Christmas concept: Smiling Santa Claus made of chocolate on white snow. Mountains in the back.

Peeling back the foil on a gold coin. Popping a perforated window on a festive calendar. Pursuing your favourite coloured wrapper around a bowl. The prize inside many little packages we open this season will be chocolate.

The secret of chocolate’s almost universal appeal is in the mouthfeel the “snap” of the first bite, how the chocolate melts and how it coats the inside of the mouth. Despite being a consumer experience, mouthfeel is strongly related to materials science – chocolate being a complex composite material.

Our instruments hold the key to many of these secrets by providing an understanding of particle size and rheological properties.

Particle Size

The textural component is critical: as consumers we prefer a smooth chocolate to a “gritty” one, and we tend to assume a smooth chocolate is a more luxurious product. Extensive consumer testing by chocolate manufacturers over decades, has established that particles of cocoa solids, sugar and milk are detected as a gritty mouthfeel at sizes over 30µm. However, the particle grinding processes in chocolate manufacture are expensive, lengthy and energy intensive so large scale manufacturers try to optimise their processes to achieve the required particle size as efficiently as possible. This optimisation is underpinned by regular particle size measurements, which are increasingly performed by laser diffraction instruments.
The Mastersizer 3000 Chocosizer kit provides for fast, reliable chocolate particle size analysis as part of routine process and quality control. It includes all the components required for chocolate sample preparation and measurement based on the Mastersizer Hydro MV or Hydro SM dispersion units.


While particles affect grittiness, the flow properties of the fat phase (cocoa butter, which may be mixed with other fats) control how the chocolate coats the mouth and influences the perception of flavour. The flow, or rheological, properties of the chocolate also have significant impact on the chocolate manufacturing process. Reducing the particle size increases viscosity, potentially causing blockages as the liquid chocolate is piped through the factory. The final product may be a bar, or tablet, of solid chocolate, or the chocolate may be used in an enrobing process to surround a filling centre. Chocolate for enrobing processes is often optimised to achieve good coverage and may have a different recipe than chocolate for tablets.

The most fundamental rheological measurement made on  chocolate is a viscosity measurement, although deeper insights can be gained by measuring other rheological parameters such as yield stress and viscoelasticity. These measurements can be readily made with a rotational rheometer such as Kinexus. The elastic and viscous moduli relate to the microstructural characteristics of the chocolate and can be used to probe component interactions and melting characteristics. The yield stress is related to how the chocolate will coat the moulds and how well the chocolate will cling to the walls of the mould or slump before it sets. Extending conventional rotational rheometry by employing the axial capabilities of the Kinexus also enables mastication, or the action of chewing, to be simulated. From this test, the hardness, yield stress, coating of the roof of the mouth and stickiness of the chocolate can be assessed.

The texture, or mouthfeel, of chocolate is critical for the consumer perception of product quality and being able to correlate objective analytical measurements with expensive and subjective sensory testing is the ultimate goal. Although it is difficult to fully replicate the complex processes occurring in the mouth,  it is at least possible to use analytical measurements to optimize key textural aspects – laser diffraction for optimising particle size and rheometry for optimising flow characteristics. It is even possible to use a rheometer to simulate mastication and help predict or control the structural changes in the chocolate as it is consumed.

To discover more about the technology behind your festive chocolate we recommend the following: