The 3:30-300 rule for beer is a guideline that states how the temperature affects the freshness of the beer.
For example when beer is stored at 90 F for 3 days, or 72 F for 30 days or in a refrigerator at 38 F for 300 days, the beer will experience roughly the same amount of flavor degradation due to oxidation, which shows that colder storage of the beer will extend the shelf life and quality of the beer significantly.
Heat and time accelerate the oxidation of the beer, which can give the beer flavors like wet cardboard, metallic notes or even a general lack of flavor.
The 3:30-300 rule is especially relevant for craft beers, which often have more delicate hop aromas and flavors that are easily lost with poor storage.
The 3:30-300 rule for beer also emphasizes that keeping the beer cold, even in a standard fridge, is far superior to leaving it at room temperature or in a hot environment.
The breakdown of the 3:30-300 rule for beer includes.
2 days at 90 F such as in a car trunk, the high heat will rapidly age the beer, making the beer taste stale or off quickly.
30 days at 72 F or when beer is stored at room temperature, the beer experiences flavor loss at a moderate pace, but still much faster than cold storage.
And when beer is stored for 300 days at 38 F in the refrigerator, the cold and consistent temperatures slow down oxidation and keep the beer tasting fresh and bright for much longer.