SCIENCE ON TAP
INTRODUCTION TO THE SCIENCE OF BEER AGING
By Charlie Hoxmeier, PhD, Brewmaster November 2, 2017
Aging a beer is a test of patience. You planned and brewed the perfect beer for aging, but how do you know if the flavors will become more rounded or if they will become dull and muddy? Twelve months later, your big spiced chocolate apricot stout is out on your table; your bottle opener in hand. Anticipation builds as you reflect on the plethora of aldehydes, ketones, esters and lactones swimming around your glass. Who am I kidding, it was more along the lines of: “hot-damn, I forgot about this beer, let’s drink it.”
Chemically speaking, beer is a water-ethanol solution containing hundreds of different molecules. These molecules come from the grain, hops, water, and yeast that we brew with. The process of mashing, boiling, and fermenting introduces new compounds and changes others. Everyone should be familiar with the primary process of glucose being converted to ethanol and carbon dioxide. However, we will discuss what happens to your beer after fermentation. And, since we are not making a neutral spirit, we have a wide variety of molecules and processes to contend with.
The components of freshly bottled beer are not in balance. Instead, they are in a dynamic state of change, under the influence of their environment. Consequently, molecules are subjected to many reactions during storage, the product of which determine the sensory characteristics of the beer. Anyone who has found an old, dusty homebrew or participated in a vertical tasting of the same beer over many years can appreciate how flavors can change, for better or worse. Since homebrewers don’t usually have access to the equipment used to measure the presence and abundance of hundreds of molecules in their beer (usually mass spectrometry (MS) or nuclear magnetic resonance (NMR)), we rely on our own sensory equipment (schnoz and kisser to be technical).
I emphasize the sensory aspect of aging in order to relate the molecular evaluation of beers to what most of us are tasting. An article describing the dynamics and influence over time of dimethyl trisulfide and gamma-nonalactone would be useful for some, but it is easier to relate to descriptions of the change in oniony and fruity/peachy flavors instead. For pallet training, I recommend a simple sensory test using your favorite BMC beer. This process can dramatically improve your recognition of flavors in beer and likely make you a better homebrewer. These products can usually be found at the grocery store or local homebrew shop (try Hops & Berries in Fort Collins):
● Acetaldehyde: imitation apple flavoring (8 drops/12 oz. beer)
● Diacetyl: imitation butter flavor (4 drops/12 oz.)
● Acetic: vinegar (25 drops/12 oz.)
● Lactic: lactic acid (20 drops/12 oz.)
● Astringent: Tannin (⅛ tsp into ¼ cup water/ 6 pack) - this is a tricky one
● Phenolic: chloraseptic or sore throat spray (8 drops/12 oz.)
● Dimethyl sulfide: canned corn (¼ cup strained liquid/12 oz.)
● Stale: small piece of cardboard in 12 oz.
● Meaty/Soy: soy sauce (5 drops/12 oz.)
At the very least, take two bottles from the same batch and store them at different temperatures for an advanced aging experiment. Put one bottle in the fridge for 10 days and hold the other bottle at 100F for 10 days (or as warm as you can get it).
Over time, particular compounds associated with particular flavors increase and decrease in a dynamic fashion, as different compounds form and degrade. For example, the compound (E)2-nonenal (an aldehyde), despite varying reports of its specific contribution, may be responsible for the cardboard flavor in stale beer. But is our beer actually stale because this compound is measured at a particular level? Not necessarily. We evaluate our beer based on taste. Of course, we are tasting the different contributions of these molecules, but the presence of off-flavor-associated molecules do not necessarily constitute an actual off-flavor in aged beer. This may be because we expect the beer to taste a certain way.
The formation and degradation of many compounds is possible over time. However, their relevance to beer aging is determined by the reaction rates under practical storage conditions (eg. temperature) and the taste of the brewer. This is where things become subjective. Different people prefer different flavors, and even beyond that, people perceive flavors differently. An objective measure of the abundance of molecular features may not coincide with the subjective taste of the brewer.
In 1977, C.E. Dalgliesh came up with a chart generalizing the flavor changes in beer over time. The ribes flavor and aroma is the aroma of Blackcurrant (Ribes nigrum), described as slightly fruity, but mostly like cat-piss, thanks to thiols (skunk and garlic aroma) and terpenes (why pine sap and hops smell so strongly). From this chart, we can see that bitterness and ribes declines over time while sweetness and staleness increases over time. However, since we are not tasting these components individually, we need to think about the contribution of all the flavors at a particular time. Taking all compounds into consideration, how will the flavor of the beer at time 1 be perceived compared to time 2? What are the flavor thresholds of the compounds that are responsible for the particular flavors?
Now, here you are thinking, “well, that depends...” Of course it does. Beer aging depends on many things. In terms of the environment, what can be controlled? Light exposure, temperature, and stability come to mind. The best place to age your beer is somewhere dark, cool, and still. If those variables are controlled, the next are process variables. The most important variable of this group is oxygen. The impact of oxygen on long term storage will require its own article. In conjunction with temperature, oxidative processes will have the most impact on the flavor stability of your beer over time. The next major dependency is style. It is well known that particular styles age better than others. But what is it about the differences between these two groups that influences how they age? What features of a beer make it more amenable to aging?