CN111912841A - Method for rapidly detecting whether content of amino acid nitrogen in brewed soy meets certain level - Google Patents
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Abstract
The invention provides a method for rapidly detecting whether the content of amino acid nitrogen in brewed soy meets a certain level, which comprises the steps of decoloring the soy, dripping an indicator, dripping monobasic strong base for neutralizing acid, dripping neutral formaldehyde for conversion, dripping the monobasic strong base with the quantity of substances of the lowest standard and the like, and directly judging whether a sample is qualified or not according to color change. The titration by instruments such as a power supply, an acidimeter and the like is not needed, the phenomena of 'sodium difference' and 'response' hysteresis of an electrode do not exist, the operation is simple and convenient, the calculation is not needed, and the result judgment is visual and accurate; the discrimination result can be obtained within a few minutes after sampling. The method has wide application range and strong practicability, and the result is completely consistent through comparison and determination with the national standard method.
Description
Technical Field
The invention relates to the quality of brewed soy sauce, in particular to a method for quickly detecting whether the content of amino acid nitrogen in the brewed soy sauce meets a certain level.
Background
The brewed soy sauce is a liquid seasoning with special color, fragrance and taste, which is prepared by taking soybeans, wheat or bran as raw materials through a certain process and microbial fermentation, has the color of reddish brown or tan, stronger sauce fragrance, fresh, salty and palatable taste and rich nutrition, and is a necessary seasoning for the public to use daily.
The national standard for the quality of brewed soy sauce products has already issued, and in the standard of GB18186-2000 'brewed soy sauce', whether high-salt liquid state fermented soy sauce or low-salt solid state fermented soy sauce is specified, the content of amino acid nitrogen (measured by nitrogen) is used as an index, and the soy sauce is divided into four grades, namely a special grade, a first grade, a second grade, a third grade and the like. In the brewed soy sauce, the content of amino acid nitrogen which is more than or equal to 0.80g/100mL is special grade; the first grade is more than or equal to 0.70g/100 mL; more than or equal to 0.60g/100mL is secondary (low-salt solid fermented soy sauce), more than or equal to 0.55g/100mL is secondary (high-salt dilute fermented soy sauce); more than or equal to 0.40g/100mL is three-grade, and the product which is lower than the three-grade is unqualified.
The first method of the national food safety standard GB5009.235-2016, "determination of amino acid nitrogen content in food", is acidimetric method, in which a soy sauce sample is treated to be alkaline (pH =8.2) with NaOH standard solution, titratable acidic substances in the sample are removed, then formaldehyde is added, the amount of acidic substances of amino acid carboxyl groups is displayed, then the final point is titrated with NaOH standard solution, a blank is simultaneously determined, the result is calculated by substituting the blank and the amount of NaOH consumed by the sample into a corresponding formula, and the quality and the corresponding grade of the brewed soy sauce are determined according to the calculation result. The national standard method has the following defects:
(1) when the acidity meter is used for measuring, NaOH standard solution is selected for titration, the soy sauce contains a large amount of salt (sodium chloride), and a large amount of sodium ions are continuously added along with the titration, so that a pH glass electrode can generate obvious sodium difference, the result is obviously lower, and errors are introduced; the national standard method does not consider the influence of the sodium difference on the measurement result.
(2) The formaldehyde reagent is often oxidized by air in the transportation and placement processes and contains formic acid, the formic acid must be treated into neutral formaldehyde, the possible interference of the formic acid is eliminated, the determination accuracy is improved, and the influence of the factor is not considered by the national standard method.
(3) The first method of GB5009.235-2016, before adding formaldehyde to convert amino acid nitrogen, the pH of the test solution is titrated to 8.2, the acidic substances existing in soy sauce are removed, after adding formaldehyde, the pH is titrated to pH9.2 by NaOH standard solution as an end point, the method breaks through the principle of 'quantity conversion and reaction of substances' in analytical chemistry quantitative analysis, and the experimental design lacks scientific basis support and has no reason.
(4) The acidity meter needs to be corrected by proper pH standard solution, and the glass electrode can be put into use after being soaked in distilled water for more than 24 hours; affecting the speed of measurement and the efficiency of operation.
(5) The acidimeter must use the power to work, to the occasion of no power, the application is restricted, can't work.
(6) The temperature change of the test solution can affect the measurement result, and the acidimeter needs to be subjected to temperature correction at any time or a temperature compensation sensor is installed through a related socket of an instrument so as to eliminate the interference of temperature, bring inconvenience to work and also affect the measurement accuracy.
(7) The national standard method has large amount of used test solution and test agent and relatively high investment cost;
(8) the operation is relatively complicated, the result judgment needs to be substituted into a formula for calculation, and the result judgment is not intuitive.
(9) From sampling, sample processing, titration to obtaining the calculation result, more than 2 hours are generally needed, and the working efficiency is low.
(10) In the titration process, the membrane potential of the pH glass electrode needs a certain time to reach equilibrium, an end point hysteresis phenomenon appears, the dripping is easy to occur, errors are introduced, and the measurement result is influenced.
Disclosure of Invention
In order to solve the problems, the invention provides a method for rapidly detecting whether the content of amino acid nitrogen in brewed soy meets a certain level, without titration by instruments such as a power supply, an acidimeter and the like, without the phenomena of ' sodium difference ' and ' response ' lag ' of an electrode, and the method is simple and convenient to operate, free from calculation and intuitive and accurate in result judgment; the discrimination result can be obtained within a few minutes after sampling. The method has wide application range and strong practicability, and the result is completely consistent through comparison and determination with the national standard method.
The object of the invention is achieved in the following way: a method for quickly detecting whether the content of amino acid nitrogen in brewed sauce meets a certain level comprises the following steps of (1) taking the brewed sauceThe oil sample a is decolorized, and a decolorized soy sauce sample b is obtained after decolorization; (2) get a solution containing V2The decolorized soy sauce sample b of the soy sauce sample a is mL, an indicator is dripped into the decolorized soy sauce sample b, the solution shows the acid color of the indicator, and then 0.050mol/L NaOH is dripped by a rubber head dropper until the solution shows the mixed color of the acid color and the alkali color of the indicator, and the solution does not fade in half minute; then adding neutral formaldehyde to completely convert amino acid nitrogen in the soy sauce sample b into an acidic substance (-COOH), wherein the solution is the acid color of the indicator; then V is added3 Slightly shaking the solution by using the mL monobasic strong alkali solution, observing the color of the solution, displaying the acid color of the indicator or the mixed color of the acid color and the alkali color of the indicator when the solution displays the acid color of the indicator or the acid-base equivalent point of the solution, wherein the sample is qualified brewed soy sauce which accords with a certain grade if the solution does not fade for half minute, and the sample is unqualified brewed soy sauce which does not accord with the certain grade if the solution displays the alkali color of the indicator and does not fade for half minute; the concentration of the monobasic alkali solution is equal to the lowest concentration of the grade of the soy sauce sample a, and V2=V3。
The monobasic strong alkali solution is sodium hydroxide solution, potassium hydroxide solution or lithium hydroxide solution.
