Clamshell packaging for airtight packaging of foodstuff, which packaging comprises: - a tray folded out of a single sheet of material, preferably cardboard, which tray has at least two adjacent pool parts (2, 3), each pool part (2, 3) having a bottom wall (4, 13) and upstanding walls (5, 6, 7, 8, 14, 15, 16, 17) arranged around the circumferential edge of the bottom wall (4, 13), wherein the at least two adjacent pool parts (2, 3) are hingedly connected to each other along an upper edge of a respective upstanding wall (7, 14), such that the at least two adjacent pool parts (2, 3) can be hinged on top of each other to form a closed box, - a circumferential, horizontal flange (9, 10, 11, 18, 19, 20) arranged to the tray to at least some of the upper edges of the upstanding walls (5, 6, 8, 15, 16, 17), and - a plastic barrier foil arranged in the pool parts (2, 3) and extending onto and substantially covering the circumferential, horizontal flange (9, 10, 11, 18, 19, 20) wherein the circumferential, horizontal flange (9, 10, 11, 18, 19, 20) comprises a seal zone extending along the full length of the flange (9, 10, 11, 18, 19, 20), wherein the seal zone is line symmetrical along the hinge line at the upper edge of the connected walls (7, 14), characterized in that the circumferential, horizontal flange (9, 10, 11, 18, 19, 20) is composed out of a number of abutting flange parts (9, 10, 11, 18, 19, 20), each flange part (9, 10, 11, 18, 19, 20) being arranged to a respective upper edge of an upstanding wall (5, 6, 8, 15, 16, 17), and wherein each of the transitions (26, 27, 28, 29) of two abutting edges of the flange parts (9, 10, 11, 18, 19, 20) are positioned line asymmetrically along the hinge line, such that none of the transitions (26, 27, 28, 29) overlap with each other when the at least two adjacent pool parts (2, 3) are hinged on top of each other. - a tray folded out of a single sheet of material, preferably cardboard, which tray has at least two adjacent pool parts (2, 3), each pool part (2, 3) having a bottom wall (4, 13) and upstanding walls (5, 6, 7, 8, 14, 15, 16, 17) arranged around the circumferential edge of the bottom wall (4, 13), wherein the at least two adjacent pool parts (2, 3) are hingedly connected to each other along an upper edge of a respective upstanding wall (7, 14), such that the at least two adjacent pool parts (2, 3) can be hinged on top of each other to form a closed box, - a circumferential, horizontal flange (9, 10, 11, 18, 19, 20) arranged to the tray to at least some of the upper edges of the upstanding walls (5, 6, 8, 15, 16, 17), and - a plastic barrier foil arranged in the pool parts (2, 3) and extending onto and substantially covering the circumferential, horizontal flange (9, 10, 11, 18, 19, 20) wherein the circumferential, horizontal flange (9, 10, 11, 18, 19, 20) comprises a seal zone extending along the full length of the flange (9, 10, 11, 18, 19, 20), wherein the seal zone is line symmetrical along the hinge line at the upper edge of the connected walls (7, 14), characterized in that the circumferential, horizontal flange (9, 10, 11, 18, 19, 20) is composed out of a number of abutting flange parts (9, 10, 11, 18, 19, 20), each flange part (9, 10, 11, 18, 19, 20) being arranged to a respective upper edge of an upstanding wall (5, 6, 8, 15, 16, 17), and wherein each of the transitions (26, 27, 28, 29) of two abutting edges of the flange parts (9, 10, 11, 18, 19, 20) are positioned line asymmetrically along the hinge line, such that none of the transitions (26, 27, 28, 29) overlap with each other when the at least two adjacent pool parts (2, 3) are hinged on top of each other.