In times like these, it can happen that breweries have more raw materials in stock than planned: When sales fall, inventories rise. This situation leaves many a brewery faced with new questions, for example with regard to hops: “How long will they stay fresh?” and “How should we store them?”
The search for answers quickly leads to the HSI (Hop Storage Index) that measures the oxidation of alpha and beta acids. This measurement is based on the non-specific spectrophotometric standard procedure according to ASBC 6A/12 or EBC 7.13. The index shows a unitless ratio of oxidized to fresh hops. The rule of thumb goes like this: The higher the index, the higher the oxidation of the bitter compounds. Accordingly, a value of 0.25 means a transformation of zero percent, while a value of 0.310 means ten percent oxidation – at least in theory.
Although it was introduced more than 50 years ago, this tool is the only one to date to have established itself for measuring hop freshness. Unfortunately, it is used not only to estimate alpha and beta acid loss, but also as an indicator of the overall condition of the hops. Experts warn, however: Never trust the HSI blindly! This is because recently published scientific papers have shown that the old rule of thumb does not measure up to modern requirements.
First, the HSI is variety-specific. Hallertau Mittelfrueh, Hersbruck, Tettnang and their descendants such as Tradition, Perle and Herkules have an average baseline value of <0.25. For Saphir, Northern Brewer and Saaz, however, the baseline values are between 0.275 and 0.300, whereas for Aurora, Celeia, Lublin and Marynka they don’t even start till >0.300. Also, the HSI increase differs according to variety, but behaves in the same variety-specific manner at cool and warm storage temperatures. Conclusion: An HSI assessment must always be carried out on the basis of the variety-specific baseline value and the rate of ageing.
Second, high HSI values do not automatically mean correspondingly lower brewing quality, as brewing trials with Citra and Galaxy have shown. As the hops age, both varieties lose not only bitter compound content but also oil content. However, analysis shows that it is mainly the myrcenes and to a lesser extent the caryophyllenes and humulenes that degrade as the hops age, and they are of hardly any importance for dry-hopping aroma. Interestingly, there was no evidence of an increase of oxidized sesquiterpenes (spicy and woody aromas) in the ageing hops, but there was in the finished beer – first in sensory terms and then through subsequent analysis.
Sensory analysis of the beers shows that in accordance with the reduced oil content the flavor profiles displayed less fruitiness and greater intensity of spicy and woody aromas – both of which are significantly more pronounced in Galaxy than in Citra hops. The intensity of bitterness is greater in beers with aged hops due to the age-related increase in humulinone levels. At the same time, there is a decline in quality of bitterness – albeit less so in Citra than in Galaxy.
Similar differences can be found in brewing trials with Perle and Saaz. All in all, beers hopped with Galaxy and Saaz are more severely affected in both analytical and sensory terms than those hopped with Citra and Perle. Conclusion: If HSI values are elevated, brewing quality should also be regarded in variety-specific terms and not on the basis of analytical values alone. Here, too, sensory evaluation is crucial.
As far as storage is concerned, time, temperature, and oxygen all promote the ageing of hop cones. In addition, the HSI value rises in proportion to the pressure on the hop bales because the lupulin glands are crushed and thus exposed to oxygen. Pellets, on the other hand, protect the valuable hop components. If the pellets are also foil-packed using inert gas and stored in cool conditions, the rise in HSI value over a period of years is negligible – so much so that BarthHaas has now extended the shelf life stated for hop pellets to five years.
