Isohumulones

Beer taste can be spoiled by contaminating bacteria or yeasts. The most common bacteria are lactic and acetic acid producers and T ymomonas. Wild yeasts can be anything other than the intended strain S. uvarum is considered a contaminant of ale fermentations and S. cerevisiae a contaminant of lager fermentations. The common wild yeast contaminants are S. diastaticus and species of Picbia, Candida and Brettanomjces. It may be noted that the flavor of beer may be improved by the ability of yeast to adsorb bitter substances extracted from hops, such as humulones and isohumulones. 

Further supporting evidence for the importance of lipophilicity in bitter response is provided by the taste of isohumulone (118), the principal, bitter-tasting component of beer, and some of its derivatives. Isohumulone can exist in both cis and trans forms. Clarke and Hilderbrand reported that the cis form, having a partition coefficient of 0.78, is more bitter than... 

Reduction of the isoprenyl groups of isohumulone and p-isohumulone to tetrahydro- and hexahydro-isohumulone, respectively, increases the lipophilicity of the resultant products. A corresponding increase in their bitterness was observed. A similar increase in bitterness was reported for hydroquinine following reduction of the vinylic double bond of quinine (117). Because the stereochemistry of the quinines does not have any significant effect on the bitterness, this change must be due to change in the relative lipophilicity of the compound. 

Humulus lupulus L. She Ma (Hop) (female flower, unripe fruit) Humulone, resin, lupulone, choline asparaginer, lupulin, isohumulone, isovaleric acid.33-450 This herb is toxic. Inhibit the growth of tubercle bacillus and arrest tuberculosis. 

The requirements for the analysis of the iso-a-acids are diverse and, to a large extent, dictate the resolving power required of the chromatography. The requirement to separate cis/trans pairs of the iso-a-acids is often driven by research projects. Hughes (16) used a multicomponent mobile phase and a NovaPak C18 cartridge to obtain resolution of the five iso-a-acids in beer within 7 minutes (Fig. 3). The separation of cw-isohumulone (often the major iso-a-acid in beer) from frans-isoadhumulone (most minor of the six iso-a-acids) usually proves to be difficult. Other workers have successfully resolved the six compounds, and micro HPLC has proved particularly effective (34). 

HA Thornton, J Kulandai, DB Hawthorne, TE Kavanagh. Preparation of a frans-isohumulones standard. J Inst Brew 96 367, 1990. 

The product was previously identified in beer by Andrews (1987) and in human sweat by Polak et al. (1988), and characterized as sunstruck or lightstruck off-flavor. The cause has been attributed to the degradation of bitter isohumulones from hops under the influence of ultraviolet light (Bondeel et al., 1987). By photofragmentation trans-isohumulone liberates a prenyl radical which can trap a thiol radical, producing an undesired flavor for beer. Holscher et al. (1992) have also shown that 3-methyl-2-buten-l-thiol and 3-mercapto-3-methylbutanol are principally formed under pyrolytic conditions in roast model reactions of prenyl alcohol and sulfur-containing amino acids. 

Bondeel, G.M.A., De Keukeleire D. and Verzele M. (1987) The photolysis of trarc.v-isohumulone to dehydrohumu-linic acid, a key route to the development of sunstruck flavour in beer. J. Chem. Soc. Perkin Trans, i, 2715-17. 

Fig. (14 a). Chemical structure of humulones contained in hops and isohumulones produced by isomerization of corresponding humulones during the heating process of beer brewing. 

Fig. (14 b). Dose-dependent anti-hyperglycemic effects of humulones (a mixture of 65% humulone, 24% cohumulone, 9% adhumulone, and 2% other a-acids) and isohumulones 6 hr after intraperitoneal injection in streptozotocin-diabetic mice. Isohumulones were prepared from humulones in 0.1% DMSO solution by heating at 60°C for 1 hr. The percentage fall of blood glucose level was evaluated from the expression of (b-a)/(b-85) x 100, where b and a are blood glucose levels before and after injection of the compounds, respectively, and 85 mg/dL is the mean blood glucose level in the fasted state of normal mice. The values represent means S.E.M. (n = 3-7). 

The structures assigned to isohumulone (71), humulinic acid (73), and 4-acetylhumulinic acid (74) each contain two chiral centres and should exist as two pairs of enantiomers. Since natural (R) (—)-humulone is a single enantiomer only two diastereoisomeric forms are found these are the cis and... 

The iso-cL-acids. These are mainly isohumulone, isocohumulone, and iso-adhumulone. 

Isohumulone. The mixture of cis- and /rcw -isohumulone. Similarly iso-cohumulone refers to the mixture of cw- and /r /i -isocohumulone and isoadhumulone refers to the mixture of cis- and treats- isoadhumulone. cis-Isohumulone. The iso-a-acid with empirical formula C21H30O5. It is an oil with the higher partition coefficient in a phase system of a hydrocarbon and a buffer, and contains an isovaleryl side-chain. Cis means that the 3-methyl-2-butenyl side-chain and the tertiary hydroxyl group are on the same side of the ring. 

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