CN113252652B

CN113252652B - Preparation method of accurate indication film for monitoring freshness of sashimi

Info

Publication number: CN113252652B
Application number: CN202110517067.2A
Authority: CN
Inventors: 王立娟; 刘若婷
Original assignee: Northeast Forestry University
Current assignee: Northeast Forestry University
Priority date: 2021-05-12
Filing date: 2021-05-12
Publication date: 2023-03-24
Anticipated expiration: 2041-05-12
Also published as: CN113252652A

Abstract

A preparation method of an accurate indication film for monitoring the freshness of sashimi relates to a preparation method of a freshness indication film and aims to solve the technical problems that the existing freshness indication film is poor in mechanical property and unobvious in color development. The method comprises the following steps: 1. preparing quaternary amination straw powder; 2. preparing grafted bromothymol blue rice straw powder; 3. adding carrageenan and sodium carboxymethylcellulose into water, uniformly stirring, adding the grafted bromothymol blue rice straw powder and sorbitol, and continuously stirring to obtain a film forming solution; and carrying out film casting and film forming on the film forming solution, and drying to obtain an accurate indication film for monitoring the freshness of the sashimi. The indicating film can indicate the freshness of the sashimi in real time through four obviously different colors presented under different TVB-N values. The method can be used in the field of intelligent labels.

Description

Preparation method of accurate indication film for monitoring freshness of sashimi

Technical Field

The invention relates to a preparation method of a freshness indicating film, belonging to the field of material processing.

Technical Field

The body is stabbed with beautiful shape, fresh raw materials, tender and delicious taste of meat and pungent seasonings, and attracts more and more attention. In recent years, thorns have gradually moved from japanese cuisine stores into a large number of medium-to-high grade chinese restaurants. The material commonly used for stabbing bodies is fish, and is the latest fresh fish. However, for customers, whether the fresh raw fish fillet is used by the stabbing body or not can not be directly judged from the appearance and the smell, the traditional raw and fresh meat monitoring method takes chemical analysis as a main means, but the analysis method has the defects of long analysis time, complex process, damage to samples and the like, and belongs to destructive detection. Some common nondestructive testing techniques rely on information technologies such as computers and sensors to accurately and objectively detect and evaluate the quality of fresh meat, and although quick, the techniques are not practical for consumers from the perspective of cost and portability. Therefore, it is imperative to develop an inexpensive, portable, visually accurate indicator label for monitoring the freshness of fresh meat.

In recent years, with the shortage of petroleum resources and the increasing awareness of environmental protection, researchers have been working on the preparation of biodegradable films using chitosan, sodium alginate, starch, carrageenan, carboxymethyl cellulose, and the like. Carrageenan has been widely used as a food additive or stabilizer in various food applications because of its wide source, non-toxic, biocompatible, biodegradable properties, etc. The characteristic of brittleness and hardness of the Cara glue film makes the Cara glue film easy to degrade and has poor mechanical property. In addition, natural pigment such as prunus davidiana anthocyanin is used as a response substance of freshness, the color change is small, and the freshness of food materials cannot be clearly reflected.

Disclosure of Invention

The invention provides a preparation method of an accurate indication film for monitoring the freshness of sashimi, aiming at solving the technical problems of poor mechanical property and unobvious color development of the existing freshness indication film.

The preparation method of the accurate indication film for monitoring the freshness of the sashimi comprises the following steps:

1. preparation of quaternized rice straw powder (CRSP):

adding the rice straw powder into a NaOH solution with the mass percentage concentration of 20-25%, stirring for 2-2.5 h, and filtering to obtain swelling cellulose; adding NaOH solution with the mass percentage concentration of 10-12% and epoxy chloropropane into swelling cellulose, reacting for 7-8 h under the condition that the water bath temperature is 65-68 ℃, filtering, and washing a solid phase substance with ethanol; adding an ethanol solution of triethylamine into the solid phase substance, reacting for 4-4.5 h at the temperature of 75-78 ℃, filtering, sequentially cleaning the solid phase substance by using distilled water, a dilute alkali solution and a dilute acid solution, finally washing the solid phase substance to be neutral by using distilled water, and drying to obtain quaternary ammonium straw powder;

