AUTOMATED REPORTING ON THE QUALITY OF HOPS AND HOP PRODUCTS

The quality of a hop variety or a hop product can readily be assessed by a fully automated sequence of selective extraction, fractionation and quantitative analysis. To illustrate the elegance of the method, nine hop varieties and three hop extracts were compared with respect to the content of important marker compounds in the hop oils and of the hop acids. Supercritical fluid extraction at different densities of carbon dioxide was applied to extract selectively, the hop oils and the hop acids, respectively. The hop oils were further fractionated into an apolar and a polar fraction by solid phase extraction and consecutive elution with n-hexane and ethyl acetate. Separation and identification were achieved by capillary gas chromatography coupled to mass spectrometry. Myrcene, β;-caryophyllene, α;-humulene and β;-farnesene in the apolar fraction, linalool, undecan-2-one, tridecan-2-one and humuladienone in the polar fraction were selected for quantitative evaluation of the respective hop oils. Sulphur-containing compounds were revealed by capillary gas chromatography using sulphur-selective atomic emission detection. Complete separation and quantification of all hop α;-acids and β;-acids was effected by microemulsion electrokinetic chromatography coupled to diode array detection .     

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 RESIN ANALYSIS BY HIGH-PERFORMANCE LIQUID CHROMATOGRAPHY ON MICROPARTICULATE SILICA

A high-performance liquid chromatographic separation of hop resins on a commercially available microparticulate silica has been developed. The chromatography involves a new mechanism in hop resin separation. A solvent containing di-n-butylammonium acetate is used. The method gives good separation of α-acid, iso-α-acid, β-acid and humulinic acid, and has been used for the quantitative analysis of α-acid and iso-α-acid in hop extracts and isomerised hop extracts.     

QUANTITATIVE DETERMINATION OF HOP BITTER SUBSTANCES AND THEIR DERIVATIVES IN HOP EXTRACTS BY THIN LAYER CHROMATOGRAPHY

A method is described which allows the separation of hop bitter components and derivatives present in hop extracts by thin-layer chromatography on a mixture of silica gel and cellulose. This method can be used to evaluate the purity of hop extracts, isomerized hop extracts and complementary base extracts.     

ANALYSIS OF HOPS AND HOP PRODUCTS FOR α-ACIDS OR ISO-α-ACIDS

Two methods based on the resolution of mixtures of hop compounds by chromatography on Sephadex columns have been adopted by E.B.C. and A.S.B.C. as ‘International Methods’.     

THE APPLICATION OF SALT FORMATION IN THE CHEMISTRY AND TECHNOLOGY OF HOP RESINS

In natural product chemistry the satisfactory isolation of chemical compounds of scientific and/or commercial significance normally follows a pattern of fractionation which involves the utilization of a range of physical and chemical properties of the compounds present in the mixture. Vapour pressure and solubility are the physical properties traditionally made use of in the initial stages of fractionation. Chemical properties normally enable somewhat more specific fractionation to be achieved. One chemical property that is widely used is the ability of a large number of compounds to form salts. In the case of the hop resins it is this specific chemical property which has played the most significant role in their isolation, characterization and commercial utilization. The historical development of this aspect of hop chemistry has been reviewed.     

ESTIMATION OF α- AND β-ACIDS IN HOP EXTRACTS BY HPLC

A procedure relying on high performance liquid chromatography for the estimation of α-acids and β-acids in hop extracts has been collaboratively tested by members of a Sub-Committee of the Institute of Brewing Analysis Committee and is recommended for use. No significant differences were found between precision values obtained using peak height and peak area measurements. For α-acids, the mean repeatability (r₉₅) and reproducibility (R₉₅) values were 1-3 and 2-4% m/m respectively over the range 41–62% m/m. For β-acids they were 0-9 and 2-0% m/m respectively over the range 11 to 35% m/m. The precision values were judged to be independent of concentration.     

PREPARATION OF HOP EXTRACTS RICH IN PARTICULAR CONSTITUENTS*

When a column containing powdered hops is extracted with liquid carbon dioxide, chromatographic separation of hop components occurs. They are extracted in the order essential oils, β-acids, α-acids; and the separation is enhanced when finely milled hops are extracted. Early fractions (～0·5 hour) contain a high proportion of the available essential oils when hops are extracted at ?20°C and such extracts are suitable as a replacement for dry hops. Fractions can be obtained from extractions at ～7°C which are rich in α-acids and contain low levels of β-acids. Small amounts of fats and waxes are normally present in fractions collected towards the end of a run when seeded hops are extracted.     

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.     

