:

Answer 1

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.

Answer 2

A nicely done experiment!

Consider hand-warmers containing sodium acetate trihydrate, $\ce{NaCH3COO.3H2O}$. When heated above the melting point and then cooled, the compound does not quickly solidify, but can be greatly supercooled. 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, 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... 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. ;-)