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The relevance of Hop Storage Index (HSI) for hop usage
McMillan, J., Zunkel, M., Muñoz Insa, A., Schönberger, C.

The increased use of aroma hops for dry hopping has become a common technique used by brewers to impart hoppy aromas and flavours into beer. The increased demand for aroma hops has led to hop quality becoming more important. Brewers want to ensure the freshest hops are used, however the quality of hops can decrease during storage. The common hop quality indicator is hop storage index (HSI). In this study, two varieties of hops (Citra® and Galaxy®) with three different HSI classifications: fresh (< 0.32), aged (0.41 - 0.50) and overaged (> 0.61) were evaluated. A pale ale was brewed and dry hopped with each variety at the different HSI classifications. The key aim was to assess the impact of HSI on total oil content and beer quality in terms of hoppy aroma and flavour quality. The total essential oil content and profile of hop oil components were analysed. Sensory and analytical analysis was conducted on all beer samples. The total essential oil content decreased as the HSI increased. The increase in HSI changed the profile of hop oil components causing significant decreases in myrcene and leading to an increase in oxidation components such as caryophyllene oxide, humulene epoxide I and humulenol II. Increase in the vegetal attribute for both varieties were significant, most likely due to the formation of sulphur compounds. Bitterness intensity and perceived bitterness increased as the HSI increased, whereas the bitterness quality was reduced. Humulinone concentration in beer increased alongside the HSI value. Beers dry hopped with hops which had a HSI value > 0.41 led to aroma, flavour and bitterness deviations in comparison to beers brewed with fresh hops. While HSI doesnt measure total oil content of aged hops, it does have a negative correlation on the total essential oil content, indicating that the hop quality has deteriorated.

Descriptors: Hop Storage Index (HSI), dry hopping, sensory, beer quality, hop quality

BrewingScience, 76 (November/December 2023), pp. 147-158