2. preparation of the bromothymol grafted blue rice straw powder (BTB-CRSP):

adding a dilute alkali solution of bromothymol blue into quaternary amination straw powder, stirring for 2-2.5 h by using a magnetic stirrer at the temperature of 40-45 ℃, then performing suction filtration, and washing a solid phase substance by using water until filtrate is neutral to obtain grafted bromothymol blue straw powder (BTB-CRSP); it is straw powder sensitive to volatile basic total nitrogen (TVB-N);

3. preparation of indicator film (CC-xBTB/CRSP):

adding carrageenan and sodium carboxymethylcellulose (CMC) into water with the temperature of 75-78 ℃ according to the mass ratio of (2-2.5) to 1, stirring for 1-1.5 h, then adding the grafted bromothymol blue rice straw powder (BTB-CRSP) and sorbitol, and continuously stirring for 1-1.5 h to obtain a film forming solution; and pouring the film forming solution into a mold, drying for 12-15 h at the temperature of 60-63 ℃, and removing the film to obtain the accurate indicating film for monitoring the freshness of the sashimi.

Furthermore, the fineness of the straw powder in the first step is 100-120 meshes;

further, in the first step, the mass ratio of the straw powder to the NaOH solution with the mass percentage concentration of 20% -25% is 1: (25-30);

further, in the first step, the mass ratio of the straw powder to the NaOH solution with the mass percentage concentration of 10% -12% is 1: (25-30);

furthermore, the ratio of the mass of the straw powder to the volume of the epichlorohydrin in the first step is 1g: (25-35) mL; cellulose in the straw powder is subjected to alkali treatment to obtain sodium cellulose, and C6-site active hydroxyl in a cellulose molecular structure and epichlorohydrin are subjected to etherification reaction.

Furthermore, in the step one, the ethanol solution of triethylamine is prepared by using ethanol as a solvent, wherein the volume percentage of triethylamine is 34-35%. And carrying out quaternization reaction on the triethylamine and the etherified cellulose in the straw powder to obtain cationized straw powder for anchoring the bromothymol blue indicator.

Furthermore, in the first step, the mass ratio of the straw powder to the ethanol solution of triethylamine is 1: (14-15);

furthermore, the dilute alkali used for solid phase washing in the step one is NaOH solution with the concentration of 0.1 mol/L;

further, the diluted acid used for solid phase washing in the step one is HCl solution with the concentration of 0.1 mol/L;

furthermore, in the second step, the diluted alkaline solution of bromothymol blue is prepared by adding bromothymol blue into a sodium hydroxide solution with the concentration of 0.09 mol/L-0.11 mol/L and mixing, wherein the concentration of the bromothymol blue is 0.9 g/L-1.1 g/L;

furthermore, in the third step, the mass ratio of the total mass of the carrageenan and the sodium carboxymethyl cellulose to the water is 1: (39-40);

furthermore, in the third step, the mass of the bromothymol blue rice straw powder (BTB-CRSP) grafted in the film forming solution accounts for 0.25-1% of the sum of the mass of the carrageenan and the mass of the sodium carboxymethylcellulose;

furthermore, in the third step, the mass of sorbitol in the film-forming liquid accounts for 45-50% of the total mass of carrageenan and sodium carboxymethylcellulose;

according to the invention, the agricultural waste rice straw is subjected to modified grafting, and the rice straw powder is anchored with the bromothymol blue indicator on one hand, so that the bromothymol blue indicator is prevented from being released; on the other hand, the barrier property of the indicating film is reduced by adding straw powder, so that the indicator is contacted with salt-based volatile total nitrogen more quickly and responds in time. The modified grafted rice straw powder is added into carrageenan/CMC to prepare an accurate indicating membrane, and the indicating membrane has different color responses under different pH values, so that the freshness of fish can be monitored in real time. At different TVB-N values, the indicator film can indicate the freshness of the fish meat in real time by showing four distinct colors.

