Carbonyl compounds
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The carbonyl compounds, mainly aldehydes derived from Strecker degradation, are detrimental to beer flavour [2,3].[1]
The substances responsible for aged character in beer, notably a series of carbonyl compounds, tend to have low flavour thresholds and so display their impact when present in very small quantities.[2]
Carbonyl compounds are partly responsible for aged/oxidative character in beer and wine.
Until a few years ago, the (E)-2-nonenal compound responsible for the cardboard-like off-flavor was considered the main compound responsible for the aging of a lager beer [31]. Today, other key aging aldehydes have been identified, such as furfural, hydroxymethylfurfural, methylfurfural, and lipid oxidation products such as hexanal, heptanal and hexanal, as well as the Strecker aldehyde group: benzaldehyde, methional, 2-methylpropanal, 2-methybutanal, 3-methylbutanal, and phenylacetaldehyde [32,33]. Narziß et al. [34] also discovered a promoting effect of oxygen during beer aging on Strecker aldehydes.[3] 3-methylbutanal and 2-methylbutanal are considered to be responsible for the "malty" character, while methional and phenylacetaldehyde are key compounds of the flavor of "aged" beer. The concentration of these aldehydes had a positive correlation with the amount of oxygen present in the bottle [32].
Furfural is very high in aged pasteurized beers or in dark beer, where the value can reach 1000 µg/l or over after 6 months of storage [40–43], causing sharper, harsher, more lingering bitterness and increased astringency [42].[3]
It is known that different carbonyl functions of aldehydes and ketones can also be bound to sulphur dioxide during fermentation and the flavour compounds are not taste-active any more. In this case there should be equilibrium between free SO2 and bound SO2. During the consumption of SO2 while beer storage the compounds are set free and get taste active again [2, 13, 15, 31, 40, 59]. The velocity of those releases is influenced by the SO2-content and SO2-consumption rate during storage which, in turn, is influenced by wort boiling conditions and reactions occurring during boiling e.g. under participation of carbohydrates as demonstrated in the results of this study.[4]
The carbonyl profile was also measured for the bottled beers stored at 0, 25, or 40°C by GC-ECD. In contrast to the results obtained by forced-aging (11,12) and the total carbonyl assay, there was no difference in the rate of formation of any carbonyl compounds measured for the beers treated with 1 mM (+)–catechin or 1 mM ferulic acid and stored in bottles at 0, 25, or 40°C. The majority of carbonyl compounds (peaks 1–3, 5, 6, 8, 11–13, 16–22, and 24) did not respond to heat or storage time. Some of these peaks have been tentatively identified based on retention time as formaldehyde, acetaldehyde, acetone, 3-methyl-2-butenal, trans-2-octenal, and undecanal. There was some evidence of a slight decrease in concentration for acetaldehyde during storage, which is consistent with large molecular weight carbonyls being formed by aldol condensation reactions. Another group of compounds (peaks 4, 7, 9, 10, 14, and 15) increased with storage time and heating and included compounds such as furfural, hexanal, trans-2-nonenal, and 5-hydroxymethylfurfural (Fig. 8).[5]
undecanal did not increase during storage, irrespective of temperature. However, it did increase upon addition of air before heating. The presence of (+)–catechin reduced its rate of formation. There were other unidentified carbonyl compounds that eluted in the same region of the chromatogram as undecanal that responded in a similar way. As these compounds are likely to be long-chain carbonyls, they would be expected to have lower flavor thresholds. In combination, these compounds may contribute to stale flavors in beer aged under exposure to air.[5]
