trans-iso-α-acids

Trans-iso-alpha-acids are the stereoisomers with a trans configuration among iso-alpha-acids and form part of iso-alpha-acids, which are the principal bitter compounds of beer. Under certain packaging conditions, they have been reported to behave as an unstable fraction during storage.

In beer stored in brown glass bottles, trans-configured iso-alpha-acid isomers show a marked and highly specific decrease, while the corresponding cis-iso-alpha-acids are barely affected. In a separate study using NMR and LC-MS, the key conversion product accompanying these storage-related changes has been structurally confirmed to derive solely from the trans isomer, and was identified as a previously unknown product.

However, this cis/trans selectivity depends on packaging conditions. During storage in polyethylene terephthalate (PET) bottles, which are known for oxygen permeability, both isomers degraded rapidly regardless of cis/trans configuration. In corresponding model oxidation experiments as well, oxidation of cis- and trans-isohumulone occurs in both configurations, generating both cis and trans hydroperoxide and hydroxyl forms.

In a separate HPLC kinetics study as well, storage-induced degradation of commercial lager was analyzed centered on the degradation of 3 trans compounds: trans-isohumulone, trans-isocohumulone, and trans-isoadhumulone, and the activation energy for each compound was calculated. In dark beer, trans-isocohumulone degradation showed higher activation energy than the other 2, whereas in pale beer and non-alcoholic beer all 3 showed comparable levels. However, the abstract does not show a direct comparison with the cis isomers.

There is a study in which the cis/trans ratio in fresh beer differed depending on the hop product used in brewing. In a study that brewed a 20 hL Pilsener by 4 methods, 2 brews each, the ratio was 2.2 for both regular pellets and inline pre-isomerization, 4.5 for isomerized pellets, and 4.0 for isomerized extract. All products were added at the start of a 100 °C, 75-minute boil, and the measured value for fresh beer was 28–33 BU. However, the form of addition and the amount of alpha acid and iso-alpha-acid also differed by product, so this is not an isolated comparison of the ratio alone.

When the same 4 methods were evaluated by 12 trained panelists, the bitterness intensity of fresh beer was 5.8–6.0 on a 0–9 scale. In samples stored at 28 °C for 2 months and 4 months, no decrease in bitterness intensity was confirmed, nor was there a significant preference for the methods with a higher cis/trans ratio. The chemical analysis extended to 4, 8, and 12 months at 5 °C and 28 °C, while the sensory testing extended only to 2 and 4 months at 28 °C — the timepoints differ. This test does not show that a higher cis/trans ratio improves human bitterness stability.

In another study of 8 naturally aged commercial lager brands, the bitterness tracked over time by a professional panel fell with age, but the difference was smaller than the differences among the 8 fresh brands, and it could not be sufficiently explained by the 5 iso-alpha-acid peaks separated by HPLC alone. Whether only the peaks corresponding to the trans forms decreased cannot be confirmed from the abstract, and a decrease in the trans peaks cannot be treated as the cause of the sensory change.