The accurate indicating film for monitoring the freshness of the sashimi has the following advantages:

(1) The invention takes cheap, biocompatible and biodegradable carrageenan as a main film forming substrate, adds CMC to improve the mechanical property of the carrageenan, and prepares a high-precision indicating film by doping rice straw powder of a grafting indicator;

(2) The high-precision indicating film material prepared by the invention has the tensile strength of 18.87-22.35 MPa and the elongation at break of 22.16-32.33%, and improves the mechanical property of the pure carrageenan film;

(3) The indicating film prepared by the invention is rapid and obvious in color development in solutions with different pH values, and when the indicating film is used for monitoring the freshness of fish, the indicating film rapidly responds with the increase of a TVB-N value, in an experiment for monitoring the putrefaction of fish, after the standing time is only 3 hours (the TVB-N value is changed from 8.845mg/100g to 9.47mg/100 g), the indicating film is changed from yellow to turquoise, is placed for 10 hours to be changed into cyan (10.619 mg/100 g), and is placed for 29 hours to be changed into dark blue (18.214 mg/100 g), the freshness of fish is indicated in real time, the slight change of the TVB-N value can be accurately captured, obviously different color responses can be rapidly made, and a consumer can intuitively know the freshness degree of fish through the obvious color change of the indicating film, and the indicating film is used for the field of intelligent labels.

Drawings

FIG. 1 is XRD patterns of Rice Straw Powder (RSP), naOH-treated rice straw powder (NaOH treated-RSP) \ quaternized rice straw powder (CRSP), and grafted bromothymol blue rice straw powder (BTB-CRSP) in example 1;

FIG. 2 is an infrared spectrum of Rice Straw Powder (RSP), naOH-treated rice straw powder (NaOH treated-RSP) \ quaternized rice straw powder (CRSP), and bromothymol-grafted blue rice straw powder (BTB-CRSP) in example 1;

FIG. 3 is an infrared spectrum of CMC, carrageenan/CMC film of example 1 and a precision indicating film (CC/BTB-CRSP film) prepared in example 1 for monitoring the freshness of sashimi;

FIG. 4 is a graph showing color test conditions of the precision indicating film for monitoring the freshness of sashimi prepared in examples 1 to 4 and the indicating film prepared in example 5;

FIG. 5 is a graph showing ultraviolet transmittance of the precision indicating film for monitoring the freshness of sashimi prepared in examples 1 to 4 and the indicating film prepared in example 5;

FIG. 6 is a haze chart of the precision indicating film for monitoring the freshness of sashimi prepared in examples 1 to 4 and the indicating film prepared in example 5;

FIG. 7 is the TVB-N value of fish meat and the color of the indicator film CC-1BTB/CRSP as a function of the standing time.

Detailed Description

The following examples are used to demonstrate the beneficial effects of the present invention.

Example 1: the preparation method of the precise indication film for monitoring the freshness of the sashimi comprises the following steps:

1. preparation of quaternized rice straw powder (CRSP):

crushing rice straws, screening and screening the rice straws by a 100-mesh sieve, adding 2 g of rice straw powder into 50mL of NaOH solution with the mass percentage concentration of 20%, stirring for 2h, and filtering to obtain swelling cellulose; adding 50mL of NaOH solution with the mass percentage concentration of 10% and 60mL of epoxy chloropropane into swelling cellulose, reacting for 7 hours at the water bath temperature of 65 ℃, filtering, and washing a solid phase substance with ethanol; adding 28g of ethanol solution of triethylamine into the solid phase, reacting for 4 hours at the temperature of 75 ℃, filtering, sequentially washing the solid phase with distilled water, naOH solution with the concentration of 0.1mol/L and HCl solution with the concentration of 0.1mol/L, finally washing the solid phase with distilled water to be neutral, and drying for 5 hours at the temperature of 60 ℃ to obtain quaternary amination straw powder; wherein the ethanol solution of triethylamine is prepared by taking ethanol as a solvent, and the volume percentage concentration of triethylamine is 34%;

2. preparation of the bromothymol grafted blue rice straw powder (BTB-CRSP):

stirring for 2 hours at the temperature of 40 ℃ by using a magnetic stirrer, then carrying out suction filtration, washing a solid phase substance by using water until the filtrate is neutral, and drying for 5 hours at the temperature of 60 ℃ to obtain the grafted bromothymol blue rice straw powder (BTB-CRSP); it is straw powder sensitive to volatile basic total nitrogen (TVB-N);

