Hops don’t like to get old, either

The brewing value of hops is determined by the amount and composition of its secondary metabolites, α-acids, β-acids, and essential oils which are synthesized and accumulated in the lupulin glands. The amount and quality of these metabolites is influenced by the variety and growing conditions as well as harvest and further processing, packaging, and transport. Exposure to elevated temperatures and oxygen during these processes will be detrimental to the brewing value (1). When hops age, chemical changes occur that reduce their brewing value. As long as the hop cone is on the hop bine growing in the field, no oxidation happens. As soon as the hop plant is harvested, the clock starts ticking before hops are packaged. The enemies of hops are high storage temperatures and time, as well as whether they are stored in oxidative or non-oxidative conditions.
 

What happens during oxidation?

The bitter taste in beer originates mainly from the isomerization of the α-acids into iso-α-acids. The α- and β-acids found in hops are composed of different homologs with similar chemical structures. The main homologs in the α-acid fraction are co-humulones, n-humulones, and adhumulones. These three substances can represent up to 98-99% of the total α-acids. Analogously, the main homologs found in the β-acids are known as co-lupulones, n-lupulones, and adlupulones.
When hops oxidize, the α-acids and β-acids oxidize to humulinones and hulupones, respectively. During hop storage, several oxidation products arise. Taniguchi et al. (2) showed a decrease in α- and β-acids when storing hops at 60 °C for 48 hours and the increase of mainly humulinones and hulupones. Similar to α-acids and β-acids, humulinones and hulupones were found as a mixture of the homologues co-, ad, and n-humulinones and co-, ad, and n-hulupones respectively. Taniguchi et al. (2) were also able to identify two additional oxidation compounds, 4′-hydroxy-allo-n-humulinone and 4′-hydroxy-allocohumulinone and suggest their origin to be derived from the degradation of humulinones. Hao (3) summarized the formation pathway of other oxidation products found in hops such as abeo-isohumulone, humulinones, 40-hydroxy-allohumulinones, 40-hydroxyallo-cis-humulinones, cis-oxyhumulinic acids, scorpiohumuli-nols A/B, dicyclohumulinols A/B, hemiacetal of tricycloperoxyhumulone A, tricycloperoxyisohumulone A, deisopropyltricycloisohumulone, tricyclooxyisohumulones A and B.
The degree of change of the valuable components in hops additionally depends on the extent of physical damage to the lupulin glands during harvesting, baling, kilning, processing, packaging methods and material, and the subsequent storage temperature (4). Oxidation not only has an impact on the bittering properties imparted by the α- and β-acids but also on hop aroma. Upon oxidation, the amount, quality, and composition of the essential oil fraction will be altered. During aging, hops lose a large portion of their volatile fraction of the essential oil due to evaporation, degradation, polymerization or oxidation. Beatson et al. (5) indicates that the amount of essential oil from hops stored at room temperature can be reduced by 34-53 % within 6 months in the presence of oxygen. 
Rutnik et al. (6) studied the changes in the oil fraction during storage under aerobic and anaerobic conditions and stated that during storage, not only does evaporation of compounds occur, but other biotransformations also takes place. The main changes described in the literature are a decrease in the concentration of hydrocarbons, which leads to an increase in oxygenated compounds during aging; a decrease in the concentration of terpenes, which results in an increase in the concentration of some esters and oxides; an increase in certain carbonyl compounds as well as sesquiterpenes. Moreover, Rutnik et al. (6) determined that the absence of myrcene is a good indicator for old and improperly stored hops, as myrcene was completely lost in the stored samples.
Previous to pelletization, hops are milled into powder which crushes the bract and bracteoles as well as the lupulin glands where the α-, β-acids and essential oils are accumulated. If hop pellets are not packaged properly (ex. inert gas atmosphere, high quality packaging foils and proper seals), milling leaves the crushed lupulin glands exposed to oxygen and therefore a quicker oxidation will take place compared to that of intact lupulin glands, i.e., hop cones. As a consequence, the hop cones have been proved to be more stable in aerobic conditions than pellets (6).
 

What does this mean for the brewer?

In general, oxidation reduces the brewing value of hops. Oxidation of α-acids and β-acids result in a decreased bittering potential due to the change in bitterness intensity and quality. In addition, oxidized α-acids or humulinones are highly soluble in wort, not able to isomerize during the boiling process, and have shown a lower bitterness intensity. Conversely, oxidized β-acids contribute to bitterness, but in a less favorable way.
Some studies have estimated the bitterness intensity of humulinones and hulupones, however the exact value is still not agreed upon. Compared to iso-α-acids, the estimated bitterness intensity of humulinones and hulupones range between 35%-66% and 50%-84%, respectively (7–9). Despite conflicting data, all estimations suggest that humulinones and hulupones are less bitter than iso-α-acids, and thus it can be concluded that oxidation reduces the bittering potential of hops. Hulupones are generally found at a lower concentration in beer compared to humulinones.
In addition, oxidation affects not only the α- and β-acids, but also the hop oil composition. Oxidation of hop aroma compounds, primarily the essential oils of hops, occurs at a rate similar to that of the hop bitter acids. The shift in the ratios of the components in the essential oil fraction will result in a change in the aroma of the hops and also in the aroma and flavor that they impart in beer.
 

