Patterns on melted, then solidified chocolate

Question

Observation

I have melted some chocolate (in a bain-marie, or water-bath) in a bowl, and then turned off the stove. I let the hot and liquid chocolate stand there, letting it cool for a few hours at room temperature. Then (it was solid, albeit not very hard) I covered it with aluminium foil, and put it in the fridge (ca $6^\circ\,\mathrm{C}$). When I took it out the next day, I saw those patterns that look like tiny footprints. Each "track" was about $2$-$4\,\mathrm{mm}$ wide. The arrow indicates where I pulled out the spoon, which left a little cusp. Note how the "footprints" converge there. The bowl is about $15\,\mathrm{cm}$ in diameter.

Possible causes that are unlikely

Insect Footprints. Even though it looks a lot like it, I think that insect footprints are an unlikely cause, though, for the following reasons:

1. It's deep winter where I live, and there has not been any fly or similar large insect in my flat for months.
2. The patterns were not visible before I put the chocolate in the fridge, and it's even more unlikely that an insect lives inside the fridge at those low temperatures (most insects lose the ability to move, and eventually die).
3. Other people have observed the exact same phenomenon, see [1](https://www.gutefrage.net/frage/hilfe-sind-das-tierspuren) and [2]. In 1, a pastry chef claims that he has seen this before, and that he rules out insects.

But I cannot be $100\,\%$ sure, since I have not watched it all the time.

Droplets falling from the aluminium foil. Since the foil has inevitable crinkles, a possibility would be that small droplets would fall from those crinkles onto the chocolate, causing regular patterns. However, this would not explain why there always are two parallel rows of droplets (which also appear in very regular "step-widths").

Question

What causes those patterns? Based on my considerations so far, I am inclined to think that this is a Physics phenomenon, which has to do with how chocolate solidifies, or how the "dew" on the surface forms. However, I am aware that most times when people report patterns on the surface of cooled-down chocolate it looks more crystalline, for example here, and that this looks nothing like it. Overall, I am very unsure about where to place this.

Comment: This is not a cooking-related question - it is to understand which physical processes produce such patterns in a substance like chocolate.

Answer 1

Understanding this in detail could probably take you from a science fair project to a PhD, but my best guess is that these are chocolate sugar blooms preferentially forming on vortex streets that were distorted and then frozen into the chocolate on cooling.

Sugar blooms form in chocolate when exposed to moisture, and they often occur when chocolate is stored in a refrigerator. Since chocolate is a complex emulsion, exactly were the blooms nucleate may depend on slight variations in the local physical and chemical makeup of the chocolate.

If you look closely at the tracks in the image above, as well as the images cited in the question as well as others that can be found on Reddit or elsewhere, you will notice that the spots are not side by side in the tracks, but instead alternate - left, right, left, right, … - as you go along the tracks. This immediately makes me think of a Karmen vortex-street, similar to this image by Jürgen Wagner from Wikpedia

Such a vortex street could form in the chocolate as eddies are shed on alternate sides of a stirring stick:

If the chocolate make-up, temperature, cooling rate, and stirring mechanics are just right, the centre of each eddy could slightly concentrate sugar or create a physical difference in the chocolate. Each "defect" could then be the nucleus for a bloom when the chocolate is subsequently stored in the cool damp environment of a refrigerator. Depending on the how the chocolate was stirred, a complex network of tracks could then become visible. It makes sense that some of the tracks converge where the spoon pulled out, since it is presumably the movement of the spoon that created the vortex streets.

To confirm (or not) this hypothesis would require some careful experimentation. I can't do it myself because my home science Health and Safety Committee (i.e. my spouse) is concerned that I would eat all the experiments with serious detrimental consequences for my weight and blood pressure.

Answer 2

I think three things are playing important role here,

• grain structure of chocolate surface
• sugar bloom
• fat bloom
• solubility of fat in water

Let me present my hypothesis

Due to cooling two things happened i) moisture evaporated from surface ii) surface started to solidify forming grains.

That's how grains look

Now sugar bloom happens when water get evaporated and sugar crystals remain on surface, the white part on surface of chocolate is sugar bloom. Fat bloom happens when fat underneath the surface gets up on the surface through porous region and accumulates there. The grain boundaries served as path for fat to travel up and it got accumulated near the grain boundaries. The pattern formed is very similar to grain structure. The dark part is fat.

Let's clear some questions.

Why are the patches round? Because fat is insoluble in water hence forms a sphere (cohesive forces win over adhesive forces etc.). That gives the circular shape to them.

Why they appear like footprint? They don't! Zoom the image in question there are multiple dots scattered randomly on trails (grain boundaries). They are more like Milky Way appearing in night sky.

Why pattern is formed on walls of container? Chocolate used maybe rich in fat hence the thin layer was sufficient to provide enough fat.

If someone can film timelapse of chocolate cooling down then we can get evidence. Also leaving the chocolate harden fully can expose grain structure.