Flavor stability

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This article will focus on the changes that occur in beer after fermentation completes.

"Flavor stability" refers to how resistant it is to changing flavor during production or storage. The flavor deterioration may be a loss of desirable flavors/aroma such as lowered hop character, or it may be the appearance of off-flavors such as stale notes or cardboard flavor.

In contrast to wine, changes in beer flavor during storage are generally negatively conceived and unwanted. These changes include both deterioration of flavor and formation of off-flavors. While the storage and handling of the final beer are critical, all the steps in the brewing process affect the beer flavor stability, including the quality of raw materials and the initial stages of brewing. Oxidative reactions are one of the main causes of beer flavor instability.

Flavor stability of beer is of considerable importance in today’s demanding beer market and has become one of the highest technical challenges facing the brewer. The formation of carbonyl compounds such as trans-2-nonenal attributed to aged beer flavor during storage results in the flavour instability of beer (Varmuza et al., 2002), which impairs the beer quality. Strecker degradation of amino acids, oxidation of alcohols to aldehydes or carboxylic acids, lipid oxidation, enzyme degradation of lipids and aldol condensation of aldehydes are widely accepted as the reaction mechanisms responsible for the formation of staling aldehydes and ketones with low flavour threshold (Vanderhaegen et al., 2006). Numerous strategies, such as avoidance of oxygen pick-up and excessively prolonged heating during brewing, selection of barley with low lipoxygenase potential, adjusting the brewing process parameters and addition of antioxidants, have been adopted in brewing practice to inhibit the formation of carbonyl compounds and improve the beer flavor stability (Bamforth, 2000). Avoidance of oxygen uptake in brewing and package has been regarded as the highly effective and low-risk means for improving beer flavor stability (Bamforth, 1999).

Fresh beer is inherently unstable, as it is not in a state of chemical equilibrium. The poor (flavour) stability causes beer to change and turn unpalatable rather fast; especially considering the detrimental effects of transport and unrefrigerated storage. And even though beer will technically not expire and become unsafe to drink, its lacking stability is a modern brewer’s biggest headache. The flavour changes that occur over time are almost always unpleasant and unintended. Adverse storage conditions with time can also cause haze defects and lack of foam in the beer. A big culprit in beer staling is oxygen, as oxidation is the main cause of beer turning ‘aged’ [3, 4]. That is why breweries go to great lengths avoiding any unnecessary oxygen pickup during brewing and why good bottling practices are so important.

There is no universally accepted terminology for the flavor changes that occur in beer and certainly no surety that any two beers will age in exactly the same way, to give the same flavor notes in identical proportions. "Gently"-flavored lagers and strong ales are not predisposed to the selfsame flavor changes. If we are to generalize in pursuit of simplification, then one of the first changes is a perceptible decline in bitterness, and the beer may be perceived as harsh. There will also be a decline in fruity/estery and floral notes. Some beers will develop a ribes (blackcurrant buds, tomcat urine) aroma and most beers are claimed to develop a wet paper or cardboard character. Bready, sweet, toffee-like, honey, earthy, straw, hay, woody, winey and sherry-like are all notes that have been reported ( Drost et al., 1971; Meilgaard, 1972; Dalgliesh, 1977 ; Whitear, 1981 ). Fresh beer generally tastes better.

In theory, any perceptible change in flavor that renders a beer different from that expected for the beer in question amounts to flavor instability. For the most part, discussion is of carbonyl compounds. It was Hashimoto (1966) who first reported on the substantial increase in the level of carbonyl compounds in ageing beer. Thereafter, Palamand and Hardwick (1969) first described the development of E-2-nonenal (cardboard-like aroma), which above all other compounds is the one most frequently referred to in the context of staling. However, many other compounds may change in their amount, taking them either above or below their flavor threshold and thus registering as a change in perceived flavor. As many as 600–700 substances contributing to the flavor of beer can be detected by the human taste and olfactory system, some at extremely low concentrations.

Over-emphasis is placed on E-2-nonenal. Several studies have show that cooler temperatures prevent its appearence within a reasonable timeframe (a few months).

The biochemical processes which occur during beer storage proceed simultaneously but at different rates. The extent of these reactions depends on the storage conditions and interaction of pathways. Beer contains compounds with antioxidant properties such as reducing sugars, Maillard reaction products, vitamins and phenolic compounds (Oñate-jaen et al., 2006).

Beer flavour stability is influenced by several factors, some of them are disputed, but the oxidation certainly plays a crucial role in flavor [2]. Minimizing the formation and activity of reactive oxygen species (ROS: O2, HOO•, H2O2 and HO•) in beer and wort is definitely a first step to get better beer flavour stability [2,5].

