RHEOLOGICAL MODELING of POTATO FLOUR DURING EXTRUSION COOKING

A generalized model for predicting the effects of shear rate, temperature, moisture content, time-temperature history and strain history on viscosity has been evaluated for extruded potato flour doughs. an Instron Capillary Rheometer and a 50 mm Baker Perkins co-rotating twin screw extruder were used to evaluate all effects incorporated in the model, except strain history. the power law model was used to describe shear rate effects in the range 10–10000 s^?1. the generalized model fit observed data for temperatures of 25–95°C and moisture contents ranging from 22 to 50%, wet basis. Since potato flour by its manufacturing process is pregelatinized, it was unnecessary to evaluate the effects of time-temperature history. Strain history was found to have an insignificant influence on the viscosity.     

Off-line capillary rheometry of corn starch: Effects of temperature, moisture content and shear rate

A capillary rheometer was used to determine the rheological behaviour of corn starch with moisture contents ranging from 27 to 37 g/100 g (wet basis), at temperatures of 85, 100 and 120 °C, and true wall shear rates ranging from 100 to 2000 s⁻¹. It was found that the rheology of the system followed a pseudoplastic law and the interactions between the processing variables such as temperature, moisture content and shear rate were well correlated with viscosity for the experimental range of conditions considered in this work. The apparent viscosity decreased as moisture content, temperature and shear rate increased in agreement with previous work.     

BACK EXTRUSION and SIMULATION of VISCOSITY DEVELOPMENT DURING STARCH GELATINIZATION

A generalized apriori theoretical model relating apparent viscosity of protein dough to several independent variables was used to model gelatinized starch dispersions. Independent variables in the original model were shear rate, temperature, moisture content, temperature-time history and strain history. the model is applied here to corn starch dispersions gelatinized using various temperature-time treatments. Apparent viscosity of a 13.7% gelatinized cornstarch solution at constant shear rate and strain history was measured at 20 C using a back extrusion technique. Activation energy of gelatinization was estimated as 210 kJ/mol (50 kcal/mol) over the range 81–95 C. the activation energy decreased in the range 95–105 C.     

Degradation Profiles of Potato Starch Melts Through a Capillary Tube Viscometer

The effects of time‐temperature and strain history on potato starch melts at 150 °C were investigated by use of a capillary tube viscometer. Reciprocating and single extrusions were performed in this study. Shear stress at single extrusion decreased gently as initial heating time increased, while shear stress at reciprocating extrusion decreased rapidly as the number of extrusion strokes increased. A high degree of cold water solubility was obtained by reciprocal extrusion at lower moisture content. From the results of gel filtration it could be concluded that starch molecules were depolymerized by high and reciprocal shear stress. According to these results time‐temperature history was more effective on the depolymerization of starch molecules and degradation of starch granules at higher moisture content, while strain history was more effective at lower moisture content.     

Degradation of Wheat Starch in a Single-Screw Extruder: Mechano-Kinetic Breakdown of Cooked Starch

Wheat starch was processed in a 19 mm diameter split-barrel laboratory extruder under varying conditions of barrel temperature (79–121^oC), screw speed (50, 100 rpm) and moisture content (25, 30%). Final product and samples removed from along the barrel length were examined for changes in molecular size by dilute solution viscometry. Final products were also examined for paste viscosity and estimated degree of cook. A mathematical model was developed which relates the residence time of the starch in the barrel, the nominal shear stress acting on the starch and the degree of cook of the product to the extent of molecular degradation of the starch. Solution viscometry was found to be more rapid than gel permeation methods previously used to determine the extent of molecular changes.     

Yield Stresses in Cooked Wheat Starch Dispersions

Wheat starch dispersions of 8–15% concentrations were cooked in the temperature range 60°–75°C for periods up to 75 min. Viscosities of the cooked dispersions were determined in a rotational viscometer at 60°C and 23°C. No yield points were observed in the viscosity behavior at 60°C. At 23°C, viscosity measurements showed yield points in the range 200–3.000 dynes/cm² dependent on cook time, cook temperature, concentration and cooling history. Plots of log viscosity versus log shear stress were useful in establishing the existence of yield points where the usual plots of shear rate versus shear stress were ambiguous.     

