J. Tayeb

Interrelationships between thermomechanical treatment and macromolecular degradation of maize starch in a novel rheometer with preshearing

Using a newly developed rheometer with preshearing, maize starch was subjected to several thermomechanical treatments that involved varying temperature (140–180°C), rotation rate (200–700 r/min) and processing time (5–40 s). Processed starches lost their native A-type crystalline structure to yield typical E-and V-structures. The degree of macromolecular degradation, expansion and water solubility were determined for the different products. The results indicated that these products are similar to those obtained from extrusion-cooking. By computing the melting and shearing process of starch in the rheometer, the quantity of energy imparted to the sample was evaluated and correlated to intensity of thermomechanical treatment. An interrelationship between degree of macromolecular degradation and intensity of treatment has been clearly established; intrinsic viscosity decreased with increasing thermomechanical energy input, whereas the water solubility index increased.

Relationship of extrusion variables with pressure and temperature during twin screw extrusion cooking of starch

Abstract Temperature and pressure inside a twin screw pilot-scale extruder (Clextral BC 45) were measured. The product — maize starch, and screw configuration — transport elements and one reverse screw element before the die, were chosen to be very simple. Axial profiles were determined: the increase in temperature up to 150–200°C is linked with the position of the reverse screw element, the increase in pressure up to 50–80 bar was mainly due to the die. Of the process variables studied, water content and feed rate had the greatest effect. When feed rate and screw speed were changed simultaneously in order to keep the fill fraction constant, the temperature remained steady whereas pressure at the die increased, changing the equilibrium point of the machine. Interpretation of these observations is suggested in terms of conduction, convection, friction, shear and pressure build-up in polymer flow.

A specific slit die rheometer for extruded starchy products. Design, validation and application to maize starch

A novel in-line rheometer, called “Rheopac”, has been designed and built in order to study the rheological behaviour of starchy products or, more generally, of products sensitive to a thermomechanical treatment. It is based on the principle of a twin channel, using a balance of feed rate between each of them, in order to make local shear rate vary in the measuring section without changing the flow conditions into the extruder. A wide range of shear rate could be reached and measurements were performed more swiftly than with a classical slit die. The viscous behaviour of maize starch was studied by taking into account the influence of the thermomechanical history, which modified the starch degradation and thus led to important variations in the viscosity. Experimental results were satisfactorily compared to previously published models.

Computer simulation of starchy products' transformation by twin-screw extrusion

Abstract A computer program based on a comprehensive model of carotating twin-screw extrusion has been developed. Apart from the specific study of each section of the extruder, it allows a wide range of simulations of the complete process for starchy products. Mixing performances of kneading elements were computed. Axial profiles of filling ratio, temperature, pressure and specific shear energy (computed specific energy — CSE) were predicted, and good agreement was found with experimental results and other theoretical studies where available. The program was validated for many different extrusion conditions on the basis of the comparison between computed and measured pressure and apparent viscosity at the die, specific mechanical energy and CSE. The value of CSE in predicting the transformation of starchy products is emphasized and an example of the use of computer simulation for process scale-up is given.

Computation of residence time and energy distributions in the reverse screw element of a twin-screw extrusion-cooker

Abstract The isothermal flow of a Newtonian material through the reverse screw element (RSE) of a twin-screw extrusion-cooker was computed. The distribution of a tracer, whose units were incremented in time and energy along their trajectories, was simulated. This resulted in residence time and energy distributions. Both distributions are important in order to quantify the treatment received by the food material during the flow through the RSE and therefore the corresponding level of transformation. The influence of several variables (screw speed, feed rate, geometry) was evaluated. The width of the axial slots was shown to be very efficient in regulating the mechanical treatment of the material.

An improved thermal model for the solid conveying section of a twin-screw extrusion cooker

Abstract A new analysis of mass and heat transfer in the partially fielled conveying section of a twin-screw cooker extruder is presented. An original hypothesis, suggested by observations made during dead-stop experiments, was introduced into the thermal model. A layer of material is assumed to be located in the clearance between the barrel and the tip of the screw flight. This layer is heated by conduction from the temperature-controlled barrel, and releases some energy to the solid powdery material transported in the screw channel. In parallel, the screws are assumed to be the origin of a large fraction of the heat supplied to the product. Complete temperature profiles along the solid converying section were computed. The influence of different process parameters was evaluated. The results prove to be qualitatively satisfactory and in agreement with measurements. The limits of this model come from the fact that more complex phenomena (including internal friction between granules) have to be taken into account as soon as the screw channel is completely filled.