It has been well demonstrated that high O2 levels into final package reduce the shelf-life of beer [6]. Moreover, the presence of transition metal ions (Cu+ and Fe2+) promotes the formation of ROS acting as electron donors. Consequently, process and technological parameters should be improved to minimize wort and beer oxygen pick-up as well as low amount of copper and iron. Modern fillers are designed to keep oxygen level in the packaged beer as low as possible, aiming for a maximum pick-up of 100 ppb during filling. Moreover, antioxidants may be used in the beer (depending on the local legislation), especially sulphur dioxide and ascorbic acid [5,7,8].

Beer flavour changes during storage as a result of an increase in the amount of several compounds among which the most significant are carbonyl compounds mainly generated via oxidation of higher alcohols, autoxidation of unsaturated fatty acids, enzymatic degradation of unsaturated fatty acids, Maillard reactions, and oxidative degradation of isohumulones. This process can be greatly slowed down by the addition of substances that act against the oxidation (antioxidants) [7].

Over the course of beer aging, a decrease in bitterness and the appearance of typical aging aroma attributes (sweet and cardboard-like notes) are observed (Vanderhaegen, Neven, Verachtert, & Derdelinckx, 2006). Karab´ın et al. (2014) described a significant decrease in bitter iso-α-acids during 5 months of beer storage depending on the storage conditions (light, temperature) (Karab´ın et al., 2014). Degradation of iso-α-acids depends on stereochemistry, trans-iso-α-acids showed a marked decay whereas the cis-isomers were comparably stable.

Flavor stability is, besides colloidal stability, one of the most important indicators of beer quality. The two properties are narrowly connected since the colloidal stability purports the polyphenol removal, a big factor in flavor stability.

It is clear that during aging of beer, either for extended storage or forced-aging, there are two mechanisms for the development of carbonyl compounds. The first involves storage time and heat, whereas the second involves storage time, heat, and exposure to air. The addition of (+)–catechin and ferulic acid to beer during forced-aging altered the rate of formation of carbonyl compounds formed predominantly by this second mechanism. It would appear from these results that the addition of the antioxidants (+)–catechin and ferulic acid to beer has no impact on carbonyl formation when oxygen levels are <0.1 ppm, irrespective of the storage temperature. However, the addition of air to the package followed by heating resulted in the formation of additional carbonyl compounds. The presence of (+)–catechin or ferulic acid in the beer altered the rate of formation of some of these additional carbonyl compounds. The antioxidants (+)–catechin and ferulic acid have no effect on the rate of formation of carbonyl compounds that increase during storage, especially trans-2-nonenal, furfural, and 5-hydroxymethylfurfural. The carbonyl profile during storage is different from that obtained during forced-aging in the presence of air, and additional carbonyl compounds, including undecanal, are formed. These results are consistent with two mechanisms of carbonyl formation during storage of beer. The first occurs with low levels of oxygen and involves, in part, trans-2-nonenal formation. This focuses attention further upstream on the stages where trans-2- nonenal is formed. The second mechanism occurs with addition of oxygen, which results in the formation of additional carbonyl compounds, including undecanal. The antioxidants (+)–catechin and ferulic acid were effective at reducing the rate of formation of carbonyl compounds in beer with high levels of oxygen but not in beer stored with low levels of oxygen.

Heat (i.e. storage temperature) greatly affects changes to beer compounds, especially at >25°C.

While it is preferred that flavour improves during the maturation process, formation of undesirable flavours inevitably occurs during beer storage. More problematic is that occurrence of aged-flavours varies from one beer style to another, with lager beer seeming especially sensitive50,118,119. Of the many chemicals involved in beer flavour modification, a few key groups have been identified: diketones, sulphur compounds, aldehydes and volatile fatty acids17,29,50,119. In general, beer aging results in decreased bitter taste, increased sweet taste and increased caramel, ribes (black currant), and toffee-like aromas. Carbonyl compounds such as trans-2- nonenal (cardboard aroma) form during beer storage from the oxidation of fatty acids and have been attributed to aged-beer flavour due to their very low flavour thresholds49,80. Other carbonyls have also been used as chemical indicators of beer oxidative flavour development even though they typically exist at concentrations below the human detection threshold in beer. Compounds such as acetaldehyde, 2-furaldehyde (furfural), 5-hydroxymethyl2-furaldehyde (5-hydroxymethyl furfural or 5 –HMF) and β-damascenone are considered useful chemical stalingindicators because their concentrations increase alongside increases in oxidative flavours during beer aging70,103,112.

Flavour instability resulting from beer storage remains one of the most important quality problems in the brewing industry. Although research has focused on aged beer flavour stability via a multitude of analytical methods, it remains very difficult to comprehensively and accurately evaluate the aging flavour of beer; no single compound or measurement exists to adequately address the multifaceted course of aging. Beer aging is caused primarily by oxidative reactions that transform into products associated with compromised product quality.

