Preparation and application of freshness indicator ink based on astaxanthin

Abstract

The present invention discloses a method for preparing and applying a freshness indicating ink based on astaxanthin, and belongs to the field of non-destructive monitoring of food. The present invention extracts astaxanthin from shrimp shells, and mixes it with other edible ingredients to prepare ink, including xanthan gum, edible oil, sucrose, etc., to prepare a diffusion-proof, highly viscous, edible freshness indicating ink. The present invention also provides an application of the ink in indicating the freshness of pork, which can realize that as the freshness of pork changes during storage, the ink will produce significant color changes accordingly, and has the advantages of real-time, easy use, and no damage to the integrity of food.

Description

Technical Field

[0001] The invention relates to the preparation and application of freshness indicating ink based on astaxanthin, and belongs to the technical field of non-destructive monitoring of food. Background Art

[0002] In recent years, with the improvement of living standards and the enhancement of health awareness, people pay more attention to the quality and safety of food. Freshness is one of the important parameters for evaluating food quality and safety. Commonly used traditional freshness detection methods, such as sensory analysis, chemical analysis, food microbiology detection, etc., are complicated to operate, cumbersome pre-treatment, time-consuming, and not convenient for on-site instant detection. At present, the development of freshness indicators is rapid. It provides information on the reduction of freshness of food due to physical and chemical changes or microbial reproduction through visual color changes. It is more intuitive, rapid and sensitive than traditional detection methods.

[0003] As a non-edible part, shrimp shells generate a large amount of shrimp shell waste every year. It is difficult to degrade under natural conditions. The large amount of accumulation not only burdens the environment, but also causes unnecessary waste of resources. At present, most of the shell waste is directly treated as garbage, and a small part is used to produce feed after simple crushing. Although this practice can reduce certain environmental pressures, it cannot effectively improve it. Therefore, it is of great significance to effectively utilize shell waste, develop utilization methods based on shell waste, and realize "turning waste into treasure". Studies have shown that astaxanthin in shrimp shells is a carotenoid protein composed of protein subunits and astaxanthin molecules. It is a color-changing protein. Just as shrimp turns bright red and orange when cooked, it is because this pigment-binding protein (i.e., astaxanthin) is denatured, and the covalent bonds, hydrogen bonds, and hydrophobic bonds that bind astaxanthin are destroyed, resulting in astaxanthin being free, thereby showing red. However, astaxanthin is highly water-soluble, and direct use on the surface of food will cause large-scale diffusion or even shedding. Therefore, the application of astaxanthin in the food field is greatly restricted.

[0004] At present, there are methods for making freshness-indicating inks. For example, patent CN110415603A discloses a method for making a printed food freshness smart label, which uses hydroxyethyl cellulose as a binder, glycerin as a plasticizer, and water-based polyurethane (DF-893) as a defoamer. After printing and drying, a layer of silicone oil is applied. Patent CN113549404A discloses attaching time-varying inks to smart labels to indicate freshness. The time-varying inks are prepared using permanent yellow, acrylic resin, lignin-modified phenolic resin, saturated polyester resin, etc. The ingredients added in the above patents are non-edible and are not suitable for use on food surfaces, which limits their wide application. Patent CN116183598A discloses using anthocyanins and beet greens as natural indicators to reflect freshness; however, the preparation method is complicated and the detection effect is poor.

[0005] Therefore, preparing an edible, highly adhesive, and color-changing indicator ink based on astaxanthin has high practical and economic value, and broadens the application scenarios of astaxanthin. Summary of the invention

[0006] To solve the above problems, the present invention develops a freshness indicator label based on astaxanthin in shrimp shells, and utilizes the growth and reproduction of microorganisms in the storage process of food, secreting metabolites such as proteases, so that the conformation of astaxanthin, a binding protein, changes, and the bound astaxanthin is released, thereby causing color changes, which has certain application potential in real-time monitoring of the freshness of fresh meat. The present invention extracts astaxanthin from shrimp shells, and utilizes the characteristics of the binding protein that changes color under different conformations to develop a natural non-toxic freshness indicator to achieve rapid, non-destructive, and real-time monitoring of food freshness. On this basis, the present invention uses the mixing of pigment protein with ink to improve adhesion, and mixes astaxanthin with ink to prepare a diffusion-resistant, highly adhesive freshness indicator ink. The present invention fills the application gap of astaxanthin in shrimp shells in food, and further broadens the application of shrimp shell waste.

