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On the contribution of malt quality and the malting process to the formation of beer staling aldehydes: a review

2021; Wiley; Volume: 127; Issue: 2 Linguagem: Inglês

10.1002/jib.644

2050-0416

Weronika Filipowska, Barbara Jaskula‐Goiris, Maciej Ditrych, Paula Bustillo Trueba, Gert De Rouck, Guido Aerts, Chris Powell, David J. Cook, Luc De Cooman,

Horticultural and Viticultural Research

Journal of the Institute of BrewingVolume 127, Issue 2 p. 107-126 Review articleOpen Access On the contribution of malt quality and the malting process to the formation of beer staling aldehydes: a review Weronika Filipowska, Corresponding Author weronika.filipowska@kuleuven.be orcid.org/0000-0001-9602-0222 KU Leuven, Faculty of Engineering Technology, Department of Microbial and Molecular Systems (M2S), Laboratory of Enzyme, Fermentation and Brewing Technology, Technology Campus Ghent, Gebroeders De Smetstraat 1, Ghent, 9000 Belgium International Centre for Brewing Science, School of Biosciences, University of Nottingham, Sutton Bonington Campus, Sutton Bonington, Leicestershire, LE12 5RD UKCorrespondence to: Weronika Filipowska, KU Leuven, Faculty of Engineering Technology, Department of Microbial and Molecular Systems (MS), Laboratory of Enzyme, Fermentation and Brewing Technology, Technology Campus Ghent, Gebroeders De Smetstraat 1, 9000 Ghent, Belgium. Email: weronika.filipowska@kuleuven.beSearch for more papers by this authorBarbara Jaskula-Goiris, orcid.org/0000-0002-8215-3590 KU Leuven, Faculty of Engineering Technology, Department of Microbial and Molecular Systems (M2S), Laboratory of Enzyme, Fermentation and Brewing Technology, Technology Campus Ghent, Gebroeders De Smetstraat 1, Ghent, 9000 BelgiumSearch for more papers by this authorMaciej Ditrych, orcid.org/0000-0001-8186-9346 KU Leuven, Faculty of Engineering Technology, Department of Microbial and Molecular Systems (M2S), Laboratory of Enzyme, Fermentation and Brewing Technology, Technology Campus Ghent, Gebroeders De Smetstraat 1, Ghent, 9000 BelgiumSearch for more papers by this authorPaula Bustillo Trueba, orcid.org/0000-0002-5938-7129 KU Leuven, Faculty of Engineering Technology, Department of Microbial and Molecular Systems (M2S), Laboratory of Enzyme, Fermentation and Brewing Technology, Technology Campus Ghent, Gebroeders De Smetstraat 1, Ghent, 9000 BelgiumSearch for more papers by this authorGert De Rouck, orcid.org/0000-0001-9092-8205 KU Leuven, Faculty of Engineering Technology, Department of Microbial and Molecular Systems (M2S), Laboratory of Enzyme, Fermentation and Brewing Technology, Technology Campus Ghent, Gebroeders De Smetstraat 1, Ghent, 9000 BelgiumSearch for more papers by this authorGuido Aerts, KU Leuven, Faculty of Engineering Technology, Department of Microbial and Molecular Systems (M2S), Laboratory of Enzyme, Fermentation and Brewing Technology, Technology Campus Ghent, Gebroeders De Smetstraat 1, Ghent, 9000 BelgiumSearch for more papers by this authorChris Powell, International Centre for Brewing Science, School of Biosciences, University of Nottingham, Sutton Bonington Campus, Sutton Bonington, Leicestershire, LE12 5RD UKSearch for more papers by this authorDavid Cook, orcid.org/0000-0002-4967-3287 International Centre for Brewing Science, School of Biosciences, University of Nottingham, Sutton Bonington Campus, Sutton Bonington, Leicestershire, LE12 5RD UKSearch for more papers by this authorLuc De Cooman, KU Leuven, Faculty of Engineering Technology, Department of Microbial and Molecular Systems (M2S), Laboratory of Enzyme, Fermentation and Brewing Technology, Technology Campus Ghent, Gebroeders De Smetstraat 1, Ghent, 9000 BelgiumSearch for more papers by this author Weronika Filipowska, Corresponding Author weronika.filipowska@kuleuven.be orcid.org/0000-0001-9602-0222 KU Leuven, Faculty of Engineering Technology, Department of Microbial and Molecular Systems (M2S), Laboratory of Enzyme, Fermentation and Brewing Technology, Technology Campus Ghent, Gebroeders De Smetstraat 1, Ghent, 9000 Belgium International Centre for Brewing Science, School of Biosciences, University of Nottingham, Sutton Bonington Campus, Sutton Bonington, Leicestershire, LE12 