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Accurate methodology to determine slip velocity, yield stress and the constitutive relation for molten chocolate

Talansier, Emeline, Bacconnier, Audrey, Caton, François, Chastel, Carine, Costa, Lucy, Gunes, Deniz Z., Roux, Denis C.D.
Journal of food engineering 2019 v.244 pp. 220-227
chocolate, equipment, liquids, melting, protocols, shear stress
Concentrated soft materials like molten chocolate often has the ambiguity of behaving like a solid or a liquid depending on the external constraints. Determination of the constitutive relation is often unreliable and consequently the yield stress value too. This difficulty of determining yield stress, particularly when using a standards protocol such as the IOCCC for chocolate, is a consequence of the presence of wall slip. Assessment of slip velocity using Yoshimura and Prud'homme's method most often leads to erroneous results. We propose to tackle this problem by using an easy and reliable methodology applied to a commercial chocolate in a melted state with, as equipment, a plate-plate rotational rheometer.This method allows precise determination of the yield stress value, the constitutive relation and the slip velocity versus the shear stress. Furthermore, it provides an explanation to the apparent Newtonian plateau and thickening behavior of the raw flow curve.