Filled snack production has been considered from several point of views. In this work. the effect of the operational variables on the cereal mixture constituting the external part of this product has been studied. A typical flour blend vas characterised and used in a previous developed model to simulate the extrusion conditions and to show process damage. Chemical and rheological methods are proposed as experimental techniques capable of measuring the damage induced by the process. The link between chemical contents and mechanical damage was proposed on the basis of the thermal history and of the mechanical power input. The simulation results obtained by a new proposed extrusion model applied to a real twin co-extruder, made it possible to predict damage such as the lysine decrease, and to show, the effect of different screw speeds. It appeared that the computed results are in,in acceptable agreement with industrial evidence. (C) 2002 Elsevier Science Ltd. All rights reserved.