Intelligent pH indicating membrane based on whey protein

Abstract

The invention discloses an intelligent pH indicating membrane based on whey protein and a preparation method of the intelligent pH indicating membrane. The intelligent pH indicating film is prepared by taking whey protein and pectin as raw materials, firstly constructing a homogeneous phase mixed solution of pectin and whey protein by adopting an ultrasonic technology, then adding a purple sweet potato anthocyanin extract as a pH indicator, promoting sufficient compounding of anthocyanin with whey protein and pectin matrix under the driving of ultrasonic waves, and finally drying. The substrates of the intelligent pH indicating film are whey protein and pectin, the pH indicator is purple sweet potato anthocyanin extract, and all the substrates are edible substances, so that the intelligent pH indicating film is high in safety and good in biodegradability and biocompatibility; in the preparation process, an ultrasonic technology is utilized to promote sufficient action of whey protein and pectin and to promote tight compounding between anthocyanin and a matrix, so that the stability of the anthocyanin is protected, and the anthocyanin is not easy to degrade in the storage process and has relatively high color change sensitivity. Other non-edible chemical reagents are not added in the preparation process of the pH intelligent indicating film, and the process is green, environment-friendly, simple and easy to implement; the prepared intelligent pH indicating film can realize intelligent indication of the spoilage process of seafood products, and has a wide application prospect.

Description

Technical Field

[0001] The invention belongs to the field of material science, and in particular relates to a pH intelligent indicator film based on whey protein and a preparation method thereof. Technical Background

[0002] During storage, packaged foods will produce volatile basic nitrogen, NH 3 Metabolites such as , which in turn lead to changes in the pH value of the internal environment of the food packaging, and the degree of change is related to the freshness of the food. The freshness of food not only determines the price of the commodity, but also affects consumers' willingness to buy, and is one of the important factors affecting food quality and safety. Therefore, smart indicator packaging films that can automatically monitor, record and track the changes in the freshness of food during storage and convey the freshness of food to consumers through visual perception have attracted the attention of researchers, especially pH smart indicator films.

[0003] pH smart indicator film is a new type of indicator packaging material or packaging auxiliary material, which can sense the internal environment of food packaging and change color to reflect the freshness of food. In order to monitor the freshness of food without opening the package, pH-sensitive compounds can be doped into the packaging matrix. As the internal environment of the package changes, the packaging film will undergo a color reaction, and the freshness of food can be monitored in real time through color changes. However, the development of pH smart indicator packaging materials that can be used for food is difficult. On the one hand, packaging materials have high requirements for mechanical strength. Although chemically synthesized macromolecules have excellent mechanical properties, their non-degradability poses certain environmental risks, and their inedibility poses safety risks, which limits their application in packaging materials. Although natural macromolecules have certain viscosity and film-forming properties, their mechanical properties are poor and it is difficult to meet the requirements of packaging materials. On the other hand, chemically synthesized pH indicators have high color change sensitivity, but their safety is controversial; natural pH indicators, such as anthocyanins, alizarin, betaine, etc., have poor stability and insensitive color development, and their application in pH smart indicator packaging films is limited.

[0004] In recent years, the application of natural macromolecules in food packaging materials has become a hot topic, especially the combination of polysaccharides and proteins. Whey protein is a large class of proteins in milk and a by-product of cheese processing. Whey protein is a mixture of various protein components and is negatively charged under neutral conditions. Whey protein is rich in essential amino acids. Because the ratio of amino acids it contains meets the needs of the human body, it is recognized as one of the high-quality protein supplements for the human body. It has high digestibility, high bioavailability and extremely high nutrition. Whey protein has a high affinity for anthocyanins, and it can spontaneously bind to anthocyanins through non-covalent bonds, thereby improving the stability of anthocyanins. Pectin is a safe, edible, biodegradable natural polysaccharide extracted from plant peels. It is mainly composed of α-1-4-D galacturonic acid units. It is negatively charged under neutral conditions and can be used as a gelling agent, thickener and stabilizer. It is widely used in the food industry. In addition, pectin has a high melting point and can form a relatively dense structure after film formation, so it has good air / water vapor insulation properties, but the mechanical strength of the film prepared by it still cannot meet the relevant requirements. In addition, when polysaccharides and proteins are used as mixed matrices to prepare packaging films, polysaccharides and proteins with the same charge are easily phase-separated under the action of repulsion, resulting in stratification. It can be seen that how to construct a stable polysaccharide-protein mixed system is the prerequisite for using it in packaging film materials.

