Free fatty acid content determination method and application

Abstract

The invention discloses a method for measuring the content of free fatty acid and application. The determination method comprises the following steps: mixing a fatty acid extraction solution of a to-be-determined sample with a copper soap developing solution and an organic phase solution for reaction, and then determining, the copper soap developing solution is a mixed solution containing pyridine derivatives and copper salt. Compared with pyridine used in a traditional copper soap complex colorimetric method, the pyridine derivative used in the method has a better and more sensitive color development effect in free fatty acid detection, and meanwhile, the color development effect on short-chain fatty acid is better; the measuring error caused by the fact that the color development intensity of fatty acid with medium-short carbon chain length is obviously weaker than that of fatty acid with long carbon chain when the traditional copper soap complexing colorimetric method is used for measuring mixed types of fatty acid can be reduced, and the method is more favorable for being applied to grain and oil storage quality evaluation and determination of free fatty acid.

Description

Technical Field

[0001] The invention belongs to the technical field of food detection, and in particular, relates to a method for determining free fatty acid content and an application thereof. Background Art

[0002] Acid value is a food safety indicator that reflects the total content of free fatty acids (FFA) in edible oils and fats. The freshness and degree of oxidation of oils and fats can be judged based on the free fatty acid content. The higher the free fatty acid content, the worse the quality. At the same time, the level of free fatty acids has become increasingly important as one of the important criteria for determining the storage quality of grain crops. The free fatty acid content of rice and bean crops that have deteriorated due to improper storage will increase significantly. Therefore, the free fatty acid content has also become a sensitive marker for judging the early quality deterioration of rice and bean crops. At present, the standard technology for determining acid value at home and abroad is acid-base titration technology, and the current standard method for determining the free fatty acid content of rice (GB / T 20569-2006 Rules for Determining the Storage Quality of Rice) also uses acid-base titration. This method is easy to operate, but it has the following disadvantages: 1. The experimental method has high requirements on the operator, the titration end point is difficult to determine, and the accuracy of the results is also limited due to equipment limitations; 2. The titration method consumes a large number of samples, and the test effect is not very good for small doses of samples; 3. The titration process is time-consuming and is not conducive to large-scale testing.

[0003] The copper soap complex colorimetry is a method that can specifically determine the content of long-chain free fatty acids in oils and fats, and is used in the detection of a variety of oils and fats. The technical principle of the traditional copper soap complex colorimetry is: the sample containing free fatty acids is dissolved in a hydrocarbon solvent as the organic phase, and the complex ion dye solution containing pyridine and copper ions is used as the aqueous phase to carry out a liquid-liquid biphasic reaction. During the reaction, the complex ions in the aqueous phase will react with various free fatty acids in the organic phase to form a trinity (pyridine-copper ions-fatty acid root ions) copper soap complex. The ternary copper soap complex is easily soluble in the organic phase and will show a special indigo color. There is a characteristic absorption peak near the wavelength of 700nm. The intensity of the absorption peak and the concentration of the copper soap complex conform to the Lambert-Beer law, so that it can be accurately quantified and then the concentration of free fatty acids can be calculated. This method is simpler and faster than the acid-base neutralization titration method, but there is also room for improvement. When measuring mixed fatty acids, the color intensity of short-chain fatty acids is significantly weaker than that of long-chain fatty acids in the traditional copper soap complex colorimetry, which will lead to significant measurement errors. In view of the above shortcomings, it is urgent to propose an improved new copper soap complex colorimetry for the determination of free fatty acid content in grain and oil foods. Summary of the invention

[0004] The purpose of the present invention is to overcome the above defects and shortcomings in the prior art and provide a method for determining the content of free fatty acids.

[0005] The second object of the present invention is to provide applications of the determination method.