[0007] The first object of the present invention is to provide a freshness indicating ink based on astaxanthin, and the preparation method of the freshness indicating ink based on astaxanthin comprises:

[0008] (1) Mix water and sucrose, add a thickener, stir evenly, and add edible oil to prepare ink; the mass ratio of water, sucrose, thickener and edible oil is 40-60:12-20:5-8:18-20.

[0009] (2) The ink and astaxanthin are mixed evenly to prepare a freshness indicating ink based on astaxanthin; the mass ratio of the ink to astaxanthin is 2:1 to 1.5.

[0010] In one embodiment, the thickener includes: one or more of xanthan gum and gellan gum; the edible oil includes: one or more of sunflower oil, soybean oil, rapeseed oil, corn germ oil, and olive oil.

[0011] In one embodiment, the stirring conditions in step (1) are 45-55° C. and 300-500 rpm.

[0012] In one embodiment, the method for preparing astaxanthin in step (2) comprises:

[0013] (1) Grind shrimp shells, add boric acid solution, stir, filter, transfer to EDTA solution, and stir;

[0014] (2) filtering, adding ammonium sulfate to the filtrate, stirring to dissolve; centrifuging to obtain a precipitate, and redissolving the precipitate in a buffer solution;

[0015] (3) adding ammonium sulfate to the buffer solution and centrifuging to remove the precipitate;

[0016] (4) Increasing the concentration of ammonium sulfate and preparing astaxanthin by centrifugation.

[0017] In one embodiment, the method for preparing astaxanthin in step (2) comprises:

[0018] (1) Grind the shrimp shells, stir and soak them in boric acid solution for 1 to 3 hours, and then stir and soak them in EDTA solution for 6 to 12 hours;

[0019] (2) filtering, adding ammonium sulfate to the filtrate to adjust the saturation, stirring until completely dissolved, centrifuging, collecting the precipitate; and redissolving the precipitate in phosphate buffer;

[0020] (3) adding ammonium sulfate to the buffer to adjust the solution saturation, centrifuging, and removing precipitated interfering proteins;

[0021] (4) The ammonium sulfate saturation in the supernatant is increased, and the supernatant is centrifuged again to obtain a precipitate, which is astaxanthin.

[0022] In one embodiment, liquid nitrogen is added when grinding the shrimp shells.

[0023] In one embodiment, the concentration of the boric acid solution in step (1) is 0.2-0.5 mol / L; and the ratio of shrimp shell to boric acid solution is 1 g: 10-40 mL.

[0024] In one embodiment, the mass fraction of the EDTA solution in step (1) is 10% and the pH is 7.0.

[0025] In one embodiment, the concentration of ammonium sulfate added in step (2) is 2.95 to 3.26 mol / L; the concentration of ammonium sulfate added in step (3) is 1.33 to 1.84 mol / L; and the concentration of ammonium sulfate added in step (4) is 2.95 to 3.26 mol / L.

[0026] In one embodiment, the stirring and soaking are both performed at 4°C.

[0027] In one embodiment, the concentration of the phosphate buffer is 0.05 mol / L and the pH is 7.0.

[0028] In one embodiment, the centrifugation temperature is 4°C and the centrifugation speed is 10000 g / min.

