An apparatus and method for detecting acetic acid and lactic acid in Daqu (a type of starter culture) and fermented mash.

Abstract

This invention relates to the field of separation, analysis, and detection technology, specifically an apparatus and method for detecting formic acid and acetic acid in baijiu (Chinese liquor) starter and mash. The apparatus includes a rotary thermal desorption sampler, a headspace thermal desorption sampler, pulse valve I, pulse valve II, ion migration tube I, and ion migration tube II. The gas paths are as follows: one path connects to the rotary thermal desorption sampler via pulse valve I and then to ion migration tube I; the other path connects to the headspace thermal desorption sampler via pulse valve II and then to ion migration tube II. Drift gas is connected to dopant gas and then enters the ion migration tube. After sample treatment with methanol, samples for acetic acid detection are injected via the rotary thermal desorption sampler, and samples for lactic acid detection are injected via the headspace thermal desorption sampler. This invention allows for the separate separation and analysis of lactic acid and acetic acid components, reducing the mutual interference of simultaneous detections. Samples only require methanol extraction for analysis, saving four times the time.

Description

Technical Field

[0001] This invention relates to the field of separation, analysis and detection technology, and specifically to an apparatus and method for detecting formic acid and acetic acid in baijiu daqu (a type of starter culture) and mash. Background Technology

[0002] The production of Daqu liquor prioritizes quality, requiring a high alcohol yield. The yield depends on the fermentation quality, and the main factors influencing this quality are the temperature, acidity, water, and starch content of the mash before it enters the fermentation pit. Water and starch are relatively easy to control, while temperature and acidity are difficult to control but are crucial. High initial pit temperature leads to rapid temperature and acidification; high initial pit acidity hinders fermentation and results in a low yield. Conversely, while low initial pit acidity may be beneficial for yield, it lacks the matrix for esterification, resulting in insufficient aroma components and poor product quality.

[0003] The acids in liquor are mainly composed of organic acids, primarily derived from acetic acid, propionic acid, butyric acid, lactic acid, hexanoic acid, and higher acids produced during the fermentation process. Most of these exist in a free state, with a small portion existing as salts. When calculating total acidity, organic acids are the primary factor, converted to acetic acid content. Organic acids are generally weak acids; only a small portion of weak acids ionizes in aqueous solutions, with the majority remaining as unionized molecules. For example, the acid content of a strong acid in aqueous solution should show a good linear relationship with the pH value. However, since most acids in liquor are organic, their types and contents are not fixed due to the influence of fermentation, distillation, and other factors. Even within the same brewing group, different fermentation pits will produce liquors with inconsistent values. Gas chromatography is commonly used to detect flavor components in baijiu; however, due to the thermal instability of lactic acid, it cannot be accurately quantified using gas chromatography. Because lactic acid readily decomposes upon heating in the vaporization chamber, mass spectrometry analysis only detects its decomposition product, dioxane. Therefore, direct injection analysis is not possible, and other methods must be employed. Lactic acid is an important acid, and its detection is often done quantitatively using colorimetric methods employed in conventional chemical analysis; however, it does not fall under the concept of acidity in food analysis. According to literature reports, methods for detecting lactic acid in baijiu include liquid chromatography, benzyl esterification, ion chromatography, and near-infrared spectroscopy. While each method has its advantages, they also suffer from complex pretreatment, cumbersome operation, and inaccurate quantification. Finding a simple, rapid, and accurate method for detecting lactic acid in baijiu is of great guiding significance for the production of baijiu base liquor and the improvement of baijiu quality. Summary of the Invention

[0004] To address the problems existing in the prior art, this invention provides an apparatus and method for detecting acetic acid and lactic acid in Daqu (a type of starter culture) and fermented mash, including an extraction method for acetic acid and lactic acid in Daqu and fermented mash, a sample introduction device, and a separation and detection device. Acetic acid and lactic acid are separated separately based on temperature gradient thermal desorption technology, enabling separate qualitative and quantitative analysis and detection of target samples of acetic acid and lactic acid.

