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Article

Development of Gelatin/Zein Electrospun Nanofiber Films Containing Purple Sweet Potato Anthocyanin for Real-Time Freshness Monitoring of Aquatic Products

by
Chenyu Wang
1,†,
Aixia Huang
2,†,
Jiaxuan Fang
3,
Shuangdie Li
3,
Siyu Wu
1,
Di Sun
1,
Qingbao Ma
1,
Zhongjie Yu
1,
Yu Liu
1,3,* and
Wei Jiang
1,3,*
1
Key Laboratory of Key Technical Factors in Zhejiang Seafood Health Hazards, College of Food and Pharmacy, Zhejiang Ocean University, Zhoushan 316022, China
2
Key Laboratory of Healthy Freshwater Aquaculture, Ministry of Agriculture and Rural Affairs, Key Laboratory of Fish Health and Nutrition of Zhejiang Province, Zhejiang Institute of Freshwater Fisheries, Huzhou 313001, China
3
Institute of Innovation and Application, Zhejiang Ocean University, Zhoushan 316022, China
*
Authors to whom correspondence should be addressed.
†
These authors contributed equally to this work.
Coatings 2025, 15(1), 79; https://doi.org/10.3390/coatings15010079
Submission received: 17 December 2024 / Revised: 9 January 2025 / Accepted: 11 January 2025 / Published: 13 January 2025
(This article belongs to the Special Issue New Advance in Nanoparticles, Fiber, and Coatings)
Browse Figures
Versions Notes

Abstract

:
In the present study, an electrospinning freshness monitoring film prepared by gelatin/zein loading with purple sweet potato anthocyanins (PSPA) was produced to track the freshness state of Penaeus vannamei. The electrospun nanofiber films with the gelatin and zein weight ratio of 1:0, 3:1, 2:1, and 1:1 were named GA, GZA 3:1, GZA 2:1, and GZA 1:1, respectively. The impacts of zein concentration on the electrospun nanofiber film properties were investigated. SEM results showed that a smooth surface was observed for the electrospun nanofiber films. As the zein content increased, the average diameter decreased. No new characteristic peaks were shown by FTIR and XRD, indicating the good compatibility between gelatin, zein, and PSPA. The incorporation of zein decreased the swelling ratio (from completely dissolved to 100.7%) and water solubility (from 100% to 30%) and increased the water contact angle (from 0° to 113.3°). The GA, GZA 3:1, GZA 2:1, and GZA 1:1 had apparent color changes to NH3 and demonstrated good stability and reversibility. Furthermore, the freshness states (fresh, sub-fresh, and spoiled) of Penaeus vannamei storage at 4 °C could be effectively distinguished by GZA 3:1 by showing different colors (from pink to grayish purple to blue). Consequently, GZA3:1 exhibited improved hydrophobicity and pH sensitivity and has great potential in real-time monitoring of aquatic product quality.

