Preparation and thermal properties of Heyns compounds based on two different amino acids

Yu Li; Longxin Wang; Qianrui Zhao; Yueqi Xu; Yihan Hu; Bingxiang Wang; Jingyi Hu; Pengze Wang; Miao Lai

doi:10.25259/AJC_164_2025

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Original Article

2025

:18;

1642025

doi:

10.25259/AJC_164_2025

Preparation and thermal properties of Heyns compounds based on two different amino acids

Yu Li^a, Longxin Wang^a,, Qianrui Zhao^b, Yueqi Xu^c, Yihan Hu^a, Bingxiang Wang^a,, Jingyi Hu^a, Pengze Wang^d,*, Miao Lai^a,*

aFlavors and Fragrance Engineering & Technology Research Center of Henan Province, College of Tobacco Science, Henan Agricultural University, Zhengzhou 450046, China

bHubei Tobacco Company Yichang Company, Yichang, 443000, China

cPingdingshan Branch of Henan Tobacco Company, Pingdingshan, 467000, China

dTianchang International Tobacco Co., Ltd., Xuchang, 461000, China

* Corresponding authors: E-mail addresses: 15249690519@163.com (P. Wang), laimiao@henau.edu.cn (M. Lai)

Received: 2025-02-22, Accepted: 2025-05-21, Epub ahead of print: 2025-07-14, Published: 2025-07-23

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Abstract

This study synthesized two Heyns compounds: 2-L-tyrosine-2-deoxy-D-glucose (Glu-Tyr) and 2-L-aspartic acid-2-deoxy-D-glucose (Glu-Asp), through reactions of D-fructose with L-tyrosine or L-aspartic acid, respectively. Structural confirmation was achieved via Fourier transform infrared spectroscopy (FTIR), nuclear magnetic resonance (NMR), and high-resolution mass spectrometry (HRMS), with yields of 61.4% for Glu-Tyr and 78.2% for Glu-Asp.Thermogravimetric (TG) analysis revealed that Glu-Asp underwent thermal decomposition at a lower temperature than Glu-Tyr. Both compounds exhibited strongly linear thermal release kinetics under varying heating rates. pyrolysis gas chromatography/mass spectrometry (Py-GC/MS) analysis demonstrated distinct flavor profiles: Glu-Tyr primarily released floral phenolic esters, whereas Glu-Asp generated baking aroma substances at 350°C and Caramel aromatic substances at 900°C. Application assessments indicated that Glu-Asp showed potential for traditional cigarettes, while Glu-Tyr exhibited superior compatibility with heated tobacco products. Headspace solid-phase microextraction-GC/MS (HS-SPME-GC/MS) further revealed a time-dependent increase in both the diversity and concentration of volatile compounds during heating.These findings establish a theoretical framework for the targeted application of Heyns compounds in flavor engineering.

Keywords

Flavor substance differences

Heyns compound synthesis

Thermal property analysis

Tobacco flavor modulation

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1. Introduction

The Maillard reaction occurs between the carbonyl group of reducing sugars and the amino group of amino acids [1-5]. The intermediate products generated vary depending on the type of reducing sugar involved. Aldoses react with amino acids to form Amadori rearrangement products, while ketoses yield Heyns rearrangement product [6]. Mass spectrometry (MS/MS) methods can be employed to identify unknown types of intermediates [7]. Maillard reaction could produce substances that have an important impact on the color, flavor and quality of food, such as pyrazines with baking aroma, furans with fruity, floral, bean and grassy aroma, benzaldehyde with cherry-like aroma, and unpleasant alkylpyridines, etc [8-13]. Maillard reaction products possess antimicrobial properties. Lang synthesized Maillard reaction products from ε-polylysine and chitooligosaccharides, which can inhibit S. putrefaciens, thereby aiding in food preservation [14]. Studies have shown that the derivatization of proteins through the Maillard reaction can further enhance the functional properties of proteins and increase their nutritional value in the food industry [15]. This strategy also has potential applications in the field of animal nutrition [16]. In addition to their antibacterial and nutritional functions, the primary role of Maillard reaction products is flavor enhancement. Huang et al. utilized the Maillard reaction to prepare a meat flavor additive using hydrolyzed soybean meal and xylose as raw materials [17]. Qiu elucidated the flavor-enhancing effect of the Maillard reaction on food by analyzing the changes in the flavor of Lentinula edodes hydrolysates after the Maillard reaction [18]. Abalone viscera hydrolysate can be used as a seasoning, but it has a slightly bitter taste. Huang et al. demonstrated that the flavor of the hydrolysate was significantly improved after the Maillard reaction, using abalone viscera as the raw material [19]. Different reducing sugars also influence the flavor of Maillard reaction products [20].

The flavor of fully Maillard reaction flavorings is intense but highly volatile, resulting in a short duration of aroma retention during thermal processing and storage. The reaction intermediates, Amadori and Heyns compounds, exhibit relatively stable physicochemical properties at room temperature and maintain high reactivity under heating conditions, readily completing subsequent Maillard reactions to rapidly generate volatile flavor compounds such as furans, pyrroles, and pyrazines. Therefore, they can serve as substitutes for fully Maillard reaction flavorings [21]. Numerous reports have documented the synthesis and analysis of Heyns compounds. Kuan Li synthesized N-(2-deoxy-D-glucose-2-yl)-L-histidine using L-histidine and D-fructose as raw materials, providing detailed descriptions of the preparation, characterization, and thermal release of aroma [22]. M. Abul Haider Shipar demonstrated that the reaction of Α-Fru and glycine under alkaline conditions was more favorable for generating Heyns compounds compared to the reactions involving β-Fru, O-Fru, and glycine [23]. Philipp Bruhns discovered that D-fructose and γ-aminobutyric acid can produce 2-amino-2-deoxy-3-ketose, a substitute for Heyns compound, at 50°C with a moisture content of less than 50% [24]. Pusen Chen facilitated the conversion of the L-threonine-D-xylose Amadori rearrangement product into the Heyns by adding exogenous threonine at room temperature, employing isotope labeling to confirm that this transformation enhances the generation of pyrazine substances [25].

