Accurate Evaluation of the Flow Properties of Molten Chocolate: Circumventing Artefacts

Abstract

Methods to characterize the flow behavior of molten chocolate have been set by International Confectionery Association (ICA) in 2000, however, there is no consensus on an accurate method and it is often followed by misinterpretation of the results. This issue is also influenced by flow phenomena generally recurring in yield-stress materials, such as transient effects, gap size effects, apparent wall slip and edge fracture. The state of the art in the field of rheology allow for more precise techniques to circumvent such rheometric artefacts. Nevertheless, a combined approach to accurately determine the flow properties of different types of molten chocolate targeting the prevention of all possible flow artefacts is still not found in the literature. In this paper, two technologically distinct molten dark chocolates are employed on different combinations of flow curve protocols, measuring geometries and viscosity functions to accurately determine their flow properties. Based on the evaluation, a wider shear rate range (𝛾̇= 0.01 -100 s -1 ) with sufficient measurement time per shear rate (> 120s) is recommended for an accurate flow curve protocol, combined with a roughened surface geometry with radii ratio < 1.10. Four-parameter mathematical models, such as the Windhab model or a modified Herschel-Bulkley model, have been proven to be more accurate in further acquiring important flow parameters, but special attention should be given to the physical meaning of some parameters. A comparison between experimental flow curve data, model-fitted data and steady-state data gained from a constant shear rate test confirmed the suitability of these recommendations for both chocolates. The proposed alternative method allows for accurate measurements at lower shear rates, which significantly improves the accuracy to obtain important parameters such as yield stress as well as detecting and preventing rheometric artefacts.