Online pretreatment method for real-time measurement of aroma substances in tea processing

Abstract

The application discloses an online pretreatment method for real-time measurement of aromatic substances in a tea processing process. The method comprises the following steps: heat tracing and airflow shearing of a wet hot carrier gas to obtain a homogeneous mixed gas; passing the mixed gas into a cold trap tube with a multi-adsorbent bed layer to adsorb and enrich at low temperature; switching through a six-way valve to remove residual moisture by reverse blowing with inert gas at low temperature; and switching through the six-way valve again to rapidly pulse the temperature of the cold trap tube, so that the aromatic substances are instantaneously desorbed and carried into a gas chromatography-mass spectrometry instrument for analysis. Through the synergistic effect of heat tracing and shearing, staged low-temperature enrichment, low-temperature blowing and pulse heating, the technical problem of real-time capture of trace aromatic substances in a high-humidity dynamic processing environment is solved, high-time-resolution monitoring of the dynamic change of aroma in the tea processing process is realized, and the method can be used for tea quality control and process optimization.

Description

Technical Field

[0001] This invention relates to the field of sample pretreatment technology, specifically to an online pretreatment method for real-time measurement of aromatic substances in tea processing. Background Technology

[0002] Aroma quality is a core indicator of tea's sensory evaluation, directly determining its economic value. The formation of characteristic aromas is a dynamic, non-linear chemical transformation process, primarily occurring during thermal processing stages such as fixation, drying, and roasting. Real-time monitoring of the generation and decay of these aromatic substances is crucial for accurately controlling processing endpoints and stabilizing tea quality. However, existing technologies face insurmountable bottlenecks: traditional tea aroma analysis methods, such as headspace solid-phase microextraction (HS-SPME) and solvent-assisted flavor evaporation (SAFE), are all offline or offline-online hybrid methods. These require sampling, cooling, weighing, and extraction enrichment at specific processing points, a process that is time-consuming and cannot match the rapid chemical changes that occur within seconds during tea processing. More importantly, existing offline pretreatment technologies have a blind spot that is difficult for those skilled in the art to notice—a lack of adaptability to the characteristics of the "semi-finished" matrix in the middle of processing. Conventional methods are mostly optimized for finished dry tea (moisture content <5%, stable cell structure). However, during tea processing, especially in the late fixation and initial drying stages of rolling, the moisture content of the leaves is still as high as 10%-40%, the cells are in a semi-damaged state, and the matrix exhibits high humidity and high adhesion colloidal characteristics. In this dynamic matrix, aroma extraction suffers from a severe "matrix competitive adsorption" effect. That is, a large number of highly polar water molecules and oligosaccharide fragments generated in the middle stage will preferentially occupy the active sites of the extraction fibers or adsorbents, resulting in a sharp decline in the enrichment efficiency of trace target aromatic substances (such as sulfur-containing compounds and pyrazines) and even false negatives. At the same time, the sampling process of offline methods disrupts the continuity of processing. After the sample leaves the thermal reaction environment, some unstable intermediates (such as Maillard reaction intermediates) will undergo reverse reactions or degradation, resulting in the detection results failing to truly reflect the instantaneous state of the processing site. Therefore, developing an online pretreatment method that can overcome interference from high-humidity matrices, achieve second-level time resolution, and stably capture unstable intermediates is a key issue that urgently needs to be addressed in this field to solve the technical challenge of real-time capture and unbiased analysis of trace aromatic substances and unstable intermediates in high-humidity dynamic processing environments. Summary of the Invention

[0003] The technical problem to be solved by this invention is to develop an online pretreatment method that can overcome interference from high-humidity matrices, achieve second-level time resolution, and stably capture unstable intermediates, thereby solving the technical challenge of real-time capture and unbiased analysis of trace aromatic substances and unstable intermediates in high-humidity dynamic processing environments.

[0004] To solve the above-mentioned technical problems, the technical solution adopted by the present invention is as follows:

[0005] An online pretreatment method for real-time measurement of aromatic substances in tea processing includes the following steps:

[0006] Step a: Extract the hot and humid carrier gas containing aromatic substances from the tea processing equipment in real time, heat and keep the hot and humid carrier gas, and then process the heat-supplied carrier gas through airflow shearing. Use the shearing force generated by the high-speed airflow to break and separate the water mist particles entrained in the carrier gas to obtain a homogeneous mixed gas.

[0007] Step b: The homogeneous mixed gas obtained in step a is passed into a cold trap tube filled with a multi-adsorbent bed and adsorbed and enriched under low temperature conditions of -10℃ to 5℃.

