From Hop Harvest to the Brewery

When evaluating hop quality, many use the Hop Storage Index (HSI) as a simple analysis to get an indication of how much the hop has been aged. The rule of thumb is – the higher the HSI, the poorer the hop variety was harvested, stored, and processed. Although HSI can be useful tool, it is important to understand that the value should not be the sole indicator to understand the total hop quality.
Hop Storage Index is a well-known and simple measurement using the ASBC HOPS 6A/12 or EBC 7.13 non-specific spectrophotometric method that is the standard measurement of alpha-acids for all hops in the United States. Specifically, the measurement was created to evaluate the degradation of alpha-acids and beta-acids by measuring the ratio of absorbance at 275 nm to 325 nm of an alkaline methanol solution of a non-polar extract of hops (1).
In Part 1 of this series, HSI was described as having a theoretical starting value of 0.250 which is the value that represents a 0% transformation. A hop with an HSI of 0.310 contains a transformation of 10%. Measuring harvest fresh whole cone hops or pellet hops can even display a negative percent transformation if the results are <0.250, which can happen, although normally not found in freshly processed pellet hops.
Although the economic factor of the loss of alpha-acids is important for breweries storing and using hops, the HSI carries the burden of being interpreted as the sole indicator of total hop quality. This premise leads to the improper evaluation of many hops that possess no lower quality based on their high HSI value. The HSI can be used as a hop quality indicator but necessitates hop variety specific considerations. This article will describe the various factors that can influence HSI and how best to interpret the values.
 

Harvest Fresh Hop Storage Index
Harvest fresh HSI is influenced by the hop variety, harvest year, time of harvest, and kilning conditions. Based on these factors, looking at every harvest year and variety, the variation can be significantly different.
A study was internally completed analyzing eleven hop harvests of sixteen different European hop varieties: Hallertau Mittelfrüh, German Hersbrucker, Tettnang Tettnanger, Spalter Select, German Tradition, German Saphir, German Perle, German Northern Brewer, German Magnum, German Taurus, German Herkules, Czech Saaz, Slovenian Aurora, Slovenian Celeia, Polish Lubliner and Polish Marynka. In Table 1, the mean, minimum and maximum values are shown for these varieties in each harvest year, analyzed as whole cone hops directly after harvest.
Year-to-year variations occur between varieties, while most harvest-fresh HSI values remain below 0.300. The varieties Hallertau Mittelfrüh, Hersbrucker, Tettnang Tettnanger, Select, Tradition, Perle, Magnum, Taurus and Herkules can be grouped with a mean in their harvest-fresh values ≤0.275. Saphir, Northern Brewer, and Czech Saaz show mean values between 0.275 and 0.300. Aurora, Celeia, Lubliner and Marynka contain an mean HSI of >0.300.

Table 1: Mean, minimum and maximum of thirteen European hop varieties from 2009-2019. The varieties Aurora (2012-2019), Celeia (2013-2019), and Marynka (2009-2012) were collected only for the indicated years. The colors indicate the minimum to maximum values: dark green < light green < yellow < orange < dark orange < red.

The maximum values above 0.250 do cause problems with the evaluation of HSI, as the percent transformation then begins for many varieties not at a theoretical 0.250 but at a significantly higher value, which delivers a false interpretation of the percent transformation. In some harvest years, lots were analyzed with a maximum value of >0.400 for harvest fresh Celeia, Lublin and Marynka. Based on this data, the typical classification for HSI values cannot be realistically used to define all hop varieties and their equivalent quality at these HSI values.
The time of harvesting hops and its correlation to HSI was investigated by Cocuzza, et al. (2). They looked at three different aroma and three bitter varieties over two harvest years in Germany. The varieties investigated were Magnum, Herkules, Taurus, Perle, Tradition and Saphir. The bitter varieties clearly showed a higher HSI the later they were harvested over the time of 40 days, showing a maximum difference of 0.100 between early and late harvested hops. For the aroma varieties, the differences were lower between the varieties, sometimes showing no significant differences between the time period of 28 days.
When looking at the kilning temperatures of hops, there are significant differences in the HSI. The kilning temperatures of 140°F (60°C), 150°F (65°C), 160°F (71°C) and 170°F (77°C) were investigated using US Cascade whole cone hops (3). The researchers saw a clear relationship that the higher the kilning temperature, the higher the HSI. The transformation was 15% higher between 140°F (60°C) and 170°F (77°C).

