Neutral-alkaline pH indicator capable of being recognized by naked eyes as well as preparation method and application of neutral-alkaline pH indicator

Abstract

The invention relates to the field of detection, and discloses a neutral-alkaline pH indicator capable of being recognized by naked eyes as well as a preparation method and application of the neutral-alkaline pH indicator. According to the neutral-alkaline pH indicator, anthocyanin in different fruit peels is extracted, compounding is conducted on the basis of the pigment three-primary color theory, a blend of summer black grape peel source anthocyanin and black wolfberry peel source anthocyanin is selected as an effective active substance to prepare the neutral-alkaline pH indicator, and the pH indicator indicates various color changes within the neutral-alkaline range (pH 6.0-10.0); the naked eye recognition contrast ratio is high, and the highest fine indication precision reaches 0.2 pH indexing. The invention also performs application detection of volatile biogenic amines such as trimethylamine in the food field, and the result shows that the pH indicator has high human friendliness on the basis of naked eye identification, high matching degree and high precision, does not generate food-borne food safety risks, and can be suitable for detection in the food field.

Description

Technical Field

[0001] The present invention relates to the field of detection, and particularly to a neutral to alkaline pH indicator that can be visually identified, and its preparation method and application. Background Art

[0002] The pH value plays a crucial role in the food industry and is one of the core elements to ensure food quality and safety. The adjustment of the pH value is crucial for the color, taste, and stability of nutritional components of food. For example, the pH value of fruit juice beverages needs to be strictly controlled to prevent the oxidation loss of nutritional components and maintain a moderate sour and sweet taste. During food processing, the precise control of the pH value can directly affect the growth environment of microorganisms, thereby effectively inhibiting the reproduction of harmful bacteria and extending the shelf life of food. For example, in the production of fermented foods, an appropriate low pH value can promote the growth of beneficial bacteria such as lactic acid bacteria and endow the food with unique flavors and textures. The pH value of fresh meat is usually between 5.8 and 6.2. When its pH value ≥ 6.7, such meat products are generally considered to have deteriorated. The physiological pH of live fish is usually 7.3, but after the fish dies, the pH value will drop to 6.0 - 6.8. The safety of water quality is judged by measuring the pH value of drinking water (6 - 8) and seawater (8 - 9). Therefore, the precise management of the pH value, especially the indication of neutral to alkaline pH values (6 - 10), is an important manifestation of the specialization and refinement of the food industry and is a key link to ensure food safety, improve product quality, and promote the sustainable development of the industry.

[0003] The currently popular pH value testing methods are mainly divided into the following two categories: (1) pH electrode detection based on the electrochemical principle. The glass electrode is a typical representative and is currently the most widely used pH electrode. For example, in Chinese Patent CN219477016U, a protection device is prepared to prevent the glass electrode body from being corroded and extend its service life. However, affected by the surrounding usage environment, there are large errors. In addition, its portability and ease of use limit its application; (2) pH value detection based on the optical principle. For example, pH indicators are loaded on different carriers. For example, in Chinese Patents CN116297185A and CN113484317A, the dyes used in the above patents are mostly industrial dyes, which have certain pollution. Compared with the first type of glass electrode, they have the advantages of simple use and fast testing. However, the colors corresponding to the pH indicator values loaded on different carriers are not only greatly affected by the subjectivity of the tester but also affected by their background colors, and the testing accuracy is relatively low. And in the current research, the detection effect for the neutral to alkaline range is not ideal. The glass electrode is not convenient enough, the detection accuracy of different carriers combined with pH indicators is insufficient, and it will also contaminate food.

[0004] In the prior art, anthocyanins and other substances are also used as indicators, but various problems also exist. For example, in Chinese Patent CN 118347995 A, blueberry anthocyanins are used as indicators, but their color change is single and difficult to be recognized by the naked eye; in Chinese Patent CN 116297185 A and Chinese Patent CN 113484317 B, pyridine-based substances, methyl orange and other chemicals that are harmful to the human body are used in the preparation processes of the above two patents respectively; in Chinese Patent CN 117843992 A and Chinese Patent CN 115897066 A, both patents have the problem that the color change is not obvious, especially the pH change in the medium and alkaline ranges.

