HVG Germany: Hopfen

Hop oils

Here we can only address this interesting subject superficially within this framework. The hop aroma is very complex and cannot be described by one or a few substances as in the case of many plants and fruits (e.g. vanilla, berries).

Aroma substances from the malt and the yeast metabolism prevail in the beer. The hop aroma characteristics in the beer differ from those of the hops before the dosage. We are very familiar with the hop oils which are present in the leaf hops, but know little about the changes which occur throughout the entire brewing process. Epoxy and conversion products of the hitherto more than 300 oils form a mixture of substances relevant for the aroma, which can individually only be detected with difficulty but which as a mixture have a considerable influence on the aroma of the beer.

A problem in the analysis are the very low concentrations of these oxidation and conversion products in the hop oils which are in the area of ppb and ppt. Despite state-of-the-art technology the usual analysis equipment meets it's accuracy limits here. There are also hardly any pure substances to calibrate the euipment with.

The hop flavour in the beer bouquet are often described as citrussy, flowery, estery, fruity, spicy or sharp. Hop oil fractions with the characteristics mentioned are sold commercially. It Is assume that increased sulphurous ester compounds of some hop varieties could give the beer an onion or garlic tang. Also aroma substances attached to polyphenols and glycosides are also responsible for the hop bouquet.

There is a substance which is nowadays used as a reference for the aroma hopping: linalool. This terpene alcohol can be relatively easily proved in the beer and can be regarded as a guiding substance for the hopping. Late hop dosages in the brewhouse quantitatively bring linalool and other aroma substances into the beer. Linalool is not a substance specifically found In hops. It can not only be found in hops but also in e.g. coriander or marjoram.

The hop character Is linked with oxidized terpenes. Sesquiterpenes are susceptible to auto-oxidation. At the same time epoxides such as e.g. caryophyllene-4.5-epoxide are formed. Humulene di-epoxides can be found not only in hops but also in the beer.

At the Technical University of Munich-Weihenstephan linalool, alpha-terpienol and humulenol II were definitely identified by GC-MS as aroma components in the beer originating from hops. Their threshold values should be at about 10 µg / l, 500 µg / l, 2,400 µg / l liegen. Other substances such as myrcenol, 5.5 dimethyl-2(5H) furanon, alpha- and beta-eudesmol and selinenol could be assigned with a high degree of probability to other peaks of the chromatogram.

This brief excursion into the hop chemistry is only intended to demonstrate how complex the subject is. We shall not go into it deeper here as even a summary of the present literature would take up a lot of pages and anyway would not even produce statements of universal validity on the hop aroma.