阅读说明：本技术 一种基于低场核磁共振技术快速无损检测干燥过程中绿茶含水率及干燥程度的方法 (Method for rapidly and nondestructively detecting moisture content and drying degree of green tea in drying process based on low-field nuclear magnetic resonance technology ) 是由 杨艳芹 袁海波 江用文 邓余良 董春旺 李佳 滑金杰 王近近 于 2020-11-10 设计创作，主要内容包括：一种基于低场核磁共振技术快速无损检测干燥过程中绿茶含水率及干燥程度的方法,属于化学分析技术领域。该方法通过建立低场核磁T2弛豫谱总信号幅值与茶样含水率的模型方程：y=8E-05x–0.0143,拟合度R～2=0.967,检测干燥过程中待测茶样含水率时,先对其进行低场核磁共振分析,并进行归一化处理,得到低场核磁T2弛豫谱总信号幅值,代入上述模型方程,从而得到待测茶样的含水率。本发明方法将低场核磁共振分析技术应用于绿茶干燥过程,具有非破坏性和非侵入性,不会对样品造成物理破坏和化学污染,为茶叶在制品水分状态在线检测和品质控制提供了重要的技术支撑。(A method for rapidly and nondestructively detecting the water content and the drying degree of green tea in a drying process based on a low-field nuclear magnetic resonance technology belongs to the technical field of chemical analysis. The method comprises the following steps of establishing a model equation of the total signal amplitude of the low-field nuclear magnetic T2 relaxation spectrum and the water content of the tea sample: y =8E-05 x-0.0143, degree of fitting R 2 And =0.967, when the water content of the tea sample to be detected in the drying process is detected, low-field nuclear magnetic resonance analysis is firstly carried out on the tea sample, normalization processing is carried out to obtain the total signal amplitude of the low-field nuclear magnetic T2 relaxation spectrum, and the total signal amplitude is substituted into the model equation to obtain the water content of the tea sample to be detected. The method applies the low-field nuclear magnetic resonance analysis technology to the drying process of the green tea, has non-destructiveness and non-invasiveness, does not cause physical damage and chemical pollution to the sample, and provides important technical support for online detection and quality control of the moisture state of the tea product.)

1. A method for rapidly and nondestructively detecting the water content and the drying degree of green tea in a drying process based on a low-field nuclear magnetic resonance technology is characterized in that a model equation of the total signal amplitude of a low-field nuclear magnetic T2 relaxation spectrum and the water content of a tea sample is established, when the water content of the tea sample to be detected in the drying process is detected, low-field nuclear magnetic resonance analysis is firstly carried out on the tea sample to be detected to obtain the total signal amplitude of the low-field nuclear magnetic T2 relaxation spectrum, the total signal amplitude is substituted into the model equation to further obtain the water content of the tea sample to be detected, when the detected total signal amplitude of the T2 relaxation spectrum is less than or equal to 1053.75, namely the corresponding water content is:

y = 8E-05x – 0.0143

wherein x represents the total signal amplitude of the low-field nuclear magnetism T2 relaxation spectrum, y represents the water content of the tea sample, E represents the exponential power with 10 as the base in the scientific calculation method, and the fitting degree R²=0.967。

2. The method for rapidly and nondestructively detecting the water content and the drying degree of green tea in the drying process based on the low-field nuclear magnetic resonance technology as claimed in claim 1, which is characterized by comprising the following steps:

(1) weighing 6 parts of tea sample with the same mass, and performing tea sample preparation and water content determination in the drying process;

(2) performing low-field nuclear magnetic resonance analysis on the 6 parts of dried tea samples to obtain a total signal amplitude of a low-field nuclear magnetic T2 relaxation spectrum corresponding to the unit mass of the tea samples;

(3) fitting the total signal amplitude of the low-field nuclear magnetic T2 relaxation spectrum with x and the water content with y to obtain a model equation with the fitting degree of 0.967;

(4) and (4) taking the tea sample to be detected as a sample, carrying out low-field nuclear magnetic resonance analysis and detection to obtain the total signal amplitude of the T2 relaxation spectrum, and substituting the total signal amplitude into the model equation in the step (3) to obtain the water content of the tea sample to be detected.

