Cardboard flavour

The formation of off-flavours in beer has been reviewed [40], Autoxidation of the lipids present in beer produces carbonyl compounds with very low taste thresholds. In particular, linoleic acid is oxidized to trihydroxyoctadecenoic acids (Table 22.7) which break down into 2-/mAz.y-nonenal. This aldehyde and related compounds impart a cardboard flavour to beer at very low concentrations. Other carbonyl are formed from the lipids in beer by irradiation with light including the C9, Cjo, and Cu-alka-2,4-dienals (thresholds 0 5, 0 3 and 0 01 ppb respectively) [40]. The level of diacetyl and pentane-2,3-dione in a range of commercial beers is given in Table 22.11. Quantities in excess of 0 15 ppm impart a buttery flavour more noticeable in lagers than in ales. Bacterial contamination and petite mutants of yeast result in high levels of diacetyl. The sulphur compounds characterized in beer are listed in Table 22.19 with some threshold data. Dimethyl sulphide is the major volatile... 

Simple furans are partly responsible for the so-called cardboard flavour of beer, particularly derivatives of furfural (53) (see also 7.2.1.). Dihydrofurans and tetrahydrofurans and/or the corresponding furanones occur in hop oil (54,55) and in the oxidation mixtures of hop bitter acids (51,56,57). The isolated tetronic acid is the first hop derivative in this series. 

Widder, S. and Grosch, W. 1994. Study on the cardboard off-flavour formed in butter oil. Z Lebensm.-Unters. -Forsch.A 198 297-301. 

The most important precursors for lipid oxidation are unsaturated fats and fatty acids like oleic (18 1), linoleic (18 2), linolenic (18 3) and arachidonic acid (20 4). The more unsaturated ones are more prone to oxidation. Lipid peroxidation and the subsequent reactions generate a variety of volatile compounds, many of which are odour-active, especially the aldehydes. That is why lipid oxidation is also a major mechanism for thermal aroma generation and contributes in a great measure to the flavour of fat-containing food. Lipid oxidation also takes place under storage conditions and excessive peroxidation is responsible for negative aroma changes of food like rancidity, warmed-over flavour, cardboard odour and metallic off-notes. 

Widder S. and Grosch W. (1997) Precursors of 2-nonenals causing the cardboard off-flavour in butter oil. Nahrung... 

Autoxidation of linoleic acid gives rise to three isomers of trihydroxy-octadecenoic acid. The concentrations found in beer (Table 22.7) are higher than those found for linoleic acid itself [47]. All three acids are potential precursors of 2-trans-nonmdX which is an important component of the cardboard or paper flavour of stale beer. 

The cardboard like flavour that occurs in stale beer is thought to arise fi-om the fi ee radical mediated oxidation of various constituents in beer. The characteristic odour and taste are caused by decomposition products finm the free radical process. Similar processes occur in many foods, but in beer, these off-flavour products can be detected by the consumer even at very low concentrations. 

The so called lagtime assay uses forced oxidation combined with EPR spin trapping to effectively measure the antioxidant activity of a beer and to even predict a beers shelf life. It has been well established that the time in minutes (i.e., lagtime) before a dramatic EPR signal increase occurs, correlates with the time in days that are required for a sensory panel to detect the characteristic cardboard off-flavour. A parameter known as the T150 (the EPR intensity at time = 150 minutes) is another metric that is used to evaluate the resistance of the beer to oxidation. The T150 value is particularly used in ale-style (top-fermented) beers which do not have a lagtime. 

Methional is also the key compound responsible for the defect of alcohol-free beers described as worty flavour. The other off-flavour in milk is a cardboard-like or metallic flavour, which develops in milk with a prolonged exposure to light. Compounds responsible for this off-flavour are secondary lipid oxidation products, such as hexanal, pentanal and some other substances formed by riboflavin-catalysed photooxidation of fatty acids. 

Descriptive terminologies for the odours of pyridines use terms such as green, bitter, astringent, roasted, burnt, pungent, solvent and fishy, none of which could be considered desirable. Their presence in some food commodities, such as beer and whisky, is disagreeable and associated with a cardboard, oxidised and harsh flavour. In roasted coffee, pyridines may contribute to a pleasant smell that is, however, less pleasant than the smell of pyrazines. 

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