A cap assembly having a body, a sealing membrane and a retaining ring. A spout engagement channel is positioned on the bottom surface of the body. An opening extends through the body and includes a recessed circumferential channel and a membrane engaging flange. The sealing membrane covers the opening and includes a body engaging flange and a pierceable surface. The sealing membrane is positioned upon the membrane engaging flange so that the body engaging flange and the membrane engaging flange are in overlying engagement. The retaining ring includes a tab, a sealing membrane engagement surface and an inner wall structure. The retaining ring is positioned in overlying engagement with the sealing membrane. Extension of the tab into the recessed circumferential channel releasably maintains said overlying engagement and biases the retaining ring against the membrane. The inner wall structure of the retaining ring extends radially inward beyond the membrane engaging flange.