3. FOOD ChEMISTRy & bIOTEChNOLOGy 3.1. Lectures

Chem. Listy, 102, s265–s1311 (2008) Food Chemistry & Biotechnology
P103 RELATIONShIP bETwEEN xANThOhuMOL,
POLyPhENOLS, AND FLAVONOIDS CONTENT
IN hOP LEAVES AND CONES wITh REGARD
TO CuLTIVAR AND VEGETATION PERIOD
eVA üRGEOVá, PETER KULACS, MáRIA VAŠKOVá
and ľUDOVíT POLíVKA
Department of Biotechnologies, Faculty of Natural Sciences,
University of SS Cyril and Methodius in Trnava, Nám. J.
Herdu 2, 917 01 Trnava, Slovakia,
eva.urgeova@ucm.sk
Introduction
Some secondary metabolites are present in plant cells
in the form of precursors and are activated after pathogen attack
1 . Secondary metabolites having biocide effect include
isoprenoids, phenol substances, organic sulphur compounds,
saponins, alkaloids, proteins, and peptides 2 .
In this perspective, increasing attention is being paid to
the plant called common hop (Humulus lupulus L.). The plant
is diecious. Wild-growing hop grows on wet soils, mainly in
riparian zones, in shrubberies, on the edges of canals and in
similar places 3,4 . All the main components are contained in
lupuline glands 3,5 , although some components, e.g. essential
oils, were also identified in the leaves of the plant 6 . Concerning
the chemical composition, hop resins are prenylated derivates
of floroglucinol, comprising alpha-bitter acids and beta-bitter
acids 5 . In recent years, antibiotic 7 , antifungal 8 , and antioxidant
activities of humulone 9 have been identified. One of the
big groups of substances contained in the hop plant is the
group of polyphenol compounds 10 , phenol acids, chalkones,
flavanols, flavonols, antocyanides, catechines and proantocyanides.
Flavonoids as a group of organic compounds display
a wide range of biological and pharmacological properties 11 ,
including antiviral, antimicrobial, antioxidant, and anticancer
properties 12,13 . Stevens 14 identified three main groups of
flavonoids in the hop plant including prenylflavonoids. The
content of prenylated flavonoids in the hop plant fluctuates
between 0.2–0.6 % 15 . The chemical composition of hop is not
constant; it changes with time and depends on the conditions
of post-harvest storage 4,15 . Regarding the chemical composition,
an important role was played by such factors as the plant
development stage, location, growing practices, weather, and
pressure of pathogens. Langezaal 6 reports that in hop leaves
there are essential oils in much lower concentrations than
in cones. To our knowledge, the content of low-molecular
polyphenyls in hop leaves has not been studied yet.
Experimental
M a t e r i a l s a n d M e t h o d s
We tested the samples of hop, cones and leaves taken
from the Gene Bank of the Slovak Republic in the Institute
of Plant Production, Piešťany (SCPV–VÚRV). The samples
of different cultivars of hop were dried and homogenized.
We used the following cultivars of hop: Osvald’s clones 31,
(K-31), and 72 (K-72), Bor, Sládek, Aromat, Zlatan, and Pre-
s810
miant. We collected the samples before flowering and at the
end of vegetal period.
The content of phenol substances was determined by
Singelton’s method 16 . The amount of 0.2 ml of methanol
extract was mixed with l ml of Folin-Ciocalteu’s reagent
and 0.8 ml of 7.5 % (w/v) na 2 CO 3 . Absorbance was measured
at 750 nm wave-length after 30 minutes of incubation
in dark. The content of polyphenols was compared with the
absorbance of galic acid.
Flavonoids were determined by Rakotoarison’s
method 17 . 1 ml of methanol extract was added to 1 ml of methanol
solution of AlCl 3 . 6H2 O, 2% (w/v), and absorbance was
measured after 10 minutes at 394 nm. Quercetin was taken as
the standard.
Xanthohumol was determined by HPLC. Dry material
was homogenized with methanol in a sonificator (Bandelin
Electronic, Germany) for about 10 minutes, and then purified
by centrifugation and filtration on a column (Waters, USA).
Eluate (5 µl) was injected into the HPLC instrument and detected
at 370 nm. The mobile phase consisted of 900 ml of
methanol with 100 ml of water. The analytical column was
nucleodur Sphinx RP EC 150/4.6 (5 µm). Xanthohumol was
purchased form Sigma, Germany.
Results
The concentration ranges of phenol substances, flavonoids
and xanthohumol in hop cultivated in the Slovak Republic
are summarized. The content of polyphenols and flavonoids
was monitored during the vegetal period of plant growth. All
cultivars showed different contents of secondary metabolites
during the vegetal period, their content in leaves decreased
from the end of June to September (Table I).
Table I
Polyphenols content in hop leaves during the vegetal period
Cultivar Polyphenols [mg g –1 DM]
06/07 * 09/07 *
K-31 8.83 1.22
Bor 5.00 2.25
Sládek 5.76 2.04
K-72 7.46 0.99
Aromat 6.56 0.98
Premiant 5.52 0.90
Zlatan 6.23 1.82
* 06/07 and 09/07 – date of sampling
Phenol substances were in the interval between
0.90–2.25 mg g –1 of dry matter in leaves, and from 4.0 to
21.1 mg g –1 of dry matter in cones (Table II) at the end of the
vegetal period.
The content of polyphenols was dependent on the type
of cultivar and on climatic conditions as well. Comparison
of the contents of polyphenols in extracts from hop cones in
2006 and 2007 is shown in Fig. 1. The content of phenol substances
in leaves did not basically relate to the content of phe-