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Chapter 9

Formation of Reactive Fragmentation Products during the Maillard Degradation of Reducing Sugars − A Review M. A. Glomb* and C. Henning Martin-Luther-University Halle-Wittenberg, Institute of Chemistry, Food Chemistry, Kurt-Mothes-Str. 2, D-06120 Halle (Saale), Germany *E-mail: [email protected]

The amine induced fragmentation of reducing sugars plays a pivotal role during the formation of browned flavors. The shorter-chained products are reactive intermediates in the further reaction cascades leading to taste-active, odor-active, or browned compounds, or they are flavor-active as such. The three mechanisms of fragmentation are retro-aldol reactions, hydrolytic β-dicarbonyl cleavage and hydrolytic oxidative α-dicarbonyl cleavage. A fourth mechanism, namely hydrolytic α-dicarbonyl cleavage, present in the older Maillard literature was outruled as void. The present mini review summarizes the up to date knowledge on mechanisms, which are discussed in detail for the degradation of 1-deoxyglucosone and ascorbic acid, respectively. For the latter, the amine catalyzed breakdown was comprehensively investigated on the basis of isotopic labeling experiments with 13C-ascorbic acid isotopomers and 18O2 atmosphere. These results can also be transferred to other reducing sugars present in foods.

Introduction The important role of the Maillard reaction during thermal food processing has been accepted for a long time (1, 2). Reducing sugars are degraded via the direct reaction with amines or via amine triggered acid-base catalysis. As both structural groups are present in almost every food system in substantial amounts, this means that Maillard reaction describes the interaction of the two © 2016 American Chemical Society Granvogl et al.; Browned Flavors: Analysis, Formation, and Physiology ACS Symposium Series; American Chemical Society: Washington, DC, 2016.


major constituents of foods, carbohydrates and nitrogen containing compounds such as proteins, beside fats. The results are manifold (Figure 1). Other than the generation of odor- and taste-active structures as desired, but also off-flavors, the formation of melanoidines leads to color, while reductone structures can significantly alter the shelf life of foods. Potentially anutritive consequences arise from the generation of mutagenic or cancerogenic structures especially at high temperatures used, e.g., during frying or roasting. Already starting at moderate conditions, glycation leads to modification of essential amino acids and thereby decreases the biological value of proteins. However, carefully controlled protein modification can also lead to increased digestibility and can lead to improvement of desirable technological properties such as hydration or gel formation.

Figure 1. Modern view of the Maillard reaction However, the most challenging aspect of the Maillard reaction is that unlike reactions in organic synthesis where two or a small manageable amount of educts lead to a clear product spectrum of almost quantitative yield, here, a plethora of structures is formed, most of them far below the ppm range. Thus, in reality, the so-called Maillard reaction comprises of a whole reaction cascade with major mechanistic aspects described by enolization, dehydration, elimination, cyclization, fragmentation, and redox reaction including both oxidation and reduction. Today it is accepted, that α-dicarbonyl compounds are the central intermediates of the Maillard reaction, by which most if not all products of the intermediate and advanced reaction stages can be explained. This understanding has changed the definition of the term Maillard reaction. In the modern view, it is the reaction of the α-dicarbonyl compounds with amines and the resulting consequences, regardless of their origin. Thus in foods, in addition to the degradation of reducing sugars, α-dicarbonyls resulting from fermentative processes (e.g., methylglyoxal) (3), from fat autoxidation (e.g., glyoxal) (4), and from the oxidation of phenolic compounds (i.e., o-quinones) (5) have to be considered. 118 Granvogl et al.; Browned Flavors: Analysis, Formation, and Physiology ACS Symposium Series; American Chemical Society: Washington, DC, 2016.

Initial and Intermediate Phase


Due to their high reactivity the profil of α-dicarbonyls formed during the amine-catalyzed degradation of a specific reducing sugar can only be studied after stabilizing derivatization reactions. The most common trapping reaction used in the field is the reaction with o-phenylenediamine leading to quinoxalines (6, 7).

