Open Access
Ghosh, Vikramaditya
Graduate Program:
Food Science
Doctor of Philosophy
Document Type:
Date of Defense:
May 30, 2003
Committee Members:
  • Ramaswamy C Anantheswaran, Committee Chair
  • Gregory Ray Ziegler, Committee Chair
  • John Neil Coupland, Committee Member
  • John L Duda, Committee Member
  • John D Floros, Committee Member
  • diffusion
  • heterogeneous media
  • mass transfer
  • water vapor permeability
The mechanism of moisture migration through chocolate-flavored coatings was investigated in this study. An approach, developed by Weisz (1967) to understand diffusion in heterogeneous materials, was used to elucidate the mechanism of moisture migration through chocolate-flavored coatings. In chocolate-flavored coatings, the sucrose and cocoa powder particles are embedded in the continuous fat phase. It was hypothesized that the unsteady-state diffusivity of moisture through the coatings could be estimated from the diffusivity of water through the continuous fat phase and the partition coefficient of moisture between the hydrophilic particles and the fat phase. Therefore, to predict moisture diffusivity through a chocolate-flavored coating, the moisture sorption isotherms for all the constituents of the coating and the diffusion coefficient through the fat phase are required. The moisture adsorption isotherms for sugar, cocoa powder, coconut oil, and coconut oil + 0.5% lecithin were obtained by equilibration over saturated salt solutions. It was found that up to an aw of 0.85, cocoa powder adsorbed more moisture than any other component. The presence of lecithin increased the moisture adsorption capacity of oil. The moisture adsorption isotherm for a coating made with 70% coconut oil + 0.5% lecithin and 30% sugar was also determined. The equilibrium moisture content, for this coating, at each water activity level was higher than the individual constituents, i.e., sugar or coconut oil + 0.5% lecithin. One possible reason for this observation is that there is a layer of moisture present between the surface of the sugar particles and the polar regions of lecithin. The diffusion coefficient, of moisture diffusing through fat, was obtained by time-lag experiments. The predicted diffusion coefficient through a coating containing fat and varying amounts of sucrose and cocoa powder were determined. Experiments were conducted with coatings containing different amounts of sucrose and cocoa powder to validate the model. For coatings containing just coconut oil and cocoa powder, the diffusion of moisture in the dispersed phase, i.e. cocoa powder, occurred through the cocoa powder particle. When lecithin was added, the water molecules diffused through the cocoa powder particles as well as along their surfaces. When coatings contained sucrose and lecithin and the water activity was 0.85, the sucrose molecules dissolved in the migrating moisture and were transported to surface, where upon evaporation of the water sucrose crystals were deposited. With diffusion of moisture there were structural changes that altered the diffusivity of moisture through the coating. The structural changes occurred because the sucrose particles dissolved in the migrating moisture and cocoa powder particles swelled in the presence of moisture. The effect of sucrose, cocoa powder, emulsifier, fat type, and storage environment on the water vapor permeability (WVP) of a chocolate-flavored coating was also studied. In addition, optical microscope images of cocoa powder and SEM images of the structure of two different coatings were obtained. A coating containing more than 20% cocoa powder (w/w) significantly increased the WVP; a coating containing 60% sucrose significantly decreased the WVP. There was an increase in the WVP with an increase in the absolute value of the relative humidity across the film. The increase in WVP at higher absolute humidity was caused due to structural changes in the coatings. The WVP of fat based films decreased with an increase in the solid fat content (SFC)