[0005] Therefore, there is an urgent need for a pH indicator that is convenient, fast, can be directly used for food detection, has high precision and is easy to identify, and can be applied to the medium and alkaline ranges. Summary of the Invention

[0006] To solve the above technical problems, the present invention provides a preparation method and application of a highly precise medium and alkaline pH indicator based on the "pigment primary colors" theory and compounded with multiple anthocyanins, which can be recognized by the naked eye. One is to solve the problems that the pH indicator has an unclear color change, insufficient discrimination, and inaccurate indication matching under medium and alkaline conditions, fill a gap in the application of pH indicators in the medium and alkaline ranges, and can also replace the existing pH indicators that contaminate food. By detecting the pH value changes of volatile biogenic amines (such as trimethylamine (TMA), ammonia (NH3)) generated during the spoilage of meat and aquatic products, the degree of food spoilage can be evaluated.

[0007] According to the color change characteristics of anthocyanins at different pH values, the present invention uses the "pigment primary colors" theory to compound anthocyanins, prepares a highly precise and easy-to-identify medium and alkaline pH indicator based on natural anthocyanins, and applies it to the detection of pH values. The "pigment primary colors" theory is to mix three colors, cyan, magenta, and yellow, in proportion to produce other colors. Anthocyanins extracted from plants will react in an acidic or alkaline environment, and then produce a color response. Anthocyanins exhibit the characteristics of pH response, and with their rich color changes, no pollution to food, and sensitivity to pH values, they can replace traditional chemical pH indicators and be used for food detection, such as monitoring the freshness of meat, dairy products, and aquatic products.

[0008] The present invention is realized through the following technical solutions: A medium and alkaline pH indicator that can be recognized by the naked eye, which is a medium and alkaline pH indicator based on natural anthocyanins. The effective active substance of the medium and alkaline pH indicator is derived from the pericarp with a high content of natural anthocyanins, and the pericarp is one or more of grape skin, mangosteen skin, black wolfberry skin, etc. The medium and alkaline pH indicator is particularly suitable for the detection in the pH range of 6-10.

[0009] Preferably, the neutral alkaline pH indicator is a mixture of anthocyanins from summer black grape skin and anthocyanins from black wolfberry skin, and the mass ratio of anthocyanins from summer black grape skin and anthocyanins from black wolfberry skin is 1:2, 1:1, 2:1 or 3:1, preferably 0.95-1.05: 0.95-1.05 (more preferably 1:1) at pH 6-10 and a detection index of 0.5 pH. Its color indication process is lilac pink → grape purple → dark purple → dark blue → gray → soft blue → earth yellow → olive green → brown. At pH 9-10 and a detection index of 0.2 pH, the mass ratio of anthocyanins from summer black grape skin and anthocyanins from black wolfberry skin is further preferably 0.75-0.85:1.2 (preferably 0.8:1.2), and its color indication process is dark dark green → dark blue → dark indigo → dark blue-purple → nearly black → dark green.

[0010] The method for screening a neutral alkaline pH indicator that can be identified by naked eyes comprises the following steps: S1: Extraction of effective active substances: extract effective active substances from various fruit peels with high natural anthocyanin content, mix the effective active substances with the extract, pass through a column, elute, and dry the eluate to obtain the effective active substances; S2: pH response of a single effective active substance: Dissolve multiple effective active substances in water to prepare anthocyanin solution, mix the pH 6-10 buffer solution with the anthocyanin solution and let it stand, and measure the color difference value; S3: Screening of three primary colors of anthocyanins: Determine the color difference L, a, b values ​​of each anthocyanin at pH = 7, and select the three primary colors of anthocyanins; S4: Compounding of neutral alkaline indicators: According to the theory of "three primary colors of pigments", the three primary colors of anthocyanins are compounded in different proportions to prepare pH indicators; the ∆E of each pH value is compared to obtain the ratio with the most obvious color change.

[0011] Preferably, in step S1, the method for preparing fruit peels rich in anthocyanin effective active substances is: wash grape skins, mangosteen skins, black wolfberry skins and other fruit peels rich in anthocyanin effective active substances, place them in an oven at 45-55°C and dry them to constant weight, and the dried raw materials are crushed and sieved and placed in brown bottles for storage away from light; the grape skins are selected from summer black grape skins, Kyoho grape skins and Cresson grape skins, etc.