René Krause found that generating Heyns compounds in the Maillard reaction is challenging [26], the content of Heyns compounds in sugar cookies and Amadori compounds in glucose cookies was 33% and 63%, respectively. Given the role of Maillard reaction intermediates, Heyns compounds could be artificially added in food production and processing to modify and enhance flavor [6,27]. Thus, it is essential to explore the pyrolytic properties and flavor characteristics of various Heyns compounds. However, previous explorations had primarily focused on the formation mechanisms of flavor compounds in the complete Maillard reaction system, while fewer reports had addressed the formation mechanisms of the fresh flavor characteristics associated with the Maillard reaction intermediate, Heyns compounds. The synthesis of Heyns compounds with different amino acids (tyrosine and aspartic acid) and their thermal degradation kinetics and pyrolysis products were rarely reported in previous literature. Therefore, this paper utilized lysine and aspartic acid as raw materials to react with fructose to synthesize two Heyns compounds. Their structures were identified using spectroscopic methods, while thermogravimetric analysis, thermal release kinetic analysis, and thermal cracking product analysis, along with headspace solid-phase microextraction flavor analysis, were employed to investigate the differences in thermal properties between the two compounds. This research provides a foundation for understanding the flavoring mechanism and precise application of Maillard reaction intermediates.

2. Materials and Methods

2.1. Reagents and instruments

D-fructose (99%, purchased from Shanghai McLean Biochemical Technology Co., Ltd., China), L-tyrosine, L-aspartic acid, and L-aspartate potassium salt (99%, purchased from Hebei Bailingwei Super Fine Materials Co., Ltd., China), as well as anhydrous methanol, anhydrous ethanol, potassium hydroxide, and glacial acetic acid (AR grade, purchased from Tianjin Fuyu Fine Chemical Co., Ltd., China) were utilized in the study. All reactions were conducted in an air atmosphere.

The following instruments were utilized in the study: a Fourier transform infrared spectrometer (Thermo Fisher Scientific Nicolet iS20, USA), a nuclear magnetic resonance instrument (BRUKER AVANCE III 400 MHz, Germany), a high-resolution mass spectrometer (AB SCIEX Triple TOF 5600+, USA), a thermogravimetric analyzer (STA 449 F3, Netzsch, Germany), a thermal cracker (CDS 5250T, USA), a headspace instrument (SPME Arrow, CTC Analytics AG, Zwingen), and a gas chromatography mass spectrometer (Agilent 7890A/5977B, USA).

2.2. Synthesis of Glu-Tyr and Glu-Asp

2.2.1. Synthesis of Glu-Tyr

The synthesis method for Glu-Tyr was based on the literature method [28], with appropriate modifications made. The synthesis route is illustrated in Figure 1.

Adding 4.676 g of potassium hydroxide and 100 mL of anhydrous methanol to a three-necked flask, and stirred using a magnetic stirrer until the potassium hydroxide was fully dissolved. Next, incorporated 15.114 g of L-tyrosine and allowed the reaction to proceed at room temperature for 30 min. Subsequently, added 9 g of anhydrous D-fructose and 175 mL of anhydrous methanol, stirring magnetically, and refluxed the mixture for 2 h. After refluxing, cooled the solution to room temperature and concentrated it to 50 mL. Following this, filtered the solution to obtain the filtrate, and cooled it to -20°C. While stirring, added absolute ethanol dropwise, then filtered with suction to collect the precipitate. This recrystallization step was repeated twice using a methanol, followed by vacuum drying to yield white crystalline fructosyl tyrosine potassium salt.

Added 50 mL of anhydrous methanol and 3 g of fructosyl tyrosine potassium salt to the three-necked flask. After stirring magnetically for 30 min, introduced 1 mL of glacial acetic acid and allowed the mixture to reflux for 1 h. Subsequently, terminated the reaction and cooled the mixture to 0°C. Then, added 30 mL of absolute ethanol and performed a pumping operation. After filtration, collected the precipitate, dried it under vacuum, and recrystallized it in anhydrous methanol to obtain white solid Glu-Tyr with a yield of 61.4%.

2.2.2. Synthesis of Glu-Asp

The synthesis route of Glu-Asp is illustrated in Figure 2. Began by adding 14.1 g of potassium hydroxide and 150 mL of anhydrous methanol into a three-necked flask. Stirred the mixture magnetically until the potassium hydroxide was fully dissolved. Next, incorporated 43 g of L-aspartate potassium salt and allowed the reaction to proceed at room temperature for 30 min. Following this, introduced 450 mL of anhydrous methanol and 27 g of anhydrous D-fructose, and continued stirring magnetically while refluxing the reaction for 3 h. Once the reaction was complete, cooled the mixture to room temperature and concentrated the reaction solution to 150 mL. After filtration, collected the filtrate and cooled it to 0°C. While stirring, added absolute ethanol dropwise, then filtered with suction to collect the precipitate. This recrystallization step was repeated twice using a methanol, followed by vacuum drying to yield white crystalline fructosyl aspartate dipotassium salt.

Added 60 mL of anhydrous methanol and 6 g of fructosyl aspartic acid dipotassium salt to the three-necked flask. After stirring magnetically for 30 min, introduced 2 mL of glacial acetic acid and refluxed the mixture for 1.5 h. Upon completion of the reaction, cooled the mixture to 0°C and added 40 mL of anhydrous water. Following suction filtration of the resulting ethanol, collected the precipitate, dried it under vacuum, and recrystallized in anhydrous methanol to obtain a white solid of Glu-Asp. The yield achieved was 78.2%.

2.3. Structural identification

Glu-Tyr and Glu-Asp were identified using FTIR, ¹H NMR, ¹³C NMR, and HR-MS, respectively.

2.4. Thermogravimetry-derivative thermogravimetry (TG-DTG) analysis

Mixed 0.05 mol L-tyrosine with 0.05 mol fructose (referred to as GT-mix) and 0.05 mol L-aspartate potassium salt with 0.05 mol fructose (referred to as GA-mix), respectively, and placed each mixture in separate high-purity alumina crucibles. Set the linear heating rate to 20°C/min, with a temperature range of 30°C to 800°C, and conduct the test in a nitrogen atmosphere [29].

The heat release kinetics experiment was conducted using a non-isothermal method. Five portions of Glu-Tyr and Glu-Asp, each weighing 10 mg, were weighed and placed in ten high-purity alumina crucibles. The linear heating rates employed were 5°C/min, 10°C/min, 20°C/min, 40°C/min, and 80°C/min, respectively. By analyzing the TG-DTG curve, the comprehensive heat release index (CRI) of the thermal decomposition process can be obtained, which can characterize the total heat release intensity of the material and its stability features under heating conditions [30]. The calculation formula is as follows.

(1)

\begin{matrix} C R I = \frac{D T G_{m a x}}{T_{m a x} \times (T_{f} - T_{i})} \end{matrix}

In the formula, T_i and T_f denote the initial and final release temperatures of the samples, as determined by the TG-DTG method. T_max indicates the temperature at which the maximum weight loss rate occurs, while DTG_max refers to the reaction temperature corresponding to this maximum weight loss rate.