1. Introduction

Intelligent packaging that could conduct real-time evaluations of food quality and safety has recently attracted increasing attention and investigation in the field of food packaging [1]. Aquatic products such as Penaeus vannamei are prone to decay during storage and transportation. Protein decomposition produces a large amount of basic volatile organic amines, leading to a pH increase in the packaging [2]. Based on the pH changes within the packaging, pH-sensitive freshness indicators could monitor the freshness status of aquatic products by displaying different colors [3,4,5]. Electrospinning is a flexible and simple method widely used in the production of non-woven fiber films. The films produced by electrospinning have unique characteristics, such as high porosity, surface area ratio, and excellent adjustability [6], which were beneficial for capturing volatile substances in the packaging and suitable for preparing the pH-sensitive freshness indicator.
The freshness monitoring films produced by electrospinning include two parts: pH-sensitive dyes and supporting matrixes. There are two types of dyes: natural dyes and synthetic dyes [7]. Synthetic dyes, such as bromothymol blue and polyaniline, are high-cost and potentially harmful to health [8]. As a natural, non-toxic, and eco-friendly dye, anthocyanins are safer and cheaper to use in intelligent food packaging. The purple sweet potato anthocyanins (PSPA) are highly thermostable and photostable and have a good color response to changes in pH value. Its chemical structure is abundant in cyanidins and peonidins in the form of monoacylation and diacetylation. Because of the acylation form, PSPA could be a potential substitute for synthetic colorants to produce high-quality foods with higher safety and green labels [9].
Polysaccharides and proteins, which are biodegradable and safe biomacromolecules, usually serve as a supporting matrix for electrospun nanofiber film [10,11]. Gelatin was widely used as a supporting matrix of electrospun nanofiber film, which is a natural biomacromolecule with good biodegradability and compatibility. However, gelatin will switch between the sol-gel state after absorbing water, which leads to rapid degradation of the electrospun film structure upon contact with water and limits the application of gelatin film. Compounding with hydrophobic polymers was an effective approach to enhance the water resistance of gelatin films. Zein is a type of edible, biodegradable, and biocompatible protein that can electrospun with other biomacromolecules and increase the hydrophobicity of the films. By adding zein, the water resistance of the gelatin nanofiber film might be improved. However, it is noteworthy that the zein film is not transparent or colorless. The color of zein will affect the color response of zein-based freshness monitoring film [12]. Moreover, the sensitivity of the pH-sensitive freshness indicator would decrease as the hydrophobicity of the films increased [13]. The balance between hydrophobicity and sensitivity of the gelatin/zein-based pH-sensitive freshness indicator should be investigated.
In this study, a film with a good water resistance property and a noticeable color response to Penaeus vannamei in different freshness states was produced based on gelatin, zein, and PSPA. The impacts of zein concentration on the film’s structure, physicochemical properties, color response, reversibility, and stability were further studied. The real-time effectiveness of the films was investigated by monitoring the freshness states of 4 °C stored Penaeus vannamei.

2. Materials and Methods

2.1. Materials

PSPA was obtained from Xi’an Zhilong Mechanical and Electrical Technology Development Co., Ltd. (Xi’an, China). Zein was obtained from Aladdin Biochemical Technology Co., Ltd. (Shanghai, China). Gelatin was purchased from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). Penaeus vannamei (weight: 20 ± 3 g) was obtained from a market (Zhoushan, China) in May 2024 and delivered to the laboratory with a foam box and crushed ice within 1.0 h. All analytical reagent grades were analytically pure.

2.2. Films Preparation

The 35% (w/v) gelatin and 35% (w/v) zein solutions were obtained by adding 7 g of gelatin and 7 g of zein in 80% (v/v) acetic acid with stirring, respectively. The electrospinning solution was obtained by mixing gelatin and zein solution in a designed volume ratio and adding PSPA (9%, w/w). Then, the electrospinning solution was stirred for 30 min, and 10 mL was loaded into the spring. The receiving distance was 20 cm, the flow rate was 1.0 mL/h, and the voltage was 17 kV, respectively. The electrospun nanofiber films with the gelatin and zein weight ratio of 1:0, 3:1, 2:1, and 1:1 were named GA, GZA 3:1, GZA 2:1, and GZA 1:1, respectively.

2.3. Characterization

A Sigma 300 SEM (Carl Zeiss AG, Jena, Germany) was used to observe the microstructures of samples according to a previous method [14]. The SEM images were set to 30.00 KX magnification. The Fourier transforms infrared spectroscopy (FTIR) spectra of samples were obtained using an FTIR spectrometer (IRAFFINITY-1S, Shi-madzu Corporation, Nakagyo-ku, Kyoto, Japan). Scan the spectrum at a resolution of 4 cm−1 within the range of 4000 to 650 cm−1 [15]. The crystal phase of samples was investigated by X-ray diffractometer (MiniFilex 6, Rigaku Corporation, Japan) according to a previous method [15]. The thermal stability of samples was measured by a thermogravimetric analyzer (DTG-60, Shimadzu Corporation, Japan) according to a previous method [16].

