EFFECT OF UNIMODAL PARTICLE SIZE AND PULP CONTENT ON RHEOLOGICAL PROPERTIES OF TOMATO PUREE

The vane method in controlled shear stress mode was used to determine the yield stress and the shear rate—shear stress data of tomato purees containing 10–35% pulp of two different average particle sizes: 0.34 and 0.71 mm. Consistency index and apparent viscosity increased significantly with pulp content and decreased with average particle size. The effect of pulp weight fraction (P) on relative viscosity (η_r) could be described by the single parameter equation: η_r= [1 – (P/A)]⁻², while the effect of particle diameter on η_r could be described using Peclet number. Magnitudes of yield stress determined directly by the vane method were higher than those obtained by using the Casson model, and were proportional to the square of pulp content. Reduced Casson yield stress—P data on purees of both particle sizes followd a single curve. Effects of pulp content and particle size on vane yield stress and apparent viscosity were evident from the correlation forms with high values of R².     

Comparison of rheological models for determining dark chocolate viscosity

Parameters in chocolate rheology, namely shear viscosity and yield stress, are important in manufacture and directly influenced by product particle size distribution (PSD) and composition. The Casson model was the standard confectionery industry strategy to quantify rheological properties of molten chocolate until in 2000, the International Confectionery Association recommended the use of interpolation data to describe viscosity. The two strategies are compared and correlated in defining rheological properties of molten dark chocolates prepared using different PSD, fat and lecithin content. Rheological parameters were determined using a shear rate-controlled rheometer and data examined using correlation, regression and principal component analyses to establish their inter-relationships. Correlation and regression analyses showed high correlation ( r  = 0.89–1.00) and regression coefficients ( R ² = 0.84–1.00). The newer International Confectionery Association technique gave higher correlation and regression coefficients than the Casson model, but multivariate principal component analysis showed that the two models were highly related and either could effectively quantify dark chocolate viscosity parameters.     

Flow Behavior of Milk Chocolate Melt and the Application to Coating Flow

ABSTRACT: The rheological properties of chocolate, especially shear viscosity and yield stress, are important control parameters for enrobing processes in confectionery manufacture. The rheological parameters of molten milk chocolate were measured at 42°C during steady pipe flow using a magnetic resonance imaging (MRI) viscometric method. The experimental method combines shear rate values obtained from an MR velocity image and shear stress values obtained from an independent pressure drop measurement. The experimental factors were emulsifier type and emulsifier level. The rheogram data were fit by the Casson model to yield the Casson yield stress and plastic viscosity. The Casson yield stress ranged from 1.9 to 15.0 Pa; the Casson viscosity ranged from 6.0 to 14.6 Pa s as a function of emulsifier content. The rheological parameters were incorporated into a drainage theory model to predict coating thicknesses in the enrobing process. The model was solved numerically and yielded good approximations to the experimental values that were between 1.1 to 2.7 mm.     

YIELD STRESS OF STRUCTURED AND UNSTRUCTURED FOOD SUSPENSIONS

Yield stresses of 40 model suspensions of apple pulp particles with unimodal and bimodal particle size distribution in water, of 13 commercial food suspensions, and of 11 prepared apple sauce samples were determined by the vane method (_0V), and by application of the Casson (_0C) model. For the unstructured (US) apple pulp suspensions magnitudes of _0V and _0C were nearly equal, but for the structured commercial and prepared apple sauce samples, magnitudes of _0V were much higher than _0C. The ratio of _0V/_0C can be used to determine the shear diameter and the degree of structure of a suspension. Energy dissipation due to viscous drag was also higher for the structured suspensions than for the US suspensions. The results suggest that some trends observed with rheological properties of nonfood suspensions may not be applicable to structured food suspensions.     

DETERMINATION OF CHOCOLATE VISCOSITY

A critical appraisal of the Casson model for measuring chocolate versus a new proposed method is performed. The new method is validated with both research grade rheometers and factory grade viscometers. This includes elements of measurement uncertainty. In this way, a new reference method is validated for the viscosity of chocolate and related cocoa products using factory grade viscometers. It involves recording: (1) the value of the stress at a shear rate of 5 s ^-1 to represent the yield stress of chocolate, (2) the value of the viscosity at a shear rate of 40 s ^-1 to represent the high shear viscosity, and (3) the difference between the viscosity measured at a shear rate of 40 s ^-1 during the ramp up and down in shear rate to represent thixotropy. Results relate to the solid to liquid glass transition of dispersions under applied stress, and it is shown that they reflect existing theories of stress induced formation and rupture of fractal aggregates, for particle crowded colloidal suspensions with low interaction energy.     