The reaction rate equation was kinetically approximated using the Coats-Redfern method [31,32]. Additionally, in conjunction with the model-free thermal analysis kinetics method at multiple heating rates, the Kissinger-Akahira-Sunose (KAS) model-free function integration method was employed to investigate the heat release kinetics of Glu-Tyr and Glu-Asp [33]. Furthermore, the variation of activation energy with respect to the heat release conversion rate is calculated under conditions of equal conversion rate.

2.5. Py-GC-MS analysis

The heating temperatures for heat-not-burn cigarettes and conventional cigarettes are approximately 350°C and 900°C [34], respectively. Based on this, we set the two pyrolysis temperatures at 350°C and 900°C to explore the potential application of synthesized Heyns compounds in cigarette production.

Weighed 2 mg of the sample and placed it in a specialized quartz tube designated for the thermal cracking instrument. After sealing both ends of the cracking tube with quartz wool, transferred the tube to the thermal cracking instrument for the cracking process. The resultant product was analyzed directly using GC-MS.

Cracking conditions: Cracking conditions include a transfer line temperature of 280°C, a valve box temperature of 280°C, and an adsorption trap temperature of 280°C. The pyrolysis temperatures were set to range from 30°C to 350°C (held for 10 seconds) and from 30°C to 900°C (held for 10 seconds), with a heating rate of 20°C/ms. The testing atmosphere consists of 10% oxygen and 90% nitrogen.

Chromatographic conditions: Chromatographic conditions involved a DB-5ms chromatographic column (30 m × 250 μm, 0.25 μm), with an inlet temperature of 250°C. The temperature rising program starts at an initial temperature of 40°C, maintained for 1 min, and then raised to 280°C at a rate of 5°C/min, held for 10 min. The carrier gas used was helium, with a flow rate of 1.0 mL/min, an injection volume of 1 μL, and a split ratio of 100:1.

Mass spectrometry conditions: Mass spectrometry conditions utilize an electron impact (EI) ionization source with an electron energy of 70 eV. The ion source temperature was set to 230°C, the quadrupole temperature to 150°C, and the transmission line temperature to 250°C. The solvent delay was 2.5 min, and acquisition was performed in full scan mode, covering a mass acquisition range of 35 to 1000 m/z.

2.6. HS-SPME/GC-MS analysis

Weighed 1 mL each of the prepared 0.04 mol/L Glu-Tyr, Glu-Asp, GT-mix, and GA-mix solutions and placed them in the headspace bottle. Then, added 10 µL of 1,2-dichlorobenzene (0.00025 µg/mL) as an internal standard. Select a 50/30 μm divinylbenzene/carboxen/polydimethylsiloxane (DVB/CAR/PDMS) extraction head (2 cm in length), and inserted it into the headspace bottle 1 cm above the sample after aging. Conduct adsorption and sampling at 70°C for 0, 5, 10, 15, and 20 min, respectively, before injecting the samples.

The GC conditions utilized a DB-5MS capillary column with dimensions of 30 m × 0.25 mm × 0.25 μm. Helium was employed as the carrier gas, with a flow rate set at 1.5 mL/min and operated in a no-split mode. The initial column temperature was held at 40°C for 3 min, followed by a ramp to 80°C at a rate of 5°C/min. Subsequently, the temperature was increased to 160°C at a rate of 10°C/min, where it was maintained for 0.5 min. The temperature was then raised to 175°C at a rate of 2°C/min, and finally increased to 230°C at a rate of 10°C/min, with a hold time of 7 min.

The MS conditions employed were as follows: EI ionization source with an electron energy set at 70 eV, a filament emission current of 35 μA, and an ion source temperature maintained at 200°C. The interface temperature was adjusted to 250°C, while the detector voltage was fixed at 1000 V. The scanning mass range was established between 35 m/z and 500 m/z.

Quantification of volatile compounds involves utilizing a single internal standard method to compare the peak areas of both volatile compounds and internal standards [35,36]. This comparison facilitates the calculation of the content of the volatile compounds. The calculation formula is as follows:

(2)

\begin{matrix} W_{n} = \dot{f} \times \frac{A_{n} \times m_{s}}{A_{s} \times V} \end{matrix}

In the formula, ‘ $\dot{f}$ ‘ represents the relative correction factor, ‘ $W_{n}$ ‘ denotes the concentration of the compound (μg/mL), ‘ $A_{n}$ ‘ indicates the peak area of the compound, ‘ $A_{s}$ ‘ refers to the peak area of the internal standard, ‘ $m_{s}$ ‘ represents the mass of the internal standard, and ‘ $V$ ‘ signifies the volume of the sample.

2.7. Sensory quality evaluation

The synthesized Heyns compounds Glu-Tyr and Glu-Asp were combined in a 5% (w/w) anhydrous ethanol solution. A microsyringe injected the solution into regular cigarettes and heated cigarettes separately. The experimental group received 10 μL per cigarette, while the control group was treated with an equal amount of ethanol. The samples were equilibrated at a temperature of (22±2)°C and a relative humidity of (60±5)% for 48 h before sensory evaluation. Five professional tasters conducted sensory evaluations to assess the differences in aroma, smoke concentration, irritation, aftertaste, and other sensory aspects between cigarettes supplemented with Heyns compounds and the control group [37].

3. Results and Discussion

3.1. Structural characterization

Two synthetic Heyns compounds were characterized using FTIR, ¹H NMR, ¹³C NMR, and HR-MS. The spectral curves were presented in Figures S1-S8. The results were summarized in Tables S1 and S2. The FTIR analysis of Glu-Tyr revealed a vibration peak at 3206.92 cm⁻¹, indicative of the presence of an -OH group, and a peak at 2953.33 cm⁻¹ associated with -NH and -CH groups. Additionally, peaks associated with -COOH, -CH, -CN, and -C-O-C were observed at 1592.57, 1367.49, 1241.37, and 1099.84 cm⁻¹, respectively. For Glu-Asp, the FTIR results showed the -OH vibration peak at 3422.63 cm⁻¹ and -COOH peak at 1607.04 cm⁻¹. The -CH vibration was detected at 1391.83 cm⁻¹, while -CN and -C-O-C peaks appeared at 1148.51 cm⁻¹ and 1044.5 cm⁻¹, respectively. By analyzing the ¹H NMR, ¹³C NMR, and HR-MS spectral data of both Glu-Tyr and Glu-Asp, in conjunction with the FTIR results, it was confirmed that the structures of Glu-Tyr and Glu-Asp are consistent with the target compound.

