Preparation of PVA / DES-anthocyanin gel material for intelligent indication packaging

Abstract

The invention discloses preparation of a PVA / DES-anthocyanin gel material for intelligent indication packaging, and belongs to the technical field of food packaging materials. Dissolving PVA (polyvinyl alcohol) in hot water at 90 DEG C; preparing a DES system and adding an anthocyanin extracting solution to obtain an anthocyanin-containing DES system; the preparation method comprises the following steps: uniformly mixing an anthocyanin-containing DES system with a PVA solution to obtain a mixed solution, pouring the mixed solution into a mold, and performing freezing-unfreezing circulation treatment to form stable hydrogel. PVA in the mixed solution is calculated according to the mass percent concentration of 7%-9%, and the mass ratio of the PVA solution to the anthocyanin to the DES system is 100: (0.06-0.60): 10. Ammonia volatile alkaline gas is released in the meat decay process, the color of anthocyanin changes, and real-time visual monitoring of the food freshness is achieved.

Description

Technical Field

[0001] This invention belongs to the field of food packaging materials technology, and specifically relates to a pH-responsive smart indicator material, which can be used in the fields of real-time monitoring of food freshness, smart packaging, and quality management of fresh products. Background Technology

[0002] Currently, visual monitoring of food safety and quality is receiving increasing attention, and the application of smart packaging materials, especially color-responsive indicator materials, in perishable foods such as meat is highly favored. Among them, pH-responsive materials based on natural anthocyanins have become a current research hotspot due to their intuitive color changes, natural sources, and high edibility.

[0003] However, existing anthocyanin-based indicator membranes or gels still have the following problems in practical applications:

[0004] (1) Poor stability of anthocyanins: They are easily affected by external factors such as pH, temperature, light and metal ions, resulting in unstable color development, discontinuous color changes or poor recognizability.

[0005] (2) Insufficient performance of carrier matrix: Currently used natural polymers such as chitosan and gelatin may have problems such as low mechanical strength, poor flexibility and easy cracking in hydrogels;

[0006] (3) Poor dispersibility of anthocyanins: They tend to aggregate in gel systems with poor hydrophilicity or unstable structure, resulting in uneven response;

[0007] (4) Lack of support from green solvent systems: Traditional carriers have weak stability in loading anthocyanins. Although natural deep eutectic solvents (DES) have been developed, their efficient synergistic effect in hydrogel construction has not yet been fully realized.

[0008] Therefore, there is an urgent need to develop a novel smart gel material that combines a green preparation path, good pH response, strong mechanical properties, and high application safety. Summary of the Invention

[0009] The purpose of this invention is to provide a smart hydrogel based on a PVA (polyvinyl alcohol) / DES (natural deep eutectic solvent)-anthocyanin composite system, which can be used as a smart visual monitoring material for the freshness of meat products.

[0010] A method for preparing a PVA / DES-anthocyanin gel material for smart indicator packaging includes the following steps:

[0011] (1) Dissolve PVA (polyvinyl alcohol) in hot water at 90°C;

[0012] (2) Prepare the DES system and add anthocyanin extract to obtain the DES system containing anthocyanins;

[0013] (3) The anthocyanin-containing DES system is uniformly mixed with the PVA solution to obtain a mixed solution, which is then poured into a mold and subjected to a freeze-thaw cycle to form a stable hydrogel.

[0014] PVA (polyvinyl alcohol): a scaffold material for building gel networks; DES (natural deep eutectic solvent): used as a stabilizing carrier for anthocyanins and a gel performance modifier; anthocyanins: derived from natural plant extracts such as butterfly pea flowers or blueberries, used as pH-responsive color change indicators.

[0015] In step (3), the PVA in the mixed solution is calculated as a mass percentage solution of 7%-9% (preferably 8%), and the mass ratio of PVA solution, anthocyanin and DES system is 100:(0.06-0.60):10, preferably 100:(0.4-0.56):10.

[0016] Application method:

[0017] The obtained gel material is cut and attached to the inner wall of meat packaging or made into modified atmosphere packaging on top; the meat spoilage process is monitored in real time during storage, and it is suitable for various perishable foods such as pork, beef, chicken and aquatic products.

[0018] It exhibits a continuous color change pattern from "acidic purple to neutral blue to alkaline cyan / green" within the pH range of 3–10, which can be used to visually detect the process from freshness to spoilage.