5RD UKCorrespondence to: Weronika Filipowska, KU Leuven, Faculty of Engineering Technology, Department of Microbial and Molecular Systems (MS), Laboratory of Enzyme, Fermentation and Brewing Technology, Technology Campus Ghent, Gebroeders De Smetstraat 1, 9000 Ghent, Belgium. Email: weronika.filipowska@kuleuven.beSearch for more papers by this authorBarbara Jaskula-Goiris, orcid.org/0000-0002-8215-3590 KU Leuven, Faculty of Engineering Technology, Department of Microbial and Molecular Systems (M2S), Laboratory of Enzyme, Fermentation and Brewing Technology, Technology Campus Ghent, Gebroeders De Smetstraat 1, Ghent, 9000 BelgiumSearch for more papers by this authorMaciej Ditrych, orcid.org/0000-0001-8186-9346 KU Leuven, Faculty of Engineering Technology, Department of Microbial and Molecular Systems (M2S), Laboratory of Enzyme, Fermentation and Brewing Technology, Technology Campus Ghent, Gebroeders De Smetstraat 1, Ghent, 9000 BelgiumSearch for more papers by this authorPaula Bustillo Trueba, orcid.org/0000-0002-5938-7129 KU Leuven, Faculty of Engineering Technology, Department of Microbial and Molecular Systems (M2S), Laboratory of Enzyme, Fermentation and Brewing Technology, Technology Campus Ghent, Gebroeders De Smetstraat 1, Ghent, 9000 BelgiumSearch for more papers by this authorGert De Rouck, orcid.org/0000-0001-9092-8205 KU Leuven, Faculty of Engineering Technology, Department of Microbial and Molecular Systems (M2S), Laboratory of Enzyme, Fermentation and Brewing Technology, Technology Campus Ghent, Gebroeders De Smetstraat 1, Ghent, 9000 BelgiumSearch for more papers by this authorGuido Aerts, KU Leuven, Faculty of Engineering Technology, Department of Microbial and Molecular Systems (M2S), Laboratory of Enzyme, Fermentation and Brewing Technology, Technology Campus Ghent, Gebroeders De Smetstraat 1, Ghent, 9000 BelgiumSearch for more papers by this authorChris Powell, International Centre for Brewing Science, School of Biosciences, University of Nottingham, Sutton Bonington Campus, Sutton Bonington, Leicestershire, LE12 5RD UKSearch for more papers by this authorDavid Cook, orcid.org/0000-0002-4967-3287 International Centre for Brewing Science, School of Biosciences, University of Nottingham, Sutton Bonington Campus, Sutton Bonington, Leicestershire, LE12 5RD UKSearch for more papers by this authorLuc De Cooman, KU Leuven, Faculty of Engineering Technology, Department of Microbial and Molecular Systems (M2S), Laboratory of Enzyme, Fermentation and Brewing Technology, Technology Campus Ghent, Gebroeders De Smetstraat 1, Ghent, 9000 BelgiumSearch for more papers by this author First published: 30 March 2021 https://doi.org/10.1002/jib.644AboutSectionsPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onEmailFacebookTwitterLinked InRedditWechat Abstract Despite decades of extensive research, beer flavour instability remains a challenge for both brewing and malting industries. Malt impacts the brewing process as well as the quality of the final beer. It also affects the stability of beer flavour, as it delivers to the brewing process various compounds with the potential to compromise the desired flavour characteristics of beer. These include staling aldehydes and their precursors, such as amino acids, reducing sugars, α-dicarbonyls and bound-state aldehydes. In general, the content of these compounds depends on barley variety and quality, the malting regime and final malt quality. Malt that represents a low potential for beer staling, i.e. that has low values of Kolbach Index, heat load, colour, LOX activity, Strecker aldehydes, transition metal ions and high antioxidative activity, leads to beer with enhanced flavour stability. However, the consistent production of malt with the desired quality remains challenging. Approaches to achieve this include adjustment of steeping and germination conditions, allowing control of grain modification and thus, the reservoir of aldehydes precursors. Also, the application of alternative kilning technologies may reduce the applied heat load, responsible for the formation of staling aldehydes and triggering development of the oxidising free radical species. This review provides an evaluation of current knowledge on the contribution