Summarizing, it would appear that trans-2-nonenal is still formed under packaging conditions capable of delivering oxygen levels <0.1 ppm. The antioxidants (+)–catechin and ferulic acid did not affect the rate of formation. This indicates that the potential to form this staling compound is already developed before packaging. In beer exposed to air, other long-chain carbonyl compounds, including undecanal, are formed which could contribute to the perception of an oxidized stale flavor in beer. The addition of (+)– catechin to beer reduced the rate of formation of some of these oxygen-induced carbonyl compounds.[5]
An important type of change during beer staling, is caused by aroma-active carbonyl compounds, which can be formed by radical reactions.[6]
Long chained fatty acids are prevalent in malt and hops. The turbidity during lautering as well as trub separation determines the amount of fatty acids of the wort which give rise to carbonyls in the finished beer.[7]
Established literature indicates that the beer stale-flavours result from the formation of unsaturated, volatile carbonyl compounds, e.g. 2-methylbutanal (MB), 3-MB, phenylacetaldehyde, benzaldehyde, 2-furfural (2-F), hydroxymethylfurfual, and trans-2-nonenal. Among the pathways, known to be involved in the formation of these carbonyls, are the Strecker degradation of amino-acids, the melanoidin-mediated oxidation of higher alcohols, the autoxidation of unsaturated fatty acids, and the aldol condensation of short-chain aldehydes.[8]
agents which bind carbonyls can strip the aged character from beer[9]
| Compound | Taste Threshold (µg/L) | Flavor | |
|---|---|---|---|
| Fatty acid oxidation products | (E)-2-nonenal | 0.11 | cardboard, cucumber, leathery |
| Hexanal | 88 | bitter, winey | |
| Maillard reaction products | 2-furfural | 15157 | boiled meat, caramel-like |
| 5-hydroxymethylfurfural | 35784 | bready, caramel | |
| Strecker degradation products | 2-methylpropanal | 86 | grainy, varnish, fruity |
| 2-methylbutanal | 35 | bitter almond, apple, malty | |
| 3-methylbutanal | 46 | cherry, chocolate, malty | |
| Phenylacetaldehyde | 100 | hyacinth, honeyed, fruity | |
| Methional | 0.5 | boiled potato, slightly spicy | |
| Benzaldehyde | 515 | bitter almond, cherry stone | |
| ?? | Heptanal | 105 | bitter, wine-like |
| Octanal | 60 | bitter, orange skin | |
| Nicotinic acid ethyl ester | 4555 | cereal-like, medicine-like | |
| γ-nonalactone | 607 | coconut, vanilla, glue | |
| Acetaldehyde | 1114 | green apple, fruity | |
Lipoxygenase (LOX) catalyses the oxidation of polyunsaturated fatty acids, notably linoleic acid, to hydroperoxides ( Doderer et al., 1992 ). In turn these are substrates for hydroperoxide isomerase ( Schwarz and Pyler, 1984 ; Zimmerman and Vick, 1970 ) and hydroperoxide lyase ( Kuroda et al., 2003 ). An ensuing sequence of non-enzymic reactions leads to the production of unsaturated carbonyl compounds, including E-2-nonenal (formerly known as trans -2-nonenal). It is argued that hydroperoxides produced upstream in malting and brewing survive into the fi nished beer and progressively decay to release stale character.[9]
In the aldol condensation, separate aldehydes or ketones react to form larger carbonyl species. This is a plausible route through which E-2-nonenal may be produced, by a reaction between acetaldehyde and heptanal ( Hashimoto and Kuroiwa, 1975 ). Proline may act as a catalyst.[9]
Cyclic acetals can be formed by the condensation of 2,3-butanediol with carbonyls such as acetaldehyde (Peppard and Halsey, 1982).[9]
Sulfite is capable of forming addition complexes with carbonyl containing compounds, the resultant "adducts" display no perceptible flavor at the concentrations likely to be found in beer ( Barker et al., 1983 ). It has been suggested that carbonyls produced upstream bind to the sulfite produced by yeast, thereby carrying through into the finished beer, to be progressively released as SO2 is consumed in other (as yet unknown) reactions ( Ilett and Simpson, 1995 ). It has been suggested that the greater significance of sulfite for protecting against staling is through its role as an antioxidant ( Kaneda et al., 1994 ). In this regard, Dufour et al. (1999) indicate that SO2-carbonyl binding actually occurs through the C=C of the unsaturated aldehyde, rather than at the carbonyl group and, as such, is non-reversible.[9]