[0006] The above-mentioned object of the present invention is achieved through the following technical solutions:

[0007] The present invention first provides a method for determining the content of free fatty acids, comprising the steps of mixing a fatty acid extraction solution of a sample to be determined with a copper soap colorimetric solution and an organic phase solution for reaction and then determining the content of free fatty acids; the copper soap colorimetric solution is a mixed aqueous phase solution containing a pyridine derivative and a copper salt, the pyridine derivative is a pyridine derivative in which the 4-position of pyridine is substituted by a C2 to C6 straight-chain alkyl group, and the organic phase solution is a colorless organic phase solution that is immiscible with the copper soap colorimetric solution.

[0008] In the present invention, a pyridine derivative connected with a carbon chain is used to replace pyridine in the traditional copper soap complex colorimetric method. Studies have found that the pyridine derivative has a better and more sensitive color development effect than pyridine in the detection of free fatty acids, and has a better color development effect on short-chain fatty acids. It can reduce the measurement error caused by the traditional copper soap complex colorimetric method when measuring mixed types of fatty acids, in which the color development intensity of fatty acids with medium and short carbon chains is significantly weaker than that of fatty acids with long carbon chains.

[0009] Furthermore, the specific determination process after the reaction is consistent with the existing copper soap complex colorimetric method, that is, the absorbance value of the solution after the reaction is measured at 710nm, and it is substituted into the standard working curve drawn in advance to calculate the free fatty acid content.

[0010] Preferably, the standard working curve is obtained by preparing standard solutions of known different concentrations using oleic acid as a standard substance for measurement, and then plotting the measured absorbance value as the ordinate and the oleic acid molar concentration as the abscissa.

[0011] Furthermore, the pyridine derivative is selected from any one of 4-ethylpyridine, 4-butylpyridine or 4-hexylpyridine.

[0012] Preferably, the pyridine derivative is 4-butylpyridine.

[0013] Furthermore, the fatty acid extract solution of the sample to be tested is obtained by extracting free fatty acids from the sample to be tested containing fatty acids using an organic solvent.

[0014] Preferably, the sample is selected from edible oils and fats, oil products, oil-containing foods, oil crops or rice.

[0015] Preferably, the sample is selected from vegetable oil, legume crops or rice.

[0016] Preferably, the sample is sesame oil, soybean or rice.

[0017] Preferably, the organic solvent is selected from cyclohexane or ethanol.

[0018] Preferably, when the sample is rice, ethanol is used to extract free fatty acids in the sample. Using ethanol to extract free fatty acids in rice samples is consistent with the current rice storage quality judgment rules (GB / T 20569-2006), avoiding differences in the extracted samples due to different extraction solvents, reducing errors caused by subsequent testing, and making the results closer to national standard testing standards. At the same time, using ethanol extraction is less toxic than solvents such as cyclohexane, and is more economical and environmentally friendly.

[0019] Furthermore, the copper salt is selected from copper acetate.

[0020] Furthermore, the pH of the copper soap color developing solution is 7.0-7.2.

[0021] Preferably, the copper soap color developing solution is prepared by adjusting the pH value using sodium acetate.

[0022] Preferably, the copper soap color developing solution is prepared by adding a pyridine derivative to a 0.05 mol / L copper acetate solution until the concentration reaches 0.1 mol / L, and adjusting the pH of the solution to 7.0-7.2 using anhydrous sodium acetate.

[0023] Furthermore, the organic phase solution is selected from cyclohexane, n-hexane or isooctane.

[0024] As a preferred embodiment, the method for determining the free fatty acid content comprises the following steps:

[0025] S1. Extracting free fatty acids from the sample to be tested using an organic solvent to obtain a fatty acid extract solution of the sample to be tested;

[0026] S2. Mix the sample fatty acid extract solution with the copper soap colorimetric solution and the organic phase solution for reaction, centrifuge and take the supernatant to measure the absorbance at 710 nm; wherein the copper soap colorimetric solution is a mixed aqueous phase solution containing a pyridine derivative and a copper salt, the pyridine derivative is a pyridine derivative in which the 4-position of pyridine is substituted by a C2 to C6 straight-chain alkyl group, and the organic phase solution is a colorless organic phase solution that is immiscible with the copper soap colorimetric solution;

[0027] S3. Substitute the absorbance value obtained in S2 into the standard working curve drawn with oleic acid as the standard, obtain the molar mass concentration of oleic acid equivalent to the total free fatty acid concentration in the fatty acid extract solution of the sample to be tested, and then calculate the free fatty acid content in the sample to be tested.

