Once a coffee bean is roasted, it is either packaged immediately for sale, or ground and then packaged for sale. Packages used are typically either an airtight plastic container or vacuum-sealed wrapping, or a folded-over bag with a pressure relief valve. Each type of package has its own advantages. The airtight containers allow maximum freshness of the bean and prolong its shelf life, whilst the valve allows excess carbon dioxide, and other gases to escape. The carbon dioxide, as much as 10 L/kg of coffee for dark-roasted coffee, is not released because it is harmful to the flavour—quite the contrary, it protects the beans against oxidation, but excess pressure could damage the container. Reported experience is that a few days of carbon dioxide release is needed between roasting and brewing for best results. There is also informed opinion that storage of freshly roasted beans in a can pressurized with nitrogen gas, with excess pressure buildup vented via a relief valve, provides optimal storage for extended periods, while promoting a beneficial aging effect related to distribution of natural oils by the pressure.