Figure 2. α-Dicarbonyl spectrum of glucose degradation Thus, we were able to establish the major structures for the Maillard chemistry of glucose (8), maltose (9), and ascorbic acid. Figure 2 depicts the spectrum from glucose including α-dicarbonyls with the intact C6-carbon backbone, but also a significant extent of degradation from C5- down to C2-fragments. Exceptions are 1-amino-1,4-dideoxyglucosone and 1,4-dideoxyglucosone, which show the intact C6-skeleton of monosaccharides, but in contrast are rather typical for the degradation of disaccharides, where they are formed in significant higher amounts. The formation of α-dicarbonyl structures with the intact carbon backbone of the original starting reducing sugar has been studied from the early days of Maillard research and has been extended ever since. Figure 3 summarizes the early stages of the reaction. Here, glucose reacts with the ε-amino group of lysine to form an imine, which is the open-chained form of the N-glycoside. Isomerization by enolization reactions, also called Amadori rearrangement, then leads to an aminoketose or Amadori product via an 1,2-enaminol. The latter intermediate gives 3-deoxyglucosone after dehydration, but because of its electron rich nature, it also has to be defined as the main precursor to give glucosone after oxidation. Even though isomerization reactions have been long reported for alkaline or amine 119 Granvogl et al.; Browned Flavors: Analysis, Formation, and Physiology ACS Symposium Series; American Chemical Society: Washington, DC, 2016.


catalyzed reaction mixtures, we know today that enolization can proceed through the entire carbon backbone also under moderate down to physiological conditions. Elimination of lysine from the 2,3-enediol gives 1-deoxyglucosone, elimination of water from C-3 position of the 5,6-enediol results in Lederer´s glucosone (N6-(3,6dideoxyhexos-2-ulos-6-yl)-lysine) in substantial amounts (10).

Figure 3. Initial stage of glucose degradation

120 Granvogl et al.; Browned Flavors: Analysis, Formation, and Physiology ACS Symposium Series; American Chemical Society: Washington, DC, 2016.


The resulting α-dicarbonyls are much more reactive than the original starting sugar. However, there are hugh differences in reactivity within this group itself. E.g., 1-deoxyglucosone and glucosone give after enolization a corresponding α-oxo-enediol function. Thus, this reductone feature explains the high redox activity of both and is the basis for their high potential to fragment to low molecular products of even much increased reactivity. Taken together, this means that specifically compounds with a reductone moiety are the key to understand and to direct the complex Maillard pathways of browned flavors. The short-chained fragments can be odor-active themselves or are critical intermediates in the syntheses of important flavors or in color formation. Beside others, especially butane-2,3-dione (diacetyl), methylglyoxal, 1-hydroxy-2-propanone (hydroxyacetone/acetol), acetaldehyde, glycolaldehyde, and formaldehyde were identified as important reactants in mechanistic studies to lead to major odor generalists. As an example, the current literature for the furaneol formation complex is depicted in Figure 4 (11–15). Cyclization of 1-deoxyglucosone leads to a furanoic hemiacetal, which on one hand can eliminate water at C-6 to give acetylformoin (1). This structure then is reduced by, e.g., reductone chemistry to result after cleavage of a second water molecule in furaneol. On the other hand, the initial hemiacetal can dehydrate at C-1 to furan-3-one to cleave off formaldehyde and yield norfuraneol. This structure represents basically a switch to the Maillard chemistry of pentoses, where it is formed in high yields from 1-deoxypentosones. The reversibility of this aldol type reaction is verified by detection of furaneol in pentose reaction mixtures (2), but also by the identification of homofuraneol, i.e., the condensation product with acetaldehyde (3). Regardless of furaneol or of homofuraneol, a reduction is needed. This key step can be avoided, if methylglyoxal is reacted with acetol. Here, aldol condensation builds up the required C6-carbon backbone (4). Isomerization leads to a 1,6-dideoxy-2,3-hexodiulose, which after loss of water directly gives furaneol. This putative intermediate in combination with non-redox chemistry also explains why furaneol is formed in high yields from 6-deoxyhexoses (5) such as rhamnose (6-deoxy-l-mannose) or fucose (6-deoxy-l-galactose).

