TEMPERING-INDUCED NANOSTRUCTURE CHANGES IN COMMERCIAL DARK CHOCOLATE AS OBSERVED BY DSC, LV-SEM AND AFM

In chocolate industry, tempering is known to be a critical step for manufacturing final products with well defined organoleptic properties such as dark color, brightness, snaps, shelf-life properties and good  melting properties (in mouth but not in hands). This procedure is mainly based on time-temperature cycles allowing controlled cocoa butter phase transformations [1] from liquid phase to metastable phases I (mp around 16 to 18 °C) and II (mp around 22 to 24°C) which transform slowly into phases III (mp around 24 to 26°C) and IV (mp around 26 to 28°C) and then into the form V, a more stable polymorph (mp 32-34°C) in well-tempering optimal conditions. Under unfavourable storage conditions, such as temperature cycling, fat blooming is developed at chocolate surface [2].

We used complementary experimental approaches for monitoring changes in calorimetric behaviour and optical properties of a commercially available dark chocolate (85% cocoa) which was submitted to different tempering-cycles depending on different  holding times at different temperatures between 55°C (lipid crystal melting), 22 °C (room temperature) and  4°C (refrigerator storage temperature)

Data obtained by differential scanning calorimetry (DSC), low vacuum scanning electron microscopy (Low Vacuum-SEM) and atomic force microscopy (AFM), provided quantitative parameters (temperature and heat of melting of different crystal polymorphic forms) and corresponding qualitative information (crystal morphology and  surface roughness) on fat blooming as related to nano-scale topographic evolution  and bloom development under temperature-cycling