How can the HSI be correctly assessed for brewing purposes?

As soon as hops have been picked and sufficiently dried, a race against time begins, or more precisely against oxidation, as the oxidation reactions reduce the quality of the hops. The number of oxidation reactions is impressive (1). The oxidation of alpha-acids produces humulinones, which add an additional, albeit less intense, bitterness to the beer with dry hopping (2). However, oxidation reactions can destroy a large proportion of the bitter substances and also the hop oils (3). In the process, volatile aroma components are often lost, while others absorb oxygen and can therefore increase a variety of oxygen-containing flavor compounds such as various esters. In order to stop these reactions as quickly as possible, hops are stored cool as soon as possible after harvesting and processed into hop pellets. In the form of pellets, the hops are stored in an inert gas atmosphere and are protected from oxidation. Protection from oxygen is even more important in the form of pellets, as some of the lupulin glands, which have been damaged during milling, would be exposed to oxygen without protection.

The HSI value is used to assess oxidation (4). This is a non-specific photometric method that measures the ratio of oxidised and non-oxidised bitter compounds. However, the method is only suitable for pellets, not for hop extract and also not for isomerised hop products. The HSI can lie between 0.25 and theoretically 1. The higher is the value, the stronger the ageing (oxidation) is assessed. To date, no other method for assessing the freshness of hops has been able to establish itself. Higher HSI values are generally understood to mean poorer quality, but it is not that simple. Years of measurements have shown that HSI values should be understood on a variety-specific basis (5). It has also always been assumed that high HSI values equate to lower brewing quality of the hops, an assumption that has so far been lacking in brewing trials and objective assessments.

It is certain that factors such as variety, harvest year, harvest time and kilning conditions determine the "starting point" of the HSI. Hop varieties can be clustered according to the "starting values" of the HSI. The varieties Hallertau Mittelfrüh, Hersbrucker, Tettnang Tettnanger, Select, Tradition, Perle, Magnum, Taurus and Herkules can be grouped together and have an HSI starting value of ≤0.275 (variety Cluster A). Saphir, Northern Brewer, and Saazer have an average value between 0.275 and 0.300 (variety cluster B). Aurora, Celeia, Lubliner and Marynka, on the other hand, start with an average HSI value of >0.300 (variety cluster C). The increase in the HSI value also depends on the variety. This variety-inherent tendency remains when the storage temperature changes. For example, Perle, a variety with a low "starting" HSI and a low increase in HSI at cool temperatures, also shows a more restrained increase in HSI value at room temperatures compared to the variety Celeia, which starts with a high HSI value and also reaches higher HSI values more quickly at cold temperatures. With the knowledge that the HSI values should be assessed on a variety-specific basis or by variety cluster, the original categorisation of the HSI values can no longer be universally valid.  Table 1 shows the original interpretation of the HSI values Tab. 2 a slightly modified version from 2003. Table 3 would be a proposal for a future version of the table in which different HSI initial values and different transformation values are used depending on the variety cluster.

 

Table 1: Original Table with degree of transformation and aging degree according to the HSI (6)

Table 2: modified Table with degree of alpha-acid deterioration and aging degree according to Forster

Table 3: Proposal for future understanding of Transformation degree and HSI value depending on variety Cluster 

How do the HSI values of the hops affect brewing?

In order to investigate the influence of the HSI value on the quality of a beer brewed with it, the US variety Citra® and the Australian variety Galaxy® were used in the form of T90 pellets. The pellets were flushed with oxygen to achieve the corresponding HSI values. It is not surprising that the concentration of alpha-acids decreases with increasing HSI values. For Citra, the HSI was adjusted to 0.430 and 0.610, which resulted in a loss of 15% and 38% alpha-acids, respectively. For Galaxy®, the HSI was “aged” to 0.420 and 0.630, which resulted in a loss of 16% and 47%, respectively. The oil content is 2 ml/100g for Citra® and drops by 40% to 1.2 ml/100g. Galaxy® has an initial content of 2.3 ml/100g and drops by 50% to 1.15 ml/100g.
Detailed GC analysis showed that the content of myrcene in particular decreases with increasing ageing in both varieties, as do caryophyllene and humulene to a lesser extent. On the other hand, an increase in oxidised sesquiterpenes or other compounds could not be shown. As myrcene, caryophyllene and humulene are hardly important for the typical flavor of Citra® and Galaxy ®, this may not be a great loss. 
These varieties were used to brew a top-fermented ale. The bitterness of the beers was adjusted with CO2 extract. No hops were added for late hopping. The brew was divided into smaller batches and each batch was dry hopped equally with 5 g/L hops. The addition rate of 5 g of hops/L naturally means less hop oil/litre for the beers with the aged hop additions (see table 4).
 

