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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.

The entrance pressure drop during extrusion cooking of corn grits was measured using a cylindrical die viscometer attached to a single screw extruder and compared with results obtained using low-density polyethylene (LDPE). The cylindrical die viscometer had a length to diameter ratio of 40 with half-entry angles of 30, 37.5, 45 and 90° with the horizontal. The entrance pressure drop at the die was measured as a function of extrusion temperature, product moisture content and the die entry angle. Results indicate that the flow behavior of corn grits and the entrance pressure drop were affected by product moisture content, process temperature and the shear history in the extruder. Entrance pressure drop also increased with wall shear stress for plastic melt, but for food biopolymer, the increase was observed provided shear history effects were minimized. Entrance correction increased with apparent shear rate for LDPE, but the reverse was true for corn meal. Using Cogswell's analysis, corn grits exhibit severe extension thinning behavior in entry flow.

Uniaxial extensional viscosity of cornmeal at moisture contents of 30 and 35% (dry weight basis) and temperature of 160 and 180 o C was estimated from entrance pressure drop measurements during extrusion cooking. The value of the sheat component of the entrance pressure drop was an order of magnitude below that of the extensional component. Food doughs exhibited sheat thinning and extension thinning behavior during processing conditions used. Increasing moisture and temperature decreased extensional viscosity at highet extension rates. The Trouton ratio ranged from 25 to 50 and was affected by product moisture content, barrel temperature, and deformation rate

Volumetric flowrate was studied in the die of a single screw extruder. Variables included the ratio of die length to die diameter, barrel to die diameter, barrel and die temperature, screw speed and product moisture. Die conductance was determined graphically and analytically. Results indicated that for a constant pressure drop across the die, increasing temperature, die diameter or moisture increased flowrate. Dimensional analysis was used to study the effect of each parameter on the flow through the extruder die. An expression for temperature rise due to viscous dissipation for isothermal flow in the die was developed and nomographs are presented.

Entrance pressure drop of extruder dies increased with a decrease in the ratio of barrel-to-die diameter during the extrusion of corn. For the low Reynolds number flow studied (1.77 x 10−3≤ Re ≤ 9 x 10−5) the viscous component, obtained from Weissberg's analysis, contributed 2% to 40% of the entrance pressure drop and increased as the barrel-to-die diameter ratio decreased. The friction factor for laminar fully developed flow through the die followed the expression f = 16/Re'. Fully developed flow within the die was achieved at a die length-to-diameter ratio of 9.