Humulone is a chemical compound found in hops (Humulus lupulus). It is an α acid that, together with structurally related compounds, plays a significant role in the brewing process and contributes to the bitterness, flavour, and aroma of beer. This compound, together with many others, constitutes hop resins [1]. Isohumulones are constitutional isomers of humulone (i.e., they have the same molecular formula but different atom connectivity) obtained during the brewing process.
Humulone and isohumulones: the chemistry behind bitterness
Humulone and other α and β acids [2] are formed within hops as specialized metabolites, and their concentrations in different hop varieties vary, contributing to the diversity of beer flavours [3]. The choice of hops, as well as the timing of hop additions and the brewer’s skill in manipulating the brewing process can significantly influence the final taste and aroma of the beer. This versatility allows brewers to craft a wide array of beer styles.
When hops are added to the boiling wort (the liquid extracted from malted barley), heat triggers a process known as isomerization. This process converts humulone and related α acids into iso-α acids, including isohumulones [3]. Iso-α acids are more soluble in beer and are the primary contributors to beer’s bitterness [4]. They possess tensioactive properties, thereby stabilizing the beer foam, and protect beer against micro-organisms [4].
It is important to note that flavour and aroma, however, are determined and affected also by several other chemicals.
Health benefits
Hop bitter acids exhibit an inhibitory effect on bacteria, playing a major role in enhancing the preservation of beer, although there are also many other parameters that make beer an unfavourable medium for the growth of most microorganisms [5]. The antibacterial properties against gram positive bacteria of α and iso-α acids are well known [4]. Each hop resin acid is differentially effective. The antibacterial activities of β and α acids are higher than that of iso-α acids. However, because of their lower solubility in beer, their actual efficiency is lower than that of iso-α acids [6].
Humulone and related compounds from hops have been studied for potential health benefits, including anti-inflammatory and antioxidant properties [2,4,6]. There is some preliminary in vitro evidence to suggest that these compounds may have anti-cancer properties [4]. Hop extracts containing humulone and isohumulone have been investigated for their potential to improve metabolic health and for their role in promoting cardiovascular health and preventing certain diseases [4], although more research is needed in this area. Some studies suggest that these compounds may have a positive impact on bone health, preventing osteoporosis [4]. The female inflorescences of Humulus lupulus have been used traditionally to promote relaxation and sleep. While several compounds have been pointed out as possible candidates for explaining these effects, humulone and other bitter acids seem to be particularly interesting candidates [2].
It is important to note that while the current results are promising, more research is needed in this area to draw definitive conclusions regarding the health benefits of hop-derived α and iso- α acids. It is also worth mentioning that, while hops and hop-derived products may offer health benefits, excessive consumption of alcohol, such as beer, can have detrimental effects on health.
Ecological role
Humulone, along with other chemical compounds found in hops, may play a crucial role in protecting the plant from pests and diseases. These compounds are part of the plant’s natural defence mechanisms and have several functions in protection and defence. Humulone has antimicrobial activity and many iso-α-acids have been suggested to have insecticidal and insect-repelling properties [2,7]. However, it is necessary to highlight that hop resin is a complex mixture of numerous chemicals and that the defensive properties are likely due to several different compounds.
Finally, it must be taken into account that the specific composition and concentration of these compounds can vary among hop varieties, and that they are influenced by factors such as environmental conditions and agricultural practices. As a result, different hop varieties may have varying levels of resistance to pests and diseases.
Note: beer stein is a traditional beer mug.
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