CREEP AND DYNAMIC RHEOLOGICAL BEHAVIOR OF TOMATO CONCENTRATES: EFFECT OF CONCENTRATION AND FINISHER SCREEN SIZE

Based on laser diffraction, pulp particles > 900 μm were detected in tomato concentrates; particles greater than 100–200 μm were found in serum from a concentrate. Creep-compliance and dynamic rheological data showed that viscoelastic properties of 21 °Brix tomato concentrates using a 0.84 mm finisher screen were higher than those from a 0.69 mm screen. Both types of viscoelastic properties were sensitive to increase in concentration in that they increased with increase in Brix of the concentrates. Magnitudes of instantaneous modulus (G₀= 1/J₀) of the tomato concentrates were of the same order of magnitude as the storage modulus (G') at 1 rad s^-1.     

Rheological Properties of Tomato Concentrates as Affected by Particle Size and Methods of Concentration

Shear rate-shear stress data were obtained on tomato concentrates made from juices that were produced using finisher screen openings (FSO): 0.020, 0.027, 0.033, and 0.045 in. In general, the apparent viscosity of the concentrates at a shear rate of 100 set^-1 increased with increase in FSO. However, concentrates made from juice using a 0.027 in FSO had the highest apparent viscosity. Magnitudes of yield stress of concentrates increased in direct proportion to FSO. Apparent viscosities of concentrates made by evaporating tomato juice were lower than those obtained either by evaporating the serum or by reverse osmosis concentration of the serum.     

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.     

FLOW BEHAVIOR AND FUNCTIONAL PROPERTIES OF MALT EXTRACT CONCENTRATES

Flow characteristics and functional properties of malt extract concentrates were investigated. Flow behavior was studied using a coaxial viscometer over shear rates of 0.6–145.8 s^-1. The shear rate-shear stress data followed the power law, Casson, Herschel-Bulkley and Bingham plastic models. Herschel-Bulkley values for flow behavior index which ranged from 0.73–0.94, and presence of yield stress indicated the Bingham plastic nature of malt extract concentrates. The activation energy of flow for various concentrates was in the range of 14.8–17.7 kcal/gmole. Magnitudes of Reynold numbers (0.0–49.9) for all concentrates were lower than 2100. Temperature, yield stress and plastic viscosity considerably influenced the flow of concentrates in straight pipes. The pressure drop and power requirements for flow of concentrates in pipes at 50°–60°C were lower than those at 30°C, but negligibly different from those at 80°C. Coefficients of correlation between the Bingham plastic values for pressure drop and yield stress (r = 0.86, P < 0.01), and plastic viscosity (r = 0.97, P < 0.01) were highly significant. Hulled barley and corn concentrates containing high amounts of reducing sugars, showed low viscosities and yield stresses, and required desirably low power for flow in pipes at 50°–60°C. The high protein concentrates of wheat and triticale malts showed high whip volume and good foam stability. Wheat concentrate was superior in fermentability. Hulless barley and finger millet malts produced concentrates with high viscosities which might facilitate their utilization in gums, candies and other such products.     

Intrinsic Viscosity of Tomato Serum as Affected by Methods of Determination and Methods of Processing Concentrates

Magnitudes of intrinsic viscosity of tomato serum determined from the slopes of three equations were not identical, but they showed common trends. Intrinsic viscosities of serums from concentrates obtained by evaporation of juice were lower than those obtained by evaporation of serum and reverse osmosis concentration of serum. Their magnitudes also were lower for serums from diluted and sheared concentrates.     

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.     