3. The method for rapidly and nondestructively detecting the moisture content and the drying degree of green tea in the drying process based on the low-field nuclear magnetic resonance technology as claimed in claim 2, wherein the step (1) is specifically as follows: the tea sample is sequentially and naturally spread indoors, roller-type enzyme deactivation, slight rolling and rough fire drying treatment are carried out, 6 parts of tea sample with the same mass is weighed and respectively placed in a stainless steel sieve tray, the stainless steel sieve tray is placed in a dryer at 85 ℃ and is respectively dried for 5min, 10min, 20min, 30min, 40min and 50min and then taken out, the weight of the 6 parts of tea sample is measured, the water content of the tea sample is measured by a rapid water content meter, and the average value of the tea sample is obtained after three times of repeated measurement.

4. The method for rapidly and nondestructively detecting the moisture content and the drying degree of green tea in the drying process based on the low-field nuclear magnetic resonance technology as claimed in claim 2, wherein the step (2) is specifically as follows: respectively weighing 1g of the 6 dried tea samples, placing the tea samples into a magnet coil, optimally setting low-field nuclear magnetic resonance conditions, acquiring nuclear magnetic resonance signals by adopting a CPMG sequence to obtain a CPMG attenuation map, inverting the measured transverse relaxation time frequency image by software to obtain an inversion map, obtaining a corresponding transverse relaxation time distribution curve and a relaxation peak area, and normalizing test data to obtain a total signal amplitude of the low-field nuclear magnetic T2 relaxation map corresponding to a unit mass of tea sample.

5. The method for rapid nondestructive testing of the moisture content and the drying degree of green tea in the drying process based on the low-field nuclear magnetic resonance technology as claimed in claim 4, wherein the low-field nuclear magnetic resonance condition is as follows: the nuclear magnetism type is a permanent magnet, the generation frequency of a nuclear magnetism generator is 23 MHz, and the pulse width of 90 degrees P1: 10 mu s; 180 ° pulse width P2: 19.52 mu s; receiver bandwidth SW: 333.333 KHz; time delay DL 1: 0.06524 ms; analog gain RG 1: 20db of; digital gain DRG 1: 3, pre-amplification gain PRG: 3; control parameter RFD of start sampling time is 0.08ms, time delay DL 1: 0.06524 ms; 2000, complete relaxation of the sample signal; oversampling wait time TW: 1000 ms; setting the repeated sampling times NS: 64.

6. the method for rapid nondestructive testing of moisture content and dryness of green tea in drying process based on low-field nuclear magnetic resonance technology as claimed in claim 2, wherein the model equation in step (3) is: y =8E-05 x-0.0143, degree of fitting R²=0.967。

7. The method for rapidly and nondestructively detecting the moisture content and the drying degree of green tea in the drying process based on the low-field nuclear magnetic resonance technology as claimed in claim 2, wherein the step (4) is specifically as follows: and (3) detecting by using a low-field nuclear magnetic resonance analysis method to obtain the total signal amplitude of the low-field nuclear magnetic T2 relaxation spectrum of the sample to be detected, normalizing to obtain the total signal amplitude of the low-field nuclear magnetic T2 relaxation spectrum of the tea sample to be detected in unit mass, and substituting the total signal amplitude into the model equation obtained in the step (3) to obtain the water content of the tea sample to be detected.

Technical Field

The invention belongs to the technical field of chemical analysis, and particularly relates to a method for rapidly and nondestructively detecting the moisture content and the drying degree of green tea in a drying process based on a low-field nuclear magnetic resonance technology.

Background

The green tea is one of six major tea types in China, is also the tea type with the largest yield and sale amount and the most advanced processing technology in China, and can be divided into fried green tea, baked green tea, steamed green tea and sun-dried green tea according to different manufacturing processes. According to the different characteristics of the green tea aroma such as type, smell and the like, the green tea is subdivided into different aroma types such as flower aroma, faint scent, chestnut aroma and the like. The tea aroma is the peculiar aroma of tea formed by combining different aromatic substances in different concentrations and comprehensively acting on olfactory nerves. More than 300 kinds of green tea aroma components are separated and identified at present.

