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.     

Relationship between rheological,textural and melting properties of dark chocolate as influenced by particle size distribution and composition

In dark chocolate, rheological properties during processing are influenced by particle size distribution (PSD), fat and lecithin contents with consequential effects on finished texture and melting characteristics. Multivariate regression, correlation and principal component analyses (PCA) were used to explore their interrelationships. A 4 × 3 × 2 factorial experiment was conducted with varying PSD [D ₉₀ (90% finer than this size) of 18, 25, 35 and 50 μm], fat (25, 30 and 35%) and lecithin (0.3 and 0.5%). Rheological properties (yield stress and apparent viscosity), textural properties (firmness, index of viscosity and hardness) and melting index (duration) were respectively measured using shear rate-controlled rheometer, TA.HD Plus texture analyzer and differential scanning calorimetry. The PSD, fat and lecithin contents significantly influenced all rheological, textural properties and some melting characteristics. Increasing particles sizes reduced yield stress, apparent viscosity, firmness, index of viscosity, hardness and melting index of products with greatest influence with 25% fat and 0.3% lecithin, reduced with increasing fat and lecithin contents. There were high correlation (r = 0.78–0.99) and regression coefficients (R ² = 0.59–0.99) among the rheological, textural and melting index indicating their high inter-relationships. In PCA, the rheological, textural and melting index accounted for >95% variance in the data.     

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.     

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.     

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.     

Microstructure and mechanical properties related to particle size distribution and composition in dark chocolate

Composition in dark chocolate was varied and the effects determined on microstructure, using light microscopy, and mechanical properties of molten and tempered chocolates, using a TA.HD Plus Texture Analyser. Compositional parameters were particle size distribution (PSD) (D₉₀ of 18, 25, 35 and 50 μm), fat (25%, 30% and 35%) and lecithin (0.3% and 0.5%) contents. Micrographs revealed wide variations in sugar crystalline network structure and inter-particle interaction strengths related to PSD and fat level. Samples containing 25% fat had more crystal agglomerates, well flocculated with greater particle-to-particle interaction strengths than those with higher (30% and 35%) fat contents. Increasing the D₉₀ to 35–50 μm caused broadening of the PSD, with particles becoming coarser, which were similar at all fat levels. Mechanical analysis showed that PSD, fat and lecithin content significantly influenced firmness of molten chocolate and hardness of solid (tempered) chocolate with significant interactions among factors. Particle size was inversely correlated with firmness (1235–173 g) and hardness (7062–5546 g). Greatest effect of PSD was with 25% fat and 0.3% lecithin. With higher fat and lecithin contents, the PSD influence was reduced. It was concluded that PSD, fat and lecithin contents and their interactions were central to mechanical properties of dark chocolates.     

Effect of lecithin concentration on properties of sucrose-free chocolate masses sweetened with isomalt

Rheological properties such as Casson viscosity and yield value strongly affect costs and efficiency of the chocolate making process. The addition of a proper amount of lecithin to the blend of chocolate ingredients results in a decrease of both the aforementioned parameters. However, overdosing of lecithin leads to worse sensory attributes and flow properties of the chocolate mass. Our studies on rheological properties of isomalt-containing dark chocolate masses and sucrose-containing milk chocolate masses revealed that the critical lecithin concentration was 0.9 g/100 g; and the Casson yield value of both the types of chocolate blends was enhanced above this content. The same phenomenon, i.e. chocolate thickening, was observed at lecithin concentrations of 0.4 g/100 g and 0.6 g/100 g for milk chocolate masses sweetened with isomalt, and for dark chocolate masses sweetened with sucrose, respectively.     