[0005] To achieve the above-mentioned objectives, the present invention adopts the following technical solution:

[0006] This invention provides a device for rapid detection of acetic acid and lactic acid. The device includes a rotary thermal desorption sampler, a headspace thermal desorption sampler, pulse valve I, pulse valve II, ion migration tube I, and ion migration tube II. The gas paths of the device are as follows: one path connects to the rotary thermal desorption sampler via pulse valve I and then to ion migration tube I; the other path connects to the headspace thermal desorption sampler via pulse valve II and then to ion migration tube II. The drift gas is connected to dopant gas and then enters the ion migration tube.

[0007] In the above technical solution, the carrier gas source for the acetic acid detection device flows into the rotary thermal desorption sampler via pulse valve I, and the rotary thermal desorption sampler is connected to the ion migration tube I; the carrier gas source for the lactic acid detection device flows into the headspace thermal desorption sampler via pulse valve II, and the headspace thermal desorption sampler is connected to the ion migration tube II via a sampling tube.

[0008] In the above technical solution, the headspace thermal desorption sampler further includes a sampling tube and a sampling bottle, the sampling bottle is covered with a heating element, and the heating element is connected to a temperature control device for temperature control.

[0009] In the above technical solution, the gas circuit working mode is that the pulse valve is open for 30-60 seconds during sample injection, and closed in other states.

[0010] This invention also provides a rapid detection method for acetic acid and lactic acid in baijiu daqu (a type of starter culture) and mash. The method uses the apparatus described in claim 1 for detection. The sample is respectively subjected to low-temperature thermal desorption by a rotary thermal desorption sampler into ion migration tube I, and high-temperature thermal desorption by a headspace thermal desorption sampler into ion migration tube II for detection.

[0011] In the above technical solution, the low-temperature thermal desorption temperature is 50-60℃, and the high-temperature thermal desorption temperature is 150-180℃.

[0012] In the above technical solution, the sample is pretreated by methanol extraction before injection, and the concentration of the sample in methanol is 1-1000 ng / μl. The sample is diluted during detection.

[0013] In the above technical solution, the dopant is further defined as acetone, or butanone, or a mixture of acetone and butanone.

[0014] In the above technical solution, methanol is further added to the dopant, and the volume ratio of methanol to dopant is 1:1 to 1:2.

[0015] In the above technical solution, the standard curve for quantitative analysis of acetic acid is a fitting of different concentrations and signal intensities, and the standard curve for quantitative analysis of lactic acid is a common logarithmic fitting of different concentrations and signal intensities.

[0016] Because the lactic acid and acetic acid contents in Daqu (a type of starter culture) and fermented mash differ significantly, and their thermal desorption temperatures differ, the products formed at different thermal desorption temperatures also differ considerably. Therefore, the method of this invention employs a dual-injection system for the same sample to separate acetic acid and lactic acid for analysis. This reduces interference between them and improves the accuracy of target sample measurements.

[0017] The beneficial effects of this invention are:

[0018] 1. The method in this invention utilizes the concept of programmed temperature rise to separate and analyze lactic acid and acetic acid components separately based on different thermal desorption temperatures, thereby reducing mutual interference when they are detected simultaneously.

[0019] 2. Each carrier gas path is connected to a pulse valve to switch and control the carrier gas supply, avoiding real-time transmission of carrier gas to dilute the sample gas, which can improve the sensitivity of lactic acid detection.

[0020] 3. Ion mobility spectrometry is used to analyze the lactic acid and acetic acid content in Daqu and fermented mash. It does not require complicated and harsh sample pretreatment, but only methanol extraction is required, which reduces the separation time of the chromatographic column and saves the analysis time by 4 times. Attached Figure Description

[0021] Figure 1 Schematic diagram of the detection device of the present invention;

[0022] Figure 2 This is a qualitative analysis chromatogram of acetic acid detected by ion mobility spectrometry.

[0023] Figure 3 This is a chromatogram for qualitative analysis of lactic acid using ion mobility spectrometry.