of the malting process and malt quality to the formation of beer staling aldehydes. © 2021 The Authors. Journal of the Institute of Brewing published by John Wiley & Sons Ltd on behalf of The Institute of Brewing & Distilling. Introduction Contribution of free and bound aldehydes to beer flavour instability The microbiological, colloidal, foam, colour and flavour stability of beer are considered to be critical quality parameters influencing drinkability and brand acceptance by the consumer (1, 2). Moreover, according to the European law, beer – as a food product – needs to retain its properties until the 'best before' date, when stored properly (3). Unfortunately, various chemical reactions take place in the closed beer package, resulting in a change to the sensorial perception of beer over time, which starts almost instantly upon packaging. The most significant changes are the increase in off-flavours (e.g. cardboard associated with trans-2-nonenal) and the loss of pleasant flavour attributes (such as bitterness or ester character due to degradation of iso-α-acids and acetate esters, respectively) (4-11). Losses in esters also reduce their well known masking effect, thereby leading to an even more pronounced perception of off-flavours (8, 9, 12). Similarly, the synergistic effect caused by the sum of the intensities of beer ageing indicators allows the perception of off-flavours even when their concentrations in beer do not exceed their individual flavour thresholds (8-10). Exposure to high temperatures, light, vibrations during transport and/or contact with oxygen, as well as certain transition metal ions, accelerate the rate of beer staling (13-17). Unravelling the chemistry behind these changes, and thus learning how to control the rate at which flavour change develops, is the key to prolonging beer freshness. Measures adopted to improve beer flavour stability are considered most effective when applied downstream and close to the packaged product (18). However, the fact that staling precursors are developed upstream in the raw materials and brewhouse operations, means that brewers with 'best in class' flavour stability control measures are currently looking to these upstream stages to better understand the source of major staling precursors. One area of active research is to focus on the raw materials. Malt, as a major brewing ingredient, delivers to the brewing process various compounds, which can contribute to beer staling – i.e. amino acids, proteins, enzymes, reducing sugars and staling aldehydes (19-28). The content of these compounds in the malt is influenced by factors such as barley variety and malting process, which also directly affect malt quality. Therefore, this review discusses current knowledge on the impact of malt quality and malting process conditions on beer flavour deterioration with regard to beer staling aldehydes. Free aldehydes In the late 1960s, the search for potential beer staling markers pointed to aldehydes as a class of compounds of paramount importance, as their increase coincides with the appearance of off-flavours during beer ageing (29). Moreover, free aldehydes show flavour-active properties and very low flavour thresholds, for example, trans-2-nonenal can be perceived at 0.03 μg/L, methional at 4.2 μg/L and 2-methylbutanal at 45 μg/L, when spiked individually to a lager beer (8). Further studies led to the identification of the most relevant aldehydes resulting from various chemical pathways, which are indicators of lager beer staling – the so-called 'marker aldehydes' (30). The most frequently reported are hexanal, trans-2-nonenal, furfural, 2-methylpropanal, 2-methylbutanal, 3-methylbutanal, methional and phenylacetaldehyde (9, 19, 30-33). These aldehydes may arise through de novo formation and/or due to their release from a bound-state. Various authors (9, 34-45) have thoroughly discussed the possible reactions of de novo formation during malting and brewing. Marker aldehydes (see section 'The most relevant formation pathways of marker aldehydes in relation to malt') can arise through the following mechanisms: (1) lipid and fatty acid oxidation (auto- and enzymatic oxidation); (2) Maillard reactions; (3) Strecker degradation (Strecker degradation of amino acids in a strict sense, Strecker-like reactions, direct Strecker aldehyde formation from Amadori compounds, direct oxidation of amino acids); (4) oxidative degradation of isohumulones; (5) aldol condensation of short chain aldehydes; (6) oxidation of higher alcohols; (7) secondary oxidation of long chain aldehydes and (8) secretion by fermenting yeast. Bound-state aldehydes Aldehydes can also be present in the so called 'bound-state' forms. Free aldehydes are prone to binding due to the high electronegativity of the double bonded oxygen atom. Therefore, the electron deficient carbonyl carbon is likely to be attacked by nucleophiles (46, 47). This may result in binding with other compounds, such as bisulphites (38, 39), cysteine (48, 49) or other amino acids (40). During beer storage and under specific conditions inside the beer package (pH, storage temperature, vibrations during transport), those adducts may dissociate, releasing free aldehydes, thus causing an increase in off-flavours (9, 35, 50). Unravelling the chemistry behind bound-state aldehydes is crucial in order to better understand beer staling. The topic of aldehyde-adduct formation (the so-called 'binding') is complex since numerous chemical pathways are possible. Beer staling aldehydes are of different origin, chemical structures and properties, and they may react with various nucleophiles at different rates and under different reaction conditions (such as pH value, temperature, presence of oxygen). The binding behaviour of selected beer staling aldehydes was thoroughly studied by Baert et al. (48, 49). The authors demonstrated that the nucleophilic addition of cysteine or bisulphite to the carbonyl group of an aldehyde is affected by the electrophilic character of an individual aldehyde (see structures in Table 1). Accordingly, the electrophilicity of the carbon atom of the carbonyl group is influenced by the nature of the R group of the aldehyde (RCHO). In particular, the R group can be aliphatic (either saturated or unsaturated) or aromatic and because of this, the electrophilic character of aldehydes will vary. Thus, aldehydes substituted with an aromatic R group (e.g. phenylacetaldehyde) are less prone to binding to nucleophilic compounds compared to saturated aliphatic aldehydes (e.g. 2-methylpropanal) because the conjugated system of the aromatic substituent is decreasing the electrophilicity of the carbon atom of the carbonyl group. For the same reason, α, β-unsaturated aliphatic aldehydes such as trans-2-nonenal are less prone to binding nucleophilic compounds compared to saturated aliphatic aldehydes. Moreover, for saturated aliphatic aldehydes, the well known inductive effect should be taken into account when comparing their reactiveness towards nucleophilic compounds. For instance, the inductive effect caused by the methyl group present at the 2-position of the R group, as is the case for 2-methylpropanal and 2-methylbutanal, reduces the readiness for binding nucleophiles to some extent (51). Baert et al. (49) and Bustillo Trueba et al. (52) reported on the influence of the pH of beer (4.4), wort (5.2) and malt (6.0) on the binding behaviour of aldehydes. A general trend of lower affinity to binding at lower pH, regardless of the nucleophile (cysteine or bisulphite) could be seen. This is because the more acidic pH (4.4) enhances protonation of the carbonyl oxygen causing enolisation, which reduces the aldehyde readiness towards binding. Cysteine and bisulphite reactivity was hardly affected by the pH of the model solution (4.4-6.0), due to the relatively high pKa values of the cysteine amino group (pKa=10.8) and the sulfhydryl group (pKa=8.3) (51). Table 1. Molecular structures of free aldehydes and their corresponding cysteinylated forms, after Bustillo Trueba et al. (52) Aldehyde Molecular Structure Cysteinylated Aldehyde Molecular Structure 2-methylpropanal 2-isopropylthiazolidine-4-carboxylic acid 2-methylbutanal 2-(sec-butyl) thiazolidine-4-carboxylic acid 3-methylbutanal 2-isobutylthiazolidine-4-carboxylic acid hexanal 2-pentylthiazolidine-4-carboxylic acid furfural 2-(furan-2-yl) thiazolidine-4-carboxylic acid methional 2-(2-(methylthio)ethyl) thiazolidine-4-carboxylic