Impact of particle size distribution on rheological and textural properties of chocolate models with reduced fat content

ABSTRACT:  With an increasing consumption of lipids nowadays, decreasing the fat content in food products has become a trend. Chocolate is a fat-based suspension that contains about 30%wt fat. Reducing fat content causes an increase in the molten chocolate viscosity. This leads to 2 major issues: difficulties in the process and a loss of eating quality in the final product, reported to have poor in-mouth melting properties, remain hard, and difficult to swallow. Literature shows that optimizing the particle size distribution (PSD), that is, having one with an increased packing fraction, can decrease the viscosity of highly concentrated suspensions. This study focuses on the impact of the PSD and fat content on the rheological properties, melting behavior, and hardness of chocolate models (dispersions of sugar in fat). We show that optimizing the PSD while reducing the fat content to a critical amount (22%wt) can decrease the viscosity of the molten material and reduce the hardness of the crystallized chocolate models. Melting in the mouth, characterized by an in vitro collapse speed, is faster for the samples with an optimized PSD. The decrease in the viscosity by optimizing the PSD in systems with a constant fraction of medium phase is based on the decrease of interparticle contact, reducing the particle aggregates strength, and structure buildup during flow or meltdown. In its crystallized state, the particle network is less interconnected, providing less resistance to breakage and meltdown.     

Effect of cocoa butter replacement with a β‐glucan‐rich hydrocolloid (C‐trim30) on the rheological and tribological properties of chocolates

BACKGROUND: The food industry has been facing the challenge of developing low‐fat and low‐calorie food products due to rising health awareness of consumers. To meet this consumer demand, an oat β‐glucan‐rich hydrocolloid (C‐trim30) was evaluated as a cocoa butter substitute in chocolates. The effects of C‐trim30 on the rheological, tribological, and textural properties of chocolates were investigated. RESULTS: The viscosity of molten chocolates increased with increasing levels of C‐trim30. Flow behaviors analyzed using the Casson model showed that the Casson viscosity and yield stress increased with increasing concentration of C‐trim30 in the chocolate. Tribological tests on a ball‐on‐flat tribometer showed a reduction in boundary coefficients of friction, with increasing C‐trim30. In addition, hardness of chocolates showed that replacement of cocoa butter with C‐trim30 produced chocolates with softer texture. CONCLUSIONS: The cocoa butter replacement with C‐trim30 up to 10% produced soft chocolates with improved boundary lubrication properties. Also, the chocolate prepared by replacing the cocoa butter with C‐trim30 resulted in a product with a lower caloric value and increased health benefits from the oat β‐glucan. Copyright © 2008 Society of Chemical Industry     

Influence of some bulk sweeteners on rheological properties of chocolate

Chocolates with reduced calories have become popular among consumers and manufacturers. One way of manufacturing chocolate with reduced calories is to replace sucrose with some alternatives. Effects of different bulk sweeteners (maltitol, isomalt, and xylitol) with different particle size intervals (PSI) (106-53, 53-38 and 38-20 μm) on rheological properties of molten chocolate were investigated. The best model that fit the rheological data was Herschel-Bulkley model. Maltitol resulted in similar rheological properties of chocolate compared to sucrose and thus can be a good alternative. Isomalt resulted in higher plastic viscosity while maltitol resulted in higher yield stress than others. As the particle size increased the plastic viscosity and the yield stress increased. The differences in rheological properties of chocolate with different bulk sweeteners were caused by differences in solid volume fraction and particle size distribution (PSD). A substitute with large particle size should be chosen to replace sucrose for improving rheological properties of chocolate, but the particle size should be small enough to obtain good sensory properties.     

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 o_y 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 o_y values. When the multiple vane method was employed, start-up speeds below 0.23 rpm had no significant effect on o_y. 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.     

Physical characterization of low-calorie chocolate formulations

Development of a high-quality low-calorie chocolate needs the use of the most appropriate ingredients that could substitute sugar without negatively affecting several product properties. In this study, sucrose-reduced chocolates sweetened with sucralose and stevia by using bulking agents were investigated in relation to their rheological, textural and sensory attributes. Dark, milk and white chocolates with different amounts of sweeteners were formulated. The Casson model best fitted to the rheological data. In dark chocolates, partial substitution of sucrose with stevia (DCSSt) gave similar plastic viscosity and yield stress values with control samples (DCS). Hardness measurements also supported these results. DCSSt sample was again found to be very similar to control in tested sensory attributes when assessed by a consumer panel. The data indicated that it was possible to manufacture chocolate by partial replacement of sucrose with stevia without adversely affecting its important rheological, textural properties and sensory acceptance.     