NON-IMPLICATION OF HOP α- AND β- ACIDS AS SOURCES OF SULPHUR COMPOUNDS IN BEER

Whereas hop oil terpenoids can give rise to organoleptically undesirable sulphur compounds in beer brewed using hops dressed on the bine with sulphur, the hop resin α- and β-acids and their transformation products appear incapable of reactions with sulphur under analogous conditions. The evidence indicates that the hop resins are not potential sources of sulphur compounds in beer     

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.     

SOLUBILITY OF HOP α- AND β-ACIDS IN LIQUID CARBON DIOXIDE*: ADDENDUM: SOLUBILITY OF PESTICIDES IN LIQUID CARBON DIOXIDE

A procedure is described for determining the solubility of hop α- and β-acids in liquid carbon dioxide. Results have shown that the optimum temperature range for the extraction of hops with liquid carbon dioxide is +5 to +10°C. A number of pesticides used by hop growers are appreciably soluble in liquid carbon dioxide.     

HOP PRODUCTION 1958–1983

This paper reviews the changes in hop production over the past 25 years. The fluctuating pattern of acreage and yield in the principal producing countries is recorded and related to political and marketing changes. Changing brewery requirements, partly related to the development of processed hops, have increased the demand for high α-acid cultivars, but hop aroma continues to be an important criterion. New cultivars are not only meeting brewers' needs but also those of growers to whom disease resistance is particularly important. Changes in production methods have included increased mechanisation particularly related to harvesting, improved drying equipment, non-cultivation techniques, mist propagation and, on an experimental basis, growing hops on low wirework.     

THE DETERMINATION OF α- AND β-ACIDS IN HOPS AND HOP PRODUCTS USING HPLC

A rapid reversed phase HPLC method for the analysis for α- and β-acids in hops and hop products is described and has been evaluated. The method uses citric acid in the eluent as a complexing agent to overcome the irreversible adsorption effects shown by some columns, thus allowing optimum eluent pH to be selected. The precision of the method for analysis of hop extract has been determined giving relative standard deviations of 1·0% and 2·1% for α- and β-acids respectively. General agreement with results obtained using a polarimetric α-acids analysis method for hop extracts and hops has been demonstrated.     

KINETICS OF HOP STORAGE

After an initial ‘delay period’ the storage losses of α- and β-acids follow a first-order rate equation and not a zero-order equation as has been suggested by others.     

HOP INDUSTRY ADVISORY COMMITTEE: ANALYSIS OF HOPS OF THE 1975 CROP

Analytical values for α-acid content of eleven varieties of hops from the 1975 crop are presented in relation to the district of production. For most of the varieties the variation within growths was lower than in the previous year. With amended sampling rates, the tolerance limits of ±0·5 % were achieved for all hop varieties and the results are acceptable for commercial purposes. Some α-acid values for seedless hops from the Seedless Hop Trials are included in the Report and comparisons with seeded hops are shown over a three year period.     

CAMBRIDGE PRIZE LECTURE. STUDIES ON THE SENSITIVITY OF LACTIC ACID BACTERIA TO HOP BITTER ACIDS

Hop bitter acids act as mobile-carrier ionophores. They inhibit the growth of beer-spoilage bacteria by dissipating the transmembrane pH gradient. Their activity is pH dependent. Low pH favours antibacterial activity but high pH reduces it. Resistance to hop bitter acids is a stable character, associated only with beer-spoilage lactic acid bacteria. Hop-resistant organisms can maintain a larger transmembrane pH gradient and ATP pool than can hop-sensitive organisms. Prior exposure of bacteria to trans-isohumulone does not influence the degree of resistance to hop bitter acids. However, in some strains, exposure to trans-isohumulone does induce the ability to spoil beer. The chemistry of these compounds is more complex than previously thought. In aqueous solutions, such as beer, hop acids bind to metal ions and may be covalently hydrated.     

ISO-α-ACIDS CONTENT OF ISOMERIZED HOP EXTRACTS

Two chromatographic procedures for the analysis of iso-α-acids in isomerized extracts have been tested. Although one of the methods—a variant of the Hansen, Hetzel & Miller procedure—performed better than the other, it still falls short of the demands of commercial transactions. The methods available for the assay of iso-α-acids are reviewed.     

HOP INDUSTRY COMMITTEE ANALYSIS OF HOPS OF THE 1984 CROP

Analytical values for α-acid content of thirteen varieties of hops from the 1984 crop are presented in relation to the district of production. For all varieties, except Yeoman, the tolerance aim of ±0·5% was achieved. The production of Yeoman has almost doubled this year and the inclusion of young hops with mature hops in some growths has led to increased within growth variation and failure of this variety, by a small margin, to remain within the limits of the tolerance aim. Apart from this, the variability of the crop compared well with previous years and the results are acceptable for commercial purposes. The sampling rates have been considered and no changes have been recommended.