Using USAXS to predict the under-tempered chocolate microstructure

Abstract

Chocolate is a manufactured product enjoyed worldwide. Over the years, manufacturers have learned how to appeal to humans using this rich-fat food that arouses all the senses. Good quality chocolate is recognized by its smoothness, a slow melt in the mouth, and a snap when bitten, and described as well-tempered. This work compares dark chocolate samples manufactured to obtain under- and well-tempered chocolate, where under-tempered does not show all the physical properties desired by consumers. The microstructure was studied using the ultra small angle X-ray scattering (USAXS) technique, complemented by small and wide angle X-ray scattering to identify the polymorphs. It was observed that under- and well-tempered chocolates exhibited differences in the q-region ~ 2 × 10−5 Å−1 < q < ~1.5 × 10−4 Å−1, which correspond to spatial length scales from 32 µm to 3.2 µm. The differences are manifested in the value of the mass fractal dimension, D, obtained when the USAXS data were fitted using the Unified Fit model (Irena software). The characteristic length scale at which these differences were observed falls in length scales detected by humans in the oral cavity. This work proposes that a D = 2.1 characterizes an under-tempered 70% dark chocolate while a D = 2.3 characterizes a well-tempered 70% dark chocolate. This work also presents a simple model that describes the disintegration of those aggregates formed by the basic scatter units for under- and well-tempered chocolate. The model proposes that aggregates formed in under-tempered chocolate persist after the bulk chocolate has melted, which can be perceived as grittiness. However, the model proposes that the aggregates for well-tempered chocolate melt at the same or lower temperatures than the bulk chocolate melting temperature; hence no grittiness is perceived. The model is supported by the observation that the heat of transition for the under-tempered chocolate is smaller than that of the well-tempered case.