[0005] Anthocyanins are water-soluble flavonoid compounds that are widely available, safe, non-toxic, and easy to obtain, and have excellent antibacterial and antioxidant properties. Anthocyanins are usually very abundant in vegetables and fruits, such as purple sweet potatoes, purple cabbage, mulberries, blueberries, etc.; but their stability is poor, especially sensitive to temperature, pH and light, and they are very easy to degrade. Therefore, improving the stability of anthocyanins and increasing their sensitivity to pH are the prerequisites for using them in pH smart indicator films.

[0006] Low-frequency ultrasonic technology is a non-thermal processing technology that has emerged in recent years. Ultrasonic waves can produce cavitation effects, thermal effects and microjet effects in liquid matrices, causing strong turbulence, generating higher shear forces, and increasing the frequency of collisions between particles, thereby weakening particle agglomeration and changing their physical properties. The team's previous research found that ultrasonic-assisted technology can promote the non-covalent binding of small molecules of milk protein and anthocyanin, thereby increasing the binding constant of the two. In addition, under the action of ultrasonic waves, negatively charged polysaccharides and proteins can form a uniformly dispersed water-in-water (W / W) system through electrostatic repulsion, thereby improving the stability of the polysaccharide-protein mixed system.

[0007] Based on the above research conclusions and theoretical basis, the present invention increases the negative charge of whey protein and pectin by pH fine-tuning, thereby increasing the repulsion between the two; then uses ultrasonic technology to construct a pectin-whey protein W / W homogeneous system, and then adds glycerol, using glycerol's smaller steric hindrance and more hydroxyl groups to form a non-covalent cross-linking effect between pectin and whey protein, which effectively plays a plasticizing role; then adds purple sweet potato anthocyanin extract, and drives anthocyanin to combine with whey protein and pectin through ultrasound to form a dense network structure, which not only stabilizes anthocyanin and maintains the detection sensitivity of anthocyanin to pH, but also effectively improves the mechanical properties of the film, increases the air / water vapor barrier performance, and further plays a plasticizing role. The pH intelligent indicator film prepared by this technology has excellent color reversibility and pH sensitivity, and can realize real-time monitoring of the freshness of shrimp during storage. Summary of the invention

[0008] The technical problem to be solved by the present invention is, on the one hand, to effectively improve the stability of anthocyanins as pH indicators, and on the other hand, to develop degradable and even edible film materials that can meet the requirements of food packaging, thereby providing the food industry with a highly stable and highly sensitive pH intelligent indicator film, thereby realizing real-time monitoring of the freshness of some foods and providing consumers with accurate information on the freshness of food that can be identified by the naked eye.

[0009] The technical solution adopted by the present invention is: A pH intelligent indicator membrane based on whey protein, the specific preparation steps are as follows: (1) Extraction of purple sweet potato anthocyanins: Cut fresh purple sweet potatoes without spoilage into slices with a thickness of 0.5-0.8 cm, place in an oven at 37-45°C and dry for 12-24 h, grind with a grinder and pass through an 80-200 mesh sieve to collect the powder; add the purple sweet potato powder to 60-80% ethanol at a ratio of 1:10-1:15 (m / v), stir magnetically at 2000-3000 r / min in a 37-45°C water bath for 4-6 h, filter with a Buchner funnel, and concentrate the filtrate by rotary evaporation in the dark until it becomes viscous; freeze-dry the concentrate to obtain the purple sweet potato anthocyanin extract, and store it in a refrigerator in the dark at 4°C for later use; (2) Pretreatment of the film-forming matrix: Pectin was dissolved in water under magnetic stirring at 2000-3000 r / min, and fully hydrated overnight to prepare a solution with a mass concentration of 3.5-4.5%. Whey protein powder was added in a ratio of pectin: whey protein of 3.5:1-4.5:1. The mixture was magnetically stirred at 2000-3000 r / min in a 45-55°C water bath and 0.5-1.0 mol / L sodium hydroxide was slowly added dropwise. The pH of the mixed solution was adjusted to be stable at 7.5-8.5. After the whey protein was fully dissolved, 100 mL was taken for ultrasonic treatment. The diameter of the ultrasonic probe was 12.7 mm, the depth of immersion in the center of the sample was 2-4 cm, the ultrasonic frequency was 20 kHz, the amplitude was 20-40%, the ultrasonic time was 6-10 min, and the pulse mode was on for 3-5 s and off for 2-4 s to obtain a film-forming matrix solution. (3) Preparation of pH indicator membrane matrix: glycerol is added to the membrane matrix solution in step (2) at a ratio of 1.0-2.0 g per 100 mL of solution, magnetically stirred at 2000-3000 r / min for 1-2 h at 45-55° C., and cooled to room temperature; purple sweet potato anthocyanin powder extracted in step (1) is then added at a ratio of 300-1000 mg per 100 mL of solution, magnetically stirred at 2000-3000 r / min for 1-2 h at room temperature to fully dissolve the anthocyanins, and then the mixture is ultrasonically treated, with an ultrasonic probe having a diameter of 12.7 mm, an immersion depth of 2-4 cm in the center of the sample, an ultrasonic frequency of 20 kHz, an amplitude of 20-40%, an ultrasonic time of 2-4 min, and a pulse mode of on for 3-5 s and off for 2-4 s, to obtain a pH intelligent indicator membrane matrix solution; (4) Preparation of pH indicator film: Accurately weigh 10-12 g of the film-forming matrix in step (3) and pour it into a culture dish with a diameter of 90 mm. Place the culture dish in an oven at 45-55°C to dry for 5-7 h. Peel off the dried film from the culture dish to obtain a pH intelligent indicator film. 1. The present invention extracts anthocyanin from purple sweet potato to prepare a natural pH indicator; utilizes pH fine-tuning technology to weaken the aggregation of whey protein under weak alkaline conditions, increase the negative charge on the molecular chains of whey protein and pectin, and then increase the electrostatic repulsion between pectin and whey protein, making it easier to form a uniform and stable system. Ultrasonic drive is used to promote the dispersion of pectin and whey protein, change the structure, and form a W / W system; then glycerol is dispersed in the pectin-whey protein composite system, and cross-linking is formed between pectin molecules and whey protein molecules through hydrogen bonds, which effectively plays a plasticizing role, increases the network structure density of the matrix, and then improves the mechanical strength of the film.

