Contents of prenylflavonoids in Czech hops and beers

Prenylflavonoids are natural substances contained in hops. Thanks to their remarkable bioactive effects they have recently become a subject of medical and pharmaceutical research. The most important prenylflavonoid of hops is xanthohumol (X). Other representatives of the same group contained in hops are desmethylxanthohumol (DMX), isoxanthohumol (IX) and 8-prenylnaringenin (8-PN). Among others, anti-cancer, anti-inflammatory, oestrogen and anti-microbial effects were proved in these substances. With respect to taxonomy, prenylflavonoids belong to polyfenols of the chalcon series. They actually represent a transition between hop resins and polyfenols. Together with resins and essences, they are made in lupulin glands. This characteristic is used to analytically determine them. Xanthohumol and humulon inhibit the process of calcium absorption in bones and thus work against osteoporosis. Prenylflavonoids enter into biochemical reactions of degradation in number of xenobiotics and help to remove them from the organism. At present, 8-Prenyl-naringenin is considered one of the most effective phytooestrogens. The contents and composition of hop prenylflavonoids depends on variety, hop ripeness, storage conditions and the processing method after harvest. Xanthohumol forms up to 90% of the total prenylflavonoids contained in hops. During beer production, xanthohumol isomerises to isoxanthohumol, which is the most important prenylflavonoid in beer. Similarly, desmethylxanthohumol isomerises to 8-prenylnaringenin and 6-prenylnaringenin (6-PN).

Analytical determination of hop prenylflavonoids is carried out by liquid chromatography, usually at the same time as alpha and beta analysis of bitter acids together with UV or weight detector. To determine xanthohumol and DMX in hops and isoxanthohumol in beer, a UV detector is sufficient. To determine minority prenylflavonoids, a weight detector (LC/MS) must be used. The contents of xanthohumol and desmethylxanthohumol in Czech varieties of hops have been regularly determined for several years at the Hop Research Institute in Ratec. The typical contents of the named prenylflavonoids in Czech hops are listed in Table 1. The values listed in Table 1 are the results of measuring a few dozen hop samples of each variety from all hop-growing regions for the past four years. The contents of xanthohumol in Czech hops varies between 0.25 to 1.1% of weight. The highest contents of xanthohumol can be found in the Agnus variety. The Sládek variety is remarkable for its high ratio between the contents of xanthohumol and alpha acids. In this respect, this variety beats all commonly grown foreign hops. This parameter is, from a brewing point of view, more important than the absolute contents of xanthohumol as it shows the amount of prenylflavonoid brought into the brewing process during hopping. The lowest contents of xanthohumol were measured in Saaz hops; however, considering the relatively low contents of alpha acids, the ratio X/alpha is comparable with the Sládek variety. Desmethylxanthohumol occurs in Czech hops in amounts of 0.05 to 0.25% of the total weight. The most desmethylxanthohumol can be found in the Sládek variety, the least in the Bor and Premiant varieties. A positive correlation was proved between the contents of alpha acids and prenylflavonoids.

Table 2 shows the contents in prenylflavonoids in the selected Czech beers. The evaluation performed during the course of 2004 included light draught as well as lager beers, one sample of diabetic beer and one sample of black beer. The results show that it is isoxanthohumol, which appears in the highest amount in beer. The concentrations vary considerably, from trace amounts up to 2mg/l. The contents of minority prenylflavonoids do not exceed 0.1mg/l. The highest contents of prenylflavonoids among the tested set of Czech beers was found in Pilsner Urquell. The decisive factor determining the level of prenylflavonoids in beer is hopping, i.e. the time scheme of hop additions, selection of varieties and hop products. If carbon-dioxide-based hop extracts or so-called "down-stream" products are used for hopping, then the contents of prenylflavonoids in beer is insignificant. This fact is also confirmed by some values in Table 2. Lager beers, usually hopped by a smaller proportion of extracts, contain more isoxanthohumol than draught beers. It is known that during brewing, significant losses of prenylflavonoids occur. Monitoring of isoxanthohumol contents in semi-finished products and finished beer in two Czech breweries proved that the greatest losses occur during the cooling of hopped wort, fermentation and beer filtration. The total loss of isoxanthohumol (hot hopped wort => finished beer) amounted from 50 to 60%.

Table 1: Contents of xanthohumol, desmethylxanthohumol and alpha acids in Czech hops

Despite great losses during the production process, beer remains the most important source of prenylflavonoids in human nutrition. Beer is actually a very healthy drink if consumed regularly and in a reasonable amount. It contains a wide range of beneficial substances, from sugars, amino acids, and polyfenols to minerals and vitamins. Therefore, it is not way off the mark to compare beer to "liquid bread."

Table 2: Contents of prenylflavonoids in a selection of Czech beers