[0029] The second object of the present invention is to provide an application of astaxanthin in preparing a freshness indicating ink, wherein the preparation method of the astaxanthin is as follows:

[0030] (1) Grind shrimp shells, add boric acid solution, stir, filter, transfer to EDTA solution, and stir;

[0031] (2) filtering, adding ammonium sulfate to the filtrate, stirring to dissolve; centrifuging to obtain a precipitate, and redissolving the precipitate in a buffer solution;

[0032] (3) adding ammonium sulfate to the buffer solution and centrifuging to remove the precipitate;

[0033] (4) Increasing the concentration of ammonium sulfate and preparing astaxanthin by centrifugation.

[0034] In one embodiment, the concentration of the boric acid solution in step (1) is 0.2-0.5 mol / L; and the ratio of shrimp shell to boric acid solution is 1 g: 10-40 mL.

[0035] In one embodiment, the mass fraction of the EDTA solution in step (1) is 10% and the pH is 7.0.

[0036] In one embodiment, the concentration of ammonium sulfate added in step (2) is 2.95 to 3.26 mol / L; the concentration of ammonium sulfate added in step (3) is 1.33 to 1.84 mol / L; and the concentration of ammonium sulfate added in step (4) is 2.95 to 3.26 mol / L.

[0037] The third object of the present invention is to provide a method for detecting the freshness of food, the method comprising: applying astaxanthin or the astaxanthin-based freshness indicator ink according to any one of claims 1 to 6 to the surface of food, and judging the freshness of the food by detecting the total color difference ΔE;

[0038] The method for judging the total color difference ΔE is as follows: when ΔE is less than 10, the food is fresh; when ΔE is between 10 and 20, the food is relatively fresh; when ΔE is greater than 20, the food is spoiled.

[0039] In one embodiment, the food includes: meat, fish, and shrimp.

[0040] A fourth object of the present invention is to provide a freshness indicating product, the product comprising any one of the above-mentioned astaxanthin-based freshness indicating inks or astaxanthin.

[0041] Beneficial effects of the present invention:

[0042] The present invention extracts astaxanthin from shrimp shells and utilizes the characteristic that the binding protein changes color under different conformations to develop a natural non-toxic freshness indicator, thereby realizing rapid, non-destructive and real-time monitoring of food freshness. On this basis, astaxanthin is mixed with ink to prepare a diffusion-resistant and highly adhesive freshness indicator ink. The present invention fills the application gap of astaxanthin in shrimp shells in food and further broadens the application of shrimp shell waste.

[0043] Specific:

[0044] (1) The present invention utilizes the fact that during the storage of food, due to the growth and reproduction of microorganisms, proteases and other metabolites are secreted, which changes the conformation of astaxanthin, a binding protein, thereby causing color changes, thereby achieving visual monitoring of the freshness of food. Compared with pH-type color developers (such as anthocyanins and astaxanthin), the color change process is related to conformational changes rather than just pH changes, and has better accuracy in practical applications.

[0045] (2) The present invention selects edible raw materials such as sucrose, sunflower seed oil, and xanthan gum to prepare ink, and mixes them with astaxanthin, so that the obtained astaxanthin indicator ink has good adhesion and stability, which solves the problem that pure astaxanthin is highly water-soluble and easy to fall off and spread over a large area when applied on the surface of food.

[0046] (3) When in use, the astaxanthin indicator ink is in direct contact with the food, and the color change speed is faster than that of indirect contact. The indication result is not easily affected by environmental factors and has good accuracy.

[0047] (4) The present invention uses edible asparagus blue protein in shrimp shells as a developer, and other materials used to make the ink are also edible ingredients, which has the characteristics of being natural, non-toxic and edible.

[0048] (5) The salting-out method is used to extract asparagus blue protein from shrimp shells, which is simple to operate and low in cost. The shrimp shells, which are considered to be waste, can be analyzed and utilized by extracting beneficial substances to increase their added value and turn waste into treasure. This not only helps to reduce environmental pollution and promote the recycling of resources, but also has important significance for the healthy development of the shrimp industry. BRIEF DESCRIPTION OF THE DRAWINGS

[0049] Figure 1 The SDS-PAGE electrophoresis diagram and molecular weight of the astaxanthin prepared in Example 1 are shown.