[0010] 2. The present invention uses ultrasound-assisted technology to promote the non-covalent interaction between purple sweet potato anthocyanin extract and whey protein and pectin, further enhancing the density of the matrix network structure, thereby improving the mechanical properties of the membrane; on the other hand, the close combination with the matrix effectively improves the structural stability of anthocyanins, making it less likely to degrade during subsequent processing, storage and use, effectively maintaining the color change sensitivity of the membrane.

[0011] 3. The present invention utilizes extracted natural purple sweet potato anthocyanins as pH indicators, and promotes the close combination of anthocyanins and the matrix through ultrasonic driving, and evenly distributes them in the matrix network structure, giving the film excellent ultraviolet barrier properties, antibacterial properties and water vapor barrier properties, so that it not only has a pH intelligent indication function, but can also be used for food packaging.

[0012] In summary, the present invention uses cheap and readily available purple sweet potato as raw material, extracts anthocyanins as natural pH indicator, uses pH fine-tuning and ultrasonic treatment technology to achieve the construction of a whey protein-pectin stable system, and then uses ultrasonic technology to promote the effective combination of anthocyanins with the matrix, thereby improving the stability of anthocyanins and the mechanical strength of the film, and providing a highly sensitive pH intelligent indicator film for the food industry. The pH indicator film has a simple processing process, a wide range of applications, and broad market prospects. BRIEF DESCRIPTION OF THE DRAWINGS

[0013] Attached Figure 1 The appearance of the pH intelligent indicator film prepared by the embodiment of the present invention is shown in FIG. 1 , and a film without purple sweet potato anthocyanins is used as a control.

[0014] Attached Figure 2 The chromaticity parameters, water vapor permeability, breaking strength and breaking elongation of the pH intelligent indicator film prepared by the embodiment of the present invention are shown.

[0015] Attached Figure 3 The ultraviolet absorption spectrum of the pH intelligent indicator film prepared by the embodiment of the present invention.

[0016] Attached Figure 4 The color changes of the pH intelligent indicator membrane prepared by the embodiment of the present invention at different pH values.

[0017] Attached Figure 5 The degradability of the pH intelligent indicator membrane prepared by the embodiment of the present invention.

[0018] Attached Figure 6 The antibacterial property of the pH intelligent indicator membrane prepared by the embodiment of the present invention.

[0019] Attached Figure 7 The invention discloses an intelligent indication of the freshness of shrimps during storage by using the pH intelligent indicator film prepared by the embodiment of the invention, a relationship curve between the freshness (pH) and the storage time, and a relationship curve between the color difference (ΔE) and the freshness (pH).

[0020] The colorimetric determination method of the pH intelligent indicator film in the embodiment is as follows: the color parameters (L 0 * -Brightness, a 0 * - redness, b 0 * - yellow-blue degree). The total color difference (ΔE) is calculated as follows: Where: L=88.02, a=-3.05, b=-2.99 are the color parameters of the standard white board; L 0 * 、a 0 * , b 0 * is the color parameter of the film.

