Please be sure to check out the first part of this article, where I described how the oxidative breakdown of hop bitter acids leads to the formation of several smaller acidic products.

The 1979 paper by Williams and Wagner (mentioned in the first part of this article) studied how the cleavage of hop bitter acids over time result in increased levels of Isobutyric, 2-Methyl Butyric, and Isovaleric acids. The graph below shows the data they gathered:

While the increase in acid levels may not seem significant from a brief glance at the chart, realize that the data is still showing an average increase of 30%-40% in acid concentration over a mere four month period. It would be nice to see what the data would have looked like if the storage period was for a year or more!

A second chart prepared by the authors also suggests that the hopping method employed by the brewer makes a big difference in the emergence of these three acids:

Dramatic differences in acids levels are clearly seen between different hopping methods.

The process that eventually converts these three acids into the “winy” flavored esters isovalerate and 2-methyl butyrate is called (appropriately enough) an esterification reaction. An esterficiation reaction typically occurs between and acid and an alcohol (ethanol in this case), check out my article on esters in beer for more info. The amount of esters that are eventually formed tend to be directly proportional to the levels of the three precursor acids that exist in the beer.

So what does this info do for those of us cellaring our beers? For one, if you happen to have some idea of the method the brewer used to hop their beer, you can check out the second chart to have some expectation of what levels of winy flavors you’ll eventually find in your beer. Secondly, we know that the particular cellaring temperature will have a big effect on this entire process. Both the oxidative and esterfication reactions are temperature dependent. If you want to save your hop flavor, keep your beer as cold as possible! Similarly, if the winy flavors are starting to get a bit too potent in your cellared beer, move it to fridge and chill it down. On the other hand, to promote some winy/brandy flavors in your brew you should be keeping it at normal cellar temperatures (or even higher for that matter).

Jason

jason@brewbasement.com

http://www.brewbasement.com