What beer attribute is fixed in bottled beer but can be affected in draft beer?

The reason carbonation is fixed in bottled beer but can be affected in draft beer relates to the different practices of carbonation in these formats. Bottled beer typically undergoes carbonation in the bottle itself, whether through natural processes like bottle conditioning or by forced carbonation before bottling. This process creates a stable level of carbonation that remains consistent as long as the bottle is sealed.

In contrast, draft beer, which is served from kegs, can have its carbonation level influenced by various factors such as the type of gas used (usually CO2 or a mixture of CO2 and nitrogen), the pressure of the gas system, and the temperature at which the beer is served. If the draft system is not properly maintained or calibrated, this can lead to variations in carbonation levels from one serving to the next, affecting the mouthfeel and overall drinking experience.

The other attributes listed—flavor, color, and aroma—are not fixed in the same way. These aspects can change over time due to factors like light exposure, oxidation, and temperature, but the carbonation level in bottled beer is generally stable due to the closed environment of the bottle.