A generalized theory linking barrel fill length and air bubble entrapment during extrusion of starch

A theory linking barrel Fill length and air bubble entrapment during extrus ion of starch has been proposed. Experimental data suggested that when barr el fill length defined as length of barrel filled with material pressurized to at least 689 kPa) was greater than the melting region-to-dic barrel dis tance, then air bubble entrapment (as measured by porosity) in the unexpand ed extrudate was favored; if the reverse was true, then air bubble entrapme nt was hampered. Bubble number density in the unexpanded extrudate was not only a function of the total entrapped air (porosity) but also of the type of starch used (pregelatinized versus native amylopectin) where pregelatini zed amylopectin gave much higher bubble number density values than the nati ve counterpart. Screw speed increased bubble number densities at constant e ntrapped air, which seemed to suggest bubble break-up by increased shear of the rotating screws. (C) 2001 Elsevier Science Ltd. All rights reserved.