Abstract: A container closure system for use in lyophilization processes employing depressurization to initiate nucleation is provided. The present container closure system includes a container and a closure element disposed proximate to and adapted for sealing the opening of the container. The closure element includes a first sealing surface adapted for sealably contacting an interior surface of the container and a second sealing surface adapted for sealably contacting the top surface of the container when the container closure system is in a closed position. The closure element also includes a plurality of closure legs extending from the second sealing surface that define a plurality of side vents between each of the closure legs when the container closure system is in the open position. The length, thickness, number, and shape of the legs are chosen to produce an effective total side vent area that is at least 50% of the area of the container opening.