Beer Aging

This is based on an original research article published in Food Chemistry (1).

The Food Science Club recently visited to Yeungling’s (and Yeungling at Wikipedia), and so I was very interested in this article on the effects of long-term storage on beer quality. The authors, who are based at the Center for Malting and Brewing Science in Belgium took eight local brewed beers, bottled with a low oxygen content in the dark at 20 ^oC for a year. They studied the formation of staling compounds as well as carrying out a sensory analysis.

Three of the beers were Pilsner-type lagers and the other five were specialty beers. They differed in alcohol (ethanol) content (5.09 – 10.6%), pH (4.29 -4.61), color (9.2-47.4 EBC*), and bitterness (16.4 – 32.4 EBU*). The lightest colored beer had the highest pH and lowest alcohol content. Conversely the darkest beer did not have the lowest pH, bitterness or color. Interestingly, there was no correlation between any of these factors, even bitterness and pH did not correlate.

Fifteen aging markers in the beers were measured, as shown in the table below. The change in sensory quality was also measured using a trained sensory panel of 10 members [wouldn’t you like that job?] who evaluated the general aging characteristics, they also rated the beer on the basis of stale flavors including Sherry/Madeira, cardboard, solvent, old hops, red fruit and caramel.

Thus, another reason why I am interested in this topic is due the involvement of the Maillard reaction in beer stability. Maillard reaction intermediates are formed during barley germination and subsequent thermal treatment which is more severe for specialty beers than for lagers. Also denser worts are used for these beers and when worts are boiled Maillard reaction intermediates are formed. It also explains why these beers are darker – the more Maillard reaction taking place the greater the color formation. Unfortunately, the presence of these Maillard intermediates leads to the formation of beer stale flavors leading to caramel, burnt and Sherry/Madeira like off-flavors.

While Maillard off-flavors were most common in specialty beers, the other off-flavors had a less clear relationship with beer type. The more bitter beers, both specialty and lager, had more old hop flavors as the iso-α-acids, 3 methyl butyric acid and 2 methyl butyric acid degrade to form 4-methylpentan-2-one and 3pentan-2-one and react with ethanol to form ethyl 3-methyl butyrate and ethyl 2-methyl butyrate. So the higher the original bitterness level, which usually means more hops added, and the higher the alcohol content the more of these compounds are formed and the more old hop notes are sensed in the beers. Esters are formed by yeast fermentation leading to a fruit background flavor. During storage these esters degrade leading to a lower background flavor intensity which, in turn, leads to perception of increased stale flavor.

Darker beers had caramel, burnt and Sherry/Madeira like stale flavors and showed the greatest tendency to age. Beers with the highest alcohol level had the least tendency to age, according to the result from the sensory panel. The high ethanol content may mask off-flavors. One solution to protecting dark beers from storage damage is to reduce the temperature and time the wort and malt are heated.

Another solution might be to drink up the beer so that it is not stored for more than a year.

References
Vanderhaegen, B., F. Delvaux, et al. (2007). “Aging characteristics of different beer types.” Food Chemistry 103(2): 404-412. (Link: restricted access)

* I don’t know what EBU and EBC stand for either. I assume the EBC is European Brewing Convention as that was one of the references. Perhaps c=color and u=units.