[0008] Step c: After the adsorption and enrichment are completed, the cold trap tube is back-purged with inert gas while maintaining a low temperature by switching the six-way valve. This selectively removes and drains the co-adsorbed residual moisture, while the target aromatic substance remains on the adsorbent.

[0009] Step d: After purging, the cold trap tube is rapidly heated to 280-300℃ at a heating rate of 40-60℃ / s by switching the six-way valve, so that the adsorbed aromatic substances are instantly desorbed and transferred to the gas chromatograph-mass spectrometer by backwashing with carrier gas in a narrow pulse form for analysis.

[0010] The six-way valve has three working positions: in the sampling position, it connects the sample inlet to the cold trap tube inlet and the cold trap tube outlet to the sampling pump, allowing the sample gas to pass through the cold trap tube in the forward direction for adsorption and enrichment; in the purging position, it connects the inert gas source to the cold trap tube outlet, allowing the inert gas to pass through the cold trap tube in the reverse direction for purging and dehydration; in the injection position, it connects the carrier gas source to the cold trap tube outlet and the cold trap tube inlet to the injection port, allowing the carrier gas to pass through the cold trap tube in the reverse direction to carry the desorbed aromatic substances into the analytical instrument.

[0011] Furthermore, in the above-mentioned online pretreatment method for real-time measurement of aromatic substances in tea processing, the extraction flow rate of the hot and humid carrier gas in step a is 50-200 mL / min, the temperature of the heat tracing and insulation is 110-130℃, and the airflow shearing treatment is achieved through a shear chamber with a Venturi structure, where the airflow velocity is accelerated to 5-10 m / s.

[0012] Furthermore, in the above-mentioned online pretreatment method for real-time measurement of aromatic substances in tea processing, the shear chamber is made of quartz glass and consists of an inlet constriction section, a throat section, and an outlet diffusion section in sequence along the airflow direction. The inlet constriction section has a length of 12 mm, an inlet diameter of 6 mm, and a constriction angle of 21°. The throat section has a length of 2 mm and a diameter of 2 mm. The outlet diffusion section has a length of 18 mm, an outlet diameter of 6 mm, and a diffusion angle of 12°.

[0013] Furthermore, in the above-mentioned online pretreatment method for real-time measurement of aromatic substances in tea processing, the multi-adsorbent bed in step b sequentially includes, along the airflow direction: a hydrophobic highly cross-linked polystyrene resin for adsorbing high-boiling-point volatile components, graphitized carbon black for adsorbing terpene compounds, and a carbon molecular sieve for adsorbing low-boiling-point small-molecule sulfur-containing compounds.

[0014] Furthermore, in the above-mentioned online pretreatment method for real-time measurement of aromatic substances in tea processing, the hydrophobic highly crosslinked polystyrene resin is Porapak Q with a particle size of 80-100 mesh; the graphitized carbon black is Carbopack B with a particle size of 60-80 mesh; and the carbon molecular sieve is Carbosieve S-III.

[0015] Furthermore, in the above-mentioned online pretreatment method for real-time measurement of aromatic substances in tea processing, the adsorption and enrichment time in step b is 1-5 minutes.

[0016] Furthermore, in the above-mentioned online pretreatment method for real-time measurement of aromatic substances in tea processing, the inert gas in step c is high-purity nitrogen, the purging flow rate is 20-50 mL / min, the purging time is 30-90 seconds, and the temperature of the cold trap tube is maintained at 0-10℃ during the purging process.

[0017] Furthermore, in the above-mentioned online pretreatment method for real-time measurement of aromatic substances in tea processing, the carrier gas in step d is helium, the backwash flow rate is 1.5-3 mL / min, and the peak width of the narrow pulse is less than 3 seconds.

[0018] Furthermore, in the above-mentioned online pretreatment method for real-time measurement of aromatic substances in tea processing, the tea processing includes fixation, drying or roasting steps; the aromatic substances include terpenoids, pyrazines, sulfur-containing compounds and Maillard reaction intermediates.

