No abstract

No abstract

The effect of screw speed, flow rate, barrel temperature and added moisture on physical characteristics of high moisture extruded rice starch was studied. The product physical characterization was made by measuring density, final moisture content and the maximum load force in a puncture test. Rice flour containing approximately 1.5% protein and 6% moisture was mixed with 55-65% (w/w) water and extruded in a co-rotating twin screw extruder with a slit die. A two level factorial design was used to analyse each parameter. It was found that added moisture was the most important variable affecting the force-deformation curve maxima, with the interaction between added moisture and barrel temperature exhibiting a less significant effect. Added moisture and flow rate each affected density, with the interaction between added moisture and screw speed showing a less significant effect. Temperature and flow rate affected final moisture interactively with added moisture, which was evidently the most important variable.

A rice starch mixture pre-mixed with thermostable u-amylase was extruded in u pilot co-rotating twin-screw extrude& with the die replaced by a slit rheometer The apparent viscosity at the slit wall was determined for a variety of operating conditions. The factors studied were barrel temperature (70-l 10°C), water content (5.50-650 glkg rice mixture), flow rate (8 x lo-' to 25 x lo-" m31s) and enzyme concentration (O.Ol-0.1 g/kg). The same screw configuration and rotational speed (225 r-pm) were used. A rheological model incorporating the integrated effect of extrusion and enzymatic activity was developed with n stepwise regression of the experimental data, analysing after each step the variability of the model parameters with the different factors. It was found that for the range of experimental conditions in question the influence of each factor could be considered separately with good results, leading to a power-law type model with a constant behaviour index. The parameters obtained describing the influence of water content, temperature and shear rate were within the range of available literature data for non-enzymatic extrusion and the influence of the enzyme concentration was well described by a power-law dependency.