2.4. Physical Properties

2.4.1. Thickness

The thickness of the samples was determined with a digital micrometer (Yiyu Machinery Technology (Shanghai) Co., Ltd., Shanghai, China) according to a previous method [16].

2.4.2. Swelling Ratio (SR) and Water Solubility (WS)

The SR and WS were measured according to a previous method [17] and calculated as follows:
S R % = M 2   −   M 1 / M 1 × 100 %
W S % = M 1   −   M 3 / M 1 × 100 %

2.4.3. Water Contact Angle (WCA)

The hydrophobicity of samples was measured using a previous method [18]. 10 µL of water was carefully dropped onto the surface of the films. Using SDC-100 software (V3.1.2.0.190824), the images were collected and analyzed (Data Physics, Filderstadt, Germany).

2.5. Method for Color Recording

An intelligent mobile phone (Vivo IQOO 12) with a camera (50 million bug-eye lens) and a tripod LED lamp from Chongqing Assistance Materials Co., Ltd. (Chongqing, China) with white light and a luminance of 66% was used for taking photos of films under a fixed distance of 20 cm between camera and sample [15].

2.6. Color Response to Ammonia and Reusability

The ammonia (0.8 mol/L) response and reusability were measured according to a previous method [15]. The ΔE value was calculated as follows:
∆ E = L * − L * 2 + ( a * − a * ) 2 + ( b * − b * ) 2
where L*, a*, and b* are the initial values and L, a, and b are the values after treatment of the sample [15].

2.7. Color Stability

Samples were stored at 4 °C and 25 °C separately. Collecting the color of the film every 1 day within 20 days. After 20 days of storage, the color response of films to ammonia was measured as described in Section 2.6. The ΔE values were calculated according to Equation (3) [15].

2.8. Freshness Monitoring Application

Live Penaeus vannamei was washed with sterile water and stored in sterile food-grade packaging boxes. The electrospun nanofiber films were cut into a circle with a radius of 1.0 cm and attached to the headspace of the packaging box. Then, the packaging boxes were stored at 4 °C for 6 days. Photos of the electrospun nanofiber films were collected every 1 day, as described in Section 2.5. The color and ΔE value were analyzed as described in Section 2.6. The pH values, total volatile basic nitrogen (TVB-N), and total viable count (TVC) of shrimps were measured using the pH meter, Kjeldahl method, and plate counting method, respectively [19].

2.9. Statistical Analysis

Statistical significance was determined with one-factor analysis of variance (ANOVA) using SPSS (Statistics 20, SPSS Inc., Chicago, IL, USA) and set as p < 0.05.

3. Results and Discussion

3.1. Characterization of Electrospun Nanofiber Films

3.1.1. Morphology Characterization

The surface morphologies of GA, GZA 3:1, GZA 2:1, and GZA 1:1 were observed by SEM. As shown in Figure 1, the morphology of the nanofibers in GA was uniform cylinder. The nanofibers remained uniform and had a bead-free structure after zein addition, indicating that gelatin and zein had good compatibility and were evenly dispersed in the nanofibers. Noticeably, the average nanofiber diameter of GA was 645.0 ± 5.9 nm and reduced to 575.0 ± 12.1, 536.4 ± 6.6 and 419.1 ± 13.7 nm for GZA 3:1, GZA 2:1, and GZA 1:1, respectively. The relative molecular weight of the electrospinning solution decreased with the increase of the zein ratio, which reduced the solution viscosity and promoted the extension refinement of nanofibers, then leading to a decrease in the diameter of the nanofiber [20]. The reduction in diameter was also reported by Hajjari et al. [20] by adding zein into the electrospun zein/C-phycocyanin composite. The decrease in the diameter of nanofibers will lead to an increase in the specific surface area of electrospun nanofiber films, which improves the contact frequency between PSPA and other substances and improves the sensitivity of the nanofibers to ambient pH [19]. However, the hydrogen bond interaction between zein and gelatin will reduce the porosity of the nanofiber films [21], indicating that a high proportion of zein might compromise the contact of PSPA with volatile alkaline gases and affect the color response of films [22].

