The Underrated Number: Impacts of HSI on Beer Aroma and the Hop Over-Supply

Summary

1. The Hop Storage Index (HSI) is essential in determining hop freshness by measuring the ratio between oxidized and non-oxidized hop acids, influencing beer aroma and bitterness.
2. HSI serves as a pivotal metric for brewers, categorizing hops based on freshness and indicating the potential impact on beer aroma, flavor, and bitterness.
3. Upcoming research from Oregon State University aims to establish a metric evaluating the deterioration of hop aroma and flavor, further enhancing brewers' decision-making tools.

Introduction

This year's MBAA conference was a whirlwind of immersive discussions and revelations for brewing aficionados. The atmosphere was thick with excitement, as brewing professionals and enthusiasts delved deep into the nuanced concepts of terroir and vintage. With a lens firmly focused on the chemistry of malt and the unique regional characteristics of brewing water, one couldn't help but sense an underlying current: a growing appreciation for the "old and local," which often seemed to overshadow the appeal of the "new and foreign". However, amidst these vast topics, one term consistently took center stage in my mind – the Hop Storage Index, commonly referred to as HSI. This article intends to provide a comprehensive exploration of HSI and its profound implications for the future trajectory of the brewing landscape.

The Importance of HSI (Hop Storage Index) for Hops and Brewing

Hops, the female inflorescences of the perennial climbing plant Humulus lupulus L., hold a central place in brewing. They are the source of lupulin, a complex of resins and essential oils that bring bitterness and aroma to beer. However, understanding how hops age and how this aging affects their brewing value is essential for brewers. This understanding revolves around a crucial measure known as the Hop Storage Index (HSI).

nbsp;Alpha, Betas and More

The soft resins in hops contain alpha- and beta-acids, the primary brewing compounds, and essential oils. Alpha-acids, including co-, n-, ad-, pre-, and post-humulone, undergo isomerization during wort boiling, giving rise to the bitterness characteristic of beer. Beta-acids, on the other hand, play a less direct role in beer bitterness. Hops' freshness, quality, and the subsequent effect on beer is largely influenced by their storage conditions.

What is HSI?

The HSI value stands as an indicator of hop freshness, representing the ratio between oxidized and non-oxidized hop acids. Depending on the HSI value, hops can be categorized into different levels of freshness:

• Fresh: HSI values up to 0.32.
• Slightly Aged: HSI values between 0.33 and 0.40.
• Aged: HSI values from 0.41 to 0.50.
• Strongly Aged: HSI values between 0.51 and 0.60.
• Over-aged: HSI values over 0.61.

The Implications of HSI

1. Storage Stability of Hops: A two-year study investigating the stability of hop resins under different conditions found that anaerobic conditions at 4°C were optimal for preserving alpha-acids, beta-acids, and HSI values. Even so, significant losses occurred over time, especially under aerobic conditions at room temperature.
2. Impact on Dry Hopping and Beer Quality: The dry hopping technique has been embraced, especially by craft brewers, to impart an intense hop aroma and flavor to beer. However, the freshness of the hops used plays a pivotal role. As HSI values increase, signifying greater aging, there's a noticeable decline in hop aroma intensity and quality. Beers brewed with hops having an HSI value greater than 0.4 showed deviations in aroma and bitterness compared to those brewed with fresh hops.
3. Hop Aroma and Bitterness: As hops age, not only do the levels of essential oils change, but there's also a shift in the chemical composition. While humulinones (oxygenated products of alpha acids) increase and could add to beer's bitterness, the overall aroma from aged hops can be starkly different from fresh hops.

The HSI emerges as an essential compass. It offers insights into the environment in which hops have been stored and the subsequent impact on their quality. Research findings have underlined that hops stored in anaerobic conditions at a temperature of 4°C manifest optimal retention of their essential components. However, storage in an aerobic environment at room temperature paints a different picture, marked by noticeable degradation.