[0028] The present invention also provides the application of any of the above methods in evaluating the storage quality of grains and oils.

[0029] Furthermore, the grain and oil food is selected from edible oils and fats, oil products, oil-containing foods, oil crops or rice.

[0030] Preferably, the grain and oil food is selected from vegetable oil, legume crops or rice.

[0031] Compared with the prior art, the present invention has the following beneficial effects:

[0032] The present invention provides a method for determining the content of free fatty acids, comprising: mixing a fatty acid extraction solution of a sample to be tested with a copper soap colorimetric solution and an organic phase solution for reaction and then conducting the determination; the copper soap colorimetric solution is a mixed solution containing a pyridine derivative and a copper salt. Compared with pyridine used in the traditional copper soap complex colorimetric method, the use of pyridine derivatives in the present invention has a better and more sensitive color development effect in the detection of free fatty acids, and at the same time has a better color development effect on short-chain fatty acids, which can reduce the measurement error caused by the color development intensity of fatty acids with medium and short carbon chains being significantly weaker than that of long carbon chain fatty acids when the traditional copper soap complex colorimetric method is used to measure mixed types of fatty acids, and is more conducive to application in the determination of free fatty acids in the evaluation of the storage quality of grain and oil foods. BRIEF DESCRIPTION OF THE DRAWINGS

[0033] Figure 1 The absorbance curves of each group in the wavelength range of 600nm to 800nm ​​are compared at 0-1mmol / L oleic acid concentration.

[0034] Figure 2 Comparison of standard curves drawn for each group at 0-1 mmol / L oleic acid concentration.

[0035] Figure 3 The color development effect of different pyridine derivatives on fatty acids with different carbon chain lengths at the same fatty acid concentration (5mmol / L) is compared.

[0036] Figure 4 This is the standard curve drawn when measuring rice extract in Example 4.

[0037] Figure 5 This is the fitting curve drawn using pyridine and 4-butylpyridine when measuring sesame oil, soybean and rice samples in Example 5. DETAILED DESCRIPTION

[0038] The present invention is further described below in conjunction with the accompanying drawings and specific examples, but the examples do not limit the present invention in any form. Unless otherwise specified, the reagents, methods and equipment used in the present invention are conventional reagents, methods and equipment in the art.

[0039] Unless otherwise specified, the reagents and materials used in the following examples are commercially available.

[0040] Pyridine: Meryer, product number M02164-500mL; 4-ethylpyridine: Aladdin, product number E106225-5mL; 4-butylpyridine: Macklin, product number B895785-100mg; 4-hexylpyridine: Macklin, product number H895305-100mg.

[0041] Example 1

[0042] Determination of free fatty acid content in rice:

[0043] 1. Extraction of free fatty acids from rice:

[0044] Referring to the national standard GB / T 20569-2006 rice storage quality judgment rules, use anhydrous ethanol to extract free fatty acids in rice. After the rice sample is shelled and crushed, it is placed in a ground-mouth bottle for use. Weigh about 10g of the sample, accurate to 0.01g, and put it into a 250mL stoppered ground-mouth conical bottle, and add 50.0mL of anhydrous ethanol to it with a pipette, place it on a reverse oscillator and shake it for 10min, with the frequency set to 100 times / min. After shaking, let the sample stand for 1min~2min, and then put folded filter paper in a glass funnel for filtration. Discard the first few drops of filtrate and collect more than 25mL of filtrate.