Vane Yield Stress of Starch Dispersions

ABSTRACT: Yield stresses of 5% (wt/wt) cross-linked waxy maize, tapioca, and Amioca starch dispersions (SDs) were measured with the vane method at different rotational speeds ( N ). The static (σ_0S) and dynamic (σ_0d) yield stress values of each SD were measured before and after breaking down its structure under continuous shear, respectively. The difference (σ_0S - σ_0d) was associated with the stress required to break the internal bonds (σ_b). Values of total yield stress (σ₀=σ_0S) and σ_b, and shear rate were related by power law relationships. The contribution of viscous and network stresses were estimated from an energy balance model. Textural characteristics of the SDs were analyzed in a texture map.     

INTERNATIONAL INTER-LABORATORY TRIALS TO DETERMINE THE FACTORS AFFECTING THE MEASUREMENT OF CHOCOLATE VISCOSITY

The working group of the International Office of Cocoa, Chocolate and Sugar Confectionery (IOCCC) performed a sequence of five ring tests to improve the agreement of the standard method for measuring viscosity of chocolate between laboratories. Reporting shear stress measurements instead of using the Casson equation improved the agreement, as did standardising the method of cleaning the concentric cylinder and calibrating the viscometers. In the revised method, the standard deviation for shear stress measurement at shear rates greater than 5 s⁻¹ from 23 laboratories was less than 8%. However, the Casson yield values had a seven-fold range and the Casson plastic viscosity a two-fold range, which was unacceptably high. A new method (IOCCC 2000) has been published as a result of this work and is available from CAOBISCO in Bruxelles, Belgium.     

Chocolate yield stress as measured by oscillatory rheology

Liquid chocolate exhibits a non-Newtonian flow behavior that is conventionally characterized by a yield stress and plastic viscosity. In general, yield stress is determined by shear rheology experiments and the data are plotted as viscosity as a function of shear stress or shear stress as a function of shear rate. For the shear stress-shear rate plot, a frequently used approach to estimate the yield stress is to fit the data to one of several established models, with the Casson model being the most popular. Even though ICA (former IOCCC) recommendation is not to use the Casson model, it is still frequently applied. With the Casson model, a good fit to the experimental data for the shear rate ranging from 5 s^{− 1} to 60 s^{− 1} is realized. However, this model is unable to resolve real differences between chocolate samples that manifest at shear rate values below 5 s^{− 1}. In this study, oscillatory rheology was applied and the stress at the end of the linear viscoelastic region (LVR) was taken as an estimate for the yield stress. This method was shown to be sensitive to fat content, emulsifier concentration and type of emulsifier. Furthermore, oscillatory rheology was found to be capable to capture differences in yield stress of chocolates that were not differentiated using the Casson model fitting approach.     

Measuring shear stress at lowest possible shear rates and improving viscosity determination of fat suspensions,for example chocolates

In industrial practice, flow property results are frequently discussed due to deviations caused by the measurement method. This project intended to improve preciseness of measuring fat suspensions like chocolate masses at very low shear using a rotary viscosimeter. Results showed no impreciseness at continuous shear rates of 0.03 s^?1 and above. A new measurement profile including constant shear of 0.05 s^?1 and flow curve evaluation according to Windhab and Casson models was created. Measuring seventeen different chocolates and comparing with a previous method showed changes in numerical results. Shear stress at constant 0.05 s^?1 approximated to Casson and Windhab yield values; they were brought into agreement. Linear correlation between the various data sets at low and high shear rates gave acceptable results, thus allowing any of the models to be used in laboratory practice. Finally, repeatability at low shear was further improved after fine tuning and avoiding wall slip effects.     

EFFECT OF PROCESSING, PRESERVATION AND STORAGE ON RHEOLOGY OF GUAVA PULP

Rheological behavior of white and pink guava pulps was investigated using Rheotest 2 coaxial cylinder viscometer over shear rates of 0.6 to 145.8 s⁻¹. The shear rate-shear stress data followed the power law, Casson, Herschel-Bulkley, Michaels and Bolger models. The power law flow behavior index values were in the range of 0.14 to 0.19. Presence of measurable yield stress indicated Bingham model of the pulps. Storage of pulps for 30 weeks had negligible influence on flow behavior index values. The consistency index and yield stress values decreased with extending storage from 0 to 30 weeks. The Michaels and Bolger model distinctly described the non-Newtonian nature of cold- and hot-break pulps as well as different behavior of the pink guava pulps. The structure shear resistance values for hot-break pulps at zero shear rate were considerably higher than those of the cold-break, indicating their high pseudoplasticity.     

