I’ve just finished relocating to a new city so it seems like a good time to dust off the ol’ blog and create some content! Let’s pretend that I haven’t left you all hanging for more than a year without any new beer sensory science content and get down to it with a short literature review:
Many brewers and beer aficionados already know that one of the first ways that beer degrades as it ages is by the loss of the hop aromas which are often considered to be marquee flavors in many products and styles. As such, if one wants to know how to extend the shelf life of beer and maintain a fresh-tasting product for as long as possible some investigation into how these aromas are lost is warranted.
This paper explores the various ways that hop oils (a major source of hop aroma) are lost throughout the shelf life of beer and focuses mostly on the loss of the aromas into packaging materials like the rubbery plastic liners under the bottle caps or crowns. It was published in the Journal of the American Society of Brewing Chemists in 1988 and written by Val Peacock and Max Deinzer – a former AB chemist and hop guru, and an experienced analytical chemist from the Oregon State University chemistry department, respectively. Both of these men have been extensively involved in beer research for years, and hop research in particular, so they know their hop chemistry; I can’t think of many too many more researchers more capable of attacking this question. Let’s see what they have to say about this.
First, the researchers present data from some analyses they performed on commercially-available products: a “super premium American brand” (Beer 1), a “Central European Import” (Beer 2), and an “American product from a mini-brewery” (Beer 3). Flavors were extracted from these beers via continuous liquid-liquid extraction with dicholormethane and prepared with 2-octanol as an internal chromatographic standard. In addition to analyzing the beer itself, they removed the foamed-PVC crown liners and extracted them in hexane prior to being made up for gas chromatography/mass spectrometry analysis. Relative concentrations of analytes were calculated by finding the ratio of the amount found in the beer vs. the crown liner. Analytical results for roughly 36 flavor-active compounds (15 from hops) are presented, with concentration values for both the beer and the crown liner indicated. Overall, they found that the more polar, or less oily, the compound, the less it migrated into crown liners. Therefore, alcohols and the water-soluble esters (like isoamyl alcohol and isoamyl acetate) were not found in liners in any appreciable levels (0% and 2% found in crown liners, respectively), while the non-polar compounds, like the hop terpenes and sesquiterpenes myrcene and humulene as well as the long-chain fatty acid esters, were found only in the crown liners. Other hop aromas, like terpene alcohols, linalool, and geraniol, were only found in the beer.
In order to understand the rate of uptake of some of these compounds into the crown liners the researchers created model systems of non-carbonated 3.5% and 3.0% (v/v) ethanol/water solutions and spiked known amounts of several hop-derived compounds, then re-crowned the bottles and stored them for 18 and 28 days, respectively. In the 18-day 3.5% ABV model, 79-87% of the hop-derived hydrocarbons (myrcene, caryophyllene, and humulene) were lost to the crown liners. As was seen in the commercial beer analysis very little, if any, of the water-soluble compounds were detected in the crown liners. In the 3.0% ABV model system after 28 days of storage, the researchers found that only small amounts of the oxygenated hop compounds (alcohols, epoxides, and diepoxides) were captured by the crown liners. Some of the results ran counter to what was seen in the previous analysis, and it was speculated that either some of the compounds degraded by oxidation after they were captured by the liners, or that the 3.0% uncarbonated model system was different enough from the other beers analyzed and that this unpredictably affected the results.
Finally, the researchers looked at the rate of decomposition of four hop aroma compounds which they had spiked into a “premium American beer” (implied later to not be a pilsner): linalool, geraniol, humulenol II, and humulene diepoxide A. Beers were stored at room temperature for about 60 days to simulate warehouse and market storage. 11% of linalool was lost after 57 days, and the steep-then-level nature of the decomposition curve indicates that the degradation of linalool is not a first-order reaction and implies that there are other factors at play in the decomposition of linalool – either that there is an equilibrium that is reached or that linalool is reacting with beer components that also get depleted over time, such as oxygen. Breakdown products of linalool were analyzed in the final (57-day) sample and the amounts found only account for 10% of the lost linalool, which is somewhat puzzling – perhaps there are other breakdown products which were not realized in this study. Geraniol behaved similarly to linalool: 12% lost in 56 days, with the majority lost in the first couple weeks and few anticipated breakdown products detected. Humulenol II degraded much more rapidly than linalool and geraniol, with 66% being lost after 61 days. While the decay curve isn’t as “curvy” as the previous compounds, it still leveled off somewhat. They also found some additional compounds in the final sample which they guessed were humulenol II breakdown products, as there was none of these detected in the fresher samples, nor in the linalool/geraniol samples. GC-MS results implied that both oxidation and acid-hydrolysis were at play. Lastly, humulene diepoxide A decayed the fastest of the four compounds, where 84% of it was lost at 56 days in a nearly-linear rate. Numerous supposed degradation compounds were detected, but the reactions are so complex that identification was not feasible.
Overall, this paper provided an interesting look into a couple of the main reasons that hop aroma is lost in aging beer: adsorption/absorption into crown liners (and likely aluminum can liner material as well!) and oxidation/acid hydrolysis reactions leading to their conversion to other compounds, both flavor-active and not. When one considers both the importance of hop aroma to so many craft beers and the fragile nature of hop aroma, it seems like some attention should be paid to maintaining sufficient hop aroma over time.
Fate of Hop Oil Components in Beer. Val E. Peacock and Max L. Deinzer, Department of Agricultural Chemistry, Oregon State University, Corvallis 97331. J. Am. Soc. Brew. Chem. 46:0104, 1988.