ZnCl2-mediated practical protocol for the synthesis of Amadori ketoses

Disclosing the Nitrogen Sources via Isotope Labeling Technique and the Formation Mechanism of Pyrazine and Alkylpyrazines during the Heat Treatment of N-(1-Deoxy-d-xylulos-1-yl)-alanine and Exogenous Alanine

The formation pathway and mechanism of various pyrazines were investigated during the thermal treatment of the alanine-xylose Amadori compound (Ala-ARP) and exogenous alanine (Ala). 15N-labeled Ala was used to coheated with Ala-ARP to clarify the nitrogen sources and the respective contributions of exogenous Ala and the regenerated Ala released from Ala-ARP to different pyrazine formation. It was found that exogenous Ala exhibited a priority in capturing glyoxal (GO) to form pyrazine during the thermal degradation of ARP. Compared to the Ala-methylglyoxal (MGO) model, a lower activation energy was required for the Ala-GO reaction, where the reaction dynamics of Ala-GO followed a zero-order model. In addition to forming pyrazine, the interaction between existing exogenous Ala and GO would accelerate the thermal degradation of Ala-ARP and retro-aldolization reaction of deoxyxylosones (DXs) to α-dicarbonyls. During this process, the release of regenerated Ala and MGO was promoted. Accordingly, as GO was expended by exogenous Ala during the initial stage of ARP-Ala degradation, the condensation between regenerated Ala and MGO became intensified, leading to the generation of methylpyrazine and 2,5-dimethylpyrazine. As a result, in the thermally treated mixture of Ala-ARP and exogenous Ala, 55% of the formed pyrazine originated from exogenous Ala, while 63% of the formed methylpyrazine and 57% of the formed 2,5-dimethylpyrazine were derived from regenerated Ala (120 °C, 30 min).

Flavor Improvement of Maillard Reaction Intermediates Derived from Enzymatic Hydrolysates of Oudemansiella raphanipes Mushroom

Maillard reaction intermediate (MRI) was prepared by the enzymatic hydrolysate (EH) of Oudemansiella raphanipes and fructose. The optimal preparation condition of MRIs was obtained when the Maillard reaction parameters were as follows: fructose addition of 5%, reaction time of 60 min, and temperature of 60 °C. E-Tongue results indicated that the umami and saltiness of MRIs were greater than those of Maillard reaction products (MRPs) and EH, and the taste-enhancing ability of MRIs was even more prominent than that of MRPs. E-Nose could obviously distinguish EH, MRIs, and MRPs, and there was an obvious difference between MRPs and MRIs regarding volatile aroma compounds. A total of 35 volatile flavor substances were identified among the three samples, including 6 alcohols, 13 aldehydes, 9 ketones, 2 esters, and 5 other compounds. Overall, MRIs could avoid the production of complete reaction products with an inferior flavor, and further enhance the umami taste.

Preparation of red jujube powder with high content of Amadori compounds and higher antioxidant activity by controlling the Maillard reaction

Amadori compounds (ACs) are stable compounds produced in the early stage of the Maillard reaction (MR) with health benefits such as immunomodulatory, antithrombosis, and tumor-preventive effects. Jujube is a medicinal and edible fruit in China. It is rich in free amino acids and reducing sugar, but traditionally, little attention was paid to the formation of ACs when jujube was processed, neither the influence of ACs on health effects. In this paper, we aimed to increase ACs through controlling the MR during different heating processes of jujube powder with adjusted water content and find the most effective AC that contributed to the antioxidant effects of jujube powder. The optimal dry-heating conditions to produce ACs were as follows: The water activity was 0.294, the heating temperature was 90°C, and the time was 120 min. After processing, the ACs content of jujube powder was 18.55 ± 0.19 mg/g dry weight (DW), which was more than 100 times of those in the unheated jujube powder (0.153 ± 0.003 mg/g DW). Besides, the antioxidant activity of jujube powder after dry-heating process was higher than that of unheated one. As the most abundant AC in the dry-heated jujube powder (12.90 ± 0.75 mg/g DW), N-(1-deoxy-d-fructose-1-yl) proline (Fru-Pro) showed the highest antioxidant activity (62% of equivalent l-ascorbic acid) among 12 ACs in ferric reducing antioxidant power assay. This result provided a method to produce jujube product with high content of ACs and confirmed the positive contribution of Fru-Pro to the antioxidant activity of the jujube powder.