Flow Characterization of Peach Products During Extrusion

Suitability of Bingham, Herschel-Bulkley, Casson, and Mizrahi-Berk models, to characterize the flow behavior of peach products during extrusion was investigated. The Casson equation sufficiently described the flow of peach extrudates within the 49 to 125 s^-1 shear rate range. As concentration increased, yield stress and consistency coefficients increased. A rheological model was proposed to describe the viscosity of peach extrudates. The model incorporates the effect of shear rate by the Casson equation and the effect of concentration by a linear expression. The model provided good fit to the experimental data for peach extrudates reconstituted from drum-dried peach purees.     

Effects of particle size distribution and composition on rheological properties of dark chocolate

Control of chocolate viscosity is vital to its quality and production cost, and directly influenced by solids particle size distribution (PSD) and composition. Effects of PSD and composition on rheological properties of molten dark chocolate were investigated by varying PSD [D₉₀ (90% finer than this size) of 18, 25, 35 and 50 μm], fat 25, 30 and 35% and lecithin (0.3 and 0.5%) using a shear rate-controlled rheometer. PSD, fat and lecithin content significantly affected all rheological parameters, with significant interaction among factors. Increasing particles size gave significant reductions in Casson plastic viscosity, Casson yield value, yield stress, apparent viscosity and thixotropy, with greatest effect with 25% fat and 0.3% lecithin, which reduced with increasing fat and lecithin contents. Statistical analysis revealed that fat exerts the greatest effect on the variability in all the rheological properties followed by PSD and lecithin. PSD, fat and lecithin could be manipulated to control dark chocolate rheology, influencing quality whilst reducing production cost.     

RHEOLOGICAL INDICES OF FRUIT CONTENT IN JAMS: EFFECT OF FORMULATION ON FLOW PLASTICITY OF SHEARED STRAWBERRY AND PEACH JAMS

The effect of formulation factors on Casson yield values measured at low shear rates (0.08 ≤γ≤ 1.01s^-1) (σ₀₁) and at medium shear rates (2.58 ≤γ≤ 387.30s^-1) (σ₀₂) was analyzed in previously sheared strawberry and peach jams. Twenty three samples of each fruit jam were prepared according to a second order composite rotatable design. Composition ranges were: fruit content, 25–55%; soluble solids content, 60–70° Brix; added pectin in strawberry jams, 0.3–0.7% and in peach jams, 0.1–0.5%. Variation of σ₀₁ values in strawberry jams depended mainly on the interactions between fruit and soluble solids and between fruit and pectin, while in peach jams, it depended on fruit-soluble solids and soluble solids-pectin interactions. Patterns of change of σ₀₂ values with composition were similar to those observed for σ₀₁ in both strawberry and peach jams. Predictive power of σ₀₁ and σ₀₂ values for estimation of fruit content was low, but taken in conjunction with soluble solids content and total pectin values, 79.5% of the variability of fruit content in strawberry jams and 91.1% of same in peach jams could be explained.     

Optimisation of processing conditions and rheological properties using stephan mixer as conche in small‐scale chocolate processing

This study investigated the suitability of Stephan mixer for use as conche during small‐scale chocolate production in an attempt to reduce cost and processing time. Molten chocolate was processed using the Stephan mixer under different temperature and time combinations and Buhler Elk'Olino conche (as reference), and the flow properties (Casson plastic viscosity, Casson yield stress and thixotropy) were studied using a shear‐rate‐controlled rheometer. Optimum settings of 65 °C for 10 min at 443  g dry conching followed by 50 °C for 15 min at 443  g wet conching in the Stephan mixer resulted in similar flow properties as the reference sample. Increasing the blade rotary speed for both dry and wet conching resulted in a decrease in flow properties. The Stephan mixer proved to be suitable for conching small‐scale (approximately 1 kg) chocolate productions and could be explored as a fast and cost‐effective method for small‐scale chocolate conching processes.     