[0012] Preferably, in step S1, the extraction is performed by mixing the peel with an extracting solution, wherein the extracting solution is a mixture of anhydrous ethanol, hydrochloric acid and an aqueous solution in a ratio of 5:1:4 to 1:5:4, and then extracting in a water bath for 2 to 6 hours. Furthermore, the ratio of anhydrous ethanol, hydrochloric acid and an aqueous solution (extracting solution) is preferably 2:1:1.

[0013] Preferably, in step S1, when the extract is passed through the column, the extract is loaded onto an AB-8 macroporous adsorption resin chromatographic column, left standing for 1 to 3 h, washed with distilled water to remove water-soluble impurities such as sugars, and then eluted with a 60 to 80% ethanol solution to elute anthocyanins until the chromatographic column is colorless.

[0014] Preferably, in step S1, the drying is to concentrate the eluate under reduced pressure and then perform freeze-drying.

[0015] Preferably, in step S2, the buffer solution is a buffer solution with a pH of 6 to 10 prepared from 0.1 mol / L standard sodium hydroxide solution and 0.1 mol / L standard hydrochloric acid solution.

[0016] Preferably, in step S2, an effective active substance is weighed and placed in distilled water, and magnetically stirred to fully dissolve the effective active substance of anthocyanins in distilled water to obtain an anthocyanin solution with an effective content concentration of 0.05 to 0.30 wt%.

[0017] Preferably, in step S2, for the mixing and standing, the anthocyanin solution is added to the buffer solution, left standing for 20 to 40 s to fully undergo a color reaction, and after the reaction is stable, a portable color difference meter is used to measure the color difference value.

[0018] Preferably, in step S3, for the screening of the three primary colors of anthocyanin pigments, the color difference L, a, b values of each anthocyanin at pH = 7 are measured, converted into CMYK values through Adobe Photoshop software, and compared with the theoretical values of the "three primary colors of pigments" red (C = 0, M = 100, Y = 0), yellow (C = 0, M = 0, Y = 100), blue (C = 100, M = 0, Y = 0), and those closer to the theoretical values are selected as the three primary colors of anthocyanins.

[0019] Preferably, in step S3, for cyan (C value), anthocyanins from black goji berry skins are selected; for magenta (M value), anthocyanins from Summer Black grape skins are selected; for yellow (Y value), anthocyanins from mangosteen skins are selected. Experiments show that: the C value of anthocyanins from black goji berry skins is closest to the theoretical value of the "three primary colors of pigments", and the M value of anthocyanins from Summer Black grape skins and the Y value of anthocyanins from mangosteen skins are closest to the theoretical value of the "three primary colors of pigments".

[0020] Preferably, in step S3, an indicator with a mass ratio of 1:1 of anthocyanins from Summer Black grape skins and anthocyanins from black goji berry skins added is selected as the neutral to alkaline pH indicator. Based on the experimental results, the ratio of anthocyanins from Summer Black grape skins plus anthocyanins from black goji berry skins has the best effect, and the adjacent color difference values are all greater than 5. In most studies, a color difference value greater than 5 means that the color change can be recognized by the human eye, which also proves the characteristic of rapid visual identification of the indicator of the present invention.

[0021] Apply the above-mentioned visually recognizable neutral-to-alkaline pH indicator to food detection.

[0022] The food detection is for detecting the change in the content of volatile biogenic amines (such as TMA, NH3, etc.) in food within the neutral-to-alkaline range, applicable within the pH range of 6 to 10. Further, it can quickly and visually distinguish the change of 0.5 pH gradation within the pH range of 6 to 10 and the change of 0.2 pH gradation within the pH range of 9 to 10.

[0023] The application method of this pH indicator includes loading the pH indicator on a carrier, putting it into the food for detection, and judging the content of volatile biogenic amines inside the food according to its color change, so as to achieve the purpose of predicting the food safety and edibility.

[0024] The loading methods of the neutral-to-alkaline pH indicator include, but are not limited to, soaking, spraying, and coating.

[0025] The carriers of the neutral-to-alkaline pH indicator include, but are not limited to, paper, cloth, aerogel, hydrogel, and film.

[0026] Compared with the prior art, the present invention has the following beneficial technical effects: 1. In the neutral-to-alkaline range (pH 6 - 10), the color change is diverse to the naked eye, with high contrast and easy to identify. The indicator prepared by the present invention can present various color changes during detection, without the need for instrument assistance to distinguish. The user can directly and quickly observe and identify, and is not interfered by the background color of the test paper during the detection process, making the observation result clearer.