[0024] Figure 4 Fitting calibration curves for quantitative analysis of acetic acid using ion mobility spectrometry;

[0025] Figure 5 Fitted calibration curve for quantitative analysis of lactic acid by ion mobility spectrometry. Detailed Implementation

[0026] The present invention will be further described below with reference to specific embodiments, but this does not limit the present invention in any way.

[0027] The ion mobility spectrometer described in this embodiment uses a VUV lamp ionization source and acetone as the dopant. The ion mobility spectrometer mainly includes two sample introduction devices: a rotary thermal desorption sampler (using the thermal desorption sampler in patent 201721139511.7) and a headspace thermal desorption sampler; two ion mobility tube detectors (using the ion mobility tube in patent 201811412007.9); a signal receiving and detection system; and a gas path drying system. Before sample introduction, the sample undergoes methanol extraction pretreatment. The Daqu (fermentation starter) is weighed at 0.1-1g, the fermented mash at 10-20mg, and 2-4ml of methanol is used.

[0028] Example 1

[0029] Acetone was used as the dopant, with a low-temperature thermal desorption temperature of 50℃ and a migration tube temperature of 100℃; the bleaching gas (air) was 600 ml / min, the carrier gas (air) was 400 ml / min, and acetic acid (acetic acid standard) was injected in 5 μl.

[0030] The sample is injected into the rotary thermal desorption sampler via pulse valve I, and then into ion migration tube I. Pulse valve I is open for 30 seconds during injection and closed afterward. The ion migration spectrum is then analyzed as follows. Figure 2 As shown, the migration time of acetic acid is 3.88 ms.

[0031] Example 2

[0032] Based on the signal peak of acetic acid migration time at 3.88 ms, signal intensities at concentrations of 0.5, 1, 2.5, 5, 7.5, 10, 20, and 30 μg / mL were measured, and quantitative standard curves were fitted between different concentrations and their corresponding signal intensities, as shown below. Figure 3 As shown, within the concentration range of 0.5-30 μg / mL, the standard curve equation is Y = 1508.5 * X + 5498.4, R0 2 =0.998. The relative standard deviation (RSD) of the fit in multiple experiments was controlled within 15%.

[0033] Example 3

[0034] Acetone was used as the dopant, with a high-temperature thermal desorption temperature of 150℃ and a migration tube temperature of 100℃; the bleaching gas (air) was 600 ml / min, the carrier gas (air) was 400 ml / min, and 10 μl of lactic acid (lactic acid standard) was injected.

[0035] The sample is injected into the rotary thermal desorption sampler via pulse valve II, and then into ion migration tube II. Pulse valve II is open for 30 seconds during injection and closed afterward. The ion migration spectrum is then analyzed as follows. Figure 4 As shown, the main migration time of the lactic acid target peak is 6.24 ms.

[0036] Example 4

[0037] Based on the signal peak at 6.24 ms of lactate migration time, signal intensities at concentrations of 5, 7.5, 15, 20, and 25 μg / mL were measured. Quantitative analysis standard curves were fitted between different concentrations and the corresponding logarithmic values ​​of the processed data, as shown below. Figure 5 As shown, within the concentration range of 5-25 μg / mL, the standard curve equation is Y = 0.204*X + 5.957, R0 2 =0.9929. The relative standard deviation (RSD) of the fit in multiple experiments was controlled within 20%.

[0038] Example 5

[0039] The device design in this invention adds a pulse valve to the carrier gas path to control the airflow switch. The carrier gas flow rate (air) is 400 ml / min. In one mode, the pulse valve is always open, and in another mode, the method of this invention is used. The sensitivity of the two methods for detecting lactic acid and acetic acid is compared. The sensitivity for detecting acetic acid is increased from 0.25 μg / mL to 0.1 μg / mL; the sensitivity for detecting lactic acid is increased from 5 μg / mL to 1 μg / mL. The sensitivity can be increased by 2.5-5 times respectively.