Beer flavor staling is caused by formation or alteration of various compounds, namely esters, O-heterocyclics and carbonyls.

The radical theory of beer staling has generally been accepted. According to this theory radicals of some organic and inorganic compounds support the process of reactions that affect beer staling formation of stale flavou compounds. The effect of oxygen, reactive oxygen species (ROS), has the central role. In the sequence of radical reactions which proceed in wort and beer, and which result in the formation of stale flavor carbonyls, antioxidants play an important role, because they can directly or indirectly affect carbonyl content in beer.

It is well known that oxidation during packaging causes deterioration of beer quality, haze and flavour stability. Generally accepted opinion is that the oxygen in the headspace is incorporated into compounds in the beer, especially polyphenols, carbonyl compounds and isohumulones during storage37. However, in case of stale flavour carbonyls, more recent work did not prove confirm this opinion as no O18 oxygen isotope was incorporated into carbonyls in stored beer.

Storage time can significantly impact beer quality, such as colloidal stability and flavor. Beer aging decreases bitterness and aging off-flavors (sweet and cardboard-like notes) emerge [101]. Isomerized α-acids show significant reduction over five-month storage, where cis-isomers showed fewer changes than trans-isomers. However, this correlates with the storage conditions (light, temperature) of beer [102,103].

The parameters used in this study to perform the forced aging process are related by the brewing industry to cause undesirable sensorial changes in beer, but as observed it can not substitute the natural aging process in order to estimate the biochemical changes occurring in aged beers.

Changes is flavor during beer storage is often acompanied by the formation of haze and an increase in color. These changes result primarily from oxidation and therefore are connected to the amount of oxygen introduced during brewing, packaging, and storage.

Minimizing the formation and reducing activity of reactive oxygen species (O2–, HOO• , H2O2 and HO• ) in beer and wort, must be the first step for improving beer flavor stability. Antioxidants reduce the rate of oxidation reactions.

Beer staling has long been a prime concern for most brewers (1). Through storage, flavor appears to deteriorate greatly with time at a rate depending on beer composition (pH (2-6), oxygen (7, 8), antioxidants (9, 10), precursor concentrations (4, 6, 11-14), etc.) and storage conditions (packaging (15), temperature (16), light (17), etc.). Improvement of beer stability requires better knowledge of all chemicals involved.

The sensory properties of beer are altered during storage and aging, as a result of various chemical, physical, and sensory transformations which can affect beer quality (Guido and others 2003; Vanderhaegen and others 2006).

Packaged commercial beer is first warehoused at the brewery, then stored at the retail outlet, and finally stored at home before consumption. All three of these places have potential for suboptimal storage (i.e., temperature >4°C), thus shortening the expected shelf life. The increasing market penetration of "premium" brands has increased the haze stability problems of these beers, since there are longer time intervals between these brews because of lower turnover, they are potentially stored for longer periods of time before consumption, while consumers expect a superior product.

Flavor stability is an important parameter for breweries because the flavor of a certain beer brand should be consistent to meet the expectations of the consumers. During storage of beer, the fresh flavor notes such as bitterness, estery, and flowery will decrease while staling and vinous notes will increase.

The nature of the changes that occur during the aging of the product and the formation of new flavors and aromas in beers is quite complex and depends on the type of beer, the oxygen concentration present in the packaging, the presence of light and the storage temperature. Dalgliesh described the sensory changes in beer during its storage (Figure 2). Among the main changes observed, we highlight if the increase in aroma and sweet taste, the decrease inthe bitter taste, the increased cardboard flavor,and the decrease in the flavor called ribes ( ribes nigrum ),described as a typical aroma of black currant leaves ,wild gooseberry-like.

Model studies suggest that either partially oxidized fatty acids or bisulfite complexes may serve as precursors of the stale-flavored, unsaturated aldehydes that are produced in beer as a result of oxidation processes during aging. The oxidation process in model reactions is inhibited by ethylenediaminetetraacetic acid disodium calcium salt (EDTA), lysine, metabisulfite and 1,2-dihydroxypolyphenol species. Lysine and EDTA also inhibit the formation of aldehydes during beer aging.

Changes in beer aroma can be explained mainly by the formation of great many flavor active substances, though mainly carbonyls. As the flavour thresholds, particularly of the long chain aldehydes are very low, only minimal quantities are necessary to impair beer flavor.

A decrease in oxidative wort stability caused by boiling conditions (time, temperature) or e.g. carbohydrate addition or e.g. oxygen entry during brewing and beer storage can be responsible for a faster SO2-consumption rate during storage and acceleration in release of staling aldehydes like 2-/3-methylbutanal from sulfite carbonyl complexes.

