I have always found it fascinating that in the US dark chocolate is often referred to as hard chocolate, because milk chocolate is actually much harder than hard chocolate. It seems softer because it has a lower melting point, just below body temperature, which gives it a softer mouthfeel.

Using rheology to characterize chocolate

The tools of rheology  provide an excellent way of analyzing and characterizing the fundamental material properties of chocolate relating to its manufacture, storage and (most importantly of all in my opinion) eating experience. Smoothness, viscosity, and cohesiveness are all qualities affecting chocolate that can be easily characterized using a rotational rheometer, such as our Kinexus system.

Viscosity, the measure of a fluid’s resistance to flow, is important, because, as a single property, it influences both chocolate manufacture and final eating experience. For example, by measuring chocolate across a range of shear rates, it is possible to work towards the high viscosity at low shear that  results in  optimal ‘stand up’ – an industry term for chocolate’s ability to hold a shape without slumping during hardening. This makes it ideal for forming all of those wonderful shapes seen in the best chocolate boxes.

Obviously at this point we’ve all got the image of molten chocolate lodged in our minds because melting chocolate is the most usual way in which we shape it, However, take a look at this lovely piece of research from the University of Cambridge

who have demonstrated that you can actually cold extrude flexible chocolate at room temperature into a cord that can be knotted, or into a sheet that can be folded. Imagine the possibilities that this can open up for chocolate processing and forming…….as well as gaining an understanding of the material properties and processing parameters that opens up this particular rheological window of course!

Particle size affects viscosity….and your enjoyment of chocolate!

In conventional processing the size of particles within the chocolate is one of the parameters used to control chocolate properties, including  ‘stand up’, and is modified at three stages of preparation:

  • Blending – mixing of cocoa solids, cocoa butter, fat, sugar, and (optionally) milk with any additional emulsifiers.
  • Conching – grinding of the blended liquid chocolate to improve smoothness (and which, in the process, reduces particle size).
  • Tempering – the fine temperature control used to ensure that cocoa butter crystals in the chocolate are uniformly small, giving the final product a fine sheen and crisp bite.

A finer particle size at the end of these processes  results in low shear viscosity and finer, smoother chocolate (milk or dark) for us all to enjoy!