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For aficionados, the most important identifier on the label of a tequila bottle is "100% agave" or "100% agave azul" - cien por ciento de agave azul. This means it is made only from the blue agave plant, with only agave sugars used for fermentation, and was approved by a government inspector to ensure purity.
Updated May, 2011
If you have a large selection of tequilas, and you want to store them to enjoy at a later date, you should keep a few things in mind.
First, unless the top is completely sealed, it will slowly lose alcohol. Bottles sealed with wax are the least likely to evaporate. Corks dry out, and shrink, so they will lose the most. natural corks also break and fray, so they can be less effective for sealing bottles than artificial corks.
Some of the fancier bottles may present a challenge for storage because of their shape and cap.
If you plan to put a tequila away for a long time, consider purchasing the plastic sealers used in the home-wine business or some heat-activated shrink wrap. These can usually be shrunk with a hair dryer or painter's heat gun.
Air contains oxygen, and oxygen will affect the tequila, and cause it to lose flavour and aroma. The more the air in the bottle, the greater the effect of the oxidation. The speed of the oxidation depends on many things, including the tequila, the seal on the bottle, the temperature of the storage, and how airtight the seal on the bottle is.
Remember that when you open a bottle of tequila, even for a single sip, you subject its contents to oxidization, just as you would a bottle of wine. This can rob the tequila of its agave flavours in a matter of a few weeks to a month. Tequila doesn't last indefinitely. You have to drink it soon - usually within 1-3 months - otherwise it starts to lose its zest.
You can always lay down an unopened bottle to keep the cork moist, but if it is opened, you should periodically tip the bottle to wet the cork in the tequila to keep it swollen. Also, watch for leakage when you lay a bottle down, because the cork may be dry and not expand immediately. Some bottles are sealed with wax on top of the cork (or a plastic wax substitute). These usually help keep an unopened bottle airtight.
Light and heat are also working against you. Bottles stored in the light or warmth will lose more alcohol and change their taste faster than those stored in a cool, dark place.
That's another thing to watch for in stores selling tequila: don't buy the bottles in the window display.
But cool doesn't mean cold: cold tequila will not have the aromatics of a room-temperature tequila.
How much will a tequila lose? That depends on both the conditions and your expectations. In my experience, change of flavour can be noticed three months after opening a bottle, and is more pronounced at six months. While I have not measured the alcohol content in my opened bottles, there is a noticeable loss of that alcohol sharpness is some older bottles, suggesting evaporation has reduced the alcohol content.
However, these tequilas are not necessarily wasted: you can use them for cooking or margaritas.
You can also store tequilas in your own barrels (see aging) but exercise caution: small barrels can make a tequila very woody in a short time. Also, you may want to lightly mist these barrels so the wood does not dry out and allow the tequila inside to evaporate.