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Tuesday, August 23, 2016

Homebrew Science: Hops Part I, Alpha and Beta-Acids

By Max Spencer

Without hops, we would not have the incredible variety of beers available to us. Hops (Humulus lupulus) are the primary way to add bitterness to beer in today’s world. While hops can also provide a wide variety of aromas and flavors from earthy to fruity, the historical importance of hops being added to beer stems from the same compounds that grant them their bittering abilities. Spoilage of beer used to be a much larger problem in the past than it is today. Before hops were used in beer, a wide variety of herbs and spices were added for flavor and bittering. If you have ever tasted a gruit or a sahti, you have enjoyed one such traditional ale without hops. However, none of the plants used in these ancient ales possess the same antibacterial properties that hops do (7).

In brewing, the word “hops” refers to the flower of the hop plant. Hop flowers (also called strobiles) contain lupulin glands that produce a sticky resin full of many different organic compounds and essential oils. Alpha-acids (α-acids) and beta-acids (β-acids) present in the resin are responsible for the bitterness found in finished beer, and are capable of ruining a bacterium’s day. These acids cause the cytoplasm (the quintessential guts of a cell) to leak out, and inhibits a large range of cellular functions including: cellular respiration (the way cells burn fuel); the synthesis of RNA, DNA and proteins; etc. (1,2,3). These effects only work on gram-positive bacteria, as gram-negative bacteria have a thick outer cell wall that shields them, hence why beer can still be infected after the addition of hops.

In their unaltered state, alpha-acids and beta-acids do not provide much bitterness. Alpha-acids contribute far more bitterness to a finished beer relative to beta-acids, and in order to “activate” the alpha-acids, hops must be boiled. The heat of boiling cause the alpha-acids to isomerize, which is a fancy way of saying that they change configuration; no atoms or pieces of the alpha-acids are lost, they merely shift and form new compounds (4,6). These new compounds, called iso-alpha-acids, are much more water soluble, provide far greater bitterness, stabilize beer foam, and provide the antibacterial properties discussed before. The length of time that hops are boiled affects the amount of alpha-acids that are isomerized, allowing for control of bitterness.

Beta-acids do not isomerize when boiled and are not very water soluble, so very little of the beta-acids present in hops end up in finished beer. The bitterness in beta-acids is activated by oxidation rather than isomerization, and this quality can actually help protect other compounds in beers from oxidizing over time, prolonging the shelf-life and allowing for longer aging processes (5). However, the bitterness derived from beta-acids tends to be harsher and generally unpleasant in high concentrations.

Every aspect of beer plays an essential role, or roles, in the finished product. Hops are at the heart and soul of what makes modern beer distinct from historical ales, and provide many benefits to the liquid sustenance that bring us all together as lovers of beer. From stouts to IPAs, from lagers to barleywines, we all have hops to thank for our beer.

Literature cited

1. Behr, J and R.F. Vogel. 2009. Mechanism of hop inhibition: Hop ionophores. J. Agric. Food Chem. 57:6074-6081.
2. Behr, J and R.F. Vogel. 2010. Mechanisms of hop inhibition include the transmembrane redox reaction. Applied and Environmental Microbiology 76:142-149.
3. Behre, K. 1999. The history of beer additives in Europe – a review. Vegetation History and Archaeobotany 8:35-48.
4. Jaskula, B., P. Kafarski, G. Aerts and L.D. Cooman. 2008. A kinetic study on the isomerization of hop α-acids. J. Agric. Food Chem. 56, 6408-6415.
5. Krofta, K., S. Vrabcova, A. Mikyška, M. Jurková, T. Čajka and J. Hajšlova. 2013. Stability of hop beta acids and their decomposition products during natural ageing. Acta Horticulturae 1010:221-230.
6. Malowicki, M.G. and T.H. Shellhammer. 2005. Isomerization and degradation kinetics of hop (Humulus lupulus) acids in a model wort-boiling system. J. Agric. Food Chem. 53:4434-4439.
7. Teuber, M. and A.F. Schmalrek. 1973. Membrane leakage in Bacillus subtilis 168 induced by the hop constituents lupulone, humulone, isohumulone and humilic acid. Arch. Microbiol. 94:159-171.

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