COMPARISON OF TIME-INTENSITY WITH CATEGORY SCALING OF BITTERNESS OF ISO-α-ACIDS IN MODEL SYSTEMS AND IN BEER*

A 17-point non-numerical category scale indicated a direct, linear relationship between average perceived bitterness and concentration of iso-α-acids in water and in a commercial lager. Time-intensity (T-I) tracings yielded additional information: the rate of increase in bitterness after taking the sample into the mouth; the rate of decrease after reaching maximum intensity; the events associated with swallowing; and total duration, which ranged from 13 to 42 s across judges. The T-I curves revealed a burst of bitterness intensity immediately after swallowing, which was proportional to the concentration of iso-α-acids for water solutions, but not for beer. Addition of 2.6% ethyl alcohol to the lager enhanced bitterness, particularly at low levels of added iso-α-acids, whereas addition of 2.0% glucose reduced bitterness in the control as well as in beer with added iso-α-acids. In water, 20 and 30 ppm of iso-α-acids were more bitter and had a longer duration than in beer. Among judges there were marked differences in the patterns of the T-I tracings, but there was excellent reproducibility within judges across replications.     

IONIZATION BEHAVIOUR OF HOP COMPOUNDS AND HOP-DERIVED COMPOUNDS

The ionization behaviour of colupulone, (?)-humulone, trans-isohumulone and trans-humulinic acid was investigated using a number of techniques. When a potentiometric technique was employed, precise estimates of pKa could not be obtained for any of the compounds, since a drift in the observed pKa values occurred during the titrations. Approximate, or equilibrium pKa values for the most acidic function of each of the compounds were: colupulone, 6.1; (?)-humulone, 5.0; trans-isohumulone, 3.1; trans-humulinic acid, 2.7. Evidence from spectroscopic measurements (UV, ¹H-NMR, ¹³C-NMR), conductiometric experiments and measurements of the pH dependency of solvent partition behaviour and aqueous solubility of the compounds is consistent with the possibility that, in aqueous solution, compounds such as trans-isohumulone are present as covalent hydrates. It is likely that reversible hydration occurs at one or more carbonyl groups. As a consequence of this behaviour, no single pKa value is sufficient to describe the behaviour of any one hop compound or hop-derived compound. The potential significance of this phenomenon is discussed.     

QUANTITATIVE ANALYSIS OF HOPS AND HOP EXTRACTS BY HIGH PRESSURE ION EXCHANGE CHROMATOGRAPHY

A new analysis for hop acids in hops and hop extracts is described. It is based on recent developments in high pressure liquid chromatography using pellicular anion exchange column filling material. Quantitative evaluation of α-acids, β-acids and oxidation products in hops and hop extracts is carried out by standard addition of pure humulone. The α-acids are completely separated from other hop substances before quantitation. The results of α-acids determinations must therefore be more accurate than was formerly possible and they are compared with conductometric titration results which are systematically higher. This is to be expected as it becomes more and more obvious that paper strip and conductometric analysis are not selective enough and determine fractions as “α-acids” which are in fact oxidation products of the hop acids.     

HOP OIL CONSTITUENTS DERIVED FROM THE OXIDATION OF COLUPULONE

Several spiroacetal and furanone derivatives have been detected in the volatile oxidation products formed by heating colupulone with gaseous oxygen. This finding explains the occurrence of high levels of these compounds in hops which have undergone a prolonged period of storage.     

RAPID SPECTROPHOTOMETRIC METHOD FOR DETERMINATION OF THE BITTERING VALUE OF ISOMERIZED HOP EXTRACTS

A method is described which allows the rapid determination of the bittering value of isomerized hop extracts. The method is also applicable in the case of isomerized hop extracts containing impurities such as α-acids and β-acids.     

COMMERCIAL BREWING AND STORAGE TRIALS WITH YEOMAN HOPS

Commercial brewing trials carried out with Yeoman hops from the 1981 crop have shown that these hops are suitable for producing both ales and lager beers. Seeded Yeoman hops show good storage characteristics and lost only 15·3% of their initial Lead Conductance Values after 44 weeks storage at ambient temperature.     

GAS CHROMATOGRAPHY AND GAS CHROMATOGRAPHY-MASS SPECTROMETRY OF SOFT RESINS AND RELATED COMPOUNDS

The α-acids, β-acids, iso-α-acids, hulupones and humulinic acid have been separated by gas chromatography utilising the cool on-column injection technique on various bonded phase capillary columns. The α-acids were separated into cohumulone and humulone; the β-acids into colupulone and lupulone; and the hulupones into cohulupone and hulupone. Iso-α-acids were separated into four peaks; cis-isocohumulone, trans-isocohumulone, cis-isohumulone and trans-isohumulone. Flame ionization detection (FID) and chemical ionization mass spectrometry (CIMS) were used for chromatographic detection. Best results were achieved by employing a DB-1 fused silica capillary column with isooctane as the sample solvent .