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 hydroxyl free radical which rapidly oxidizes components of the beer to free radicals. Beer-derived free radicals react further to perpetuate a chain reaction that results in carbonyl end products such as aldehydes and ketones. It is these carbonyl compounds that give rise to the “cardboard like” flavor of stale beer.
Antioxidants in beer help them resist free radical oxidation. And although oxidation is inevitable over time, it can be minimized by optimizing brewery operations and storage conditions to provide maximum antioxidant content in the packaged beer. The EPR oxidation profile provides an analytical measure for evaluating the overall antioxidant status of your beer at each stage of the brewing process. The EPR data is also useful for predicting the shelf life of a finished beer before it goes through costly packaging and distribution.
EPR Lag Time Assay:
The lag time assay uses an EPR (electron paramagnetic resonance) spectrometer to measure free radical formation in beer. The beer is heated to 60ºC to force its oxidation and a special reagent called a spin trap is added to capture free radicals as they form. Trapped free radicals are measured using the e-scan EPR spectrometer. Initially, antioxidants in the beer quench free radicals before they can be trapped. However, as time progresses, the antioxidants run out and a dramatic increase in free radical trapping occurs. The initial antioxidant quenching causes a characteristic “lag” in the time profile of EPR signal intensity. The duration of the lag time is directly related to the antioxidant content of the beer and is longer for beers with good shelf life stability. The lag time assay is used to test the effects of various processing steps on the final antioxidant content in a beer. For example, oxygen pick-up in unfinished product within the brewery promotes oxidation and lowers antioxidant levels. Filtration media often contain metals such as iron or copper which act as pro-oxidants in the finished beer.
Correlating EPR Oxidation Profiles with Shelf Life:
The real advantage of the EPR method lies in its timeliness. Most breweries rely on sensory analysis studies to determine the shelf life of their beer. Although tasting is the true measure of staleness, it is not always a practical or timely method for evaluating the effect of several process changes. Sensory studies are not only confounded by their subjectivity, but also require weeks, even months to perform. Recently, it has been demonstrated that a beer’s EPR lag time strongly correlates with data from sensory analysis (Uchida et al. (1996) Am. Soc. Brew. Chem. 54, 198-204). That is, beers with long lag times require longer periods before sensory panels taste the oxidative off-flavor. The brewery first establishes a correlation between the EPR lag time of a beer (in minutes) and the time (in days) before oxidative off-flavor is detected by the sensory panel. Once the correlation is established the brewer can use the lag time assay to predict shelf life in beer.