[0050] Figure 2: This is a graph showing changes in the freshness index of pork during storage at 4°C in Example 2; wherein the red line indicates the maximum limit of the pork freshness index specified in the national standard; A is a graph showing changes in pH of pork during storage at 4°C; B is a graph showing changes in the total bacterial count (TVC) of pork during storage at 4°C; C is a graph showing changes in the volatile basic nitrogen (TVB-N) content of pork during storage at 4°C; and D is a graph showing the total color difference curve of the indicator ink during the pork freshness detection process.

[0051] Figure 3 This is a physical picture of the ink color change during the pork freshness detection process when the astaxanthin freshness indicator ink in Example 3 is used.

[0052] Figure 4 These are the effect diagrams of inks prepared using gum arabic and guar gum in Comparative Examples 1 to 2; wherein, Figure A is the ink prepared using gum arabic, and Figure B is the ink prepared using guar gum.

[0053] Figure 5 These are the effect diagrams of the inks prepared using anthocyanins and astaxanthin in Comparative Examples 3 to 4; wherein, Figure A is the ink prepared using anthocyanins, and Figure B is the ink prepared using astaxanthin. DETAILED DESCRIPTION

[0054] The preferred embodiments of the present invention are described below. It should be understood that the embodiments are for better explaining the present invention and are not used to limit the present invention.

[0055] The instruments involved in the present invention are:

[0056] Magnetic stirrer (IKA, Germany), FE28 pH meter (Shanghai Mettler-Toledo Instrument Co., Ltd.), ST40R centrifuge (Thermo Fisher Scientific), DC-1006 low-temperature constant temperature bath (Nanjing Xianou Instrument Manufacturing Co., Ltd.), K9840 semi-automatic Kjeldahl nitrogen analyzer (Shandong Haineng Scientific Instrument Co., Ltd.).

[0057] Detection method:

[0058] 1. Ink color difference detection method:

[0059] Select fresh pork with skin of uniform appearance, cut it into 3.5cm×3.5cm×1cm squares in a clean bench, apply the prepared ink on the surface of the pig skin, place it in a disposable culture dish, cover it with a lid, and refrigerate it at 4°C. Use a camera in a photo studio to take pictures of the color change of the ink at 0, 2, 4, 6, and 8 days of storage time. And measure the L, a, and b values ​​of the astaxanthin indicator ink, and use the following formula to calculate the total color difference (ΔE).

[0060]

[0061] Wherein, L0, a0, b0 are the initial color difference values ​​measured at 0d, and L, a, b are the color difference values ​​measured at different storage times.

[0062] 2. The pH value, total bacterial count (TVC), and volatile basic nitrogen content (TVB-N) of pork are used to evaluate the freshness of pork. The specific operation is as follows:

[0063] (1) pH value: mince the pork evenly, weigh 5 g, add 50 mL of 0.1 mol / L potassium chloride solution, homogenize for 5 min, let stand, and take the supernatant to measure the pH value using a pH meter.

[0064] (2) Total colony count (TVC): According to GB 4789.2-2022 "National Food Safety Standard for Food Microbiological Examination - Determination of Total Colony Count", the colony test piece produced by 3M was selected for the determination of the total colony count. The collected sample was homogenized, 1 mL was dropped onto the surface of the colony test piece, evenly coated and incubated at 37°C for 48 hours to record the total colony count. All results are shown as the total colony count in 1 g sample (CFU / g sample).