Beer 2 (a dark beer, 44 EBC) had no ESR lag phase and produced an extremely high rate of radical formation compared with the other beers; this has also been observed previously with other types of dark beer. i.e. dark beers are less flavor stable.

Green (wet) hops can help maintain flavor stability over a very long period in commercial beer (e.g. up to 5 years)!

The ingress of oxygen during all stages of brewing and maturation should be limited to ensure that beer maintains maximum flavour shelf life (up to 52 weeks). However, even under reduced oxygen conditions, non-oxidative flavour modification reactions such as esterifications, etherifications, Maillard reactions and glycoside and ester hydrolysis may still occur in bottled beer due to production of OH● via the Fenton reaction or during thermally or photochemically induced homolysis of some weak bonds of organic beer molecules61. Regardless, it is generally thought that aged beer flavor depends heavily on the oxidative degradation of beer compounds by reactive oxygen species (ROS).

The colour modification is one of most important beer aging indicators, mainly due to oxidation and consequent degradation of polyphenols and the formation of Maillard compounds during storage, especially at warm storage temperature [21,22].

The mixture of reversible and irreversible reactions is the main feature of beer ageing, which is supported by temperature, light and various oxidation agents such as oxygen in the presence of catalysers. Beer reductones and the Maillard reaction products play an important role in beer ageing to be natural electron donors. The degradation products of sugars contribute to non-oxidative browning of aminoacids and proteins by rearrangement and elimination pathways which generate deoxydicarbonyl compounds, such as deoxyglucosone and methylglyoxal. Paradoxically they can act as prooxidants as well as antioxidants enabling electron exchange (3, 4). On the other hand polyphenols can also take part in ageing beer via higher alcohols oxidation in the presence of Cu2+ and oxygen (5). Some of them have an amine oxidase-like activity which is similar to oxidative ability of the Maillard reaction products (6). Polyphenols easily undergo oxidation generating reactive oxidised products which reacts with beer proteins forming haze (7).

phenols in the bottle are progressively degraded to oxidized analogs with unexpected properties. Polymerization of small flavonoids to tannoids could be induced by acetaldehyde (excreted by yeast or issued from ethanol oxidation) through formation of ethyl bridges between flavanols.(177) Opening of oxidized phenol rings has been proposed as an alternative mechanism of degradation.(178) In one study, lager beer aging experiments conducted with a stable non-radioactive oxygen isotope (18O2) made it possible to visualize the incorporation of oxygen. 18O was present in 6.5% of the polyphenols after 5 days at 40°C but in only 0.6% after 9 months at 20°C.(109) Especially in natural aging, huge amounts of 18O isotope were recovered in the water fraction, indicating that polyphenols were also oxidized to quinones.

Compounds formed during beer storage

Class	Compounds
Aldehydes	Acetaldehyde
	E-2-Nonenal
	E-2-Octenal
	E,E-2,4-Decadienal
	E,E-2,6-Nonadienal
	2-Methylbutanal
	3-Methylbutanal
	Benzaldehyde
	2-Phenylacetaldehyde
	3-(Methylthio) propionaldehyde
Ketones	E-β-Damascenone
	Diacetyl
	3-Methyl-2-butanone
	4-Methyl-2-butanone
	4-Methyl-2-pentanone
	2,3-Pentanedione
Cyclic acetals	2,4,5-Trimethyl-1,3-dioxolane
	2-Isopropyl-4,5-dimethyl-1,3-dioxolane
	2-Isobutyryl-4,5-dimethyl-1,3-dioxolane
	2-Sec butyl-4,5-dimethyl-1,3-dioxolane
Heterocylic compounds	Furfural
	5-Hydroxymethylfurfural
	5-Methylfurfural
	2-Acetylfuran
	2-Acetyl-5-methylfuran
	2-Propionylfuran
	Furan
	Furfuryl alcohol
	Furfuryl ethyl ether
	2-Ethoxymethyl-5-furfural
	2-Ethyoxy-2,5-dihydrofuran
	Maltol
	Dihydro-5,5-diemethyl-2(3H)-furanone
	5,5-Dimethyl-2(5H)-furanone
	2-Acetylpyrazine
	2-Methoxypyrazine
	2,6-Dimethylpyrazine
	Trimethylpyrazine
	Tetramethylpyrazine
Ethyl esters	Ethyl-3-methylbutyrate
	Ethyl-2-methylbutyrate
	Ethyl-2-methylpropionate
	Ethylnicotinate
	Diethyl succinate
	Ethyl lactate
	Ethyl phenylacetate
	Ethyl formate
	Ethyl cinnamate
Lactones	γ-Nonalactone
	γ-Hexalactone
S-compounds	Dimethyl trisulphide
	3-Methyl-3-mercaptobutylformate

Yeast is known to release a range of enzymes with potential impact on product quality. Included amongst these are the glycosidases, the substrates for which include complexes of carbohydrate with several significant hop aroma components ( Biendl et al., 2003 ). If these enzymes remain in beer (e.g. if beer is not pasteurized) then conceptually there may be a progressive change in hop character over time. Chevance et al. (2002) showed β-glucosidase enhanced the release of (E)-β-damascenone in beer.

