Bioactive substances in hops and beer - (01/07)
The contents of prenylflavonoids (xanthohumol, desmethylxanthohumol, isoxanthohumol) were investigated in Czech and world hop cultivars during the period 2000-2004. The concentrations of xanthohumol varried between 0.2 and 1.2% (w/w) with the highest amounts found in the high-alpha hop cultivars Taurus, Admiral and Agnus. The highest ratio of xanthohumol/alpha acids, greater than 10-1, was determined for Sladek, Czech hybrid aroma cultivar. A positive correlation was observed for the xanthohumol and alpha acids. Fresh hops contain also small amount of isoxanthohumol up to 2.10-2 % (w/w). Desmethylxanthohumol was present in hop cultivars in the range of 0.05 to 0.30% (w/w). The contents of prenylflavonoids, predominantly isoxanthohumol, in Czech beers were usually between 0.3 and 1.9 mgL-1. Low isoxanthohumol levels were found if hop CO2 extracts were used. Prenylflavonoids were very minor constituents in draught or diet beers. Losses of isoxanthohumol on beer production were highest during wort cooling and trub removal.
Bioactive substances in hops and beer
Hops have a long history in herbal history. They have been used to treat a variety of complaints. Hops have been used for the relief of insomnia, as herbal antibiotics and antiflammatory agens. Hops also have a reputation for ability to treat menstrual disturbances. Recent research in bioactive effects revealed many remarkable properties of many compounds contained in hops. Pharmaceutical companies have taken increasing interest in herbal remedies and hops have been considered as a potential source of new-plant derived medicines. Practically all hops secondary metabolites exhibit more or less pronounced bioactive effects (Kondo, 2003).
Antioxidative activity of hop bitter acids and their analogues has a cancer preventive effect since they can quench free radicals which cause oxidation of the DNA in the body and thus cause genetic defects (Tagashira, 1995). Humulon has been known for its preventive effect against osteoporosis (Tobe, 1997). Humulon hinders the growth of certain leukemia cells especially with the combination of vitamine D. Humulones (a-acids) were found to be present in beer at concentration levels of 150-200 mgL-1, while b-acids were absent (Hofte, 1998). The results of one recent study confirmed that beer components are protective against the genotoxic effects of heterocyclic amines on target organs associated with tumorigenesis „in vivo“ (Arimoto-Kobayashi, 2005). Isohumulones prevent the developments of non-insulin dependent diabetes and hyperlipidemia. Oral administration of isohumulones may improve insulin sensitivity in patients with type 2 diabetes (Kondo, 2003). Antibiotic activities were detected not only in hop resins but in the essential oils as well. Althought less performant than traditional antiseptics, hop oils components are able to eliminate specifically some inauspicious microorganism of the skin, pathogens or saprophytes with high levels of odour (Chaumont, 1997).
Recently the high attention has been devoted to hop prenylflavonoids, plant secondary metabolites that are prominently present in hops (Humulus lupulus L.), (Stevens, 1997). The most important hop prenylflavonoids are xanthohumol (X), desmethylxanthohumol (DMX), isoxanthohumol (IX), 6-prenylnaringenin (6-PN) and 8-prenylnaringenin (8-PN). Importance of prenylflavonoids consists in wide range of biological effects (Stevens, 1999a; Miranda, 1999). Xanthohumol enters biochemical pathways of many xenobiotics and promote to eliminate them out of organism. For example X at micromolar levels stimulates the activity of detoxifying enzyme quinonereductase. Stimulating effect of some enzymes contributes to cancer prevention (Miranda, 2000). Hops are also source of phytoestrogens. 8-Prenylnaringenin is the most phytoestrogen currently known (Milligan, 1999). Phytoestrogens are plant-derived substances with oestrogenic activity. They are thought to protect against several chronic diseases or breast and prostate cancer (Walker, 2000). The composition of prenylflavonoids largely depends on the variety. The ripening stage and storage conditions constitutes up to 90 % of the mixture of prenylflavonoids (Biendl, 2002). During beer brewing X is isomerized to IX, hence this is the most prevalent prenylflavonoid in beer (Fig. 1). Bioactivity of isoxanthohumol is usually lower than of xanthohumol. It is compensated by higher concentrations of IX and easy availability in beer. Similarly DMX is converted to 6-PN and 8-PN. Beer is the most important source of prenylflavonoids in the human diet (Stevens et. al, 1999b).