Figures S1-S8

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Table S1

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Table S2

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3.2. TG-DTG analysis

3.2.1. Analysis of TG results

The TG curves of Glu-Tyr, GT-mix, Glu-Asp, and GA-mix, as illustrated in Figure 3(a), demonstrated that Glu-Tyr and Glu-Asp could degrade at lower temperatures compared to GT-mix and GA-mix. Specifically, Glu-Asp began to lose weight at approximately 56°C, while Glu-Tyr started to lose weight at around 113°C. In contrast, the raw material mixtures GT-mix and GA-mix, which were used to synthesize the two Heyns compounds, commenced weight loss at 163°C and 171°C, respectively. Figure 3(b) presents the DTG curves of Glu-Tyr, GT-mix, Glu-Asp, and GA-mix, illustrating the relationship between weight loss rate and temperature for the four samples at a heating rate of 20°C/min. It was evident that the weight loss rate of Glu-Asp exceeds that of Glu-Tyr prior to 146°C. GT-mix and GA-mix initiated decomposition at 163°C and 171°C, respectively, with consistent trends in their weight loss rates. In comparison to GA-mix, Glu-Asp began to decompose in the early stages of heating, achieving its maximum weight loss rate at 228°C, after which the decomposition rate gradually declines and levels off at 520°C, indicating an overall decomposition rate lower than that of GA-mix. Notably, Glu-Tyr’s initial decomposition temperature and the temperature at which it reached its maximum decomposition rate occur earlier than those of GT-mix, at 126°C and 269°C, respectively. The decomposition rate peaks at 269°C before decreasing, and around 550°C, the rate approached zero, although slow decomposition continues. Under the maximum weight loss rate, the order is T_GT-mix > T_GA-mix > T_Glu-Tyr > T_Glu-Asp. Since Glu-Tyr and Glu-Asp contain N-glucosamine bonds in their structures, they ccould decompose into smaller molecular substances at lower temperatures compared to GT-mix and GA-mix. The initial weight loss temperature of Glu-Asp was significantly lower than that of Glu-Tyr. It was speculated that this difference could be attributed to two factors: first, Glu-Asp was hygroscopic, and the evaporation of water at lower temperatures resulted in weight reduction. Second, Glu-Asp’s side chain contained two carboxyl groups, enabling decarboxylation to occur at relatively low temperatures [38]. In contrast, the side chain of Glu-Tyr comprised a phenolic hydroxyl group, which had a larger molecular weight and higher polarity, along with strong hydrogen-bond-forming ability. This structural feature likely enhanced thermal stability through intermolecular hydrogen bonding, thereby delaying bond cleavage [39]. The observed weight loss behavior aligned with the structural identification results.

(a) TG and (b) DTG curves of Glu-Tyr, GT-mix, Glu-Asp and GA-mix.

Glu-Asp exhibits a decreased decomposition rate after the weight loss approaches 0.65, a phenomenon similar to findings in previous studies [40,41]. It is speculated that there are two possible reasons for this phenomenon. Firstly, it may be related to the incomplete decarboxylation of the two carboxyl groups on its side chain during thermal weight loss, where the connection of these two carboxyl groups forms macromolecular substances that are difficult to pyrolyze. Secondly, during its pyrolysis process, the pyrolysis products react with each other, forming substances that are difficult to degrade, leading to the occurrence of such a phenomenon.

3.2.2. The effect of temperature rise rate on heat release parameters

Figure 4 presents the TG and DTG curves for Glu-Tyr and Glu-Asp at various heating rates. The TG curve could be seen that Glu-Tyr had little mass loss before 115°C, and there was almost no change in weight. The weight loss rate initially increases before decreasing, with the degradation rate stabilizing around 530°C. In contrast, Glu-Asp exhibited a significant weight loss beginning at approximately 62°C. The TG curve for Glu-Asp revealed two distinct peaks, with the weight loss rate demonstrating a pattern of initial increase followed by a decrease, then another increase, and a subsequent decrease, with degradation occurring around 500°C. Both the TG curves for Glu-Asp and Glu-Tyr approach the high-temperature end as the heating rate escalates. Concurrently, the peak temperature of the thermal weight loss rate on the DTG curve exhibits an upward trend.

TG and DTG curves of Glu-Tyr (a and b) and Glu-Asp (c and d) at different heating rates.

The parameters involved in the release process of the compounds Glu-Tyr and Glu-Asp at various heating rates are presented in Table 1.

Table 1. Heat release characteristic parameters of compounds Glu-Tyr and Glu-Asp at different heating rates.

Name	β/(°C/min)	T_i/°C	T_max/°C	DTG_max/(%/min)	T_f/°C	T_f-T_i	T_max*(T_f-T_i)	CRI/ (%/(min×°C²))
Glu-Tyr	5	115.9	253.1	3.60	507.1	391.2	99012.72	0.00364
	10	124.2	260.4	5.72	554.4	430.2	112024.08	0.00511
	20	126.3	269.9	12.78	550.7	424.4	114545.56	0.01116
	40	129.7	281.2	26.88	585.6	455.9	128199.08	0.02097
	80	130.1	288.3	55.63	600.20	470.1	135529.83	0.04105
Glu-Asp	5	62.1	213.4	2.34	369.6	307.5	65620.5	0.00357
	10	63.0	222.2	4.48	467.1	404.1	89791.02	0.00499
	20	67.6	228.9	7.54	520.3	452.7	103623.03	0.00728
	40	69.2	229.5	14.90	551.7	482.5	110733.75	0.01346
	80	70.3	252.3	21.15	604.1	533.8	134677.74	0.01570

The T_i, T_max, and T_f values of the compounds Glu-Tyr and Glu-Asp exhibited an increase with rising heating rates, demonstrating a nonlinear trend. Additionally, both DTG_max and CRI values increased as the heating rate was elevated. CRI uses formula (Eq. 1) to calculate. Specifically, the DTG_max of the compound Glu-Tyr rose significantly from 3.60%/min to 55.63%/min, while the CRI increased from 0.00364%/(min×°C²) to 0.04105%/(min×°C²). Similarly, the DTGmax of the compound Glu-Asp increased from 2.34%/min to 21.15%/min, and the CRI rose significantly from 0.00357%/(min×°C²) to 0.01570%/(min×°C²).