The amino acid cysteine will progressively release dimethyl sulfide from dimethyl sulfoxide in final package ( Bamforth, 1985 ). Residual yeast (in naturally conditioned products) will also do this and will reduce sulfur dioxide to hydrogen sulfide ( Walker and Simpson, 1994 ). Compounds responsible for the ribes character, 3-methyl-3-mercaptobutyl formate (Schieberle, 1991) and 4-mercapto-4-methyl-penta-2-one (Tressl et al., 1980) are also produced on storage. Peppard (1978) and Gijs et al. (2002) have reported the development of DMTS in beer from various precursors.

Changes in ester levels - Stenroos (1973) and Neven et al. (1997) reported a decrease in the level of isoamyl acetate during the storage of beer. However, a range of other esters (see Table 3.1 ) increase in quantity during storage (Bohnan, 1985b; Gijs et al., 2002; Lustig et al., 1993; Miedaner et al., 1991; Williams and Wagner, 1978).

Flavour characteristics of beer are an important – if not the most important – criterion for evaluation of beer quality. Yet, with time, beer changes its flavour – the fresh attributes, as e.g. overall bitterness quality or fruity aromas decline, whereas staling-related off-flavours appear (e.g. sherry, caramel, cardboard flavours) [1]. One of the major contributors to beer staling are aldehyde compounds [2], whose concentrations elevate during beer aging [3, 4]. Staling aldehydes are carbonyl compounds of relatively low molecular masses and high volatility. A very specific characteristic of these compounds is their low threshold level that can be perceived in the ppb, or even sub-ppb range, e.g. trans-2-nonenal [2, 5]. In general, aldehydes may arise via two mechanisms: de novo formation and the release from bound-state adduct forms. Several chemical pathways have been proposed for the de novo formation of aldehydes, of which the most relevant are oxidation of unsaturated fatty acids, Maillard reactions and Strecker degradation of amino acids [5–9]. The latter is often subcategorised under Maillard reactions, which include e.g. reactions of α-dicarbonyls, α-unsaturated carbonyls or Amadori compounds with amino acids [5]. Strecker aldehydes may also be formed as a result of direct oxidative degradation of amino acids [10]. On the other hand, aldehydes can also be converted into non-volatile adduct forms with e.g. bisulfite, cysteine, or other amino acids (imine formation), which, over time, can split up again and thereby release the free aldehydes [5, 11].

Beer flavour stability may be influenced by a number of factors, however, generally, it is discussed in the context of the influence of the raw materials and the brewing process. Malt, the brewing raw material used in large quantities, contains a variety of staling precursors e.g. amino acids, lipids, but also aldehydes as such [12–15]. On the other hand, malt is a more or less rich source of beer endogenous antioxidants, which inhibit the rate of oxidation reactions [16]. From the perspective of the brewing process, the most important factors influencing final beer flavour stability are: exposure of wort/beer to oxygen, amount of heat-load applied, wort/beer pH, and contact with transition metal ions [10, 17, 18]. Nonetheless, the impact of applied technologies during each step of wort production is obviously also important. For instance, milling regime, mashing-in pH and temperature have been reported as important control-point parameters for fatty acid oxidation [19]. Lipoxygenases present in malt – enzymes catalysing oxidative degradation of unsaturated fatty acids – markedly decrease in their activity when mashing-in at pH of 5.2 and a temperature of 63 °C is applied [17, 19, 20]. Moreover, quick and effective performance of mash filtration has been reported to positively correlate with improved beer flavour stability [18, 19]. During wort boiling, aldehydes evaporate, however, due to the application of substantial heat-load, new ones are also formed via e.g. Strecker degradation of amino acids or Maillard reactions [21–23]. On the other hand, lipid oxidation has been reported to hardly proceed during wort boiling [23]. Performance of wort clarification is also relevant, as the wort is still exposed to heat-load and the separated hot-trub contains significant amount of aldehyde precursors (e.g. lipids or aldehydes bound to insoluble trub particles) [19, 22, 24].

Among various countermeasures to prevent beer from staling, the avoidance of oxygen pick-up has been considered to be the key technology. Two other approaches have been also considered to be effective for improving beer flavor stability. One is the regulation of precursors for some aldehyde staling flavor. The other is the suppression of the rate of oxidative reaction in beer. For the former, one example is the avoidance of carry-over of unsaturated fatty acids into pitching wort, which might be precursors of stale flavor (5,12,13,17). For the latter, increasing the antioxidant activity of beer itself is a key strategy (2,10,23). Beer flavor stability is expected to be practically improved by increasing the level of sulfite, which is considered to be one of the important antioxidants produced during fermentation.

low oxygen brewing (hot side) improves flavor stability.

