Hop bittering substances

A Forster. Investigation by means of HPLC of hop bittering substances in beer during different ageing conditions. Proc Conv Inst Brew (Aust NZ Sect) Brisbane 20 109-117, 1988. 

R Franiau, R Mussche. Quantitative determination of hop bitter substances and their derivatives in hop extracts by thin layer chromatography. J Inst Brew 80 59-67, 1974. 

Most substances dissolved in the medium surrounding yeast cells diffuse freely through the cell walls to the plasma membranes. There is adsorption of certain materials by the outer layers of the cell walls during the diffusion. For instance, hop bitter substances, polyphenols, and nitrogenous compounds of wort tend to be adsorbed. The plasma membrane isolates the living cell or protoplast from its environment and controls the movement of materials in and out of the cell. Substances which are soluble in lipid, an important constituent of the membrane, tend to penetrate readily. Thus, unbranched long-chain fatty acids and a-oxo-acids penetrate more quickly than the corresponding short-chain acids which are less soluble in lipid. If the acids are dissociated, they enter the cell either slowly or not at all. 

Viable yeasts in pressed or slurried form may be autolysed or plasmolysed to yield extracts. Surplus brewers yeast is usually debittered (the adsorbed hop bitter substances removed) by treatment with alkali before processing. 

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. 

By far the most important constituents of hops are the bitter substances, humulones (a-acids), lupulones (P-acids), and then oxidation products. 

The extraction of the bitter substance of hops is a complex process. During boiling and subsequent fermentation, large losses of bitter substances are incurred, and only 25—30% of the bitter substances in hops are present in the finished beer. The amount of loss depends on composition of the wort, pH, etc. Much of these bitter substances is adsorbed by the warm sludge and is lost during its separation. Wort boiling takes place in the wort kettle or copper of various constructions. An example appears in Figure 7. 

Dried raw hops contain about 13 to 23% bitter substances, as... 

Many bitter substances have been mentioned as substitutes for hops, but they are very seldom found in beer. 

Fractions of three groups of substances used in beer production are extracted from hops using supercritical C02. The first fraction, the so-called oil essence, was obtained via extraction with C02 at a density of 0.30 g mL 1 and a temperature of 50°C. Bitter substances were collected as the second fraction at a C02 density of 0.70 gmL 1 (50°C) that fraction overlapped only slightly the third and last fraction of neutral fats, extracted at a C02 density of 0.90gmL 1 (50°C). 

The content of bitter substances is closely related to the dosage of hop. The flavouring is steamed down to the desired concentration by cooking it for 70-120 min. The protein coagulates, different valuable hop components are diluted and the enzymes inactivated. 

Beer flavor. Primary odor and taste substances from malt and hops determine the type of beer bitter substances (see humulone) and aroma substances of hops characterize Pilsener beer, a relatively high content of Furaneol provides the caramel note of dark beer. The most important aroma substances in light full beer are... 

Humulone (7), from Humulus lupulus (Cannabaceae), is partly responsible for the bitter flavor in beer. About 35 ppm of hop-derived substances contibute a specific bitter taste and hoppy flavor to beer. Hops also contribute for the microbiological stability and the stability of foam of beer (De Keukeleire, 1991). However, the activity comes not from the native hop compounds, but from iso-a-acids produced by boiling the wort-containing hops (De Keukeleire et al., 1992 Bonded et al., 1987). 

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 ... 

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. 

Wort boiling with hops or hop products is done in a brew kettle (hop kettle) in which the initial and subsequent worts from the lautering step are collected. Addition of hops is adjusted according to the type and quality of beer desired. The quantity (in hop cones/hectoliter) for fight lager beer is 130-150 g for Dortmund-type beer, 180-220 g for Pilsener beer, 250-400 g for dark Munich beer, 130-170 g and for malt beer and dark bock beer, 50-90 g. The critical factor is the content of bitter substances in the hops selected. The utilization of the bitter substances (a-acids) is only 30-35%. 

Hops accumulate bitter substances called bitter adds in the mature female inflorescences (cones). The level of bitter substances usually amounts to about 5 20% of the cones weight (water content... 

In addition to bitter substances, hop cones contain a number of monoterpenes and sesquiterpenes (0.5 3%) that carry their own characteristic aroma. Some non-volatile components present in the hop, such as polyphenols (3-6%), contribute to a full mouth feel during beer tasting. Important non-volatQe components of hop cones are also prenylated flavonoids that show oestrogenic and anti-carcinogenic effects. 

In 1952 Rigby and Bethune detected by counter-current distribution (CCD) the heterogeneous character of the hop iso-alpha acids, the bitter substances in beer (42). They also showed that the hop alpha acids themselves were not homogeneous. After 100 transfers in the two-phase system iso-octane aqueous buffer pH 8.0 a new compound was isolated, namely cohumulone (43). 

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. 

The primary aromatic substances in beer are derived from raw materials (barley or hops) that confer the beer s typical odour and taste. Bitter acids of hops have a bitter taste (see Section 8.3.5.1.3), but hop cones also contain 0.3-1% m/m of terpenoids (60-80% of hop essential oil), which have a considerable influence on the smell of beer. The main components of aromatic hop oils are sesquiterpenic hydrocarbons in which a-humulene, P-caryophyllene and famesene dominate. The major monoter-penic hydrocarbon is myrcene. For example, the essential oil content of fine aromatic varieties, such as Saaz, is 0.8% m/m, of which 23% is myrcene, 20.5% a-humulene, 14% famesene 6% and P-caryophyUene. Significant components of the hop aroma in beer are mainly isomeric terpenoid monoepoxides resulting from autoxidation and diepoxides of a-humulene and fS-caryophyUene, but also other terpenoids. Important components of hops odour are also various alcohols (such as geraniol and hnalool), esters (ethyl 2-methylpropanoate, methyl 2-methylbutanoate, propyl 2-methylbutanoate and esters of terpenic alcohols, such as geranyl isobutanoate), hydrocarbons, aldehydes and ketones formed by oxidation of fatty acids, such as (3E,5Z)-undeca-l,3,5-triene, (Z)-hex-3-enal, nonanal, (Z)-octa-l,5-dien-3-one, their epoxides, such as ( )-4,5-epoxydec-2-enal and sulfur compounds. Other important components of hops are so-called polyphenols (condensed tannins) that influence the beer s taste and have antioxidant effects. Less important compounds are waxes and other hpids. Hop products, such as powder, pellets and extracts (by extraction with carbon... 

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