3.1.2. FTIR Analysis

The possible interactions between molecules in the films were investigated by FTIR and shown in Figure 2a. The characteristic peaks of GA were interpreted as follows: the broad absorption band observed around 3281 cm−1 was N-H stretching vibration of amide A, the C-H stretching vibrations at approximately 2937 cm−1, at approximately 1632 and 1528 cm−1 were the stretching vibration of amide I (C=O stretching) and amide II (C-N stretching), respectively [23], the N-H bending and C-N stretching combination at 1448 cm−1, C-H deformation of the methyl group at 1333 cm−1, and the C-N stretching and N-H bending combination 1239 cm−1 (amide III) [23]. The characteristic peaks of GZA 3:1, GZA 2:1, and GZA 1:1 were similar to GA, with a little change in the amplitude, and some peaks shifted. Compared with pure proteins such as gelatin or zein, there was no peak split in all the electrospun nanofiber films, indicating a uniform structure of films or a phase separation. However, there was no clear evidence of phase separation in the present study. The spinning solution formed smooth and uniform nanofibers under the electric field (Figure 1), indicating that all substances are uniformly distributed in nanofibers and have good compatibility.

3.1.3. X-Ray Diffraction (XRD) Analysis

As shown in Figure 2b, the crystalline structures of GZA3:1, GZA2:1, and GZA1:1 showed similar diffraction patterns, with a broad peak (amorphous fractions) at 2θ = 20–25°, indicating good compatibility between gelatin, zein, and PSPA. The low crystallinity of electrospun nanofiber films can be attributed to two aspects: acid degradation of ordered structures, and the electrospinning process hinders the crystallization of polymers and promotes the formation of amorphous polymer structures [24]. Furthermore, the crystallinity index of GA was 33.2%, and those of GZA 3:1, GZA 2:1, and GZA 1:1 increased to 37.0%, 41.2%, and 48.7%, respectively, which is due to the mixture of gelatin and zein facilitates the transition from disorder to order, resulting in higher crystallinity [24,25] or molecular side effects.

3.1.4. Thermogravimetric Analysis (TGA)

As shown in Figure 2c, three zones of weight loss of the electrospun nanofiber films were observed in the TGA curves. In approximately the range of 30–150 °C, the first weight loss is possible because of the evaporation of water and acetic acid. The second weight loss was approximately in the range of 150–200 °C, which could possibly be attributed to the decomposition of PSPA [16]. The third weight loss was in approximately the range of 200 °C to 600 °C, which was attributed to the decomposition of polymers [26]. The residual substances above 600 °C are attributed to inorganic compounds produced by thermal degradation [26]. When the temperature was greater than 300 °C, the degradation rate of pure zein was faster than that of pure gelatin, and the residue decreased with the increasing weight percentage of zein in the blend film, suggesting that gelatin had higher thermal stability above 300 °C. In addition, when GA was compared to pure gelatin, adding PSPA enhanced the thermal stability of gelatin-based films. Similar improvements were found previously [27].
The DTG curves can be used to study the changes in sample weight loss rate during heating up. As shown in Figure 2d, the degradation behaviors of GZA 3:1, GZA 2:1, and GZA 1:1 were similar to those of GA. The weight loss rate of electrospun nanofiber films was slightly decreased in the first stage, increased in the second stage, and decreased in the third stage with the addition of zein. The Tmax of gelatin was 323.6 °C. When adding PSPA, the Tmax of GA was slightly decreased to 322.0 °C, which was consistent with the related studies [28]. The Tmax of GZA 3:1, GZA 2:1 and GZA 1:1 was 317.2 °C, 322.9 °C, and 310.2 °C, respectively. Compared to GA, the Tmax for each kind of gelatin/zein/PSPA electrospun nanofiber films was lower, which indicating the addition of zein slightly reduced the films’ thermal stability.

