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.