3.2.3. Analysis of heat release dynamics

As illustrated in Figure 4, the heating rate significantly influences the heat release of both compounds. To established a dynamic reaction model for the heat release process of Glu-Tyr and Glu-Asp, the Coats-Redfern method was employed to identify the most suitable reaction model and kinetic parameters. This approach also enabled the calculation of the enthalpy change (ΔH), Gibbs free energy (ΔG), and entropy change (ΔS) associated with the reaction system during the heat release process. The results of these calculations are presented in Table 2. The second-order reaction model F2 [42] effectively characterizes the exothermic processes of Glu-Tyr and Glu-Asp across various heating rates, with R² values exceeding 0.97. The activation energy (E) for Glu-Tyr across all heating rates ranges from 17.64 to 19.73 kJ/mol, while for Glu-Asp, it ranges from 6.50 to 7.76 kJ/mol. Figures 5(a) and (b) display the fitting curves of the F2 reaction model for Glu-Tyr and Glu-Asp at different heating rates, demonstrating a strong linear correlation within the 20% to 70% exothermic conversion rate range. The activation energies of both Glu-Tyr and Glu-Asp increase with rising temperature. Notably, due to differences in their side-chain structures, Glu-Asp exhibited a lower initial decomposition temperature than Glu-Tyr, consequently requiring less activation energy than Glu-Tyr at the same heating rate.

Table 2. Heat release dynamics parameters of Glu-Tyr and Glu-Asp at different heating rates.

Name	β/ (°C/min)	Fitted equation	Reaction model	R²	E/(kJ/mol)	A/(min^-1)	△H/((kJ/mol)	△G/ (kJ/mol)	△S/(kJ/mol/K)
Glu-Tyr	5	y = -2137.4x - 9.2757	F2	0.9766	17.77	2.28×10⁵	15.67	79.41	-0.2519
	10	y = -2122.3x - 9.4005	F2	0.9836	17.64	2.57×10⁵	15.48	81.12	-0.2521
	20	y = -2200.4x - 9.3943	F2	0.9843	18.29	2.64×10⁵	16.05	84.17	-0.2524
	40	y = -2332.5x - 9.2541	F2	0.9822	19.39	2.44×10⁵	17.05	88.12	-0.2527
	80	y = -2374.3x - 9.2884	F2	0.9811	19.73	2.57×10⁵	17.34	90.27	-0.2529
Glu-Asp	5	y = -783.19x - 11.717	F2	0.9832	6.50	9.60×10⁵	4.74	58.18	-0.2504
	10	y = -915.03x - 11.561	F2	0.9871	7.60	9.60×10⁵	5.76	61.48	-0.2508
	20	y = -951.44x - 11.576	F2	0.9752	7.91	1.01×10⁶	6.00	63.46	-0.2510
	40	y = -962.38x - 11.63	F2	0.9757	8.00	1.08×10⁶	6.09	63.71	-0.2511
	80	y = -933.75x - 11.757	F2	0.9774	7.76	1.19×10⁶	5.66	69.20	-0.2518

Coats-Redfern fitting curves of (a) Glu-Tyr and (b) Glu-Asp at different heating rates.

The KAS method, which employs multiple heating rates, was utilized to calculate the activation energies of Glu-Tyr and Glu-Asp at heat release rates ranging from 0.2 to 0.7. Figures 6(a) and (b) illustrated the behavior of the compound Glu at various heat release rates. The linear fitting curves obtained from the KAS method for both Glu-Tyr and Glu-Asp exhibit strong linear correlations, with R² values exceeding 0.95. The fitting equations and corresponding activation energy values for Glu-Tyr and Glu-Asp were presented in Table 3. The activation energies for the two compounds were found to be between 142.87 and 233.79 kJ/mol for Glu-Tyr, and between 71.24 and 357.60 kJ/mol for Glu-Asp. Under conditions of 20% to 60% weight loss and at the same conversion rate, Glu-Tyr required more energy than Glu-Asp, a finding that was corroborated by the Coats-Redfern method, as illustrated in Figures 4(c) and (d). It was evident that when the weight loss of Glu-Asp approaches approximately 0.65, the weight loss rate begins to plateau and nears cessation. Consequently, Glu-Asp necessitates the absorption of additional energy to sustain pyrolysis, which subsequently results in a decrease in the conversion rate. This also results in a higher energy requirement for Glu-Asp pyrolysis than Glu-Tyr at a conversion rate of 0.7.

KAS fitting curves of compounds (a) Glu-Tyr and (b) Glu-Asp when the heat release rate is 0.2∼0.7.

Table 3. Fitting results of Glu-Tyr and Glu-Asp heat release kinetics based on KAS mode.

Conversion rate	Glu-Tyr			Glu-Asp
Conversion rate	Fit the equation	R²	E/(KJ/mol)	Fit the equation	R²	E/(KJ/mol)
0.2	y = -17184x + 24.518	0.9744	142.87	y = -8569.6x + 9.2119	0.9582	71.24
0.3	y = -25351x + 37.129	0.9946	210.77	y = -21470x + 31.521	0.9787	178.50
0.4	y = -20930x + 26.822	0.9617	173.79	y = -20919x + 25.693	0.9817	173.92
0.5	y = -20472x + 23.206	0.9744	170.21	y = -22062x + 23.532	0.9690	183.42
0.6	y = -34340x + 40.136	0.9773	285.50	y = -16446x + 11.515	0.9620	136.73
0.7	y = -28120x + 27.775	0.9830	233.79	y = -43012x + 38.024	0.9903	357.60

3.3. Analysis of Py-GC/MS results

3.3.1. Analysis of pyrolysis products

To investigate the release of pyrolysis products from Glu-Tyr and Glu-Asp at elevated temperatures, Glu-Tyr, GT-mix, Glu-Asp, and GA-mix were thermally cracked at 350°C and 900°C, respectively. The NIST17 mass spectral library was employed to characterize the cleavage products. After filtering out anomalous data and low-matching results, the thermal cracking of Glu-Tyr, GT-mix, Glu-Asp, and GA-mix at 350°C yielded 19, 37, 31, and 32 distinct species, respectively. Under the conditions of 900°C, the thermal cracking produced 60, 45, 65, and 43 species, respectively. The primary substances were identified, and the product contents at 350°C and 900°C were normalized and presented in Figures 7(a) and (b), respectively. And the corresponding chromatograms were presented in Figures S9-S16. The content of lysis products at different temperatures was shown in Tables S3-S4. Figure 7 illustrates that at 350°C, Glu-Tyr, GT-mix, Glu-Asp, and GA-mix generated 11, 8, 22, and 15 substances, respectively, while at 900°C, 22 and 18 substances were produced, along with 46 and 14 substances.