Hop Storage Index and Storage of Hops in Bales
Once hops are in bales, the most important factors for determining the quality and value of hops are time, temperature and the exposure to oxygen. This storage time is important to the increase of HSI as well as losses of alpha-acids. At this stage, hops are stored cold before processing into further hop products - hop pellets and CO2 extract. These products provide a stable form that protects the valuable compounds of hops in an inert atmosphere (N2 or CO2).
In addition to the storage conditions, the compression of US Cascade whole cone hops in the bale has been shown to have an influence on the HSI value. A high compression pressure crushes the lupulin glands and exposes them to oxygen (3). The lupulin glands contain all the valuable alpha- and beta-acids and hop oil that needs to be protected by an intact lupulin gland. At the beginning of storage, the HSI did not significantly change, although over the time of twelve months a 15% higher transformation was showed compared to hop bales baled with a lower pressure. In this case, the US bale weight of 195 lb (88 kg) for regular weigh bales, and 298 lb (135 kg) for heavy weight bales was applied. 
Once the whole cone hops are in bales, they should be stored cold, which is the industry standard until further processing. At cold temperatures, the speed of degradation is slowed. Even at the lowest temperatures that are realistically possible for the amount of bales, this is not enough to completely stop the degradation reactions.
Looking closer at the storage of bales over time, internal studies of four different European varieties – Perle, Saphire, Aurora and Celeia - were analyzed at three different HSIs stored at cold and ambient temperatures over a period of ten months. Samples were taken and analyzed every month from September to June for HSI and alpha-acids.
To be expected, the HSI of hops stored at ambient temperature significantly increased in all conditions, and equivalently the alpha-acids decreased, respectively. Independent of the initial HSI, whether low, medium or high, the HSI did not increase at a quicker rate at a higher initial HSI.
The behavior of the HSI was hop variety dependent. Aurora, Perle, and Saphir, whether stored cold or at ambient temperatures, showed a similar relative percent increase in HSI. The exception was Celeia, which showed the largest relative differences between September to June for both the HSI increase and alpha-acid decrease (Table 2).
 

Table 2: The relative difference of HSI and alpha-acids between September to June of baled hops at three different HSI values stored at cold (3°C) and ambient (25°C) temperatures.

Hop Storage Index in Packaged Hop Pellets
The majority of the world’s hop supply is pelletized to be used in breweries. This key process ensures to protect the valuable compounds from further degradation or other unwanted chemical reactions during storage.
Internal studies were undertaken to evaluate two varieties German Perle and US Citra. These were evaluated in packaged inert gas atmosphere at both cold (3°C) and ambient (25°C) temperatures for a period of 48 months. The HSI, alpha- and beta-acids (ASBC 6A/12), as well as the humulone and lupulone homologes were measured (HPLC or EBC 7.7) (Table 3).
For Perle and Citra, HSI at cold temperatures only increased a negligible amount of 7% and 9%, respectively. This can also be reflected in the alpha-acid loss, irrespective of the method of analysis, where the losses were between 2-8% for both varieties. This shows the excellent stability of packaged hops in inert gas stored at cold temperatures.
At ambient temperatures, the variety stability varietal differences. The HSI increased significantly more in Citra at 99% compared to Perle at 69%. Oppositely, the alpha-acid decrease of Citra shows a lower rate of degradation compared to Perle. The lupulone (+homologes) negligibly decrease at ambient temperature. This can only point to other auxillary bittering compounds affecting the non-specific HSI measurement.

Table 3: The relative decrease of alpha-acids (EBC 7.5), increase of HSI, and decrease of alpha- and beta-acids (ASBC 6A/12), and EBC 7.7 analysis of humulone and lupulone homologes of German Perle and US Citra at cold (3°C) and ambient (25°C) temperatures over the storage time of 48 months.

Hop Storage Index for all hop products?
The previous data covered whole cone and normal (T90) pellet hops. When analyzing enriched hops, by the removal of the coarse fraction of the milled hops, the HSI values are lower than their equivalent normal pellets. Furthermore, HSI cannot be applied to other hop products. Although CO2 extract can be analyzed, it does not provide relevant information. During extraction of hop pellets, polyphenols are not extracted to the final product, which creates a lower HSI value compared to the pellets used for the CO2 extraction. Isomerized pellets and other hop products are unable to be measured for HSI.

Can Hop Storage Index be used for hop quality?
The HSI value should be used with caution. The assessor should be knowledgeable about the processing of hops to pellets and factors that influence the HSI. The hop variety needs to be taken into consideration when evaluating HSI values.
To evaluate whether a hop pellet has been incorrectly stored, an HSI analysis should be completed before use of the hop. The HSI at time of processing compared to the HSI at the time of purchasing or brewing will be able to give a good indication of its storage conditions.

Conclusion
Hop Storage Index was developed over 50 years ago and is still actively used in the brewing industry. This article has described factors that influence Hop Storage Index from the harvest to use in the brewery. During the harvest, some factors including the hop variety, harvest year, time of harvest, and kilning conditions affect the harvest fresh HSI value. Data shows that many hop varieties can be grouped into three different harvest fresh values with some varieties showing year-over-year average values >0.300.
The pressure of the baling affects the HSI, where the higher the baling pressure showed a higher HSI after storage. After baling whole cone hops and storing in bales, the main factor that affects the HSI (and alpha-acid losses) is the storage temperature. Finally, once hops are pelletized and packaged in inert gas, the increase of HSI is negligible over many years when stored cold.
HSI does not have the ability to be the final quality measurement for hops. If a brewery does want to know the hop quality from the HSI analysis, it should be completed before use in the brewery to understand the storage conditions when the initial packaged HSI is known. The hop variety needs to be taken into consideration for the final evaluation. In this way, each hop variety can be determined individually for its use and evaluated for each beer brewed.

Literature     
1.    Nickerson GB, Likens ST. Hop storage index. Journal of the American Society of Brewing Chemists. 1979;37(4):184–7. 
2.    Cocuzza, S., Lutz, Anton, Müller-Affermann, Konrad. Influence of Picking Date on the Initial Hop Storage Index of Freshly Harvested Hops. Master Brewers Association of the Americas Technical Quarterly. 2013;50(2):66-71. 
3.    Weber, K., Jangaard, N., Foster, R. Effects of Postharvest Handling on Quality of Storage Stability of Cascade Hops. Journal of the American Society of Brewing Chemists. 1979;37(2):58-60.