A closure mechanism for a beverage preparation machine comprising a lower part and an upper part, the upper part and lower part being movable relative to one another from an open position in which a beverage cartridge can be received in the closure mechanism and a closed position in which the beverage cartridge is fixedly retained between the upper part and lower part, wherein in the closed position the closure mechanism contacts a top surface and a bottom surface of the beverage cartridge at or near a centre of the beverage cartridge to exert a compressive force of greater than 50N.