FLOW BEHAVIOUR OF CONCENTRATED ORANGE JUICE: MATHEMATICAL TREATMENT

Based on a model of a suspension of interacting particles in a pseudoplastic solvent, a rheological equation was developed which could describe the flow properties of concentrated orange juice. The function was obtained by modifying Casson’s equation and has the following form: The magnitude of the parameter k_o is affected by the concentration of the suspended particles, the concentration of soluble pectin and by conditions favorable for pectin gel formation. The other two parameters, k and m, are determined mainly by the solvent properties. The mechanism responsible for the flow behaviour of the concentrated orange juice under various conditions could be predicted and explained by the properties of the components in the system.     

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.     

RHEOLOGICAL AND TEXTURE STUDIES OF BUTTER3

Four commercial butters (conventional batch and Westfalia, Contimab and Cherry-Burrell continuous churned) were stored at two temperatures and evaluated periodically for rheological and textural properties. Evaluation methods included four mechanical tests (quasistatic shear, quasistatic uniaxial compression, Penetrometer, and extrusion) and sensory ratings of spreadability, firmness, and other texture parameters. The first three mechanical methods produced quasistatic shear yield stress values that had close agreement. For quality control the three tests can be considered equivalent. Shear stress-strain rate data from extrusion testing developed using the Rabinowitch-Mooney equation could be represented well by the Casson model. Casson yield stress values were an order of magnitude lower than the quasistatic values. The power law equation with a yield stress also represented the butter flow well, and all four butters had similar power law exponents (approx. 0.56). The Cherry-Burrell butter had much higher quasistatic and Casson model constants than the other butters. It was also the most temperature sensitive and most stable with storage time. Temperature-shear rate equivalent effects on shear stress were demonstrated by developing shift factors. The quasistatic yield values for the butters best predicted sensory spread-ability and hardness. The changes in flow properties (Casson yield stress and Casson slope) correlated with aging.     

FLOW PARAMETERS OF SELECTED COMMERCIAL SEMI-LIQUID FOOD PRODUCTS2

The rheological characteristics of samples of commercial apple sauce, baby food (banana and peach), mustard and ketchup were evaluated by two successive shearing cycles in a coaxial viscosimeter. It was found that generally, both the Herschel-Bulkley and the modified Casson equations were equally good mathematical representatives of the experimental flow curves, especially after the sample has already been sheared once. The original Casson equation was generally a lesser appropriate model especially for the first flow curve. Time and shear effects on the flow curve features, and on the magnitude of the yield stress, were also studied. The results showed that although all the effects were roughly on the same order of magnitude, they varied considerably between the different types of products and within the same products of different manufacturers.     

RHEOLOGICAL CHARACTERIZATION OF SALAD DRESSINGS. 1. STEADY SHEAR, THIXOTROPY AND EFFECT OF TEMPERATURE

Shear stress-shear rate data of five commercial salad dressings at 2.0, 10.0, 18.3, 26.7, 32.2 and 43.3°C were obtained using a concentric cylinder viscometer. The power law and Casson equations were applicable for all products (r ≫ 0.98). The Arrhenius equation was applicable for the effect of temperature on the apparent viscosity at 100 s^−l (r ≫ 0.97). Rheological data were obtained at a constant shear rate of 275s^−l at 2.0 and 10.0° C. The results show that the Weltman equation was better than the Hahn equation in describing the thixotropic behavior of the five salad dressings.     

RHEOLOGICAL PROPERTIES OF CLOUDY AND CLARIFIED JUICE OF MALUS FLORIBUNDA AS A FUNCTION OF CONCENTRATION AND TEMPERATURE

The rheological behaviour of cloudy and clarified juice of Malus floribunda was studied as a function of solid concentration, in a range of 45–70 Brix, at nine temperatures (5–50C). Cloudy and clarified juices are slightly thixotropic at 65–70 Brix and 5–10C. At lower concentrations and higher temperatures the thixotropy is low and can easily be eliminated by shearing. The power law model was applied to fit the pseudoplastic behaviour depending on the concentration level. Cloudy juice has pseudoplastic behaviour for concentrations higher than 45 Brix. Clarified juice is pseudoplastic at 65 Brix for temperatures lower than 25C and Newtonian at higher temperatures and for solid concentrations lower than 60 Brix. The activation energy for flow evaluated with the Arrhenius-Guzman equation, increases with solid concentration and ranged from 10.8 to 3.45 kcal gmol⁻¹ for cloudy juice and 15.3 to 7.9 kcal gmol⁻¹ for clarified juice. A nonlinear correlation accounting for temperature and concentration effects on the consistency coefficient was identified.     