3.2. Physical Properties of the Electrospun Nanofiber Films

Thickness, WCA, SR, and WS were important indices for the application performance of electrospun nanofiber films [29] and were investigated in Table 1. As the ratio of zein increased, the thickness of the electrospun nanofiber films increased significantly (p < 0.05), which is probably due to the hydrophobic nature of zein reducing the free volume between the gelatin chains to form a more compact network [30].
WCA can characterize the hydrophilicity and hydrophobicity of the film surface. When WCA is greater than 90°, the film surface is hydrophobic, and when WCA is less than 90°, the film surface is hydrophilic [15]. The WCA of GA was 0°, which might attributed to the high specific surface area of GA promoting water penetration and dissolution [31]. As the zein content increased in the electrospun nanofiber films, the WCA of GZA 3:1, GZA 2:1, and GZA 1:1 increased to 68.7°, 82.9°, and 113.3°, respectively, which might be due to the decrease of O-H groups. Hydrogen bond interaction between gelatin and zein occurs through free hydroxyl and free amino groups; this interaction reaches the maximum when the ratio of the two proteins is 1:1, causing an increase in the hydrophobic groups and showing a hydrophobic surface of GZA 1:1 [23].
The SR and WS reflect the water resistance property of the electrospun nanofiber films. The WS of GA was 100%, and the SR of GA could not be detected due to the strong hydrophilicity of gelatin [16]. As the ratio of zein increased, the SR of GZA 3:1, GZA 2:1, and GZA 1:1 decreased to 204.9%, 181.5%, and 100.7%, and the WS of GZA 3:1, GZA 2:1, and GZA 1:1 decreased to 58%, 52%, and 30%, respectively. A higher SR and WS of electrospun nanofiber film might cause early release of color indicators and poor morphological stability during application [32]. Adding zein to the electrospun nanofiber film could improve the hydrophobicity, which was consistent with the results of WCA and might enhance the morphological stability.

3.3. The Sensitivity, Reusability, and Stability of the Electrospun Nanofiber Films

3.3.1. The Sensitivity to Ammonia Gas

During the spoilage process of aquatic products, volatile nitrogen compounds are generated. In order to simulate the color response of electrospun nanofiber films to these compounds, ammonia gas response experiments were conducted [15]. After exposure to ammonia for 4 h (Figure 3a), the color of GA, GZA 3:1, GZA 2:1, and GZA 1:1 changed from pink to yellow-green. The acetic acid in the electrospun nanofiber film led to an acidic environment and caused an initial pink color of the electrospun nanofiber films. The color changes of the electrospun nanofiber films in an ammonia environment were due to the reversible structural transformation of anthocyanins when the environmental pH changes from acidic to alkaline, which causes protonation and deprotonation of the phenolic hydroxyl group [33]. Furthermore, as the content of zein increased, the color of the film became lighter, which might be caused by the zein in electrospun nanofiber films affecting the halogenated color-changing ability of PSPA through the formation of intermolecular interactions [12].

