Yield Stresses in Molten Chocolates

Yield stress and flow properties of four molten chocolate samples at 40C were measured using coaxial cylinder and vane fixtures. The possibility for slip to occur during measurement was demonstrated by significant differences (p < 0.05) in Gasson parameters when measured between coaxial cylinders of different radius ratios. Significant differences in the Casson flow parameters (p ≤ 0.05) were also found when samples were sheared for 12 min as compared to 30 min. A piecewise regression technique, which fitted the Casson model to the low shear rate ranges and the Bingham model to higher shear rate ranges, could be used to best describe the flow behaviour of chocolate melts. Stress relaxation produced very low oy values, which were believed to be an artifact of the measuring fixture and instrument. In the case of the single vane method, use of different start‐up speeds had a marginal but significant effect (p < 0.05) on oy values. When the multiple vane method was employed, start‐up speeds below 0.23 rpm had no significant effect on oy. The multiple vane method proved to be simple and more rapid than obtaining the steady shear data and then fitting the Casson flow model, and thus may be more suitable for routine yield stress measurements of molten chocolate for quality assurance purposes.