Fragmentation Mechanisms Figure 5 visualizes the four fragmentation mechanisms quoted in Maillard literature. However, as shown by a recent review, mainly in the older reports the chemistry proposed was more a cut through the molecule than a solid final argumentation (16). This is especially true for the hydrolytic α-cleavage, which is one of the most cited fragmentations in early Maillard research. Based on an intramolecular disproportionation of an unknown mechanism, this hypothetical pathway leads to the formation of carboxylic acids and aldehydes without change of stereochemistry. However, the corresponding counterparts were either missing or quantitatively in a total misbalance.



Figure 4. Formation of furaneol from 1-deoxyglucosone, 1-deoxypentosone, 6-deoxyhexoses, and C3-fragmentation products

These facts prompted Davidek et al. to conduct an in-depth study using simple α- and β-diketo alkane reaction mixtures (17). Indeed, there were absolutely no fragments monitored to verify hydrolytic α-dicarbonyl cleavage reactions. This, also in light of the dissatisfying literature background, led to the conclusion that the proposed mechanism is not existent. Instead, α-dicarbonyls in single reaction systems proved to be rather stable and as a minor pathway the authors monitored the two quantitatively fitting carboxylic fragments of an alternative hydrolytic oxidative α-scission route. By the use of isotopically labeled reagents, they successfully transferred this novel pathway to the chemistry of 1-deoxyglucosone, however, as a negligible side reaction. More importantly, they were able to conclusively support the underlying oxidative α-dicarbonyl cleavage mechanism, which was shown to be a major pathway for ascorbic acid Maillard degradation by us and is discussed in detail further below for the ascorbic acid degradation cascade. 122 Granvogl et al.; Browned Flavors: Analysis, Formation, and Physiology ACS Symposium Series; American Chemical Society: Washington, DC, 2016.


Figure 5. Maillard sugar fragmentation mechanims

A mechanism, which is second most used are retro-aldol type reactions. For this scission, a β-hydroxycarbonyl moiety is prerequisite, which is true for most carbohydrates, but also for the much more reactive α-dicarbonyl compounds discussed herein. In theory, it is the reverse of aldol condensations, which indeed can be monitored in aqueous systems. Both, aldol and retro-aldol type reactions are frequently used in organic syntheses, however, under non-aqueous conditions. This led to the assumption that retro-aldol reactions might be a major mechanism to argue the formation of short-chained carbonyl intermediates in water based systems as foods, too.

Figure 6. Retro-aldol fragmentations 123 Granvogl et al.; Browned Flavors: Analysis, Formation, and Physiology ACS Symposium Series; American Chemical Society: Washington, DC, 2016.


The most prominent examples for this hypothesis are found for the formation of methylglyoxal during Maillard sugar degradation (18, 19). As shown in Figure 6, 1-deoxyglucosone and 3-deoxyglucosone are proposed as the main educts, both leading to methylglyoxal and glyceric aldehyde as the corresponding counterpart. However, investigations of our work group on these two C6-dicarbonyls conducted at moderate conditions up to 50 °C (20) and also by others (16) at higher temperatures indeed led to the identification of methylglyoxal, but only as a minor product of negligible concentration. In an attempt to clarify the concept of aldol mechanisms, the reaction of diacetyl with formaldehyde led to the formation of 1,4-dideoxypentosone in aqueous conditions (21). The authors then used this fact for the inverse argumentation of diacetyl formation from 1,4-dideoxyosones as an important odorant in Maillard reaction systems. However, when we synthesized authentic 1,4-dideoxyglucosone as a direct proof of concept, absolutely no diacetyl nor the hypothetic counterpart glycolaldehyde were found during amine catalyzed degradation at moderate temperatures (9). In contrast, 1,4-dideoxyglucosone turned out to be rather stable. Thus, it must be concluded that retro-aldol fragmentation is of minor importance for reaction mixtures up to 50 °C, if not non-existent. Degradation of 1-Deoxyglucosone When Davidek et al. monitored β-diketo alkane reaction mixtures in above basic studies on fragmentation, they found these class of molecules to be rather reactive in comparison to α-dicarbonyls and to be significantly degraded by the forth fragmentation mechanism, the hydrolytic β-dicarbonyl cleavage route (22). As shown in Figure 5, the molecule gets hydrated first at one of the carbonyl functions, which triggers fragmentation by the second carbonyl to give a carboxylic acid and an enol. The enol then isomerizes to a ketone. Transferred to the degradation of reducing sugars, the intermediate enol presents an enediol to result in a ketose. The authors then used this mechanism to explain the release of major quantities of acetic acid from 1-deoxyglucosone. Hydrolytic β-dicarbonyl cleavage reactions have occasionally been used before in Maillard literature, but from today´s perspective have to be evaluated as the quantitatively most important mechanism to explain the generation of short-chained fragments during sugar degradation, already working readily at moderate conditions down to ambient temperature. This conclusion is based to a large extent on the recent understanding of the 1-deoxyglucosone degradation cascade, which explains most, if not all short-chained carbonyl degradation products in hexose reaction mixtures. Thus, 1-deoxyglucosone has evolved to the single major intermediate in hexose Maillard chemistry directing the mechanistic course. The investigations were based on our successful synthesis of authentic 1-deoxyglucosone to in-depth study the specific degradation of this highly reactive structure (23). Figure 7 summarizes the pathways for reactions of 1-deoxyglucosone with amines in phosphate buffered mixtures at temperatures of 37 °C up to 50 °C (20, 24, 25). Cylization of the hydroxyl group at C-5 and the carbonyl function at C-2 gives the furanoic structure already discussed above for the chemistry of furaneol 124 Granvogl et al.; Browned Flavors: Analysis, Formation, and Physiology ACS Symposium Series; American Chemical Society: Washington, DC, 2016.