Influence of time-temperature history and strain history on the melt rheology of soy protein isolate at an elevated temperature

Reciprocating extrusion and single extrusion experiments were performed using a completely air-tight capillary extrusion viscometer in order to elucidate the effects of time-temperature history and strain history on the melt rheology of soy protein isolate (SPI) at an elevated temperature (140°C). Both histories produced a decrease in the measured pressure drop during extrusion and these effects were strongly dependent on the sample moisture content. For moisture contents above 41% (dry basis), the time-temperature history effect was predominant more in decreasing the measured pressure drop, whereas the strain history effect was more influential at moisture contents below 30% (d.b.)     

Shear Induced Starch Conversion During Extrusion

Using a capillary rheometer and a singlerscrew extruder, waxy corn starch was extruded at a relatively low temperature range. Notable conversions induced by shear energy were obtained in very short residence times (from several sec to 1 min). Conversions up to > 70% were also achieved. Kinetic studies showed that conversion caused by shear energy was more efficient than that by thermal energy. The shear rate constant, k, was an exponential function of shear stress and a linear function of shear rate. The minimum shear stress required to cause starch conversion was from 10⁴ to 10⁶ N/m² for 100°C to 21°C. In such “shear” experiments, the maximum temperature rise due to viscous thermal dissipation, measured experimentally and calculated theoretically, was too low to cause any significant thermal cooking of starch. The shear converted starch extrudates were unpuffed and had shapes of traditional pasta.     

A VISCOSITY MODEL FOR A COOKING DOUGH1

A five parameter semi-empirical model is developed describing the apparent viscosity of a cooking dough as a function of applied shear rate, temperature, and time-temperature history. Such a model will be useful in the mathematical simulation of the extrusion cooking process. Other factors which can affect the apparent viscosity of a cooking dough are also discussed. The viscosity model is verified and the constants evaluated for a typical dough by combining the results of Amylograph and capillary viscometer experiments.     

EVALUATION OF THE HOLE PRESSURE METHOD TO MEASURE THE FIRST NORMAL STRESS DIFFERENCE OF CORN MEAL DOUGH DURING EXTRUSION COOKING

Rheological properties of food dough melt, such as steady shear viscosity (η) and first normal stress difference (N₁), were measured using an in-line slit die rheometer attached to a laboratory model single-screw extruder. The flow rate through the rheometer was varied by altering the screw speed. Alternatively, a sidestream valve, to vary the flow rate through the rheometer at fixed screw speed, was also used to obtain rheological data. The slit die rheometer was tested with corn meal at three different experimental conditions: 25% and 35% moisture contents at 180C barrel temperature and 35% moisture content at 150C barrel temperature. N₁ was measured using the hole-pressure and exit-pressure method. Exit pressure measurements were found to be erratic and unreliable. Hole pressure increased monotonically with increasing flow rate and was found to be affected by the processing history. The magnitude of the hole pressure ranged between 2% and 21% of pressure of the flush mounted transducer. For the experimental conditions used in this study, N₁ ranged from 3 × 10⁴ to 6 × 10⁵ Pa for shear rates between 30 to 400 s⁻¹. Possible sources of errors in the hole pressure measurements are discussed.     

Shear and extensional viscosities of hard wheat flour dough using a capillary rheometer

The rheological behaviour of hard wheat flour dough was investigated for a high shear rate range (10-10⁴ s⁻¹) using a capillary rheometer. The shear viscosity obtained from capillary measurements demonstrated a shear thinning behaviour of dough and described by a power law model with a power law index n = 0.38, a consistency coefficient K = 1387 Pa sⁿ, and a coefficient of determination R² = 0.997. The extensional viscosity obtained from capillary measurements for a high extensional rate range (0.16-154 s⁻¹) showed a tension thinning behaviour and described by a power law model with a power law index m = 0.38, a consistency coefficient K_E = 353,000 Pa s^m, and a coefficient of determination R² = 0.977. Torsional measurements using a parallel plates rheometer for a shear rate range (10⁻³-20 s⁻¹) demonstrated the time or strain dependence of wheat flour dough, no steady state shear viscosity of dough, and the shear viscosity increased with time or strain to a maximum value and then decreased, suggesting a breakdown or rupture of the dough structure.     

