1. According to the invention, natural waste crab shells are used as a cross-linking agent, as the main component of the cross-linking agent is calcium carbonate, gluconolactone can be used as a calcium ion releasing agent to promote the cross-linking of calcium ions in crab shell powder and sodium alginate to form gel, and the prepared hydrogel is dyed by using a bromothymol blue and methyl red compound indicator to obtain the indicating hydrogel which can be used in chilled meat packaging, and different color changes occur according to the putrefaction condition of fresh meat in the storage process, so that the method has important significance in the field of chilled meat freshness detection.

2. The preparation method and application of the hydrogel for indicating the freshness of chilled meat according to claim 1 are characterized by comprising the following steps:

(1) crushing the waste crab shells by using an ultrafine crusher, collecting the crushed crab shells for later use, stirring the crushed crab shells in a low-concentration sodium hydroxide solution at the temperature of 60 ℃ for 6 hours to remove protein, then filtering and washing the mixture until the pH value is neutral, adding absolute ethyl alcohol, stirring the mixture at room temperature for 6 hours to decolor the mixture, and filtering and drying the mixture to obtain the crab shell powder.

(2) Preparing a sodium alginate aqueous solution with the mass fraction of 2%, stirring until the sodium alginate aqueous solution is completely dissolved, adding a certain amount of crab shell powder prepared in the step (1), and stirring for 6 hours at room temperature.

(3) And (3) adding a certain amount of glucolactone into the mixed solution obtained in the step (2), stirring for 3min at room temperature, pouring into a mold, and crosslinking for 12h at 4 ℃ to obtain the sodium alginate hydrogel.

(4) Dissolving bromothymol blue and methyl red in 70% ethanol according to a certain proportion, and adding distilled water for dilution to enable the total mass percent of dry substances to reach 0.01%, thereby obtaining the compound coloring agent.

(5) And (4) soaking the hydrogel obtained in the step (3) into a composite indicator for 24h, taking out the hydrogel, and then sucking the indicator remained on the surface by using filter paper to obtain the pH-sensitive sodium alginate indicating hydrogel.

3. The method as claimed in claim 2, wherein the sodium hydroxide used for deproteinization in step (1) is 5% by weight.

4. The method for preparing hydrogel for indicating freshness of chilled meat according to claim 2, wherein the mass ratio of the sodium alginate to the crab shell powder in the step (2) is 2: 1.

5. The method as claimed in claim 2, wherein the gluconolactone is present in the amount of 0.9% by weight in the step (3).

6. The method for preparing hydrogel for indicating freshness of chilled meat according to claim 2, wherein the mass ratio of bromothymol blue to methyl red in the step (4) is 3: 2.

Background

The pork is indispensable in daily life of people and is very popular with consumers, wherein the pork is the meat with the largest consumption in China and accounts for more than 60 percent of all meat. The cooled meat contains various nutrient substances such as protein and the like, and the cooled meat often goes bad during the storage process at 4 ℃, so that the taste and the nutritive value of the cooled meat are damaged, diseases are easily caused, and the health of people is harmed, so that the intelligent package capable of intuitively monitoring the freshness of the meat in time is produced.

The freshness indicating material functions to monitor or react with metabolites in the fresh food resulting from microbial growth (e.g., organic acids, carbon dioxide, volatile basic nitrogen or sulfur derivatives), and to determine the quality of the chilled meat within the package by a significant color change as the volatile material dissolves in the water within the matrix. However, the coloring agent in the traditional indicating material is generally single indicator such as bromothymol blue, methyl red or anthocyanin extract, and the pH variation range is limited to a certain extent. In addition, the substrate is generally some films such as cellulose film and starch film, which have certain hydrophilicity, but the sensitivity is reduced under the condition of low humidity inside the package, and the indicating effect is not obvious.

The hydrogel is a novel high polymer material and has the advantages of good sealing property, degradability, moisture retention and the like. The water content in the water-based food freshness detection reagent is high, so that the water-based food freshness detection reagent is a good matrix for food freshness detection. However, the conventional hydrogel also has certain disadvantages, such as non-natural sources of raw materials for preparation, complicated preparation process, and the like. Sodium alginate is a safe natural anionic polysaccharide, and can be crosslinked with divalent metal cations to form hydrogel. Compared with other hydrogel materials, the sodium alginate hydrogel is nontoxic and high in biocompatibility, and is widely applied to various technical fields such as food packaging and the like. Crab shell is a natural waste, the main component of which is calcium carbonate, which can be used as a natural calcium source after being crushed, deproteinized and decolored, and Ca released after gluconolactone is dripped²⁺Can be crosslinked with sodium alginate to form an egg box structure, thereby obtaining the sodium alginate hydrogel which is simple to prepare, green and high in strength. Will make intoAnd dyeing the obtained hydrogel by using a bromothymol blue and methyl red compound dyeing agent to obtain the hydrogel for indicating the freshness of the chilled meat.