The soy sauce sample a is original soy sauce which is a soy sauce product sold in the market or produced in factories; the original soy sauce is Li Jinji delicious seafood soy sauce/light soy sauce, Haitian soy sauce superior dark soy sauce, Li Jinji choice light soy sauce, Li Jinji bright dark soy sauce, and optionally straw mushroom dark soy sauce, gold symbol dark soy sauce and Luhua natural fresh soy sauce.
The indicator is purple sweet potato pigment.
The preparation method of the purple sweet potato pigment comprises the following steps: taking purple sweet potatoes, grinding into paste, adding distilled water according to the material-liquid ratio of 1:5, transferring into a 100mL colorimetric tube, extracting with the aid of ultrasonic waves, wherein the ultrasonic wave power is 500W, the extraction temperature is 55 ℃, the extraction time is 15-20min, and then filtering to obtain purple sweet potato pigment; the pH of the distilled water used was 6.8.
The step (1) of decolorizing the soy sauce comprises the following steps: transferring 1.00mL of the original soy sauce sample into a 25mL colorimetric tube by using a liquid transfer gun, adding 9.00mL of water for dilution, adding 0.30g of activated carbon, slightly stirring by using a glass rod for decolorization, and filtering after 2min to obtain a filtered clarified liquid which is the decolorized soy sauce sample b.
In the step (2), 1.00mL of soy sauce sample b is taken by a pipette and is placed in a 10mL colorimetric tube, 2 to 3 drops of natural purple sweet potato pigment are dropwise added, at the moment, the solution is red or red-orange, 0.050mol/LNaOH standard solution is dropwise added by a rubber head dropper until the color is orange-gray, and 2.00mL of neutral formaldehyde is added, at the moment, the solution is red or red-orange.
The acid color of the purple sweet potato pigment is red or reddish orange, the alkali color of the purple sweet potato pigment is grayish blue, green soil color or yellow brown, and the mixed color of reddish orange and grayish blue of the indicator is shown when the acid-base equivalent point of the identified solution is orange gray.
The amount of the residual acid substances in the solution of each grade product is more than red orange, and the red orange is more than orange gray; the remaining monobasic strong alkali in the solution has a yellow-brown color greater than that of the smectite, and a smectite color greater than that of the grayish blue.
Has the advantages that: compared with the prior art, the method for rapidly detecting whether the content of the amino acid nitrogen in the soy sauce is qualified scientifically sets the concentration and the amount of NaOH required for identifying soy sauce products at all levels according to GB18186-2000, grading parameter indexes of the amino acid nitrogen in the standard for brewing soy sauce and a reaction principle of equal quality, and preferably selects a decolorant and a natural color indicator. The method does not need to use a power supply, an acidimeter and other instruments for titration, does not have the phenomena of 'sodium difference' and 'response' hysteresis of an electrode, and reduces the consumption of the required test solution and the standard solution by about 10 times. The method for rapidly detecting whether the content of amino acid nitrogen in the brewed soy meets a certain level is simple and convenient to operate, calculation is not needed, and the result judgment is visual and accurate; the discrimination result can be obtained within a few minutes after sampling. The method has wide application range and strong practicability, and the result is completely consistent through comparison and determination with the national standard method.
Drawings
Fig. 1 is the color change of purple sweet potato pigment in different pH solutions, from left to right: the pH was 2.00, 3.00, 4.00, 5.00, 6.00, 7.00, 8.00, 9.00, 10.00, 11.00, 12.00, and 13.00, respectively. The color varied from red (pH 2, 3), red-orange (pH 4, 5, 6, 7), orange-gray (pH 8), gray-blue (pH 9), green-earth (pH 10, 11) to yellow-brown (pH 12, 13).
FIG. 2 is a graph showing the effect of the amount of activated carbon on soy sauce decolorization.
FIG. 3 is the effect of time on soy sauce decolorization.
Detailed Description
A method for rapidly detecting whether the content of amino acid nitrogen in brewed soy meets a certain level comprises the following steps of (1) taking a brewed soy sample a, decoloring, and obtaining a decolored soy sample b after decoloring; (2) get a solution containing V2The decolorized soy sauce sample b of the soy sauce sample a is mL, an indicator is dripped into the decolorized soy sauce sample b, the solution shows the acid color of the indicator, and then 0.050mol/L NaOH is dripped by a rubber head dropper until the solution shows the mixed color of the acid color and the alkali color of the indicator, and the solution does not fade in half minute; then adding neutral formaldehyde to completely convert amino acid nitrogen in the soy sauce sample b into an acidic substance (-COOH), wherein the solution shows the acid color of the indicator; then adding V3 mL monobasic strong base solution, slightly shaking and observing the color of the solution, wherein the acid color of the indicator is displayed when the solution is displayed as the acid color of the indicator or the acid-base equivalent point of the solution is displayed as the mixed color of the acid color and the alkali color of the indicator, and the color of the sample is not faded within half a minute, so that the sample is qualified brewed soy sauce which meets a certain level; if the solution shows an alkaline color of the indicator and does not fade for half a minute, the sample is a non-conforming brewed soy sauce that does not meet the rating; the concentration of the monobasic alkali solution is equal to the lowest concentration of the grade of the soy sauce sample a, and V2=V3。
The monobasic strong alkali solution is sodium hydroxide solution, potassium hydroxide solution or lithium hydroxide solution.
The soy sauce sample a is brewed soy sauce original shape, and the soy sauce original shape is a soy sauce product sold in the market or produced in factories; the original soy sauce is Li Jinji delicious seafood soy sauce/light soy sauce, Haitian soy sauce superior dark soy sauce, Li Jinji choice light soy sauce, Li Jinji bright dark soy sauce, and optionally straw mushroom dark soy sauce, gold symbol dark soy sauce and Luhua natural fresh soy sauce.
The indicator is purple sweet potato pigment.
The preparation method of the purple sweet potato pigment comprises the following steps: taking purple sweet potatoes, grinding into paste, adding distilled water according to the material-liquid ratio of 1:5, transferring into a 100mL colorimetric tube, extracting with the aid of ultrasonic waves, wherein the ultrasonic wave power is 500W, the extraction temperature is 55 ℃, the extraction time is 15-20min, and then filtering to obtain purple sweet potato pigment; the pH of the distilled water used was 6.8.
The step (1) of decolorizing the brewed soy sauce comprises the following steps: transferring 1.00mL of the original soy sauce sample into a 25mL colorimetric tube by using a liquid transfer gun, adding 9.00mL of water for dilution, adding 0.30g of activated carbon, slightly stirring by using a glass rod for decolorization, and filtering after 2min to obtain a filtered clarified liquid which is the decolorized soy sauce sample b.
In the step (2), 1.00mL of soy sauce sample b is taken by a pipette and is placed in a 10mL colorimetric tube, 2 to 3 drops of natural purple sweet potato pigment are dropwise added, at the moment, the solution is red or red-orange, 0.050mol/LNaOH standard solution is dropwise added by a rubber head dropper until the color is orange-gray, and 2.00mL of neutral formaldehyde is added, at the moment, the solution is red or red-orange.
