Solid mould surface characteristics in relation to chocolate adhesion and the influence of processing conditions

Crystallization of cocoa butter in the correct polymorphic form (Form V) leads to a volumetric contraction of tempered chocolate during solidification and aids in the ease of the demoulding process. Specific steps during chocolate bar manufacturing may result in an increased adhesion between chocolate and mould surface causing intermittent problems in demoulding. Adhesion is an important physical phenomenon commonly observed in many food-related situations. With respect to chocolate adhesion it is expected that the balance of the adhesion force between the chocolate and the mould and the cohesion force within the chocolate itself determines the stickiness at the chocolate-mould interface during demoulding. The research presented investigated the effect of surface thermodynamics and processing conditions on the observed extent of adhesion of (aerated) chocolate to four different mould materials (quartz glass, stainless steel, polycarbonate, and polytetrafluoroethylene (PTFE)). Surface energy of solid mould materials was calculated from experimental surface tension and contact angle data. An experimental set-up build around a Texture Analyser was developed for the experimental surface adhesion force determination, using a simple separation test between the solidified chocolate and a mould probe. Process conditions specific to the moulding and demoulding phases of the commercial chocolate manufacturing process have been investigated using this set-up. Surface energy (thermodynamics) has been shown to be the major factor controlling the adhesion between chocolate and a mould material. Chocolate-mould adhesion can be minimized if the total surface energy of the mould material is < 30 mN m-l ,and the electron donor component ~15 mN m- l. High surface energy materials are assumed to produce more compact crystal networks with a resulting increase in crystal-crystal interactions being responsible for difficulties demoulding. Processing parameters had a significant impact on the crystallization and solidification processes, and are therefore regarded as the key determining factors of chocolate-mould interactions. Demoulding can be optimised by pre-heating the mould under controlled environmental conditions (% RH. 25-30 ℃) and by applying a cooling temperature of 10-15 °C. Significant differences were observed between standard and aerated chocolate systems. It was shown that aeration lowers the cohesive or mechanical strength of the chocolate sample. consequently reducing the surface adhesion. Possible mechanisms proposed to impact surface adhesion of aerated chocolate, are the heat transfer coefficient of the mould material and the presence of water vapour at the mould surface which interacts with the CO2 gas used for the chocolate aeration. Edible coatings can reduce the surface adhesion, but often have negative effects on chocolate surface characteristics. Further optimisation is required before edible coatings can be applied as a surface modification technique.