Melting Chocolate – FOR SCIENCE!

The Ultimate Fondu Melting Pot

[Patrick Herd] was in Sweden recently and decided to help out a team of high school students in the International Young Physicist Tournament — The challenge? Chocolate Hysteresis.

Chocolate what? When chocolate melts, it doesn’t actually re-solidify at it’s melting point — in fact, it’s quite below that. The challenge here is figuring out a scientific way of measuring the time (and temperature) it takes to return to a solid state. This in itself is kind of tricky considering you have to accurately measure the temperature and be able to empirically tell if its solid or liquid.

The first scientific apparatus they came up with was the Chocolate Rig V1 – a very simple peltier heated and cooled calorimeter. They used an Arduino to control the temperature and a motor shield to power the peltier plate. It kind of worked but they discovered it was difficult to assess the physical state of the chocolate. This is when [Patrick] started doing some research and discovered rotary viscometry.

It works by determining the shear force in a fluid by rotating some kind of object in it, with sensor feedback measuring the torque. This was quite a bit more challenging to create than their first rig, but they pushed onward anyway.

The sensor they built to measure the shear force is quite ingenious. They are rotating the heated crucible with a shear probe stuck in the middle — string is wrapped around the probe and attached to a force gauge. When the chocolate is liquid it spins around the probe with little to no rotation — but as soon as it hardens up, the probe starts rotating with the chocolate which in turn pulls on the force gauge.

The entire project is quite fascinating, so if you’re into scientific experimentation, you should definitely give his whole blog post a good read!

15 thoughts on “Melting Chocolate – FOR SCIENCE!

  1. Nice post, thanks James. I am sure this sort of data is critical to the new 3d Printing machines coming out soon. I bet there are tons of experiments going on in labs all over the world at the moment! I just finished a post covering what is going on in 3d Printing Chocolate here I am sure the ambient temperature at your location would have a big affect on the operational characteristics on your particular 3d Chocolate Printing machine. Things like start up time, quality and resolution.

  2. at it’s melting point –> at its melting point

    I’m so tired of seeing this on HaD !
    Is it so darn difficult to learn and remember when to use IT’S and when to use ITS ?

    Yeah, I know !
    As usual there will be those idiots commenting that spelling doesn’t matter. That’s just an excuse because they’re (yes, not “their” or “there” !) either too stupid or too lazy to care about proper spelling.

    Know you’re sh*t !

    Sorry, meant to say :
    Know your sh*t !

  3. The rotation may delay the solidifying of the chocolate, so I would suggest (without reading the blog post, maybe they did it) to heat it up, spin like crazy and then cool down to a set temp (or time at temp) and then test the rheology. This will take much more time, so can be done after you have established a rough time/temp estimate with the rotating-while-cooling test. An industry (oil, not chocolate) standard for testing may apply where you stir your fluid at the temperature you want to measure, the stop the spinning for 10minutes (or around the time you notice it takes), then just move the cup slowly to see if you have created a gel-like structure.

  4. I’d be inclined to place a heated weight on top of the chocolate, and measure the temperature of the weight when it drops. Care would need to be taken to make sure the weight was the correct size and shape so it didn’t float, but I’m sure it wouldn’t be too difficult. Maybe a wide conical shape to disperse any melting chocolate, but care would need to be taken so the tip did not heat up at a different rate to the rest of the thermal mass. The rate of fall could even be measured if need be, to determine viscosity as well as initial softening and melting temperatures.

    To find the temperature at which it re-solidifies, repeatedly raise then lower the weight gently and measure the resistance to movement until it comes to a complete stop.

    I’d be inclined to place a heated weight on top of the chocolate, and measure the temperature of the weight when it drops. Care would need to be taken to make sure the weight was the correct size and shape so it didn’t float, but I’m sure it wouldn’t be too difficult. Maybe a conical shape to disperse any melting chocolate. The rate of fall could even be measured if need be.

    1. Another idea would be heating the chocolate in a container then measuring the temperature when it starts to drip through a hole in the bottom. You could either measure the drips from below, or place the chocolate under pressure via an extruder on top and measure when it starts moving.

      To find the temperature where it re-solidifies, pump the chocolate through pipes in a water bath, and measure the temperature when it stops flowing. The amount of pressure needed to pass the chocolate through the pipe could help establish the viscosity. The chocolate could even be passed through multiple pipes kept at different temperatures, to see how far it passed until it stopped flowing.

      Apologies for the accidental double-post above, I hit Ctrl-P twice on a copied paragraph then forgot to delete it.

  5. Chocolate is tricky stuff to get to solidify right. Part of it is melting the chocolate properly. Because if you screw that up you’re done before you even started. Then to get solid, tempered chocolate, that has to be done when the chocolate is at rest. I don’t think it will really happen while you are still stirring. Oh, and if you get water in your chocolate while it is melted, you’re done then too. Then the chocolate seizes up, and is ruined as a result.

  6. I was going to criticize them for not making use of the existing scientific methods for calculating melting point, but after thinking for a second I realized it’s not that simple.

    Normally, you do one of two approaches. In the first option, you put a tiny sample into a very small glass tube, and you watch the sample as the tube is heated within a device at known temperature. Since it’s so small, it’s at a fairly even temperature (no stirring required), and you just visually determine when it starts to turn into a liquid.

    In the second option, you put the sample into an oven and record two values: the temperature and the energy you’re putting into the oven (the oven is small and efficient). When you put in energy at a constant rate put you don’t get a temperature change (technically, the slope of the temperature change decreases), that’s the melting point.

    However, neither of these two approaches is designed to handle cooling, and it seems like it would be non-trivial to adapt them to work in the reverse direction. For example, visually determining when the sample becomes solid would be much harder than watching a crystal lose its shape.

    1. Let the molten chocolate cool on its own and plot its temperature over time. The resolidification point would be the part of the curve that has a flat line. The liquid phase change to solid and release energy there by maintaining the temperature.
      (latent heat of fusion)

      This is similar to having melting ice in a cup of water maintaining its temperature.

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