Tribology of Chocolates

Credit: This article is the summary of ‘Insights into the Multiscale Lubrication Mechanism of Edible Phase Change Materials’ by Siavash Soltanahmadi, Michael Bryant, and Anwesha Sarkar.

Introduction

When chewing, there is often a shift from one stage to another in the dynamic interaction between the food being chewed and the surfaces in the mouth, moving from the initial stage of licking to a stage where saliva is mixed in. This change takes place at various scales, including the micro level (cellular), meso level (papillae), and macro level. However, the relationships and lubrication performance across these different stages and scales are not well understood because there are no testing methods that accurately replicate the conditions of human oral tissues. The results of this study were ground-breaking and broke away from traditional lubrication theories. It showed that as food is being chewed, the lubrication mechanism changes from being dominated by solid fats (when there is a lack of saliva) to being dominated by aqueous lubrication (when there is a high amount of saliva), which led to a three-fold increase in friction. At the mesoscale, the mechanisms involved the binding of cocoa butter between confined cocoa particles and the merging of emulsion droplets in the melted and saliva-mixed stages, respectively.

Fig- 1 Chocolate tribology shown in the paper[1]

Methodology

In this study, they have used a newly developed 3D biomimetic tongue-like surface that resembles the properties of a real human tongue to investigate the lubrication mechanisms of dark chocolate at both the tongue-scale and single-papilla-scale during different stages of oral processing, including licking and saliva-mixed stages. This was the first study to examine the solid lubrication of chocolate before its phase transition and the results of this study were obtained through in situ tribomicroscopy. In this research, they have visual evidence of cocoa particles being trapped at the point of contact. In traditional tribology, solid particles can become trapped between two surfaces during friction, but they often cause abrasive wear through indentation. Previous studies have shown that chocolate particles can indent soft plastic surfaces, like polytetrafluoroethylene, and affect friction, but it is still unclear if they can have the same effect on elastic materials or the human tongue.

Fig-2 Tribological setup used in this study[1].

Conclusions

In this study, they have presented the first comprehensive examination of the multiscale lubrication mechanism of chocolate, which is an edible phase change material containing solid particles. This was the first study to support relevant theoretical considerations when the material undergoes a phase change and mixes with saliva during different stages of oral processing. Also, this study provides the closest approximation to the real tongue-palate interaction compared to previous works. They have found that at the tongue scale, the fat content of dark chocolate had the greatest impact on lubrication behaviour across different stages of oral processing. At the single-papilla scale, the solid lubricity of chocolate was linked to its melted-state lubricity, but this relationship was disrupted when saliva was present.

They have used techniques such as tribomicroscopy, to observe that the cocoa particles were confined at the contact interface for melted chocolate, where classical lubrication theories cannot fully explain this tribological behaviour. The entrainment of solid particles into the contact can increase friction, provided that hydrodynamic forces are strong enough to support particle entrainment. The multiscale characterization techniques developed in this study can be used as a reference for understanding the tribological behaviour of phase change materials when they undergo transformation.

Reference

[1] Soltanahmadi, S., Bryant, M. and Sarkar, A., 2022. Insights into the Multiscale Lubrication Mechanism of Edible Phase Change Materials. ACS Applied Materials & Interfaces.