Tanning Properties of Flavanols in Barley and Hops Measured by Reaction With Cinchonine Sulphate in Relation to Haze Formation in Beer

Abstract

The flavanoid polyphenol extracts from barley and hops were each separated into six fractions by adsorption chromatography on Sephadex LH20. These fractions were further characterised by several analytical methods, including high-performance liquid chromatography and a colorimetric measurement of polymerisation index. The tanning powers of the fractions were graded according to their reactivities with cinchonine sulphate solution in a standardised turbidometric test. Whereas, almost 75% of the flavanols from Ark Royal barley were non-tanning oligomers almost 96% of the flavanols from Bullion hops were polymeric tannins. Reactivity of most of the barley flavanols with cinchonine sulphate was increased greatly by oxidation with peroxidase and hydrogen peroxide. Some effects of polymerisation, caused by enzyme action or by exposure to air, on oxidisable polyphenols (nontannins) were measured using ( + )-catechin. procyanidin B3 and prodelphinidin B3 in model systems. These, and other measurements on experimental and commercial beers indicated that oxidation of simple flavanols from barley produced polymers with tanning properties. In contrast, the hop flavanols when extracted apparently in their native forms, were capable of co-precipitating with polypeptides in beer. Treatment of beers with different stabilising agents, such as Polvelar AT and silica hydrogel, retarded haze formation by restricting 'proteinpolyphenol' interactions.Key words: flavanols, haze, high-performance liquid chromatography. oxidation, polyphenols. tannins. IntroductionAlthough it is recognised that non-biological haze in beer may be generated through several possible routes,30 the popular and time-honoured concept is that the tanning of polypeptides" by flavanoid39 polyphenols is the primary cause. Monomeric and dimcric flavanols14"1920"-43 such as are present in barley,39 have relatively weak tanning powers themselves but can be transformed19 either by oxidation or acid-catalysis into polymeric tannins. Modern chromatographic methods28-3"6 have confirmed earlier claims9-20 that beers invariably contain pools of flavanoid haze precursors. Fortunately, acceptable colloidal stability can be achieved by a variety of methods,2"'■4246-52 not the least of which is the prevention of oxidation.41 Owing, perhaps, to the suc cesses of contemporary stabilisation procedures interactions of the heterogeneous beer 'proteins' and polyphenols have not been studied in detail. While probing the chemical mechanism of some local processing features known to influence colloidal stability a need was seen for a realistic experimental model of protein-polyphenol interactions in wort and beer. Consequently, it was decided to elaborate on the investigations made with selected simple flavanols141619 and include more examples from the wide range of flavanoids"-36-18 obtainable from barley and hops.The work described herein preceded an investigation of protein-polyphenol interactions which will be reported later.37 At the outset it was necessary to isolate flavanols fr...

Effect of the Removal of Sensitive Proteins and Proanthocyanidins on the Colloidal Stability of Lager Beer

“…As an alternative to sample pretreatment, the use of more specific detection with the electrochemical detector was preferred. The amounts of monomeric and dimeric flavanols measured were similar to those reported by others (7,18,26,30,31,32) and the significance of such low concentrations (1-5 mg/L) in beer is in their effects on colloidal stability (2,7,8,17,22,24,35,37,40). Whereas it is certain that such small amounts of dimeric flavanols in beer can considerably increase the rate of chill haze formation, it seems that much larger concentrations of catechins may be required to exert the same destabilizing effect (2,7,8,22,37).…”

“…For several years it has been known that PVPP adsorbs from beer a complex mixture of phenolic substances and nitrogenous constituents (24). Our analyses of unstabilized beer showed that the only simple flavanoid constituents were prodelphinidin B3, procyanidin B3, catechin, and epicatechin.…”

“…Monomeric flavanols, flavonol glycosides, phenolic acids, 4dimethylaminocinnamaldehyde, and HPLC-grade solvents were obtained from sources mentioned previously (23)(24)(25)(26)(27)(28)(29).…”

Reducing power of hop cultivars and beer ageing

“…In order to remove humulones, 1 g of ground hop pellets was ®rst extracted three times with 7 ml diethyl ether (super purity solvent, Romil, Loughborough, England) by shaking for 15 min and sonicating for 5 min (McMurrough & Hennigan, 1984). After centrifugation (3500 g, 10 min), the supernatant was removed.…”

Characterization of Stabilized and Unstabilized Beers

“…For stabilization Protesal at 0.05 g/L was used. Test A was conducted under the same conditions as Control A with the use of stabilizing agents such as Polyclar AT (an insoluble, high-molecular weight, cross-linked form of polyvinylpolypyrrolidone) which is reported to remove phenolic impurities (Helin 1980;Siebert et al 1981;McMurrough and Hennigan 1984). PVPP was used at 0.2 g/L and silica hydrogel at 0.5 g/L.…”