Binding of carbonyls by amino groups in proteinaceous species: A similar scenario to SO2 is understood to occur with carbonyl compounds entering into reversible Schiff base formation with amino groups, including proteinaceous species in the grist ( Lermusieau et al., 1999 ).[9]
Yeast is capable of reducing carbonyl compounds ( Peppard and Halsey, 1981 ). These of course include the well appreciated reactions leading from acetaldehyde to ethanol and diacetyl to acetoin and butanediol. But many other aldehydes and ketones produced upstream will be reduced, leading to the belief by some that upstream production of such carbonyls is unimportant. Various enzymes may be involved in this reduction ( Collin et al., 1991 ; Debourg et al., 1993, 1994 ; Laurent et al., 1995 ).[9]
The four principal candidates as sources of staling carbonyls are unsaturated fatty acids, higher alcohols, iso-a-acids, and amino acids. Higher alcohols cannot be oxidized during wort production, because they are formed only at the fermentation stage. Equally, iso-ar-acid oxidation cannot take place before the kettle boil.[12]
The most frequently cited pathway by which staling aldehydes are produced is through the oxidation of unsaturated fatty acid to form hydroperoxides, which are subsequently transformed enzymically and then nonenzymically to the lower molecular weight unsaturated aldehyde (E )-2-nonenal that has pronounced cardboard character. However, it is naive to believe that nonenal is the only contributor to stale character—diverse unpleasant compounds containing the carbonyl (C=O) group are found in beer and they are produced via other pathways. Alcohols in beer can be converted to their equivalent aldehydes. This is a reaction catalyzed by melanoidins, substances often referred to as antiox- idants but which can have undesirable impacts also. Iso-α-acids are oxidized with the formation of carbonyl substances from their side chains. The reduced derivatives do not do this. The Strecker degradation comprises a reaction between an amino acid and an α-dicarbonyl compound, such as the intermediates in browning reactions. The amino acid is converted into an aldehyde with one fewer carbon atom. The reaction is believed to be catalyzed by certain polyphe- nols, showing again that some classes of substances may have both beneficial and adverse roles to play. Different carbonyl-containing substances produced in the types of reac- tion listed above can react together in the “aldol condensation” to form larger, different carbonyl compounds; for example, (E )-2-nonenal can come from the reaction between acetaldehyde and heptanal. Proline, which is abundant in beer, can catalyze such reactions.[13]
Aldol Condensation. It was observed in model solutions that unsaturated aldehydes with a low flavor threshold can be formed by aldol condensation of saturated aldehydes with a higher flavor threshold, for example, (E)-2-nonenal from heptanal and acetaldehyde. Amino acids such as proline are thought to act as catalysts.146 The general aldol condensation is shown in Figure 13, with heptanal and acetaldehyde as an example. Besides the formation of (E)-2-nonenal, several aldol condensations have been reported, such as the reaction of two molecules of 3-methylbutanal giving 2-isopropyl-5-methyl-2-hexenal,147 as well as the reaction of phenylacetaldehyde with acetaldehyde, 2-methylpropanal, 2-methylbutanal, 3-methylbutanal, and hexanal, yielding 2-phenyl-2-butenal, 4-methyl-2-phenyl-2-pentenal, 4-methyl-2-phenyl-2-hexenal, 5-methyl-2-phenyl-2-hexenal, and 2-phenyl-2-octenal, respectively.121 Other combinations might occur as well, giving rise to a myriad of branched aldehydes with potentially totally different flavor attributes. However, the extent to which these reactions take place is unclear. For example, the yield of heptanal and acetaldehyde aldol condensation in model solutions was shown to be only about 0.2%. Combined with the generally low heptanal level in beer (order of magnitude 1 μg L−1 ), the influence of this pathway on (E)-2-nonenal concentrations, and aldehyde concentrations in general, during beer aging under normal conditions is questionable.[11]