3. preparation of indicator film (CC-xBTB/CRSP):

adding 7 g of carrageenan and 3 g of sodium carboxymethylcellulose (CMC) into 400mL of distilled water with the temperature of 75 ℃, stirring for 1h, then adding 0.025 g of bromothymol-grafted blue rice straw powder (BTB-CRSP) and 5g of sorbitol, and continuing stirring for 1h to obtain a film-forming solution; and pouring the film forming solution into a mold, drying for 15 hours at the temperature of 60 ℃, and removing the film to obtain an accurate indicating film for monitoring the freshness of the sashimi, which is recorded as CC-0.25BTB/CRSP.

Example 2: the difference between the example and the example 1 is that the mass of the grafted bromothymol blue rice straw powder added in the third step is 0.05g, the other steps are the same as the example 1, and the obtained accurate indicating film for monitoring the freshness of the sashimi is recorded as CC-0.5BTB/CRSP.

Example 3: this example differs from example 1 in that the mass of the grafted bromothymol blue straw powder added in step three was 0.075 g, otherwise the same as example 1, and the resulting precision indicating film for monitoring the freshness of sashimi was designated as CC-0.75BTB/CRSP.

Example 4: the difference between this example and example 1 is that the mass of the grafted bromothymol blue rice straw powder added in the third step is 0.1 g, and the other steps are the same as example 1, and the obtained accurate indicating film for monitoring the freshness of the sashimi is recorded as CC-1BTB/CRSP.

Example 5: the difference between the example and the example 1 is that the weight of the grafted bromothymol blue rice straw powder added in the step three is 0g, the other steps are the same as the example 1, and the obtained film material is marked as CC-0BTB/CRSP.

Fig. 1 is XRD patterns of Rice Straw Powder (RSP), naOH treated rice straw powder (NaOH treated-RSP) \ quaternized rice straw powder (CRSP), and grafted bromothymol blue rice straw powder (BTB-CRSP) in example 1. For the rice straw powder, stronger diffraction peaks exist at 2 theta =15.7 degrees, 22.1 degrees and 35.0 degrees, and belong to cellulose I type; after alkali swelling, diffraction peaks at 2 theta =12.3 degrees and 20.5 degrees belong to cellulose type II, diffraction peaks at 2 theta =22.1 degrees belong to cellulose type I, and the obtained alkali swelling straw powder is a mixture of cellulose type I and cellulose type II. The strong alkali treatment can convert the cellulose I type into the cellulose II type, and the cellulose in the rice straw is prevented from being completely converted from the cellulose I type into the cellulose II type due to the existence of lignin in the rice straw powder; after cation modification, the original cellulose I-type characteristic peak completely disappears, and the cellulose II-type characteristic peak is obtained when 2 theta =20.5 degrees, which indicates that the cellulose in the prepared quaternary ammonium straw powder is II-type; the adsorption of BTB did not affect the crystalline structure of the quaternized rice straw powder.

FIG. 2 shows the IR spectra of the Rice Straw Powder (RSP), naOH-treated rice straw powder (NaOH treated-RSP) \ quaternized rice straw powder (CRSP) and bromothymol-grafted blue rice straw powder (BTB-CRSP) of example 1, which are 3351, 3356 and 3362cm in length, as shown in FIG. 2 ^-1 Nearby absorption peaks, both representing the stretching vibration of hydroxyl-OH, are located at 2920 and 2986cm ^-1 The absorption peak at (B) represents the C-H stretching vibration. 1463cm can be observed after the rice straw powder is modified ^-1 The peak intensity at the position increases, and the absorption band is caused by the vibration of the carbon skeleton containing the quaternary amine group. After adsorption of BTB, the-OH absorption peak slowed, indicating a decrease in hydroxyl content. At the same time, in the infrared spectrogram of the bromothymol blue rice straw powder (BTB-CRSP) grafted, 1330cm ^-1 A new absorption peak appears nearby, which is a characteristic peak of S = O, demonstrating that BTB has been successfully introduced onto the backbone of straw powder, confirming the chemisorption of bromothymol blue by straw powder.

