Beer - Hops For Bitterness
Hops are typically available in three different forms: whole, plugs, and pellets. A given supplier may not stock all three forms. No matter what form they are in, hops begin to deteriorate as soon as they are harvested. Certain varieties keep better than others, but none immune to deterioration. The main causes of deterioration are oxygen, heat, light, and time. Upon deterioration, the alpha acids and essential oils are broken down. This causes a loss in bittering power and negatively affects hop flavor and aroma. Thus, the manner in which hops are stored from the time of harvest until the time they are used is critical to preserving those characteristics of hops which are of value to brewers.
Hops are harvested once each year in late August or early September. The year of harvest is typically marked on the package. If it is not, you really have no idea how old the hops are. Hops for a particular year's harvest may not be available at supply shops until the end of that year or the beginning of the next. The following year's hops won't be available until the end of that year. Thus, you will always be buying hops that are somewhat dated. Just be sure you don't buy hops harvested in a prior year if newer hops are available and don't buy hops without a date. If you can see the hops through the packaging, look for hops that are mostly a green shade. Although, some browning is unavoidable, hops should be more green than brown. The exact shade will vary with variety.
Since temperature is a factor in deterioration, hops should always be stored in the freezer, whether at the shop or at your house. The warmer the storage temperature, the faster they deteriorate. Don't buy hops stored at room temperature. Also, consider the packaging of hops. The best packaging will shield hops from the harmful effects of oxygen and light. You shouldn't be able to smell hops through the package. If you can, the package is highly permeable to oxygen. Hops packaged in thin bags that are similar in texture to a zip lock bag will fall under this category. The best quality hops will be vacuum or inert gas packed in an oxygen barrier material. Oxygen barrier bags are generally heavier, stiffer, and thicker than zip lock type bags. They are also shinier. Aluminized or foil bags are also a good choice. These, as they are opaque, also provide a barrier to light.
Once you get the hops home, you can do your part by storing them appropriately. Keep them in the freezer. Don't open the packages until you are ready to use them. Partially used open packages or hops that were not packaged properly from the start can be repackaged for storage. Brewers who have access to a home sealer can reseal open packages. Another alternative if you have a home kegging system is to store hops in mason jars or heavy plastic jars under carbon dioxide. Recycled plastic food containers made of "PET" are good for this. Look on the bottom of containers for the designation "PET" or "PETE". Simply flush the container with carbon dioxide, fill with hops, add more gas being careful not to displace the hops, and attach the lid. If you don't have a sealer or a kegging system, fill a mason jar, plastic container, or heavy plastic bag as full as possible with hops, seal, and store in the freezer. This will minimize air space and should provide some degree of protection.