Extract kits - If you don't plan to brew immediately, break up and store the components of extract kits separately, as per below.
Dry Malt Extract (DME) - DME stores well. Store protected from moisture and rodents/vermin, and free from excess humidity or heat. DME is very stable, and the only risks are slight darkening with time from Maillard reactions, and more rapid darkening plus caking if exposed to humidity or moisture. Resealing open bags of DME with tape after squeezing out the air is effective, as is transferring the bag DME into an airtight container.
Liquid Malt Extract (LME) - LME does not store well. LME is prone to rapid darkening and change in flavor (for the worse) from oxidation as well as Maillard reactions. LME should be stored very cold to slow this process down. Freezer is best, followed by refrigerator. Try to reduce any head space to slow down oxidation and freezer burn. Ensure that there is enough room for expansion before freezing so your container does not burst.
Active Dry Yeast - Store in the refrigerator. Keeps very well. Storing unopened packs in the freezer is probably fine pursuant to some in-house research conducted by Lallemand. Oxygen, humidity, and heat are the enemies of active dry yeast, and will prevent some yeast from reviving. A refrigerated, vacuum-sealed sachet will last about long as an unopened sachet. An open sachet, folded over and sealed in a sanitary ziploc bag with the air squeezed will be good for about 1-2 weeks per the late Dr. Clayton Cone. *Note: do not sanitize the bag and introduce moisture that way. New, unused plastic ziploc bags are sanitary from the factory.
Commercial Liquid Yeast Cultures [...]
Harvested Yeast Slurry [...]
Yeast Starter - [...]
Hops, dried - Pellets and cones will last a year or longer without dropoff in alpha acid or aroma character if stored either in an oxygen free package (nitrogen-flushed, mylar barrier package, for example) at room temp, or if frozen (with air contact reduced to a reasonable minimum). If you have a vacuum sealer, the best practice is to store dried hops in vacuum-sealed bags or jars in the freezer. If not, dried hops ought to store well at freezer temps in ziploc bags with the air squeezed out. Immersing the bags in water when sealing may apply exterior pressure that will help the air to come out.
Fresh Hops - [...]
Grain, unmilled - [...]
Grain, milled - Milled grain does not rapidly expire notwithstanding some 'old wives tales' from homebrewers to the contrart. Milled grain should be stored in the same way as unmilled grain. If stored well, it should last at least 6 months without signs of degradation per Briess Malting. Roasted, milled grain may be free from degradation up to 12-18 months according to the same source.