In FIG. 3, a to d respectively show CMC and Carla in example 1Infrared spectra of gums, carrageenan/CMC films and the precision indicator film for monitoring sashimi freshness (CC/BTB-CRSP film) prepared in example 1. In a, 3300, 2930 and 1606cm ^-1 Peaks at (D) belong to-OH, C-H and C = O,1457, 1640cm, respectively ^-1 Belong to COO ^- Stretching and vibrating of the group; b spectrum is positioned at 3300 and 2930cm ^-1 The peak belongs to-OH and C-H groups in carrageenan, and is 1240cm ^-1 Is treated as a stretching vibration peak of sulfate group, 848cm ^-1 The peak at (A) corresponds to the characteristic absorption peak of the d-galactose-4-sulfonic acid group. After carrageenan and CMC are mixed into a membrane according to the proportion of 7 to 3 (C spectrogram), the intensities of OH and C-H peaks are enhanced, and characteristic peaks of carrageenan and CMC can be found. Due to the introduction of BTB-CRSP (d spectrum), a stronger-OH absorption peak is shown, and due to the action of hydrogen bonds among components, the-OH absorption peak is shifted to a low wave number direction, and the absorption peak of C-H is also strengthened.

Color test conditions of the precision indicating film for monitoring the freshness of sashimi prepared in examples 1 to 4 and the indicating film prepared in example 5 are shown in table 1 and fig. 4.

Table 1 color of the indicating films prepared in examples 1 to 5

Film sample	L*	a*	b*	△E
					CC-0BTB/CRSP	88.75	2.20	-12.57	-
CC-0.25BTB/CRSP	86.41	-0.70	2.98	15.99
					CC-0.5BTB/CRSP	84.93	-0.88	7.71	20.86
CC-0.75BTB/CRSP	84.41	-1.34	7.12	20.48
					CC-1BTB/CRSP	80.29	-2.98	19.57	33.64

As can be seen from table 1, as the amount of the modified straw powder added was increased, the chromaticity parameter a was gradually changed from +2.20 to-2.98, the chromaticity parameter b was increased from-12.57 to +19.57, and Δ E was increased from 15.99 to 33.64, indicating an increase in the yellow-green color of the film. From the photograph corresponding to the film of fig. 4, the color of the film gradually changed to yellow-green in accordance with the increase in the amount of BTB/CRSP added, which corresponds to the change in the chroma thereof.

The mechanical properties of the precision indicating film for monitoring the freshness of sashimi prepared in examples 1 to 4 and the indicating film prepared in example 5 are shown in table 2.

Table 2 mechanical properties of the indicator films prepared in examples 1 to 5

Film sample	Stress (MPa)	Elongation at Break (%)
			CC-0BTB/CRSP	20.39	22.16
CC-0.25BTB/CRSP	21.90	27.68
			CC-0.5BTB/CRSP	22.35	32.33
CC-0.75BTB/CRSP	19.80	31.30
			CC-1BTB/CRSP	18.87	25.70

As can be seen from table 2, with the increase of the modified straw powder, the stress and the elongation at break of the film material both increased and then decreased, and at the addition amount of 0.05g, the maximum stress of 22.35MPa and the elongation at break of 32.33% of the film were indicated. The reason is that after a small amount of the rice straw powder is added, a large amount of hydroxyl carried by the rice straw powder, carrageenan and CMC more fully form hydrogen bonds, and an external force is difficult to damage a formed multi-hydrogen bond network structure. When the using amount of the rice straw powder is too large, the hydrogen bonds reach a saturated state, and the rice straw powder is agglomerated. When an external force is applied, stress defects occur at the place of agglomeration, so that the tensile strength is reduced.

The ultraviolet transmittance curves of the precision indicating film for monitoring the freshness of sashimi prepared in examples 1 to 4 and the indicating film prepared in example 5 are shown in fig. 5, and the haze of the films with different amounts of powdered rice straw is shown in fig. 6. As can be seen from fig. 5, as the amount of straw powder added increases, the uv transmittance of the indicator film becomes lower and lower, and the haze increases from 0.65% to 16.3%.