The acid color of the purple sweet potato pigment is red or reddish orange, the alkali color of the purple sweet potato pigment is grayish blue, green soil color or yellow brown, and the mixed color of the reddish orange color and the grayish blue color of the purple sweet potato pigment is orange gray; when the sample is identified according to the method of the step, the red color is more than red orange, and the red orange color is more than orange gray; the remaining monobasic strong alkali in the solution has a yellow-brown color greater than that of the smectite, and a smectite color greater than that of the grayish blue.
The present invention is described in detail below with reference to specific embodiments, it should be noted that the embodiments are only used for further illustration of the present invention, and should not be construed as limiting the scope of the present invention, and those skilled in the art can make modifications and adaptations of the present invention based on the above-mentioned disclosure. In this application, the solution color was observed after gently rocking, and gently rocking was the degree of rocking of titration in-process conventionality.
Preparation of purple sweet potato pigment (using distilled water or purified water)
Cutting 10g-15g of fresh purple sweet potatoes (a purple sweet potato sample is purchased from Hongzhai town of the Erqidistrict of Zhengzhou city in 2019, 10 months and 30 days, is very fresh when being purchased, and has purple to deep purple color) into small pieces, placing the small pieces into a mortar, grinding the small pieces into paste, adding distilled water according to the material-liquid ratio of 1:5, transferring the paste into a 100mL colorimetric tube, performing ultrasonic assisted extraction with the power of 500W, wherein the extraction temperature is 55 ℃, and the extraction time is 10-20 min. And then filtering the mixture into a dropping bottle to obtain the purple sweet potato pigment. The pH of the distilled water used was 6.8.
Taking 12 clean 100mL small beakers, adding about 40mL distilled water respectively, adding 0.10mol/L hydrochloric acid or 0.10mol/L sodium hydroxide solution dropwise on an acidimeter carefully, adjusting the pH to be 2.00, 3.00, 4.00, 5.00, 6.00, 7.00, 8.00, 9.00, 10.00, 11.00, 12.00 and 13.00 respectively, then adding 4 drops of purple sweet potato pigment filtrate dropwise, shaking uniformly, transferring into a 25.00mL colorimetric tube respectively, and observing the color change of the purple sweet potato pigment in the solutions with different pH values, as shown in figure 1, from left to right: the color varied from red (pH 2, 3), red-orange (pH 4, 5, 6, 7), orange-gray (pH 8), gray-blue (pH 9), green-earth (pH 10, 11) to yellow-brown (pH 12, 13).
Secondly, preparing and calibrating NaOH standard solution and adjusting concentration
1. Setting of concentration
The amino acid is neutral inner salt, and when neutral formaldehyde is added, the amino acid presents basic amino (-NH)2) The compound is combined with formaldehyde, the alkalinity disappears, and the compound presents acidic carboxyl (-COOH), can react with strong base standard solution and other substances, and is particularly suitable for measuring amino nitrogen in fermentation liquor. Each amino group (-NH)2) Containing one nitrogen. In both high-salt dilute-state fermented soy sauce and low-salt solid-state fermented soy sauce, the content of amino acid nitrogen (calculated by nitrogen) is used as a main physicochemical index of products at all levels, and when neutral formaldehyde is added after a sample is treated to convert the amino acid nitrogen, the lowest concentration of nitrogen, namely the lowest concentration of required amino acid, and the lowest concentration of carboxyl after conversion are required by the products, so that the lowest concentration of carboxyl (-COOH) in qualified products at all levels is as follows:
third-stage:
and (2) second stage:
(for high salt liquid state fermented soy sauce)
First-stage:
special grade:
therefore, according to the principle of 'reaction of equal mass', the invention prepares NaOH (or KOH) strong base standard solution with the same concentration at the corresponding grade, and the NaOH (or KOH) strong base standard solution and carboxyl (-COOH) in the sample have 'reaction of equal mass', and after the reaction is finished, if the carboxyl of the sample is remained, the sample is a qualified product at the corresponding grade; if the carboxyl is insufficient and strong alkali is left, the brewed soy sauce is unqualified product with corresponding grade. If the amount of carboxyl (-COOH) in the sample is exactly the same as the amount of the substance of the strong base added, no residue remains after the neutralization reaction, and the theoretical end point of the lowest index of the required grade is reached, the solution presents a mixture of red-orange and grayish-blue color, i.e. orange-gray, as the indicator, which is extremely rare. After the disturbing color of the soy sauce is removed by the decolorizing agent, the soy sauce can be quickly screened and judged by a proper indicator.
2. Preparation, calibration and regulation of NaOH standard solutions with different concentrations
Firstly, the required amount of NaOH (A.R) is weighed by an electronic balance, and the required amount is dissolved and determined. Calibrating by using a potassium hydrogen phthalate reference substance which is dried to constant mass, carrying out parallel calibration for 4 times, taking an average value after no suspicious value is detected, and calculating and adding required solid NaOH (A.R) if the concentration is lower than the target concentration; if the concentration is higher than the target concentration, the required distilled water is calculated and added.
(1) Preparation, calibration and regulation of 0.2857mol/L NaOH standard solution
Quickly weighing 5.714 g of NaOH (A.R, analytical purity) in a small beaker by using an electronic balance, dissolving the NaOH in about 80mL of distilled water, cooling the solution to room temperature, quantitatively transferring the solution into a 500mL volumetric flask, fixing the volume to a scale by using the distilled water, and uniformly mixing the solution for later use.
Accurately weighing 0.6g-1.2g (accurately to 0.0001 g) of potassium hydrogen phthalate reference reagent which is dried to be constant in mass at 105 ℃ on an analytical balance, adding about 40mL of distilled water to completely dissolve the reagent, adding 4 drops of purple sweet potato pigment prepared by the method, calibrating with the sodium hydroxide solution in a treated basic burette, titrating until the red orange color just disappears and becomes orange gray and does not fade for half a minute, and recording the volume V of the consumed sodium hydroxide solution1(mL), while taking 40mL of distilled water for blank test and color comparison, the concentration was calculated according to the following formula:
C(NaOH)=（m×1000÷204.22）/（V1-V0）
wherein, C (NaOH) is the mass concentration of sodium hydroxide standard solution, mol/L; m is the mass g of the potassium hydrogen phthalate reference reagent which is accurately weighed; 204.22 is the molar mass of potassium hydrogen phthalate, g/mol; v0Volume of sodium hydroxide solution consumed as blank (mL); v1The volume of sodium hydroxide solution (mL) consumed to calibrate a mass m of potassium hydrogen phthalate reference reagent.
Performing 4 times of parallel calibration, taking an average value after no suspicious value is detected, and calculating and adding required solid NaOH if the concentration is lower than 0.2857 mol/L; if the concentration is higher than 0.2857mol/L, the required distilled water is calculated and added until the target concentration is reached after recalibration.
The standard potassium hydroxide solution of 0.2857mol/L can also be prepared for work, and the sodium hydroxide standard solution with lower price is selected in the invention, so the investment cost is relatively reduced.
(2) Preparation, calibration and regulation of 0.3929mol/L NaOH standard solution
Rapidly weighing 7.858 g of NaOH (analytically pure) in a small beaker by using an electronic balance, dissolving the small beaker by using about 80mL of distilled water, cooling the solution to room temperature, quantitatively transferring the solution into a 500mL volumetric flask, fixing the volume to a scale by using the distilled water, and uniformly mixing the solution for later use.
