Monitoring and quantifying of oil migration in cocoa butter using a flatbed scanner and fluorescence light microscopy

Abstract

Oil migration from a nile red stained phase composed of a mixture of peanut oil and chemically interesterified hydrogenated palm oil into a cocoa butter matrix was quantified by the use of a flatbed scanner and epifluorescence light microscopy. A concentration gradient of linoleic acid (C18:2) in the cocoa butter phase confirmed that nile red migrated with the oil from the creamy mixture. Scanner and microscopic images were acquired every day during 7 days of storage at 20 °C. Scanner image processing and analysis included color space transformation and Gaussian smoothing which allowed for the measurement of the gray scale intensity variation due to oil migration. The kinetics of oil migration followed a one phase exponential association model described by: Y = Y max ∗ ( 1 - e - kt ) which could be related to capillary rise. The half-time of the process (about 1 day) illustrated the rapidity of oil migration within the first three days and equilibrium was reached after four days only. Quantification of the mass transferred at specific times and at equilibrium (saturation) illustrates that diffusion may also play a role in oil migration. In addition, fluorescence microscopy revealed that small and less stable crystals observed initially transformed into larger crystals after 7 days of storage. Moreover the presence of stain around the crystals suggested these may have grown and trapped oil within their structure. This study demonstrated that cocoa butter structural changes may have influenced oil migration, and that both diffusion and capillary rise mechanisms may be operational during oil migration through cocoa butter. This new imaging technique for visualizing structure and quantifying oil migration seems promising.