Crystallization Kinetics of Cocoa Fat Systems: Experiments and Modeling

Isothermal crystallization kinetics of unseeded and seeded cocoa butter and milk chocolate is experimentally investigated under quiescent conditions at different temperatures in terms of the temporal increase in the solid fat content. The theoretical equations of Avrami based on one-, two- and three-dimensional crystal growth are tested with the experimental data. The equation for one-dimensional crystal growth represents well the kinetics of unseeded cocoa butter crystallization of form α and β′. This is also true for cocoa butter crystal seeded milk chocolate. The sterical hindrance due to high solids content in chocolate restricts crystallization to lineal growth. In contrast, the equation for two-dimensional crystal growth fits best the seeded cocoa butter crystallization kinetics. However, a transition from three- to one-dimensional growth kinetics seems to occur. Published data on crystallization of a single component involving spherulite crystals are represented well by Avrami's three-dimensional theoretical equation. The theoretical equations enable the determination of the fundamental crystallization parameters such as the probability of nucleation and the number density of nuclei based on the measured crystal growth rate. This is not possible with Avrami's approximate equation although it fits the experimental data well. The crystallization can be reasonably well defined for single component systems. However, there is no model which fits the multicomponent crystallization processes as observed in fat system

Rheometry for large-particulated fluids: analysis of the ball measuring system and comparison to debris flow rheometry

For large-particulated fluids encountered in natural debris flow, building materials, and sewage treatment, only a few rheometers exist that allow the determination of yield stress and viscosity. In the present investigation, we focus on the rheometrical analysis of the ball measuring system as a suitable tool to measure the rheology of particulated fluids up to grain sizes of 10mm. The ball measuring system consists of a sphere that is dragged through a sample volume of approximately 0.5l. Implemented in a standard rheometer, torques exerted on the sphere and the corresponding rotational speeds are recorded within a wide measuring range. In the second part of this investigation, six rheometric devices to determine flow curve and yield stress of fluids containing large particles with maximum grain sizes of 1 to 25mm are compared, considering both rheological data and application in practical use. The large-scale rheometer of Coussot and Piau, the building material learning viscometer of Wallevik and Gjorv, and the ball measuring system were used for the flow curve determination and a capillary rheometer, the inclined plane test, and the slump test were used for the yield stress determination. For different coarse and concentrated sediment-water mixtures, the flow curves and the yield stresses agree well, except for the capillary rheometer, which exhibits much larger yield stress values. Differences are also noted in the measuring range of the different devices, as well as for the required sample volume that is crucial for applicatio

Influence of flowing fluid property through an elastic tube on various deformations along the tube length

The study of fluid flow characteristics in collapsible elastic tubes is useful to understand biofluid mechanics encountered in the human body. The research work presented here is aimed at thoroughly investigating the influence of both Newtonian and/or non-Newtonian fluids (low and high shear thinning) during steady flow through an elastic tube on various tube deformations, which enables understanding of the interaction between wall motion, fluid flow, and intestinal transmembrane mass transfer as a crucial contribution to a mechanistic understanding of bioaccessibility/bioavailability. It is observed that for a given steady volume flow rate, the tube is buckled from an elliptical shape to a line or area contacted two lobes as the critical external pressure is increased. The downstream transmural pressure is found to get more negative than that at the upstream as the outlet pressure decreased due to stronger tube collapse resulting in a reduced cross-sectional area. The experimental results depict that the tube cross-sectional area decreased by only about a factor of one for PEG (polyethylene glycol) and about a factor of six for both CMC (carboxymethyl cellulose) and PAA (polyacrylamide) from the undeformed one under an applied external pressure of 105 mbar. The corresponding maximum velocity increased by a factor of two during steady flow of shear-thinning fluids. The shear-thinning behavior of both CMC and PAA solutions is clearly observed at a constant flow rate of 17 ml/s as the tube cross-sectional area decreased due to an increase in compressive transmural pressure. In addition, the viscosity of PAA is drastically decreased due to its high shear-thinning behavior than that of the CMC under the same applied external pressure