The water vapor barrier properties of the precision indicating film for monitoring the freshness of sashimi prepared in examples 1 to 4 and the indicating film prepared in example 5 are shown in table 3.

TABLE 3 film Water vapor Barrier Performance with different amounts of BTB-CRSP added

Film sample	WVP(g m ^-1 s ^-1 Pa ^-1 ×10 ^-10 )
		CC-0BTB/CRSP	1.12±0.055a
CC-0.25BTB/CRSP	1.29±0.005b
		CC-0.5BTB/CRSP	1.30±0.005b
CC-0.75BTB/CRSP	1.32±0.065b
		CC-1BTB/CRSP	1.40±0.050c

As can be seen from Table 3, when the amount of BTB-CRSP added was increased from 0 to 1%, the WVP of the membrane was increased from 1.12X 10 ^-10 g m ^-1 s ^-1 Pa ^-1 Increase to 1.4 × 10 ^-10 g m ^-1 s ^-1 Pa ^-1 The reason is that the straw powder is dispersed in the CC composite membrane, so that the pores are enlarged, the original compact structure is reduced, the permeation of water vapor is facilitated, and the response speed of the indicating membrane is improved.

The colorimetric parameters and color changes of the prepared precise indicator film for monitoring the freshness of sashimi in example 4 in different pH buffers are shown in table 4. Where Δ E was calculated as the average chroma value of the CC-1BTB/CRSP smart indicator film as a control. The photographs show that the CC-1BTB/CRSP film appears yellowish at pH =1 to 3, yellowish at pH =4 to 7, greenish at pH =8 to 9, bluish-green at pH =10 to 11, and deep blue at pH =12 to 13. As analyzed from the colorimetric parameters, L is not significantly regular, the colorimetric parameters a and b are not significantly regular when pH =1 to 3 and pH =11 to 13, and the parameter a decreases with increasing pH at pH =4 to 10, indicating that the indicator film gradually changes to green on the red-green axis; the parameter b decreases with increasing pH, indicating a gradual shift towards blue on the yellow-blue axis. The indicating film has obvious color change under different pH values (TVB-N), and can be used for accurately monitoring the freshness of the sashimi in real time.

TABLE 4 CC-1BTB/CRSP indicates the colorimetric parameters of the membranes in different pH buffer solutions

The freshness of the raw fish fillet is monitored in real time by the accurate indicating film CC-1BTB/CRSP for monitoring the freshness of the raw fish fillet prepared in example 4. The color change of the film was observed during the experiment and subjected to a colorimetric test. The TVB-N value of the fish meat at the time of the color change of the film is measured, and the research indicates the application value of the film in the precise monitoring of the freshness of the sashimi, and the result is shown in Table 5.

TABLE 5 colorimetric change of indicator film at different monitoring times

Monitoring time (h)	L*	a*	b*	△E
					0	80.29	-2.98	19.57	-
3	72.80	-8.88	7.24	15.59
					10	69.39	-11.36	-4.38	28.03
29	63.52	-13.73	-11.83	37.20

As is clear from table 5, the a value of the indicator film gradually decreased with increasing resting time of the fish meat, indicating that the color of the film on the red-green axis shows a tendency to change from red to green, while b was observed to gradually decrease and decrease from a positive value to a negative value, indicating that the color of the film on the yellow-blue axis has a tendency to change from yellow to blue. Fig. 7 shows the TVB-N values of fish meat and the color of the indicator film as a function of the resting time. Fresh fish meat used in the study, i.e., after 0h of standing, was initially monitored as pale yellow in color and had a TVB-N value of 8.845mg/100g. As time increases, the TVB-N value also increases. After standing for 3 hours, the color of the film changed to grass green, at which time the TVB-N value of the fish meat was 9.47mg/100g. After standing for 10h, the color of the indicator film is changed into blue-green, the TVB-N value of the fish meat is 10.619mg/100g, and the fish meat is not fresh and belongs to a sub-fresh state. GB2733-2015 shows that the TVB-N is 20mg/100g to represent that the fish meat is deteriorated, when the indication film is placed for 29 hours, the blue-green color of the indication film is changed into dark blue, and the measured corresponding TVB-N value is 18.214mg/100g, which indicates that the indication film can timely display a signal that the fish meat is about to deteriorate. From this, it can be seen that the precision indicating film CC-1BTB/CRSP for monitoring the freshness of sashimi prepared in example 4 can indicate the freshness of fish meat in real time.