Figures S9-S16

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Table S3

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Table S4

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Comparison of Glu-Tyr, GT-mix, Glu-Asp and GA-mix cleavage products at (a) 350°C and (b) 900°C.ST

Under cleavage conditions of 350°C, 6 heterocyclic substances cleaved by Glu-Tyr included three pyrazines (0.94%), one pyrrole (0.15%), and two furans (4.07%), collectively accounting for 5.14%. Additionally, phenolic substance, 4-ethyl phenol, accounted for 1.99%. Three ester substances (0.22% methyl p-hydroxyphenylpropionate, 0.95% methyl cinnamate, and 65.26% p-cresol acetate) together constituted 66.48%. Finally, one type of alcohol substance, benzyl alcohol, accounted for 6.42%. GT-mix yielded 6 types of heterocycles, including three pyrazines (1.52%), one pyrrole (3.18%), and two furans (23.23%), which collectively accounted for 27.94% of the total. Additionally, two types of phenolic compounds were identified: 4-ethylphenol (9.07%) and p-methylphenol (10.04%), contributing to 19.11% of the total. Furthermore, one type of ketone, hydroxyacetone, was present, accounting for 4.14%.

Most of the 22 products cleaved by Glu-Asp at 350°C are heterocyclic compounds, which include 20 heterocyclic compounds: 4 pyridines (5.38%), 7 pyrazines (16.79%), and 8 pyrroles (20.03%), collectively accounting for 43.26%. Additionally, there is 3-methyl-2-cyclohexen-1-one (ketone substance), which accounts for 1.04%, and 4-pentenoic acid, which constitutes 2.03%. In contrast, GA-mix cleavage produces 14 types of heterocycles: 3 pyridines (6.29%), 6 pyrroles (21.45%), and 5 pyrazines (17.77%), together accounting for 45.50%. There is also ketone compound (hydroxyacetone), which accounts for 1.11%.

At 900°C, Glu-Tyr decomposed into two types of esters, comprising 46.98% p-cresol acetate and 0.53% dioctyl adipate, which together accounted for 47.51% of the total. Additionally, it yielded ten types of heterocycles, including four types of pyrroles (0.14%), four types of pyrazines (0.58%), and two types of furans (24.18%), collectively representing 24.90%. Furthermore, ten types of phenols were produced, accounting for 22.57% of the total. In contrast, the GT-mix resulted in five types of heterocycles at 900°C, consisting of four types of pyrroles (0.89%) and one type of furan (0.21%), which together accounted for 1.10%. It also generated one type of alcohol (p-hydroxyphenylethanol), representing 2.20%, one type of ester (p-cresol acetate), accounting for 1.78%, and eleven types of phenols, which comprised 84.34% of the total.

Glu-Asp cleaves 33 types of heterocyclic species at 900°C, including 10 types of pyrrole species (19.80%), 5 types of pyrazines (7.41%), 4 types of pyridines (7.38%), 1 type of pyran species (1.28%), and 6 types of furan species (6.31%), as well as 5 types of indole species (5.97%). This accounts for a total of 58.55%. Additionally, 8 types of species of ketones contribute 17.94%, while 4 types of species of phenols account for 4.34%, 1 species of alcohol accounts for 3.04%, and 1 species of hydrocarbons (1,2-dimethylcyclohexene) accounts for 2.64%. In contrast, GA-mix cracks 11 types of heterocycles at 900°C, comprising 3 types of pyrroles (28.09%), 5 types of pyrazines (28.93%), 1 type of furan (1.25%), and 1 type of pyridine (2.64%), among others. This results in a total of 63.91%. Furthermore, GA-mix includes 2 types of ketones, accounting for 3.45%, and 1 type of alcohol (phenylethanol), accounting for 3.41%.

In comparison to the cleavage products obtained at 350°C, Glu-Tyr generated a greater variety of heterocyclic substances at 900°C, with the relative content of these substances rising significantly from 5.14% to 24.90%. Additionally, there was a notable increase in phenolic substances, with nine new species identified, resulting in a content proportion that rose from 1.99% to 22.57%. While the amount of p-cresol acetate decreased slightly, benzyl alcohol was not detected. Conversely, the heterocyclic substances produced by GT-mix at 900°C were nearly identical to those at 350°C; however, their content decreased markedly, from 27.94% to 1.1%. The number of phenolic substances also increased by nine, with their proportions rising from 27.94% to 84.34%.

Compared to the cleavage products at 350°C, Glu-Asp exhibits 11 additional types of heterocyclic substances at 900°C. The new products primarily consist of furans (4 types) and indoles (5 types), although the overall heterocyclic content is slightly lower. Notably, the ketone content has increased significantly, rising from 1.04% to 17.94%. The main heterocyclic substances generated from the cleavage of GA-mix at 900°C remain consistent with those observed at 350°C, specifically pyrroles and pyrazines. While the variety of pyrroles has decreased, their content has increased, resulting in an overall rise in the content of heterocyclic substances.

At 350°C, the primary cleavage product of Glu-Tyr was p-cresol acetate, while the main cleavage products of GT-mix, Glu-Asp, and GA-mix consist predominantly of heterocyclic substances. The principal cleavage products of GT-mix were furans, whereas the cleavage products of Glu-Asp and GA-mix were primarily composed of pyrazines and pyrroles. At 900°C, the main cleavage products of Glu-Tyr include p-cresol acetate, furans, and phenols. For GT-mix, the predominant cleavage products were phenols. The majority of cleavage products from Glu-Asp and GA-mix were also heterocyclic substances, with Glu-Asp primarily yielded pyrroles and ketones, while GA-mix predominantly produces pyrazines and pyrroles. P-cresol acetate, heterocyclic substances, phenols, and ketones are all significant aroma components. Among these, p-cresol acetate is noted for its strong narcissus-like floral aroma [43,44], which is complemented by a rich animal scent, making it suitable for enhancing nutty aromas in food flavors. The p-cresol acetate released from the cleavage of Glu-Tyr and GT-mix is associated with the tyrosine structural unit. At a temperature of 900°C, the breakdown of the ester bond in p-cresol acetate leads to an increase in the cleavage products of phenolic substances. The heterocyclic substances released from the cleavage of Glu-Asp and GA-mix are linked to glucose structural units, with Glu-Asp capable of releasing a greater quantity of ketone aroma substances at 900°C. Consequently, the cleavage products of Glu-Tyr and Glu-Asp compounds differ significantly from the physical mixture of their raw materials, and the flavor substances generated from the cleavage of different Heyns compounds vary considerably. The type and concentration of the cleavage products are heavily influenced by the cleavage temperature.

Glu-Asp generates roasted aroma substances at 350°C, potentially serving as a precursor for bread or coffee flavor. By utilizing microencapsulation technology to control the thermal release sequence, the aroma complexity of baked goods is enhanced [45]. Meanwhile, the high-temperature pyrolysis esters of Glu-Tyr exhibit floral characteristics, which could potentially serve as a flavor-enhancing module for cigarettes to mask undesirable odors and reduce irritation [46].

