Mechanism of Pyrazine Formation Intervened by Oxidized Methionines during Thermal Degradation of the Methionine-Glucose Amadori Compound

Mechanism of pyrazines and thioethers formation promoted by high oxygen concentration in the methionine-glucose Maillard reaction system

BACKGROUND

The typical aroma compounds in methionine-glucose Maillard products often undergo further degradation and polymerization during storage and thermal processing, leading to flavor dispersion and aroma distortion. It is crucial to identify measures that enhance typical aroma substances in such flavor matrices.

RESULTS

The effect of oxygen on the flavor formation of methionine-glucose thermal reaction system was explored by determining typical flavor substance contents and flavor differences. Compared with the oxygen concentration in the air (21%), a high oxygen concentration (30% and 40%) effectively promoted the formation of typical flavor substances. Pyrazines increased by 44% and thioethers increased by 13%. The reaction process and the content of key substances were both measured to explain the involvement of oxygen. It was found that high oxygen concentration increased the reaction efficiency of glucose and methionine and promoted the formation of α-dicarbonyl compounds, including glyoxal, methylglyoxal and 3-deoxyglucosinone. Moreover, a glyoxal-methylglyoxal-methionine model system was established to verify the effect of oxygen intervention on the formation of pyrazines and thioethers generating from α-dicarbonyl compounds. It was confirmed that a high oxygen concentration promoted the consumption of glyoxal and methylglyoxal, which were more readily converted into pyrazines and thioethers without forming melanoidins.

CONCLUSION

A high concentration of oxygen promoted the formation of pyrazines and thioethers during the Maillard reaction of methionine and glucose, and effectively inhibited the occurrence of browning. The present study provides a new concept for the typical flavor enhancement of methionine-glucose Maillard reaction products. © 2024 Society of Chemical Industry.

Effect of lysine on the cysteine-xylose Maillard reaction to form flavor compounds

To understand flavor formation mechanisms in complex meat-like Maillard systems, effect of lysine on cysteine-xylose reaction to form flavors was studied. GC-MS and GC-O analyses found lysine of 1 times cysteine concentration led to the greatest amount of sulfur-containing meaty compounds while more additional lysine caused more pyrazine compounds. LC-MS analysis showed lysine competed with cysteine to form the early-stage intermediate of Lys-Amadori compounds and accelerated conversion of 2-threityl-thiazolidine-4-carboxylic acids to Cys-Amadori compounds from the cysteine-xylose reaction. Reaction rates based on UV 294 and 420 nm absorbance, browning color, and consumption of cysteine and xylose suggested addition of lysine continuously accelerated the Maillard reaction at intermediate and final stages. Pearson correlation analysis revealed less reaction rates and Lys-Amadori compounds formed could cause more meaty compounds and thereby exposed formation pathways of important aroma compounds. This work can provide guidance for optimizing meat or meat product composition to improve meat flavor.

Promoted formation of pyrazines by targeted precursor addition to improve aroma of thermally processed methionine-glucose Amadori compound

The methionine/glucose (Met/Glc) and methionine/glucose-derived Amadori rearrangement product (MG-ARP) models were established to analyze their differences in flavor profiles and aroma potentiality. The principal component analysis revealed the advantage of MG-ARP in the formation of low temperature-induced processing flavor. MG-ARP exhibited superior potential in the rapid formation and high intensity of processed flavor than the Met/Glc except for the inefficiency in pyrazine production. The extra-added Glc tended to react with recovered Met to compete against α-dicarbonyl compounds to suppress the Strecker degradation and pyrazine formation. The additional Met effectively improved the precursor availability and facilitated the conversion of C6-α-dicarbonyl compounds to short-chained α-dicarbonyl compounds for pyrazine formation rather than their dehydration and cyclization to generate furans. The oxidation of Met favored the nonoxidative carbohydrate degradation leading to MGO formation and the aldolization of dihydropyrazines, which synergistically enriched the varieties of pyrazines, especially for the promoted formation of long-chain substituted pyrazines.