Claims

1. A preparation method of an accurate indication film for monitoring the freshness of sashimi is characterized by comprising the following steps:

1. preparing quaternary amination straw powder: adding the rice straw powder into a NaOH solution with the mass percentage concentration of 20-25%, stirring for 2-2.5 h, and filtering to obtain swelling cellulose; adding NaOH solution with the mass percentage concentration of 10-12% and epoxy chloropropane into swelling cellulose, reacting for 7-8 h under the condition that the water bath temperature is 65-68 ℃, filtering, and washing a solid phase substance with ethanol; adding an ethanol solution of triethylamine into the solid phase substance, reacting for 4-4.5 h at the temperature of 75-78 ℃, filtering, sequentially cleaning the solid phase substance by using distilled water, a dilute alkali solution and a dilute acid solution, finally washing the solid phase substance to be neutral by using distilled water, and drying to obtain quaternary ammonium straw powder;

2. preparing the grafting bromothymol blue rice straw powder: adding a dilute alkali solution of bromothymol blue into quaternary amination straw powder, stirring for 2-2.5 h by using a magnetic stirrer at the temperature of 40-45 ℃, then performing suction filtration, and washing a solid phase substance by using water until filtrate is neutral to obtain grafted bromothymol blue straw powder;

3. preparation of an indicator film: adding carrageenan and sodium carboxymethylcellulose into water with the temperature of 75-78 ℃ according to the mass ratio of (2-2.5) to 1, stirring for 1-1.5 h, and then adding the grafted bromothymol blue rice straw powder and sorbitol, wherein the mass of the grafted bromothymol blue rice straw powder in the film-forming liquid accounts for 0.25-1% of the mass sum of the carrageenan and the sodium carboxymethylcellulose; the mass of the sorbitol accounts for 45-50% of the sum of the mass of the carrageenan and the mass of the sodium carboxymethyl cellulose; continuously stirring for 1-1.5 h to obtain a film forming solution; and pouring the film forming solution into a mold, drying for 12-15 hours at the temperature of 60-63 ℃, and removing the film to obtain the accurate indicating film for monitoring the freshness of the sashimi.

2. The preparation method of the precise indicator film for monitoring the freshness of the sashimi according to claim 1, wherein the fineness of the straw powder in the first step is 100-120 meshes.

3. The preparation method of the precise indication film for monitoring the freshness of the sashimi as claimed in claim 1 or 2, wherein the mass ratio of the straw powder to the NaOH solution with the mass percentage concentration of 20-25% in the first step is 1: (25 to 30).

4. The preparation method of the precision indicator film for monitoring the freshness of the sashimi according to claim 1 or 2, wherein the mass ratio of the straw powder to the NaOH solution with the mass percentage concentration of 10% -12% in the step one is 1: (25 to 30).

5. The manufacturing method of the precise indicating film for monitoring the freshness of the sashimi according to claim 1 or 2, wherein the ratio of the mass of the straw powder to the volume of the epichlorohydrin in the first step is 1g: (25-35) mL.

6. The method for preparing the precise indication film for monitoring the freshness of the sashimi according to claim 1 or 2, wherein the ethanol solution of triethylamine in the first step is prepared by using ethanol as a solvent, the volume percentage of the triethylamine is 34-35%, and the mass ratio of the straw powder to the ethanol solution of triethylamine is 1: (14 to 15).

7. The method for preparing the precision indicating film for monitoring the freshness of the sashimi according to claim 1 or 2, wherein the diluted alkaline solution of bromothymol blue in the second step is prepared by adding and mixing bromothymol blue into a sodium hydroxide solution with a concentration of 0.09 mol/L-0.11 mol/L, wherein the concentration of the bromothymol blue is 0.9 g/L-1.1 g/L.

8. The preparation method of the precise indication film for monitoring the freshness of the sashimi according to claim 1 or 2, wherein the mass ratio of the total mass of carrageenan and sodium carboxymethylcellulose to water in the step three is 1: (39 to 40).