in yields up to 50 mol-%. In lower amounts, up to 18 mol-%, the pyranoic ring structures like the γ-pyranone can be found. However, both structures are transient intermediates to be degraded again in the further course of the reaction. In addition, as cyclic reductone structures, the concentrations also depend on the presence of oxygen. However, the key step to explain the high reactivity of 1-deoxyglucosone is the corresponding α-oxo-enediol structure, allowing (1) the isomerization to 1-deoxy-2,4-hexodiulose or (2) the oxidation to give a 2,3,4-tricarbonyl compound. In the first place, the resulting β-dicarbonyl moiety is hydrated at C-2 (1a), to fragment to give acetic acid and a C4-enediol intermediate as the counterpart. Under our experimental setup, we were able to detect up to 65 mol-% yield for acetic acid inline with publications of Davidek’s team, who reported up to 85 mol-% at cooking temperature (22). While carboxylic acids must be considered as stable endproducts accumulating with reaction time, the C4-enediol is a reactive intermediate to give 3-deoxytetrosone, erythrulose, 1-deoxytetrosone, or tetrosone after oxidation all being transient, very reactive intermediates themselves. As a measure for this C4-chemistry, erythrulose was formed in maximum concentrations of 16 mol-% and was again degraded. As possible products, lactic acid, glyceric aldehyde, acetaldehyde, and glycolaldehyde were identified. As a second pathway from the β-dicarbonyl isomer, position C-4 is hydrated to give two C3-fragments (1b), acetol counterparted by glyceric acid from the lower part in concentrations up to 16 mol-%. Alternatively to isomerization, the reductone intermediate can be oxidized (2). The β-dicarbonyl reactivity of the resulting 2,3,4-tricarbonyl structure is better envisioned after hydration at the C-3 position, reminiscent of the situation in ninhydrine. This means that from this educt the hydrolytic β-cleavage leads to two carboxylic acids, lactic acid from position C-1 to C-3 and glyceric acid from the lower part. Under oxidative conditions, the lactic acid concentrations increased to 16 mol-%, which supported the proposed oxidation of 1-deoxyglucosone. In addition, stable isotope labeling experiments verified 8 mol-% to stem from the C-1 to C-3 position with the methyl group solely assigned to position C-1 inline with the proposed mechanism. The remaining 8 mol-% were assigned to the C-3 to C-6 part of the carbon skeleton resulting from above C4-intermediates, most likely via β-scission again. Taken together, depending on the reaction conditions, we were able to verify up or more than 80 mol-% of the degradation of 1-deoxyglucosone based on the hydrolytic β-dicarbonyl cleavage mechanism by monitoring carboxylic acids as stable Maillard end products. In further strong support of this mechanism, we identified the parallel formation of the lysine amide derivatives, i.e., N6-acetyl lysine, N6-glycerinyl lysine, and N6-lactoyl lysine, however at much lower concentrations. In this case, the ε-amino function of lysine forms a hemi aminal, which then induces the aminolytic β-cleavage to give this stable class of amide advanced glycation endproducts. An alternative cleavage of above pathway (2) initiated by a 2,3-dihydate intermediate from the 2,3,4-tricarbonyl was excluded, as no erythronic acid was detected. This fact also undermined the possibility of a distinct oxidative α-dicarbonyl cleavage route starting directly from 1-deoxyglucosone to explain the significant acetic acid generation. 125 Granvogl et al.; Browned Flavors: Analysis, Formation, and Physiology ACS Symposium Series; American Chemical Society: Washington, DC, 2016.