[0021] The water vapor permeability determination method of the pH smart indicator film in the embodiment is as follows: the composite film is cut into a circular sample with a diameter of 7.5 cm, placed on a test cup containing 8.0 mL of distilled water, and then placed in a water vapor permeability tester and tested at 25°C and 90% RH.

[0022] The method for determining the breaking strength and breaking elongation of the pH intelligent indicator film in the embodiment is as follows: an electronic universal testing machine is used to determine the breaking strength and breaking elongation of the film. Before the test, the composite film is cut into rectangular samples of 2.0 cm×7.0 cm, the initial holding distance is 50 mm, and the moving speed is 10 mm / min.

[0023] The UV-visible spectrum measurement method of the pH intelligent indicator film in the embodiment is as follows: the UV spectrum of the film is measured using a UV-1780 UV-spectrophotometer, and the composite film is cut into a 4.5 cm × 1.0 cm rectangular sample, which is vertically attached to the inner side of a quartz cuvette, with an empty cuvette as a control.

[0024] The method for determining the degradation of the pH intelligent indicator membrane in the embodiment is as follows: take an appropriate amount of soil from the garden and place it in a flowerpot, bury the membrane 5 cm below the soil, spray 50 mL of water to soak the soil, take out several pieces of the membrane every 2 days, carefully clean the adhering soil and take pictures.

[0025] The method for determining the antibacterial property of the pH intelligent indicator membrane in the embodiment is as follows: Escherichia coli (E.coli) and Staphylococcus aureus (S.aureus) are selected as typical strains of Gram-negative / Gram-positive bacteria. The membrane is cut into squares (64 mm 2 ) and placed in a bacterial suspension (100 μL, 1 × 10 5CFU / mL) for 12 hours, then add 900 μL of LB medium to the surviving bacteria, incubate at 37°C for 12 hours, and use a UV-visible spectrophotometer to detect the optical density of the bacterial suspension at 600 nm (ODtest), with the optical density of the non-film bacterial suspension (ODcontrol) as the control. The following formula is used to calculate the antibacterial rate of the membrane.

[0026]

[0027] In the embodiment, the pH intelligent indicator film is used to intelligently indicate the quality change of shrimp during storage as follows: cut the film into a semicircle and place it inside the cover of a culture dish; place fresh shrimp in the culture dish, cover the cover with the indicator film, and place it in a 4°C refrigerator, dynamically observe the color change of the film and take pictures, and at the same time measure the chromaticity of the film and calculate the color difference (ΔE), and measure the pH of the shrimp; fit the relationship curve between the color difference and pH. It is reported that pH greater than 7.7 is spoilage. DETAILED DESCRIPTION

[0028] In order to better understand the present invention, the present invention is further described below in conjunction with the accompanying drawings and embodiments, but the scope of protection claimed by the present invention is not limited to the scope described in the embodiments.

[0029] Example 1, a pH intelligent indicator membrane based on whey protein, the specific preparation steps are as follows: (1) Extraction of anthocyanins from purple sweet potatoes: Cut fresh purple sweet potatoes without spoilage into slices with a thickness of 0.5 cm, place them in an oven and dry them at 37°C for 24 h, grind them with a grinder and pass them through an 80-mesh sieve to collect the powder; add the purple sweet potato powder to 60% ethanol at a ratio of 1:10 (m / v), stir them magnetically at 2000 r / min in a 37°C water bath for 6 h, filter them with a Buchner funnel, and concentrate the filtrate by rotary evaporation in the dark until it becomes viscous; freeze-dry the concentrate to obtain the purple sweet potato anthocyanin extract, and store it in a refrigerator in the dark at 4°C for later use; the anthocyanin content in the extract was measured by the pH differential method to be 1.36±0.06 mg / g.

