Brewing up crystal structures: Hoppy bitterness
- Published: Jan 15, 2013
- Author: David Bradley
- Channels: X-ray Spectrometry
During brewing, beer obtains its bitter flavour from the bitter iso-alpha acids that come from hops. In the journal Angewandte Chemie, scientists now report that they have used X-ray crystallography to determine the absolute configurations of these humulones and isohumulones, as well as several of their derivatives.
During the brewing process, beer gains a bitter flavour from the acids that are derived from the fermented hops, the flower heads of the plant Humulus lupulus. Now, chirality expert Werner Kaminsky of the University of Washington, Seattle, and colleagues have reported the crystal structures and absolute configurations of these acids, the humulones and isohumulones. They reported details in Angewandte Chemie this month as well as data on several of the derivatives of these compounds.
Humulones are bacteriostatic bitter substances from H. lupulus and are the primary flavour-producing agent in the wide range of alcoholic beverages commonly referred to as beer, they also act as natural preservatives. In the brewing process, beer wort (the mashed extract of barley) is heated together with hops, which leads to numerous rearrangement products. Some breweries use tetrahydro-iso-alpha acids extracted from hops. Intriguingly, these compounds have been investigated as anti-inflammatory agents as well as for their potential in reducing weight gain.
When humulones rearrange, their six-membered ring is converted into a five-membered ring and once this occurs there may also be a rearrangement of two side groups leading to both cis and trans forms. Until now, it was not known on which side of the molecule the side groups sit in the cis-isohumulones and in trans-isohumulone, and which side group points up and which points down? The team also wanted to know the absolute configuration, i.e. the handedness of the chiral carbon in the original six-membered ring.
Kaminsky worked with scientists from KinDex Therapeutics in Seattle to successfully answer these questions using X-ray crystallography. The process was rather complicated given that the isomerization process of humulones results in a large number of very similar compounds that had to be separated, purified, and their acids converted into suitable salts for crystallisation.
Interestingly, the absolute configurations of the hops bitter acids as determined by Kaminsky and his co-workers are not the same of those reported previously by other chemists who used indirect methods including the Horeau method and the Cotton effect to determine the chirality of the compound. This suggests that those indirect methods may not be as reliable as was once thought and could have implications for studies of other compounds in which they were used.
Ultimately, the absolute configuration of the hop-derived compounds is of interest not only to brewers and perhaps even to beer drinkers, but may have health implications for those who enjoy the occasional dark mild or blonde. As mentioned earlier, the bitter acids from hops have been implicated in a positive way in the inflammatory response, in weight loss and even in diabetes and some forms of cancer. The effects of different hop acids have not been investigated thoroughly, partly because of the myriad such compounds that exist and that have not until now been determined in details. The study by Kaminsky and colleagues pins down the acids and should provide biomedical and clinical researchers with a baseline array of hop acids to study and to correlate configuration with biochemical and health effects.
Angew Chem Int Edn, 2013, online: "Absolute Configuration of Beer's Bitter Compounds"
Article by David Bradley
The views represented in this article are solely those of the author and do not necessarily represent those of John Wiley and Sons, Ltd.