Accurately weighing 0.8-1.6 g (accurately to 0.0001 g) of reference reagent potassium hydrogen phthalate which is dried to be constant in mass at 105 ℃ on an analytical balance, adding about 40mL of distilled water to completely dissolve the potassium hydrogen phthalate in a clean conical flask, adding 4 drops of purple sweet potato pigment prepared by the method, calibrating with the sodium hydroxide solution in a treated basic burette, titrating until the color of the purple sweet potato is just disappeared to orange gray from red orange, keeping the color of the purple sweet potato unchanged for half minute as an end point, and recording the volume V of the consumed sodium hydroxide solution1(mL), while taking 40mL of distilled water for blank test and color comparison, the concentration was calculated according to the following formula:
C(NaOH)=（m×1000÷204.22）/（V1-V0）
wherein, C (NaOH) is the mass concentration of sodium hydroxide standard solution, mol/L; m is the mass g of the potassium hydrogen phthalate reference reagent which is accurately weighed; 204.22 is the molar mass of potassium hydrogen phthalate, g/mol; v0Volume of sodium hydroxide solution consumed as blank (mL); v1The volume of sodium hydroxide solution (mL) consumed to calibrate a mass m of potassium hydrogen phthalate reference reagent.
Performing 4 times of parallel calibration, taking an average value after no suspicious value is detected, and calculating and adding required solid NaOH if the concentration is lower than 0.3929 mol/L; if the concentration is higher than 0.3929mol/L, the required distilled water is calculated and added until the target concentration is reached after recalibration.
The standard potassium hydroxide solution of 0.3929mol/L can also be prepared for work, and the sodium hydroxide standard solution with lower price is selected in the invention, so the investment cost is relatively reduced.
(3) Preparation, calibration and regulation of 0.4286mol/L NaOH standard solution
Quickly weighing 8.5720 g of NaOH (analytically pure) in a small beaker by using an electronic balance, dissolving the small beaker by using about 80mL of distilled water, cooling the solution to room temperature, quantitatively transferring the solution into a 500mL volumetric flask, fixing the volume to a scale by using the distilled water, and uniformly mixing the solution for later use.
Accurately weighing 0.8-1.8 g (accurately to 0.0001 g) of reference reagent potassium hydrogen phthalate which is dried to be constant in mass at 105 ℃ on an analytical balance, adding about 40mL of distilled water to completely dissolve the potassium hydrogen phthalate in a clean conical flask, adding 4 drops of purple sweet potato pigment prepared by the method, calibrating with the sodium hydroxide solution in a treated basic burette, titrating until the red orange color just disappears and becomes orange gray, keeping the color from fading in half a minute as an end point, and recording the volume V of the consumed sodium hydroxide solution1(mL), while taking 40mL of distilled water for blank test and color comparison, the concentration was calculated according to the following formula:
C(NaOH)=（m×1000÷204.22）/（V1-V0）
wherein C (NaOH) is the mass concentration of the sodium hydroxide standard solution, mol/L; m is the mass g of the potassium hydrogen phthalate reference reagent which is accurately weighed; 204.22 is the molar mass of potassium hydrogen phthalate, g/mol; v0Volume of sodium hydroxide solution consumed as blank (mL); v1The volume of sodium hydroxide solution (mL) consumed to calibrate a mass m of potassium hydrogen phthalate reference reagent.
Performing 4 times of parallel calibration, taking an average value after no suspicious value is detected, and calculating and adding required solid NaOH if the concentration is lower than 0.4286 mol/L; if the concentration is higher than 0.4286mol/L, the required distilled water is calculated and added until the target concentration is reached after recalibration.
0.4286mol/L potassium hydroxide standard solution can be prepared for work, the invention selects the sodium hydroxide standard solution with lower price, and the investment cost is relatively reduced.
(4) Preparation, calibration and regulation of 0.5000mol/L NaOH standard solution
Quickly weighing 10.00 g of NaOH (analytically pure) in a small beaker by using an electronic balance, dissolving the NaOH in about 80mL of distilled water, cooling the solution to room temperature, quantitatively transferring the solution into a 500mL volumetric flask, metering the volume to a scale by using the distilled water, and uniformly mixing the solution for later use.
Accurately weighing 1.0g-2.0g (quasi-quality) of standard reagent potassium hydrogen phthalate baked to constant mass at 105 ℃ on an analytical balanceTo ensure that 0.0001 g) is put into a clean conical flask, about 40mL of distilled water is added to completely dissolve the purple sweet potato pigment, 4 drops of the purple sweet potato pigment are added, the purple sweet potato pigment is calibrated by the sodium hydroxide solution in a treated basic burette, the titration is carried out until the red orange color just disappears and becomes orange gray, the fading is not carried out in half a minute, and the volume V of the consumed sodium hydroxide solution is recorded1(mL), while taking 40mL of distilled water for blank test and color comparison, the concentration was calculated according to the following formula:
C(NaOH)=（m×1000÷204.22）/（V1-V0）
wherein C (NaOH) is the mass concentration of the sodium hydroxide standard solution, mol/L; m is the mass g of the potassium hydrogen phthalate reference reagent which is accurately weighed; 204.22 is the molar mass of potassium hydrogen phthalate, g/mol; v0Volume of sodium hydroxide solution consumed as blank (mL); v1The volume of sodium hydroxide solution (mL) consumed to calibrate a mass m of potassium hydrogen phthalate reference reagent.
Performing parallel calibration for 4 times, taking an average value after no suspicious value is detected, and calculating and adding required solid NaOH if the concentration is lower than 0.5000 mol/L; if the concentration is higher than 0.5000mol/L, calculating and adding the required distilled water until the target concentration is reached after recalibration.
And a potassium hydroxide standard solution with the concentration of 0.5000mol/L can be prepared for work, and the sodium hydroxide standard solution with lower price is selected, so that the investment cost is relatively reduced.
(5) 0.5714mol/L NaOH preparation, calibration and adjustment
Rapidly weighing 11.428g of NaOH (analytically pure) in a small beaker by using an electronic balance, dissolving the small beaker by using about 80mL of distilled water, cooling the solution to room temperature, quantitatively transferring the solution into a 500mL volumetric flask, fixing the volume to a scale by using the distilled water, and uniformly mixing the solution for later use.
Accurately weighing 1.2-2.5 g (accurately to 0.0001 g) of standard reagent potassium hydrogen phthalate which is dried to be constant in mass at 105 ℃ on an analytical balance, adding about 40mL of distilled water to completely dissolve the standard reagent potassium hydrogen phthalate in a clean conical flask, adding 4 drops of purple sweet potato pigment prepared by the method, and using the sodium hydroxide in a treated basic buretteThe solution is calibrated, titrated until the red orange color just disappears and becomes orange gray color and does not fade for half a minute, and the volume V of the consumed sodium hydroxide solution is recorded1(mL), while taking 40mL of distilled water for blank test and color comparison, the concentration was calculated according to the following formula:
C(NaOH)=（m×1000÷204.22）/（V1-V0）
wherein C (NaOH) is the mass concentration of the sodium hydroxide standard solution, mol/L; m is the mass g of the potassium hydrogen phthalate reference reagent which is accurately weighed; 204.22 is the molar mass of potassium hydrogen phthalate, g/mol; v0Volume of sodium hydroxide solution consumed as blank (mL); v1The volume of sodium hydroxide solution (mL) consumed to calibrate a mass m of potassium hydrogen phthalate reference reagent.
