Homebrew Science: Sour Hour!

By Max Spencer

So, what in the hell makes something sour? The simple answer is organic acids. Acids are found in almost every food and drink we ingest, including the beer we love — even non-sour beer. Organic acids are present in malted grains and are created by yeast and other microbes during fermentation². This is why the pH of beer is relatively low (~4 for non-sour beer, down to ~3 for sour beer). It is beneficial to have acids in beer. Acidity augments flavor, affects viscosity, hinders microbial growth, stabilizes flavor and has an influence on perceived hop bitterness^2,4.

This still doesn’t explain what makes something sour. Is there really such a huge difference between a pH of 4 and a pH of 3? Yep, but it’s more complicated than that. pH is a measurement of free protons (H+; hence the H in pH) in a solution. Acids lower the pH of a solution by releasing protons when dissolved. The pH scale is logarithmic, so each number represents a 10x increase in the concentration relative to the number before it. The last thing we need to know about pH is that the scale works in the opposite direction you expect it to — the lower the number, the higher the concentration. So, a pH of 3 is 10x more acidic than a pH of 4, meaning it has 10x the concentration of H+. Despite this, two different beers that share a pH can be vastly different in their sour character and intensity. pH plays a role, but this tells us that there is more to it^1,3,4.

Organic acids are comprised of at one least carboxyl group (-COOH) attached to an endless variety of structures. The carboxyl group is where our free protons come from, as the hydrogen on the carboxyl group gladly gives up its electron in exchange for a life of free lovin’ in solution (-COO- and H+). Studies have found that solutions of different acids at the same pH do not result in the same intensity of sourness^1,3. The sour character is also distinct between different acids. For example, acetic acid — the main component of vinegar — has an unmistakable vinegar character to our taste buds, while lactic acid tends to be described as “clean” and “tart”¹. This reveals to us that it is not only the pH of a beer that causes sourness, but the identity of the acid. The structure attached to the carboxyl group clearly influences beer in a way that is detected by us through our sense of taste — specifically, our taste for sour^1.3,4. Despite knowing that different organic acids result in various sour flavors and intensity, the mechanism behind why and how is still unclear⁴. Ph.D. project, anyone?

Now that we have a better understanding of sourness, how do we get to sour beer? There’s an easy way, and a complex way. The easy way is to add an organic acid to beer until it tastes the way you want. Homebrew shops — including little ol’ us — carry lactic acid solutions that can be used to lower the pH of any beer and add that “tart” flavor common to most sour beers. The traditional way is to use microbes. Brewers, including homebrewers, have access to non-yeast microbes that are used alongside yeast to create sour beers. The two most popular microbes used are Lactobacillus and Pediococcus. They are closely related genera of bacteria that create lactic acid and other byproducts during fermentation, creating a sour character that yeast is incapable of developing on its own. Microbes add a layer of complexity to the flavor of sour beers that pure acid solutions don’t provide. A Belgian lambic or Flanders red would not be the same without the massive diversity of microbes that perform their collective metabolic magic. Microbes create a smörgåsbord of organic acids that can accentuate fruit and malt characters already present in beer and provide a swathe of flavors ranging from funky to fruity^5,6.

The major disadvantage of using microbes is the length of time it takes for their character to develop. It can take months or even years for a sour beer to fully develop, but techniques such as kettle souring can drastically shorten the length of time required to achieve sourness via microbes. A kettle soured beer merely takes hours to days for souring to occur. Another disadvantage is the potential for cross contamination. Breweries and homebrewers alike fear occasional infected batches. Using microbes intentionally increases that chance by bringing them into the same space as clean beers. Despite this, if you practice solid sanitation technique or maintain a second set of plastic equipment the chance of infection stays at the same level it would be if you only fermented clean beers.

Whether you’re a homebrewer or someone who just enjoys drinking beer I hope you can go forth with a new appreciation for all things sour. Maybe take a crack at making your first sour, or buy yourself a nice Belgian masterpiece crafted by tradition and dedication. Either way, let us enjoy the spoils of — intentionally — sour beer together. Cheers!

"I went on a diet, swore off drinking and heavy eating, and in fourteen days I lost two weeks." –Joe E. Lewis

Literature cited

1. Hartwig, P. and M.R. McDaniel. 1995. Flavor characteristics of lactic, malic, citric, and acetic acids at various pH levels. Journal of Food Science 60(2):384-388. 
2. Li, H. and F. Liu. 2015. Changes in organic acids during fermentation. J. Am. Soc. Brew. Chem. 73(3):275-279.
3. Makhlouf, G.M. and A.L. Blum. 1972. Kinetics of the taste response to chemical stimulation: a theory of acid taste in man. Gastroenterology 63:67-75. 
4. Neta, E.R.C., S.D. Johanningsmeier, and R.F. McFeeters. 2007. The chemistry and physiology of sour taste—a review. Journal of Food Science 72(2):R33-R38.
5. Snauwaert, I, S.P. Roels, F.V. Nieuwerburg, A.V. Landschoot, L.D. Vuyst and P. Vandamme. 2016. Microbial diversity and metabolite composition of Beligan red-brown acidic ales. International Journal of Food Microbiology 221:1-11.
6. Spitaels, F. A.D. Wieme, M. Janssens, M. Aerts, H. Daniel, A.V. Landschoot, L.D. Vuyst and P. Vandamme. 2014. The microbial diversity of traditional spontaneously fermented lambic beer. PLOS ONE 9(4):1-13.

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.