Figure 7. 1-Deoxyglucosone degradation cascade Degradation of Ascorbic Acid Our research on the major impact of hydrolytic β-dicarbonyl cleavage reactions on 1-deoxyglucosone fragmentation prompted us to initiate mechanistic investigations on L-threo-ascorbic acid degradation as another reducing carbohydrate with a reductone α-oxo-enediol moiety (26). Shin and Feather reported that in this case 2,3-diketogulonic acid is the central intermediate, because regardless if the reaction was started from this structure or from ascorbic 126 Granvogl et al.; Browned Flavors: Analysis, Formation, and Physiology ACS Symposium Series; American Chemical Society: Washington, DC, 2016.


acid, the product profile was identical (27). This means that ascorbic acid is first oxidized reversibly to dehydroascorbic acid to hydrolyze irreversibly to 2,3-diketogulonic acid, which then initiates fragmentation. Ascorbic acid degradation was studied in presence of lysine in phosphate buffered reactions from ambient up to moderate temperatures (50 °C). All fragmentation products were assigned 100% to the exact position in the original C6-carbon backbone by the use of single 13C-labeled ascorbic acid isotopomers as shown in Figure 8. Exceptions were oxalic acid, glycolic acid, and methylglyoxal, which originated 90% from C-1 to C-2, 50% from C-2 to C-3, and 70% from C-4 to C-6, respectively. As stable end products, all major carboxylic acids were quantified, whereas reactive intermediates as carbonyl or dicarbonyl structures were assessed after derivatization. Decarboxylation is a well-known fragmentation mechanism of ascorbic acid leading to products with a C5-carbon skeleton. This reaction starts after formation of a hydrate at the C-2 position, which now clearly identifies the carboxylic acid function of 2,3-diketogulonic acid as an excellent leaving group in β-position to the C-3 carbonyl function. The resulting C5-enediol intermediate can isomerize to give the stereoisomeric xylonic and lyxonic acids as stable end products, or alternatively cleave off water to give xylosone to be further degraded. This decarboxylation is very similar to the hydrolytic β-dicarbonyl cleavage mechanism. To initiate this, 2,3-diketogulonic acid has to isomerize through the molecule to give 2,4- and 3,5-diketogulonic acid. For the 2,4-isomer, hydration at C-2 followed by fragmentation gives oxalic acid and a C4-enediol to give erythrulose, 3- and 1-deoxytetrosone, and tetrosone, as already detailed above for the d-stereochemistry of 1-deoxyglucosone. In significantly lower amounts, the 3,5-isomer is degraded to give tartronic acid and a C3-enediol that can isomerize to glyceric acid or cleave off water to result in methylglyoxal. However, quantitation of all major fragments showed that only 28 of a total of almost 60 mol-% of oxalic acid could be attributed to the β-scission of the 2,4isomer, as the C4-counterpart erythrulose only gave a maximum of 28 mol-%. The only other C4-fragment referring to the original C-2 to C-6 carbon backbone of ascorbic acid was threonic acid with 25 mol-%. This implied a major hydrolytic oxidative α-dicarbonyl cleavage route based on the mechanism detailed in Figure 5. This pathway is started by the addition of molecular oxygen at the carbonyl function, which can be generated, e.g., by photochemical processes in presence of sensitizers or by decay of hydroperoxide species generated, e.g., from Maillard reactions themselves or from fat autoxidation. The resulting alkoxyradical intermediate then gives a hydroxyl-hydroperoxide by single electron transfer reactions to result in an asymmetric acid anhydride after a Bayer-Villiger type rearrangement reaction. Spontaneous hydrolysis ends in two carboxylic acids, i.e., 2,3-diketogulonic acid is cleaved to oxalic acid and threonic acid. To verify this mechanism, we used two approaches, (I) incubation with 18O2 and (II) identification of the corresponding lysine carboxylic amides. Due to the cleavage of the hydroxyperoxide, only one label of the dioxygen is incorporated into the anhydride intermediate, which means carboxylic acids solely resulting from the oxidative α-scission must be 50% labeled by definition. Thus, we were able to 127 Granvogl et al.; Browned Flavors: Analysis, Formation, and Physiology ACS Symposium Series; American Chemical Society: Washington, DC, 2016.