RHEOLOGICAL PROPERTIES OF NONCOHESIVE APPLE DISPERSION WITH HELICAL AND VANE IMPELLERS: EFFECT OF CONCENTRATION AND PARTICLE SIZE

The proportionality constant, k_s, between shear rate, γ, and agitation velocity, N, for a helical ribbon‐screw (HRS) agitator was 17.8. Using the HRS agitator, values of consistency index K and the flow behavior index n of 14 apple pulp suspensions at seven different solids concentrations and two average particle diameters 0.71 mm and 1.21 mm were determined; in addition, values of the Casson viscosity η_c and yield stress σ_OC were also calculated. The magnitudes of K increased and of n decreased with increase in pulp concentration. Experimental values of the vane yield stress, σ_Oν, measured with a six‐blade vane increased with increase in pulp content. The values of σ_OC obtained using the Casson model were close to the experimental values σ_Oν. The effect of particle size on the relative viscosity, η_r, was correlated with Peclet number.     

FLOW PROPERTIES OF TOMATO CONCENTRATES

The flow properties of over seventy concentrates made from four cultivars of tomatoes were determined with a concentric cylinder viscometer. The apparent viscosity (100 s⁻¹, 25°C) of the concentrates of each variety was proportional to the 2.5 power of the concentration (% total solids). The apparent viscosity of the Nova cultivar obeyed the Arrhenius temperature relationship; the activation energy for the concentrates was 2.3 ± 0.3 kcal/mole. The simple power law and the Casson model described satisfactorily the flow data of the concentrates but the power law gave higher correlation coefficients. The Herschel-Bulkley model with the Casson yield stress was also satisfactory but the correlation coefficients were lower than for the simple power law. The consistency index of the power law model also showed a power dependence on the concentration. The natural logarithm of the yield stress and the concentration could be correlated by a quadratic equation.     

COMPARISON OF IN-LINE CONSISTENCY MEASUREMENT OF TOMATO CONCENTRATES USING ULTRASONICS AND CAPILLARY METHODS

This study focuses on the development of a noninvasive, in‐line rheometer using ultrasonics and its comparison with data from a traditional capillary viscometer. The ultrasonic based pointwise viscosity measurement technique combines a measurement of the velocity profile and the pressure drop to determine the shear rate and the shear stress distributions, respectively in a pipe. The experiments were carried out using tomato concentrates at 8.75%, 12.75%, and 17.10% total solids content. The flow system consisted of a 53.2 mm diameter acrylic tube, a positive displacement pump and two pressure transducers. Multiple shear viscosity‐shear rate data were recorded under actual pipeline conditions from a single combined measurement of the velocity profiles using ultrasonics and the pressure gradient, using the pressure transducers. The samples showed shear thinning behavior and a yield stress. Power law and Casson models were used to fit the data and both obtained R² values higher than 0.99. The yield stress was also directly determined from the velocity profiles. Shear viscosity versus shear rate data of the 12.75% total solids sample were obtained by capillary rheometry at four different flow rates. These showed very‐good agreement with those obtained using the velocity profile technique.     

Role of Pulp Content and Particle Size in Yield Stress of Apple Sauce

A paddle mixer in conjunction with a rotational viscometer was used to determine the yield stress and the shear rate-shear stress data of apple sauce. Magnitudes of yield stress obtained by extrapolation of the shear rate-shear stress data according to Casson, Herschel-Bulkley, and Mizrahi-Berk models were considerably lower than the experimental values. The experimental values were closer to the Bingham yield values obtained by extrapolation of the straight line portion of the shear rate-shear stress data. Pulp content and average particle size of the solids affected the magnitudes of the yield stress.     

Classification of Rice Starch Amylose Content from Rheological Changes of Starch Paste after Cold Recrystallization

Following storage at 4°C for 24 h (recrystallization) rice starch pastes with low and medium amylose content exhibited a lower increase in consistency index, shear stress and Casson plastic viscosities than pastes with high amylose content. On the other hand, high amylose rice starch showed a higher degree of retrogradation than rice starch of lower amylose content.     

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.     