[0027] 2. High matching degree and high precision. The present invention can accurately and significantly reflect the color response of every 0.5 pH gradation change within the pH range of 6 to 10 by using different compounding methods and ratios. In the pH range of 9 to 10, the present invention can accurately reflect the change of 0.2 pH gradation, further reflecting the high precision of the present invention. Compared with traditional pH indicators, it can more accurately reflect the pH change, is more sensitive to pH, and has a more accurate detection matching degree.

[0028] 3. Good stability and human-friendly nature, and can be used for food field detection. The present invention is prepared by using natural anthocyanins. Anthocyanins are green and pollution-free, non-toxic to the human body, and will not contaminate food, having a wide prospect in the food detection field. The present invention demonstrates diverse pH color change response characteristics, especially in the neutral-to-alkaline pH value range, and can accurately reflect the pH change difference of 0.5 units. In the pH range of 9 to 10, this patent can even distinguish the pH change of 0.2 units, thus highlighting its high-precision measurement ability. The present invention will have an obvious color change (color difference > 5) when reaching the limit node of trimethylamine, and can be applied to the detection of food fields such as aquatic products.

[0029] 4. Simple to manufacture with wide material sources. The manufacturing method of the present invention is of low difficulty, and the preparation materials are easy to purchase. Even non-professionals can quickly master the preparation after understanding.

[0030] 5. Promote resource recycling. The materials used in the present invention can be grape skins, mangosteen skins, other agricultural and sideline products or agricultural waste, and their added value can be increased through recycling and reprocessing. Description of the Drawings

[0031] In order to more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the following will briefly introduce the drawings required to be used in the embodiments. Obviously, the drawings in the following description are only some embodiments of the present invention. For those skilled in the art, without creative efforts, other drawings can also be obtained based on these drawings.

[0032] Figure 1 It is a comparison chart with the theoretical values of "the three primary colors of pigments"; Figure 2 It is a chart for screening the optimal ratio; the ratio of each pigment is 1:1 or 1:1:1; Figure 3 It is a color change chart of Summer Black grape skin-derived anthocyanin: black wolfberry skin-derived anthocyanin = 1:1 (pH response of the colorimetric system in the pH range of 6 - 10 (pH 0.5 graduation)); Figure 4 It is a color change chart of Summer Black grape skin-derived anthocyanin: black wolfberry skin-derived anthocyanin = 0.8:1.2 (pH response of the colorimetric system in the pH range of 9 - 10 (pH 0.2 graduation)); Figure 5 It is the actual application effect (trimethylamine concentration detection); Figure 6 It is the actual application effect (ammonia concentration detection). Detailed Description of the Invention

[0033] To more clearly elaborate the purpose, technical solutions and advantages of the present invention, the following will be described in detail in combination with the drawings and specific embodiments. The drawings shown only represent the exemplary embodiments of the present invention and do not limit the implementation manner thereof. The present invention can be implemented in various forms, and its design concept and core technology are not limited by the embodiments shown in the drawings. The purpose of providing these embodiments is to more conveniently enable those skilled in the art to understand the principles, structures and functions of the present invention, so as to better master and apply its technical solutions. The terms used in this specification are only for describing specific embodiments and do not constitute a limitation to the present invention.

[0034] Example 1: Preparation of effective active substances of Summer Black grape anthocyanin S1: Peel the skins of Summer Black grapes, wash them, and then dry them in an oven at 50 °C until constant weight. After drying, the effective active substances of anthocyanins are crushed respectively, passed through a 40-mesh sieve, and finally stored in a brown bottle away from light. S2: Mix the effective active substances of anthocyanins obtained above with a solution of absolute ethanol: hydrochloric acid: water = 2:1:1, and extract in a water bath for 3 h. Load the extract onto an AB-8 macroporous adsorption resin chromatography column (1.5×60 cm) and let it stand for 2 h. S3: After standing, wash the water-soluble impurities such as sugars with distilled water, and then elute the anthocyanins with 70% ethanol solution (5 column volumes (CV); flow rate: 2 mL / min) until the inside of the chromatography column is colorless. Finally, concentrate the eluate under reduced pressure and then perform freeze-drying (model LGJ-12A, Beijing Sihuan Qihang Technology Co., Ltd., China) to obtain the effective active substances of Summer Black grape anthocyanins.