Table 4: Hop addition (in oil equivalent) and results of sensory difference testing.

Triangle tastings revealed significant sensory differences. Only the test with the Citra beer with fresh hops compared to the beer with aged hops was not significant. The flavor profiles of the beers show that the intensity of the fruitiness of the beers decreases and the intensity for spicy and woody increases. This is more pronounced in the beers with the Galaxy variety than in the beers with the Citra variety. The GC analyses of the beers confirm the sensory results in the sense that the oxidised sesquiterpenes known to cause spicy/woody aromas increase in concentration in the beers with aged hops (see figures 1 and 2).

Figure 1: BarthHaas Hopsessed® attributes quantitative descriptive analysis of Citra® beers with their p-values

Figure 2: BarthHaas Hopsessed® attributes quantitative descriptive analysis of Galaxy® beers with their p-values.

Flavor intensity/quality, bitterness intensity/quality in the beers and preference of the beers were also assessed. There was a preference for the beers with fresh hops, where the aroma intensity was also rated the highest. The bitterness intensity was rated higher for the beers with the aged hop samples, which can be explained by the increasing humulinone content in these beers. However, the bitter quality of these beers was rated lower.
 

Table 5: Rating of the Citra® and Galaxy® beers (8).

Previous brewing trials working with the varieties of Perle and Saaz yielded in similar results. Here the HSI values of Perle were 0.270 vs 0.470 and for Saaz 0.310 and 0.490. Here we can already see the different starting HSI values of two varieties from different HSI clusters. The brewing trials combined bittering, aiming for 20 mg IAA/L, whirlpool hopping with 5 g/L and Dry Hopping at 2 g/L. Also, here results show significant differences between the beers with hops of low and elevated HSI values and higher bitterness intensity in the beers dry hopped with aged hops. However, the bitterness quality in the beers only hopped for bitterness purpose was rated as equal, independent of the HSI value of the used hop. 

Summary

The freshness evaluation of hops should be carried out on a variety-specific basis with the awareness that certain hop varieties already start with a higher HSI value. Furthermore, the increase in HSI values during storage under oxygen or at warm temperatures is also variety-specific. Even if higher HSI values do not have a negative influence on beer bitterness, they do result in losses in terms of fruitiness of hop flavor, aroma quality, bitterness intensity and bitterness quality when used for dry hopping. 
Optimal storage of hops must exclude oxygen. Poor storage conditions reduce the alpha-acid and oil content and therefore also the bitterness and flavor profile of the beers brewed with them. The increase in bitterness intensity can be attributed to humulinones (oxidised alpha-acids). Brewing trials show an individual behaviour for each variety. Beers brewed with varieties such as Saaz or Galaxy were more affected by higher HSI values than beers brewed with Perle or Citra. These trials also showed that the differences in the flavor profile of beers hopped with HSI values of 0.320 and 0.430 for Citra® and 0.310 and 0.410 for Galaxy® were smaller than the differences in the flavor profiles of beers hopped with hops with HSI values of 0.610 and 0.630.
The HSI is valuable information for the brewer, but it should not be considered in isolation but with the knowledge of which factors it influences. Sensory analysis is still the method of choice for determining the quality of hops. A trained tasting panel can easily and reliably work out quality differences in aroma. Together with the HSI value, the value of the total oil and the alpha-acids, the brewer can judge the quality of his hops very well.

References

1)    Taniguchi Y, Matsukura Y, Ozaki H, Nishimura K, Shindo K. Identification and quantification of the oxidation products derived from α-acids and β-acids during storage of hops (Humulus lupulus L.). Journal of agricultural and food chemistry. 2013;61(12):3121–30.  
2)    Algazzali V, Shellhammer T. Bitterness intensity of oxidized hop acids: Humulinones and hulupones. Journal of the American Society of Brewing Chemists. 2016;74(1):36–43.  
3)     Rutnik K, Ocvirk M, Košir IJ. Changes in hop (Humulus lupulus L.) oil content and composition during long-term storage under different conditions. Foods. 2022;11(19):3089.  
4)    Nickerson GB, Likens ST. Hop storage index. Journal of the American Society of Brewing Chemists. 1979;37(4):184–7.  
5)    Zunkel M,.oral presentation at Young Scientist Symposium, April 2016, Chico, USA
6)    Zunkel M, Kritisch betrachtet – HSI Messmethode und Einflussfaktoren, der Doemensianer, (1), 2021: 41-43
7)    Forster A, The quality chain from hops to hop products, EBC Proceedings 29th Congress, 2003
8)    McMillan J, The relevance of Hops Storage Index (HIS) for Hop Usage, Brewing Science, 2023, accepted, in print