Effect of Methionine on the Thermal Degradation of N-(1-Deoxy-d-fructos-1-yl)-methionine Affecting Browning Formation

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.

Light-Colored Maillard Peptides: Formation from Reduced Fluorescent Precursors of Browning and Enhancement of Saltiness Perception

The browning formation and taste enhancement of peptides derived from soybean, peanut, and corn were studied in the light-colored Maillard reaction compared with the deep-colored reaction. The fluorescent compounds, as the browning precursors, were accumulated during the early Maillard reaction of peptides and subsequently degraded into dark substances, which resulted in a higher browning degree of deep-colored Maillard peptides (MPs), especially for the MPs derived from corn peptide. However, the addition of l-cysteine in light-colored Maillard reaction reduced the formation of deoxyosones and short-chain reactive α-dicarbonyls, thereby weakening the generation of fluorescent compounds and inhibited the browning of MPs. Synchronously, the peptides were thermally degraded into small peptides and amino acids, which were consumed less during light-colored thermal reaction due to its shorter reaction time at high temperature compared with deep-colored ones, thus contributing to a stronger saltiness perception of light-colored MPs than deep-colored MPs. Besides, the Maillard reaction products derived from soybean and peanut peptides possessed an obvious "kokumi" taste, making them suitable for enhancing the soup flavors.

Factors influencing on the formation of dimethyl disulfide and dimethyl trisulfide in model systems

The generations of dimethyl disulfide (DMDS) and dimethyl trisulfide (DMTS) in a binary or ternary model system including lipids, free amino acids and Maillard reaction products (MRPs) were studied. Various factors affecting the formation of DMDS and DMTS indicated that cysteine (Cys) and Cys MRPs could effectively decrease not only the concentrations of methionine (Met), DMDS and DMTS, but also the pH level. Rapid drops in pH limited the formation of DMDS and DMTS during Met thermal degradation. Quantitative analyses of DMDS and DMTS at acidic aqueous solutions revealed that the mixtures of MRPs derived from Cys and xylose (Xyl) had the best inhibition effect on the formation of DMDS and DMTS. The low level of DMDS and DMTS and the increasing level of furfuryl methyl sulfide and 2-thiophenecarboxaldehyde during storage indicated that MRPs derived from Cys and Xyl could effectively not only decrease the concertation of DMDS and DMTS, but also promote the development of thiophene and sulfur substituted furan. Thus, this study implied that MRPs derived from Cys/Xyl could be applied as effective substances to control the formation of DMDS and DMTS and improve the production of volatile compounds with meat-like aroma.

Colorants and Antioxidants Deriving from Methylglyoxal and Heterocyclic Maillard Reaction Intermediates

The Maillard reaction is well known for producing antioxidant compounds alongside colored substances. Low-molecular-weight antioxidant intermediates such as maltol (MAL) or norfuraneol (NF) are well described, but it is still unclear which of these Maillard intermediates are the precursors of antioxidant and colored melanoidins-the so-called late stage Maillard reaction products. This study aimed to provide novel insights into the correlation between browning potential and antioxidant properties of reaction products formed during the heat treatment of prominent Maillard reaction intermediates. It was achieved by the incubation of binary reaction systems composed of methylglyoxal (MGO) or NF in combination with furfural (FF), MAL, and pyrrole-2-carbaldehyde (PA) at pH 5 and 130 °C for up to 120 min. Overall, it could be shown that the formation of colored products in the binary NF reaction systems was more efficient compared to those of MGO. This was reflected in an increased browning intensity of up to 400% and a lower conversion rate of NF compared to MGO. The colorants formed by NF and FF or PA (~0.34 kDa and 10-100 kDa) were also found to exhibit higher molecular weights compared to the analogue products formed in the MGO incubations (<0.34 kDa and 10-100 kDa). The incorporation of NF into these heterogenous products with FF and PA resulted in the preservation of the initial antioxidant properties of NF (p < 0.05), whereas no antioxidant products were formed after the incubation of MGO.

Effect of Maillard Reaction Products Derived from Cysteine on the Formation of Dimethyl Disulfide and Dimethyl Trisulfide during Storage

The effect of Maillard reaction products derived from cysteine on dimethyl disulfide (DMDS) and dimethyl trisulfide (DMTS) was evaluated in the ternary mixture (methionine, cysteine, and xylose) and binary mixture (methionine and 2-threityl-thiazolidine-4-carboxylic acid) during 56 days storage. Changes in concentrations revealed that the methionine/cysteine/xylose model showed lower concentrations of DMDS and DMTS than those of the binary mixture. Antioxidant ability and labeled isotopomer proportion information indicated that low levels of DMDS and DMTS in the ternary mixture were attributed to not only 2-furfurylthiol and 2-methyl-3-furanthiol (MFT) reacting with methanethiol but also the ternary mixture having stronger antioxidant activity. Correlation analysis demonstrated that MFT reacting with methanethiol and strong antioxidant ability are considered major factors controlling the formation of DMTS and DMDS, respectively. Research on the reaction mechanism of the rate-limiting step would provide the basis for preventing the development of DMDS and DMTS during storage.