The shelf life of packaged pasteurized beer is essentially determined by either the appearance of haze or the deterioration of the flavor. Both of these phenomena are the result of nonbiological oxidation processes that involve active oxygen species, such as H2O2, HO• , and HOO• /O2 •- (Dadic, 1984; Bamforth et al., 1993; Uchida and Ono, 1996). The colloidal instability of beer (i.e., the formation of haze) is mainly caused by the formation of insoluble complexes between proteins and oxidized polyphenols. Lowering the concentration of the phenolic proanthocyanidins in beer, e.g., by cold filtration or treatment with polyvinylpolypyrrolidone (PVPP), can efficiently delay the formation of haze during storage (McMurrough et al., 1996; McMurrough et al., 1997). The oxidation reactions involve Fenton reactions which are dependent on oxygen and iron or copper ions, and minimizing the content of these compounds in the packaged beer, have a positive effect on the stability of the flavor (Irwin et al, 1991; Narziss et al., 1993).

Flavor quality is, of course, very important in light of the general appreciation of consumers of a particular beer brand, but also important is the flavor stability of the brand they are accustomed to. Not all flavors associated with aging are necessarily regarded as off-flavors, and sometimes they are even preferred by the drinker. When a certain brand fails to meet the expectations of the consumer; for example, when the expected flavor is that of the fresh beer and the presented product shows aged flavors (or vice versa), it can lead to rejection of the brand.18−24 Conversely, more flavor-stable beer allows greater flexibility in terms of the length of supply chain and temperature management in logistics.

An attempt was made by Dalgliesh24 to generalize the sensory evolution of beer flavor during storage. Numerous papers make reference to the so-called Dalgliesh plot, and variations on this theme have been published as well, for example, by Zufall et al.28 (Figure 1). As the aging pattern will differ between different beers, the depicted curves will vary in relative intensities and times.24 In lager beers, for example, cardboard flavor is said to be the principal stale flavor. This negative attribute appears after a lag period and increases over time.24 According to some, the cardboard flavor decreases again when aged even further.28 This off-flavor may, however, not be perceived in aged specialty beers. Apart from cardboard flavor, aging beer may develop sweet, toffee-like, caramel, and burnt-sugar aromas, as well as a sweet taste. Also, a typical “ribes” flavor may appear very rapidly, but the intensity decreases upon further aging. This odor resembles the smell of crushed leaves and stems of black currant (Ribes nigrum) or flowering currant (Ribes sanguineum) and can also be referred to as “catty”. 24 After very long aging, woody, wineand whiskey-like notes can be detected as well. Also, sherry/ madeira-like, solvent-like, metallic, earthy, straw, bread crust, and cheesy flavors can be detected in some cases.18,21,24,28,30,31 Staling is not only characterized by an increase of undesired aging flavors, but the decrease of pleasant fresh flavors plays an important part as well. The loss of these positive flavor attributes, such as floral, fruity, and estery aromas, also comprises a loss in masking effect of negative flavor aspects.4,18 Sulfury notes decline very rapidly. Bitterness becomes harsher, astringency develops, and mouthfulness decreases.

As generally recognized, many chemical reactions still take place during beer storage, indicating that freshly bottled beer is not in a state of chemical equilibrium. Moreover, bottled beer is not a perfectly closed system (e.g., oxygen ingress, light irradiation). It is stated, as a rule of thumb derived from the Arrhenius equation, that a temperature increase of 10 °C approximately doubles the rate of chemical reactions.18,19,21,23,24 However, it was seen empirically that the degrees of flavor staling were comparable when beer was stored for 5 days at 37 °C, for 22 days at 30 °C, and for 42 days at 25 °C.33 Therefore, to slow the chemical reactions in beer and prevent staling, it is advisible to maintain the lowest temperature possible for beer storage, while also taking into account other factors such as haze formation.

Often, (E)-2-nonenal has been cited as the most important stale compound of lager beer, because its concentration was seen repeatedly to increase during aging to levels above the flavor threshold (approximately 0.03 μg L−1 according to Saison et al.4 ), causing cardboard/papery notes.13,18,19,33,42−44 This attribute was first described by Burger et al.45 in the 1950s. Over the years, it became clear, however, that (E)-2-nonenal is just a part of the bigger picture of staling and that the overall stale flavor is caused by a myriad of compounds.