Cysteine-Induced pH-Dependent Formation of Thiols and Sulfides or 2-Acetylthiazole and Pyrazines during Thermal Treatment of N-(1-Deoxy-d-xylulos-1-yl)-alanine

The influence of pH was studied on volatile flavor formation during thermal treatment of an Amadori rearrangement product (ARP) with or without the addition of cysteine (Cys). The formation of thiols and sulfides or 2-acetylthiazole and pyrazines induced by Cys during thermal degradation of ARP was pH-dependent. At low pH levels, the hydrolysis of Cys to hydrogen sulfide (H₂S) was promoted, giving rise to the increase of thiols and sulfides with an obvious meaty aroma. However, alkaline conditions were beneficial for enhancing the cyclization or transformation of imine to the enol structure, which strengthened the formation of 2-acetylthiazole and pyrazines with a roasted and nutty aroma. The imine was derived from the nucleophilic addition of Cys and methylglyoxal (MGO) and subsequent decarboxylation. At pH 8, Cys-induced variation of the flavor profile was weakened during thermal degradation of ARP. Accordingly, the combinational effect of pH and added Cys could be beneficial for achieving the desirable flavors during thermal processing of ARP.

Advanced Glycation End-Products and Their Effects on Gut Health

Dietary advanced glycation end-products (AGEs) are a heterogeneous group of compounds formed when reducing sugars are heated with proteins, amino acids, or lipids at high temperatures for a prolonged period. The presence and accumulation of AGEs in numerous cell types and tissues are known to be prevalent in the pathology of many diseases. Modern diets, which contain a high proportion of processed foods and therefore a high level of AGE, cause deleterious effects leading to a multitude of unregulated intracellular and extracellular signalling and inflammatory pathways. Currently, many studies focus on investigating the chemical and structural aspects of AGEs and how they affect the metabolism and the cardiovascular and renal systems. Studies have also shown that AGEs affect the digestive system. However, there is no complete picture of the implication of AGEs in this area. The gastrointestinal tract is not only the first and principal site for the digestion and absorption of dietary AGEs but also one of the most susceptible organs to AGEs, which may exert many local and systemic effects. In this review, we summarise the current evidence of the association between a high-AGE diet and poor health outcomes, with a special focus on the relationship between dietary AGEs and alterations in the gastrointestinal structure, modifications in enteric neurons, and microbiota reshaping.

Formation of Amadori compounds in LIGAO (concentrated pear juice) processing and the effects of Fru-Asp on cough relief and lung moisturization in mice

LIGAO (concentrated pear juice) has been used for more than 1000 years to treat respiratory complaints such as cough and expectoration in China, but the study of the mechanism of its antitussive effects and ability to moisten the lungs is limited. This study found that the content of Amadori compounds (ACs) and other nutrients changed during LIGAO processing. Furthermore, N-(1-deoxy-D-fructos-1-yl)-aspartic acid (Fru-Asp), the most abundant and characteristic AC in LIGAO, was prepared and studied. The antitussive test revealed that Fru-Asp could significantly reduce the frequency of cough and prolong the cough latent period in mice. A high dose of Fru-Asp (250 mg kg^-1) in mice provided better therapeutic activities than that of dextromethorphan hydrobromide tablets (30 mg kg^-1). In the Fru-Asp pretreated group, Fru-Asp significantly alleviated inflammation in LPS-induced acute lung injury mice. Fru-Asp can significantly decrease the levels of TNF-α and IL-β in mice by 11%. Additionally, Fru-Asp exhibited angiotensin-converting enzyme (ACE) inhibitor activity (IC₅₀ = 0.242 mM). The contribution and health benefits of Fru-Asp on cough relief were first reported in this study, which also substantiated it as a functional component of LIGAO. The results provided the basis for future research on the health effects of ACs and a method to improve the added value of LIGAO and other pear products.

Amadori Reaction Products of Theanine and Glucose: Formation, Structure, and Analysis in Tea

Amadori rearrangement products (ARPs) derived from the Maillard reaction between theanine and glucose (ARP 1), as well as pyroglutamic acid and glucose (ARP 2), were identified by liquid chromatograph tandem mass spectroscopy methods. The effects of initial reactant ratio, temperature, pH, and heating time on ARP generation were analyzed. The formation of both ARPs was most favored under 100 °C, while an alkaline environment slightly promoted the generation of ARP 1 and acidic conditions contributed more to ARP 2 formation. The decomposition of ARP 1 was suggested to be the predominant formation mechanism of ARP 2. Preparation, purification, and structure identification of ARP 1 were conducted, with its structure confirmed as 1-deoxy-1-l-theanino-d-fructose. The contents of ARP 1 in green, black, dark, white, yellow, and Oolong teas were quantitatively determined, of which black teas contained the highest levels of ARP 1, possibly due to the high glucose content and processing techniques.