How to measure hop aging?

The standard method in the brewing industry to measure hop aging is the Hop Storage Index (HSI). The HSI is used to estimate losses of α-acids and β-acids during storage and handling (10). HSI is measured using a non-specific spectrophotometric method (Method ASBC Hops 6A + 12) (11) and is a unitless ratio that indicates the difference between fresh hop acids and oxidized hop acids (see equation below). First, the hops are extracted with an organic solvent, and then the absorbance is measured at 325 nm, the maximum absorbance for hop acids, as well as at 275 nm, the maximum absorbance for oxidized hop acids.

HSI = A₂₇₅ / A₃₂₅

During natural oxidation, the amount of oxidized hop acids and thus the absorbance at 275 nm increases, while the concentration of hop acids and the absorbance at 325 nm decreases. The higher the HSI, generally the more the bitter compounds have been altered by the aging processes (12) and there will be a measurable loss of alpha acids.
According to literature, HSI values can be used to categorize hops into categories of freshness according to the HSI value, as seen in Table 1.

Table 1: Degree of transformation and aging degree according to the HSI

Table 1 can be an orientation for most hop varieties but does not hold true for all hop varieties. This topic will be covered in the second part in this HSI article series. The degree of transformation is a theoretical value that can be understood as the difference of a HSI value compared to 0.25.
The HSI is the simplest method to measure the oxidation of the hops, but it has some limitations. HSI is suitable for pellets and hop cones as well as can be measured for CO2 extract, however the values obtained when measuring CO2 extract are not comparable with those of cones or pellets. A freshly produced CO2 extract will show a HSI of about 0.25 and will increase more rapidly than in pellets if the product is not stored properly. The main reason is that CO2 extraction "rejuvenates" a hop with a high HSI because nonpolar components such as humulinones and hulupones are not dissolved. Furthermore, since the UV spectrum of iso-α-acids also shows a maximum at 275 nm, this method is unsuitable for isomerized products as well as for ethanol extract (13). For these products, the “real” α-acid content should be measured using HPLC.
Some authors have suggested alternative methods to analyze the oxidation of hops. Garden et al. (14) evaluated the use of NIR to measure α-acids, β-acids as well as the HSI of hops. Their results showed the advantages of NIR being fast, easy to perform, with little or no sample preparation or operator training. Also, it is a simple and solvent-free method, but it also showed limitations in its accuracy when predicting HSI values for hop samples that exceeded 0.30.
Jerkovic and Collin (15) found that the presence of cis-resveratrol in hops by means of HPLC could be an indicator of hop freshness. Lermusieau and Collin (16) determined the aroma oxidation products via GC and proved that both the ratios of humulene to humulene + humulene epoxides (I, II, and III) and the volatile compound ratio of bergamotene to farnesene are good indicators of the freshness of hop samples. However, due to the required equipment none of these methods are well established in the hop or brewing industries.
 

Aging and its economic implications

There are two reasons why the brewer is obligated to use old hops (not aged!). First, in years of a hop surplus, some brewers may buy more than what they need, and this oversupply needs to be used by the “best by” date. Second, in years of a shortage, brewers may find themselves in the situation of having to buy hops from an older crop. However, as a number of environmental factors, in addition to harvest year impact the HSI (17), the same variety, harvested in previous years can have a lower HSI than hops form the current harvest.
Moreover, most hops are sold or bought based on the kg of α-acids (kg-α price) in the product. For all these reasons, it is of interest to the hop merchant, as well as to the brewer, to avoid aging and thus α-acid oxidation through the supply chain. To avoid oxidation, the hop merchant should dry the baled hops properly for further storage and processing, store them in cold as soon as possible, and package hop pellets with inert gas such as N2 and/or CO2 (which is standard practice in the industry). Once the hops are in the hands of the brewer, they should store the product cold (or at the recommended temperature by the supplier in the case of extracts), and once opened, use it as soon as possible. If the merchant as well as the brewer apply the best practices in handling, storage, and processing, hop oxidation can be reduced to a minimum.
To avoid selling or using aged hops, it is in merchants’ and brewers’ interest to analyze the current α-acid content and the Hop Storage Index.
 

Conclusions

Hop and hop product quality relies on the use of best industry practices for harvesting, storing, and processing. Three main factors to be considered are an oxygen-free atmosphere (packaging), low storage temperatures (for cones and pellets), and use within the “best by” date. Moreover, to ensure a good quality of hops and hop products, hops should be harvested, stored, and processed according to best practices. If this is ensured, there will be insignificant, if any, changes in the α-acid, β-acid, and essential oil content of the hops.
If the quality of the product is compromised or the brewer has a shortage of hops and needs access to older crops, the HSI can give insight into aging processes and changes in the composition and the brewer could adjust their hopping regime if necessary.
 

Literature

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