HSI and Oilsnbsp;

Hop essential oil plays a pivotal role in the brewing process, influencing the aroma profile of beer. However, during storage, the chemical makeup of hops undergoes significant alterations, affecting their quality. The study led by Ksenija Rutnik and her team sought to understand the variations in hop oil content and its chemical composition over two years. Their examination spanned four distinct storage conditions: anaerobic, aerobic, cold room (4 °C), and room temperature, with hops in both cone and pellet forms sourced from Slovenia. Through steam distillation and GC-FID analysis, it was discerned that hops stored aerobically at room temperature saw the most pronounced decline in essential oil content. Different hop oil compounds exhibited varied change patterns across the storage conditions. Notably, hops stored in an anaerobic environment at cooler temperatures fared best in retaining their quality. Furthermore, hop pellets were found to be more susceptible to oxygen-induced degradation than hop cones, while hop cones were more affected by higher temperatures. Importantly, apart from storage conditions, the variety and form of the hops also influenced the extent of these changes.?

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Oversupply of Hops and the HSI Relationship

The importance of HSI becomes particularly pronounced during periods of hop oversupply. When the market is flooded with hops, brewers and suppliers may need to store these hops for extended periods before they are used in brewing. As hops age, their quality begins to deteriorate, leading to potential shifts in the aroma, flavor, and bitterness profile of the beers they produce. The HSI serves as a critical measure of hop freshness, providing insights into how much the hop acids have oxidized over time. A reliable indicator like HSI becomes invaluable for brewers, allowing them to make informed decisions about which hops to use and when, ensuring the consistency and quality of their beers. In times of oversupply, understanding and monitoring HSI helps in maximizing the utility of stored hops and mitigating potential losses due to hop aging.

Brewers and HSI

Brewers, when equipped with the Hop Storage Index (HSI) of specific hop lots from their suppliers, gain a valuable metric that aids them in making informed brewing decisions. Using this information, brewers can select hop lots that align best with their desired flavor, aroma, and bitterness profiles for particular brews. For instance, they might opt for fresher hops with a lower HSI for beers where a bright, vibrant hop aroma is paramount, while possibly considering lots with a slightly higher HSI for brews where subtle aging characteristics might be deemed favorable. In essence, the HSI allows brewers to optimize the quality and consistency of their beers by selecting hop lots that best match the intended characteristics of their brews.

Future Research

In the near future, Dr. Thomas Shellhammer's laboratory at Oregon State University will embark on groundbreaking research. Their aim is to pinpoint a metric analogous to the "HSI for Aroma" that evaluates the degradation of aroma and flavor characteristics in hops. Currently, the HSI predominantly focuses on alpha acids and other compounds that might not significantly impact aroma and flavor. As a result, this present metric might not be as influential for brewers when assessing stored hops. This new research aims to fill that gap, offering a more comprehensive evaluation tool.

Conclusion

The brewing landscape is continually evolving, influenced by a blend of tradition, innovation, and scientific exploration. As we've gleaned from discussions at the MBAA conference and the extensive look into the Hop Storage Index, understanding the intricacies of hops storage and their impact on the final brew is paramount for brewing excellence. The future promises even more nuanced insights as research from esteemed institutions like Oregon State University aims to refine and enhance our understanding of hop aroma and flavor preservation. As brewers, consumers, and enthusiasts, staying abreast of these advancements will not only enhance the quality of the beer we produce and consume but will also pay homage to the age-old craft of brewing, ensuring its legacy thrives in the modern age.

References

1. Rutnik, K., Ocvirk, M., & Ko?ir, I.J. (2023). The Stability of Hop (Humulus lupulus L.) Resins during Long-Period Storage. Plants, 12(4), 936. https://doi.org/10.3390/plants12040936
2. Rutnik, K., Ocvirk, M., & Ko?ir, I.J. (2022). Impact of Hop Freshness on Dry Hopped Beer Quality. Foods, 11(9), 1310. https://doi.org/10.3390/foods11091310
3. Rutnik, K., Ocvirk, M., & Ko?ir, I.J. (2022). Changes in Hop (Humulus lupulus L.) Oil Content and Composition during Long-Term Storage under Different Conditions. Foods, 11(19), 3089. https://doi.org/10.3390/foods11193089
4. MBAA National Conference 2023, Seattle, WA, Personal Communication, October 7, 2023