Control Formation of Furans and Pyrazines Resulting from Dual Glycation Sites in Nα,Nε-Di(1-deoxy-d-xylulos-1-yl)lysine via Elevating Thermal Degradation Temperatures

Lysine (Lys) glycated by xylose (Xyl) at α-NH2 [Nα(1-deoxy-D-xylulos-1-yl)lysine (Nα-Xyl-Lys ARP)] or ε-NH2 [Nε-(1-deoxy-D-xylulos-1-yl)lysine (Nε-Xyl-Lys ARP)] significantly impacted the thermal degradation pathways of Amadori rearrangement products (ARPs). Nα-Xyl-Lys ARP was found to undergo retro-aldolization on the sugar fragment more readily to form glyoxal/methylglyoxal than Nε-Xyl-Lys ARP. Furans and pyrazines formation during the degradation of the diglycated lysine [Nα,Nε-di(1-deoxy-d-xylulos-1-yl)lysine (Nα,Nε-di-Xyl-Lys ARP)] was delayed at 120 °C relative to Nε-Xyl-Lys ARP. This was attributed to the complex degradation of Nα,Nε-di-Xyl-Lys ARP, which slowed the substantial formation of deoxypentosones and the effective release of Lys. At 140 °C, the dual glycated Nα,Nε-di-Xyl-Lys ARP was more conducive to promoting the redistribution of electrons and facilitating molecular rearrangement. This accelerated the efficient decomposition of dual glycated groups in Nα,Nε-di-Xyl-Lys ARP and enabled glyoxal to actively participate in Strecker degradation. Thus, the production of furans and pyrazines was substantially increased, and the variety of pyrazines was expanded from three types to eight types. An appropriate increase to pH 7.5 effectively avoided the overprotonation of hydroxyl and amino groups (pH 5.5), simultaneously enhancing furans and pyrazines yield while minimizing the formation of pyridines under alkaline conditions.

Accelerated Degradation of DiXyl-α,ε-Lys-ARP via Interaction between Extra-Added Xylose and Monosubstituted Lys-ARPs during Maillard Reaction

Lysine (Lys) is capable of forming a di-substituted Amadori rearrangement product (ARP) with xylose (Xyl), designated as diXyl-α,ε-Lys-ARP. DiXyl-α,ε-Lys-ARP degradation was characterized by two steps: Initially, Xyl-α- and Xyl-ε-Lys-ARP were formed through elimination or hydrolysis at specific Nα/Nε positions of the corresponding enol and imine intermediates, which were then further degraded to dicarbonyl compounds and regenerated Lys. Xyl-α- or Xyl-ε-Lys-ARP had a reactive free amino group (ε-NH2 or α-NH2), both of which were still highly reactive and able to undergo further reactions with Xyl. Therefore, the diXyl-α,ε-Lys-ARP/Xyl model system was established to explore the impact of extra-added Xyl on diXyl-α,ε-Lys-ARP degradation behavior. Extra-added Xyl remarkably affected the degradation pathway of diXyl-α,ε-Lys-ARP by capturing the Xyl-α- and Xyl-ε-Lys-ARP to regenerate diXyl-α,ε-Lys-ARP. This interaction between Xyl and mono-substituted Lys-ARPs promoted the shift of chemical equilibrium toward the degradation of diXyl-α,ε-Lys-ARP, thereby accelerating its degradation rate. This degradation was markedly facilitated by the elevated temperature and pH values. Interestingly, the yield of Xyl-α- and Xyl-ε-Lys-ARP was particularly dependent on the pH during diXyl-α,ε-Lys-ARP degradation. Xyl-ε-Lys-ARP was the dominant product at pH 5.5-7.5 while Xyl-α-Lys-ARP possessed a relatively higher content under weak alkaline conditions, which was related to the reactivities of the Nα/Nε positions under various reaction conditions.

Exogenous Alanine Promoting the Reaction between Amadori Compound and Deoxyxylosone and Inhibiting the Formation of 2-Furfural during Thermal Treatment

The involvement of exogenous alanine was observed to inhibit the generation of 2-furfural during the thermal degradation of the Amadori rearrangement product (ARP). To clarify the reason for the reduced yield of 2-furfural triggered by exogenous alanine, the evolution of the precursors of 2-furfural formed in the ARP model and ARP-alanine model was investigated, and a model including ARP and 15N-labeled alanine was used to differentiate the role of endogenous and exogenous alanine in the degradation of ARP. It was found that the condensation between ARP and 3-deoxyxylosone could occur during thermal treatment. Nevertheless, the interaction of ARP with 3-deoxyxylosone exhibited an accelerated pace in the presence of exogenous alanine. In this way, exogenous alanine blocked the recovery of endogenous alanine while simultaneously enhancing the consumption of ARP and 3-deoxyxylosone during the Maillard reaction. Hence, the yield of 2-furfural was diminished with the interference of exogenous alanine. Furthermore, the promotion of the reaction between ARP and deoxyxylosone induced by exogenous alanine blocked their retro-aldolization reaction to short-chain α-dicarbonyls (α-DCs) and consequently resulted in a lack of pyrazine formation during the ARP degradation. The present study provided a feasible method for the controlled formation of 2-furfural during the thermal treatment of ARP derived from alanine.

