Filled Snack Production by coextrusion-cooking: 3. A rheological based method to compare filler processing properties

Filled snacks are new industrial co-extruded products made by a dough shell and an internal filler constituted by salty or sweet cream. Due to different rheological characteristics of fillers and dough, it is difficult to choose operational variables capable of guaranteeing a complete filling. This work aims to determine the optimal rheological properties of both filler and dough to make an optimal product without undesired voids. A typical dough and several fillers have been considered and rheologically characterised by means of dynamic oscillation, steady shear flow and creep tests. A dynamic temperature ramp test at 1 Hz was performed to determine temperature dependency of the tested material. A time-temperature shifting technique based on a modification of the Cox-Merz rule and on weak gel model was applied to extend the flow curve range and to overcome temperature experimental difficulties. The obtained data, expressed by a power law correlation, were inserted in a numerical algorithm previously developed, and the internal swelling of the dough followed by a recovery was computed during mixed flow inside the extrusion head. It was suggested that a greater recovery implies a better filling capability because the filler may follow the dough during swelling. Simulations indicated that a complete filling was obtained only for the materials exhibiting a quite stable network during the flow. The experimental results obtained by preparing different industrial filled snacks confirmed the simulative predictions. Quantification of the network shear stability was determined by comparing the loss modulus (G″) and shear viscosity (η) values. In particular G″ versus frequency and η versus shear rate were fitted by a power law model and the best fillers must present a ratio between their exponents close to 1.