Air bubble nucleation during starch extrusion

The effect of extrusion conditions on formation of air bubbles (nuclei) in amylopectin melt has been studied during the production o unexpanded extrudates. Also determined was the effect that these nuclei have on the cellular structure of the heat-expanded product.;Five extrusion variables were investigated to create different extrusion conditions. Air bubble entrapment was characterized by porosity and bubble number density of the unexpanded extrudate. A wide range of bubble number density values was obtained depending on the extrusion conditions and the type of amylopectin used. Those extrusion conditions that promoted nuclei formation involved a high mass flow rate, low screw speeds and the use of pregelatinized amylopectin.;The proposed mechanism for air bubble entrapment is as follows. The granular material being conveyed through the extruder contains numerous air-filled pores. When it reaches a certain barrel section it undergoes melting. Some of the air-filled pores in the granular material are converted to air bubbles in the melt. The entrapment of air as bubbles in the melt is related to the existence or not of a mass-filled section in the melting region. Such a filled section will form a mass barrier between the feeder port and the melting region and will prevent air from escaping during the melting transition. The resulting unexpanded extrudate will be characterized by a high porosity and a high bubble number density. On the other hand, if the barrel section between the feeder port and the melting region were stated, then air will have an open route for escape. Thus, extrudates with low porosities and low bubble number densities will result.;Unexpanded extrudates with high bubble number densities produced expanded structures with fine cells; whereas, those with low bubble number densities resulted in expanded products with coarse cellular structure.