Disclosure of Invention

The invention aims to provide a preparation method and application of hydrogel for indicating freshness of chilled meat, which takes natural waste crab shells as a crosslinking initiator and aims to utilize gluconolactone as Ca²⁺Releasing agent prepared by adding Ca into crab shell powder²⁺Released to be crosslinked with sodium alginate, thereby forming hydrogel. Bromothymol blue and methyl red are two acid-base indicators and can sensitively detect CO in environment₂Concentration, CO₂Dissolved in water to become H₂CO₃And further dissociate into H^﹢With HCO₃ ^-The two indicators are colored. The freshness indicating type hydrogel is obtained by dyeing the bromothymol blue and methyl red compound dyeing agent, the pH value color change range of the hydrogel can be enlarged, and a foundation is laid for the research of the hydrogel in the fields of food packaging, food freshness monitoring and high polymer materials.

In order to achieve the purpose, the method comprises the following steps:

(1) carrying out superfine grinding on crab shell powder, adding a low-concentration sodium hydroxide solution, stirring for 6 hours at 60 ℃ for deproteinization, filtering and washing the obtained crab shell powder until the pH value is 7, adding absolute ethyl alcohol, stirring for 6 hours at room temperature for decoloration, and filtering and drying to obtain crab shell powder.

(2) Preparing a sodium alginate aqueous solution with the mass fraction of 2%, adding a certain amount of the crab shell powder prepared in the step (1) after the sodium alginate aqueous solution is completely dissolved, and stirring for 6 hours at room temperature.

(3) And (3) stirring a certain amount of gluconolactone in the mixed solution obtained in the step (2) for 3min, pouring the mixture into a mold, and crosslinking at 4 ℃ for 12h to obtain the natural sodium alginate hydrogel.

(4) Dissolving bromothymol blue and methyl red in 70% ethanol according to a certain proportion, and adding distilled water for dilution to enable the total mass fraction of dry substances to reach 0.01%, thereby obtaining the compound indicator.

(5) And (4) immersing the sodium alginate hydrogel obtained in the step (3) into a composite indicator for 24h, taking out, and then, drying the indicator remained on the surface by using filter paper to obtain the freshness indication type hydrogel.

The invention also includes such features:

the mass fraction of sodium hydroxide used for deproteinization concentration in (1) is 5%.

The mass ratio of the sodium alginate to the crab shell powder added in the step (2) is 2: 1.

The mass fraction of the gluconolactone added in the step (3) is 0.9%.

The mass ratio of bromothymol blue to methyl red in the step (4) is 3: 2.

The color change condition of the indicating hydrogel in buffers with different pH values and different CO contents of the indicating hydrogel at 4 ℃ are adopted₂Color change in a concentrated gas environment. The practical performance of the invention is characterized by the detection of the electronic nose, the change of the total number of bacterial colonies and the change of the actual color of the gel during the storage process of the cooled meat at 4 ℃.

The invention has the beneficial effects from the steps: firstly, sodium alginate used as a matrix of the indicating hydrogel prepared by the invention is a biodegradable material, and has the advantages of no toxicity, high biocompatibility, high safety and the like. The invention uses the crab shell powder from nature as a cross-linking agent, provides a calcium source for the ionic crosslinking of the sodium alginate hydrogel and increases the comprehensive utilization rate of agricultural and sideline products. Thirdly, the invention uses glucolactone as Ca²⁺Releasing agents, H, dissociated after dissolving gluconolactone in water⁺Can slowly remove Ca in crab shell²⁺Released and then evenly cross-linked with sodium alginate to form an egg box structure, and the effect is better than that of other organic acids. Fourth, the bromothymol blue and methyl red compound indicator is used in the invention, and the pH sensitivity of the indicator is superior to that of other single indicators. The freshness indicating type hydrogel prepared by the invention is simple to prepare, good in water retention and very good in pH sensitivity, has an obvious indicating effect when being applied to chilled meat tray packaging, and is superior to a traditional freshness indicating type material.

