Structural basis for the rheology of molten chocolate: a multi-technique approach

Abstract

Chocolate comprises a dense suspension of solids, mainly sucrose with cocoa and milk solids, in a continuous fat phase of cocoa butter stabilised by surfactants, namely lecithin (mostly phospholipid), and sometimes polyglycerol polyricinoleate (PGPR). The surfactants favourably affect the rheology of molten chocolate reducing energy cost during production. Lecithin reduces viscosity, however, undesirably increases the yield stress at high concentrations, whereas, PGPR is primarily used to reduce the yield stress. When combined, the viscosity and yield stress are reduced further than with either surfactant individually. Whilst this implies a modified inter-particle interaction, the molecular mechanism by which these modifications occur were previously poorly understood. This work provides a mechanism on the basis of molecular scale structural information for this co-operative rheological effect, opening up the potential to rationally select or design alternative surfactants and improve manufacturing of chocolate at a lowered fat content.

Small Angle Neutron (SANS) and X-rays (SAXS) Scattering show that lecithin and PGPR form micellar structures in triglyceride oil. Lecithin forms extended inverse cylindrical micelles that exhibit lamellar arrangements at high concentration or upon ageing. The addition of PGPR disrupts these ordered structures and decreases the aspect ratio of the cylindrical micelles. The adsorption of these micelles at the solid/fat interface leads to a modification of the inter-particle interactions which in turn changes the rheology.

The solid/fat interface in chocolate has been investigated using two complimentary model systems: a dense suspension of sucrose in triglyceride oil stabilised by lecithin and PGPR and an analogous model system based on an extended planar sucrose film. The model suspensions, comprising 65% w/w of sucrose in Glyceryl Triocotanoate (GTO) and Glyceryl Trioleate (TO) with 0.8% w/w total surfactant, exhibit similar rheology to molten chocolate and have been used to characterize the adsorption and structure of the interfacial surfactant films using SANS and SAXS. Varying the PGPR content whilst keeping the total surfactant amount constant shows that the adsorbed surfactant interfacial film thickness increases with increasing PGPR fraction. In the lecithin rich suspensions the sucrose grains are in contact decorated by lecithin, giving rise to a fractal interface. On increasing the PGPR fraction the particles are pushed out of contact, resulting in a smooth particle interface. Spin Echo SANS studies show that sucrose-sucrose correlation length also increases, consistent with the picture in which the sucrose particles are pushed further apart for the surfactant compositions containing more PGPR.

A planar model system comprising a sucrose film spin-coated onto a silicon/silicon oxide substrate mounted into a flow cell has been used for Quartz Crystal Microbalance with Dissipation Monitoring (QCM-D) and X-ray and Neutron Reflectivity (XRR and NR, respectively) studies. Triglyceride oil containing lecithin and PGPR and their binary mixtures in the same molar concentration as chocolate has been flowed across the planar sucrose surface and the adsorption of the surfactants characterised using QCM-D. The adsorbed amount, found using the frequency shift, is comparable to the surfactant amount adsorbed in model suspensions found using small angle scattering. The dissipation shift increases with increasing PGPR fraction of the surfactant mixture, showing that the layers become more extended and diffuse. Structural details, investigated using NR and XRR, show that lecithin forms a compact phospholipid multilayer (5-7 monolayers) extending ~10 nm and PGPR forms a solvated polymer layer of ~30 nm. In binary mixtures the PGPR intercalates into the lecithin and the whole structure swells to ~50 nm. Using these calculated thicknesses, the viscoelastic properties of the adsorbed surfactant films have been modelled by co-fitting the frequency and dissipation shift observed in QCM-D to the Voigt model. The interfacial surfactant structure and the viscoelastic properties of the interfacial films are then used to explain the role of different surfactants in the rheology of molten chocolate suspensions.

Based on this work we propose that lecithin layers reduce friction between sucrose grains, reducing the high-shear viscosity due to their high load bearing capacity whilst maintaining the fluid characteristic of the interfacial layer. The compact fractal interfacial layer implies that the sucrose grains are still sufficiently close in the suspension for Van der Waals adhesion to maintain a yield stress. PGPR incorporates into the lecithin layer, swelling it. This osmotically pushes the grains apart, decreasing Van der Waals interactions and reducing the yield stress giving the desired liquid-like flow.