The main preparation procedures of the green tea comprise fresh leaves, spreading, enzyme deactivation, rolling and drying. Drying is the main processing process of tea moisture loss, and the moisture loss is accompanied with the generation of aroma and flavor substances, so that the tea has the special qualities of color, aroma, taste, shape and the like. The drying plays an important role in fixing the quality and has an important influence on the postnatal storage of the tea, which is mainly reflected in the water content. The national standard (GB/T14456.1-2017) requires that the moisture content of green tea (pan-fired green tea, baked green tea and steamed green tea, but not sun-dried green tea) is less than or equal to 7 percent. Mujie believes that in the drying process, the fire and the water loss degree of the tea leaves are properly controlled, and good fragrance can be obtained. In addition, the water content can also be used as an index of the degree of drying temperature and time. At present, methods for detecting the water content of tea mainly comprise a normal-pressure constant-temperature drying method, a time prediction method, an exhaust humidity detection method and the like. However, the detection methods have the defects of complicated operation, long time consumption and the like to different degrees, so that the rapid nondestructive detection of the water content of the tea leaves becomes a problem to be solved urgently.

Low-Field Nuclear Magnetic Resonance (LF-NMR) is also known as Low-resolution NMR, i.e. Nuclear Magnetic Resonance with a Magnetic Field strength below 0.5T, is usually used for measuring physical properties of substances, is mainly used for detection of lipid content in food, research on moisture content in food and its existing state, and has the advantages of rapidness, no damage, accuracy, and the like. The principle is that the relaxation time of hydrogen protons in different existing states in a magnetic field is different, and the relaxation time of the hydrogen protons in a sample is analyzed to obtain the information related to the distribution condition and the water migration of water in the sample. At present, the technology is widely used for researching the water distribution state and change in the processing process of pork, nuts, common fruits and vegetables and other food. Zhang Shunkun et al applied the transverse relaxation time T2 inversion spectrum of low-field nuclear magnetic resonance to analyze the internal moisture change process of carrot slices at different drying temperatures. Hanne et al demonstrated a strong correlation between pork water retention as measured by conventional methods and transverse relaxation time T2 as measured by NMR, demonstrating that low-field nuclear magnetic resonance techniques are an effective method for pork water retention. The Chenwenyu and the like determine the change rule of water in the drying process of the macadimia nuts by utilizing a low-field nuclear magnetic resonance technology, analyze the change conditions of free water, semi-combined water and combined water in the drying process, find out the relation between the water content and a transverse relaxation peak, and provide an important technical basis for rapidly predicting the water content in the drying process of the macadimia nuts. At present, the application research of the low-field nuclear magnetic resonance technology in the field of tea is very little, and the application of the transverse relaxation spectrum to the detection of the water content of a product in the drying process of green tea is not reported.

Disclosure of Invention

Aiming at the defects in the prior art, the invention aims to design and provide a method for rapidly and nondestructively detecting the water content and the drying degree of green tea in the drying process based on a low-field nuclear magnetic resonance technology. The method utilizes a transverse relaxation time inversion spectrum technology of a low-field nuclear magnetic resonance technology to rapidly detect the water content of the green tea in the drying process in a nondestructive mode.

A method for rapidly and nondestructively detecting the water content and the drying degree of green tea in a drying process based on a low-field nuclear magnetic resonance technology is characterized in that a model equation of the total signal amplitude of a low-field nuclear magnetic T2 relaxation spectrum and the water content of a tea sample is established, when the water content of the tea sample to be detected in the drying process is detected, low-field nuclear magnetic resonance analysis is firstly carried out on the tea sample to be detected to obtain the total signal amplitude of the low-field nuclear magnetic T2 relaxation spectrum, the total signal amplitude is substituted into the model equation to further obtain the water content of the tea sample to be detected, when the detected total signal amplitude of the T2 relaxation spectrum is less than or equal to 1053.75, namely the corresponding water content is:

y = 8E-05x – 0.0143

wherein x represents the total signal amplitude of the low-field nuclear magnetism T2 relaxation spectrum, y represents the water content of the tea sample, E represents the exponential power with 10 as the base in the scientific calculation method, and the fitting degree R²=0.967。

The method for rapidly and nondestructively detecting the water content and the drying degree of the green tea in the drying process based on the low-field nuclear magnetic resonance technology is characterized by comprising the following steps:

