Final answer:
The CO2 pressure regulator must be applied and adjusted by a draft trained professional to ensure proper carbonation and pressure of beer in a keg. Accurate adjustment affects beer quality and safety due to the pressurized nature of kegs.
Step-by-step explanation:
The item that should be applied and adjusted by a draft trained professional when dealing with a keg is the CO2 pressure regulator. This device is critical in maintaining the right amount of carbonation and pressure within the keg. If the pressure setting is incorrect, it can result in beer that is either too flat or too foamy. Additionally, an incorrect pressure setting can cause safety issues due to the highly pressurized nature of CO2 tanks and kegs.
Adjusting the CO2 pressure regulator is a delicate task that involves specific knowledge and experience, which a draft trained professional possesses. This professional knows how to adhere to safety standards while setting the regulator to ensure that the beer pours correctly and that the quality of the beer is maintained.