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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.
Once bought, the method of storage used depends on the type coffee purchased. Green beans store the best in cooled airtight containers, and can easily last in this state for a year without losing flavor. Roasted whole beans are best stored in airtight containers out of the light. The best material choices for the container are ceramic, or opaque glass. Plastic and metal may alter the flavor of the coffee bean. In addition, for the first week of storage, containers should be opened or vented by a relief valve to release the carbon dioxide gas that will be produced by the roasted beans to prevent the gas from changing the quality of the coffee. Whole bean roasted coffee stored in this manner will last for about two weeks. The advisability of freezing roasted beans is controversial. Those advocating freezing believe that the flavour can then last for one to two months. If beans are frozen, leaving them frozen until brewing best preserves the flavour of the coffee. Frozen beans will grind the same as unfrozen beans, but refreezing beans alters the quality of the coffee. Coffee grounds are stored in metal containers that are non-reactive airtight ceramic or glass containers, like roasted beans. Due to increased total surface area of coffee grounds, the grounds go stale in days, rather than weeks. In addition, freezing has no effect in increasing the storage life of coffee grounds.
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