[0035] Example 2: Preparation of effective active substances of Kyoho grape anthocyanins Replace the raw materials in Example 1 with Kyoho grape skins, and keep the other steps unchanged to obtain the effective active substances of Kyoho grape anthocyanins.

[0036] Example 3: Preparation of effective active substances of Crimson Seedless grape anthocyanins Replace the raw materials in Example 1 with Crimson Seedless grape skins, and keep the other steps unchanged to obtain the effective active substances of Kyoho grape anthocyanins.

[0037] Example 4: Preparation of effective active substances of black wolfberry anthocyanins Replace the raw materials in Example 1 with black wolfberry fruit skins, and keep the other steps unchanged to obtain the effective active substances of black wolfberry anthocyanins.

[0038] Example 5: Preparation of effective active substances of mangosteen anthocyanins Replace the raw materials in Example 1 with mangosteen fruit skins, and keep the other steps unchanged to obtain the effective active substances of mangosteen anthocyanins.

[0039] Example 6: pH response test of natural anthocyanins S1: Conduct a single anthocyanin pH response test on the effective active substances obtained in Examples 1-5 to detect their color changes. Prepare a buffer solution with a pH of 6-10 using 0.1 mol / L standard sodium hydroxide solution and 0.1 mol / L standard hydrochloric acid solution. Weigh 0.5 g of the effective active substances of Summer Black grapes, Kyoho grapes, Crimson Seedless grapes, black wolfberries, and mangosteen anthocyanins in Examples 1-5 respectively, place them in 50 ml of distilled water, and then stir for 10 min under magnetic stirring at 600 r / m to dissolve the anthocyanins fully in the distilled water. S3: Use a pipette to measure 6 mL of buffer solution with a pH of 6 - 10 into 5 10 - mL centrifuge tubes respectively. Then measure 1 mL of the effective active substances of anthocyanins from Summer Black grapes, Kyoho grapes, Crimson Seedless grapes, black goji berries, and mangosteens and dissolve them into the above - mentioned 5 centrifuge tubes. Let it stand for 30 s to fully undergo a color reaction. S4: After the reaction stabilizes, use a portable colorimeter (model RM200QC, X - rite (Shanghai) Color Technology Co., Ltd., USA) to measure the color difference value, and finally make the anthocyanin concentration of each reaction solution 0.17 wt%.

[0040] Example 7: Color confirmation of natural anthocyanins S1: According to the pH response of the single - peel anthocyanins in Example 6, select the color difference L, a, b values of the anthocyanins from Summer Black grapes, Kyoho grapes, Crimson Seedless grapes, black goji berries, and mangosteen peels at pH = 7. S2: Convert them into CMYK values through Adobe Photoshop CC 2018 software and compare them with the theoretical values of the "three primary colors of pigments": red (C = 0, M = 100, Y = 0), yellow (C = 0, M = 0, Y = 100), blue (C = 100, M = 0, Y = 0). Select the ones that are closer to the theoretical values as the "three primary colors of anthocyanins".

[0041] The comparison result shows that the C value of the anthocyanins from black goji berry peels is closest to the theoretical value of the "three primary colors of pigments", the M value of the anthocyanins from Summer Black grape peels is the highest, and the Y value of the anthocyanins from mangosteen peels is the highest and closest to the theoretical value, as Figure 1 shown. For all three colors, select the ones that are closest to the theoretical values of the "three primary colors of pigments". Finally, the "three primary colors of anthocyanins" selected are: for cyan (C value), use the anthocyanins from black goji berry peels; for magenta (M value), use the anthocyanins from Summer Black grape peels; for yellow (Y value), use the anthocyanins from mangosteen peels.