Inhibition of Maillard reaction during alkaline thermal hydrolysis of sludge

The Maillard reaction (MR) occurs during the alkaline thermal hydrolysis (ATH) of sludge, which affects the quantity and quality of recovered protein. In this paper, four different sulfites were added to investigate their inhibitory effects on melanoidin production. The results showed that sulfites inhibited melanoidin production during ATH of sludge and the inhibitory rate increased with their concentration. At a concentration of 5.71 g/L, the inhibitory rates of NaHSO₃ on melanoidin were 63.27%. Furthermore, the 3D-EEM (Three-Dimension Excitation-Emission-Matrix) fluorescence spectroscopy and protein testing data showed that the inhibition of melanoidin production was accompanied by an increased protein concentration, and protein increased with increasing sulfites concentration. A 2.5-fold increase in protein concentration with Na₂S₂O₄ significantly enhanced the quantity of protein recovered. Therefore, the addition of sulfite during ATH of sludge reduces the amount of non-biodegradable melanoidin, which in turn benefits protein recovery.

Angiotensin-Converting Enzyme (ACE) Inhibitory Activity and Mechanism Analysis of N-(1-Deoxy-d-fructos-1-yl)-histidine (Fru-His), a Food-Derived Amadori Compound

N-(1-Deoxy-d-fructos-1-yl)-histidine (Fru-His), one of the Amadori compounds, widely presents in processed foods, and its potential functional activities have attracted extensive attention in recent years. In this work, the angiotensin-converting enzyme (ACE) inhibitory activity and mechanism of Fru-His were investigated. The IC₅₀ value of Fru-His was 0.150 ± 0.019 mM, and there was no obvious degradation of Fru-His after digestion simulation, showing that Fru-His has good ACE inhibition and digestive stability. Fru-His was a competitive inhibitor according to the enzyme inhibition kinetic analysis. The interaction between ACE and Fru-His occurred spontaneously mainly through hydrogen bonding, and the process was accompanied by fluorescence quenching and the alteration of the secondary structure of ACE. The molecular docking data supported the above results. Fru-His was attached to ACE's S1 active pocket through hydrogen bonds and interacted with zinc ions in active sites. The present study demonstrates that food-derived Fru-His has the potential to relieve hypertension.

Vacuum Dehydration: An Excellent Method to Promote the Formation of Amadori Compounds (ACs, N-(1-Deoxy-d-fructos-1-yl)-amino Acid) in Aqueous Models and Tomato Sauce

Amadori compounds (ACs; N-(1-deoxy-d-fructos-1-yl)-amino acid) are superior flavor precursors and potential functional ingredients in food processing. In this study, vacuum dehydration as an excellent and universal method for the formation of ACs in both simulation systems and food processing was revealed. In total, 12 amino acids referring to all six categories were selected to conduct simulated reactions with glucose in aqueous models. At 90 °C, yields of 11 ACs were significantly increased by vacuum dehydration, reaching 4-198 times compared to a heat sealing reaction in aqueous systems, and formation of 5-hydroxymethyl-2-furaldehyde (5-HMF) and browning were slower than that by a dry powder reaction. In particular, the yields of Fru-Arg, Fru-His, and Fru-Glu reached 87.03, 90.73, and 89.88 mol %, respectively. The order of promotion effect was acid ACs > basic ACs > unique ACs > polar neutral ACs > aliphatic ACs > aromatic ACs. The excellent effect was mainly attributed to the control of water activity (Aw) and pH, which enabled the models to reach the optimal reaction state quickly by adjusting the vacuum degree at mild temperatures. The method was also applied to AC enrichment in tomato sauce processing; the AC content could rise to 30.72 mg/g, which was more than 17 times than those in samples without vacuum dehydration and two commercial tomato sauces.