assign the formation of erythronic acid, glycolic acid, and glyceric acid fully to this pathway, while xylonic acid, lyxonic acid, and tartronic acid showed absolutely no label incorporation. This means that the later are formed solely by the β-cleavage reaction. As the single exemption, mass spectrometric analysis of oxalic acid showed 25% labeling, which means that one half is formed by the α-oxidative route with threonic acid as the quantitatively fitting counterpart. The other half has to be allocated to the β-scission with erythulose as the corresponding fragment.

Figure 8. Ascorbic acid degradation cascade 128 Granvogl et al.; Browned Flavors: Analysis, Formation, and Physiology ACS Symposium Series; American Chemical Society: Washington, DC, 2016.


As a second verification, we identified the parallel formation of the corresponding threonyl, glycerinyl, and glycolyl lysine amides. In this case, the anhydride intermediate in Figure 5 is attacked by the ε-amino group of lysine to give the amides as an alternative aminolytic oxidative α-dicarbonyl cleavage route, however, at significantly lower yields. In summary, as depicted in Figure 8, we were able to verify 75 mol-% of the amine catalyzed degradation of ascorbic acid by three mechanisms: 12 mol-% to decarboxylation, 32 mol-% to the hydrolytic β-dicarbonyl cleavage and 31 mol-% to the hydrolytic oxidative α-dicarbonyl scission. Depending on the reaction conditions, up to 10 mol-% additional degradation to products of unknown mechanistic background was identified.

Conclusion Even more than 100 years after the first reports on amine catalyzed degradation of reducing sugars by Louis Camille Maillard, the reaction cascade termed thereafter remains exciting (28). The field has come to the understanding that α-dicarbonyl compounds and especially their short-chained degradation products are the key to understand the confusing diversity of Maillard products as browned flavors. Research in the last 5 years has unravelled hydrolytic β-dicarbonyl cleavages as the main fragmentation route of reducing sugars based on conclusive mechanistic investigations, although it had been suggested before on a rather descriptive way. It has become clear that central intermediates as 1-deoxyglucosone for hexoses must show a tautomeric α-oxo-enediol reductone moiety to be especially prone to the β-scission route. The second important mechanism, as verified in detail for ascorbic acid reactions, is the hydrolytic oxidative α-dicarbonyl cleavage. This fragmentation includes (I) the addition of singlet oxygen at the respective carbonyl function and opens the question how this activation of oxygen is substantiated. The answer obviously must be also the key to understand the immense difference to 1-deoxyglucosone. Here, inspite of the same reductone character, oxidative α-dicarbonyl scission only plays a minor role. Furthermore, this fragmentation also includes (II) one electron transfer reactions and again underlines that Maillard reactions provide redox chemistry with both two and one electron transitions. The other famous example for the later facet is the Namiki pathway, which proceeds via a pyrazinium radical cation (29, 30). Whatever, one major fundamental fact from the research described above is that the long propagated alternate hydrolytic α-dicarbonyl cleavage is devoid of any mechanistic basis and has to be ruled out for fragmentation reactions. Another aim of future mechanistic studies on Maillard reactions must be to clarify the role of retro-aldol fragmentations, a concept which has to be challenged from today´s perspective. This also includes the unexpected conclusion that even today there is no coherent concept to explain the formation of methylglyoxal as an important intermediate in the formation of many browned flavors. 129 Granvogl et al.; Browned Flavors: Analysis, Formation, and Physiology ACS Symposium Series; American Chemical Society: Washington, DC, 2016.

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