- Beer is one of the most extensively consumed beverages world-wide and it is almost always brewed with hops (Humulus lupulus, L.). Hops provide beer with bitterness, aroma, flavor and texture and also enhance specific beer properties such as foam stability, clarity (colloidal stability), color, flavor stability and microbial stability. Hops are a dioecious species, with female plants producing the hop strobilus (cone). The cone is an inflorescence, which is the entire part of the plant that holds the flowers. Hop cones contain lupulin glands (the source of the hop bittering resins), essential oils, and polyphenols (prenylflavonoids).
Beer prenylflavonoids such as the flavan-3-ols and their condensed products, the proanthocyanidins, represent a class of readily oxidizable compounds capable of hindering or preventing the oxidation of other molecules present in beer. Flavan-3-ols and proanthocyanidins have recently gained significant consideration as potential beer flavor modifiers and/or stabilizers. However their roles in beer flavor stability have not yet been fully realized.
In this study polyphenols were extracted from spent hop (Humulus lupulus L. cv Galena) solid materials and dosed into commercial lager beers. Chemical analysis of the fresh and aged beers confirmed an anti-staling effect of the dosed polyphenol extract as measured by antioxidant capacity assays: FRAP, DPPH• and ESR. The polyphenol rich extract was subjected to phloroglucinolysis and analyzed via reverse-phased chromatography/mass spectroscopy-electrospray ionization (RP-HPLC/MS-ESI) to determine flavonoid content. C-18 RP-HPLC analysis of the extract revealed that it was 99% phenolic in nature, with a procyanidin mean degree of polymerization (mDP) of 2.72.
Based on these findings beers were subsequently brewed with and without hop products (Humulus lupulus L. cv Galena) to target the effect of the complete hop (pellets), hop bittering acids only (CO₂ extract), hop polyphenols only (spent hop solids) and no hop components (Control) on beer flavor and flavor stability. Spent Hop and Pellet Hop beers scored highest in antioxidant potential as measured by the FRAP assay, howeer ESR results were contradictory. Even after force-aging, Pellet hopped beers were lowest in total aldehydes and Control beers were highest in total aldehydes, indicating a protective effect for whole hop products on staling aldehyde formation. Sensorially, the Spent Hop and Pellet Hop beers were characterized by high Piney and Tropical fruit notes, with significant increases occurring after force-aging. The Control beers were rated as being higher in Overall Aroma Intensity, and were judged as being high in Cardboard aroma after force-aging.
Preliminary findings from the brewing trials indicated that significant changes in polyphenol levels occur during accelerated aging. The brewing trial was therefore repeated and beers were profiled for phenolic content and investigated for changes in phenolic content during aging. Beer polyphenols were extracted with Sephadex LH20 resin and subjected to phloroglucinolysis to reveal subunit composition and proanthocyanidin mDP. Although the sephadex extracts were phenolic in nature, proanthocyanidins only accounted for up to 2% of the total phenolic material. Total flavanoid and proanthocyanidin content of the beers increased initially during storage, with eventual decreases occurring after 6 weeks of storage at 30°C. Beers high in hop polyphenols did not suppress the loss of iso-alpha acids during aging and were once again assessed as least flavor stable of the beers by ESR T150.