From elastic deformation to flow in tempered chocolate

Abstract

We characterise the yield behaviour and flow curve of a commercially available, molten, tempered milk chocolate with a fat content of 30%, from rheological measurements performed with a rotational rheometer fitted with a four-bladed vane spindle in a large-gap setup. We show that the yield behaviour can be captured by the viscoelastic White-Metzner model, which indicates that elastic deformation precedes flow as the shear stress is increased. Following an initial fluidisation cycle, the shear modulus of the tempered chocolate increases by a factor sixty to G = 4.4 × 10 4 Pa. We find that the Bingham model provides the best fit to our extensive flow curve data, where the yield stress is τ yield = 36.1 ± 0.6 Pa and the Newtonian plateau viscosity is µ B = 8.7 ± 0.02 Pa s.

Fluidisation of yield stress fluids under vibration

“…For τ ≥ τ y , the resulting flow curve of stress as a function of strain rate was found to be best fitted by a two-parameter Bingham model (see table I for parameter values). For τ < τ y , the material deformed elastically, and the measured et al [4] for tempered chocolate, which we present in Appendix A. They were carried out repeatedly over a period of several months and did not reveal measurable aging of the material, consistent with previous studies [19,31].…”

“…Similar viscoelastic oscillations are also observed transiently in the chocolate drop for sufficiently large forcing but they become weaker as the drop spreads. This reduction in overall strain is consistent with an increase of the elastic modulus, which is known to occur in chocolate over successive fluidisation cycles through mesoscopic reorganisation of the material [4]. The strong viscoelastic effects observed in Carbopol result in a striking reduction in spreading:…”

“…The chocolate was tempered using a Model Prima tempering machine (FBM, Italy) which delivered chocolate at its outlet at a temperature of (28.2 ± 0.3) • C. The temper level was determined prior to deposition of each drop onto the substrate, using an EXOTHERM 7400 temper meter (Systech Analytics SA, 170 Switzerland). The tempered chocolate has been extensively characterised by Bergemann et al [4], who performed rheological measurements using a Brookfield R/S-Plus (SST) rheometer with a four-bladed vane spindle rotating in a large-diameter cylindrical glass beaker. In that study, shear was applied cyclically by successively increasing and decreasing the angular velocity or the torque with constant rate.…”

“…Values of angular velocity and torque were converted into shear rate ˙ and stress τ , re- [20] to flow curves measured experimentally. In the case of chocolate, the coefficients were obtained from a two-parameter fit to the Bingham model (n = 1) [4]. G is the elastic shear modulus of the material obtained with constant-shear rheometry for τ < τ y in chocolate by [4].…”

“…In the case of chocolate, the coefficients were obtained from a two-parameter fit to the Bingham model (n = 1) [4]. G is the elastic shear modulus of the material obtained with constant-shear rheometry for τ < τ y in chocolate by [4]. For Carbopol we list the values measured by [18,19] with oscillatory shear rheometry in the limit of an approximately elastic response (τ τ y ).…”