Degradation of Bitter Acids. During wort boiling, the α-acids (six-carbon ring compounds, also called humulones) derived from hop products are heat-isomerized to the bitter tasting iso-α-acids (five-carbon ring compounds, also called isohumulones) (Figure 12). Previous studies demonstrated that, during beer aging, especially trans-iso-α-acids are prone to degradation, whereas cis-iso-α-acids remain largely unaltered, even after prolonged storage. Furthermore, the ratio of trans- over cis-iso-α-acids showed a good correlation with the observed decrease in bitterness intensity and quality over time.31,135−142 In particular, a lower pH and a higher temperature appear to negatively affect trans-iso-α-acid stability.138,141 Among myriad degradation products, a variety of volatile carbonyl products (e.g., 2-methylpropanal, 2-methylbutanal, 3-methylbutanal; Figure 12) was formed from these bitter acids in model solutions.143 The exact aldehyde-producing degradation mechanism is, however, still unclear. Hashimoto et al.144 reported that beer brewed without hops hardly develops a characteristic stale flavor profile, not even after prolonged storage. This would indicate that hop product degradation might be an important stale flavor formation mechanism. This view is, however, contradicted by the results of more recent research by De Clippeleer et al.145 They separated cis- and trans-iso-α-acids from a commercial isomerized hop extract on pilot scale and dosed these bittering principles to unhopped beer in milligrams per liter concentrations. After forced aging in the dark at 30 °C, results confirmed the higher instability of trans-iso-α-acids compared to cis-iso-α-acids. However, the formation of 2-methylpropanal, 2-methylbutanal, and 3-methylbutanal could not be linked to hop product degradation, because the levels of these aldehydes increased to a similar extent, whether the beer was unhopped, hopped with commercial isomerized extract, hopped solely with cis-iso-α-acids, or hopped solely with trans-iso-α-acids. From these results, it can be concluded that stale aldehyde formation from iso-α-acid degradation must be of minor importance, if relevant at all, compared to other mechanisms.

The cause of increasing levels of (E)-2-nonenal during beer aging remains unclear. To estimate the relevance of (E)-2-nonenal release from a bound state, the concept of “nonenal potential” was introduced already more than two decades ago. According to Drost et al.,75 the nonenal potential is a forcing test that determines the potential of a wort to form (E)-2-nonenal under beer conditions. this procedure represents a way to determine the amount of (E)-2-nonenal formed during the production process and that is subsequently bound reversibly in an adduct. Adduct formation will reduce the volatility of (E)-2-nonenal and therefore will prevent it from evaporation during the wort production process. Additionally, as an adduct, (E)-2-nonenal will be insensitive to the reducing activity of yeast during fermentation (see further). Consequently, in its bound state, (E)-2-nonenal may remain present throughout the production process and end up in the final beer. Because analytical aldehyde detection methods are often based on volatilization of the compounds, this bound (E)-2-nonenal will be obscured and undetectable as such. The same accounts for the sensory perception of (E)-2-nonenal.158 However, under the specific conditions during beer storage (beer pH, storage temperature), adducts may degrade, releasing (E)-2-nonenal, causing cardboard flavor and rendering the beer stale.42,158−162 Several studies support this hypothesis and point to the release of (E)-2-nonenal from a bound state during beer aging. For example, a close correlation was observed between the nonenal potential of clarified wort and the (E)-2-nonenal concentration in both naturally aged and forced-aged beer.4 mashing may contribute around 30% of the (E)-2-nonenal in aged beer, whereas wort boiling contributes about 70% of (E)-2-nonenal. Furthermore, other studies excluded trihydroxy fatty acids as (E)-2-nonenal precursors in the bottled beer67 and proved that lipid oxidation has no significant activity in bottled beer, because 18O2 isotopes in the headspace were not incorporated into the carbonyl fraction. It is reasonable to assume that, besides the fatty acid oxidationderived aldehyde (E)-2-nonenal, other staling aldehydes may form a similar “potential” during the beer production process and that (part of) these aldehydes are already present in a bound state in fresh beer. Indeed, based on several tests using the Strecker degradation inhibitor o-diaminobenzene, added to beer samples, and 13C-labeled amino acids, spiked to filtered wort and beer samples, it has been reported that approximately 15% of total Strecker degradation aldehydes present in aged beer appear to be the result of de novo formation during storage, whereas about 85% seems to be derived from adducts, preformed during wort production.165 The individual Strecker aldehydes showed, however, a different behavior; for example, 70% of phenylacetaldehyde was estimated to be derived from wort boiling and clarification, compared to practically 100% of 3-methylbutanal and methional. The two adduct formation mechanisms considered to be most important are imine formation and bisulfite adduct formation.

The presence of fatty acids in beer is typically associated with decreased flavour stability and it has been suggested that the staling factor 2-trans-nonenal, which gives a distinctive cardboard taste to beer at very low concentrations, may arise through the oxidation of linoleic acid in beer.