acid phenylacetaldehyde 2-benzylthiazolidine-4-carboxylic acid trans-2-nonenal (E)-2-(oct-1-en-1-yl)thiazolidine-4-carboxylic acid Regarding the release of flavouractive free aldehydes, until now, indirect methods under extreme conditions were applied to measure the dissociation of bound-state aldehydes. In 1983, Baker et al. (38) presented the release of carbonyl compounds from their corresponding bisulphite adducts. In 1990, Drost et al. (53) introduced the concept of 'nonenal potential' as an indicator of the possible release of trans-2-nonenal from a bound-state form. In 2015, Baert et al. (49) used 4-vinylpyridine as an aldehyde 'releasing agent', to demonstrate that the bound aldehydes are present in fresh beers. Only recently the development of analytical methodologies has allowed a direct determination of cysteinylated aldehyde adducts in model solutions (52), and somewhat later, in malt, brewing and beer samples (28). Bustillo Trueba et al. (52) conducted a detailed study investigating the chemical behaviour of cysteinylated aldehydes in model solutions. The results showed that the degradation rate of an adduct depends on the 2-substitution pattern (i.e. the nature of the R group) of the thiazolidine ring and on the pH value of the medium, e.g. at malt pH (6.0) decomposition of cysteine adduct was slower than at beer pH (4.4). Under the acidic conditions, the nitrogen atom may be protonated, leading to destabilisation of the thiazolidine ring and ring opening. Sensitivity to pH was also previously reported for bisulphite adducts, as Kaneda et al. (39) demonstrated that carbonyl compounds are present in a bound-state when SO3- is in the nucleophilic form (pH range 3-6). Conversely, imine adducts are more stable at higher pH (since an increase of pH raises Schiff base concentration), whereas a lower pH, similarly to the exposure to heat, promotes dissociation of the complex (40). Therefore, at the pH of malt (6.0), bound-state aldehydes may be formed more easily than in beer and their stability may also be higher. An overview of possible interactions between saturated and unsaturated aldehydes and cysteine, an amine or bisulphite respectively, is shown in Figure 1. Figure 1Open in figure viewerPowerPoint An overview of the possible interactions between saturated (e.g. methional) and α-unsaturated (e.g. trans-2-nonenal) aldehydes and cysteine, an amine or bisulphite, after Baert et al. (49). The significance of free and bound-state aldehydes in beer ageing The debate as to what extent bound-state forms may be responsible for beer staling is still ongoing. Regarding de novo formation, it has been suggested that aldehydes 'reappear' during beer ageing. Wietstock et al. (45) demonstrated that supplementation of fresh beer with leucine, isoleucine and phenylalanine in the presence of oxygen, leads to higher concentrations of the corresponding Strecker aldehydes (2-methylbutanal, 3-methylbutanal and phenylacetaldehyde, respectively) upon beer ageing for 30 weeks at 20°C. This indicates that de novo formation of Strecker aldehydes may indeed occur in a closed beer package and is enhanced by the presence of oxygen. Similar outcomes were obtained by Gibson et al. (54), who added amino acids to fresh beer and observed an increase in Strecker aldehydes after forced ageing. Furthermore, Rangel-Aldao et al. (55) reported on the relevance of α-dicarbonyls (intermediate products of Maillard reactions) to aldehyde formation while storing beer at elevated temperatures (28°C). The authors determined lower levels of furfural and 5-hydroxymethyl furfural (5-HMF) in beers with the addition of an α-dicarbonyl trapping reagent. This is in agreement with Rakete et al. (44), who indicated that Maillard reactions resulting in the formation of furfural, occur to some extent during forced ageing of beer (two weeks at 50°C) since intermediates necessary for the reaction are present in beer. On the other hand, it has been suggested that the conditions in a closed beer package do not promote de novo formation. For example, Lermusieau et al. (40) compared the content of trans-2-nonenal in oxygen free and oxygen receiving ageing beers. The results indicated that the increase in trans-2-nonenal over time