Engine oil and chocolate

The understanding of complex flow behaviour, such as that of oils within car engines, is now being applied in the UK to ensure quality control in the production of chocolate and other confectionery.

The common factor in these substances is that they are both complex fluids. When these complex fluids experience rapid changes in pressures, they can behave in a similar fashion.

The UK team has made progress in providing manufacturers with a fresh way of characterising the flow properties of complex fluids such as chocolate. Because of its highly complex nature, conventional approaches to this problem have often proved unsuccessful.

The team have developed an instrument called the pulse-resonance rheometer which incorporates a number of well-known principles in a new way. Not only was it successful with chocolate, it also provides an important part of gel-making and jelly-making in the world of confectionery.

This research stemmed from an interest in the rheological properties of complex fluids, such as lubricants found in car engines. Focusing on properties such as viscosity, the team found that if lubricants were subjected to extremes of stress in the bearings, they could be subjected to negative pressure. As a result, the liquid could become sufficiently stretched to allow bubbles to form thus causing the oil to fail in its lubricating properties. In response to this, researchers developed techniques involving the propagation of pressure waves in liquids that allowed the monitoring of tensile strength.

A variation of these techniques was applied to the flow of chocolate or confectionery along a production line. It involved changing the approach from using high-frequency wave propagation in the low ultrasonic, to an audio frequency range.

At the point of transition from liquid to solid, the material has a combination of rheological properties in that it carries some of the liquid state and some of the solid state. Because of its complexity, it can be difficult to measure the viscosity of chocolate reliably and this is where the pulse-resonance rheometer is useful.

The practical benefit of getting this right is the creation of a rheological 'fingerprint' of the product. If tasters using their ultra-sensitive mouths find a difference in two batches of chocolate, instruments such as the wave-propagation techniques will allow users to track down the faulty component in the production line.