A GENERALIZED VISCOSITY MODEL FOR EXTRUSION of PROTEIN DOUGHS1

A theoretical model to predict the apparent viscosity of protein doughs during thermal processing involving heat induced denaturation is presented. the model allows viscosity prediction based on the effects of temperature-time history, strain history, temperature, shear rate and moisture content. Data from several sources have been considered in investingating the performance of the model. the approach facilitates an understanding of the mechanisms associated with protein texturization during extrusion.     

ENGINEERING ANALYSIS OF SOY DOUGH RHEOLOGY IN EXTRUSION

Effects of moisture level, temperature and shear rate on the rheological properties of defatted soy dough during extrusion were studied on a Brabender laboratory extruder. Entrance pressure loss through the extruder capillary die as well as viscosity of the cooked and compressed soy dough were expressed as a function of added moisture, shear rate and temperature through multiple regression analysis. Process conditions involved in this study were (a) temperature from 100 to 160°C; (b) added moisture from 22 to 32% and (c) shear rate from 50 to 10,500 sec^?1. Data were collected at steady state. It was found that in the shear rate range of 50 to 1,000 sec^?1 and temperature range of 100 to 160°C the viscosity of the cooked and compressed soy dough with 32%, 25% and 22% added moisture ranged from 10,000 to 500 poise and 20,000 to 200 poise and 20,000 to 1,000 poise respectively.     

RHEOLOGICAL BEHAVIOUR OF COCOA DISPERSIONS

The steady-shear viscosities of cocoa mass and cocoa suspensions were measured over a wide range of temperature and cocoa powder content. The effects of gasification, moisture and lecithin contents were also investigated. The viscosity of cocoa mass followed the Quemada or the Casson model. Both viscosity and yield stress decreased with increasing temperature. Degasification also lowered the viscosity and yield stress. The viscosity of concentrated cocoa suspension increased with increasing powder content and exhibited shear-thinning behaviour, and followed the Kreiger and Dougherty model. While the relative intrinsic viscosity was nearly constant, the maximum volume fraction increased with increasing shear stress or shear rate. Addition of moisture in suspension increased the viscosity due to partial gelation of starch; while addition of lecithin decreased the viscosity due to the lesser number of flocs in the suspension.     

Enhancement effect on apparent viscosity of aqueous tragacanth gum dispersions promoted by sugars

Flow curves of aqueous dispersions of tragacanth gum (T) with sucrose and glucose at different temperatures were determined using a controlled‐stress rheometer. The effect of sodium chloride without or with sucrose (at the highest content) on the rheology of T dispersions was evaluated. The presence of sucrose and glucose promoted a noticeable enhancement impact on the apparent viscosity of aqueous T dispersions, which depended on sugar type/content, shear rate and temperature. In all cases, the glucose addition led to the largest enhanced viscosities at low shear rates (<10 s^?1) and temperature. The joint action of sugar and salt exhibited a notable effect on apparent viscosity at low shear rates, softening the strong shear‐thinning behaviour of T samples. Flow curves of T in the presence of sugars were satisfactorily described by the Cross‐Williamson model, being semi‐empirical correlations of the model parameters with ingredients content and temperature stablished.     