is not caused by lipid oxidation in the beer package, but it is due to the release of its free form from a bound-state. Moreover, Maillard reactions leading to de novo formation of e.g. furfural are favoured at conditions where pH is higher than typical beer pH values (for example, in malt) (46). To date, free aldehydes originating from both potential pathways – de novo formation and release from a bound-state form – are considered to be contributors to beer flavour deterioration. Suda et al. (42) reported that 85% of Strecker aldehydes determined in aged beers are derived from the wort, whereas 15% originate from de novo formation in packaged beer. Furthermore, both free and bound-state aldehydes, might be delivered to the brewing process with the raw materials and/or could be formed during beer production (28). Formation of imine adducts may occur during malting (56, 57) and brewing (50), whereas bisulphite adducts might be formed during fermentation or downstream (38, 53, 58). In summary, the above studies (28, 38, 42, 53, 56-58) point to the relevance of the malting and brewing process in the formation of bound-state aldehydes, as well as to malt as an essential source of beer staling compounds and their precursors. The most relevant formation pathways of marker aldehydes in relation to malt Malt provides aldehydes to the brewing process directly but also offers a variety of their precursors (Figure 2). The formation pathways of marker aldehydes are complex and consist of numerous steps, which strongly depend on the reaction conditions (e.g. pH, temperature, presence of substrates) and can lead to various intermediates and final products. This section focuses on reactions taking place in malt (or analogous conditions) and leading to the formation of marker aldehydes, namely: hexanal, trans-2-nonenal, furfural, 2-methylpropanal, 2-methylbutanal, 3-methylbutanal, methional and phenylacetaldehyde. These compounds represent the end products of typical formation reactions, e.g. oxidation of unsaturated fatty acids, Maillard reactions and Strecker degradation of amino acids. Figure 2Open in figure viewerPowerPoint Compounds contributed by malt to the brewing process, which may potentially affect beer flavour stability. Hexanal and trans-2-nonenal The main chemical pathway leading to the formation of hexanal and trans-2-nonenal is the oxidation of unsaturated fatty acids via autoxidation or catalysed by enzymes. In the enzymatic pathway (Figure 3), linoleic acid (C18:2) and linolenic acid (C18:3), representing up to 60 and 10% of the total fatty acids in malt (59), are released in the presence of water from triacylglycerols by lipase (pH optimum 6.8) (60). The resulting free fatty acids are oxidised by lipoxygenase (LOX-1 and LOX-2, pH optimum 6.5) (61) to hydroperoxy fatty acids. LOX-1 yields 9-hydroperoxyoctadeca-10,12-dienoic acid (9-LOOH), whereas LOX-2 produces 13-hydroperoxyoctadeca-9,11-dienoic acid (13-LOOH). During malt kilning, most of the LOX activity is destroyed as both enzymes are heat-sensitive. However, LOX-1 is more heat resistant than LOX-2, thus the formation of 9-LOOH proceeds at a higher rate (62). Subsequently, 9- and 13-LOOH are subject to enzymatic degradation to mono-, di- and trihydroxy fatty acids followed by non-enzymatic breakdown resulting in carbonyl compounds. The pathway of 9-LOOH leads to trans-2-nonenal, whereas 13-LOOH yields hexanal (1). Another possible oxidation pathway of linoleic and linolenic acid esterified in triacylglycerol is by LOX-2, also leads to the formation of carbonyl compounds (1, 59). Figure 3Open in figure viewerPowerPoint An overview of enzymatic oxidation leading to the formation of hexanal and trans-2-nonenal, according to Vanderhaegen et al. (1). The continuous line refers to reactions at a high rate, whereas the dashed line relate to pathways that proceed at a slower rate. Regarding autoxidation, in the cascade of reactions, unsaturated fatty acids can be oxidised by reactive oxygen species and via lipid peroxyl radicals into lipid hydroperoxides (9-LOOH and 13-LOOH) (Figure 4) (64). Again, various compounds may be formed from these precursors in enzymatic and non-enzymatic reactions leading to hexanal and trans-2-nonenal. The