Hop leaves that are discarded during beer brewing are packed with antioxidant polyphenols that could protect against dental disease, say researchers.
The findings come after Japanese scientists assessed the chemical make-up of hop leaves (also known as bracts), following previous reports that they contain substances that may be useful in battling dental cavities and gum disease. The team identified three completely new compounds in samples of the bracts, in addition to identifying many other previously known substances in hops for the first time.
"Three new compounds, one known compound identified for the first time in plants, and 20 known compounds that have not been reported in hops, were found," the Japanese team revealed."The hop bract extract also contained an abundance of highly oligomeric proanthocyanidins, which consisted of B-type procyanidin structures."
Every year thousands of tons of hops are harvested for beer brewing globally. However, since the bracts are not used for making beer, they are generally discarded - thus meaning there is potentially a large amount of bracts that could be used for other applications.
Led by Yoichi Shibusawa from Tokyo University of Pharmacy and Life Sciences, the team explained that extracts from these bracts have previously been shown to stop the bacteria responsible for these dental conditions from being able to stick to surfaces - while also preventing the release of certain bacterial toxins.
Writing in the Journal of Agricultural and Food Chemistry, they noted that this previous research has suggested "a possibility for the beneﬁcial and industrial use of the hop bracts that are unutilized from beer brewing."
"However, unlike lupulin glands, the complete composition of the polyphenolic compounds in hop bracts has yet to be fully elucidated. Therefore, we started to perform research on the comprehensive separation and structural analyses of polyphenolic compounds from a hop bract extract," wrote Shibusawa.
The Japanese scientists used a novel type of chromatography to analyse the chemical composition of hop bracts.
"Over 100 types of constituents were effectively isolated from only 25 g of extract in high yields by high-speed countercurrent chromatography followed by hydrophilic interaction chromatography and reversed-phase high performance liquid chromatography," they explained.
In their analysis Shibusawa and his colleagues identified three totally new compounds in addition to one already-known compound that was identified for the first time in plants. Twenty compounds that were already well known but had not been reported from hops were also identified in the bract extract.
"Most of the compounds were hydrophilic glycosylated and/or esterified analogues of abscisic acids, hydroxycinnamic acids, flavonols, lignans, hydroxybenzoic acids, or carotenoid," explained the team.
Bracts were also found to contain substantial amounts of proanthocyanidins, which are healthful antioxidants.
They suggested that further studies which investigate the biological effects of the newly identified compounds is now needed in order to maximise the industrial potential of hop bracts.
Source: Journal of Agricultural and Food Chemistry
Published online ahead of print, doi: 10.1021/jf405544n
"'Comprehensive Separation and Structural Analyses of Polyphenols and Related Compounds from Bracts of Hops (Humulus lupulus L.)"
Authors: Yoshihisa Tanaka, Akio Yanagida, et al