Modeling Shearing Resistance of Powdery Starch for Simulation Studies of Extrusion Cooking Processes

Modeling extrusion cooking of starch requires the use of appropriate intrinsic kinetic equations. Knowledge of shear stress profile in a particulate system such as that in the transition zone of an extruder is crucial for calculation of degrees of conversion (gelatinization and melting) of starch. Shearing resistance of moistened powder starch as affected by temperature, normal pressure, effective shear rate, moisture content, and degree of starch conversion was studied in a model system. A modified vane shear testing device was used for experimental measurement of each individual effect. Within the experimental ranges studied (temperature 30 to 80°C, moisture content 16 to 35%, effective shear rate 0 to 550 s⁻¹, starch conversion 0 to 0.36 (36%), and normal pressure 0 to 83 kPa) parameter values in each functionality were evaluated. A single mathematical model useful for calculating shear stress was obtained by combining these effects in a single relationship. A good correlation was shown when the calculated shear stress was plotted against the experimentally measured. The methodologies developed here are useful for the studies of powdery food materials. The potential application of this correlation to the simulation of extrusion processes was discussed.     

A multiple-step slit die rheometer for rheological characterization of extruded starch melts

Extrusion cooking is a very important process in the field of cereal and snack manufacturing. The rheological properties of the starch based matrix strongly influence this process. A newly developed online rheometer was mounted on a twin screw extruder in order to measure these properties. It was possible to obtain viscosity of wheat flour and corn grits at typical extrusion cooking conditions over a shear rate range of three decades. Flow curves for varying screw speed and water content at a constant thermomechanical history of the starch were measured. Also the temperature dependence of the apparent viscosities could be determined showing activation energies in the range found for synthetic polymers. Furthermore, additional pressure drops that occur at step changes of the slit height were detected. Thus, it was possible to evaluate visco-elastic properties of extruded starch melts at different extrusion cooking conditions.     

Rheological behaviour of sago (Metroxylon sagu) starch paste

Rheological behaviour of gelatinized sago starch solution was studied over the shear rate range of 13.61–704 s⁻¹ at various concentration and temperature ranges. A power law equation was used to describe the rheological behaviour of the starch solution, while the effect of temperature was evaluated by the Arrhenius equation. The effect of starch concentration on apparent viscosity was studied using the exponential model describing the relationship between apparent viscosity and concentration. Consistency index (κ) increased with concentration and decreased with the increase of temperature. Flow behaviour indices (η) were within the range of 0.495–0.559 which indicated the pseudoplastic nature of gelatinized sago starch. The amount of starch and shear rate affect activation energy (ΔE). Depending on the shear rate and concentration, activation energy varied from 0.619 to 1.756 kcal mol⁻¹. A mathematical relationship correlating the various parameters (temperature, concentrations, shear rates) was tested for its significance and validity.     

Thermal and rheological properties of breadfruit starch

The thermal and rheological properties of breadfruit starch were studied using DSC and 2 different rheometers. It was found that the gelatinization temperature of starch with excess moisture content (>70%) was at approximately 75 °C. A new endotherm was detected at about 173 °C when the moisture content was lower than required for full gelatinization of the starch. A detailed examination revealed that this endotherm represented the melting of amylose-lipid complexes. Breadfruit starch paste exhibited shear-thinning fluid characteristics, and good thermal and pH stability. The setback viscosity of the breadfruit starch was lower than that of potato and corn starches. The rheological properties of the breadfruit starch paste was well described by the Herschel-Bulkley model at a shear rate of 0 to 100 s(-1), where R(2) is greater than 0.95, and it behaved like a yield-pseudoplastic fluid. Both the storage modulus and loss modulus of the paste initially increased sharply, then dropped after reaching the gelatinization peak. Breadfruit starch gel showed both flexibility and viscosity. Suspension with 6% starch content exhibited very weak gel rigidity; however, this increased significantly at starch contents above 20%.     

磷酸化醋酸酯淀粉流变学特性的研究

采用流变仪研究了低取代度醋酸酯淀粉及其不同磷酸化程度产物的糊的流变特性。结果表明：所有的样品糊均呈现假塑性流体特征。在相同的温度下，剪切应力随剪切速率的增加而增大；在同一剪切速率下，样品糊的剪切应力随温度的上升而降低，剪切应力随取代度的增加而升高，表观黏度随着取代度的增加而上升。当DS〉0．0416时，随着取代度的提高，剪切稀化也增强。