Unit for sterilizing web-fed material on a machine for packaging pourable food products

A unit for sterilizing a web (2) of packaging material on a machine (1) for packaging pourable food products, the unit having a bath (7) containing a sterilizing agent, in which the web (2) is fed continuously; a process chamber (8) connected to an outlet (12) of the bath (7), and housing drying means (17) for removing residual sterilizing agent from the web (2); an aseptic chamber (25) communicating with the process chamber (8) via an opening (27) for passage of the web (2), and in which the web (2) is folded and sealed longitudinally to form a tube (29) which is filled continuously with the product for packaging; and an air processing circuit (24) for controlling process conditions; the circuit (24) draws air from the process chamber (8) through an orifice (54), the opening of which is adjustable, during operation of the machine, by a shutter (55) to maintain a pressure value greater than a predetermined minimum value in the aseptic chamber (25).