(1) weighing 6 parts of tea sample with the same mass, and performing tea sample preparation and water content determination in the drying process;

(2) performing low-field nuclear magnetic resonance analysis on the 6 parts of dried tea samples to obtain a total signal amplitude of a low-field nuclear magnetic T2 relaxation spectrum corresponding to the unit mass of the tea samples;

(3) fitting the total signal amplitude of the low-field nuclear magnetic T2 relaxation spectrum with x and the water content with y to obtain a model equation with the fitting degree of 0.967;

(4) and (4) taking the tea sample to be detected as a sample, carrying out low-field nuclear magnetic resonance analysis and detection to obtain the total signal amplitude of the T2 relaxation spectrum, and substituting the total signal amplitude into the model equation in the step (3) to obtain the water content of the tea sample to be detected.

The method for rapidly and nondestructively detecting the water content and the drying degree of the green tea in the drying process based on the low-field nuclear magnetic resonance technology is characterized in that the step (1) specifically comprises the following steps: the tea sample is sequentially and naturally spread indoors, roller-type enzyme deactivation, slight rolling and rough fire drying treatment are carried out, 6 parts of tea sample with the same mass is weighed and respectively placed in a stainless steel sieve tray, the stainless steel sieve tray is placed in a dryer at 85 ℃ and is respectively dried for 5min, 10min, 20min, 30min, 40min and 50min and then taken out, the weight of the 6 parts of tea sample is measured, the water content of the tea sample is measured by a rapid water content meter, and the average value of the tea sample is obtained after three times of repeated measurement.

The method for rapidly and nondestructively detecting the water content and the drying degree of the green tea in the drying process based on the low-field nuclear magnetic resonance technology is characterized in that the step (2) specifically comprises the following steps: respectively weighing 1g of the 6 dried tea samples, placing the tea samples into a magnet coil, optimally setting low-field nuclear magnetic resonance conditions, acquiring nuclear magnetic resonance signals by adopting a CPMG sequence to obtain a CPMG attenuation map, inverting the measured transverse relaxation time frequency image by software to obtain an inversion map, obtaining a corresponding transverse relaxation time distribution curve and a relaxation peak area, and normalizing test data to obtain a total signal amplitude of the low-field nuclear magnetic T2 relaxation map corresponding to a unit mass of tea sample.

The method for rapidly and nondestructively detecting the water content and the drying degree of the green tea in the drying process based on the low-field nuclear magnetic resonance technology is characterized in that the low-field nuclear magnetic resonance conditions are as follows: the nuclear magnetism type is a permanent magnet, the generation frequency of a nuclear magnetism generator is 23 MHz, and the pulse width of 90 degrees P1: 10 mu s; 180 ° pulse width P2: 19.52 mu s; receiver bandwidth SW: 333.333 KHz; time delay DL 1: 0.06524 ms; analog gain RG 1: 20db of; digital gain DRG 1: 3, pre-amplification gain PRG: 3; control parameter RFD of start sampling time is 0.08ms, time delay DL 1: 0.06524 ms; 2000, complete relaxation of the sample signal; oversampling wait time TW: 1000 ms; setting the repeated sampling times NS: 64.

the method for rapidly and nondestructively detecting the water content and the drying degree of the green tea in the drying process based on the low-field nuclear magnetic resonance technology is characterized in that the model equation in the step (3) is as follows: y =8E-05 x-0.0143, degree of fitting R²=0.967。

The method for rapidly and nondestructively detecting the water content and the drying degree of the green tea in the drying process based on the low-field nuclear magnetic resonance technology is characterized in that the step (4) specifically comprises the following steps: and (3) detecting by using a low-field nuclear magnetic resonance analysis method to obtain the total signal amplitude of the low-field nuclear magnetic T2 relaxation spectrum of the sample to be detected, normalizing to obtain the total signal amplitude of the low-field nuclear magnetic T2 relaxation spectrum of the tea sample to be detected in unit mass, and substituting the total signal amplitude into the model equation obtained in the step (3) to obtain the water content of the tea sample to be detected.