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

Methionine (Met) oxidation was observed during thermal degradation of methionine/glucose-derived Amadori rearrangement product (MG-ARP). The effects of oxidized methionine products, methionine sulfoxide (MetSO) and methionine sulfone (MetSO₂), on pyrazine yields of the MG-ARP model were investigated. The pyrazine contents in the MG-ARP/Met and MG-ARP/MetSO models were found lower compared to those in the MG-ARP/MetSO₂ model, and the inefficiency of pyrazine formation in the MG-ARP/Met model was proposed due to the fact that Met oxidation competitively inhibited the oxidation of dihydropyrazines for pyrazine formation in spite of relatively high methylglyoxal (MGO) content. The models of MGO mixed with Met, MetSO, or MetSO₂ were established for further investigation of the mechanism for the involvement of Met oxidation in pyrazine formation. It was observed that the aldolization or carbonyl-amine reaction of MetSO with MGO was another important reason for the inhibition of pyrazine formation, except for the competitive inhibition of oxidative formation of MetSO on dihydropyrazine oxidation, and the adduct of MGO-MetSO was identified by MS/MS. These results also accounted for the phenomenon of low pyrazine yields but high yields of long-chain substituted pyrazines, which were converted from dihydropyrazines with the aldehyde involvement.

Promoted Formation of Pyrazines and Sulfur-Containing Volatile Compounds through Interaction of Extra-Added Glutathione or Its Constituent Amino Acids and Secondary Products of Thermally Degraded N-(1-Deoxy-d-ribulos-1-yl)-Glutathione

An Amadori rearrangement product (ARP) derived from ribose (Rib) and glutathione (GSH) was prepared and identified as N-(1-deoxy-d-ribulos-1-yl)-glutathione by ultraperformance liquid chromatography-tandem mass spectrometry and NMR. Thermal treatment of the ARP aqueous solution was conducted, and a relatively high temperature was found to accelerate the degradation of the ARP. The concentration of furans formed at 120 °C was more than 6.39 times that at 100 °C, and especially, the high temperature favored the formation of furfural and 4-hydroxy-5-methyl-3(2H)-furanone through deoxyosone dehydration. The promoting role of extra-added GSH or its constituent amino acids was investigated in the volatile formation during thermal processing of the ARP. Both, the added GSH and its constituent amino acids, could timely capture glyoxal (GO) and methylglyoxal (MGO) to facilitate Strecker degradation, which improved pyrazine formation. Compared with glycine and glutamic acid, cysteine was the most effective extra-added amino acid to react with GO and MGO to produce pyrazine and methylpyrazine. More importantly, the cysteine degraded from extra-added GSH effectively accelerated the generation of sulfur-containing volatile compounds through the reaction of cysteine degradation products with furans and shorter-chain α-dicarbonyl compounds.

Comparison of pyrazines formation in methionine/glucose and corresponding Amadori rearrangement product model

The generation of pyrazines in a binary methionine/glucose (Met/Glc) mixture and corresponding methionine/glucose-derived Amadori rearrangement product (MG-ARP) was studied. Quantitative analyses of pyrazines and methional revealed that MG-ARP generated more methional compared to Met/Glc, whereas lower content and fewer species of pyrazines were observed in the MG-ARP model. Comparing the availability of α-dicarbonyl compounds generated from the Met/Glc model, methylglyoxal (MGO) was a considerably effective α-dicarbonyl compound for the formation of pyrazines during MG-ARP degradation, but glyoxal (GO) produced from MG-ARP did not effectively participate in the corresponding formation of pyrazines due to the asynchrony on the formation of GO and recovered Met. Diacetyl (DA) content was not high enough to form corresponding pyrazines in the MG-ARP model. The insufficient interaction of precursors and rapid drops in pH limited the formation of pyrazines during MG-ARP degradation. Increasing reaction temperature could reduce the negative inhibitory effect by promoting the content of precursors.