[0042] Example 8: Preparation of neutral - basic pH indicator of natural anthocyanins S1: According to the "three - primary - color of pigments" compounding principle, compound the three pigments of the anthocyanins from black goji berry peels, Summer Black grape peels, and mangosteen peels screened in Example 7, that is, compound them in different ratios (1:1 / 1:2 / 1:3) of two different colors. S2: Prepare the pH indicator. The ratio of Summer Black grape peel - sourced anthocyanins to black goji berry peel - sourced anthocyanins is 1:1 / 1:2 / 1:3; the ratio of Summer Black grape peel - sourced anthocyanins to mangosteen peel - sourced anthocyanins is 1:1 / 1:2 / 1:3; the ratio of black goji berry peel - sourced anthocyanins to mangosteen peel - sourced anthocyanins is 1:1 / 1:2 / 1:3; the ratio of Summer Black grape peel - sourced anthocyanins to black goji berry peel - sourced anthocyanins is 2:1 / 3:1; the ratio of Summer Black grape peel - sourced anthocyanins to mangosteen peel - sourced anthocyanins is 2:1 / 3:1; the ratio of black goji berry peel - sourced anthocyanins to mangosteen peel - sourced anthocyanins is 2:1 / 3:1. S3: For color reaction in the medium alkaline range of pH 6 - 10, the color difference is measured using CIE color parameters (ΔE) to describe color change; the ΔE value is calculated according to the formula: ; Such as Figure 2 , from the comparison results, it can be obtained that the adjacent color differences of the ratio of anthocyanins from Summer Black grape skins and black wolfberry skins are all greater than 5. Among all the ratios, the color change is the most obvious and is most easily distinguishable by the human eye. Among them Figure 3 shows the color change diagram of anthocyanins from Summer Black grape skins: anthocyanins from black wolfberry skins = 1:1 (pH response of the colorimetric system in the pH range of 6 - 10 (pH 0.5 scale)); Figure 4 shows the color change diagram of anthocyanins from Summer Black grape skins: anthocyanins from black wolfberry skins = 0.8:1.2 (pH response of the colorimetric system in the pH range of 9 - 10 (pH 0.2 scale)).

[0043] Example 9: Food detection application experiment of the indicator (1) Response of the indicator to TMA S1: Take 0.2 g of anthocyanins from Summer Black grape skins and 0.2 g of anthocyanins from black wolfberry skins and dissolve them in 20 ml of distilled water, and pour the solution into a petri dish; S2: Put the petri dish into a 2 L sealed jar, prepare TMA concentration gradients of 50 ppm, 100 ppm, 150 ppm, 200 ppm, 250 ppm, 300 ppm, 350 ppm, and distribute the prepared gradient solutions into the 2 L sealed jar; S3: Wait for 10 min, take out the petri dish and take a photo, measure the color difference using a portable color difference meter, calculate the color difference ∆E according to the formula, and conduct multiple experiments to detect the color change under different concentrations of TMA.

[0044] According to the national food safety standard, fresh and frozen animal aquatic products (GB 2733—2015), the limit node for detecting volatile biogenic amines is 300 ppm. The results are as Figure 5 shown. When the concentration changes from 250 ppm to 300 ppm, the color changes from purplish red to dark green, which can clearly reflect the freshness of the food.

[0045] (2) Response of the indicator to NH3 S1: Take 0.2 g of anthocyanins from Summer Black grape skins and 0.2 g of anthocyanins from black wolfberry skins and dissolve them in 20 ml of distilled water, and pour the solution into a petri dish; S2: Place the petri dish into a 2L sealed jar, prepare NH3 solution concentration gradients of 50ppm, 100ppm, 150ppm, 200ppm, 250ppm, 300ppm, and 350ppm, and distribute the prepared gradient solutions into the 2L sealed jar; S3: Wait for 10 minutes, take out the petri dish and take a photo, measure the color difference value using a portable color difference meter, calculate the color difference ∆E according to the formula, and conduct multiple experiments to detect the color changes under different concentrations of NH3 respectively.

[0046] The results are as Figure 6 shown. When the concentration changes from 300ppm to 350ppm, the color changes from dark green to light grayish green. According to the threshold for detecting volatile biogenic amines in GB 2733-2015, this indicator has great potential in detecting the freshness of products.

[0047] Example 10: Comparative experiment As shown in Table 1, the indicator described in Example 9 was compared with other existing patents / products in terms of foods with a pH value in the range of 6-10 under normal temperature detection conditions, and the results are shown in Table 1.

[0048] Table 1. Comparison of pH indicator performance

[0049] The experimental results show that compared with the existing methods (electrode detection) and pH reagents, the present invention has higher sensitivity, diverse color changes, is easy to identify, can detect in a timely and rapid manner, and has good biocompatibility and is suitable for the detection of foods.

[0050] The above are only the preferred embodiments of the present invention and are not intended to limit the present invention. Any modifications, equivalent replacements, and improvements made within the spirit and principle of the present invention shall be included in the protection scope of the present invention. The above is only the preferred implementation manner of the present invention. It should be noted that for those of ordinary skill in the art, without departing from the technical principle of the present invention, several improvements and modifications can still be made, and these improvements and modifications should also be regarded as the protection scope of the present invention.