[0065] (3) Volatile basic nitrogen content (TVB-N): mince the pork and weigh 10g, add 75mL ultrapure water, shake and let stand for 30min, then add 1g MgO and measure it on a semi-automatic Kjeldahl nitrogen analyzer. Add 30mL of boric acid receiving solution to the receiving bottle, distill for 3min, add 10 drops of mixed indicator solution (the indicator solution is prepared by mixing 1 part of methyl red ethanol solution and 5 parts of bromocresol green ethanol solution) to the receiving bottle, and use 0.01mol / L sulfuric acid to titrate to purple-red, record the volume of sulfuric acid solution consumed by each sample V1 (mL), and the volume of sulfuric acid solution consumed by the test blank V2 (mL). The calculation method of TVB-N is as follows:

[0066]

[0067] Wherein, V1 and V2 represent the volume of sulfuric acid solution consumed by the experimental group and the blank group during the titration process (mL), respectively; c represents the concentration of the standard sulfuric acid titration solution (mol / L); 28 represents the mass of nitrogen equivalent to titrating 1.0mL of sulfuric acid [c(H2SO4)=1.000mol / L] standard titration solution (g / mol); m is the mass of the pork sample (g).

[0068] Example 1: Preparation of astaxanthin freshness indicator ink

[0069] 1. A method for extracting astaxanthin from Macrobrachium rosenbergii shells, comprising the following steps:

[0070] Purchase fresh Macrobrachium rosenbergii from the market, peel the shells, wash the shells with ice water, wipe the surface moisture with absorbent paper, and then add liquid nitrogen to grind the shells.

[0071] Weigh 40.0 g of ground shrimp shells, add 400 mL of boric acid solution, stir magnetically at 4° C. for 1 h (magnetic stirring speed is 300 rpm), filter, transfer the shrimp shells to 800 mL of EDTA solution, and stir magnetically for 10 h; the concentration of the boric acid solution is 0.30 mol / L, the pH is 7.0, the mass concentration of the EDTA solution is 10% (m / v; g / mL), and the pH is 7.0.

[0072] Then filter out the shrimp shells, add ammonium sulfate to the filtrate and stir thoroughly to dissolve it so that the concentration of ammonium sulfate is 2.95 mol / L, centrifuge at 10000g at 4°C for 20 min, discard the supernatant and take the precipitate; redissolve the precipitate in 50 mL of 0.05 mol / L, pH 7.0 phosphate buffer, add ammonium sulfate to the buffer so that the concentration of ammonium sulfate is 1.33 mol / L, centrifuge, remove the precipitated interfering proteins, and take the supernatant; finally, add ammonium sulfate to the supernatant so that the concentration of ammonium sulfate in the supernatant is 2.95 mol / L, centrifuge, and obtain a blue protein precipitate, which is astaxanthin.

[0073] Depend on Figure 1 It can be seen that the main protein band of astaxanthin measured by SDS-PAGE gel electrophoresis is 72-95 kDa, and the concentration of astaxanthin extracted above is 1.015 mg / g shrimp shell measured by Coomassie Brilliant Blue method.

[0074] 2. The preparation of an edible astaxanthin freshness indicator ink comprises the following steps:

[0075] In a 250mL beaker, add 30mL ultrapure water and 20g sucrose, stir magnetically at 50°C and 500rpm until fully dissolved, then slowly add 3g xanthan gum until it is not suspended or clumped; then slowly add 10g sunflower oil, continue stirring to fully mix it to obtain ink; finally, take 0.2000g ink and 0.1000g astaxanthin and mix them thoroughly to obtain edible astaxanthin freshness indicator ink.

[0076] Comparative Example 1: Preparation of ink using gum arabic

[0077] On the basis of Example 1, the xanthan gum in step 2 was changed to gum arabic (dosage 3 g), and the remaining steps were consistent with Example 1 to prepare an edible astaxanthin freshness indicator ink.

[0078] Comparative Example 2: Preparation of ink using guar gum

[0079] On the basis of Example 1, the xanthan gum in step 2 was changed to guar gum (dosage 3 g), and the remaining steps were consistent with Example 1 to prepare an edible astaxanthin freshness indicator ink.