[0019] The introduction of DES not only stabilizes anthocyanins but also acts as a tunable crosslinking agent to regulate gel properties. Anthocyanins are uniformly dispersed in the gel at the molecular level, preventing aggregation and sedimentation, and improving color development sensitivity and consistency. The DES stabilizer and PVA are blended to form a three-dimensional gel network, synergistically loading natural anthocyanins, thereby endowing the hydrogel with good flexibility, pH responsiveness, and color reversibility. By adjusting the ratio of DES to PVA, the swelling, flexibility, and response speed of the gel can be controlled. When stored with meat, as the total volatile basic nitrogen (TVB-N) value increases, the gel color changes from purplish-red to blue and finally to green, a clearly visible change that is easy to interpret.

[0020] The beneficial effects that can be achieved include:

[0021] (1) Sensitive and intuitive response: As volatile alkaline gases such as ammonia are released during the meat spoilage process, anthocyanins undergo color changes (which can be sensitively displayed within 1 minute or even 30 seconds), enabling real-time visual monitoring of food freshness.

[0022] (2) Significantly improves anthocyanin stability: DES provides a hydrogen bond network environment, which significantly enhances the dispersibility and antioxidant capacity of anthocyanins in hydrogels, and a small amount of anthocyanins can be sensitively detected.

[0023] (3) Excellent mechanical and water absorption properties: The PVA-based gel has good mechanical strength and flexibility, which supports its long-term adhesion in the packaging.

[0024] (4) Green and biodegradable: The raw materials are all food-grade, and the preparation process is environmentally friendly and non-toxic, which is in line with the development trend of green and intelligent packaging;

[0025] (5) It has good recyclability and can be reused. It is also suitable for large-scale preparation and application. Attached Figure Description

[0026] Figure 1 Flowchart for the preparation of an aqueous solution of butterfly pea flower anthocyanins;

[0027] Figure 2 Images of gels with different anthocyanin concentrations;

[0028] Figure 3 Figure showing the pH response of gels with different anthocyanin concentrations;

[0029] Figure 4 Colorimetric diagram of pH response of gels with different anthocyanin concentrations;

[0030] Figure 5 Figure showing the pH response cycle reversibility of gels with different anthocyanin concentrations;

[0031] Figure 6 Colorimetric diagram of the pH response cycle reversibility of gels with different anthocyanin concentrations;

[0032] Figure 7 Figure showing the gel atmosphere response effect at different anthocyanin concentrations;

[0033] Figure 8 Figure 1 shows the effect of hydrogels with different anthocyanin concentrations on shrimp meat preservation monitoring.

[0034] Figure 9 The graph shows the change in volatile basic nitrogen content in shrimp meat over time.

[0035] Figure 10 This is a graph showing the pH changes in shrimp meat over time. Detailed Implementation

[0036] The technical solution will now be clearly and completely described with reference to the accompanying drawings of this embodiment. Obviously, the described embodiments are only some embodiments, not all embodiments. Based on this embodiment, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this invention.

[0037] Example: Preparation of PVA / DES-anthocyanin hydrogel and its pH responsiveness test

[0038] This embodiment prepares a smart responsive hydrogel based on PVA / DES-anthocyanins using a freeze-thaw cycle method, which is suitable for visual monitoring of the freshness of meat products.

[0039] (1) Preparation of DES eutectic solvent: Choline chloride and glycerol were mixed at a mass ratio of 1:1.32 and stirred in a water bath at 80 °C to obtain a uniform and transparent liquid, which is the natural deep eutectic solvent (DES).

[0040] (2) Preparation of anthocyanin extract as follows Figure 1 30g of dried butterfly pea flowers were ground after removing the calyx, and then soaked in 200ml of 75% ethanol solution for 24 hours. After filtration, an extract containing anthocyanins was obtained. The ethanol solution was then removed by vacuum distillation in a 50℃ water bath to obtain the anthocyanins.

[0041] (3) Preparation of PVA solution: Prepare a PVA solution with a mass fraction of 8%, heat and stir at 90 ℃ until completely dissolved, and cool to room temperature.

[0042] (4) Preparation of gel solution: PVA solution: anthocyanin: DES co-crystal solvent = 100: (0.08, 0.24, 0.4, 0.56): 10 by mass and stirring until homogeneous (the concentration gradients in parentheses are A1, A3, A5, and A7 respectively).

[0043] (5) Molding and Hydrogel Formation: The obtained gel solution was poured into a petri dish mold and subjected to four freeze-thaw cycles to form a stable PVA / DES-anthocyanin smart hydrogel, such as... Figure 2 .