[0030] (2) Pretreatment of the film-forming matrix: Pectin was dissolved in water under magnetic stirring at 2000 r / min, and fully hydrated overnight to prepare a solution with a mass concentration of 3.5%. Whey protein powder was added in a ratio of pectin: whey protein of 3.5:1. The mixture was magnetically stirred at 2000 r / min in a 45°C water bath and 0.5 mol / L sodium hydroxide was slowly added dropwise. The pH of the mixed solution was adjusted to 7.5 and the whey protein was fully dissolved. Then 100 mL was taken for ultrasonic treatment. The diameter of the ultrasonic probe was 12.7 mm, the depth of immersion in the center of the sample was 2 cm, the ultrasonic frequency was 20 kHz, the amplitude was 20%, the ultrasonic time was 10 min, and the pulse mode was on for 3 s and off for 2 s to obtain a film-forming matrix solution. (3) Preparation of pH indicator membrane matrix: add 1.0 g of glycerol to the membrane matrix solution of step (2), stir the mixture under magnetic force at 2000 r / min for 2 h at 45°C, and cool to room temperature; then add 300 mg of purple sweet potato anthocyanin powder, stir the mixture under magnetic force at 2000 r / min for 2 h at room temperature to fully dissolve the anthocyanin, and then perform ultrasonic treatment on the mixture. The diameter of the ultrasonic probe is 12.7 mm, the depth of immersion in the center of the sample is 2 cm, the ultrasonic frequency is 20 kHz, the amplitude is 20%, the ultrasonic time is 4 min, and the pulse mode is on for 3 s and off for 2 s, to obtain a pH intelligent indicator membrane matrix solution; (4) Preparation of pH indicator membrane: Accurately weigh 12 g of the film-forming matrix in step (3) and pour it into a culture dish with a diameter of 90 mm. Place the culture dish in a 45°C oven and dry it for 7 h. Peel off the dried film from the culture dish to obtain a pH intelligent indicator membrane.

[0031] By the attached Figure 1 It can be seen that compared with the control film without purple sweet potato anthocyanin, the film-forming base liquid and the dried film are pink, and the chromaticity parameters are shown in the attached Figure 2 The whiteness value is 79.10±0.48. The water vapor permeability of the membrane is 0.76±0.70×10 -11 g·cm / (cm 2 ·s·Pa), which can effectively prevent the penetration of water vapor. The breaking strength of the membrane is 12.21±1.01MPa, and the breaking elongation is 17.91±0.98%, which has good mechanical strength. Figure 3 It can be seen that the film has an extremely low transmittance in the ultraviolet region, indicating that it has good ultraviolet blocking performance. Figure 4 It can be seen that the membrane is sensitive to pH, changes color quickly, and the color change is highly recognizable by the naked eye. Figure 5 It can be seen that the film has good degradability and can be mostly degraded after being buried in the soil for 6 days. Figure 6It can be seen that the membrane has a certain degree of inhibition on Escherichia coli and Staphylococcus aureus. Figure 7 It can be seen that the film has a good intelligent indication effect on the freshness of shrimp during the storage period, and its color changes from pink to light purple and then to blue-purple. The color difference (ΔE) of the film is linearly related to the pH of the shrimp, and the fitting degree is as high as 0.9361. It can be seen that the film has a good pH indication effect and can realize the intelligent indication of the freshness of shrimp during the storage period. The pH of the shrimp can be inferred by the color difference (ΔE) to determine its freshness.

[0032] Example 2, a pH intelligent indicator membrane based on whey protein, the specific preparation steps are as follows: (1) Extraction of anthocyanins from purple sweet potatoes: Cut fresh purple sweet potatoes without spoilage into slices with a thickness of 0.8 cm, place in an oven at 45°C and dry for 12 h, grind with a grinder and pass through a 200-mesh sieve to collect the powder; add the purple sweet potato powder to 80% ethanol at a ratio of 1:15 (m / v), stir magnetically at 3000 r / min in a 45°C water bath for 4 h, filter with a Buchner funnel, and concentrate the filtrate by rotary evaporation in the dark until it becomes viscous; freeze-dry the concentrate to obtain the purple sweet potato anthocyanin extract, and store in a refrigerator at 4°C in the dark for later use; the anthocyanin content was measured by the pH differential method to be 1.97±0.12 mg / g; (2) Pretreatment of the film-forming matrix: Pectin was dissolved in water under magnetic stirring at 3000 r / min, and fully hydrated overnight to prepare a solution with a mass concentration of 4.5%. Whey protein powder was added in a ratio of pectin: whey protein of 4.5:1. The mixture was magnetically stirred at 3000 r / min in a 55°C water bath and 1.0 mol / L sodium hydroxide was slowly added dropwise. The pH of the mixed solution was adjusted to 8.5 and the whey protein was fully dissolved. Then, 100 mL was taken for ultrasonic treatment. The diameter of the ultrasonic probe was 12.7 mm, the depth of immersion in the center of the sample was 4 cm, the ultrasonic frequency was 20 kHz, the amplitude was 40%, the ultrasonic time was 6 min, and the pulse mode was on for 5 s and off for 4 s to obtain a film-forming matrix solution. (3) Preparation of pH indicator membrane matrix: add 2.0 g of glycerol to the membrane matrix solution of step (2), stir the mixture under magnetic force at 3000 r / min for 1 h at 55°C, and cool to room temperature; then add 1000 mg of purple sweet potato anthocyanin powder, stir the mixture under magnetic force at 3000 r / min for 1 h at room temperature to fully dissolve the anthocyanin, and then perform ultrasonic treatment on the mixture. The diameter of the ultrasonic probe is 12.7 mm, the depth of immersion in the center of the sample is 4 cm, the ultrasonic frequency is 20 kHz, the amplitude is 40%, the ultrasonic time is 2 min, and the pulse mode is on for 5 s and off for 4 s, to obtain a pH intelligent indicator membrane matrix solution; (4) Preparation of pH indicator film: Accurately weigh 10 g of the film-forming matrix in step (3) and pour it into a culture dish with a diameter of 90 mm. Place the culture dish in a 50°C oven and dry it for 5 h. Peel off the dried film from the culture dish to obtain a pH intelligent indicator film.