Maillard Reaction Process and Characteristic Volatile Compounds Formed During Secondary Thermal Degradation Monitored via the Change of Fluorescent Compounds in the Reaction of Xylose-Corn Protein Hydrolysate

Until now, no effective method has been found to monitor the Maillard reaction process for complex protein hydrolysates. Dynamic changes in the concentration of α-dicarbonyl compounds, fluorescence intensity, and browning degree were investigated during the Maillard reaction of corn protein hydrolysates. When the fluorescence intensity reached the peak, deoxyosones would continue to be increased by ARP's degradation. However, the reaction node with the highest fluorescence intensity coincided with the turning point of the browning reaction, and the subsequent browning rate remarkably increased. Therefore, the change in fluorescence intensity could be used to monitor the degradation of ARP and the formation of browning melanoidin at different stages of the Maillard reaction of complex systems, thus effectively indicating the process of the Maillard reaction. When Maillard reaction intermediates (MRIs) with maximum fluorescent compounds were heated, the most abundant pyrazines were subsequently achieved. However, furan compounds would be progressively increased during the thermal process of MRIs with continuously enhanced browning.

Efficient Formation of N-(1-Deoxy-d-ribulos-1-yl)-Glutathione via Limited Oxidation and Degradation of Glutathione during the Atmospheric-Vacuum Thermal Reaction

The efficient preparation of the ribose-glutathione (Rib-GSH) Amadori rearrangement product (RG-ARP) as a potent precursor of meaty flavor was studied through the atmospheric-vacuum thermal reaction. Liquid chromatography-mass spectrometry (LC-MS) analysis revealed that the oxidation and degradation of GSH occurred during the preparation of RG-ARP via the atmospheric thermal reaction, especially at a low molar ratio of Rib to GSH and high reaction temperature. The RG-ARP and the ARPs derived from the products of GSH oxidation and degradation with the participation of Rib were identified by MS/MS as N-(1-deoxy-d-ribulos-1-yl)-glutathione, N-(1-deoxy-d-ribulos-1-yl)-cysteinylglycine, and N-(1-deoxy-d-ribulos-1-yl)-glutathione disulfide. The selective formation of RG-ARP was disrupted due to the multiple consumption pathways of GSH and Rib. The removal of water and the reduction of oxygen content during vacuum dehydration exhibited an obvious inhibitory effect on the oxidation of cysteinyl and the cleavage of glutamyl, limiting the oxidation and degradation of GSH. Meanwhile, the rapid evaporation of water promoted the molecular collision between the reactants, which allowed the glycation reaction of GSH to be advanced and fragmentation of RG-ARP to be inhibited at a mild dehydration temperature. Accordingly, the atmospheric-vacuum thermal reaction was proposed to limit the generation of secondary byproducts and enhance the yield of RG-ARP, enabling the RG-ARP yield to reach 49.23% at 80 °C and a molar ratio of 2:1 (Rib/GSH) for 20 min.

Exogenous Threonine-Induced Conversion of Threonine-Xylose Amadori Compound to Heyns Compound for Efficiently Promoting the Formation of Pyrazines

The involvement of exogenous threonine during the degradation of l-threonine-d-xylose Amadori rearrangement product (Thr-ARP) was found to promote the formation of pyrazines. A model including Thr-ARP and ¹⁵N-labeled l-threonine was applied to reveal the role of free threonine in Thr-ARP conversion to pyrazines. Quantitative analyses of pyrazines in the model of Thr-ARP/¹⁵N-labeled threonine showed a precedence of the endogenous threonine (formed by the degradation of Thr-ARP) over the exogenous threonine in pyrazines formation, and the ratio of ¹⁵N to ¹⁴N content in pyrazines increased significantly over time. According to the observed occurrence of the Heyns rearrangement products (HRP) derived from ¹⁵N-threonine, as well as the sharp decrease of ¹⁵N-threonine content and a rapid increase of ¹⁴N endogenous threonine at the initial stage of heat treatment, it was proposed that aldimine condensation between exogenous threonine and Thr-ARP followed by the hydrolysis led to the endogenous threonine and the generation of HRP. Then, the HRP underwent dehydration followed by hydrolysis to form exogenous threonine and deoxyxyosones, and the dehydration and hydrolysis of deoxyxyosones to form organic acids was inhibited, but the retro-aldolization of deoxyxyosones was promoted, facilitating the generation of reactive α-dicarbonyl compounds. In this way, exogenous threonine accelerated the release of endogenous threonine and α-dicarbonyl compounds and the pH decline was slowed down, which was favorable for the formation of pyrazines.