[0019] The beneficial effects of this invention are as follows: This invention, through the close integration of four steps—"heat tracing and insulation-airflow shearing," "staged adsorption-low-temperature enrichment," "low-temperature selective purging," and "rapid pulse heating-high-temperature desorption"—and the synergistic effect of precise timing switching of the three positions of a six-way valve, successfully solves the technical challenge of real-time capture and unbiased analysis of trace aromatic substances and unstable intermediates in high-humidity dynamic processing environments, a challenge that existing offline technologies cannot overcome. Specifically: the synergy of heat tracing and insulation (110-130℃) and airflow shearing (5-10 m / s) not only prevents the condensation and blockage of the humid carrier gas during transmission, but also breaks down large water mist particles through high-speed shearing force, dissociating water molecules and aromatic substances into micro-clusters, releasing the encapsulated hydrophobic molecules, and forming a homogeneous aerosol that enters subsequent stages, creating clean interface conditions for efficient adsorption; staged adsorption (Porapak Q / Carbopack B / Carbosieve)... The synergy between S-III) and low-temperature enrichment (0℃) utilizes the gradient retention capacity of the adsorbent for compounds with different boiling points in the C2-C20 range. Combined with the reduction of vapor pressure at low temperature to enhance adsorption efficiency, it achieves high-fidelity capture of the entire spectrum from easily penetrating dimethyl sulfide to high-boiling geraniol, increasing the adsorption rate of low-boiling sulfur-containing compounds from less than 20% in conventional methods to over 95%. The precise coordination of low-temperature selective purging (0-10℃) and the purging position of the six-way valve utilizes the difference in retention strength between water and target analytes on the adsorbent to selectively remove co-adsorbed water without loss of target analytes, reducing water peak interference by over 80% and significantly improving the chromatographic peak shape and signal-to-noise ratio of low-boiling components. Minute-level sampling (1-5 minutes) The synergistic effect of rapid pulse heating (50℃ / s) and instantaneous switching of the six-way valve injection position unifies long-term enrichment with instantaneous injection, allowing adsorbed aromatic substances to be introduced into the column in narrow pulses (peak width <3 seconds). This resolves the contradiction between time resolution and detection sensitivity, and for the first time captures the complete growth curve of dimethyl sulfide and the formation window of pyrazines during roasting. These four sets of characteristics are organically linked through precise timing switching of the six-way valve's sampling-purge-injection positions, ensuring seamless connection and non-interference between each step. In the high-humidity, high-temperature, and dynamically changing tea processing environment, real-time, unbiased, and high-time-resolution monitoring of trace aromatic substances (including unstable intermediates) is achieved. Attached Figure Description

[0020] Figure 1 This is a schematic diagram of the system gas path for step b of the online pretreatment method for real-time measurement of aromatic substances in tea processing according to Embodiment 1 of the present invention.

[0021] Figure 2 This is a schematic diagram of the system gas path for step c of the online pretreatment method for real-time measurement of aromatic substances in tea processing according to Embodiment 1 of the present invention.

[0022] Figure 3 This is a schematic diagram of the system gas path for step d of the online pretreatment method for real-time measurement of aromatic substances in tea processing according to Embodiment 1 of the present invention. Detailed Implementation

[0023] To explain in detail the technical content, objectives, and effects of the present invention, the following description is provided in conjunction with the embodiments and accompanying drawings.

[0024] Example 1

[0025] This embodiment focuses on the electric roasting process of Longjing green tea, aiming to monitor in real time the dynamic changes of key aroma compounds (linalool, geraniol, dimethyl sulfide, and 2,5-dimethylpyrazine) during the process of pan-frying the green leaves at a pan temperature of 180℃ from flat roasting to full drying. This embodiment employs a self-built online thermal desorption-gas chromatography-mass spectrometry system, which consists of a sampling module, a shearing module, a cold trap enrichment module, a six-way valve switching module, a thermal desorption module, and an Agilent 7890B-5977B GC-MS.

[0026] Connection relationships between various structures in the system:

[0027] Along the gas flow direction, the structures are connected in the following order:

[0028] The inlet of the sampling probe is positioned approximately 10 cm above the fumes enrichment zone of the electric frying pan. The outlet of the sampling probe is sealed to the inlet of the quartz shear chamber via a heated pipeline. The outlet of the quartz shear chamber is connected to the first port (sample inlet) of a VICI Valco (6-port 3-position valve) via a heated pipeline. The second port (sample outlet) of the VICI Valco is connected to the inlet of the cold trap tube. The outlet of the cold trap tube is connected to the third port (cold trap return) of the VICI Valco. The fourth port (sampling pump port) of the VICI Valco is connected to the inlet of a constant flow sampling pump (KNF N86KT.18 diaphragm pump), and the outlet of the constant flow sampling pump is vented. The fifth port (carrier gas inlet) of the VICI Valco is connected to a high-purity helium gas source (purity >99.999%), and the sixth port (sample outlet) of the VICI Valco is connected to the sample inlet of the gas chromatograph-mass spectrometer via a heated transmission line. A high-purity nitrogen gas source (purity >99.999%) is connected to the pipeline containing the third port (cold trap return) of the six-way valve via a three-way valve for reverse purging during the purging stage.