Claims

1. A neutral to alkaline pH indicator that can be visually recognized, characterized in that, The effective active substance of the medium-alkaline pH indicator is derived from the pericarp with high natural anthocyanin content, and the pericarp is one or more of grape pericarp, mangosteen pericarp, and black wolfberry pericarp.

2. The visually recognizable neutral to alkaline pH indicator according to claim 1, wherein The effective active substance of the medium-alkaline pH indicator is selected from a mixture of one or more of mangosteen pericarp-derived anthocyanins, Summer Black grape pericarp-derived anthocyanins, and black wolfberry pericarp-derived anthocyanins; preferably, a mixture of Summer Black grape pericarp-derived anthocyanins and black wolfberry pericarp-derived anthocyanins is used as the effective active substance.

3. The visually recognizable neutral to alkaline pH indicator according to claim 2, characterized in that, The medium-alkaline pH indicator is applicable to the detection in the pH range of 6.0 - 10.0, and can quickly and significantly indicate the pH value changes of 0.5 and 0.2 graduations on the basis of being visually recognizable; the 0.5-graduation of the medium-alkaline pH indicator is applicable to the pH range of 6.0 - 10.0, and the 0.2-graduation is applicable to the pH range of 9.0 - 10.

0.

4. The visually recognizable neutral to alkaline pH indicator according to claim 3, characterized in that, Under the conditions of pH 6.0 - 10.0 and a detection graduation of 0.5 pH, the mass ratio of Summer Black grape pericarp-derived anthocyanins to black wolfberry pericarp-derived anthocyanins is 0.95 - 1.05:0.95 - 1.05 (preferably 1:1); under the conditions of pH 9.0 - 10.0 and a detection graduation of 0.2 pH, the mass ratio of Summer Black grape pericarp-derived anthocyanins to black wolfberry pericarp-derived anthocyanins is 0.75 - 0.85:1.2 (preferably 0.8:1.2).

5. The visually recognizable neutral to alkaline pH indicator according to claim 4, characterized in that, When the pH value of the analyte ranges from 6.0 → 10.0 and the detection pH graduation is 0.5, its color indication process is lilac pink → grape purple → dark purple → dark blue → gray → soft blue → olive green → khaki → brown; when the pH value of the analyte ranges from 9.0 → 10.0 and the detection pH graduation is 0.2, its color indication process is dark dark green → dark blue → dark indigo blue → dark blue-violet → near black → dark green.

6. Use of the visually recognizable neutral to alkaline pH indicator according to any one of claims 1-5, characterized in that, It is applied to food detection, specifically for detecting the change in the content of volatile biogenic amines in food in the medium-alkaline range.

7. The application according to claim 6, characterized in that The application method of the medium-alkaline pH indicator includes loading the pH indicator on a carrier, putting it into food for detection, and judging the content of volatile biogenic amines inside the food according to its color change.

8. The application according to claim 7, characterized in that, The loading methods of the medium-alkaline pH indicator are soaking, spraying, and coating; the carriers of the medium-alkaline pH indicator are paper, cloth, aerogel, hydrogel, or film.

9. The preparation method of the visually recognizable medium-alkaline pH indicator according to any one of claims 1 - 5, characterized in that Mix different anthocyanin effective active substances in proportion to obtain the medium-alkaline pH indicator.

10. The preparation method according to claim 9, wherein, The extraction method of the effective active substance of anthocyanin is to mix the pericarp containing natural anthocyanin with the extraction solution, pass it through a column, then elute it, and dry the eluate to obtain the effective active substance; more specifically, the extraction method of the effective active substance is as follows: Wash the pericarp rich in anthocyanin respectively, place it in an oven at 45-55 °C and dry it to constant weight, crush the dried raw material and sieve it, and store it in a brown bottle away from light; the extraction solution is a mixture of anhydrous ethanol, hydrochloric acid and aqueous solution in a ratio of 5:1:4 to 1:5:4, preferably 2:1:1, and extract it with a water bath for 2-6 h; passing the extraction solution through a column means loading the extraction solution onto an AB-8 macroporous adsorption resin chromatographic column, standing for 1-3 hours, washing with distilled water, and then eluting anthocyanin with a 60-80% ethanol solution until the inside of the chromatographic column is colorless; drying is to concentrate the eluate under reduced pressure and then perform freeze-drying.

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