[0033] By the attached Figure 1 It can be seen that compared with the control film without purple sweet potato anthocyanin, the film-forming base liquid and the dried film are pink, and the chromaticity parameters are shown in the attached Figure 2 , whiteness value is 61.50±0.22, water vapor transmission rate is 4.67±1.22×10 - 11 g·cm / (cm 2 ·s·Pa), which can effectively prevent the penetration of water vapor. The breaking strength of the membrane is 13.79±1.03MPa, and the breaking elongation is 19.12±0.87%, which has good mechanical strength. Figure 3 It can be seen that the film has an extremely low transmittance in the ultraviolet region, indicating that it has good ultraviolet blocking performance. Figure 4 It can be seen that the membrane is sensitive to pH, changes color quickly, and the color change is highly recognizable by the naked eye. Figure 5 It can be seen that the film has good degradability and can be mostly degraded after being buried in the soil for 6 days. Figure 6 It can be seen that the membrane has good inhibition against Escherichia coli and Staphylococcus aureus. Figure 7 It can be seen that the film has a good intelligent indication effect on the freshness of shrimp during the storage period, and its color changes from pink to light purple and then to blue-purple. The color difference (ΔE) of the film is linearly related to the pH of the shrimp, and the fitting degree is as high as 0.9406. It can be seen that the film has a good pH indication effect and can realize the intelligent indication of the freshness of shrimp during the storage period. The pH of the shrimp can be inferred by the color difference (ΔE) to determine its freshness.

[0034] Example 3, a pH intelligent indicator membrane based on whey protein, the specific preparation steps are as follows: (1) Extraction of anthocyanins from purple sweet potatoes: Cut fresh purple sweet potatoes without spoilage into slices with a thickness of 0.6 cm, place in an oven at 41°C and dry for 18 h, grind with a grinder and pass through a 120-mesh sieve to collect the powder; add the purple sweet potato powder to 70% ethanol at a ratio of 1:12 (m / v), stir magnetically at 2500 r / min in a 41°C water bath for 5 h, filter with a Buchner funnel, and concentrate the filtrate by rotary evaporation in the dark until it becomes viscous; freeze-dry the concentrate to obtain the purple sweet potato anthocyanin extract, and store it in a refrigerator at 4°C in the dark for later use; the anthocyanin content was measured by the pH differential method to be 1.53±0.31 mg / g; (2) Pretreatment of the film-forming matrix: Pectin was dissolved in water under magnetic stirring at 2500 r / min, and fully hydrated overnight to prepare a solution with a mass concentration of 4.0%. Whey protein powder was added in a ratio of pectin: whey protein of 4.0:1. The mixture was magnetically stirred at 2500 r / min in a 50°C water bath and 0.8 mol / L sodium hydroxide was slowly added dropwise. The pH of the mixed solution was adjusted to 8.0 and the whey protein was fully dissolved. Then 100 mL was taken for ultrasonic treatment. The diameter of the ultrasonic probe was 12.7 mm, the depth of immersion in the center of the sample was 3 cm, the ultrasonic frequency was 20 kHz, the amplitude was 30%, the ultrasonic time was 8 min, and the pulse mode was on for 4 s and off for 3 s to obtain a film-forming matrix solution. (3) Preparation of pH indicator membrane matrix: add 1.5 g of glycerol to the membrane matrix solution of step (2), stir the mixture under magnetic force at 2500 r / min for 1.5 h at 50° C., and cool to room temperature; then add 700 mg of purple sweet potato anthocyanin powder, stir the mixture under magnetic force at 2500 r / min for 1.5 h at room temperature to fully dissolve the anthocyanin, and then perform ultrasonic treatment on the mixture. The diameter of the ultrasonic probe is 12.7 mm, the depth of immersion in the center of the sample is 3 cm, the ultrasonic frequency is 20 kHz, the amplitude is 30%, the ultrasonic time is 3 min, and the pulse mode is on for 4 s and off for 3 s, to obtain a pH intelligent indicator membrane matrix solution; (4) Preparation of pH indicator membrane: Accurately weigh 11 g of the film-forming matrix in step (3) and pour it into a culture dish with a diameter of 90 mm. Place the culture dish in a 50°C oven and dry it for 6 h. Peel off the dried film from the culture dish to obtain a pH intelligent indicator membrane.