Direct glucosone-based synthesis and HILIC-ESI-MS/MS characterization of N-terminal fructosylated valine and valylhistidine for validation of enzymatic HbA1c assays in the diagnosis of diabetes mellitus

Naturally occurring fructosamines are of high clinical significance due to their potential use in diabetes mellitus monitoring (quantification of fructosylated hemoglobin, HbA1c) or for the investigation of their reactivity in consecutive reactions and harmfulness towards the organism. Here we report the specific synthesis of the fructosylated dipeptide L-valyl-L-histidine (Fru-Val-His) and fructosylated L-valine (Fru-Val). Both are basic tools for the development and validation of enzymatic HbA1c assays. The two fructosamine derivatives were synthesized via a protected glucosone intermediate which was coupled to the primary amine of Val or Val-His, performing a reductive amination reaction. Overall yields starting from fructose were 36% and 34% for Fru-Val and Fru-Val-His, respectively. Both compounds were achieved in purities > 90%. A HILIC-ESI-MS/MS method was developed for routine analysis of the synthesized fructosamines, including starting materials and intermediates. The presented method provides a well-defined and efficient synthesis protocol with purification steps and characterization of the desired products. The functionality of the fructosylated dipeptide has been thoroughly tested in an enzymatic HbA1c assay, showing its concentration-dependent oxidative degradation by fructosyl-peptide oxidases (FPOX). Graphical abstract.

Interaction of Added l-Cysteine with 2-Threityl-thiazolidine-4-carboxylic Acid Derived from the Xylose-Cysteine System Affecting Its Maillard Browning

The Maillard reaction under a stepwise increase of temperature using l-cysteine as the indicator was performed to determine the formation conditions for the preparation of 2-threityl-thiazolidine-4-carboxylic acid (TTCA) which was the main Maillard reaction intermediate (MRI) derived from the xylose (Xyl)-cysteine (Cys) model system in aqueous medium. To clarify the indicating mechanism of Cys for the TTCA formation, the thermal model systems of TTCA-Cys and TTCA solutions were investigated. The browning of the final Maillard reaction products (MRPs) and concentration of downstream degradation products of MRIs indicated that the added Cys could react with TTCA to inhibit the formation of visible color via preventing the generation of dicarbonyl compounds derived from MRIs. Through HPLC analysis, it was demonstrated that added Cys affected the normal reaction pathway from TTCA to ARP and other downstream products by restoring TTCA to sugar and amino acid under heat treatment.

Preparation of N-(1-Deoxy-Α-D-Xylulos-1-Yl)-Glutamic Acid via Aqueous Maillard Reaction Coupled with Vacuum Dehydration and Its Flavor Formation Through Thermal Treatment of Baking Process

Amadori rearrangement product (ARP) derived from glutamic acid (Glu) and xylose (Xyl) was prepared by aqueous Maillard reaction. Subsequently, ion exchange chromatography, MS, and NMR were used for purification and identification, confirming that the molecular formula of ARP was C₁₀ H₁₇ NO₈ , namely N-(1-deoxy-α-D-xylulos-1-yl)-glutamic acid, with a molecular mass of 279 Da. To improve the aqueous yield of ARP, a thermal reaction coupled with vacuum dehydration was used and the yield of ARP was increased from 2.07% to 75.11%. Furthermore, flavor formation capacity of ARP by a thermal treatment simulated to a baking process was compared with Maillard reaction products, Maillard-dehydration reaction products, and Glu-Xyl mixture. The results indicated that a larger amount of volatile flavor compounds and a biscuit-like, burnt aroma was generated rapidly from the mixture of ARP and unreacted Glu-Xyl, which could be a potential flavor enhancer for baked foods. PRACTICAL APPLICATION: Maillard reaction performed in aqueous medium through thermal reaction combined with vacuum dehydration is a novel and practical technology that could be widely used to produce Maillard reaction intermediates (MRIs), such as Amadori or Heyns rearrangement products, which are regarded as significant nonvolatile aroma precursors and have stable physical and chemical properties compared with Maillard reaction products (MRPs). MRI derived from glutamic acid and xylose is a potential substitute of MRPs for flavorings preparation and shows a great capacity to generate fresh flavors in a short time at high temperature, which meets the requirements of baking foods. Therefore, the new developed method could be a promising tool for MRI preparation and application in food and flavoring industries.

