A plant has a line to convey trays and a processing chamber under modified atmospheric conditions that has a regulating chamber with gates at an entrance and a regulating chamber (9) with gates at an exit to introduce and extract trays with foodstuff. Before the input regulating chamber there is a stacking device for stacking trays, after the exit of the input regulating chamber (4) and inside the processing chamber there is an unstacking device, and transfer device are arranged to introduce stacks of trays in the processing chamber through the regulating chamber in order to provide sufficient time to regulate the atmospheric conditions in the regulating chamber without lowering the advance rate of the trays in the line.