Performing 4 times of parallel calibration, taking an average value after no suspicious value is detected, and calculating and adding required solid NaOH if the concentration is lower than 0.5714 mol/L; if the concentration is higher than 0.5714mol/L, the required distilled water is calculated and added until the target concentration is reached after recalibration.
0.5714mol/L potassium hydroxide standard solution can be prepared for work, the invention selects the sodium hydroxide standard solution with lower price, and the investment cost is relatively reduced.
Preparation of neutral formaldehyde
4-6 drops of the purple sweet potato pigment prepared by the method are dropwise added into 36-38% of analytically pure formaldehyde solution, NaOH solution is dropwise added into the formaldehyde solution while the formaldehyde solution is gently shaken, and the purple sweet potato pigment is stored in a sealed manner until the formaldehyde solution disappears from red orange and just turns to orange gray.
Fourthly, decolorization treatment and effect of soy sauce
Experimental apparatus and reagents: UV-2800AH model UV-visible spectrophotometer (Youth instruments Co., Ltd.), 200 mesh food sugar activated charcoal (Ron reagent).
The experimental steps are as follows:
(1) and (3) putting 1.00mL of the Lijinji-flavor extremely fresh original soy sauce into a clean test tube, adding 9.00mL of deionized water, and uniformly mixing for later use, wherein the sample solution is to-be-scanned.
(2) Preparing a blank liquid: putting 1.00mL of Lijinji-flavor fresh raw soy sauce into a clean test tube, adding 9.00mL of deionized water, adding 0.40g of activated carbon for treatment, standing for 2min, filtering by using a funnel, and taking the filtrate as a blank (to exclude other factors except pigment from influencing absorbance).
(3) Drawing an absorption spectrum: the sample solution diluted by 10 times is used as a test solution, a blank sample solution is used as a control, and the absorption spectrum of the test solution between 200 and 800nm is scanned by a 1cm cuvette, so that the following results are shown: maximum wavelength lambda of Lijinji flavor extremely fresh original soy saucemax=336.0 nm。
(4) Investigation of decolorization ratio: 20mL of 10-fold diluted soy sauce is respectively placed in 7 colorimetric tubes of 50.00mL, and 0.12g, 0.16g, 0.20g, 0.24g, 0.28g, 0.30g and 0.32g of activated carbon are respectively added in turn to be gently shaken and then are kept stand for 1min to 2min, and then are filtered by filter paper. The absorbance of each sample solution was measured using a 1cm cuvette with reference to a blank sample solution and incident light of 336.0 nm. The decolorization ratio of the sample was calculated as follows:
decolorization ratio,% = (a)0-A)÷A0×100% (3)
In the formula, A0The absorbance was measured when the diluted soy sauce sample was not decolorized 10 times, and the absorbance of the filtrate was measured after the diluted soy sauce sample was decolorized 10 times by adding different amounts of activated carbon. The results are shown in FIG. 2. As shown in FIG. 2, the decolorization rate of 0.30g of activated carbon is 92.56%, and a better decolorization effect can be achieved.
And weighing 0.30g of activated carbon, standing for 2min, 4min and 6min respectively, filtering, measuring absorbance according to the method, and calculating the decolorization rate. The results are shown in FIG. 3. As can be seen from FIG. 3, the decoloring effect is satisfactory from 2min to 6min, and the decoloring time is selected to be 2min in the invention to improve the working efficiency.
Fifth, the determination of the national standard method by potentiometric titration
(1) Determination by potentiometric titration with standard potassium hydroxide solution
The method comprises the following operation steps: 5.00mL of soy sauce is sucked into a 100mL volumetric flask, water is added to the volumetric flask to the scale mark, 20mL of soy sauce is sucked after uniform mixing, the soy sauce is placed into a 100mL beaker, and 60mL of water is added. Titrate to ionometer pH =8.2 with potassium hydroxide standard solution c (KOH =0.050 mol/L), note the volume of KOH consumed. 20mL of neutral formaldehyde was added, mixed well and the pH at this point was recorded. Titration was continued with KOH to pH =8.2 and the volume of KOH consumed was recorded.
Simultaneously, 20mL of water is taken, firstly, 0.050mol/LKOH is used for adjusting the pH to be 8.2, then 20mL of neutral formaldehyde is added, 0.050mol/LKOH is used for titrating the pH to be 8.2, and the consumed KOH volume is recorded and used as a blank experiment of the reagent.
The calculation formula of the content of amino acid nitrogen in the sample is as follows:
in the formula:
x represents the content of amino acid nitrogen in the sample, and g/100 mL;
V1measuring the volume of KOH consumed in the sample diluent after adding formaldehyde, mL;
V0volume of KOH consumed, mL, after addition of formaldehyde to the reagent blank;
v is the sample diluent dosage, mL;
the mass concentration of C-KOH, mol/L;
0.014-mass of nitrogen equivalent to 1.00mmol (millimole) KOH, g.
The calculation results retain three significant digits.
(2) Determination by potentiometric titration with sodium hydroxide standard solution
The method comprises the following operation steps: 5.00mL of soy sauce is sucked into a 100mL volumetric flask, water is added to the volumetric flask to the scale mark, 20mL of soy sauce is sucked after uniform mixing, the soy sauce is placed into a 100mL beaker, and 60mL of water is added. Titrate to ionometer pH =8.2 with sodium hydroxide standard solution c (NaOH =0.050 mol/L), and note the volume of NaOH consumed. 20mL of formaldehyde, which had not been adjusted to neutrality, were added, mixed well and the pH at this point was recorded. Titration was continued with NaOH to pH =9.2 and the volume of NaOH consumed was recorded.
Simultaneously, 20mL of water was adjusted to pH =8.2 with 0.050mol/LNaOH, 20mL of formaldehyde which was not adjusted to neutral was added, the solution was titrated to pH =9.2 with 0.050mol/LNaOH, and the volume of NaOH consumed was recorded as a blank of reagent.
The calculation formula of the content of amino acid nitrogen in the sample is as follows:
in the formula:
x represents the content of amino acid nitrogen in the sample, and g/100 mL;
V1measuring the volume (mL) of the strong base standard solution consumed after the sample diluent is added with formaldehyde;
V0-reagent blank experiment consumes volume of strong base standard solution, mL, after addition of formaldehyde;
v is the sample diluent dosage, mL;
c, the quantity concentration of the substance of the strong base standard solution, mol/L;
0.014-mass of nitrogen equivalent to 1.00mmol (millimole) of strong base, g.
The calculation results retain three significant digits.
(3) The results of the sample control measurements and analyses are shown in table 1 below.