[0029] The cold trap tube is a U-shaped quartz tube with an inner diameter of 2 mm and a total length of 120 mm, with an effective adsorption section length of 80 mm. Its interior is filled sequentially along the airflow direction: a quartz wool fixing layer with a thickness of 1 mm is placed at the inlet end; the first adsorbent layer is hydrophobic highly cross-linked polystyrene resin (Porapak Q, 80-100 mesh), with a filling length of 25 mm and a filling amount of 20 mg; the first and second adsorbent layers are separated by quartz wool with a thickness of 0.5 mm; the second adsorbent layer is graphitized carbon black (Carbopack B, 60-80 mesh), with a filling length of 20 mm and a filling amount of 15 mg; the second and third adsorbent layers are separated by quartz wool with a thickness of 0.5 mm; the third adsorbent layer is carbon molecular sieve (Carbosieve S-III), with a filling length of 15 mm and a filling amount of 10 mg; finally, a quartz wool fixing layer with a thickness of 1 mm is placed, followed by the outlet end. The cold trap tube is embedded in the cooling cell of the semiconductor refrigeration module (TEC1-12706 thermoelectric refrigeration chip), and the outer wall of the cold trap tube is tightly attached to the heating wire of the pulse heating module.

[0030] An online pretreatment method for real-time measurement of aromatic substances in tea processing includes the following steps:

[0031] Step a: Dynamic headspace-gas shear extraction:

[0032] The sampling probe has a built-in detachable sintered stainless steel filter with a filtration accuracy of 5 μm to intercept dust and tea leaves in the processing environment. The heating pipeline is turned on, and a PID controller precisely maintains the pipeline temperature at 120℃±2℃ to prevent moisture condensation on the pipeline walls.

[0033] Turn on the constant flow sampling pump and set the flow rate to 150 mL / min, which is then precisely adjusted using the mass flow controller. The extracted aromatic hot gas flow enters the quartz shear chamber.

[0034] The quartz shearing chamber is integrally blown from high-transmittance quartz glass and consists of three parts along the airflow direction: an inlet contraction section, a throat section, and an outlet diffuser section. The inlet contraction section is 12 mm long, 6 mm inlet diameter, and has a contraction angle of 21°; the throat section is 2 mm long and 2 mm in diameter, where the calculated airflow velocity is approximately 8 m / s; the outlet diffuser section is 18 mm long, 6 mm in outlet diameter, and has a diffusion angle of 12°. An annular groove turbulence structure, 0.2 mm deep and 0.3 mm wide, is installed on the inner wall of the rear of the throat section and the beginning of the diffuser section to induce local micro-vortices and promote the dissociation of water molecules and aromatic substances. A 0.2 mL micro-storage tank is located at the bottom of the shearing chamber to temporarily store the separated and settled liquid.

[0035] The airflow forms high-speed turbulence in the shear chamber. The shear force of the high-speed airflow at 8 m / s breaks up large water mist particles (particle size > 5 μm) entrained in the airflow, causing them to collide with the chamber wall and settle back down. At the same time, aromatic molecules are dissociated from the particulate matter, forming a homogeneous mixture in aerosol state that enters the next stage. This step lasts for 5 minutes and corresponds to a sampling point in the roasting process.

[0036] Step b: Selective adsorption-dynamic enrichment in a cryogenic cold trap (refer to...) Figure 1 ):

[0037] When sampling begins, the six-way valve switches to the sampling position (position A). The internal passages are as follows: the first port (sample inlet) is connected to the second port (sample outlet), the third port (cold trap return) is connected to the fourth port (sampling pump port), and the fifth port (carrier gas inlet) and the sixth port (sample outlet) are both closed.

[0038] The gas path is as follows: sampling probe → heat tracing pipeline → quartz shear chamber → first port of six-way valve → second port of six-way valve → cold trap tube inlet → cold trap tube (passing through the adsorbent bed in the forward direction) → cold trap tube outlet → third port of six-way valve → fourth port of six-way valve → sampling pump → venting.

[0039] At this point, the sheared gas flow enters the cold trap tube. The semiconductor cooling module precisely controls the temperature via a PID controller, with a fluctuation range of ±0.5℃, and the set temperature is kept constant at 0℃. During the 5-minute sampling period, the target aromatic substances are fractionally enriched on the adsorption bed, while gases that are not adsorbed by the multi-adsorbent bed under low-temperature conditions (-10℃ to 5℃) and do not liquefy or condense, mainly including nitrogen, oxygen, helium, argon, as well as non-target gas components and most residual moisture in the sample air, permeate and are discharged. The cumulative sampling volume recorded during the sampling period is 750 mL (150 mL / min × 5 min).