[0045] 2. Color reaction:

[0046] 2.1 Preparation of new copper soap color developing solution:

[0047] First, a 0.05 mol / L copper acetate solution is prepared, and then a pyridine derivative is added thereto until its concentration reaches 0.1 mol / L, and finally anhydrous sodium acetate is used to adjust the pH of the solution to 7.0-7.2; the pyridine derivative is a pyridine derivative in which the 4-position of pyridine is substituted by a C2-C6 straight-chain alkyl group.

[0048] 2.2 Drawing of standard working curve:

[0049] (1) Preparation of standard working solutions: using oleic acid as a standard substance, anhydrous ethanol was used to prepare standard working solutions of oleic acid with molar concentrations of 0.1, 1, 3, 5, 7 mol / mL and 10 mol / mL respectively;

[0050] (2) Color reaction: Take 18 2mL centrifuge tubes and add 200μL of the oleic acid standard working solution of different molar concentrations prepared in step (1) respectively; then, add 600μL of the new copper soap colorimetric solution and 800μL of cyclohexane respectively. Use a vortex oscillator to oscillate evenly and let it stand for 5 minutes. Then, use a centrifuge to centrifuge at a speed of 5000g for 1 minute. Take 600μL of the supernatant and use a UV-visible spectrophotometer to detect the absorbance at a wavelength of 710nm. Set up 3 parallel experimental groups for each concentration, and repeat the measurement for each sample three times; use the measured absorbance value as the ordinate and the oleic acid molar concentration as the abscissa to draw a first-order linear regression curve, and obtain the standard curve equation and determine the correlation coefficient.

[0051] 2.3 Color reaction determination of samples

[0052] Sample the rice extract from the previous step, use a pipette to transfer 200 μL into a 2 mL centrifuge tube, and then add 600 μL of the new copper soap colorimetric solution and 800 μL of cyclohexane. Use a vortex oscillator to oscillate evenly and let stand for 5 minutes, then use a centrifuge to centrifuge at 5000g for 1 minute, take 600 μL of the supernatant and use a UV-visible spectrophotometer for absorbance detection. Substitute the measured absorbance value into the pre-drawn standard working curve to calculate the free fatty acid concentration of the sample.

[0053] 3. Determination and conversion of fatty acid value:

[0054] The unit of free fatty acid content determined in step 2 is molar concentration, and conversion is required to obtain the fatty acid value in the national standard GB / T 20569-2006. The calculation formula is as follows:

[0055]

[0056] Where:

[0057] S is the fatty acid value, in mg / 100g;

[0058] c is the mass concentration of oleic acid obtained by the standard curve equation, in mol / mL;

[0059] 50 is the volume of anhydrous ethanol used for sample extraction, in mL;

[0060] 56.1 is the molar mass of potassium hydroxide in g / mol;

[0061] m is the mass of the test sample, in g.

[0062] Example 2

[0063] The hydrophobicity of pyridine derivatives will affect the solubility of the ternary complex finally formed in the organic phase, and the solubility of the ternary complex formed by the pyridine derivatives with strong hydrophobicity in the organic phase is better, so there will be a better color development effect, and the detection performed will also be more sensitive. But the hydrophobicity enhancement of pyridine derivatives will also reduce its solubility in the aqueous phase, and then affect the efficiency of its formation of the ternary complex, so the hydrophobicity of pyridine derivatives is not the stronger the better, and an optimal range needs to be determined. Due to the poor water solubility of pyridine with a longer carbon chain, it is impossible to prepare an aqueous solution to form a copper soap color developing solution, in order to determine the best hydrophobicity of pyridine derivatives, the present invention selects pyridine (Py), 4-ethylpyridine (Py-2C), 4-butylpyridine (Py-4C) and 4-hexylpyridine (Py-6C) to detect and compare the results.