EFFECT OF PECTIN CONTENT ON FLOW PROPERTIES OF MANGO PULP CONCENTRATES

The effect of pectin on the flow properties of mango pulp concentrates was studied using a coaxial cylindrical viscometer in the temperature (T) range 303-343°K. The variables were soluble solid content(C) 16-30°Brix, pectin content (C_pec, range 0.0323-0.0349 dry wt fraction) for the shear rate (γ) range 9.0-250 s^-1. Mango pulp concentrates behaved as pseudoplastic fluids in the entire pectin content range with a flow behaviour index of 0.286. A model relating shear stress () with the above variables is presented:     

AN ADAPTATION OF THE CASSON EQUATION FOR THE RHEOLOGY OF CHOCOLATE

The generalized Casson equation with exponent m was considered. The plastic viscosities and yield values corresponding to different ranges of shear rate D were calculated for different values of exponent m between 0.5 and 1. All the plastic viscosities tend to about the same value at high shear rates, while at low shear rates all the yield values tend to the same value which is supposed to be the "true" yield value. For the exponent m = 0.6, the yield value is least dependent on the shear rate range and the yield value calculated for D > 5 sec⁻¹ is generally much closer to the "true" yield value than the Casson value (m = 0.5). Therefore, choice of the exponent m = 0.6 is proposed.     

RHEOLOGY OF DIFFERENT FORMULATIONS OF MILK CHOCOLATE AND THE EFFECT ON COATING THICKNESS

This work characterized the flow properties and investigated the effect of grind, emulsifier type and emulsifier level on the coating thickness of milk chocolate. Rheological properties were characterized using a concentric cylinder viscometer according to the method recommended by the International Confectionery Association. The thickness of the chocolate on dip‐coated acrylic plates was obtained by a gravimetric method. A three‐way analysis of variance was performed for the coating thickness. All three factors were significant; the thickness of the chocolate coating was greater for fine grind, soy lecithin for emulsifier and for low concentrations of emulsifier. The thicknesses were well predicted using the model with the Casson model parameters from the rheological characterization.     

INFLUENCE OF TIME THICKENING ON THE WHIPPABILITY OF CREAMS

Flow properties of creams containing milk fat (Cream A), vegetable fat (Cream C), and vegetable fat plus milk fat (Cream B) were determined with a coaxial cylinder viscometer for a wide range of shear rates. All creams examined showed time thickening. The viscosity increase with shearing time was expressed by two stage equations as follows: (1) (1) where η_o and η_t are cream viscosity at zero and t shearing time, K₁ and K₂ are rate constants and C₁ and C₂ are constants. The first stage (Eq. 1) was assumed to occur in the course of primary clustering of the independent fat globules, and the second stage (Eq. 2) was assumed to occur in the course of coagulation of the fat globule clusters. Both K₁ and K₂ increased as shear rate increased.
At the same time, the whippability of each cream was determined with a household mixer to which was attached a strain gauge transducer unit for measuring consistency of the whipped creams. There was a tendency for a higher ratio of milk fat/vegetable fat in the creams to decrease the whipping time or to increase whippability of the creams.
Correlations of stability, whippability, and flow properties were examined. A cream which was high stability showed a low K₁ value, and a cream which has high whippability showed a high K₂ value. K₁ and K₂ values at a suitable shear rate will be quite helpful in the determination of the physical properties of cream.     

RHEOLOGICAL PROPERTIES OF HOT PEPPER-SOYBEAN PASTE

Rheological properties of fermented hot pepper-soybean paste (HPSP) were evaluated at different total solid contents (TS, 43.6-54.7%) and temperatures (10-40C). HPSP samples at 20C are highly shear thinning fluids (n=0.25-0.33) with large magnitudes of Casson yield stresses (106-573 Pa). Consistency index (K) and apparent viscosity (η_a,20) increased with increase in TS. Apparent viscosity of HPSP obeyed the Arrhenius temperature relationship. The magnitudes of activation energy (7.6-20.4 kJ/mole) for HPSP increased with increase in TS. A single equation, combining the effects of temperature and concentration on η_a,20, was used to describe flow behavior of HPSP. The time-dependent model of Weltman was found to be most applicable (R²= 0.97-0.99) for HPSP. Storage (G') and loss (G") moduli increased with increase in TS, while complex viscosity (η*) decreased. Magnitudes of G'were higher than those of G" over the entire range of frequencies (ω).