Oxidative staling of beer occurs by a free radical process. During storage, even trace amounts of transition metals such as iron or copper will catalyze the conversion of molecular oxygen to what are known as "reactive oxygen species" (ROS). One such ROS is the hydroxl free radical which rapidly oxidizes components of the beer to free radicals. Beer derived free radicals react further, creating a chain reaction that results in carbonyl end products such as aldehydes and ketones, giving beer a "cardboard like" flavor.
Antioxidants in beer help them to resist free radical oxidation and although oxidation is inevitable over time, it can be minimized by optimizing brewing operations and storage conditions to provide maximum antioxidant content in packaged beer.
The EPR oxidation profile provides an analytical measure for evaluating the overall antioxidant status of a beer at each stage of the brewing process and provides a useful prediction of shelf life of a finished beer before it goes through costly packaging and distribution.