[0040] Step c: Online drying - inert gas purging (refer to...) Figure 2 ):

[0041] After 5 minutes of sampling, the sampling pump is turned off, and the six-way valve is switched to the purge position (position B). At this time, the internal pathways are as follows: the passage connecting the third port (cold trap return) to the external three-way valve is open; the second port (sample outlet) is connected to the venting pipeline (venting through the internal passage of the valve body); and the first, fourth, fifth, and sixth ports are all closed. Simultaneously, the high-purity nitrogen gas source is turned on, and the nitrogen enters the pipeline where the third port of the six-way valve is located through the three-way valve.

[0042] The gas path is as follows: high-purity nitrogen source → three-way valve → six-way valve third port → cold trap tube outlet (reverse) → cold trap tube (reverse through adsorbent bed) → cold trap tube inlet → six-way valve second port → venting.

[0043] The purging parameters were: nitrogen purity >99.999%, flow rate 30 mL / min, purging time 60 seconds, and the semiconductor cooling module maintained at 0°C during the purging process. This step utilizes the difference in retention capacity of different components on the adsorbent at low temperature (0°C)—water has a weaker retention capacity than most target aromatic substances—to selectively remove water from the pores of the adsorbent material and weakly adsorbed water, while simultaneously protecting the heat-sensitive aromatic substances from degradation through low temperature. After purging, the residual moisture content in the cold trap tube was reduced to less than 5% of the original sample volume.

[0044] Step d: Programmed temperature ramp-pulse thermal desorption injection (refer to...) Figure 3 ):

[0045] After purging, shut off the nitrogen supply and switch the six-way valve to the sample injection position (position C). At this time, the internal pathways are as follows: the fifth port (carrier gas inlet) is connected to the third port (cold trap return), the second port (sample outlet) is connected to the sixth port (sample injection outlet), and the first port (sample inlet) and the fourth port (sampling pump port) are both closed.

[0046] Simultaneously, the pulse heating module is activated. This module uses a low-heat-capacity gold-plated quartz tube (2 mm inner diameter, 0.2 mm wall thickness) to directly wrap the adsorption bed, with heating wires wound around the outer wall of the quartz tube. The temperature of the cold trap tube is rapidly increased from 0℃ to 290℃ at a heating rate of 50℃ / s. The heating process is monitored in real time by a PID controller in conjunction with thermocouples, taking approximately 5.8 seconds in total. After reaching the set temperature, the temperature is maintained for 1 minute.

[0047] The gas path is as follows: high-purity helium source → 6-way valve fifth port → 6-way valve third port → cold trap tube outlet (reverse) → cold trap tube (reverse through adsorbent bed) → cold trap tube inlet → 6-way valve second port → 6-way valve sixth port → heat tracing transmission line → GC-MS injection port.

[0048] Helium flow rate was 2.0 mL / min (constant flow mode) to quantitatively transfer the thermally desorbed aroma substances to the gas chromatograph injection port in a narrow pulse. The desorption peak width (half-peak width) was measured to be 2.8 seconds, meeting the requirements for narrow pulse injection. Simultaneously with the six-way valve switching to the injection position, the PLC controller sent a start acquisition signal to the GC-MS, achieving time synchronization between injection and analysis.

[0049] Comparative Example 1

[0050] The solution in Example 1 differs from that in step a, the heat tracing pipeline is shut off and no heating or insulation is performed (the sampling pipeline is at room temperature of 25°C). The remaining steps are the same as in Example 1.

[0051] Comparative Example 2

[0052] The solution in Example 1 differs from that the quartz shear chamber is removed and the sampling pipeline is directly connected to the first port of the six-way valve. The remaining steps are the same as in Example 1.

[0053] Comparative Example 3

[0054] The scheme in Example 1 is different in that the cold trap tube is filled with only a single adsorbent (Porapak Q, 80-100 mesh, 45 mg), without a graded adsorption bed, and the rest of the steps are the same as in Example 1.

[0055] Comparative Example 4

[0056] The scheme of Example 1 is different in that the semiconductor cooling module is turned off in step b, and the cold trap tube is subjected to adsorption and enrichment at room temperature (25°C). The remaining steps are the same as in Example 1.

