Mixer viscometry assumptions were used to model average shear rate in a co-rotating twin-screw extruder. Screw speed was varied at 50, 100, 200, 300 and 400 rpm in an MPF-19APV twin-screw extruder for Newtonian and non-Newtonian fluids with different flow behavior indices (n). A composite screw configuration was used. Degree of fill was varied from 0.4 to 1.0. Also, for three separate all-one-type screw configurations, screw speed was varied from 50 to 400 rpm to estimate the average shear rate for 30°forwarding paddles, single-lead, and twin-lead screws individually for 1.0°fill. A modified matching viscosity technique was used to estimate average shear rate for all conditions.Average shear rate was modeled as a function of extruder constant (k′), screw speed and an empirical parameter a. The k′ increased as a power-law and a decreased linearly with degree of fill. The k′ ranged from 5.7 to 45.1 s a-1 rev -a and was independent of screw speed and flow behavior index. Alpha increased with n. Average shear rates for the composite screw configuration ranged from 10.5 to 404 s -1 for fluids with flow behavior indexes of 0.24, 0.60 and 0.67, respectively. For the separate screw configurations at fill = 1.0, average shear rate at a constant screw speed was highest for 30 forwarding paddles, followed by single-lead and twin-lead screws. Corresponding PRACTICAL APPLICATIONSPractical applications include providing a convenient method to estimate average shear rate for composite screw configurations under partial-fill conditions. Also, the study provides the basis for follow-up studies where the influence of shear on food product quality and nutritional value can be examined. 962 M. SUPARNO ET AL. AVERAGE SHEAR RATE IN EXTRUDER 963 1 6 10 15 11 100 1 . , e T n 970 M. SUPARNO ET AL.
A model was proposed to predict separate thermal and mechanical effects of extrusion cooking on thiamin retention. Thermal effects were determined by heating small samples of wheat flour mixed with 0.30% (wt/ wt) thiamin hydrochloride isothermally at 140°C, 151°C, and 161°C for different times. The calculated activation energy and rate constants at each temperature were 67.28 kJ/g mol and 0.00869/min, 0.0145/min, and 0.0224/min, respectively. The "extruder constant" was estimated as 27.7/rev, based on a matching-viscosity method. Wheat flour with 0.30% (wt/wt) thiamin was extruded at different screw speeds. Mechanical effects caused 89.7% to 94.4% of total thiamin loss. This research provides a generalized method to "fingerprint" the extrusion process.
: Kinetic parameters for thiamin degradation were obtained using 2 high‐temperature heating methods: (1) atmospheric pressure (AP) with moisture correction and (2) controlled pressure (CP). At AP conditions, 33.3% dry basis (db) moisture wheat flour with 0.35% (db) thiamin was heated in thin steel cells isothermally at 145,160, and 172°C. To obtain the moisture correction factor, a constant‐moisture study was conducted at 80°C using 6 moisture contents (6.1% to 36.9%). At CP conditions, flour at 19%, 28.2%, and 33.3% (db) moisture in double‐seamed cans was heated in a CP steam retort at 129°C. For the AP method, the corrected activation energy for 33.3% moisture content was 129.5 kJ/g‐mol and reaction rate at 80°C was 3.48×10−4 min−1. Using the CP method, the activation energy and reaction rate were 121.0 kJ/g‐mol and 9.69×10−5 min−1, respectively. Results obtained from 2 methods were not statistically different. These results illustrated that the correction method could be used as an alternative for researchers without access to controlled pressure equipment and transient heat transfer software.
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