Investigation of melt electrical conductance in food extrusion

The electrical conductance of the extruder melt was measured at the extruder die to determine changes in the extruder melt structure and the residence time distribution patterns due to changes in the extruder operating variables.; An extruder die was manufactured from stainless steel and a dielectric engineering polymer to measure the electrical conductance of the melt in the die. The die produced reproducible electrical conductance results.; When native cornstarch was exposed to dynamic barrel temperature increase, electrical conductance of the starch melt in the extruder increased by 13% with the onset of starch gelatinization and continued increasing until the end of the experiments regardless of the physical structure of starch. Whereas extruder responses showed more drastic changes to the changes in the starch structure.; Effects of the feed rate, screw speed and moisture content were investigated in a 2 x 2 x 2 factorial experimental design for cornstarch and cornmeal. Even though all three variables affected the extruder responses significantly, the electrical conductance was only affected by the moisture content for both cornstarch and cornmeal.; A residence time distribution (RTD) study was conducted by changing the feed rate and screw speed in a 3 x 3 factorial design while keeping the moisture content, screw geometry and barrel temperature profile constant. Potassium chloride was used as the salt tracer and the electrical conductance of the extruder melt was measured in the die to characterize RTDs. All RTD curves were highly skewed to the right resulting in inflated arithmetic mean residence times. Logarithmic transformation of the time scale was shown to best represent the residence time distribution in terms of mean residence time and spread of the RTD.; Increase in both the feed rate and the screw speed decreased the mean residence time but the feed rate effect was more pronounced. The specific feeding load, barrel fill and flow profile were affected by the feed rate and the screw speed in opposite directions. Spread of the normalized RTD decreased with increasing feed rate while increasing screw speed broadened RTD due to more mixing. The flow profile in the extruder barrel was a mixture of both plug and well mixed flow.