The method applies the low-field nuclear magnetic analysis technology to the green tea moisture content analysis, has non-destructive and non-invasive properties, does not cause physical damage and chemical pollution to the sample, and provides a basis for the nondestructive online detection of the sample. The method has the characteristics of rapidness, convenience and accuracy, can quickly feed back the drying state of the tea by high-efficiency detection in the actual production process, and is favorable for timely adjusting the drying process parameters; the method can realize real-time online measurement, and provides effective technical support for monitoring the real-time change of the moisture content in the drying process of the green tea.

Drawings

FIG. 1 is a low-field NMR T2 spectrum of green tea at different drying times;

FIG. 2 is a model equation of the amplitude and water content of the low-field nuclear magnetic total signal.

Detailed Description

The invention will be further explained with reference to the drawings and examples.

Example 1:

1 experimental part

1.1 instruments and devices

Model ME103E electronic balance, mettler-toledo (shanghai) instruments ltd; MesoMR23-060H-I low field nmr analyzer, ny analyzer, su; type 80 electromagnetic de-enzyming machine, yaojiang tea machine ltd, yao, city; 6CR-55 tea twisting machine, Zhejiang Shanghai mechanical Co., Ltd; model CS-90XZ baking machine, quanzhou changsheng tea machine, inc.

1.2 materials and reagents

The fresh tea leaves are collected in 4 months in 2020, Shengzhou city in Zhejiang province, the variety is Zhongcha 108, and the tenderness is from one bud to one bud and two leaves. The specific processing technology is as follows: spreading (indoor temperature is 25-28 ℃, humidity is 70-80%, moisture content is reduced to about 70%) → roller type enzyme deactivation (leaf throwing amount is 150 kg/h, 260 ℃, 75 s, moisture content is 47%) → rolling (30 kg leaf throwing amount, rotating speed is 50-52 r/min, lightly rolling is 30min, moisture content is 45%) → gross fire drying (110 ℃, 20min, moisture content is 20%) and then respectively weighing 80g, placing in a stainless steel sieve tray, then respectively drying in a box type drying machine at 85 ℃ for 5min, 10min, 20min, 30min, 40min and 50min, then taking out, measuring the moisture content by using a rapid moisture meter, and repeatedly measuring for three times to obtain the average value.

1.3 Low-field NMR detection conditions

Weighing about 1g of tea sample, putting the tea sample into a magnet coil, optimally setting low-field nuclear magnetic resonance detection conditions, and analyzing CPMG sequences with specific parameters as follows: the nuclear magnetism type is a permanent magnet, the generation frequency of a nuclear magnetism generator is 23 MHz, and the pulse width of 90 degrees P1: 10 mu s; 180 ° pulse width P2: 19.52 mu s; receiver bandwidth SW: 333.333 KHz; time delay DL 1: 0.06524 ms; analog gain RG 1: 20db of; digital gain DRG 1: 3, pre-amplification gain PRG: 3; control parameter RFD of start sampling time is 0.08ms, time delay DL 1: 0.06524 ms; 2000, complete relaxation of the sample signal; oversampling wait time TW: 1000 ms; setting the repeated sampling times NS: and 64, acquiring nuclear magnetic resonance signals to obtain a CPMG attenuation map of each sample, and inverting the measured transverse relaxation time T2 frequency image through software to obtain an inversion spectrogram. In order to improve the sampling accuracy, the test data is normalized, the influence of dry matters is eliminated, and the moisture signal amplitude corresponding to the unit mass sample is obtained.

2 results and analysis

2.1 internal moisture distribution State of Green tea during drying

The nuclear magnetic resonance transverse relaxation time spectrum of the green tea dried sample has 3 peaks in common, and the 3 peaks correspond to 3 moisture states, so as to obtain each transverse relaxation time range, wherein T21 is a combined water area mainly based on chemical adsorption; t22 is a semi-bound water region with capillary adsorption as the main component; t23 is a free water zone with poor binding capacity. Compared with other two forms of water, the free water is easier to absorb energy in a magnetic field to generate vibration, and needs longer time to recover to a ground state after the magnetic field disappears; in contrast to free water, bound water (T21) does not readily absorb energy in a magnetic field, and the time to return to the ground state is shorter, typically around 1 ms.