[0044] (1) pH response test:

[0045] The prepared hydrogels were cut into several identical squares (specifically, nine 1cm x 1cm squares were cut from anthocyanin hydrogels of the same concentration). These squares were then placed in solutions of different pH values ​​for 5 minutes, and their color changes were observed. Figure 3 And its Lab color value was directly measured using a colorimeter, such as Figure 4 Then, the color difference between the original state and the initial state is calculated and plotted (e.g., ...). Figure 5 To determine its responsiveness, the color difference is calculated using the following formula:

[0046]

[0047] Experimental results show that the PVA / DES-anthocyanin hydrogel prepared in this invention exhibits significant color responses under different pH conditions. For example... Figure 3 As shown, the hydrogel appears purplish-blue in acidic conditions, gradually turning blue-green as the pH increases, and then green under alkaline conditions; the color changes are clearly visible. Different formulations exhibit varying pH response sensitivity, with PVA / DES-A7 showing the most pronounced color change, while PVA / DES-A1 shows a more gradual change.

[0048] At the same time, utilizing ΔE color difference ( Figure 4 The overall color change range was comprehensively evaluated. The results showed that each formulation responded to varying degrees within the acidic to neutral range. However, under alkaline conditions (pH 10–11), the ΔE values ​​of PVA / DES-A7 and PVA / DES-A5 were close to 40, exhibiting extremely strong visual change effects. In contrast, PVA / DES-A1 showed smaller overall changes and was suitable for precise detection in acidic environments.

[0049] (2) pH response cycle reversibility test:

[0050] PVA / DES-anthocyanin hydrogel samples cut to 1cm × 1cm were immersed in buffer solutions of pH = 3 and pH = 10, respectively, to test their reversible color response performance under acid-base conditions. Each cycle consisted of immersing the sample in pH 3 buffer for 5 minutes, rinsing it in pH 7 neutral buffer for 1 minute, then immersing it in pH 10 buffer for 5 minutes, followed by rinsing again. This acid / base alternation process was repeated 5 times, and then the pH response test procedure was repeated to observe the hydrogel's responsiveness after multiple cycles.

[0051] Depend on Figure 5 , 6 It can be seen that although the responsiveness of the hydrogel decreased somewhat after multiple response cycles (the highest color difference decreased from around 40 to around 30, and the lowest color difference decreased to below 5), it still exhibited a strong responsiveness. This indicates that the hydrogel possesses good pH response reversibility and color memory stability, and has the potential to be used as a smart indicator material in practical scenarios such as food packaging and quality monitoring.

[0052] (3) Atmosphere response test:

[0053] To further verify the atmosphere responsiveness of the hydrogel, hydrogel samples with different formulations were exposed to simulated acidic and alkaline atmospheres. The acidic atmosphere was provided by 3% hydrochloric acid vapor, and the alkaline atmosphere by 3% ammonia vapor. Specifically, a 1cm × 1cm hydrogel sample was placed on the inner surface of a petri dish lid. A certain amount of 3% hydrochloric acid or 3% ammonia was added to the petri dish, which was then sealed and left at room temperature for a period of time before the color change of the hydrogel was observed.

[0054] The hydrogel showed a response within 30 seconds under an alkaline atmosphere and within 1 minute under an acidic atmosphere. The effects were as follows: Figure 7 As shown, in an alkaline atmosphere, the hydrogel exhibits a color ranging from light green to dark green, while in an acidic atmosphere, it rapidly turns pink or purplish-red, displaying vivid colors and a clear response. The PVA / DES-A5 and PVA / DES-A7 samples, in particular, with higher anthocyanin content, show even more pronounced color changes and higher recognizability, demonstrating excellent volatile acid-base indication capabilities.

[0055] (4) Shrimp meat preservation monitoring test:

[0056] Whole shrimp were placed in a 4 °C refrigerator and stored in the same compartment as 1 cm × 1 cm × 1 mm PVA / DES-anthocyanin indicator blocks. A0 was a colorless control, and A1, A3, A5, and A7 were formulations with different anthocyanin contents. TVB-N and pH were measured and indicator color changes were recorded at 0, 2, 4, 6, and 8 days of storage. Triple-parallel results showed that TVB-N (mg / 100 g) was 5.6 ± 0.21, 12.2 ± 0.76, 21.3 ± 2.08, 31.3 ± 2.08, and 43.0 ± 1.00, respectively; and pH was 6.70 ± 0.10, 7.03 ± 0.12, 7.33 ± 0.06, 7.60 ± 0.10, and 8.10 ± 0.10, respectively. Image recordings show that A1 / A3 / A5 / A7, which exhibit color changes, undergo a continuous transformation from initial to final color over time, while the control A0 shows virtually no significant change; the color change is most pronounced between days 4 and 6. Linear interpolation estimation shows that TVB-N ≈ 30 mg / 100 g corresponds to approximately 5.7 days of storage, at which point the pH is approximately 7.6, and the color gradation has reached a warning level that is easily readable by the naked eye. A5 / A7 show a clearer color distinction at this point, facilitating rapid identification in practical applications.

