Non-Pasteurized

For the most part, keg beer brewed and packaged in kegs in the U.S. is not pasteurized. During the packaging process, non-pasteurized draft beers are sterile filtered and chilled to the point that any surviving bacteria could ferment the beer and become dormant. Kegs are kept cold ( < 50° F ) from the brewery to the point of dispense. Draft beer dispensed from a keg should be fresh by storing as short as possible and serving cold at 38° F.  

Temperatures above 38° F may promote non-pasteurized draft beers to turn sour or cloudy. Should the temperature rise above 50° F, the dormant bacteria which ferments and spoils beer will once again become active and, subsequent growth will rapidly begin to spoil the flavor and cloud the beer.

Pasteurized

Most of the keg beer brewed and packaged outside the U.S. (Import beers) are heat pasteurized during packaging. This process kills off the bacteria that ferment and spoils the beer. Pasteurized draft beer kegs can be transported and stored at room temperature. The beer in these kegs can be flash-cooled at the point of dispense. However, most imported kegs are stored and dispensed at the same temperature (38° F) as domestic, non-pasteurized kegs.