[0057] Comparative Example 5

[0058] The scheme of Example 1 is different in that step c does not involve inert gas purging. After sampling, the sample is directly switched to the injection position to perform step d, pulsed thermal desorption injection. The remaining steps are the same as in Example 1.

[0059] Comparative Example 6

[0060] The scheme of Example 1 is different in that the cold trap tube is heated to 50°C and purged with nitrogen in step c. The rest of the steps are the same as in Example 1.

[0061] Comparative Example 7

[0062] The scheme of Example 1 is different in that a slow heating rate (10℃ / s) is used for thermal desorption in step d, and the other steps are the same as in Example 1.

[0063] Comparative Example 8

[0064] The scheme of Example 1 is different in that the final desorption temperature in step d is set to 200℃ (instead of 290℃), and the rest of the steps are the same as in Example 1.

[0065] The detailed analysis of GC / MS for Example 1 and Comparative Examples 1-8 is as follows:

[0066] The GC / MS model is Agilent 7890B-5977B, equipped with split / splitless injection ports.

[0067] Chromatographic column: HP-INNOWax capillary column (60 m × 0.25 mm × 0.25 μm), polyethylene glycol stationary phase;

[0068] Temperature program: Initially 40℃ and hold for 3 min, then increase to 230℃ at a rate of 5℃ / min and hold for 10 min, for a total running time of 51 min;

[0069] Inlet temperature: 250℃, splitless injection, inlet liner is deactivated straight-through type;

[0070] Carrier gas: Helium, constant flow mode, flow rate 1.0 mL / min;

[0071] Mass spectrometry conditions: EI source, ionization energy 70 eV, ion source temperature 230℃, quadrupole temperature 150℃, transfer line temperature 250℃;

[0072] Scan mode: Full scan mode, scan range 35-350 amu, scan rate 2 times / second, solvent delay 3 minutes;

[0073] Data acquisition and processing: Data was acquired using an Agilent MassHunter workstation, qualitative analysis was performed by searching the NIST 17 mass spectrometry library and combining it with the retention index, and semi-quantitative analysis was performed using the peak area normalization method.

[0074] The above steps were repeated once every 5 minutes (i.e., sampling for 5 minutes, purging for 60 seconds, pulse desorption injection and chromatographic analysis were performed in parallel, and the total cycle was matched with the chromatographic running time). The entire roasting process was continuously monitored (60 minutes in total), and aroma component fingerprints were obtained at 12 time points (0 min, 5 min, 10 min, 15 min, 20 min, 25 min, 30 min, 35 min, 40 min, 45 min, 50 min, 55 min).

[0075] Example 1: The dynamic changes of aroma compounds at 12 time points in Comparative Examples 1-8 are shown in the following table (in Tables 1 to 9 below, each value is a percentage of the peak area of ​​a single compound relative to the total peak area. Therefore, the unit of data should be percentage content (%)):

[0076] Table 1. Dynamic trends of aroma compounds in Example 1

[0077]

[0078] Table 2. Dynamic trends of aroma compounds in Comparative Example 1

[0079]

[0080] 1 "—" indicates that data is missing due to pipeline blockage.

[0081] Table 3. Dynamic trends of aroma compounds in Comparative Example 2

[0082]

[0083] Table 4. Dynamic trends of aroma compounds in Comparative Example 3

[0084]

[0085] Table 5. Dynamic trends of aroma compounds in Comparative Example 4

[0086]

[0087] Table 6. Dynamic trends of aroma compounds in Comparative Example 5

[0088]

[0089] Table 7. Dynamic trends of aroma compounds in Comparative Example 6

[0090]

[0091] Table 8. Dynamic trends of aroma compounds in Comparative Example 7

[0092]

[0093] Table 9. Dynamic trends of aroma compounds in Comparative Example 8

[0094]

[0095] Results Analysis

[0096] Comparative Example 1: Due to the lack of heating in the sampling pipeline, water vapor condensation caused pipeline blockage, and the experiment was interrupted after the third sampling (15 min), with only data obtained at the 0, 5, and 10 min points (see Table 2). This result indicates that heat tracing and insulation are fundamental conditions for ensuring stable transmission of high-humidity carrier gas; the absence of heat tracing will prevent the system from operating continuously, rendering all subsequent steps meaningless.

