The invention relates to a method for preparing a casing material for use in co-extruding food products comprising the steps of providing two or more casing material components; mixing the casing material components and feeding the resulting viscous gelling solution to the co-extrusion process to form an external layer on an extruded strand of food dough. Each of the casing material components in itself is not suitable for co-extrusion as an external layer and the composition of the viscous gelling solution is determined by changing the volumes of the casing material components used dependent on the required food product to be co-extruded and/or the composition of the viscous gelling solution is adjusted by changing the volumes of the casing material components dependent on the information on the properties of the food dough and/or the food product to be co-extruded. The present invention further relates to a method and a system for preparing food products by means of co-extrusion wherein such casing material is used.