Quantifying microstructural changes in chocolate under shear, heating

Popular chocolate products such as fountains, spreads, and bars are neither elastic (solid-like) nor viscous (liquid-like). Instead, chocolate shows an intermediate behavior between solid and liquid called viscoelastic because its high solid content is dispersed in a continuous phase of cocoa butter with long fatty acid chains.

At high temperature, the continuous phase is in the liquid state and chocolate can flow easily. Upon sudden decrease in temperature, the fatty acid chains form crystalline domains.

On the other hand, application of shear under ambient conditions also induces fluidity in chocolate, wherein the crystalline domains get altered and oriented. When shear is stopped, chocolate regains its high viscosity.

The viscosity change after both cooling and stopping shear seems comparable, but shearing influences the quality of chocolate – how it feels in the mouth, its softness, and its texture – to a great extent. Both processes are used to commericially render chocolate.

Bhattacharyya and Joshi quantified the rheological properties of chocolate, such as dynamic moduli and relaxation time, after cooling and shearing to explore the difference in the final crystalline structure.

“We observe the rheological properties to be significantly different under each scenario, suggesting a substantial microstructural change owing to shearing,” said author Yogesh Joshi. “This work is inspired from the structural difference observed in polymer glasses under similar scenarios.”

“Shearing disrupts the crystalline domains and orients the fatty acid chains in the direction of flow leading to homogeneous crystal formation throughout the mass,” said author Tulika Bhattacharyya.

The researchers aim to explore different ratios of chocolate ingredients. Their goal is to connect the rheological properties to the mouthfeel to quantify the eating experience.

Source: “Effect of thermal and mechanical rejuvenation on rheological behavior of chocolate,” by Tulika Bhattacharyya and Yogesh Moreshwar Joshi, Physics of Fluids (2022). The article can be accessed at https://doi.org/10.1063/5.0083335 .

This paper is part of the Kitchen Flows Collection, learn more here .