[0080] Comparative Example 3: Preparation of ink using anthocyanin

[0081] The ink is prepared by using anthocyanin, comprising the following steps:

[0082] In a 250mL beaker, add 30mL ultrapure water and 20g sucrose, stir magnetically at 50°C and 500rpm until fully dissolved, then slowly add 3g xanthan gum until it is not suspended or clumped; then slowly add 10g sunflower oil, continue stirring to fully mix it to obtain ink; finally, take 0.2000g ink and 0.1000g anthocyanin and mix them thoroughly to obtain edible anthocyanin freshness indicator ink.

[0083] Comparative Example 4: Preparation of ink using astaxanthin

[0084] The ink is prepared using astaxanthin, comprising the following steps:

[0085] Add 30 mL of ultrapure water and 20 g of sucrose into a 250 mL beaker, stir magnetically at 50 ° C and 500 rpm until fully dissolved, then slowly add 3 g of xanthan gum until it is not suspended or clumping; then slowly add 10 g of sunflower oil and continue stirring to fully mix it to obtain ink; finally, take 0.2000 g of ink and 0.1000 g of astaxanthin and mix them thoroughly to obtain edible astaxanthin freshness indicator ink.

[0086] Example 2: Ink performance testing

[0087] The edible astaxanthin freshness indicator ink prepared in Example 1 and Comparative Examples 1 to 4 was used to test its indicator properties (color difference) and pork freshness (pH, colony count, volatile basic nitrogen).

[0088] The results of pork freshness index are as follows Figure 2 As shown in the results, with the extension of storage time, the pH value, total colony count and volatile basic nitrogen of pork gradually increased, reaching pH 6.49, colony count 6.19lg CFU / g and volatile basic nitrogen content 17.05mg / 100g on the eighth day. The literature "Effect of adding tea polyphenols to coating preservatives on the storage quality of fresh pork [J]. Food Science, 2009, 30(24): 452-456." shows that pork with a pH value above 6.4 is spoiled meat. The above values ​​obviously exceed the food microbial limit of 10 specified in the national standard (GB / T 9959.2-2008). 6CFU / g and the upper limit of volatile basic nitrogen content in the national standard (GB 2707-2016) is 15mg / 100g, which means that the pork is inedible after 8 days.

[0089] The color change result of the edible astaxanthin freshness indicator ink prepared in Example 1 is as follows: Figure 3 As shown in the figure, the results show that there is no significant change in the control group ink. As the storage time of pork increases, the color of the ink in the sample group changes from blue to pink, which can intuitively reflect the color change of pork from fresh to rotten. Under 4°C conditions, the color of the ink is blue in the first 4 days of storage, and the pork is fresh at this time. On the 6th day, it turns to light pink, and ΔE reaches 19.98, and the pork is at a sub-fresh level. On the 8th day, the ink turns pink, and ΔE reaches 22.13, at which time the pork has been corrupted and deteriorated. That is, when ΔE is less than 10, the food is fresh; when ΔE is 10-20, the food is relatively fresh; when ΔE is greater than 20, the food is deteriorated.

[0090] The test results of the edible freshness indicator ink prepared in Comparative Examples 1 to 2 are as follows: Figure 4 As shown, the results show that the inks prepared using gum arabic and guar gum in Comparative Examples 1 to 2 are easy to spread and have poor viscosity;

[0091] The results of the inks prepared using anthocyanins and astaxanthin in Comparative Examples 3 to 4 are as follows: Figure 5 As shown, the results show that the ink prepared using anthocyanins turns orange after contact with pork, but as the storage time of pork increases, the pH value of pork does not change much, resulting in no obvious color change of anthocyanins, and the ink remains orange, indicating lag; the ink prepared using astaxanthin has no obvious color change, indicating that astaxanthin cannot be used to indicate freshness.

[0092] The embodiments provided above are not intended to limit the scope of the present invention, and the steps described are not intended to limit the execution order thereof. Those skilled in the art may make obvious improvements to the present invention in combination with existing common knowledge, which also fall within the scope of protection defined by the claims of the present invention.

