Fat Migration in Chocolate: Diffusion or Capillary Flow in a Particulate Solid?—A Hypothesis Paper

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Abstract
The exact mechanism of fat and oil migration in chocolate and chocolate coatings is still unknown. Nevertheless, the so‐called “diffusion equation” derived from Fick's 2nd law has been extensively used to model the phenomenon, giving the impression that molecular diffusion is the single transport mechanism. We propose that chocolate may be microstructurally regarded as a particulate medium formed by an assembly of fat‐coated particles (for example, cocoa solids, sugars crystals, and milk powder). Within this matrix the liquid fraction of cocoa fat (which increases with temperature) is likely to move under capillary forces through interparticle passages and connected pores. Based on available evidence (microstructure, kinetic data, temperature dependence of liquid fat fraction, and so on) we demonstrate that capillary forces may have an important role to play in bulk flow of liquid fat and oils. The Lucas‐Washburn equation for capillary rise fits available data under most reported experimental conditions. Detailed microstructural analysis in actual products as well as data on key parameters (surface tension, contact angle, viscosity) is necessary to confirm this hypothesis. Bulk flow due to capillary effects, highly disregarded in structured foods, should be considered as a mass transfer mechanism in liquid‐filled porous or particulate foods.

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