Isohumulone bitterness

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. 

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. 

In the normal isomerization of humulone (70) the bond between C-1 and C-6 is broken and a new bond is formed between C-1 and the carbonyl group at C-5 (Fig. 14.6). The reversed isomerization of humulone (Fig. 14.8), in which the bond between C-5 and C-6 is broken and the new bond formed between C-5 and the carbonyl on C-1, has now been found to occur and the unti-products account for about 10% of the isomerization mixture [93]. By boiling humulone in a buffer solution at pH 11-0 the cis- and tmn.r-isomers of both acetylhumulinic acid (88) are formed. The bitter tasting hop acids known [94]. In the mixture of isomerization mixture are deacylated anti-derivatives deacyl-ated anti-isohumulone (90), deacylated anti-acetylhumulinic acid (91), and deacylated onti-humulinic acid (92) [93]. Deacylated humulone (89) is readily isomerized to these products whereas the deacylation of anti-isohumulone only occurs to a limited extent (< 2 %) so it has been proposed [95] that (90), (91) and (92) are formed via deacylated humulone (89). However, the relative ease of deacylation of humulone and Deacylated derivatives of isohumulone have not been characterized in isomerization mixtures. 

In the reduction of isohumulone with sodium borohydride the carbonyl group in the isohexenoyl side-chain is reduced to a secondary alcohol to give the so-called p-isohumulones [126]. Since a new asymmetric centre is produced each isomer of isohumulone affords two p-isohumulones (97). The principal isomer from /r jfw -isohumulone (p-isohumulone A, m.p. 80°C) is reported to be 50 % as bitter as isohumulone and that from m-isohumulone (p-isohumulone Bj, m.p. 78 80 C) 80% as bitter as isohumulone [126]. p-Isohumulones are claimed to be less sensitive to photolysis, but in view of their reduced bittering power, extracts containing them are unlikely to be widely accepted. p-Isohumulones have not been characterized as normal constituents of beer but may be formed, in small amounts, from iso-a-acids during the fermentation. 

Of the individual iso-a-acids, isohumulone is concentrated into the foam to a greater extent than isocohumulone and unhopped beer bittered (21 0 BU) with isohumulone had a better head retention (2 = 132) than that bittered with isocohumulone (2 = 115) [146]. Addition of iso-a-acids to beer increases head retention but, in particular, the iso-a-acids are responsible for foam adhesion, cling or lacing. Unhopped beer does not show these effects. Traces of heavy metal ions (iron, cobalt, nickel and copper) also improve the head retention of beers but only in the presence of iso-a-acids [139]. 

The quality and intensity of the bitter taste derived from these secondary products are different. Evaluation of hops is therefore based on a determination of composition of individual a- and P-acids, rather than of the total content of bitter substances. As seen in Table 20.4, the composition varies greatly with hop origin. During the boiling of hops, humulons isomer-ize into isohumulons (cis-compounds, VII trans-compounds, VIII cf. Formula 20.2), which are more soluble and bitter than the initial compounds. The isohumulons can be further transformed into humulinic acids (IX,X), which have only about 30% of the bitterness of isohumulons. 

The odor- and taste-active substances essentially determine the type of beer. The bitter taste of Pilsener beers is produced by relatively high concentrations of isohumulons, and humulenes (including oxidation products), while larger amounts of furaneol are responsible for the caramel note of dark beers. 

Contains 0.3-1% volatile oil 3-12% resinous bitter principles composed of a-bitter acids (humulone, isohumulone, cohumulone, isocohumulone, adhumulone, prehumulone, posthumulone, and so on, with first three in predominance) and P-bitter acids (lupulone, colupulone, adlupulone, etc., in decreasing concentration) other resins, some of which... 

Efforts are also being made to find new outlets for isomerized extracts. Apparently many drinks and dairy products are improved by the addition of a faint bitter taste (41). This seems worth looking into. At one time we added pure trans isohumulone to Coca Cola with quite interesting results. 

The abeo-isohumulones are only slightly bitter and display strong foam-stabilizing properties (50). They occur in hops to the extent of 0.5-1.15% and in beer, 0.088 g to 0.160 g.T. These figures of 1967 now seem impossibly high an do require confirmation. 

In the brewery, hop is boiled with wort at a pH vaiue around 5.5. In these conditions the hop alpha acids are poorly soluble (1), but in the process they are transformed into the iso-alpha acids, which are better soluble in the wort medium (2). Consequently, only traces of alpha acids remain in beer (3) (see 5.3.). The iso-alpha acids are the hop derivatives, which contribute mainly to the beer bitter taste. In this Chapter the chemistry of the isohumulones, which are the most important iso-alpha acids, is discussed. The chemistry of the other iso-alpha acids is practically identical to that of the isohumulones. As humulone is readily available, it is easier to study the isohumulones than the other iso-alpha acids. Therefore, the isohumulones are the best known iso-alpha acids. 

The bitterness and the water-solubility of the spiro-sohumulones are lower than those of the isohumulones. They do not occur in aqueous isomerization mixtures of humulone. 

The humulinic acids are not bitter. In the commercial isomerization of humulone the formation of humulinic acids should be avoided. Nevertheless, beer that has been brewed in the conventional way contains minor quantities of the humulinic acids (39). As a result, unwanted interferences occur in the spectrophotometric determination of the bitterness (see 5.5.). The antibiotic and foam-stabilizing properties of the humulinic acids are at least 100 times less than those of the isohumulones (40,41). With high efficiency LC it is relatively easy to determine the humulinic acids content of beer. An interesting suggestion is that this could be related to beer age and thus could be a quality parameter. 

The existence of the anti-isohumulones and the anti-humulinic acids was advanced at one time in order to rationalize the existence of two isomers of most of the five-membered ring hop derivatives. The possibility of the anti-structures was rejected as soon as it was shown unequivocally that the isolated isomers were in fact epimers and not positional isomers (1,2). The existence of the anti-series of the five-membered hop bitter acids is now, however, firmly established. 

The most important property of the anti-isohumulones is the very high bitterness level, which is twice that of the isohumulones. It appears that the anti-isohumulones are the most bitter hop derivatives known today. There are reasonable indications for the occurrence of the anti-isohumulones in beer, altough the presence has not been proved unambiguously. The concentration must be at least a factor of 10 lower than that of the iso-alpha acids. 

In view of the exceptional bitterness it could be interesting to increase the ratio of the anti-isohumulones relative to that of the isohumulones in the brewery. A ratio of 2 3 can be reached by boiling humulone at pH 5.0 during several hours in water dioxane 1 1 containing catalytic amounts of magnesium salts (8). It is remarkable that in these conditions the least stable trans anti-isohumulone is formed preferentially. Nothing is known yet about other characteristics. This particular aspect of hop chemistry has not been further explored, although it could be worthwhile to do so. 

The total yield of the deacylated derivatives of the anti-series is 8.5%. When the anti-isohumulones and the anti-acetyihumulinic acids are included, the yield even exceeds 10%. This is certainly not negligible in comparison with the most important hop bitter acids, the isohumulones. 

The deacylated compounds show the highest water-solubility of all anti-derivatives. As the utilization yield is consequently high, the compounds of the anti-series occur in beer (10). Since the deacylated components of the anti-series are half as bitter as the isohumulones, the contribution to the overall bitter taste of beer will not be negligible. 

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