The Role of Copper, Oxygen, and Polyphenols in Beer Flavor Instability

An assessment of the impact of oxygen and hydrogen peroxide on mashing and wort parameters has been made on a laboratory scale. Oxygen has been stridently eliminated by using an anaerobic chamber during mash analysis. Additionally the relative importance of proanthocyanidin species has been assessed by comparing the behaviour of "conventional" malt and a malt produced from a low proanthocyanidin variety. It seems that oxygen and peroxide act independently in causing the oxidation of thiolcontaining materials and polyphenols in mashes and that oxygen is not primarily exerting its impacts through the intermediacy of peroxide. The removal of thiols (presumably at least in part through the production of disulphide bridges between proteins) and of polyphenols (presumably via polymerisation) both contribute to increased wort turbidity and decreased rates of wort separation after mashing. Three inhibitors (nordihydroguaiaretic acid, ethylenediamenetetraacetate and potassium cyanide) have been employed in an attempt to differentiate between enzymic and non-enzymic events and also to identify whether lipoxygenase and peroxidase are catalysing key events. Whilst it seems that peroxidase has a key role in catalysing the oxidation of polyphenols by H 2 O 2 , it does not appear that either peroxidase or lipoxygenase is involved in the removal of measurable thiol. Nonetheless a significant proportion of the thiol elimination is likely enzyme-catalysed. We have been unable to demonstrate the production of hydroperoxides in mashes, but added hydroperoxide is undetectable, which suggests that these materials are either lost by onward conversion or by adsorption onto spent grains.

Section: -2863(9'8-32mentioning

confidence: 99%

Laboratory-Scale Studies of the Impact of Oxygen on Mashing

Stephenson

Biawa

Miracle

et al. 2003

“…The effect of oxygen, reactive oxygen species (ROS), has the central role 39 . The mechanisms of reactive oxygen species formation catalysed by metal ions 2,14 as well as some mechanisms of formation and reactions of free radicals resulting in carbonyl formation 8,[14][15][16]18,20,48,49,51 have been explained, others are still being studied. In the sequence of radical reactions which proceed in wort and beer, and which result in the formation of stale flavour carbonyls, antioxidants play an important role, because they can directly or indirectly affect carbonyl content in beer.…”

Section: -2863(9'8-32mentioning

confidence: 99%

“…Some classes of polyphenols (dihydroxyflavans such as catechin and procyanidin B3) can protect sensitive substances from oxidation and are not able to act as prooxidants, while other classes (trihydroxyflavans, such as prodelphinidin) can act as pro oxidants in the mechanism of beer staling 14 . Both malt and hops are sources of beer polyphenol substances and the quality of raw materials can affect sensory stability 5,23 .…”

Section: -2863(9'8-32mentioning

confidence: 99%

The Role of Malt and Hop Polyphenols in Beer Quality, Flavour and Haze Stability

Mikyška

Hrabák

Hašková

et al. 2002

Pilot-scale brewing trials of a 12°P pale lager beer were conducted to look at the effect of a modified dose of hop and malt polyphenols on haze, flavour quality, and stability. Results confirmed that malt polyphenols, and particularly hop polyphenols, in the course of wort boiling, improved reducing activity values and the carbonyl content in fresh and stored beers. Hop polyphenols significantly increased reducing activity and decreased the formation of carbonyls (TBA value) in fresh and stored beer. Reduced content of malt polyphenols, combined with the use of hop CO 2 extract, caused an increase in the TBA value in beer. PVPP stabilized beers tended to be lower in reducing activity. Both malt and hop polyphenols affected the intensity of "harsh taste" in fresh beers and a significant influence from PVPP stabilization of beer was not observed. The staling degree of forcedaged beers depended on the polyphenol content in the brewhouse. Both hop and malt polyphenols had a positive impact on flavour stability. PVPP treatment of beer had a positive effect on the flavour stability of heat-aged beers. Polyphenols, especially hop polyphenols, slowed down flavour deterioration during the nine month storage period, but the primary effect was seen during the first four months of storage. Storage trials did not show any unambiguous effects for PVPP stabilization on beer flavour stability. Results confirmed the negative impact of malt and hop polyphenols on haze stability, and PVPP stabilization minimized differences in shelf life prediction values between beers prepared with the modified dose of polyphenols.

“…Strategies for avoiding the development of this type of haze may either involve a removal of protein, of polyphenol or a proportion of both. Those advocating the removal of protein (which may either be by adsorption on silica hydrogels or xerogels 8 , by precipitation with tannic acid 12 or by hydrolysis with papain 4 ) claim that this is preferable to eliminating polyphenols, as the latter are important in imparting body to beer 7 and as antioxidants 6 . Those championing the removal of polyphenols, using polyvinylpolypyrrolidone (PVPP 13 ), stress that the removal of protein is to jeopardise foam stability.…”

Section: Introductionmentioning

confidence: 99%

A Comparison of Polyvinylpolypyrrolidone (PVPP), Silica Xerogel and a Polyvinylpyrrolidone (PVP)-Silica Co-Product for Their Ability to Remove Polyphenols from Beer

Mitchell¹,

Hong²,

May³

et al. 2005

Proanthocyanidins have been characterized and levels compared in an ale and a lager using normal-phase liquid chromatographymass spectroscopy (LC /MS) after a pre-concentration step employing Sephadex LH-20 chromatography. Twenty proanthocyanidins were identified including both non-galloylated and galloylated forms of the monomers, dimers and trimers. Oligomers greater than trimer were not detected. A proportion of all classes of proanthocyanidin adsorbed non-specifically to a silica xerogel product, but polyvinylpolypyrrolidone (PVPP) and a product that comprises polyvinylpyrrolidone (PVP) on a surface of amorphous silica displayed greater capability (i.e. specificity) for binding these materials. Of the two specific adsorbents, it was the PVPP that had the greater binding capability compared to the PVP-silica co-product.