[0040] Example 6

[0041] Sample pretreatment of Daqu: Accurately weigh about 0.5000g of Daqu sample with an analytical balance and dissolve it in 2ml of methanol. Vortex the sample for 10min, centrifuge at 12000rpm for 5min and take the supernatant for analysis.

[0042] Pretreatment of fermented mash sample: Accurately weigh approximately 0.0010 g of fermented mash sample using an analytical balance and dissolve it in 2 ml of methanol. Vortex the sample for 10 min, centrifuge at 12000 rpm for 5 min, and take the supernatant and dilute it 50 times for analysis.

[0043] Using the method of this invention, and substituting the standard curves of Examples 2 and 4, the lactic acid content of the Daqu sample was finally calculated to be 5.2 mg / 100g; the lactic acid content of the fermented mash sample was 4.13 g / 100g.

[0044] For anyone skilled in the art, many possible variations and modifications can be made to the technical solutions of this invention, or equivalent embodiments can be modified based on the disclosed technical content, without departing from the scope of the technical solutions of this invention. Therefore, any simple modifications, equivalent changes, and modifications made to the above embodiments based on the technical essence of this invention without departing from the content of the technical solutions of this invention should still fall within the protection scope of the technical solutions of this invention.

Claims

1. A method for rapid detection of acetic acid and lactic acid in Daqu and fermented grains, characterized in that, The method is detected by the device for rapidly detecting acetic acid and lactic acid, and the sample is detected by a rotary disc thermal desorption sampler low-temperature thermal desorption into ion migration tube I and high-temperature thermal desorption into ion migration tube II by a headspace thermal desorption sampler; The low-temperature thermal desorption temperature is 50-60 DEG C, and the high-temperature thermal desorption temperature is 150-180 DEG C. The device comprises a rotary disc thermal desorption sampler (1), a headspace thermal desorption sampler (2), a pulse valve I (3), a pulse valve II (4), an ion migration tube I (5), and an ion migration tube II (6); the gas path of the device is connected to the rotary disc thermal desorption sampler through the pulse valve I, and then connected to the ion migration tube I, and the other path is connected to the headspace thermal desorption sampler through the pulse valve II, and then connected to the ion migration tube II; The drift gas is connected to the dopant gas and then enters the ion migration tube; The gas source carrier gas of the device for detecting acetic acid is circulated into the rotary disc thermal desorption sampler through the pulse valve I, and the rotary disc thermal desorption sampler is connected to the ion migration tube I; the gas source carrier gas of the device for detecting lactic acid is circulated into the headspace thermal desorption sampler through the pulse valve II, and the headspace thermal desorption sampler is connected to the ion migration tube II through the sampling tube; The gas path working mode is that the pulse valve is opened for 30-60 seconds during sampling, and the pulse valve is closed in other states.

2. The method for rapid detection of acetic acid and lactic acid in Daqu and fermented grains according to claim 1, characterized in that, The headspace thermal desorption sampler comprises a sampling tube (7) and a sampling bottle (8), and the sampling bottle is sleeved with a heating body (9).

3. The method for rapid detection of acetic acid and lactic acid in Daqu and fermented grains according to claim 1, characterized in that, The sample is pretreated by methanol extraction before sampling, and the concentration of the sample in methanol is 1-1000 ng / μl, and the sample is diluted during detection.

4. The method for rapid detection of acetic acid and lactic acid in Daqu and fermented grains according to claim 1, characterized in that, The dopant is acetone, or butanone, or a mixture of acetone and butanone.

5. The method for rapid detection of acetic acid and lactic acid in Daqu and fermented grains according to claim 4, characterized in that, Methanol is added to the dopant, and the volume ratio of the methanol to the dopant is 1:1-1:

2. 6.The method for rapid detection of acetic acid and lactic acid in Daqu and fermented grains according to claim 1, characterized in that, The standard curve for acetic acid quantitative analysis is fitted with different concentrations and signal intensities, and the standard curve for lactic acid quantitative analysis is fitted with different concentrations and signal intensities of common logarithm.

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