CHEMISTRY OF HOP POLYHPENOLS
Dr Max L. Deinzer, Professor, Department of Chemistry, Oregon State University, Corvallis, OR
Beer is the most widely consumed beverage in the world. Of all the herbs that have been used in beer, only the hop (Humulus lupulus L., Cannabinaceae) plant has gained widespread acceptance and is regarded as an essential raw material in the brewing industry. Hops are perennial plants grown on trellises, and different varieties are derived from breeding programs. The hop plant is dioecious and cultivated in most temperate zones of the world for its female inflorescences, which are commonly referred to as hop cones or simply hops. The female flower clusters are partly covered with lupulin glands, while male flowers have only a few glands in the crease of their anthers and on their sepals. The resin secreted by these glands contains bitter acids, essential oils, and flavonoids (flavonol glycosides, prenylflavonoids, and proanthocyanidins). Hops or hop products account for the bitter taste and the flavor of beer. In addition, hops have a favorable influence on the stability of beer foam and contribute to the microbiological stability of beer. Different hop cultivars are traditionally classified into three groups, i.e. aroma (noble) hops, e.g. cv. Saaz, Hallertau, Hersbruck, Cascade, Mt Hood, high α-bitter hops e.g. cv. Northern Brewer, Wye Target, Nugget and intermediate α-bitter hops, e.g. cv. Kent, Cluster, Brewers Gold. Modern brewing technology allows for the post-fermentation introduction of a variety of hops or hop preparations to adjust bitterness levels and to control foamability. Brews may benefit from proper selection of particular hop varieties that add subtle tastes and flavors. Among various hop constituents, hop proanthocyanidins have attracted increasing attention, and have been considered as the most reactive of hop polyphenols.
Proanthocyanidins (PAs), better known as condensed tannins, are flavan-3-ol oligomers and polymers that give anthocyanidins upon acid-mediated oxidative depolymerization reactions. There is a growing body of evidence linking these compounds with plant defense mechanisms, organoleptic characteristics, and stabililizing effects of color in plants. PAs exhibit general toxicity towards fungi, yeast and bacteria. They account for astringent properties of many commonly consumed fruits and their beverage products presumably because of interactions with salivary proteins that might contribute to astringency. They also exhibit a wide range of biological activities through their action as antioxidants that leads to protection against cardiovascular disease, immune disorders, and neurodegenerative diseases.
PAs or oligomeric PAs, names that are often used interchangeably, consist of sequences of phenolic compounds built from single monomer units called afzelechin, epiafzelechin, catechin, epicatechin, gallocatechin and epigallocatechin. Although usage of the word “oligomeric” varies somewhat, dimers, trimers and up to heptamers are generally referred to as oligomeric (n = 2~7), while larger chains are referred to as polymeric or tannins (n = 8 24,). These flavan-3-ol monomer units are sometimes esterified with gallic acid to form 3-O-gallates. Proanthocyanidins can be divided into A-type and B-type PAs. The latter are flavan-3-ol oligomers and polymers linked mainly through C4→C8 and sometimes C4→C6 bonds. A new analytical protocol based on LC/ESI-MSn was used to identify a total of 50 PAs including 3 flavan-3-ol monomers, 13 A-type and 34 B-type in hops. This is the first time to our knowledge that such an extensive and diverse group of PAs has been observed in hops.