What makes the feeling of eating chocolate so irresistible to most people?

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Scientists have decoded the physical process that takes place in the mouth when a piece of chocolate is eaten, as it changes from a solid into a smooth emulsion that many people find totally irresistible.

The study – published in the scientific journal ACS Applied Materials and Interface – did not investigate the question of how chocolate tastes. Instead, the investigation focused on its feel and texture.

Original Research: Open access.
Insights into the multiscale lubrication mechanism of edible phase change materials” by Anwesha Sarkar et al. ACS Applied Materials & Interfaces

Phase change materials (PCM) are attractive classes of materials with broad applications ranging from the energy sector (thermal management, battery applications, etc.) and photonic to the neuroinspired computing where often a change between states of matter involves a transition between amorphous and crystalline phases. (1)

The phase transition can occur when the source of heat to propel the phase change is tribological stresses or when a PCM is exposed to environmental factors in an application. Examples of the former and latter are skiing blades transforming snow to water and oral processing that transforms solid macroscopic food structures to molten mixtures of the food with the saliva (i.e., a nature-engineered biolubricant). (2)

In the case of skiing, the kinetics of phase transition due to the tribological contact determines the right surface roughness on ski blades (3) and the thickness of a film of the molten ice determines the extent of frictional forces. (4) In ingested PCM systems, where phase changes are abundant owing to processing at industrial scale (i.e., manufacturing of a chocolate) as well as the biological exposure upon consumption (e.g., temperature, salivary enzymes, etc.), mechanistic studies on the multiscale lubrication behavior of edible PCM are currently limited.

Chocolate is a classic edible PCM, which mainly consists of suspended particles (cocoa solid and sugar crystals) in crystalline cocoa butter. The oral perception of chocolates starts with either biting or licking. The biting often correlates with the bulk properties of chocolates and involves tooth–chocolate contacts, (5) largely falling into the area of fracture mechanics and is relatively well-studied. (6)

The concentration, size distribution, and shape of solid particles of chocolates along with the type and concentration of emulsifier (e.g., soy lecithin and polyglycerol polyricinoleate) influence the bulk properties, flow characteristics, and lubrication behavior of chocolates in mouth, impacting the gustation (i.e., the primary taste perception). (5,7−9)

Thinking of the stages of oral processing, the licking starts with a direct contact between the tongue and a solid chocolate followed by a gradual phase transition of the chocolate from a crystalline solid to a continuous molten fat phase containing suspended particles of cocoa and sugar.

The molten chocolate is eventually mixed with the biological fluid, i.e., saliva, (5) which gradually dissolves the sugar crystals. (10) The licking process, i.e., the solid-lubrication behavior of chocolates, remains principally unexplored.

The later stages of chocolate in mouth (i.e., molten chocolate and saliva-mixed) have been studied through sensory trials and rheological and tribological methods. Tribology has been proven to be an enabling field of study where other conventional methods (i.e., rheology, etc.) have failed to provide understanding of the tactile sensation of food systems from emulsions to semisolid foods, and deciphering food–saliva interactions. (11−14)

Consequently, oral tribology has contributed to informed design of healthy foods as well as tailored food for vulnerable populations. (11,15−17) In addition to flavor-induced retronasal characteristics of chocolates, the temporal profile of chocolates in a myriad of food–mouth interactions correlates to the frictional behavior of chocolates, which are translated to sensory attributes such as smoothness, coarseness, grittiness, etc. (11−13,15,16)

Attempts have been made to understand often unpleasant sensory attributes of dark chocolates (e.g., grittiness, pasty, mouth-coating) through tribological studies of molten chocolates and their mixtures with saliva. (8,9,18−20) These studies have provided invaluable information on the mouth-feel of chocolates, albeit using materials (e.g., smooth polydimethylsiloxane surfaces) or conditions that are far from real biophysical characteristics of tongue–palate contact. As recently evidenced with edible polymers, (21−23) the distinctive micropapillated architecture of real human tongue may have pivotal tribological consequences, which is poorly understood to date in PCM. Of more importance, the intricate nature of chocolates (as a PCM) and their interactions with saliva across solid to saliva-mixed stages have not been explored from a multiscale perspective to date.

With our recently fabricated three-dimensional (3D) biomimetic tongue-like surface, (21) which emulates the topography, deformability, and wettability of a real human tongue surface, herein, we took a multistage approach representing licking to saliva-mixed stages to decipher the lubrication mechanisms of dark chocolates at a tongue-scale as well as at a single-papilla-scale in orally relevant contact conditions.

For the first time, we investigated solid lubrication behavior of chocolates (i.e., before its phase transition) and exploited an in situ tribomicroscopy to provide insights into different stages of oral processing of chocolates, supported by theoretical considerations. In this study, we demonstrate that the classical lubrication theories have failed to fully explain the complex tribological behavior of chocolates.

The scale-dependent interplays of solid lubricity, aqueous lubrication, hydrodynamic forces, and particle entrainment as a function of the stage of processing and speed are discussed. This study offers a novel pathway to design edible PCM such as the one with a gradient design to contain a higher degree of cocoa butter at the chocolate interface, showing a promising prospect to produce low-calorie dark chocolates with pleasant mouth-feel.

Therefore, the fundamental multiscale insights provided by this work can facilitate engineering metamaterials, which undergo a phase transition when subjected to tribological stresses and application-specific factors (e.g., saliva in this study).

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