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What happens during hop aging?
The oxidative processes that take place in hops from the time of harvest affect two important groups of constituents: the α- and β-acids on the one hand and the essential oils on the other.
This means that the degradation or oxidative conversion of the bitter substances not only inhibits the bitter potential of the hops. The aroma profile also changes during hop ageing: a large proportion of the volatile fraction can be lost through evaporation, degradation and polymerization.
In particular, the extent of the changes depends on the physical damage suffered by the lupulin glands during harvesting, baling, kilning and subsequent packaging.
At room temperature and in the presence of oxygen, hops can lose up to 50 percent of their essential oils.
Hops are like beer … a fresh productWhat does hop ageing mean for the brewer?
The brewer must not only expect a reduced intensity of bitterness with aged hops. This is because oxidized α-acids are highly soluble in water and are attributed an unfavorable bitterness.
The aforementioned change in hop aroma also leads to a change in the flavor imparted into the beer by the hops.
Hop Storage Index, or: How do you measure hop ageing?
The Hop Storage Index (HSI) is measured using a non-specific spectrophotometric method (Method ASBC Hops 6A + 12). This is a unitless ratio of the oxidized hop acids (maximum absorption at 275 nm) to the fresh hop acids (325 nm).
HSI=A275/A325
If the proportion of oxidized hop acids increases during hop storage, while fresh hop acids are simultaneously broken down, the ratio increases and the HSI rises.
HSI values can be divided into different degrees of transformation, starting from a theoretical initial value of 0.25. The initial value stands for a zero-percent transformation, a hop with an HSI of 0.31 shows a ten-percent transformation. HSI values of 0.31–0.40 stand for normally stored and processed hops and a degree of transformation between 10 and 21 percent. HSI values greater than 0.60 represent hops that have passed their sell-by date, with a degree of transformation of more than 40 percent.
The measurement of HSI has its limits. The method is well suited for pellets, hop cones and also for CO2 extract. However, it should be noted that the values of the CO2 extract do not correspond to those of cones and pellets. The CO2 extraction „rejuvenates“ the HSI to a certain extent, as non-polar components, precisely the oxidized hop acids, are not extracted.
It is not always possible for breweries to use freshly harvested hopsWhy are hops from older vintages used and what needs to be considered
There are two main reasons that force brewers to use hops from older vintages (not necessarily aged!). In years with a hop surplus, some brewers buy more hops than they need. This may lead to the expiry of the best-before date. On the other hand, in years with a shortage of hops, brewers have to fall back on hops from older vintages.
The largest quantity of hops is traded on the basis of their α-acid content. It is therefore in the interest of everyone involved in the value chain to avoid oxidation in the product. Hop bales must therefore be processed by the trader as quickly as possible after harvesting, dried gently, stored cold and in a protective atmosphere (under N2, CO2).
Proper storage conditions must also be ensured in the brewery. Opened containers must be used as quickly as possible.
Caution when interpreting the Hop Storage Index
As shown, the HSI is an important, but by no means the only indicator of overall hop quality. In any case, variety-specific aspects must always be taken into account.
In addition to the hop variety, the freshly harvested HSI is also influenced by the year of harvest, the time of harvest and the kilning conditions. These factors can lead to significant variations in HSI.
In an internal study by the BarthHaas Group, in which eleven hop harvests of sixteen different varieties were examined, most of the freshly harvested varieties remained below an HSI of 0.30. The varieties Hallertau Mittelfrüh, Hersbrucker, Tettnang Tettnanger, Select, Tradition, Perle, Magnum, Taurus and Herkules could be regarded as one group with average freshly harvested values of less than 0.275. The varieties Hallertau Mittelfrüh, Hersbrucker, Tettnang Tettnanger, Select, Tradition, Perle, Magnum, Taurus and Herkules could be regarded as one group. Saphir, Northern Brewer and Czech Saaz had mean values between 0.275 and 0.30. Aurora, Celeia, Lubliner and Marynka had mean HSI values of more than 0.30.
Given these data, the typical classification of HSI values cannot realistically be used to define all hop varieties and their corresponding qualities.
It should also be emphasized that the HSI cannot be applied to all hop products. In the analysis of enriched hops – after removal of the coarse fraction of the ground hops – the HSI values were lower than in the corresponding standard pellets. As already noted, the analysis of CO2 extract also does not lead to meaningful interpretable information. Isomerized hop products cannot be measured.
With proper storage, the loss of brewing value in the hops is negligibleVariety-specific freshness assessment of hops
The Hop Storage Index was developed over fifty years ago and is still a popular measure for assessing the freshness of hops in the brewing industry. However, analyses have shown that this evaluation must always be carried out on a variety-specific basis, as some varieties already start with higher values than the theoretical initial value of 0.25. Furthermore, the increase during storage under oxygen is also variety-specific. Also the increase during storage under oxygen is variety-specific. Even if higher HSI values, again depending on the hop variety, do not have a significant negative influence on beer bitterness, there are differences in terms of fruitiness and aroma quality, bitterness quality and intensity when used for hop tamping.
Optimal storage of hops must exclude oxygen. If the hops have then been processed into pellets, the increase in HSI is negligible in cold storage.
The HSI is therefore important information for the brewer, but should never be considered in isolation, but rather with the knowledge of which factors influence it. Sensory analysis is still the method of choice for ascertaining the quality of hops. A trained tasting panel can easily and reliably work out quality differences in aroma. Together with the HSI value and values for total oil as well as α and β acids, the brewer can assess the quality of his hops very well.
This article is based on a three-part series of articles on the Hop Storage Index published in BRAUWELT by Dr. Christina Schönberger, Joshua McMillan, Mark Zunkel and Dr. Alicia Muñoz Insa, all BarthHaas Group, Nuremberg.