Claims

1. A freshness indicator ink based on astaxanthin, characterized in that: The preparation method of the freshness indicator ink based on astaxanthin comprises: (1) Mixing water and sucrose, adding a thickener, stirring evenly, and adding edible oil to prepare ink; the mass ratio of water, sucrose, thickener and edible oil is 40-60:12-20:5-8:18-20; (2) The ink and astaxanthin are mixed evenly to prepare a freshness indicator ink based on astaxanthin; the mass ratio of the ink to astaxanthin is 2:1-1.

5.

2. The freshness indicating ink according to claim 1, characterized in that: The thickener includes: one or more of xanthan gum and gellan gum; the edible oil includes: one or more of sunflower oil, soybean oil, rapeseed oil, corn germ oil and olive oil.

3. The freshness indicating ink according to claim 1, characterized in that: The stirring conditions in step (1) are 45-55°C and 300-500 rpm.

4. The freshness indicating ink according to claim 1, characterized in that: The method for preparing astaxanthin in step (2) comprises: (1) Grind the shrimp shells, add boric acid solution, stir, filter, transfer to EDTA solution, and stir; (2) Filter, add ammonium sulfate to the filtrate, stir and dissolve; centrifuge to obtain the precipitate, and redissolve the precipitate in a buffer solution; (3) Add ammonium sulfate to the buffer and centrifuge to remove the precipitate; (4) Increasing the concentration of ammonium sulfate and preparing astaxanthin by centrifugation.

5. The freshness indicating ink according to claim 4, characterized in that: In step (1), the concentration of the boric acid solution is 0.2-0.5 mol / L; the dosage ratio of shrimp shell to boric acid solution is 1 g:10-40 mL.

6. The freshness indicating ink according to claim 4, characterized in that: In step (2), the concentration of ammonium sulfate added is 2.95-3.26 mol / L; in step (3), the concentration of ammonium sulfate added is 1.33-1.84 mol / L; and in step (4), the concentration of ammonium sulfate added is 2.95-3.26 mol / L.

7. Use of astaxanthin in preparing the freshness indicating ink according to any one of claims 1 to 6, characterized in that: The freshness indicator ink is applied to the surface of food, and the food includes meat, fish, and shrimp; The preparation method of astaxanthin comprises: (1) Grind the shrimp shells, add boric acid solution, stir, filter, transfer to EDTA solution, and stir; (2) Filter, add ammonium sulfate to the filtrate, stir and dissolve; centrifuge to obtain the precipitate, and redissolve the precipitate in a buffer solution; (3) Add ammonium sulfate to the buffer and centrifuge to remove the precipitate; (4) increasing the concentration of ammonium sulfate and preparing astaxanthin by centrifugation; The concentration of the boric acid solution in step (1) is 0.2-0.5 mol / L; the ratio of shrimp shell to boric acid solution is 1 g:10-40 mL; the mass fraction of the EDTA solution in step (1) is 10% and the pH value is 7.0; the concentration of ammonium sulfate added in step (2) is 2.95-3.26 mol / L; the concentration of ammonium sulfate added in step (3) is 1.33-1.84 mol / L; and the concentration of ammonium sulfate added in step (4) is 2.95-3.26 mol / L.

8. A method for detecting the freshness of food, characterized in that: Applying the astaxanthin-based freshness indicator ink according to any one of claims 1 to 6 on the surface of food, and judging the freshness of the food by detecting the total color difference ΔE; The method for judging the total color difference ΔE is as follows: when ΔE is less than 10, the food is fresh; when ΔE is between 10 and 20, the food is relatively fresh; when ΔE is greater than 20, the food is spoiled.

9. The method according to claim 8, characterized in that The food includes: meat, fish, and shrimp.

10. A freshness indicating product, characterized in that: The product contains the astaxanthin-based freshness indicator ink according to any one of claims 1 to 6.

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