• Flavor stability. 2019. Pages 199-213 in: Brewing Techniques in Practice. Werner Back et al., eds. Fachverlag Hans Carl: Nürnberg, Germany.
• Drost, B. W., van den Berg, R., Freijee, F. J. M., van der Velde, E. G., and Hollemans, M., Flavor stability. J. Am. Soc. Brew. Chem., 48, 124–131 (1990). ]
• Bamforth, C.W. (1999). The science and understanding of the flavour stability of beer: a critical assessment. Brauwelt International, 17, 98–110.
• Bamforth, C.W. (2000). Making sense of flavor changes in beer. MBAA Technical Quarterly, 37, 165–171.
• Further Studies on the Impact of Commercial Malting and Brewing Practices on Beer Flavour Stability
• Schieberle, P.; Komarek, D. Changes in key aroma compounds during natural beer aging. ACS Symp. Ser. 2002, 836, 70–79.
• Vanderhaegen, B.; Neven, H.; Coghe, S.; Verstrepen, K.J.; Verachtert, H.; Derdelinckx, G. Evolution of chemical and sensory properties during aging of top-fermented beer. J. Agric. Food Chem. 2003, 51, 6782–6790.
• Guido, L.F.; Fortunato, N.A.; Rodrigues, J.A.; Barros, A.A. Voltammetric Assay for the Aging of Beer. J. Agric. Food Chem. 2003, 51, 3911–3915.
• Oñate-Jaén A, Bellido-Milla D, Hernández-Artiga MP (2006). Spectrophotometric methods to differentiate beers and evaluate beer ageing. Food, 97:361-369.
• Formation of Aldehydes by Direct Oxidative Degradation of Amino Acids via Hydroxyl and Ethoxy Radical Attack in Buffered Model Solutions
• Characterisation of the flavour and the chemical composition of lager beer after ageing in varying conditions
• Detailed Multivariate Modeling of Beer Staling in Commercial Pale Lagers
• The Chemistry of Beer Aging: A Critical Review
• Development of a Lexicon for Staling Aromas in North American Lager and Analysis ofConsumer Preference and Difference
• "The stability of flavour constituents in alcoholic beverages"
• Flavor Stability
• "Changes in bitterness as beer ages naturally"
• 125th Anniversary Review: The Non-Biological Instability of Beer
• Savel, J.: Reductones and Beer Ageing. MBAA Tech. Quart. 38 (2001), no. 3, pp. 135-144.
• Hoff, S., Lund, M. N., Petersen, M. A., Frank, W., and Andersen, M. L. Storage stability of pasteurized non-filtered beer. J. Inst. Brew. 119:172-181, 2013.
• Flavour Stability (slideshow)
• Carlos Silva Ferreira, Sonia Collin, Fate of Hop and Fermentation Odorants in Commercial Belgian Dry-Hopped Beers over 2 Years of Bottle Storage: Key-Role of Oxidation and Hop Esterases, Journal of the American Society of Brewing Chemists, 10.1080/03610470.2020.1843898, (1-22), (2020).
• Callemien, D., Dasnoy, S., & Collin, S. (2006). Identification of a stale-beer-like odorant in extracts of naturally aged beers. Journal of Agricultural and Food Chemistry, 54(4), 1409–1413.
• Antioxidant activity and typical ageing compounds: their evolutions and relationships during the storage of lager beers.
• Teumac, F.N., Ross, B.A., Rassouli, M.R.: Air ingress through bottle crowns. MBAA Technical Quart., 27 (4), 122 – 126, 1990.
• Vanderhaegen, B., Neven, H., Coghe, S., Verstrepen, K. J., Verachtert, H.,and Derdelinckx, G. (2003) Evolution of chemical and sensory proper-ties during aging of top-fermented beer,J. Agric. Food Chem. 51,6782–6790.
• Dalgliesh, C.E. Flavour stability. In Proceedings of the 16th European Brewery Convention Congress, Amsterdam, The Netherlands, 1 December 1977; pp. 623–659.
• Stephenson WH, Bamforth CW. 2002. The impact of lightstruck and stale character in beers on their perceived quality: a consumer study. J Inst Brew 108: 406– 9.
• Kaneda, H., Takashio, M., Osawa, T., Kawakishi, S., and Tamaki, T. Behavior of sulfites during fermentation and storage of beer. J. Am. Soc. Brew. Chem. 54:115-120, 1996.
• https://www.sciencedirect.com/science/article/abs/pii/B9780123738912000365 The Chemistry of Aging Beer (chapter in Beer in Health and Disease Prevention, 2009)
• Stewart, G.G. (2004) The chemistry of beer instability. J. Chem. Educ., 81(7), 963–968.

See also

• Oxidation
• Antioxidants
• Lipids
• Phenolic compounds
• Protein