[0057] The PVA / DES-anthocyanin gel of this invention exhibits a rapid, reversible, and visually perceptible colorimetric response to both solution pH and gas-phase acid / alkali conditions. Within the pH range of 3–10, it displays a continuous color change pattern from "acidic purple—neutral blue—alkaline cyan / green"; the A5 / A7 formulation shows the best sensitivity and color saturation. After multiple cycles of alternating acid / alkali conditions, although the maximum color difference decreases somewhat, it remains significantly higher than the visual recognition threshold (ΔE≫5), demonstrating good cycling stability and color memory. Atmosphere exposure experiments show that color development is completed within approximately 30 seconds in an acidic atmosphere and approximately 1 minute in an alkaline atmosphere, and the color reverts upon restoration to air, proving its rapid response to gas-phase alkalinity changes caused by volatile basic nitrogen. Validation of whole shrimp refrigeration applications showed that TVB-N and pH increased monotonically over time, with the indicator block color progressing synchronously from the initial color to the final color. A clear warning color appeared when TVB-N ≈ 30 mg / 100 g (approximately day 5–6, pH ≈ 7.6), with A5 / A7 showing the best differentiation. Based on this, a three-stage interpretation rule can be established: "fresh (TVB-N < 15 mg / 100 g) — critical (15–30 mg / 100 g) — unsuitable (≥ 30 mg / 100 g)". In summary, this indicator hydrogel can achieve intuitive, rapid, and low-cost in-situ monitoring of the spoilage process of shrimp and other aquatic products under non-contact conditions. It has good visualization effects, adjustable sensitivity, and application stability, making it suitable for promotion in conjunction with refrigerated / retail packaging.

Claims

1. A method for the preparation of PVA / DES-cyanidin gel material for smart indicating packaging, characterized by, The method comprises the following steps: (1) dissolving PVA (polyvinyl alcohol) in hot water at 90°C; (2) preparing a DES system and adding an anthocyanin extract to obtain an anthocyanin-containing DES system; (3) uniformly mixing the anthocyanin-containing DES system and the PVA solution to obtain a mixed solution, pouring the mixed solution into a mold, and forming a stable hydrogel through freeze-thaw cycle treatment; PVA (polyvinyl alcohol): a skeleton material for constructing a gel network; DES (deep eutectic solvent): used as a stable carrier for anthocyanin and a gel performance regulator; anthocyanin: derived from natural plant extracts such as butterfly pea flowers or blueberries, serving as a pH-responsive color-changing indicator.

2. The method according to claim 1, characterized in that In the mixed solution of step (3), the PVA is calculated as a 7%-9% mass percentage concentration solution, and the mass ratio of the PVA solution, anthocyanin, and DES system is 100: (0.06-0.60):

10.

3. The method of claim 1, wherein, In the mixed solution of step (3), the PVA is calculated as a 7%-9% mass percentage concentration solution, and the mass ratio of the PVA solution, anthocyanin, and DES system is 100: (0.06-0.60):

10.

4. The method of claim 1, wherein, In the mixed solution of step (3), the PVA is calculated as a 7%-9% mass percentage concentration solution, and the mass ratio of the PVA solution, anthocyanin, and DES system is 100: (0.06-0.60):

10.

5. The PVA / DES-anthocyanin gel material prepared according to the method of any one of claims 1-4.

6. The application of the PVA / DES-anthocyanin gel material prepared according to the method of any one of claims 1-4, which is used for visual detection of perishable food such as pork, beef, chicken, and aquatic products, and real-time monitoring of meat spoilage during storage.

7. The application of claim 6, wherein the obtained gel material is cut and attached to the inner wall of meat packaging or made into modified atmosphere packaging on the top.

8. The application of claim 6, wherein the obtained gel material exhibits a continuous color change rule of "acid violet-neutral blue-alkaline cyan / green" within a pH range of 3-10, and can be used for visual detection of the process from freshness to deterioration.

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