[0035] By the attached Figure 1 It can be seen that compared with the control film without purple sweet potato anthocyanin, the film-forming base liquid and the dried film are pink, and the chromaticity parameters are shown in the attached Figure 2 , whiteness value is 67.88±0.12, water vapor transmission rate is 3.78±0.26×10 - 11 g·cm / (cm 2 ·s·Pa), which can effectively prevent the penetration of water vapor. The breaking strength of the membrane is 11.22±0.87MPa, and the breaking elongation is 14.69±0.64%, which has good mechanical strength. Figure 3 It can be seen that the film has an extremely low transmittance in the ultraviolet region, indicating that it has good ultraviolet blocking performance. Figure 4 It can be seen that the membrane is sensitive to pH, changes color quickly, and the color change is highly recognizable by the naked eye. Figure 5 It can be seen that the film has good degradability and can be mostly degraded after being buried in the soil for 6 days. Figure 6It can be seen that the membrane has good inhibitory properties against Escherichia coli and Staphylococcus aureus. Figure 7 It can be seen that the film has a good intelligent indication effect on the freshness of shrimp during the storage period, and its color changes from pink to light purple and then to blue-purple. The color difference (ΔE) of the film is linearly related to the pH of the shrimp, and the fitting degree is as high as 0.9854. It can be seen that the film has a good pH indication effect and can realize the intelligent indication of the freshness of shrimp during the storage period. The pH of the shrimp can be inferred by the color difference (ΔE) to determine its freshness.

[0036] Example 4, a pH intelligent indicator membrane based on whey protein, the specific preparation steps are as follows: (1) Extraction of anthocyanins from purple sweet potatoes: Cut fresh purple sweet potatoes without spoilage into slices with a thickness of 0.5 cm, place in an oven at 40°C and dry for 18 h, grind with a grinder and pass through a 100-mesh sieve to collect the powder; add the purple sweet potato powder to 75% ethanol at a ratio of 1:13 (m / v), stir magnetically at 2500 r / min in a 42°C water bath for 5 h, filter with a Buchner funnel, and concentrate the filtrate by rotary evaporation in the dark until it becomes viscous; freeze-dry the concentrate to obtain the purple sweet potato anthocyanin extract, and store in a refrigerator at 4°C in the dark for later use; the anthocyanin content was measured by the pH differential method to be 1.66±0.29 mg / g; (2) Pretreatment of the film-forming matrix: Pectin was dissolved in water under magnetic stirring at 2600 r / min, and fully hydrated overnight to prepare a solution with a mass concentration of 4.0%. Whey protein powder was added in a ratio of pectin: whey protein of 4.0:1. The mixture was magnetically stirred at 2600 r / min in a 50°C water bath and 0.8 mol / L sodium hydroxide was slowly added dropwise. The pH of the mixed solution was adjusted to 8.2 and the whey protein was fully dissolved. Then 100 mL was taken for ultrasonic treatment. The diameter of the ultrasonic probe was 12.7 mm, the immersion depth into the center of the sample was 3 cm, the ultrasonic frequency was 20 kHz, the amplitude was 30%, the ultrasonic time was 7 min, and the pulse mode was on for 4 s and off for 3 s to obtain a film-forming matrix solution. (3) Preparation of pH indicator membrane matrix: add 1.6 g of glycerol to the membrane matrix solution of step (2), stir the mixture under magnetic force at 2600 r / min for 1.5 h at 50°C, and cool to room temperature; then add 500 mg of purple sweet potato anthocyanin powder, stir the mixture under magnetic force at 2500 r / min for 1.5 h at room temperature to fully dissolve the anthocyanin, and then perform ultrasonic treatment on the mixture. The diameter of the ultrasonic probe is 12.7 mm, the depth of immersion in the center of the sample is 3 cm, the ultrasonic frequency is 20 kHz, the amplitude is 30%, the ultrasonic time is 3 min, and the pulse mode is on for 4 s and off for 3 s, to obtain a pH intelligent indicator membrane matrix solution; (4) Preparation of pH indicator film: Accurately weigh 10 g of the film-forming matrix in step (3) and pour it into a culture dish with a diameter of 90 mm. Place the culture dish in a 50°C oven and dry it for 6 h. Peel off the dried film from the culture dish to obtain a pH intelligent indicator film.