Antioxidant Activity In Vitro of N-(1-deoxy-α-d-xylulos-1-yl)-Phenylalanine: Comparison Among Maillard Reaction Intermediate, End-Products and Xylose-Phenylalanine

The Maillard reaction end-products (MRPs) and intermediate (MRI, N-(1-deoxy-α-D-xylulos-1-yl)-phenylalanine) derived from xylose (Xyl) and phenylalanine (Phe) model system were synthesized in an aqueous medium, and their antioxidant activity was evaluated. Both the MRPs and N-(1-deoxy-α-d-xylulos-1-yl)-phenylalanine exhibited the Fe²⁺ chelating activity at their addition concentration of 2 to 12 mg/mL. While their reducing power, free radical scavenging activity, and inhibition effect on lipid peroxidation were observed at a lower addition concentration of only 0.2 to 1.2 mg/mL. The free radical scavenging activity was improved with the increase in their concentration, and a time-dependent scavenging effect of the MRI was also shown in the results. However, a weak chelating activity of Xyl-Phe on Fe²⁺ in a concentration-dependent manner was observed, but no significant free radical scavenging activity or reducing power of Xyl-Phe was found. Based on these results, possible free radical scavenging pathway of the MRI was proposed. As the MRI showed lower antioxidant activity than MRPs, the application of the MRI in combination with MRPs as an antioxidant was proposed in cooked or semi-processed foods to avoid the deterioration induced by oxidation. PRACTICAL APPLICATION: The Maillard reaction intermediates (MRIs) showed antioxidant activity and could be applied in foods as both antioxidants and flavor enhancer. The MRIs will probably extend the shelf-life of lipid rich foods and could be the substitute of artificial antioxidants.

Evaluation of the extent of initial Maillard reaction during cooking some vegetables by direct measurement of the Amadori compounds

BACKGROUND

During vegetable cooking, one of the most notable and common chemical reactions is the Maillard reaction, which occurs as a result of thermal treatment and dehydration. Amadori compound determination provides a very sensitive indicator for early detection of quality changes caused by the Maillard reaction, as well as to retrospectively assess the heat treatment or storage conditions to which the product has been subjected. In this paper, a hydrophilic interaction liquid chromatographic-electrospray ionization-tandem mass spectrometric method was developed for the analysis of eight Amadori compounds, and the initial steps of the Maillard reaction during cooking (steaming, frying and baking) bell pepper, red pepper, yellow onion, purple onion, tomato and carrot were also assessed by quantitative determination of these Amadori compounds.

RESULTS

These culinary treatments reduced moisture and increased the total content of Amadori compounds, which was not dependent on the type of vegetable or cooking method. Moreover, the effect of steaming on Amadori compound content and water loss was less than that by baking and frying vegetables. Further studies showed that the combination of high temperature and short time may lead to lower formation of Amadori compounds when baking vegetables.

CONCLUSION

Culinary methods differently affected the extent of initial Maillard reaction when vegetables were made into home-cooked products. © 2017 Society of Chemical Industry.

Sodium sulfite pH-buffering effect for improved xylose-phenylalanine conversion to N-(1-deoxy-d-xylulos-1-yl)-phenylalanine during an aqueous Maillard reaction

The yield of the Maillard reaction intermediate (MRI), prepared in aqueous medium, is usually unsatisfactory. However, the addition of sodium sulfite could improve the conversion of xylose-phenylalanine (Xyl-Phe) to the MRI (N-(1-deoxy-d-xylulos-1-yl)-phenylalanine) in aqueous medium. Sodium sulfite (Na₂SO₃) showed a significant pH-buffering effect during the Maillard reaction, which accounted for its facilitation of the N-(1-deoxy-d-xylulos-1-yl)-phenylalanine yield. The results revealed that the pH could be maintained at a relatively high level (above 7.0) for an optimized pH-buffering effect when Na₂SO₃ (4.0%) was added before the reaction of Xyl-Phe. Thus, the conversion of Xyl-Phe to N-(1-deoxy-d-xylulos-1-yl)-phenylalanine increased from 47.23% to 74.86%. Furthermore, the addition moment of Na₂SO₃ and corresponding solution pH were crucial factors in regulating the pH-buffering effect of Na₂SO₃ on N-(1-deoxy-d-xylulos-1-yl)-phenylalanine yield. Based on the pH-buffering effect of Na₂SO₃ and maintaining the optimal pH 7.4 relatively stable, the conversion of Xyl-Phe to N-(1-deoxy-d-xylulos-1-yl)-phenylalanine was successfully improved.

Controlled formation of flavor compounds by preparation and application of Maillard reaction intermediate (MRI) derived from xylose and phenylalanine

An effective method for preparing Maillard reaction intermediate (MRI) derived from xylose (Xyl) and phenylalanine (Phe) in aqueous medium was proposed in this study.