3.3.2. Analysis of flavor characteristics of pyrolysis products

Most of the cleavage products of Glu-Tyr, GT-mix, Glu-Asp and GA-mix at 350°C and 900°C were important aromatic components in baked goods or tobacco. Based on literature reports and spice aroma classification method [47-50], a comprehensive summary of the pyrolysis products of these four compounds at various temperatures allowed for their classification according to aroma characteristics. The predominant aroma characteristics were broadly categorized into three types: burnt sweet aroma, floral aroma, and baking aroma. The results were presented in Table 4.

Table 4. Main aroma characteristics of Glu-Tyr, GT-mix, Glu-Asp and GA-mix pyrolysis products.

Fragrance characteristics	Representative compound
Burnt sweet aroma	Hydroxyacetone, 4,6-dimethyl-2-pyrone, 2-methyl-3-(2-methylpropyl)pyrazine, 3-methyl-2-cyclohexen-1-one, Furanone, 4-pentenoic acid
Floral aroma	Phenylethyl alcohol, styrene, 2-phenylpropionaldehyde, 2-phenyl-1-propanol, allyl phenyl ether, thymol, p-hydroxyphenylethyl alcohol, p-cresol acetate
Baking aroma	2-methylpyrazine, 2-methylfuran, 2,6-dimethylpyrazine, 3-ethyl-2,5-methylpyrazine, 2,5-dimethylpyrazine, 3,5 -Diethyl-2-methyl-pyrazine, 2-ethylpyrazine, 2,3,5-trimethylpyrazine, 2,3-diethyl-5-methylpyrazine, 2-methyl-3-propylpyrazine, 2-ethyl-3,5-dimethylpyrazine, 2-ethyl-3- Methylpyrazine, 2-ethyl-6-methylpyrazine, 5-ethyl-2-methyl-pyridine, pyrrole

The classification and organization of substances with similar aroma characteristics were illustrated in Figure 8. As shown in Figure 8(a), at temperatures below 350°C, Glu-Tyr produced 65.26% of floral aroma substances, 1.13% of baking aroma substances, and 0.26% of burnt sweet aroma substances. In contrast, GT-mix yielded 7.93% of burnt sweet aroma substances and 2.52% of baking aroma substances. Glu-Asp generated 18.32% of baking aroma substances and 1.04% of burnt sweet aroma substances at 350°C. Additionally, GA-mix produced 19.03% of baking aroma substances and 1.11% of burnt sweet aroma substances. Figures. 8(b) demonstrates that under pyrolysis conditions of 900°C, the Glu-Tyr lysate contains floral and baking aroma substances, which account for 47.20% and 0.51%, respectively. GT-mix yielded 1.94% of baking aroma substances and 5.82% of floral aroma substances. Glu-Asp produced 7.21% of burnt sweet aroma substances and 6.44% of baking aroma substances. The aroma pyrolysis products of GA-mix include baking and floral aroma substances, comprising 3.57% and 3.41%, respectively.

The content of flavor substances generated by Glu-Tyr, Glu-Asp, GT-mix, and GA-mix at (a) 350°C and (b) 900°C.

At a temperature of 350°C, Glu-Tyr primarily decomposed into floral aroma compounds along with a minor quantity of baking aroma substances, whereas Glu-Asp predominantly yielded baking aroma and burnt sweet aroma compounds. In contrast, at 900°C, the concentrations of floral and baking aroma substances derived from Glu-Tyr decrease. Meanwhile, Glu-Asp continues to produce baking aroma and burnt sweet aroma substances. However, compared to the 350°C condition, there was a reduction in the amount of baking aroma substances and an increase in burnt sweet aroma substances. These findings indicated that under varying temperature conditions, the same Heyns compound could produce distinct flavor characteristics, while different Heyns compounds could yield different flavors at the same temperature [22]. And at the same temperature, Glu-Tyr and Glu-Asp have more fragrance substances and rich fragrance than their raw materials after physical mixing.

3.4. Analysis of headspace solid-phase microextraction coupled with gas chromatography-mass spectrometry (HS-SPME-GC/MS) results

To investigate the thermal flavor release of Glu-Tyr and Glu-Asp with aqueous solutions, Glu-Tyr, GT-mix, Glu-Asp, and GA-mix aqueous solutions were analyzed respectively. Volatile aroma compounds were characterized using the NIST17 mass spectral library, and any abnormal or low-matching data were excluded. The corresponding chromatograms were shown in Figures S17-S36, and the content of the product at different heating times was shown in Tables S5-S8. To highlight data differences, we normalized the outcome data using Z-scores. The comparison of results before and after heating for Glu-Tyr, Glu-Asp, GT-mix, and GA-mix was presented in Figure 9. The results indicated that the variety of volatile substances generated by Glu-Tyr, Glu-Asp, GT-mix, and GA-mix increased with prolonged heating time. Specifically, the number of volatile substances produced before heating were 3, 5, 3, and 4, respectively. After heating for 5 min, the quantities increased to 15, 13, 18, and 15 substances, respectively. After heating 10 min, the numbers were 22, 16, 20, and 16, respectively. After 15 min, 26, 21, 22 and 18 substances were generated, and after 20 min, the counts reached 27, 22, 24, and 24 substances. The volatile aroma substances generated by the four compounds when heated at 70°C for 20 min primarily consist of five aldehydes (n-butyraldehyde, phenylacetaldehyde, nonanal, decanal, and 4-methoxybenzaldehyde dimethyl acetal), four esters (methyl propionate, isobutyl propionate, methyl butyrate, and methyl 2-methylbutyrate), and three types of ketones (2-pentanone, 2,3-hexanedione, and 2-methyl-3-pentanone). The products generated by each substance during this process were largely similar, exhibiting minimal variation. It was speculated that the Maillard reaction might occur when the two samples of GT-mix and GA-mix were dissolved in water under heated conditions [51,52]. Consequently, volatile components analogous to Glu-Tyr and Glu-Asp were produced.