The main reason is that the pH of the test solution is treated to 8.2 before the national standard method adds formaldehyde to convert amino acid nitrogen, and after adding formaldehyde, the test solution is titrated to pH9.2 by NaOH standard solution, which violates the quantitative conversion and reaction principle of the substances in analytical chemistry quantitative analysis and makes the determination result higher.
Sixthly, measuring results of the original brewed soy sauce according to the first method acidity meter method of the national standard method
The samples were measured by acidimetry according to the first method of the national standard GB5009.235-2016 "measurement of amino acid nitrogen content in food", and compared with the color of the solution in the reagent tube and the commercial label value, and the results are shown in Table 2.
As can be seen from Table 2, the results of the national standard sample measurements are all higher than the commercial values.
Seventh, quick detection method for determining whether content of amino acid nitrogen in brewed soy meets certain level and result verification
(I) quick detection method for determining whether content of amino acid nitrogen in original soy sauce meets certain level and result verification
(1) Special soy sauce (amino acid nitrogen content is more than or equal to 0.80g/100 mL)
Experimental reagent: 0.5714mol/LNaOH solution, neutral formaldehyde
The method comprises the following operation steps: transferring 1.00mL of soy sauce into a 25mL colorimetric tube by using a liquid transfer gun, adding 9.00mL of water for dilution, adding 0.30g of activated carbon, slightly stirring and decoloring by using a glass rod, filtering after 2min, transferring 1.00mL of filtered clarified liquid into a 10mL colorimetric tube by using the liquid transfer gun, dropwise adding 2 to 3 drops of purple sweet potato natural pigment, wherein the solution is red, dropwise adding 0.050mol/LNaOH by using a rubber head dropper until the solution is orange gray in color, adding 2.00mL of neutral formaldehyde, wherein the solution is red, adding 0.5714mol/LNaOH solution 100 mu L by using the liquid transfer gun, slightly shaking, and observing the color of the solution, wherein the solution is red, red orange or orange gray and does not fade in half minute, so that the product is the special grade brewed soy sauce, and if the solution is grayish blue, green earth or yellow brown and does not fade in half minute, so that the quality of the product does not meet the requirements of the special grade brewed soy sauce.
Since the amino acid nitrogen and other substances are converted into acid (-COOH) by neutral formaldehyde, the acidity of the solution is greatly increased, and the pigment is red. Then adding strong alkali required by the corresponding grade product, observing the color of the solution, and performing rapid identification. The same applies hereinafter. 2.00mL of neutral formaldehyde completely converted the amino acid nitrogen in the 100. mu.L of the taken sample of the raw soy sauce into an equivalent amount of acidic substance (-COOH).
(2) First class soy sauce (amino acid nitrogen content is more than or equal to 0.70g/100 mL)
Experimental reagent: 0.5000mol/LNaOH solution, neutral formaldehyde
The method comprises the following operation steps: transferring 1.00mL of soy sauce into a 25mL colorimetric tube by using a liquid transfer gun, adding 9.00mL of water for dilution, adding 0.30g of activated carbon, slightly stirring and decoloring by using a glass rod, filtering after 2min, transferring 1.00mL of filtered clarified liquid into a 10mL colorimetric tube by using the liquid transfer gun, dropwise adding 2 to 3 drops of natural purple sweet potato pigment, dropwise adding 0.050mol/L NaOH by using a rubber head dropper until the color of the solution is orange gray, adding 2.00mL of neutral formaldehyde, red when the solution is red, adding 0.5000mol/L of LNaOH solution 100 mu L by using the liquid transfer gun, slightly shaking, observing the color of the solution, and if the solution is still red, red orange or gray and does not fade in half minute, the product is first-stage fermented and brewed soy sauce, and if the solution is gray blue, green earth or yellow brown and does not fade in half minute, the product does not meet the requirements of the first-stage fermented and brewed soy sauce.
(3) Second grade soy sauce (high salt liquid state fermentation soy sauce amino acid nitrogen content is more than or equal to 0.55g/100 mL)
Experimental reagent: 0.3929mol/LNaOH standard solution, neutral formaldehyde
The method comprises the following operation steps: transferring 1.00mL of soy sauce into a 25mL colorimetric tube by using a liquid transfer gun, adding 9.00mL of water for dilution, adding 0.30g of activated carbon, slightly stirring and decoloring by using a glass rod, filtering after 2min, putting 1.00mL of clarified liquid into a 10mL colorimetric tube, dropwise adding 2 to 3 drops of natural purple sweet potato pigment, wherein the solution is red, dropwise adding 0.050mol/LNaOH solution by using a rubber head dropper until the color is orange gray, adding 2.00mL of neutral formaldehyde, wherein the solution is red, adding 0.3929mol/LNaOH solution by using the liquid transfer gun until the solution is 100 mu L, and observing the color of the solution after slightly shaking, wherein the solution is still red, red orange or orange gray and does not fade in half minute, so that the product is a secondary high-salt dilute state fermented soy sauce, and if the solution is grayish blue, green earth or yellow brown and does not fade in half minute, the product does not meet the requirements of the secondary high-salt dilute state fermented soy sauce.
(4) Second grade soy sauce (Low salt solid fermentation soy sauce amino acid nitrogen content is more than or equal to 0.60g/100 mL)
Experimental reagent: 0.4286mol/LNaOH solution, neutral formaldehyde
The method comprises the following operation steps: transferring 1.00mL of soy sauce into a 25mL colorimetric tube by using a liquid transfer gun, adding 9.00mL of water for dilution, adding 0.30g of activated carbon, slightly stirring and decoloring by using a glass rod, filtering after 2min, putting 1.00mL of clear liquid into a 10mL colorimetric tube, dropwise adding 2 to 3 drops of natural purple sweet potato pigment, wherein the solution is red, dropwise adding 0.050mol/LNaOH solution by using a rubber head dropper until the color is orange gray, adding 2.00mL of neutral formaldehyde, wherein the solution is red, adding 0.4286mol/LNaOH standard solution 100 mu L by using the liquid transfer gun, slightly shaking, observing the color of the solution, wherein the solution is still red, red orange or orange gray and does not fade in half minute, and the product is a secondary low-salt solid fermented soy sauce, and if the solution is grayish blue, green earth or yellow brown and does not fade in half minute, the product does not meet the requirements of the secondary low-salt solid fermented soy sauce.
(5) Third-class soy sauce (amino acid nitrogen content is more than or equal to 0.40g/100 mL)
Experimental reagent: 0.2857mol/LNaOH standard solution, neutral formaldehyde
The method comprises the following operation steps: transferring 1.00mL of soy sauce into a 25mL colorimetric tube by using a liquid transfer gun, adding 9.00mL of water for dilution, adding 0.30g of activated carbon, slightly stirring and decoloring by using a glass rod, filtering after 2min, taking 1.00mL of clarified liquid into a 10mL colorimetric tube by using the liquid transfer gun, dropwise adding 2 to 3 drops of natural purple sweet potato pigment, wherein the solution is red, dropwise adding 0.050mol/LNaOH standard solution by using a rubber head dropper until the color is orange gray, adding 2.00mL of neutral formaldehyde, wherein the solution is red, adding 0.2857mol/LNaOH standard solution 100 mu L by using the liquid transfer gun, slightly shaking, observing the color of the solution, wherein the solution is still red, red orange or orange gray and does not fade in half minute, wherein the product is the three-stage fermented soy sauce, and if the solution is grayish blue, green earth or yellow brown and does not fade in half minute, the product does not meet the requirements of the three-stage fermented soy sauce, namely does not meet the national relevant standards.