Drawings

FIG. 1 color change of hydrogel in buffer solution at pH3-7

FIG. 2 hydrogel with different CO concentrations at 4 deg.C₂Color change of

FIG. 3 Total number of colonies on chilled meat during storage at 4 deg.C

FIG. 4 electronic nose detection of chilled meat during storage at 4 deg.C

FIG. 5 shows the effect of hydrogel formation on chilled meat storage at 4 ℃

Detailed Description

Examples

A preparation method and application of hydrogel for indicating freshness of chilled meat comprise the following steps:

the method comprises the following steps: crushing waste crab shells by using an ultrafine crusher, adding a 5% sodium hydroxide solution, stirring for 6h at 60 ℃ for deproteinization, filtering and washing the crab shell powder until the pH is neutral, adding absolute ethyl alcohol, stirring for 6h at room temperature for decoloration, filtering, and drying to obtain the crab shell powder.

Step two: dissolving a certain amount of sodium alginate in deionized water to obtain a sodium alginate water solution with the mass fraction of 2%. Then adding a certain amount of crab shell powder (m)_{Sodium alginate}：m_{Crab shell powder}2:1) to obtain a uniform mixed solution.

Step three: adding 0.9 mass percent of gluconolactone into the film-forming solution. Stirring for 2min, pouring into a mold, and crosslinking at 4 deg.C for 12 hr to obtain sodium alginate hydrogel.

Step four: dissolving bromothymol blue and methyl red in 70% ethanol according to the mass ratio of 3:2, and adding distilled water for dilution to enable the total mass fraction of dry substances to reach 0.01%, thereby obtaining the compound indicator.

Step five: and (3) putting the sodium alginate hydrogel into the compound indicator in the fourth step for 24 hours, then taking out the sodium alginate hydrogel, and sucking the indicator remained on the surface of the hydrogel by using filter paper to obtain the freshness indicating type hydrogel.

As can be seen from FIG. 1, after the freshness indicating hydrogel is taken out from the buffer solutions with different pH values, the hydrogel has a good shape, a remarkable color change and is easy to distinguish, the hydrogel is dark yellow when the pH value is 3-5, the hydrogel is light yellow when the pH value is 6, the hydrogel is green when the pH value is 7, and the hydrogel is changed from dark green to dark blue when the pH value is 8-11, which indicates that the prepared hydrogel has good pH sensitivity and can play an indicating role.

As can be seen from FIG. 2, the freshness indicating hydrogel is CO₂The concentration level is sensitive when indicating that the hydrogel is exposed to 20% or more of CO₂At the level, a clear color change from green to yellowish occurs.

According to the national standard, fresh meat is defined as the total number of bacterial colonies less than 4Log (CFU/g) in the process of storing the cooled meat; the total number of colonies between 4 and 6Log (CFU/g) was defined as the sub-fresh meat. Above 6Log (CFU/g) is defined as spoiled meat. As can be seen from FIG. 3, the total number of colonies in this experiment at the fourth day of storage of the chilled meat exceeded 4Log (CFU/g), indicating a gradual change from fresh meat to sub-fresh meat, and at the tenth day exceeding 6Log (CFU/g) indicating complete spoilage of the meat.

As can be seen from fig. 4, the decay process of chilled meat is often accompanied by the generation of off-flavors, which are often used by consumers as indicators for identifying the quality of chilled meat. The response values of the sensors W1W, W6S, W5S and W1S in the control group gradually increased with the increase of the storage time, which indicates that sulfides, hydrides, oxynitrides and alkanes are generated in the cooled meat during the storage process, particularly the response value of W1S is particularly obvious, and that alkanes are the most obvious components of flavor change during the storage process of the cooled meat. In addition, the W1W value gradually increased with the increase of the storage time, which indicates that the hydrogen sulfide gas generated during the deterioration of the chilled meat gradually increased.

As can be seen from FIG. 5, the color of the freshness indicating hydrogel changed from deep blue to light green and finally to yellow as the chilled meat was stored for an extended period of time, since the chilled meat gradually spoiled as the chilled meat was stored for an extended period of time, and oxygen was consumed by the proliferation of bacteria to generate CO₂While meat decay produces more H₂S gases, which accumulate above the closed vial and gradually dissolve inside the hydrogel to become acidic, discoloring the hydrogel, indicating the freshness-indicating hydrogelCan successfully detect the main volatile gas generated by the fresh pork in the putrefaction process.