It’s not just because of the exquisite taste of cocoa. Researchers at the University of Leeds (UK) have found that the ingredients in the chocolate itself interact with saliva, creating a smooth, greasy film that coats the tongue and makes the product feel creamy for as long as it remains in the mouth.

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Chocolate irresistible.

Scientists from the School of Food and Nutritional Sciences at the University of Leeds, UK, have discovered why chocolate is so irresistible, and that it is not just down to the taste, but has to do with the process in which it changes in the mouth from a solid to a smooth emulsion due to its own ingredients and the combination with saliva.

The study, whose findings are published this week in the journal ACS Applied Materials and Interface, takes an in-depth look at the physical process that takes place in the mouth when eating a piece of chocolate and the pleasure produced by its touch and texture. In particular, the lubrication behaviour of this phase change material (PCM).

We find chocolate irresistible not only because of its taste but also because of the process by which it changes in the mouth from a solid to a smooth emulsion, due to its own ingredients and the combination with saliva.

Fat plays a key role at the moment when a piece of chocolate comes into contact with the tongue, after which the solid cocoa particles are released and become important in terms of tactile sensation.

Research with an artificial tongue
The tests were conducted, using a luxury brand of dark chocolate, on a surface similar to an artificial tongue. The researchers used analytical techniques from a field of engineering called “tribology”, which studies the friction, wear and lubrication that occurs during contact between moving solid surfaces.

In this case, they tested this interaction between the ingredients in the chocolate itself and saliva. In the taste buds, “the determining mechanisms were the formation of cocoa butter bridges between the cocoa butter particles and the fatty material of the emulsion drops,” according to the study.

The authors also found that the product, on contact with the tongue, releases a fatty film that coats the tongue and other surfaces of the mouth. It is this film that makes the product feel soft the whole time it is in the mouth.

“The unprecedented results of this study, supported by landmark lubrication theories, reveal how the tribological mechanism of licking switched from solid grease-dominated lubrication (saliva-poor regime) to aqueous lubrication (saliva-dominated regime), which resulted in an increase in the coefficient of friction,” the authors note in their study, which also highlights “the formation of the distinctive hydrodynamic viscous film” on the tongue.

Removing fat but not taste
The researchers also point out that the deeper fatty material within chocolate plays a rather limited role, so it could be reduced without having an impact on the sensation of pleasure produced by consumption.

The team believes this work may help develop a new generation of chocolates that have the same sensation and texture but are healthier to eat.

This work may help develop a new generation of chocolates that have the same sensation and texture but are healthier to eat.

In addition, the physical techniques used could be applied to investigate other foods that undergo a phase change, where a substance transforms from solid to liquid, such as ice cream, margarine or cheese.

The project in which this work was carried out received funding from the European Research Council under the European Union’s Horizon 2020 research and innovation programme.