Changes in the apparent viscosity profiles of casein suspensions as affected by plant enzymes

The aim of this work was to study the degree of hydrolysis and changes in the apparent viscosity of casein suspensions as a result of various enzymes addition. Suspensions with 3, 12 and 15 g/100 mL of casein, at pH 5.2, 6.0 and 6.5 were prepared in buffer solutions. Previous standardization; plant (papain and bromelain) and animal (chymosin) enzymes were added to hydrolize the casein suspensions. A control with no enzyme addition was used. The rheological behaviour was determined using a rotational rheometer (Haake RV20), with a cone and plate geometry. The Casson and the power law equations were applied to the data. The degree of hydrolysis was a function of the enzyme, pH and casein concentration, presenting chymosin the highest values. All enzymes showed the highest activity at acidic pH. Also, some substrate inhibition was observed. All samples behaved as non-Newtonian, shear-thinning systems with a yield stress value. In all cases, a significant increase in the viscosity was observed when shifting from 3 to 12 g/100 mL. Further increase in concentration caused an opposite effect. Changes in pH of the casein suspensions affected the viscosity, presenting maximum values at pH 6.0. The Casson equation fitted the results better than the power law model.     

RHEOLOGICAL BEHAVIOR AND PHYSICOCHEMICAL CHARACTERISTICS OF GOLDENBERRY (PHYSALIS PERUVIANA) JUICE AS AFFECTED BY ENZYMATIC TREATMENT

Goldenberry (Physalis peruviana) juice was prepared and enzymatically treated with two dosages (300 and 600 ppm) of Pectinex Ultra SP‐L, then, the juice was concentrated to 30 and 40°Brix. Rheological properties of juices were studied at a wide range of temperatures (5 to 100C) and shear rate range from 0.3 to 100/s using Brookfield Digital Viscometer. The Herschel–Bulkley, Casson and Bingham rheological models were applied to describe the rheological properties of different juices. These properties are key parameters required to solve food industry problems in numerous areas such as quality control, evaluation of consumer acceptance and texture. The results indicated that juice concentrates behave as non‐Newtonian fluids (pseudoplastic) and have a definite yield stress. The Bingham and Casson, plastic viscosity, yield stress, consistency index and flow index decreased with increasing enzyme dosage and temperature. The effect of temperature on the viscosity described by means of an Arrhenius‐type equation. The activation energy for viscous flow depends on the total soluble solids. Moreover, sensory evaluation for juices was carried out. In consideration of industrial utilization, detailed knowledge on the rheological and physicochemical characteristics of cape gooseberry juice is of major importance.     

RHEOLOGICAL BEHAVIOUR OF CLARIFIED MANGO JUICE CONCENTRATES

The rheological behaviour of clarified mango juice was measured at temperatures 15–85C and concentrations 15–66 °Brix, using a rotoviscometer. Mango juice free of pectin and pulp behaves as a Newtonian fluid. The effect of temperature can be described by an Arrhenius-type equation. The activation energy for viscous flow was in the range of 1.64–8.44 kcal/g-mol, depending on the concentration. The effect of concentration was modelled better by an exponential relationship than a power-law relationship. Simple equations are proposed for describing the combined effect of temperature and soluble solids content on the juice viscosity.     

RHEOLOGY OF SALTED EGG YOLK

The influence of salt content (7-14% of weight) on the rheological behaviour of egg yolk in the temperature range 5-60°C was studied. The samples showed rheological behaviour that may be described by the power law equation. An increase in the salt content leads to an increase in the consistency index, whereas the flow behavior index shows a tendency to decrease. The effect of temperature on the consistency may be related by an Arrhenius-type equation in which the activation energy decreases as the salt content increases. The behaviour index does not vary greatly with temperature and there is a linear relation with the total solids content. A general equation describes the behaviour of egg yolk for the ranges of dry extract (D), salt content (C) and temperature studied:
     