INFLUENCE OF GLUCOSE AND FRUCTOSE ON HIGH-METHOXYL PECTIN GEL STRENGTH AND STRUCTURE DEVELOPMENT

High-methoxyl (HM) pectin/sugar gels are viscoelastic materials whose elastic (storage modulus, G′) and viscous (loss modulus, G″) characteristics can be measured by dynamic rheological technique. At 10C, over the frequency range 0.1–10 Hz, magnitudes of G′ and G″ of glucose gels were higher than those of fructose gels. Magnitudes of G′ and G″ of both sugar gels increased with pectin concentration according to power laws; the increase in G′ was higher than that of G″. Much useful information on food gels can also be obtained by following the sol-gel transition process. Complex viscosity (η*) at 1 Hz was used to follow structure development (SD) and SD rate during sol-gel transition of pectin-fructose/glucose dispersions. Integrated average viscosities and SD rates were also higher for glucose gels than for fructose gels.     

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.     

YIELD STRESS OF FOOD DISPERSIONS WITH THE VANE METHOD AT CONTROLLED SHEAR RATE AND SHEAR STRESS

The vane method was used to measure yield stresses of 15 commercial food dispersions under controlled shear stress (Cσ) and controlled shear rate (C     ) operating conditions. Magnitudes of yield stress (σ_σ) at Cσ were higher than those of yield stress (σ     ) at C     on tomato products and baby foods. There were good linear correlations ( R ²= 0.96, 0.86) between γ     and σ^σ for food dispersions with undisturbed structure (UDS) and with broken down structure (BDS). Data obtained under Cσ were also used to calculate shear moduli of foods.     

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.     

MODELING THE TEMPERATURE EFFECT ON THE FLOW BEHAVIOR OF SWEET POTATO PUREE1

A study was conducted to investigate the effect of temperature on viscosity of sweet potato (SP) puree. Rheological data of the puree were measured using a Bohlin VOR Rheometer. Shear rates employed ranged from 0.01–921 s^?1. Thermal scans were run at 15, 25, 40, 60, 75, and 90C. The puree exhibited “shear thinning” behavior with a yield stress value of 10 Pa at each temperature tested. The characteristic of the flow behavior of the puree was determined by fitting the experimental data to Herschel-Bulkley, Casson and Modified Casson models. The model with the best fit was then used to represent the combined effect of shear rate and temperature on the apparent viscosity of SP puree in a model based upon the Modified Casson model. The model was validated using data for SP puree samples at 50C. Results showed that, within the experimental conditions used in this study, the model could adequately predict apparent viscosities of sweet potato puree processed at different temperatures.     

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.     

EFFECT OF ORANGE JUICE COMPOSITION ON FLOW BEHAVIOUR OF SIX-FOLD CONCENTRATE

Abstract. The effect of the pulp content and the relative viscosity of the serum on the flow behaviour of a six-fold orange juice concentrate, as represented by the generalized Casson equation and the power law, was studied on samples prepared with a single factor varied at a time. These general flow equations, optimal in terms of multiple correlation coefficients, were established and confirmed on commercial samples. Both equations were successfully applied in predicting the flow behaviour of orange juice concentrates. The generalized Casson equation was found to be somewhat more accurate than the power law.     

Steady and dynamic shear rheology of glutinous rice flour dispersions

The steady and dynamic shear rheological properties of Korean glutinous rice flour dispersions were evaluated at different concentrations (4, 5, 6, 7 and 8%). Glutinous rice flour dispersions at 25 °C showed a shear‐thinning behaviour (n = 0.487–0.522) with low magnitudes of Casson yield stresses (σ_oc = 0.056–0.339 Pa). The magnitudes of σ_oc, consistency index (K) and apparent viscosity (η_a,100) increased with the increase in concentration. The power law model was found to be more suitable than the exponential model in expressing the relationship between concentration and apparent viscosity. The apparent viscosity over the temperature range of 25–70 °C obeyed the Arrhenius temperature relationship, with high determination coefficients (R² = 0.982–0.998), indicating that the magnitudes of activation energies (E_a) were in the range of 9.05–11.89 kJ mol^?1. A single equation, combining the effects of temperature and concentration on η_a,100, was used to describe the flow behaviour of glutinous rice flour dispersions. Magnitudes of storage (G′) and loss (G′′) moduli increased with the increase in concentration and frequency. Magnitudes of G′ were higher than those of G′′ over most of the frequency range.