[0064] Specifically, according to the preparation method of the new copper soap color developing solution in Example 1, copper soap color developing solutions 1 to 5 were prepared with blank control, pyridine (Py), 4-ethylpyridine (Py-2C), 4-butylpyridine (Py-4C) and 4-hexylpyridine (Py-6C), respectively. The experiment included comparison of the absorbance curves of each group in the wavelength range of 600nm to 800nm ​​at a concentration of 0 to 1mmol / L oleic acid, and comparison of the standard curves drawn for each group at a concentration of 0 to 1mmol / L oleic acid.

[0065] The specific experimental operation is as follows: oleic acid is used as a standard substance, and oleic acid is prepared into standard working solutions with molar concentrations of 0, 0.01, 0.1, 0.3, 0.5, 0.7 mol / mL and 1 mol / mL using anhydrous ethanol. Take 35 2mL centrifuge tubes and add 200μL of the previously prepared oleic acid standard working solutions of different molar concentrations; then, add 600μL of copper soap colorimetric solutions 1 to 5 of different formulations and 800μL of cyclohexane. Use a vortex oscillator to oscillate evenly and let stand for 5 minutes, then use a centrifuge to centrifuge at a speed of 5000g for 1 minute, take 600μL of the supernatant and use a UV-visible spectrophotometer to detect the absorbance at a wavelength range of 600 to 800nm. Set up 3 parallel experimental groups for each concentration, and repeat the measurement three times for each sample. The absorbance curves of the five color developing solution formulas at 0, 0.1, 0.3, 0.5, 0.7 mol / mL and 1 mol / mL concentrations in the wavelength range of 600-800 nm were recorded respectively; the absorbance value at 710 nm was used as the ordinate and the molar concentration of oleic acid was used as the abscissa to draw the first-order linear regression curve, and the standard curve equation was obtained and the correlation coefficient was determined. The experimental results are shown as follows: Figure 1 , Figure 2 shown.

[0066] Judging from the experimental results, the results obtained with pyridine connected with a carbon chain are better than those of the pure pyridine experimental group. The carbon chain pyridine group (Py-2C, Py-4C, Py-6C) has a characteristic absorption peak at 0.3mmol / L, while the pyridine group (Py) can only be detected at 0.5mmol / L. Among them, the linear trend of the Py-6C group at low concentration is more obvious, and the zero-point concentration of the linear equation obtained by fitting is also smaller than that of other groups.

[0067] Example 3

[0068] The present invention tests the detection effect of different copper soap color developing solution formulas on fatty acids with different carbon chain lengths, and the following experiments are conducted: butyric acid, valeric acid, caproic acid, heptanoic acid, caprylic acid, capric acid, lauric acid, myristic acid, palmitic acid and oleic acid are used as standard products, and a standard solution with a molar concentration of 5 mol / mL is uniformly prepared with anhydrous ethanol. Take 40 2mL centrifuge tubes, and add 200μL of the previously prepared oleic acid standard working solution of different molar concentrations; then, add 600μL of copper soap color developing solution 2-5 of different formulas described in Example 2 and 800μL of cyclohexane. Use a vortex oscillator to oscillate evenly and stand for 5min, then use a centrifuge to centrifuge at a speed of 5000g for 1min, take 600μL of the supernatant and use a UV-visible spectrophotometer to perform absorbance detection at a wavelength range of 600-800nm. Set up 3 parallel experimental groups for each concentration, and repeat the measurement three times for each sample. The absorbance value at 710nm was taken as the ordinate, and the carbon number of the measured fatty acid was taken as the abscissa. The data of different formula groups were normalized based on the color intensity of oleic acid, and the color development effect curves of different pyridine derivatives with different carbon chain length fatty acids at the same fatty acid concentration (5mmol / L) were drawn. The experimental results are shown in Figure 3 shown.