RHEOLOGICAL BEHAVIOUR OF KIWI FRUIT JUICE CONCENTRATES

The rheological behaviour of diluted kiwi concentrates (22.5–63.0 °Brix) over the temperature range 4 to 70C, was studied. The values below 55.7 °Brix fit the power law while above 55.7 °Brix fit the Herschel-Bulkley equation. Maximum and minimum consistency index values were 62225 and 16 mPa. s ⁿ . Concentrates showed a yield stress at 55.7 °Brix, from 5 to 11 Pa, and 63.0 °Brix, from 19 to 53 Pa. Yield stress was correlated with temperature by means of a linear equation. Arrhenius activation energies were in the range of 28.97 to 33.89 kJ/mol. The average value of the flow behaviour indexes varied from 0.85 (sample at 22.5 °Brix) to 0.59 (sample at 63.0 °Brix). Simple equations are proposed to describe the combined effect of temperature and soluble solids content on consistency index and rheological behaviour index.     

Flow Properties of Low-Pulp Concentrated Orange Juice: Serum Viscosity and Effect of Pulp Content

65° Brix serum samples from four concentrated orange juice (COJ) samples, between -19 and 30°C, were mildy shear-thinning fluids with negligible yield stress. The power law model described well the flow data. For one sample, the Powell-Eyring model also described the data. The activation energy of flow (E_a) for the samples was between 11.2–13.6 kcal/gmole. The power law parameters were determined for a 65°Brix sample as a function of pulp content (0–11%) between −19 and 30°C. At a fixed temperature, the apparent viscosity and the consistency index increased exponentially with pulp content. E_a decreased with increase in pulp content and reached a constant value of about 10.2 kcal/gmole. Data on a high-pulp (21.2%), 65° Brix, sample indicated the presence of yield stress, particularly at low temperatures. The Mizrahi-Berk model predicted reliable values of the yield stress.     

A REVIEW OF THE PHYSICOCHEMICAL APPROACH TO THE ANALYSIS OF THE STRUCTURAL VISCOSITY OF FLUID FRUIT PRODUCTS

The main goal of the physicochemical approach is to understand the flow mechanism in fluid fruit products and to control as well as “synthesize” a desired consistency. Three methods have been employed in an attempt to correlate the rheological parameters of the product with the physicochemical properties of its components. The first method uses flow and physicochemical data, obtained from samples where a single factor is varied at a time, to derive an empirical correlation. This correlation is subsequently verified on commercial samples before putting it into practical use. The second method is based on an analogy between a well analysed experimental model and a closely related commercial product. The third method is based on a theoretical model. Since to-date there are no specific models for fluid fruit products, use was made of a modification of Casson's equation which yielded satisfactory results for orange juice concentrate.     

CONTINUOUS-FLOW MICROWAVE INACTIVATION KINETICS OF PECTIN METHYL ESTERASE IN ORANGE JUICE

The use of microwave energy as an alternative to conventional continuous-flow pasteurization of orange juice was explored. One advantage of microwave heating is obvious: it can prevent surface fouling of heat exchanger, due to prevailing higher surface temperature. Microwave heating may also have the potential for intensified enzyme inactivation which is explored in this study. To elucidate conditions suitable for continuous pasteurization of orange juice, a microwave heating system was designed using a domestic microwave oven (700W) under a full power operation. The residence time of the juice in the heating coil in the microwave oven was dependent on the juice flow rate and dimensions of the heating tube. Based on time-temperature profile and predetermined enzyme inactivation kinetics, the effectiveness of microwave heating (come-up) was evaluated. The PME decimal reduction time for microwave heating were ∼38, 12, 4.0 and 1.3 s at 55, 60, 65 and 70C, respectively, as compared with ∼150 and 37s at 60 and 70C under conventional heating, respectively. Microwave heating is, therefore, substantially more effective in inactivating PME in orange juice than conventional method.     

Flow Properties of Low-Pulp Concentrated Orange Juice: Effect of Temperature and Concentration

Three 65° Brix low-pulp concentrated orange juice (COJ) samples, between −19 and 30°C were shear-thinning (pseudoplastic) fluids with negligible magnitudes of yield stress. The simple power law model fit welt the shear rate-shear stress data. For one sample, the Powell-Eyring model also described the data well. The Arrhenius model described the effect of temperature on the apparent viscosity and the consistency index of the power law model. The activation energy of flow (E_a) was 10.7 ± 0.2 kcal/g mole. The models of Harper and El Sahrigi and Christiansen and Craig were suitable for describing the combined effect of temperature and shear rate. The magnitude of E_a decreased with decrease in concentration. Apparent viscosity and K increased exponeitially with concentration.     