Fig. 1 is a low field nmr T2 spectrum of green tea at different drying times. The integral areas and area ratios of the relaxation peaks (T21, T22 and T23) reflect the moisture content in different states, and the larger the integral areas and the area ratios are, the higher the moisture content is. As can be seen from fig. 1, the green tea has the highest content of bound water, followed by semi-bound water, and the very least content of free water in the product sample during drying; furthermore, the most obvious reason for the decline of bound water during drying is probably that as the drying process advances, part of the bound water is converted into semi-bound water and then converted into free water to be lost.

2.2 Low-field Nuclear magnetic transverse relaxation Peak area to Water content relationship

Fitting is carried out by taking the obtained low-field nuclear magnetic T2 atlas total signal amplitude (T21, T22 and T23) as x and the water content as y, as shown in figure 2, the equation obtained by fitting is y =8E-05 x-0.0143, and the degree of fitting R is²=0.967。

Example 2: detecting a sample to be tested

Weighing about 1g of tea sample to be detected (drying for 35 min at the same drying temperature of 85 ℃) and placing the tea sample into a magnet coil, optimally setting low-field nuclear magnetic resonance detection conditions, wherein the specific parameters of CPMG sequence analysis are as follows: the nuclear magnetism type is a permanent magnet, the generation frequency of a nuclear magnetism generator is 23 MHz, and the pulse width of 90 degrees P1: 10 mu s; 180 ° pulse width P2: 19.52 mu s; receiver bandwidth SW: 333.333 KHz; time delay DL 1: 0.06524 ms; analog gain RG 1: 20; digital gain DRG 1: 3, pre-amplification gain PRG: 3; control parameter RFD of start sampling time is 0.08ms, time delay DL 1: 0.06524 ms; 2000, complete relaxation of the sample signal; oversampling wait time TW: 1000 ms; setting the repeated sampling times NS: and 64, acquiring nuclear magnetic resonance signals to obtain a CPMG attenuation map of the sample, and performing software inversion on the measured transverse relaxation time T2 frequency image to obtain an inversion map, wherein the total signal amplitude is 1775.521. In order to improve the sampling accuracy, the test data is normalized to eliminate the influence of dry matters, and the moisture signal amplitude 984.104 corresponding to the unit mass sample is obtained.

And substituting the equation y =8E-05 x-0.0143 to obtain the water content of the sample to be tested, which is 6.44%.

The moisture content of the tea sample is measured to be 6.12% by adopting a rapid moisture tester, the relative standard deviation is less than 5% and is 3.7%, the error is small, and the accuracy is high. The method has the advantages of high detection speed, high precision, non-destructiveness and non-invasiveness, and provides an important reference for realizing the online detection of the moisture content of the green tea.

Example 3: detecting a sample to be tested

Weighing about 1g of tea sample to be detected, putting the tea sample into a magnet coil, optimally setting low-field nuclear magnetic resonance detection conditions, and analyzing CPMG sequences with specific parameters as follows: the nuclear magnetism type is a permanent magnet, the generation frequency of a nuclear magnetism generator is 23 MHz, and the pulse width of 90 degrees P1: 10 mu s; 180 ° pulse width P2: 19.52 mu s; receiver bandwidth SW: 333.333 KHz; time delay DL 1: 0.06524 ms; analog gain RG 1: 20; digital gain DRG 1: 3, pre-amplification gain PRG: 3; control parameter RFD of start sampling time is 0.08ms, time delay DL 1: 0.06524 ms; 2000, complete relaxation of the sample signal; oversampling wait time TW: 1000 ms; setting the repeated sampling times NS: and 64, acquiring nuclear magnetic resonance signals to obtain a CPMG attenuation map of the sample, and performing software inversion on the measured transverse relaxation time T2 frequency image to obtain an inversion map, wherein the total signal amplitude is 1270.897. In order to improve the sampling accuracy, the test data is normalized to eliminate the influence of dry matters, and the moisture signal amplitude 759.833 corresponding to the unit mass sample is obtained.

And substituting the equation y =8E-05 x-0.0143 to obtain the water content of the sample to be detected of 4.64%.

The moisture content of the tea sample is measured to be 4.44% by adopting a rapid moisture tester, the relative standard deviation is less than 5% and 3.19%, the error is small, and the accuracy is high. The method has the advantages of high detection speed, high precision, non-destructiveness and non-invasiveness, and provides an important reference for realizing the online detection of the moisture content of the green tea.