[0069] In the experiment on the detection effect of fatty acids of different lengths, ideally, the closer the results on short-chain fatty acids are to those on long-chain fatty acids, the better, because this will reduce the error when detecting mixed acid samples. The experimental results show that the Py-4C group has a better short-chain fatty acid color development effect, and the Py-2C group and the Py-6C group have similar effects but are better than the Py group. My guess is that the reason why the results of the Py-4C group are better than those of the Py-6C group is that the solubility of Py-6C in the Py-6C group in water is too low, resulting in its low efficiency in combining to form a ternary complex. Short-chain fatty acids have a greater solubility in water, and the strong hydrophobicity of Py-6C reduces its chance of contact with short-chain fatty acids, resulting in a decrease in its color development effect with short-chain fatty acids.

[0070] In summary, when using pyridine derivatives to determine free fatty acids, it is a better choice to use Py-4C for detection. It has good water solubility, a lower detection limit than pyridine (Py), and better detection effect for short-chain fatty acids. Therefore, 4-butylpyridine can be preferably used to prepare the new copper soap colorimetric solution.

[0071] Example 4

[0072] 500 g of rice was purchased from the local market and its free fatty acid concentration was tested.

[0073] First, the purchased rice was dried, and the free fatty acids of the rice were extracted according to the national standard GB / T 20569-2006 for determining the storage quality of rice to obtain the rice extract, and the free fatty acid concentration of the rice was measured by acid-base titration to be x d =1.1mol / mL.

[0074] According to the method of Example 1, 4-butylpyridine was used to prepare a new copper soap color developing solution, oleic acid was used as a standard, and oleic acid was prepared into standard working solutions with molar concentrations of 1, 3 and 5 mol / mL with anhydrous ethanol. 12 2mL centrifuge tubes were taken, and 200 μL of the previously prepared oleic acid standard working solutions of different molar concentrations and the previously extracted rice extract were added respectively; Subsequently, 600 μL of the new copper soap color developing solution and 800 μL of cyclohexane were added respectively. The mixture was shaken evenly with a vortex oscillator and allowed to stand for 5 minutes, then centrifuged at a speed of 5000g for 1 minute with a centrifuge, and 600 μL of the supernatant was taken and the absorbance was detected at a wavelength range of 600 to 800 nm using a UV-visible spectrophotometer. Three parallel experimental groups were set for each concentration, and each sample was measured three times. With the measured absorbance value at 710nm as the ordinate and the molar concentration of oleic acid in the standard working solution as the abscissa, a first-order linear regression curve was drawn, and the standard curve equation and correlation coefficient were obtained. The results are shown in Figure 4 Substituting the absorbance value of the rice extract into the equation, the free fatty acid content is calculated as x t =1.044 mol / mL.

[0075] Example 5

[0076] Sesame oil, soybeans and rice were purchased from the local market. The fatty acids of the samples were directly extracted using cyclohexane, and copper soap staining solutions were prepared using pyridine and 4-butylpyridine, respectively, to detect the free fatty acid concentration by copper soap colorimetric detection.

[0077] First, the purchased soybeans and rice were dried and crushed, and about 5g of the dried and crushed samples were taken and placed in a 250mL conical flask, and 50mL of cyclohexane was added thereto with a pipette. The conical flask was placed in a reverse oscillator and oscillated at a frequency of 100 times / min for 10 minutes to extract free fatty acids. After shaking, the sample was allowed to stand for 1min to 2min, and then filtered by placing a folded filter paper in a glass funnel. The first few drops of filtrate were discarded, and more than 25mL of filtrate were collected respectively. For sesame oil samples, about 2.5g of the sample was directly taken and the volume was made up to 50mL with cyclohexane for free fatty acid extraction. After that, the free fatty acid content of each sample was measured by acid-base neutralization titration. Three parallel experimental groups were set for each sample, and each group was measured three times. The results are shown in Table 1.