And (5) result verification: the results obtained by the rapid detection method provided by the application are completely consistent with the results obtained by the national standard method, as shown in table 3.
(II) method for rapidly detecting whether the content of amino acid nitrogen in diluted brewed soy sauce is qualified or not and result verification
Diluting the original soy sauce with distilled water according to a certain proportion to obtain a sample, performing acidometry according to the first method of national food safety standard GB5009.235-2016 (determination of content of amino acid nitrogen in food) and calculating the result according to (1), performing parallel measurement for 3 times, checking that no suspicious value exists, taking an average value to report, and determining the results of national standard methods for samples diluted according to different proportions to obtain the results shown in tables 4 and 5.
1. Preparation and measurement of samples diluted one time as brewed Soy sauce
Transfer 12.50mL of soy sauce as it is to a 25.00mL volumetric flask with a pipette, dilute to the mark with water (pH = 6.80) of yibao purified water, and mix well for use. The samples were tested in parallel 3 times according to the first acidimetric method of GB5009.235-2016 "determination of amino acid nitrogen content in food", and after no suspicious values were detected, an average value was reported.
Meanwhile, transferring 1.00mL of a sample diluted by one time of the soy sauce into a 25mL colorimetric tube by using a liquid transfer gun, adding 9.00mL of water for dilution, adding 0.30g of activated carbon, slightly stirring and decoloring by using a glass rod, filtering after 2min, taking 1.00mL of decolored clear liquor by using the liquid transfer gun into a 10mL colorimetric tube, dropwise adding 2 to 3 drops of the purple sweet potato pigment prepared by the application, wherein the solution is red, dropwise adding 0.050mol/LNaOH standard solution by using a rubber head dropper until the color is orange gray, adding 2.00mL of neutral formaldehyde, wherein the solution is red, adding 100 mu L of the NaOH standard solution with corresponding concentration at each level by using the liquid transfer gun, observing the color of the solution after slight shaking, wherein the solution is still red, red-orange or orange gray and does not fade in half minute, the product is a qualified brewed soy sauce, and if the solution is grayish blue, earth-green or yellow-brown and does not fade in half minute, the brewed soy sauce does not meet the requirements of the brewed soy sauce, i.e. not meeting the relevant national standard, the color of the solution in the reagent tube and the measurement result of the national standard method are shown in the table 4.
2. Preparation and assay of samples after five-fold dilution of soy sauce as received
Transfer 5.00mL of soy sauce as it is into a 25.00mL volumetric flask with a pipette, dilute to the mark with yibao purified water of water (pH = 6.80), and mix well for use. The samples were tested in parallel 3 times according to the first acidimetric method of GB5009.235-2016 "determination of amino acid nitrogen content in food", and after no suspicious values were detected, an average value was reported.
Meanwhile, transferring 1.00mL of a sample obtained by diluting soy sauce five times original form into a 25mL colorimetric tube by using a liquid transfer gun, adding 9.00mL of water for dilution, adding 0.30g of activated carbon, slightly stirring and decoloring by using a glass rod, filtering after 2min, taking 1.00mL of decolored clear liquor by using the liquid transfer gun into a 10mL colorimetric tube, dropwise adding 2 to 3 drops of the purple sweet potato pigment prepared by the application, wherein the solution is red, dropwise adding 0.050mol/LNaOH standard solution by using a rubber head dropper until the color is orange gray, adding 2.00mL of neutral formaldehyde, wherein the solution is red, adding 100 mu L of the NaOH standard solution with corresponding concentration at each level by using the liquid transfer gun, slightly shaking to observe the color of the solution, wherein the solution is still red, red-orange or orange-gray and does not fade in half minute, the product is a qualified brewed soy sauce, and if the solution is grayish blue, green-earth color or yellow-brown and does not fade in half minute, the brewed soy sauce does not meet the requirement of the brewed soy sauce, i.e. not meeting the relevant national standard, the color of the solution in the reagent tube and the measurement result of the national standard method are shown in the table 5.
The invention sets the volume of the added standard sodium hydroxide solution to be 100 microliter, namely 0.10mL of the original brewed soy sauce for rapid identification according to the qualification index of amino acid nitrogen of the brewed soy sauce at each level and the principle of equal quality reaction. When the amount of the residual acidic substance (-COOH) in the sample solution is identified according to the method of the step, the red color is more than red orange, and the red orange color is more than orange gray; the residual monobasic strong base (-OH) in the solution has a yellowish-brown color greater than that of smectite, and a smectite color greater than that of grayish-blue.
The concentration of the monobasic strong base is scientifically designed according to the minimum requirement of the qualified amino acid nitrogen of different levels of physical and chemical indexes of brewed soy sauce. According to the quantitative reaction principle of the substances, when the volume of a taken raw soy sauce sample is determined, adding a monobasic strong alkali solution with the same volume, if the concentration of amino acid nitrogen in the taken soy sauce sample is greater than that of the monobasic strong alkali solution, the concentration of an acid substance (-COOH) generated by converting the amino acid nitrogen by using neutral formaldehyde is also greater than that of the monobasic strong alkali solution, after the reaction is finished, the acid substance is left, the product is a qualified product, and the more the acid is left, the better the quality of the product is; if the concentration of the acid substances converted from the quality of the brewed soy sauce and the like is exactly equal to the concentration of the monobasic strong alkali solution, the theoretical end point which meets the lowest requirement of the soy sauce of the corresponding grade is exactly reached after the reaction is finished, and the acid substances and the alkali substances are not left, so that the brewed soy sauce is a qualified product which meets the requirement of the lowest index; if the concentration of the acid substance (-COOH) converted from the soy sauce is less than that of the monobasic strong alkali solution, strong alkali is remained after the reaction is completed, and the-COOH is insufficient, the amino acid nitrogen in the brewed soy sauce is lower than the minimum requirement of the corresponding grade, and the brewed soy sauce is a product which does not conform to the corresponding grade. This conclusion can be quickly identified with a suitable indicator.
The weak acid is titrated by the monobasic strong base, and the titration jump always falls in the alkaline region. The size of the jump range is related to the concentration of acid and base and the size of Ka of the weak acid, such as: titrating 0.01000mol/L acetic acid with 0.01000mol/L sodium hydroxide to a breakthrough pH of 7.7 to 8.7 with a theoretical endpoint of pH 8.23 (see the book on analytical chemistry); the purple sweet potato pigment has red (pH 2, 3), red orange (pH 4, 5, 6, 7), orange gray (pH 8), gray blue (pH 9), and green earth (pH 10, 11) to yellow brown (pH 12, 13) colors.
