Halloween is just around the corner and its spooky costumes, pumpkins, and sweet-filled celebrations are all deeply rooted in history. The tradition of “souling” emerged during the medieval age, where people would go door-to-door in exchange for soul cakes, which they believed would help the souls of the deceased trapped in Purgatory to reach heaven.
In the 20th century, Halloween was primarily celebrated in North America, and the holiday’s customs began to evolve. Trick-or-treating is the evolution of “souling”, and it gained popularity during the 1920s and 1930s. Initially, the treats handed out during Halloween were homemade, including cookies, popcorn balls, and apples. However, as the confectionary industry expanded, pre-packaged sweets – especially chocolates – made their way into Halloween celebrations.
Over the years, chocolate has become synonymous with Halloween, enhancing the holiday experience for children and adults alike. While it might not seem obvious, tribology has played an incredibly important role in making sure each chocolate is as delicious and satisfying as possible!
Chocolate, tribology and rheology
Deeper into the world of chocolate, we discover a curious connection between this delicious treat and tribology, particularly how it interacts with our senses during consumption. Tribology intersects with chocolate when we explore its rheological properties and the complex interactions that happen inside our mouths.
Rheology is the branch of science concerned with the flow and deformation of materials, which plays an important role in understanding the behaviour of chocolate. The way chocolate melts, spreads, and engages with our taste buds hinges on its rheological properties. When a piece of chocolate touches your tongue, it starts a melting process, and central to this tribological phenomenon is the friction that occurs between chocolate and the surfaces of our mouths!
Tribology testing and chocolate
To analyse this friction, scientists use advanced instruments like the PCS Instruments HFRR and MTM. These instruments can be used to gauge the friction experienced by various chocolate samples as they make contact with the tongue, and soft/hard palates (the roof of your mouth).
The resulting friction curves typically exhibit a distinctive pattern. There is a rapid surge in friction as the high-viscosity chocolate melt is sheared in the contact region, followed by a decline in friction as the chocolate film disintegrates. Notably, this pattern varies with the cocoa content of the chocolate samples, especially in the case of very high (85%) and very low (Candy_EU~5%) cocoa content samples, which maintain constant friction traces over the test duration. 
The graph below is a perfect example of this characteristic ‘chocolate friction response,’ which clearly shows the close relationship between the final friction coefficient, and cocoa solids content. This leads us to an interesting question: is it possible to draw a connection between this tribological behaviour and the consumer preference for milk chocolate over dark chocolate?
The significance of such test results lies in their contribution to our understanding of the relationship between oral processing, mouth-feel, consumer satisfaction, and the popularity of a product. Further advancements in industry test methods are crucial for building our ability to simulate and understand the complex materials and interactions involved in tongue-palate contact, including the impact of saliva.
Want to know more about how food and tribology are closely linked? Find out what Stefan Baier, former associate R&D Fellow at PepsiCo had to say on the topic, in our 60 seconds in the spotlight interview.
  M.E Malone, I.A.M Appelqvist, I.T Norton, 2003 Oral behaviour of food hydrocolloids and emulsions. Part 1. Lubrication and deposition considerations, Food Hydrocolloids Vol 17, Issue 6