[0037] By the attached Figure 1 It can be seen that compared with the control film without purple sweet potato anthocyanin, the film-forming base liquid and the dried film are pink, and the chromaticity parameters are shown in the attached Figure 2 , whiteness value is 74.24±0.39, water vapor permeability is 0.95±0.02×10 - 11 g·cm / (cm 2 ·s·Pa), which can effectively prevent the penetration of water vapor. The breaking strength of the membrane is 11.96±1.12MPa, and the breaking elongation is 16.11±0.79%, which has good mechanical strength. Figure 3 It can be seen that the film has an extremely low transmittance in the ultraviolet region, indicating that it has good ultraviolet blocking performance. Figure 4 It can be seen that the membrane is sensitive to pH, changes color quickly, and the color change is highly recognizable by the naked eye. Figure 5 It can be seen that the film has good degradability and can be mostly degraded after being buried in the soil for 6 days. Figure 6 It can be seen that the membrane has a certain degree of inhibition on Escherichia coli and Staphylococcus aureus. Figure 7 It can be seen that the film has a good intelligent indication effect on the freshness of shrimp during the storage period, and its color changes from pink to light purple and then to blue-purple. The color difference (ΔE) of the film is linearly related to the pH of the shrimp, and the fitting degree is as high as 0.9146. It can be seen that the film has a good pH indication effect and can realize the intelligent indication of the freshness of shrimp during the storage period. The pH of the shrimp can be inferred by the color difference (ΔE) to determine its freshness.

Claims

1. A pH intelligent indicator membrane based on whey protein, characterized in that The method comprises the following preparation steps and process conditions: (1) Extraction of anthocyanins from purple sweet potatoes: Cut fresh purple sweet potatoes without spoilage into slices with a thickness of 0.5-0.8 cm, place in an oven at 37-45°C and dry for 12-24 h, grind with a grinder and pass through an 80-200 mesh sieve to collect the powder; add the purple sweet potato powder to 60-80% ethanol at a ratio of 1:10-1:15 (m / v), stir magnetically at 2000-3000 r / min in a water bath at 37-45°C for 4-6 h, filter with a Buchner funnel, and concentrate the filtrate by rotary evaporation in the dark until it becomes viscous; The concentrated solution was freeze-dried to obtain the purple sweet potato anthocyanin extract, which was stored in a refrigerator at 4°C away from light for later use; (2) Pretreatment of the film-forming matrix: Pectin was dissolved in water under magnetic stirring at 2000-3000 r / min, and fully hydrated overnight to prepare a solution with a mass concentration of 3.5-4.5%. Whey protein powder was added in a ratio of pectin: whey protein of 3.5:1-4.5:

1. The mixture was stirred magnetically at 2000-3000 r / min in a water bath at 45-55°C and 0.5-1.0 mol / L sodium hydroxide was slowly added dropwise. The pH of the mixed solution was adjusted to 7.5-8.5 and the whey protein was fully dissolved. Then, 100 mL was taken for ultrasonic treatment to obtain a film-forming matrix solution. (3) Preparation of pH indicator membrane matrix: glycerol is added to the membrane matrix solution in step (2) at a ratio of 1.0-2.0 g per 100 mL, and magnetically stirred at 2000-3000 r / min for 1-2 h at 45-55° C., and cooled to room temperature; purple sweet potato anthocyanin powder extracted in step (1) is then added at a ratio of 300-1000 mg per 100 mL of solution, and magnetically stirred at 2000-3000 r / min for 1-2 h at room temperature to fully dissolve the anthocyanins, and then the mixed solution is subjected to ultrasonic treatment; (4) Preparation of pH indicator film: Accurately weigh 10-12 g of the film-forming matrix in step (3) and pour it into a culture dish with a diameter of 90 mm. Place the culture dish in an oven at 45-55°C to dry for 5-7 h. Peel off the dried film from the culture dish to obtain a pH intelligent indicator film.

2. The pH intelligent indicator membrane based on whey protein according to claim 1, characterized in that: The sodium hydroxide droplet addition speed described in step (2) is 0.4~0.6mL / min; the ultrasonic treatment conditions described in step (2) are as follows: the ultrasonic probe diameter is 12.7mm, the immersion depth of the sample center is 2~4cm, the ultrasonic frequency is 20kHz, the amplitude is 20~40%, the ultrasonic time is 6~10min, and the pulse mode is on for 3~5s and off for 2~4s; the ultrasonic treatment conditions described in step (3) are as follows: the ultrasonic probe diameter is 12.7mm, the immersion depth of the sample center is 2~4cm, the ultrasonic frequency is 20kHz, the amplitude is 20~40%, the ultrasonic time is 2~4min, and the pulse mode is on for 3~5s and off for 2~4s.

Images