Shake, rattle and Rolos
15 Jan 2000
Of all the fluids handled in the process industries, chocolate is one of the most awkward. A complex suspension of sugar, cocoa and milk granules in liquid cocoa butter fat, it doesn't behave like a standard Newtonian fluid. But researchers from Nestle in York and Birmingham University's school of chemical engineering might help to give chocolate processing a boost.
Molten chocolate is often vibrated when it is moulded, the team explains, which helps to drive away air bubbles and settle the chocolate in the mould. However, unlike other non-Newtonian fluids like concrete and cement, very little research has been done on the flow properties of chocolate, so what actually happens when the fluid is vibrated is not known.
Better understanding of this topic would help confectioners achieve better quality products, and control the weight of chocolate bars more accurately.
The team set up a test rig consisting of a cup with a rotating cylinder inside it, set on a vibrating table. The space betwixt the cylinder and the inner wall of the cup was filled with chocolate, with the rotation of the cylinder providing a measurement of the chocolate's viscosity. The team varied the frequency and amplitude of the table's vibration, as well as the speed that the cylinder spun, to give a full picture of the effects of vibration on viscosity. They also experimented with golden syrup and cocoa butter, which are Newtonian fluids.
The results showed that the Newtonian fluids were not affected by vibration, but the chocolate was deeply shaken. The yield value the force needed to make the chocolate flow dropped dramatically when it was shaken, and fell steadily as the amplitude of the vibrations increased. Viscosity fell as vibration frequency increased the viscosity of milk chocolate fell by as much as 80%, while dark chocolate's fell by 92%.
The researchers think that chocolate's structure is the cause of this behaviour. At rest, the solid particles clump together, giving the molten liquid high resistance to stress and viscosity. Vibration breaks up these clumps, making the liquid runny. However, they stress, the situation is still complex; a further marathon of experiments is needed, including visualisation of flow under vibration, before chocolate manufacturers can reap the bounty of the research.