Non-Newtonian flow behaviour in narrow annular gap reactors

In this paper local flow investigations under isothermal conditions have been established for a narrow annular gap reactor (NAGR, given by a rotor/stator system with a radius ratio of ri/ro=0.8) including two wall scraper blades of different geometry. Two-dimensional laminar flow fields are considered (with Reynolds numbers below Re \u3c 82.6), based on numerical flow simulations, where validations with experimental velocity measurements are in good agreement. Comparisons of the macroscopic flow structuring behaviour are shown for Newtonian and inelastic shear-thinning fluids based on velocity profiles, secondary flows, hydrodynamic pressure contours, shear stress and energy dissipation, by varying the rotor velocity (described in terms of a characteristic rotational Reynolds number Re), the scraper blade angle (β) and the non-Newtonian flow behaviour (power-law exponent n). © 2001 Elsevier Science Ltd. All rights reserved

Effects of flow incidence and secondary mass flow rate on flow structuring contributions in scraped surface heat exchangers

The effects of flow incidence and secondary mass flow rate on flow structuring contributions in scraped surface heat exchangers was discussed. Continuous mixing and dispersing process flows produced by scraped surface heat exchangers (SSHE) in food technology influences the microstructure of multiphase food systems. A scraped surface apparatus with special narrow annular gaps including two wall scraper blades was choosen as a model process

Impact of pre-crystallization process on structure and product properties in dark chocolate

Dark chocolate microstructures with different structure densities, i.e., close-packing of the fat crystal lattice, and homogeneity i.e., evenness and connectivity of the fat crystal network, were created by βVI- seeding or conventional pre-crystallization with various degrees of temper and were evaluated with respect to storage stability. The structure characterization was conducted by measuring the strength of the cocoa butter crystal network with traction tests combined with DSC melting curves. Subsequent storage stability was evaluated with DigiEye technique for fat bloom development and gravimetrical techniques for fat/moisture migration. The two pre-crystallization processes generated significantly different structures and storage stability. Well-tempered βVI-seeding resulted in a dense and homogenous chocolate structure directly after solidification, which was optimal in order to retard fat bloom and fat migration. However, a too high structure density generated heterogeneous structures with reduced ability to withstand fat bloom. A lower structure density exhibited optimal resistance against moisture migration. © 2012 Elsevier Ltd. All rights reserved

Effect of pre-crystallization process and solid particle addition on microstructure in chocolate model systems

The microstructure of chocolate model systems was investigated at the meso (~. 10. μm), micro (~. 50. μm), and macro (0.1-1 mm) scales simultaneously, to examine effect of pre-crystallization process and/or solid particle addition on the formation of a dense structure. The structure density was quantified by measuring the diffusion rate of small molecules at different length scales. At the meso scale, fluorescence recovery after photobleaching (FRAP) was utilized to quantify local diffusion rate solely in the fat phase, whereas high-performance liquid chromatography (HPLC) measurements were made to assess the global diffusion of the same molecules at the macro scale. Both techniques were used in combination with microstructure characterization using confocal laser scanning microscopy (micro scale) and supported by differential scanning calorimeter melting curves for estimating cocoa butter polymorphism. Both FRAP and HPLC analysis generated relevant information on the effect of pre-crystallization and solid particle addition on the structure density. FRAP measurements gave detailed information on microstructure heterogeneity or homogeneity in the cocoa butter, whereas HPLC clearly revealed the impact of solid particles on the structure density. Combining the two techniques revealed that a compact and homogeneous structure obtained through optimized pre-crystallization is required at all times, i.e., immediately after cooling and throughout the product's shelf life, to retard global diffusion in confectionery systems. © 2011 Elsevier Ltd

A three-stage freezing model for liquid droplets with applications to food sprays

The development and validation of a three-stage freezing model for polymorphous materials, with applications to food sprays, is presented. In the first stage, the cooling of the droplet down to the freezing temperature is described as a convective heat transfer process in turbulent flow. In the second stage, when the droplet has reached the freezing temperature, the solidification process is initiated, which results in the release of latent heat. The amount of latent heat released is related to the amount of heat convected away from the droplet. The solidification process is monitored with a progress variable that is used to determine the specific heat capacity of the semisolid droplet. After completion of the crystallization process, in stage three, the cooling of the solidified particle is described again by a convective heat transfer process until the particle temperature is close to that of the gaseous environment. The freezing model has been validated with experimental data of a single cocoa butter droplet in an air flow. Subsequently, the model has been implemented into the computational fluid dynamics code KIVA-3 and has been validated with experimental data of a cocoa butter spray. In addition, the sensitivity of drop sizes with respect to variations in material and processing parameters has been investigated. © 2010 by Begell House, Inc