Temperature-Dependent Catalysis of Glycylglycine on Its Amadori Compound Degradation to Deoxyosone

The Amadori rearrangement product derived from xylose-glycylglycine (XGG-ARP) is reactive to be attacked by another glycylglycine to generate a xylose-glycylglycine cross-linking product (XGG-CP) as a secondary product of the ARP. In this research, the role of additional glycylglycine in the XGG-ARP degradation was studied, and the dependence of glycylglycine on temperature was further clarified. The yields of XGG-CP and its degradation products were significantly affected by the molar ratio of glycylglycine to XGG-ARP. At the similar total concentration of reactant XGG-ARP and glycylglycine, the yields of XGG-CP, 3-deoxyxylosone, and furfural were dramatically decreased as the molar ratio of glycylglycine to XGG-ARP was increased. However, when the reaction temperature was increased to 120 °C, the increased additional glycylglycine percentage showed an obvious catalytic effect on the XGG-ARP degradation to deoxyosone and thus improved the furfural yield as well. The results revealed that an increased glycylglycine dosage level enhanced both the conversion of XGG-ARP to XGG-CP and the conversion of XGG-CP to 3-deoxyosone. The high-temperature-induced unequal acceleration for XGG-CP formation and degradation at a high glycylglycine dosage further led to a catalytic effect on the ARP degradation to deoxyosone. The concentration of 3-deoxyosone was increased by 37.5% when the molar ratio of glycylglycine to XGG-ARP increased from 1:2 to 2:1 at a temperature of 120 °C.

Structural diversity and concentration dependence of pyrazine formation: Exogenous amino substrates and reaction parameters during thermal processing of l-alanyl-l-glutamine Amadori compound

The Amadori compound of glucose and l-alanyl-l-glutamine (Ala-Gln-ARP) was prepared and characterized by UPLC-MS/MS and NMR. There were no pyrazines produced by heated Ala-Gln-ARP alone due to the asynchronicity of regenerated l-alanyl-l-glutamine and α-dicarbonyl compounds. High temperature (130 °C) and long reaction time could facilitate the 2,5-dimethylpyrazine formation at a small concentration (33.4 ± 3.47 μg/L). The exogenous amino substrates would lower the formation temperature of pyrazines and make it to be generated effectively. Extra supplied l-alanyl-l-glutamine could generate 2,5-dimethylpyrazine at 110 °C, while higher temperature of 140 °C could strengthen the formation of 2,5-dimethylpyrazine (793 ± 119 μg/L) and stimulate the generation of other pyrazines, including methylpyrazine and 2,6-dimethylpyrazine. The exogenous alanine, glutamic acid, and glutamine was also beneficial to enhance the pyrazines formation, especially the glutamic acid. Furthermore, alkaline pH of thermal reaction made pyrazines increase significantly than in neutral medium and further enriched their species such as unsubstituted pyrazine and trimethylpyrazine.

Degradation of 2-Threityl-Thiazolidine-4-Carboxylic Acid and Corresponding Browning Accelerated by Trapping Reaction between Extra-Added Xylose and Released Cysteine during Maillard Reaction

2-Threityl-thiazolidine-4-carboxylic acid (TTCA), a nonvolatile precursor of flavor and color, is considered to be more stable than its isomeric Amadori compound (ARP). The degradation behavior of TTCA favors higher temperatures and pH. In order to adjust and control the thermal degradation of TTCA to improve its food processing adaptability, a TTCA-Xyl thermal reaction model was constructed to explore the effect of extra-added Xyl on the thermal degradation behavior of TTCA. The results confirmed that the extra-added Xyl was involved in the degradation pathway of TTCA and accelerated its depletion, thus promoting the formation of characteristic downstream products of TTCA including some α-dicarbonyl compounds, and consequently accelerating the browning formation. The isotope-labeling technique was further applied to confirm that the added Xyl could trap the Cys released from the decomposition of ARP and formed additional TTCA, which could promote the movement of chemical equilibrium and gradually accelerate the degradation rate of TTCA as well as melanoidins formation. The higher pH value could even promote this phenomenon.

Key Aspects of Amadori Rearrangement Products as Future Food Additives

Flavor is one of the most important factors in attracting consumers and maximizing food quality, and the Maillard reaction (MR) is highly-involved in flavor formation. However, Maillard reaction products have a big drawback in their relatively low stability in thermal treatment and storage. Amadori rearrangement products (ARPs), MR intermediates, can alternatively act as potential flavor additives for their better stability and fresh flavor formation ability. This review aims to elucidate key aspects of ARPs’ future application as flavorings. The development of current analytical technologies enables the precise characterization of ARPs, while advanced preparation methods such as synthesis, separation and drying processes can increase the yield of ARPs to up to 95%. The stability of ARPs is influenced by their chemical nature, pH value, temperature, water activity and food matrix. ARPs are associated with umami and kokumi taste enhancing effects, and the flavor formation is related to amino acids/peptides of the ARPs. Peptide-ARPs can generate peptide-specific flavors, such as: 1,6-dimethy-2(1H)-pyrazinone, 1,5-dimethy-2(1H)-pyrazinone, and 1,5,6-trimethy-2(1H)-pyrazinone. However, further research on systematic stability and toxicology are needed.