Secondary Autoxidation of Aldehydes. Unsaturated aldehydes, for example, (E)-2-nonenal, formed by one of the formerly described mechanisms can be further degraded to saturated shorter chain aldehydes (e.g., pentanal, hexanal, heptanal, octanal) by autoxidation.146 This might be an explanation for the decline of the (E)-2-nonenal concentration (and of the related cardboard flavor) during prolonged beer storage.[11]
Aldehyde Secretion by Fermenting Yeast. Yeast is able to excrete Strecker aldehydes (e.g., 3-methylbutanal, methional) during fermentation via the Ehrlich pathway.149−152 Oxoacids are formed anabolically from the main carbon source or they are derived from the catabolism of exogenous amino acids. Decarboxylation of these oxoacids yields Strecker aldehydes.153,154 As an illustration, labeled 3-methylbutanal was produced and excreted by the yeast during cold contact fermentation in a medium containing leucine-d10. 150 The contribution of this origin of aldehydes in the final beer is, however, most likely limited.[11]
Aldehydes can also bind proteins by hydrophobic interaction. In practice, this aldehyde scavenging potential of proteins might be important in the removal of aldehydes from the medium during the brewing process, for example, with the trub.[11]
See also[edit]
References[edit]
- ↑ Wu MJ, Clarke FM, Rogers PJ, et al. Identification of a protein with antioxidant activity that is important for the protection against beer ageing. Int J Mol Sci. 2011;12(9):6089–6103.
- ↑ Stephenson WH, Biawa JP, Miracle RE, Bamforth CW. Laboratory-scale studies of the impact of oxygen on mashing. J Inst Brew. 2003;109(3):273–283.
- ↑ a b De Francesco G, Bravi E, Sanarica E, Marconi O, Cappelletti F, Perretti G. Effect of addition of different phenolic-rich extracts on beer flavour stability. Foods. 2020;9(11):1638.
- ↑ Kunz T, Brandt NO, Seewald T, Methner FJ. Carbohydrates addition during brewing – effects on oxidative processes and formation of specific ageing compounds. BrewingScience. 2015;68(7):78–92.
- ↑ a b c Walters MT, Heasman AP, Hughes PS. Comparison of (+)–catechin and ferulic acid as natural antioxidants and their impact on beer flavor stability. Part 2: Extended storage trials. J Am Soc Brew Chem. 1997;55(3):91–98.
- ↑ Zufall, C., and Tyrell, Th. "The Influence of Heavy Metal Ions on Beer Flavour Stability." J. Inst. Brew., vol. 114, no. 2, 2008, pp. 134–142.
- ↑ Narziss L. Technological factors of flavour stability. J Inst Brew. 1986;92:346–353.
- ↑ Kunz T, Brandt NO, Seewald T, Methner FJ. Carbohydrates addition during brewing – effects on oxidative processes and formation of specific ageing compounds. BrewingScience. 2015;68(7):78–92.
- ↑ a b c d e f g Bamforth CW, Lentini A. The flavor instability of beer. In: Bamforth CW, ed. Beer: A Quality Perspective. Academic Press; 2009:85–109.
- ↑ Pahlisch S, Fuchs J, Harms D. Tracking staling components in beer. Brauwelt International. 2017:44–47.
- ↑ a b c d e Baert JJ, De Clippeleer J, Hughes PS, De Cooman L, Aerts G. On the origin of free and bound staling aldehydes in beer. J Agric Food Chem. 2012;60(46):11449–11472.
- ↑ Bamforth CW, Muller RE, Walker MD. Oxygen and oxygen radicals in malting and brewing: a review. J Am Soc Brew Chem. 1993;51(3):79–88.
- ↑ Lewis MJ, Bamforth CW. Chapter 12: Oxygen. In: Lewis MJ, Bamforth CW, eds. Essays in Brewing Science. Springer; 2006:131–142.