Growlers are specifically designed to preserve the carbonation and freshness of beer. The sealing mechanism is paramount. A high-quality, airtight seal prevents oxygen, a beer's worst enemy after brewing, from entering and making the beer go flat. Advanced growlers even come with CO2 cartridges and other mechanisms to maintain the beer's carbonation. However, once the growler is opened, it's best to consume the beer quickly to enjoy its optimal flavor and carbonation.