Hop bitter principles

The bitter substances from hops, which are important in brewing, are also phloroglucinol derivatives. The humulones, lupulones and corresponding iso-compounds can be separated on silica gel G layers, using weakly polar solvents [4, 132, 211, 214]. The method proposed by Grant [67] for detection of hop constituents in beer or in extracts, appears most serviceable  

Beer samples and extracts are extracted with n-hexane. The extract is carefully concentrated and taken up in methanol and the solution cooled to about 1° C so that waxes separate out. After centrifuging, the clear liquid is concentrated and can be applied directly to the silica gel GF254 layer. A mixture of 2,2,4-trimethyl-pentane-isopropanol-formic acid (83.5 + 16.5 + 0.5) is employed as solvent. The substances can be detected through their quenching of the fluorescence and can thus be isolated and submitted to further identification procedures. All absorption maxima are near 274 nm yet the specific bitter values vary greatly. The usual phenol 

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Industrial plants are working at 300 to 350 bar and 40 to 65°C. Under these conditions all the hop bitter-principles are extracted quantitatively. The content of total resins in the extract is about 90% and a part of the hard resins is extracted, which is not the case with subcritical conditions. 

The TLC plate is inspected immediately after spiraying. The hop bitter principles and oleiiropein turn yellow-brown to yellow-green (vis). 

Biosynthesis H. belongs to the group of meroterpenes, i. e., isoprene units are linked with components derived from oAer key building blocks. The biosynthesis of the ring structure of the hop bitter principles H. and lupulone proceeds through the polyketide pathway. The isoprene units are formed on the deoxyxylulose pathway. 

Humulone, the bitter principle of hops, does, however, spread with a ring apparently lying flat on the surface it forms a very remarkable incompressible film, of area 90 sq. A. (k). 

Glandular trichomes can be separated from the strobiles by sifting and this material is known as lupulin. Hops contain bitter principles (15-25%) consisting of two types of resin, hard and soft, which are important in beer production. 

The adulterations of malt liquors are numerous and varied. Sodium carbonate is added with the double purpose of neutralizing an excess of acetic arid and increasing the foam. The moat serious Multention consists in the introduction of bitter principles other than hops, and notably of etrychnine, cocculus indicus (picrotoxin), and picric acid. 

Synonyms Bitter ash Bitter wood Picrasma excelsa Quassia amara Definition Wood of Picrasma exceisa or Quassia amara, contg. bitter principle quassin Properties YIsh-wh. to bright yel. chips or fibrous coarse grains, si. odor, very bitter taste Uses Natural flavoring agent in foods and pharmaceuticals bitter tonic medicine (anthelmintic) extract as fly poison on flypaper surrogate for hops hair lotion alcohol denaturant... 

The hop resins are not very soluble in wort the solubility of humulone and lupulone is dependent on temperature as illustrated in Fig. 14.5. [63]. Thus at the pH of wort (5-0) only 40 mg of humulone is soluble in one litre of water at 25°C and 60 mg/1 at 100°C. The solubility of lupulone is much less 1-2 and 9 0 mg/1 at 25°C and 100°C respectively. If, therefore, hops containing, say, 5% of a-acid are used at a rate of 0 28 kg/hl (1 Ib/barrel), all the a-acid could go into solution at 100°C to produce a wort containing 56-6 mg of a-acid/1. However, on cooling such a wort, humulone would be precipitated on to the break from the supersaturated solution. After fermentation, when the pH drops to 4 0, and the wort is conditioned at about 0°C any humulone in excess of 5 mg/1 (Fig. 14.5) will be precipitated, and in practice only small amounts of humulone are detected in finished beer. The most important bittering principles found in beer are the iso-a-acids formed from a-acids during wort boiling. 

This method gave a better correlation with the iso-a-acid content found by countercurrent distribution than the first method which omits the acid washing stage. It was subsequently found that the acid washing stage removed three unknown bitter principles from beer brewed with old hops [112]. 

The nature of the other bittering principles in beei brewed from old hops is still largely unknown. These substances probably contain more oxygen than the iso-a-acids, are more polar, and incompletely extracted into isooctane. The abeo- %o-aL-2iC ds (p. 487) are such compounds, the concentration of which in lager beers was reported to be 88-160 mg/1. Subsequent work established the presence of polyphenols in the extracts analysed and found that the concentration of f6 o-iso-a-acids in English beers was less than 6 ppm [57]. Many other oxidation products of the hop resins have been detected in beers (see Chapter 12) but in most cases it has not been established that they are normal constituents. 

Some of the bound keto groups may be reduced by pyridine nucleotide-dependent dehydrogenases (C 2.1, see the formation of 6-methylsalicylic acid, D 3.3.1), and some of the activated CHg-groups may be alkylated by S-adenosyl-L-meth-ionine (C 3.3, see the biosynthesis of tetracyclines, D 3.3.7), or may be substituted by dimethylaUyl pyrophosphate (D 6), see the formulas of lupulone and humulone, the bitter principles of hop cones used in brewing beer (F 1). 

By the end of the 19th century, the dual properties of the flavour and preservative value of hops had become the subjects of investigation by chemists. In the 1880s the effect of the hop resins on beer spoilage bacteria had been demonstrated [1,2] and later their influence on some pathogenic organisms [3]. The resin fractions designated alpha and beta were separated and their bactericidal activities compared [4]. For a while, the resin content of a hop was used as an indication of preservative value (P.V.). It was found that the resins contained the bitter principles while the aroma was predominantly due to the essential (volatile) oil. 

This does not mean however that all problems in hop chemistry and hop utilization in the brewery have been solved. Far from it Even the simple and still fundamental question What is a good-quality hop " asked of us by a local brewer in Gent (Andre Van der Stricht) who initiated our research in 1945, has not yet received a definitive answer. Many more issues about hop and beer bitter principles could be tackled by fundamental research, some of which may be considered to be important enough to warrant the effort. 

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