Chapter 1: Hop (Humulus lupulus L.)-derived bitter acids as multipotent bioactive compounds




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НазваниеChapter 1: Hop (Humulus lupulus L.)-derived bitter acids as multipotent bioactive compounds
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Chapter 1: Hop (Humulus lupulus L.)-derived bitter acids as multipotent bioactive compounds




Adapted from: Marjan Van Cleemput,†‡ Ko Cattoor, Karolien De Bosscher, Guy Haegeman, Denis De Keukeleire, Arne Heyerick. 2009. Hop (Humulus lupulus L.)-derived bitter acids as multipotent bioactive compounds. Journal of Natural Products, Vol. 72, pp. 1220 – 1230.

Laboratory of Pharmacognosy and Phytochemistry, Ghent University-UGent, Belgium. Laboratory of Eukaryotic Gene Expression and Signal Transduction (LEGEST), Department of Physiology, Ghent University-UGent, Belgium.


1.1Introduction


The cultivation of the hop plant (Humulus lupulus L.) has a long history. By the first century BC, the great Roman naturalist Plinius described the plant as “the wolf of the willow” (“lupus salictarius”): wild hops grew around willows and strangled them, which could be compared to the wolf’s behavior towards sheep.18;102 In the 8th century AD, hop gardens commonly surrounded monasteries and the inflorescences were used for medicinal purposes.43 During the Middle Ages, brewers discovered the advantage of adding hops to the brewing kettle as a natural antiseptic and flavoring agent.30 Ever since, the hop plant has been an essential ingredient in beer brewing and about 95% of world-wide cultivated hops is destined for brewing purposes,8 with the remainder used largely for the production of phytomedicines and botanical dietary supplements. Young shoots are eaten in the spring as a culinary delicacy, particularly in Belgium.43

From a taxonomic point of view, the genus Humulus belongs to the family Cannabaceae of the order Urticales, but in 2003 it was incorporated in the order Rosales.16 This genus includes the species Humulus japonicus Siebold and Zucc., Humulus yunnanensis Hu, and Humulus lupulus L., of which the latter is almost exclusively cultivated for brewing purposes.161 Successful cultivation of hops requires optimal growth conditions, especially with respect to the length of day light, the summer temperature, the amount of rain, and the fertility of the soil. Therefore, hops are found in the moderate climatic zones of the Northern and Southern hemispheres, with Germany and the United States by far the largest producing countries.5;8;183;187 The plant is a perennial, dioecious herb, of which the shoots start to grow during spring as bines, having stout stems with stiff hairs to aid in climbing by wrapping clockwise, from 6 m up to 18 m high (Error: Reference source not found). During the summer, the inflorescences of the female plants form hop cones (strobiles, hops), which secrete a fine yellow resinous powder (lupulin) in their lupulin glands.5;18;53 Harvest occurs at the end of the summer or the beginning of autumn, when the content of lupulin is highest. Hops are collected, carefully dried to obtain a residual moisture content of less than 10%, and preferably stored in a cold room (4 °C) to minimize compositional changes. Only female plants are cultivated for brewing; moreover, in many countries it is forbidden by law to cultivate male plants in the vicinity of females. Thus, seed formation is avoided, which is believed to influence negatively hop and beer quality.5;31;65 The bitter and aroma components in the hop cones are very sensitive to oxidation. For this reason, the hop plant is frequently processed into more stable products, such as non-isomerized aerial supercritical or liquid carbon dioxide extracts, distilled hop oil fractions, and potassium solutions of pre-isomerized hop acids that can be directly added to the brewing kettle (non-isomerized products) or post-fermentation (hop oils, pre-isomerized products).28;55;123 (Figure 1 .1) (See Results, Chapter 3, for an overview on commercial processed hop products.) Hop extracts and hop oil fractions are also used as flavoring products in non-alcoholic beverages and foods.53




Figure 1.1 Different types of processed hop products (Botanix).


Since ancient times, hops have been used in folkloric medicine for their claimed anti-inflammatory, antiseptic, antidiuretic, (an)aphrodisiac, hypnotic, sedative, stomachic and properties.18;43;44;58;184 Indian tribes drank hop tea to alleviate nervousness and heated a small bag of leaves to apply in cases of earache or toothache.122 King George III slept on a pillow stuffed with hop cones to alleviate symptoms of porphyria.8;43;192 The German Commission E approved a monograph on hops for use in mood and sleep disturbances. Similar indications are described in an ESCOP (European Scientific Cooperative on Phytotherapy) monograph.8;47;48;53 Today, a wide range of over-the-counter preparations containing hop extracts or hop-derived products is available on the market, in particular for use in the phytotherapy of sleep disorders or pain relief and in postmenopausal therapy.72;150;183;184;193


Recently, investigators have been trying to identify the bioactive ingredients in hops and to elucidate the underlying molecular mechanisms by which they exert their activities. Much of attention has gone to the polyphenolic content of hops, and specific compounds, such as xanthohumol and 8-prenylnaringenin, have been identified as multipotent bioactive compounds (for detailed reviews, see Stevens and Page,170 Gerhauser,57 and Chadwick et al.21). Moreover, increasing evidence reveals that the so-called hop bitter acids (HBA), which represent up to 30% of the total lupulin content of hops, exhibit interesting effects on human health. In the present chapter, we will focus on this group of hop secondary metabolites, and will start from a phytochemical characterization of the main hop acids (including purification, quantification, use in brewing, and degradation). A detailed overview will be provided of the current evidence for the bioactivities and pharmacological properties of HBA, such as their potential to combat cancer and inflammation, effects related to metabolic syndrome, modulation of CNS activity, and bactericidal properties. Finally, the toxicological profile of the hop plant will be described comprehensively.
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