[0097] Comparative Example 2: After removing the quartz shear chamber, water mist particles directly entered the cold trap. The experiment was interrupted after the fourth sampling (20 min) due to cold trap blockage, and only data from the first four nodes were obtained (see Table 3). In the obtained 15 min node data, the dimethyl sulfide content was only 0.23% (1.23% in Example 1), and linalool was 2.56% (5.89% in Example 1), with reductions of 81.3% and 56.5%, respectively. This indicates that airflow shearing has a dual effect of physical anti-blockage and chemical dissociation: on the one hand, it breaks up the water mist through high-speed shearing to prevent cold trap blockage; on the other hand, it dissociates the clusters of water molecules and aromatic substances, releasing the encapsulated hydrophobic molecules, significantly improving adsorption efficiency, especially for low-boiling-point sulfur-containing compounds.

[0098] Comparative Example 3: When only Porapak Q was filled, the peak value of dimethyl sulfide at 15 min was 0.21% (1.23% in Example 1), a decrease of 82.9%; the peak value of 2,5-dimethylpyrazine at 40 min was 0.12% (0.78% in Example 1), a decrease of 84.6%; while the peak value of high-boiling linalool oxide II at 45 min was 0.79% (1.08% in Example 1), a decrease of 26.9% (see Table 4). This indicates that the hierarchical adsorption characteristics mainly contribute to broadening the detection spectrum, especially significantly improving the capture efficiency of easily penetrating substances such as small molecule sulfur-containing compounds and moderately polar pyrazines. The absence of carbon molecular sieves and graphitized carbon black layers will lead to severe penetration of low-boiling and medium-boiling components.

[0099] Comparative Example 4: When the cold trap adsorbed at 25°C, the peak value of dimethyl sulfide at 15 min was 0.45% (1.23% in Example 1), a decrease of 63.4%; the peak value of 2,5-dimethylpyrazine at 40 min was 0.31% (0.78% in Example 1), a decrease of 60.3%; and the peak value of linalool at 35 min was 5.64% (8.45% in Example 1), a decrease of 33.3% (see Table 5). It is evident that the low-temperature enrichment characteristic has the most significant enhancing effect on the adsorption of low-boiling-point compounds. Reducing vapor pressure and thus minimizing breakthrough is key to the quantitative recovery of trace substances.

[0100] Comparative Example 5: When the sample was injected directly without purging, the peak areas of each compound decreased slightly (0.79% for dimethyl sulfide at 15 min, a decrease of 35.8%), but the signal-to-noise ratio deteriorated significantly. The signal-to-noise ratio of dimethyl sulfide decreased from 156 to 58.3, a decrease of 62.6%, and the peak tailing was severe. The resolution of linalool oxide I / II decreased from 1.8 to 1.1 (see Table 6). This indicates that the low-temperature purging characteristic mainly contributes to the removal of moisture interference, significantly improving chromatographic resolution and detection sensitivity through selective water removal, while having a relatively small impact on the absolute recovery of the target analytes.

[0101] Comparative Example 6: When the purge temperature was increased to 50°C, the peak value of linalool at 35 min was 5.69% (8.45% in Example 1), a decrease of 32.7%; the peak value of geraniol at 45 min was 2.94% (4.89% in Example 1), a decrease of 39.9%; and the peak value of 2,5-dimethylpyrazine at 40 min was 0.41% (0.78% in Example 1), a decrease of 47.4% (see Table 7). This indicates that excessively high purge temperatures can lead to co-desorption losses of medium-boiling-point target substances. Precise control of the purge temperature (0-10°C) is the core of achieving selective dehydration by utilizing the difference in retention strength between moisture and target substances.

[0102] Comparative Example 7: When desorbed slowly at 10℃ / s, the peak width broadened by 3-4 times, and the peak value of geraniol at 45 min was 3.35% (4.89% in Example 1), a decrease of 31.5%; the resolution of linalool oxide I / II decreased to 0.9 (1.8 in Example 1), making accurate quantification impossible (see Table 8). This indicates that the rapid pulse heating characteristic ensures complete desorption and efficient chromatographic separation, and the heating rate of 50℃ / s allows the target analyte to be injected in a narrow plug shape, avoiding the residue of high-boiling-point compounds and the overlap of isomers.

[0103] Comparative Example 8: At a desorption temperature of only 200℃, the loss of high-boiling-point substances was significant: geraniol's peak value at 45 min was 1.25% (4.89% in Example 1), a decrease of 74.4%; linalool oxide II's peak value at 45 min was 0.40% (1.08% in Example 1), a decrease of 63.0%; while the peak value of low-boiling-point dimethyl sulfide at 15 min was 1.18% (1.23% in Example 1), a decrease of only 4.1% (see Table 9). This demonstrates that high-temperature desorption characteristics (280-300℃) are a necessary condition for the quantitative recovery of high-boiling-point aromatic substances, and insufficient temperature will lead to residues and cross-contamination.