Therefore, the brewed soy solution is operated according to the technology of the invention, the color result of the purple sweet potato pigment is red and reddish orange, after the reaction is finished, acidic substances (-COOH) are left before products at all levels are positioned at the theoretical endpoint meeting the requirement of minimum amino acid nitrogen of the brewed soy at the corresponding level, and the products can be immediately judged as qualified samples; if the color of the purple sweet potato pigment is orange gray, the reaction just reaches the theoretical end point meeting the requirement of the lowest amino acid nitrogen of the soy sauce of the corresponding grade, the amounts of acid and alkali substances in the solution are just equal and not excessive, and the brewed soy sauce can be judged to be just a qualified sample of the corresponding grade; if it appears grayish blue, greenish earth or yellowish brown, the reaction has exceeded the theoretical endpoint of minimum amino acid nitrogen requirement and the strong base is in excess, which is a rejected sample.
Above, if the color of the purple sweet potato pigment is orange gray, the reaction just reaches the theoretical end point required by the lowest amino acid nitrogen of soy sauce at each level, the amounts of acid and alkali substances in the solution are just equal and not excessive, and the sample can be judged to be just qualified; the end of the reaction is near the theoretical endpoint, which is rarely seen, and the resulting relative error is no more than 0.1%, i.e., the accuracy of the assay is more than 99.9% (the range of titration jump is the interval of relative error between-0.1% and +0.1% before and after the theoretical endpoint), which is the allowable range for titration reaction and assay. In a large number of specific samples identified in actual operation, the orange gray or gray blue color tone of the purple sweet potato pigment in the actual samples is not seen. In the products of manufacturers, the amino acid nitrogen is almost zero according to the minimum design process required by the national standard, and the amino acid nitrogen is not found at present.
Supplementary notes in addition: since the acidity of the solution, i.e. the pH value, determines the existing form of the acid-base indicator and the ratio of the acid color form to the base color form, the specific color of the indicator is also determined, i.e. the acidity of the solution determines the color of the acid-base indicator. Whereas the shade of the same colour is related to the concentration of the colour substance, i.e. to the amount of the relevant colour substance per unit volume, i.e. the concentration of the indicator determines the shade of the colour. At the same acidity, the shade of the solution is proportional to the amount of indicator added. If the sample with the same volume is added with only one drop of purple sweet potato pigment in the identification, the color is lighter, but the judgment of a conclusion is not influenced. Because the amount of the indicator added does not generally affect the acidity of the solution and does not affect the presence of the indicator and the ratio. The added purple sweet potato pigment is less, the same color is lightened due to less colored particles of the purple sweet potato pigment in unit volume, the same color is darker due to more added purple sweet potato pigment, but the purple sweet potato pigment is still red (including dark red, light red, orange red, pink, reddish and the like), green (including green, dark green, grass green, earthy green, light green and the like) and yellow (including brown yellow, dark yellow, green and the like), and the shades of the colors are different. So long as the result is judged correctly and the result is obtained.
While the preferred embodiments of the present invention have been described, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention.
Claims (8)
1. A method for rapidly detecting whether the content of amino acid nitrogen in brewed soy meets a certain level is characterized in that: taking a brewed soy sauce sample a, decoloring, and obtaining a decolored brewed soy sauce sample b after decoloring; (2) get a solution containing V2The decolorized soy sauce sample b of the soy sauce sample a is mL, an indicator is dripped into the decolorized soy sauce sample b, the acid color of the indicator is displayed in the solution, then 0.050mol/L NaOH is dripped into a rubber head dropper to enable the color of the solution to be the mixed color of the acid color and the alkali color of the indicator, neutral formaldehyde is added after the solution is not decolorized for half a minute, the amino acid nitrogen in the soy sauce sample b is completely converted into an acid substance, and the acid color of the indicator is displayed in the solution at the moment; then V is added3 Slightly shaking the solution by using the mL monobasic strong alkali solution, and observing the color of the solution, wherein when the solution shows the acid color of the indicator or the acid-base equivalent point, the mixed color of the acid color and the alkali color of the indicator is shown, and the sample is a qualified product which accords with a certain grade if the solution does not fade for half minute, and when the solution shows the alkali color of the indicator and the sample does not fade for half minute, the sample is an unqualified product which does not accord with the certain grade; concentration of monobasic alkali solution and Soy sauce sample aThe lowest concentration of the grade is equal, and V2=V3。
2. The method for rapidly determining whether the content of amino acid nitrogen in brewed soy sauce meets a certain level according to claim 1, wherein: the monobasic strong alkali solution is sodium hydroxide solution, potassium hydroxide solution or lithium hydroxide solution.
3. The method for rapidly determining whether the content of amino acid nitrogen in brewed soy sauce meets a certain level according to claim 1, wherein: the brewed soy sauce sample a is original soy sauce which is sold in the market or produced in factories; the original soy sauce is Li Jinji delicious seafood soy sauce/light soy sauce, Haitian soy sauce superior dark soy sauce, Li Jinji choice light soy sauce, Li Jinji bright dark soy sauce, and optionally straw mushroom dark soy sauce, gold symbol dark soy sauce and Luhua natural fresh soy sauce.
4. The method for rapidly determining whether the content of amino acid nitrogen in brewed soy sauce meets a certain level according to claim 1, wherein: the indicator is purple sweet potato pigment.
5. The method for rapidly determining whether the content of amino acid nitrogen in brewed soy sauce meets a certain level according to claim 1, wherein: the preparation method of the purple sweet potato pigment comprises the following steps: taking purple sweet potatoes, grinding into paste, adding distilled water according to the material-liquid ratio of 1:5, transferring into a 100mL colorimetric tube, extracting with the assistance of ultrasonic waves at the extraction temperature of 55 ℃ for 15-20min, and filtering to obtain purple sweet potato pigment; the pH of the distilled water used was 6.8.
6. The method for rapidly determining whether the content of amino acid nitrogen in brewed soy sauce meets a certain level according to claim 4, wherein: the step (1) of decolorizing the soy sauce comprises the following steps: transferring 1.00mL of the original soy sauce sample into a 25mL colorimetric tube by using a liquid transfer gun, adding 9.00mL of water for dilution, adding 0.30g of activated carbon, slightly stirring by using a glass rod for decolorization, and filtering after 2min to obtain a filtered clarified liquid which is the decolorized soy sauce sample b.
7. The method for rapidly determining whether the content of amino acid nitrogen in brewed soy sauce meets a certain level according to claim 6, wherein: in the step (2), 1.00mL of soy sauce sample b is taken by a pipette and is placed in a 10mL colorimetric tube, 2 to 3 drops of natural purple sweet potato pigment are dropwise added, at the moment, the solution is red or red-orange, 0.050mol/LNaOH standard solution is dropwise added by a rubber head dropper until the color is orange-gray, and 2.00mL of neutral formaldehyde is added, at the moment, the solution is red or red-orange.
8. The method for rapidly determining whether the content of amino acid nitrogen in brewed soy sauce meets a certain level according to claim 4, wherein: the acid color of the purple sweet potato pigment is red or reddish orange, the alkali color of the purple sweet potato pigment is grayish blue, green soil color or yellow brown, and the mixed color of reddish orange and grayish blue is orange gray when the acid and alkali are equal in point.
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| CN113984952B (en) * | 2021-11-12 | 2023-09-08 | 鹤山市东古调味食品有限公司 | Method for analyzing components of fermented soy sauce |
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