Effects of selected properties of ultrafiltered spray-dried milk powders on some properties of chocolate

The effects of selected properties of spray-dried milk fat powders on chocolate were determined. Milk powders produced from control or ultrafiltered (UF) milks with various levels of fat were blended with skim milk powder to give a standard 26 g fat 100 g⁻¹ powder. Particle size of the chocolate mixes after refining decreased as the fat content and free-fat content of the powders increased. Despite this, increasing fat and free-fat contents of powders reduced the Casson viscosity of the subsequent molten chocolates. Casson viscosities using powders from control or UF milks were similar, but decreased as the particle size of powders increased and particle size after refining the chocolate mix decreased. Casson yield value and hardness decreased as fat content of powders increased. Casson yield value increased with vacuole volume of powders. It is possible to alter important properties of chocolates using milk powders of varying fat contents, free-fat contents and particle sizes.     

Thermal,structural and rheological characteristics of dark chocolate with different compositions

The effect of different cocoa composition of dark chocolate samples and their ingredients on their thermal, structural and rheological characteristics was investigated. Thermal behavior was evaluated by means of differential scanning calorimetry (DSC), while the crystal morphology was observed by polarized light optical microscopy. The rheological measurements were carried out using both continuous and oscillatory experiments. The formation of more stable polymorphic structures was time and temperature dependent; and it was not affected by either cocoa composition or particle sizes. The kinetics of crystallization was determined by a step crystallization method and modeled by the Avrami equation, it was accelerated by solid particles concentration, lower particle size and lower crystallization temperatures. Negative spherulites with featherlike microstructures defined the time dependent crystal growth and were consistent with Avrami indexes of 3–4. Under continuous shear, cocoa butter was well described by the Bingham rheological model, while Casson and Carreau equations modeled the flow of cocoa liquor and chocolate samples. However the Carreau model was preferred for presenting better fittings and for predicting apparent viscosities at low and at high shear rates. From both, continuous and oscillatory experiments, it was found that composition of chocolate samples in terms of fat and nonfat cocoa solids, and sugar content, affected their rheological behavior. The solid liquid transition of chocolate samples and cocoa liquor was obtained at a yield stress of around 1 Pa from both continuous and oscillatory shear experiments.     

Functionality of inulin and polydextrose as sucrose replacers in sugar‐free dark chocolate manufacture – effect of fat content and bulk mixture concentration on rheological,mechanical and melting properties

Inulin and polydextrose are key bulk sugar replacers and have been utilised extensively as functional ingredients due to their fibre and prebiotic claims. This study investigated effects of fat content and bulk mixture concentrations of inulin and polydextrose on rheological properties, mechanical and melting characteristics of sugar‐free dark chocolates sweetened with steviol glycosides. Sucrose was replaced with polydextrose and inulin (0:100, 25:75, 50:50, 75:25, 100:0) at different fat contents (27%, 30% and 33%). Reducing fat content from 33% to 27% had similar effects on the sugar‐free dark chocolates as it did on reference chocolate. Increasing inulin concentrations with simultaneous reduction in polydextrose resulted in consistent increases in Casson plastic viscosity and decreases in Casson yield stress regardless of the fat content. These were explained by microstructural examination of the sugar replacers, which revealed wide variations in network structure. Understanding these factors would help reduce energy load from fat in sugar‐free chocolate confectionery.     

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.     

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.     

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.     

The role of particle size distribution of suspended solids in defining the flow properties of milk chocolate

The flow characteristics of molten milk chocolate, like most dense suspensions, are a function of the maximum packing fraction ((_m) of the suspended solids. Milk chocolates were prepared from mixtures of two relatively narrow coarse (d_4,3 = 17 μm) and fine (d₄,₃ = 8.5 μm) size distributions of non‐fat solids to contain 0, 25, 50, 75, and 100% of the coarse component. Solids bed density (a measure of maximum packing fraction) correlated with the apparent viscosity of milk chocolates (r = ‐0.98). The highest value for solids bed density and lowest value for apparent viscosity were measured for size distributions with 75% of solids from the coarse fraction. Casson yield value correlated with d_4,3 (r = ‐0.98) and specific surface area (r = 0.93). The maximum packing fraction of binary mixtures of two distinct sizes of non‐fat chocolate solids attained a maximum value for a specific volume ratio of the components. The maximum in _m corresponded with the minimum viscosity in molten chocolate at a given solids volume concentration.