rate of autoxidation is enhanced by high temperatures and the presence of oxidants, e.g. transition metal ions (iron and copper) (9). Malt, among other brewing raw materials, is rich in these ions, delivering up to 97.5% of iron and 94.3% of copper to the process (65). Another critical reaction from the perspective of malting is the secondary autoxidation of unsaturated aldehydes. For example, trans-2-nonenal can be autoxidised into shorter chain aldehydes, such as hexanal (66). Figure 4Open in figure viewerPowerPoint An overview of the oxidation cascade of unsaturated fatty acids, initiated by reactive oxygen species, from Bamforth and Cook (63). Furfural Furfural is one of the many products of Maillard reactions – a complex reaction chain initiated by an amine, amino acid, peptide or protein reacting with a pentose reducing sugar (Figure 5) (9, 67). The reaction is initiated by nucleophilic addition of an amino group to the reducing end of an open chain of sugar, leading to N-glycosylamine (Schiff base) formation (68). This intermediate undergoes Amadori rearrangement resulting in the formation of 1-amino-1-deoxyketose (Amadori compound), which undergoes enolisation and, depending on the pH, forms specific isomers. In the next stage, the amine is released and α-dicarbonyls are formed – 3-deoxyosone (pH 7). Upon dehydration of 3-deoxyosone followed by cyclisation of the intermediate α-dicarbonyl, and final dehydration, furfural is formed from pentose. The generated α-dicarbonyls can also act as a reactant in Strecker degradation of amino acids. The kinetics of Maillard reactions are strongly dependent on the nature and proportion of reactants, temperature, time, pH and water activity (69-71). For example, pH value affects the reactivity of amino group (pKa values around 9 or higher) and the proportion of open chain to closed chain forms of sugars (more aldose forms are present at higher pH). Also, a moderate water activity is required, allowing almost a subsequent addition and elimination of water molecule (see Figure 5). As Maillard reactions are mostly associated with the exposure of malt to high temperatures, these chemical pathways have been studied extensively with regard to dark speciality malts (72-74) and pale malts (75). Figure 5Open in figure viewerPowerPoint An overview of Maillard reactions leading to the formation of furfural, after Baert et al. (9). Reaction begins with pentose (n=2) or hexose (n=3), and yields α-dicarbonyls (3-, 4-, and 1-deoxyosones) and some heterocyclic compounds (furfural and 5-hydroxymethylfurfural (5-HMF)). 3,4-DDP - 3,4-dideoxypentosulose-3-ene; 3,4-DDH - 3,4-dideoxyhexosulose-3-ene. 2-Methylpropanal, 2-methylbutanal, 3-methylbutanal, phenylacetaldehyde and methional Strecker aldehydes may arise via several pathways. One of them is the Strecker degradation in a strict sense (Figure 6), which is a reaction between an amino acid and an α-dicarbonyl. In the case of beer staling marker aldehydes, amino acids act as precursors (valine is a precursor of 2-methylpropanal, isoleucine of 2-methylbutanal, leucine of 3-methylbutanal, methionine of methional and phenylalanine of phenylacetaldehyde (9)), whereas a variety of α-dicarbonyls are derived, among others, from Maillard reactions (76). The Strecker degradation in a strict sense is initiated by nucleophilic addition of the unprotonated amino group to the carbonyl group resulting in the formation of a hemiaminal. This unstable intermediate undergoes dehydration and subsequent irreversible decarboxylation forming an imine zwitterion. Water addition leads to an unstable amino alcohol, which breaks down into an α-ketoamine and an aldehyde (1, 9, 76, 77). Figure 6Open in figure viewerPowerPoint An overview of the Strecker degradation in a strict sense leading to the formation of 2-methylpropanal, 2-methylbutanal, 3-methylbutanal, methional and phenylacetaldehyde, according to Baert et al. (9). Alternatively, Strecker aldehydes may arise via Strecker-like reactions – between an amino acid and an α,β-unsaturated carbonyl compound (e.g. trans-2-nonenal, furfural) (77) - or by direct oxidative degradation of amino acids (78). The latter was confirmed to occur in beer (45), however, further invest