[0104] The above description is merely an embodiment of the present invention and does not limit the patent scope of the present invention. Any equivalent modifications made based on the content of the present invention specification and drawings, or direct or indirect applications in related technical fields, are similarly included within the patent protection scope of the present invention.

Claims

1. An online pretreatment method for real-time measurement of aromatic substances in tea processing, characterized in that, Includes the following steps: Step a: Extract the hot and humid carrier gas containing aromatic substances from the tea processing equipment in real time, heat and keep the hot and humid carrier gas, and then process the heat-supplied carrier gas through airflow shearing. Use the shearing force generated by the high-speed airflow to break and separate the water mist particles entrained in the carrier gas to obtain a homogeneous mixed gas. Step b: The homogeneous mixed gas obtained in step a is passed into a cold trap tube filled with a multi-adsorbent bed and adsorbed and enriched under low temperature conditions of -10℃ to 5℃. Step c: After the adsorption and enrichment are completed, the cold trap tube is back-purged with inert gas while maintaining a low temperature by switching the six-way valve. This selectively removes and drains the co-adsorbed residual moisture, while the target aromatic substance remains on the adsorbent. Step d: After purging, the cold trap tube is rapidly heated to 280-300℃ at a heating rate of 40-60℃ / s by switching the six-way valve, so that the adsorbed aromatic substances are instantly desorbed and transferred to the gas chromatograph-mass spectrometer by backwashing with carrier gas in a narrow pulse form for analysis. The six-way valve has three working positions: in the sampling position, it connects the sample inlet to the cold trap tube inlet and the cold trap tube outlet to the sampling pump, allowing the sample gas to pass through the cold trap tube in the forward direction for adsorption and enrichment; in the purging position, it connects the inert gas source to the cold trap tube outlet, allowing the inert gas to pass through the cold trap tube in the reverse direction for purging and dehydration; in the sample injection position, it connects the carrier gas source to the cold trap tube outlet and the cold trap tube inlet to the sample inlet, allowing the carrier gas to pass through the cold trap tube in the reverse direction to carry the desorbed aromatic substances into the analytical instrument; the gas flow shearing treatment is achieved through a shear chamber with a Venturi structure, where the gas flow velocity is accelerated to 5-10 m / s; The shear chamber is made of quartz glass and consists of an inlet constriction section, a throat section, and an outlet diffuser section in sequence along the airflow direction. The inlet constriction section is 12 mm long, 6 mm inlet diameter, and has a constriction angle of 21°. The throat section is 2 mm long and 2 mm in diameter. The outlet diffuser section is 18 mm long, 6 mm in outlet diameter, and has a diffuser angle of 12°. The multi-adsorbent bed in step b, along the airflow direction, sequentially includes: a hydrophobic, highly cross-linked polystyrene resin for adsorbing high-boiling-point volatile components, graphitized carbon black for adsorbing terpenoid compounds, and a carbon molecular sieve for adsorbing low-boiling-point small-molecule sulfur-containing compounds; the tea processing process includes fixation, drying, or roasting steps; the aromatic substances include terpenoid alcohols, pyrazines, sulfur-containing compounds, and Maillard reaction intermediates.

2. The online pretreatment method for real-time measurement of aromatic substances in tea processing according to claim 1, characterized in that, The extraction flow rate of the humid heat carrier gas in step a is 50-200 mL / min, and the temperature of the heat tracing and insulation is 110-130℃.

3. The online pretreatment method for real-time measurement of aromatic substances in tea processing according to claim 1, characterized in that, The hydrophobic highly crosslinked polystyrene resin is Porapak Q with a particle size of 80-100 mesh; the graphitized carbon black is Carbopack B with a particle size of 60-80 mesh; and the carbon molecular sieve is Carbosieve S-III.

4. The online pretreatment method for real-time measurement of aromatic substances in tea processing according to claim 1, characterized in that, The adsorption and enrichment time in step b is 1-5 minutes.

5. The online pretreatment method for real-time measurement of aromatic substances in tea processing according to claim 1, characterized in that, The inert gas mentioned in step c is high-purity nitrogen, the purging flow rate is 20-50 mL / min, the purging time is 30-90 seconds, and the temperature of the cold trap tube is maintained at 0-10℃ during the purging process.

6. The online pretreatment method for real-time measurement of aromatic substances in tea processing according to claim 1, characterized in that, The carrier gas in step d is helium, the backwash flow rate is 1.5-3 mL / min, and the peak width of the narrow pulse is less than 3 seconds.

Images