Some Physical,Chemical, and Rheological Properties of Sweet Sorghum (Sorghum Bicolor (L) Moench) Pekmez (Molasses)

Some physical, chemical properties and the rheological behaviour of the sweet sorghum (Sorghum bicolor L Moench) pekmez (molasses) were determined. The rheological behaviour of the sweet sorghum pekmez (concentrated sorghum juice) with different soluble solid contents (75.1, 72.4, 66.5, and 59.4 °Brix) was determined in the temperature range of 10, 20, 30, 40, and 50°C using a rotational viscometer equipped with spindle 5 at the speed (share rate) of 5, 10, 20, 50, and 100 rpm. An empirical power law model was used to describe the rheological behavior of the sweet sorghum pekmez with correlation coefficients (R²) between 0.922 and 0.986. The sweet sorghum pekmez exhibited a pseudoplastic behavior. An Arrhenius equation was used to describe the effect of temperature on viscosity and E_a value of the sweet sorghum pekmez was calculated as 31 350 J/mol. Depending on the soluble solids contents, the activation energies for flow of diluted samples vary from 52.27 to 24.50 kJmol^?1. The effect of °Brix on viscosity can be described by the power-law equation. Experimental data were fitted to power law and exponential model in order to describe the effect of temperature and soluble dry matter content. Density and Electrical conductivity were measured 1.3915 g/cm³ and 13.53 mS/cm, respectively. The color as L, a and b value were measured 19.07, + 4.0, and + 2.18, respectively.     

RHEOLOGICAL CHARACTERIZATION OF SALAD DRESSINGS 2: EFFECT OF STORAGE

The consistency index and the flow behavior index of the power law model, and yield stress according to the Casson model that describe the viscoelastic nature of a model salad dressing were determined as a function of storage time and temperature. A four parameter model was employed to describe the creep-compliance data. Major changes in the magnitudes of the rheological parameters took place during the initial seven days of storage. The changes are attributed to coalescence of oil droplets and to hydration of the food gum. Creep-compliance data on six commercial salad dressings showed significant differences in the rheological behavior among the dressings.     

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.     

Rheological behaviour of blueberry cloudy juice (Vaccinium corymbosum L.)

The rheological behaviour of blueberry cloudy juice ( Vaccinium corymbosum L.), prepared from fresh berries, was studied as a function of solids concentration, in the range 25–61 °Brix, at different temperatures (5–50 °C), with a control rate (CR) rheometer. Cloudy juice was concentrated by vacuum evaporation, and solutions at other concentrations were prepared from the concentrate by adding the liquid collected as distillate. The juices were slightly thixotropic between 45 and 61°Brix. For lower concentrations, the thixotropy was low and could be easily eliminated by shearing. The power law model was applied to fit the pseudoplastic behaviour. The activation energy for flow, evaluated with the Arrhenius–Guzman equation, increased with solids concentration and ranged from 4.6 to 24 kJ gmol^–1. A non-linear correlation, accounting for temperature and concentration effects on the consistency coefficient, was identified.     

FLOW BEHAVIOR OF CLARIFIED ORANGE JUICE AT LOW TEMPERATURES

This paper presents a study of the rheological behavior of clarified and depectinated orange juices, with different soluble solid contents, at temperatures just above their freezing point. The rheological behavior of depectinated and clarified orange juice was determined for a wide range of soluble solid contents (10–65.8° Brix) and temperatures ( − 15 to 30C). The results indicated that the samples behave like a Newtonian fluid. The effect of temperature on the juice can be described with an Arrhenius-type equation. It was found that the activation energy for viscous flow was in the range of 9.4 to 59 kJ/mol, depending on the concentration. The effect of the soluble solid content can be described by an exponential-type equation. Finally, an equation in which dynamic viscosity depends on temperature and soluble solids content gives the following expression: η =  1  ×  10 ^{- 15} exp(0.12C  +  9021/T), where T in Kelvin, C in °Brix, and viscosity in mPa·s.

PRACTICAL APPLICATIONS

It is necessary to know the relationship between viscosity with temperature and concentration, since knowledge of these parameters is of vital importance in the manipulation, calculation and design of all those unit operations during the process, mainly those based on heat and quantum transfer.