Hop packets are convenient and easy to use. Simply open the packet, weigh the required amount of hops, and add them to the brew according to the recipe. However, it is quite common not to use the entire packet, leaving some hops behind.
What happens to the bittering and aroma compounds once the packet has been opened? And how does the storage temperature affect them afterward?
Most craft beers are brewed using either hop cones or hop pellets. These two products are very similar in composition and represent the most natural form of cultivated hops. As a result, they retain all the characteristic properties of each hop variety.
In all cases, production begins with the cleaning of dried hop cones. At the end of the process, the products are packaged in oxygen-barrier, airtight bags filled with inert gas to minimize degradation and preserve their aromatic qualities over time.
Under these conditions, hop cones can be stored for up to four years and hop pellets for up to six years, provided the packaging remains unopened and the storage temperature stays below 5°C (41°F). In breweries, however, a packet is not always used entirely, and the remaining hops may be used several weeks later. It is precisely this situation that prompted storage trials focusing on the evolution of bittering and aroma compounds.
Description of the Trials
Hop pellets from two varieties—one aroma variety and one bitter variety—were stored in opened bags for six weeks at temperatures of 10°C (50°F) and 20°C (68°F). Regular analyses were then carried out to monitor their evolution. Bitter compounds were measured weekly, while aroma compounds were analyzed every two weeks using the official Analytica-EBC methods.
Evolution of Bitter Compounds
The bitter compounds in hops, particularly alpha acids, degrade relatively slowly after the packet is opened. Over several weeks, the reduction remains limited, especially when the hops are stored under refrigerated conditions.
At room temperature, a measurable decrease appears after approximately six weeks, although it remains moderate. For aroma varieties, the maximum observed loss is around 7%. For example, hops initially containing 4.0% alpha acids decline to approximately 3.7%.
By contrast, when stored under refrigeration, this degradation remains almost negligible over the same period. This confirms that bitter compounds are generally stable, especially when storage conditions are well controlled.
Evolution of Essential Oils
Essential oils, which are responsible for hop aroma, change much more rapidly and significantly. Within the first two weeks after opening, aroma hop varieties can lose between 20% and 30% of their essential oils. After six weeks, only 60% to 75% of the original oil content remains.
Degradation is therefore rapid, particularly when hops are stored at room temperature.
Bitter hop varieties also experience losses, but in a more gradual manner. After six weeks, losses are approximately 15% under refrigerated storage and up to 25% at higher temperatures. Unlike aroma varieties, the first two weeks of storage do not result in significant losses.
This difference is partly explained by their composition: bitter varieties generally contain a higher initial concentration of essential oils. As a result, the same absolute loss represents a smaller percentage decrease.
Which Aroma Compounds Are Lost?
Not all essential oils behave in the same way. The overall reduction is mainly due to the evaporation of highly volatile compounds.
Myrcene is the primary contributor to this loss. Because it is extremely volatile, it evaporates quickly as soon as the hops are exposed to air. Humulene also contributes to the reduction, although to a lesser extent.
In contrast, compounds such as linalool and caryophyllene are much more stable and remain relatively unchanged even after several weeks.
In addition, higher storage temperatures promote chemical reactions, particularly oxidation. These reactions lead to the formation of new compounds, altering the hop aroma profile despite the overall decrease in essential oil content.
Key Takeaways
Overall, bitter compounds are relatively resistant to storage in opened packets, particularly under refrigerated conditions. Aroma compounds, on the other hand, are far more sensitive and change rapidly, especially during the first few weeks after opening.
The losses mainly affect the most volatile molecules, while heat significantly accelerates degradation and transformation processes. As a result, aroma hop varieties are more fragile than bitter varieties once the packaging has been opened.
Practical Storage Recommendations
To minimize these effects, opened packets should be carefully resealed while removing as much air as possible. Refrigerated storage—or even freezing—can significantly slow down the chemical reactions responsible for quality loss.
It is also advisable to purchase packaging sizes that match actual brewing needs in order to avoid storing leftovers for extended periods. In general, the shorter the time between opening and use, the better the hops will retain their quality.
In Summary
Opened hops remain perfectly usable, but their characteristics change rapidly, particularly in terms of aroma. Depending on how they are used—whether for bittering or for aroma additions—these changes can have a greater or lesser impact on the final flavor profile of the beer.
This article is based on research conducted by Sandro Cocuzza, Brewing and Beverage Technology Engineer (Technical University of Munich / Weihenstephan) and Technical Support Manager at Simon H. Steiner, Hopfen GmbH.