[0078] Then, oleic acid was used as a standard substance, and cyclohexane was used to prepare standard working solutions with molar concentrations of 1, 3, 5, and 10 mmol / L, respectively. Then, a 0.05 mol / L copper acetate solution was prepared, and 4-butylpyridine (Py-4C) was added thereto until its concentration reached 0.1 mol / L. Finally, anhydrous sodium acetate was used to adjust the solution pH to 7.0-7.2, which was recorded as copper soap color solution I. Similarly, pyridine (Py) was used to replace 4-butylpyridine to prepare copper soap color solution II.

[0079] Take 42 2mL centrifuge tubes, add 200μL of the previously prepared oleic acid standard working solution of different molar concentrations and the extracted fatty acid extracts of each sample; then, divide them into two groups and add 600μL of copper soap colorimetric solution I and copper soap colorimetric solution II respectively, and then add 800μL of cyclohexane. Use a vortex oscillator to oscillate evenly and let it stand for 5 minutes. Then use a centrifuge to centrifuge at 5000g for 1 minute. Take 600μL of the supernatant and use a UV-visible spectrophotometer to detect the absorbance at a wavelength range of 600-800nm. Set up 3 parallel experimental groups for each concentration, and repeat the measurement for each sample three times. Use the absorbance value at 710nm measured by the two groups of colorimetric solutions as the vertical coordinate and the oleic acid molar concentration of the standard working solution as the horizontal coordinate to draw a first-order linear regression curve, and obtain the standard curve equation and determine the correlation coefficient. The results are shown in Figure 5 As shown, the 4-butylpyridine group has a better color development effect. The absorbance values ​​measured for the fatty acid extracts of each sample were substituted into the equation to calculate their respective free fatty acid contents, and the results are shown in Table 1.

[0080] Table 1 Fatty acid content test results of different samples

[0081]

[0082] The results show that the value measured by 4-butylpyridine is closer to the value measured by acid-base neutralization titration than pyridine, where the result of the pyridine group is smaller than that of the 4-butylpyridine group, and the result of the 4-butylpyridine group is smaller than that of the acid-base neutralization titration. This should be the measurement error caused by the short-chain fatty acids contained in the sample, and the use of 4-butylpyridine can reduce this error.

Claims

1. A method for determining free fatty acid content, characterized in that: The method comprises the steps of mixing a fatty acid extraction solution of a sample to be tested with a copper soap colorimetric solution and an organic phase solution for reaction and then performing the test; the copper soap colorimetric solution is a mixed aqueous phase solution containing a pyridine derivative and a copper salt, the pyridine derivative is a pyridine derivative in which the 4-position of pyridine is substituted by a C2 to C6 straight-chain alkyl group, and the organic phase solution is a colorless organic phase solution that is immiscible with the copper soap colorimetric solution.

2. The measuring method according to claim 1, characterized in that: The pyridine derivative is selected from any one of 4-ethylpyridine, 4-butylpyridine or 4-hexylpyridine.

3. The determination method according to claim 2, characterized in that: The pyridine derivative is 4-butylpyridine.

4. The measuring method according to claim 1, characterized in that: The fatty acid extract solution of the sample to be tested is obtained by extracting free fatty acids from the sample to be tested containing fatty acids using an organic solvent.

5. The measuring method according to claim 4, characterized in that: The sample is selected from edible oils and fats, oil products, oil-containing foods, oil crops or rice.

6. The determination method according to claim 4, characterized in that: The organic solvent is selected from cyclohexane or ethanol.

7. The determination method according to claim 1, characterized in that: The copper salt is selected from copper acetate.

8. The determination method according to claim 1, characterized in that: The organic phase solution is selected from cyclohexane, n-hexane or isooctane.

9. Use of the determination method according to any one of claims 1 to 8 in evaluating the storage quality of grains and oils.

10. The use according to claim 9, characterized in that: The grain and oil food is selected from edible oils and fats, oil products, oil-containing foods, oil crops or rice.

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