Figures S17-S39

Table S5

Table S6

Table S7

Table S8

The content of volatile compounds was quantified using formula (2). The volatile aroma substance contents of Glu-Tyr, Glu-Asp, GT-mix, and GA-mix at different heating durations were illustrated in Figure 10. When heated for 0 min, the volatile flavor substance contents of Glu-Tyr, Glu-Asp, GT-mix, and GA-mix were measured to be 0.0080 µg/L, 0.0074 µg/L, 0.0039 µg/L, and 0.0043 µg/L, respectively. After 5 min of heating, the contents increased to 0.0144 µg/L for Glu-Tyr, 0.0104 µg/L for Glu-Asp, 0.0121 µg/L for GT-mix, and 0.0103 µg/L for GA-mix. After 10 min, the values were 0.0192 µg/L, 0.0174 µg/L, 0.0156 µg/L, and 0.0129 µg/L, respectively. Following 15 min of heating, the contents were recorded as 0.0227 µg/L, 0.0193 µg/L, 0.0161 µg/L, and 0.0149 µg/L. Finally, after 20 min of heating, the volatile substance contents were 0.0242 µg/L, 0.0219 µg/L, 0.0180 µg/L, and 0.0204 µg/L, respectively. It can be observed that as the heating time increases, the volatile products resulting from the thermal reactions of Glu-Tyr, Glu-Asp, GT-mix, and GA-mix also increase gradually. This phenomenon aligned with prior studies indicating that the quantity of volatile products increased with prolonged heating time [35,53]. Notably, within the same time frame, GT-mix and GA-mix yield fewer flavor compounds compared to Glu-Tyr and Glu-Asp. This suggestd that the Maillard reaction occurring between GT-mix and GA-mix proceeds more slowly under these conditions, resulting in the generation of volatile substances at a rate slower than that of the intermediate compounds, ultimately leading to a lower production of volatile substances within the same time period. Furthermore, the volatile aldehydes, esters, and ketones identified in this study [47-50] were all recognized as excellent flavor compounds. Appropriate addition of these components could enhance the aromatic complexity, demonstrating the potential of Heyns compounds as natural flavor enhancers in the development of heat processed food.

Variation curve of flavor content over time.

3.5. Sensory evaluation results

After evaluation by five panelists, the balanced cigarette with added Heyns compounds was found to have increased smoke concentration, fuller aroma, significantly reduced irritation, and smoother, more fluid smoke. However, different Heyns compounds had slightly varying sensory impacts. Cigarettes with added Glu-Tyr exhibited reduced strength and enhanced sweet aftertaste compared to those with added Glu-Asp, which was related to the phenolic esters produced by the pyrolysis of Glu-Tyr [54]. Cigarettes with added Glu-Asp showed enhanced style expression without masking the inherent aroma of tobacco, which was associated with the heterocyclic compounds produced by its pyrolysis [55], making it more suitable as a flavoring additive for regular cigarettes. The heated cigarette with added Glu-Tyr exhibited moderate strength, prominent floral notes, relatively dense smoke, a noticeable aftertaste, and a significantly reduced irritation sensation. The cigarette with added Glu-Asp displayed a distinct roasted tobacco aroma, but the increase in smoke density was not significant, and it had a slight irritancy. Sensory evaluation results indicated that Glu-Asp could be prioritized for flavoring in conventional cigarettes, while Glu-Tyr was more suitable for addition in heated cigarettes.

4. Conclusions

In this study, two Heyns compounds were successfully synthesized using two amino acids (lysine and aspartic acid) and fructose, with their structures confirmed through IR, NMR, and HRMS analysis. TG-DTG analysis indicated that Glu-Asp had a lower decomposition temperature and a faster decomposition rate compared to Glu-Tyr, and the thermal release kinetics results showed a strong linear relationship under different heating rates. Pyrolysis results demonstrated that after heating to 900°C, in addition to the increase in heterocyclic compounds, phenolic compounds in Glu-Tyr and GT-mix, as well as ketones and phenolic in components Glu-Asp and GA-mix, also increased. Glu-Tyr, GT-mix, Glu-Asp, and GA-mix were dissolved in water and heated to 70°C, the variety of volatile aroma substances increased over time, and they showed significant differences. The sensory evaluation results indicated that when 10 ul 5% concentration of Heyns compounds were added to cigarettes, Glu-Asp was more suitable for conventional cigarettes, while Glu-Tyr significantly enhanced the quality and aroma of heated cigarettes. The distinct aroma substances generated under varying conditions may hold significant implications for the design and application of flavor formulations within the food industry. In this study, we systematically analyzed the flavor formation mechanism of tyrosine- and aspartic acid-derived Heyns compounds. However, due to the limited amino acid selection range of the study samples, the pyrolysis kinetics of Heyns compounds synthesized from other amino acids and the formation mechanism of key flavor compounds remain unclear and require further exploration.

Acknowledgment

We would like to thank the Natural Science Foundation of Henan Province (232300421257) provided funding for this work.

CRediT authorship contribution statement

Yu Li: writing - original draft, data curation, formal analysis. Longxin Wang: investigation, methodology and formal analysis. Qianrui Zhao: data curation and validation. Yueqi Xu: data curation and software. Yihan Hu: data curation and validation. Bingxiang Wang: data curation and validation. Jingyi Hu: data curation and software. Pengze Wang: writing - review & editing and supervision. Miao Lai: writing - review & editing and supervision.

Declaration of competing interest

The authors declare no competing interests.

Data availability

All data from this study were included in the paper and its supplementary materials.

Declaration of Generative AI and AI-assisted technologies in the writing process

The authors confirm that there was no use of artificial intelligence (AI)-assisted technology for assisting in the writing or editing of the manuscript and no images were manipulated using AI.

Supplementary material

Supplementary data to this article can be found online at: https://dx.doi.org/10.25259/AJC_164_2025.

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Preparation and thermal properties of Heyns compounds based on two different amino acids

Abstract

Keywords

Flavor substance differences

Heyns compound synthesis

Thermal property analysis

Tobacco flavor modulation

1. Introduction

2. Materials and Methods

2.1. Reagents and instruments

2.2. Synthesis of Glu-Tyr and Glu-Asp

2.2.1. Synthesis of Glu-Tyr

2.2.2. Synthesis of Glu-Asp

2.3. Structural identification

2.4. Thermogravimetry-derivative thermogravimetry (TG-DTG) analysis

2.5. Py-GC-MS analysis

2.6. HS-SPME/GC-MS analysis

2.7. Sensory quality evaluation

3. Results and Discussion

3.1. Structural characterization

3.2. TG-DTG analysis

3.2.1. Analysis of TG results

3.2.2. The effect of temperature rise rate on heat release parameters

3.2.3. Analysis of heat release dynamics

3.3. Analysis of Py-GC/MS results

3.3.1. Analysis of pyrolysis products

3.3.2. Analysis of flavor characteristics of pyrolysis products

3.4. Analysis of headspace solid-phase microextraction coupled with gas chromatography-mass spectrometry (HS-SPME-GC/MS) results

3.5. Sensory evaluation results

4. Conclusions

Acknowledgment

CRediT authorship contribution statement

Declaration of competing interest

Data availability

Declaration of Generative AI and AI-assisted technologies in the writing process

Supplementary material

References

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