3.3.2. Reusability

The reusability of electrospun nanofiber films was analyzed by reversibility detection. The ΔE values of GA, GZA 3:1, GZA 2:1, and GZA 1:1 increased to 12.3 ± 0.9, 15.0 ± 0.4, 12.9 ± 0.8, and 12.0 ± 0.3, respectively, when exposed to ammonia gas for 1 min, and returned to baseline as apart from the ammonia for 20 min (Figure 3b). In the second cycle, the ΔE value of GA, GZA 3:1, GZA 2:1, and GZA 1:1 attained 2.4 ± 0.3, 2.4 ± 0.2, 2.1 ± 0.2, and 2.1 ± 0.3, respectively (a cycle was defined as exposing the electrospun nanofiber films to ammonia for 1 min and apart from it for 20 min) [15]. In ten ammonia-air circulation experiments, the color of the electrospun nanofiber films shifted between pink and green. When apart from ammonia, the ΔE values of the electrospun nanofiber films were lower than 5, indicating that it was difficult for the naked eye to distinguish color changes [34]. The results indicated that GA, GZA 3:1, GZA 2:1, and GZA 1:1 had excellent reversibility.

3.3.3. Stability

Color stability is critical for the application of freshness monitoring films [14]. The ΔE value of the electrospun nanofiber films increased on the first day (Figure 4). As the electrospun nanofiber films were sealed storage (0% RH), the increased ΔE value on the first day might mainly be attributed to the hygroscopic property of the films led by the RH difference between the storage environment and the experiment environment (50% RH) [15]. After 20 days of storage, the color of the electrospun nanofiber films stored at 4 °C and 25 °C was similar to freshly prepared films, and the ΔE values were lower than 5, indicating good color stability [35]. The good compatibility of substances in the films and the hydrophobicity of the film might prevent the hydration and oxidation of anthocyanins, which probably leads to good color stability of the electrospun nanofiber films [36].
Furthermore, the sensitivity to ammonia gas of electrospun nanofiber films after 20 days of storage was also evaluated. GA, GZA 3:1, GZA 2:1, and GZA 1:1 still had good sensitivity to the ammonia with the ΔE values similar to the freshly prepared films after being stored for 20 days (Figure 4d). In a word, all the electrospun nanofiber films had good color stability, and the ratio of zein in the electrospun nanofiber films had no obvious effect on color stability.

3.4. Application for Penaeus Vannamei Freshness Monitoring

The volatile nitrogen compounds produced by protein decomposition during storage of Penaeus vannamei can cause an increase in pH value inside the packaging [14]. To characterize the potential of the electrospun nanofiber films for freshness monitoring of aquatic products, the relationship between the freshness state of Penaeus vannamei and the color change of the electrospun nanofiber films was investigated.
As shown in Figure 5c, the initial pH value of Penaeus vannamei was 6.9 and increased as the storage time prolonged. It is widely considered that shrimps with a pH value higher than 7.7 are completely inedible [37]. On the fourth day, the pH value of Penaeus vannamei was 7.9, indicating that Penaeus vannamei had deteriorated.
The TVB-N value is typical for assessing the freshness degree of shrimp [38]. As limited by the Chinese Standard (DB33451–2003) [15], the TVB-N content of shrimp before consumption should be below 30 mg/100 g [39]. According to the TVB-N content, the freshness state of shrimp can be divided into “fresh state” (TVB-N values < 10 mg/100 g, representing good quality), “sub-fresh state” (10 mg/100 g ≤ the TVB-N values < 30 mg/100 g, represent as near spoilage and should be eaten as soon as possible) and “spoiled state”. (TVB-N values ≥ 30 mg/100 g, represent as inedible) [39]. The results of TVB-N value changes are shown in Figure 5d. The initial TVB-N values of Penaeus vannamei were 1.5 ± 0.3 mg/100 g, and then increased to 8.0 ± 0.9 mg/100 g (fresh state), 11.6 ± 0.7 mg/100 g (sub-fresh state) and 16.6 ± 2.0 mg/100 g (sub-fresh state) at 1 d, 2 d and 3 d storage, respectively. The shrimps were completely spoiled on the fourth day (30.8 ± 1.5 mg/100 g), which was consistent with the results of pH values. TVB-N value increased substantially after 3 days of storage, which was likely due to the simultaneous effects of microbial activity and protein degradation [40].
TVC is an important indicator to evaluate shrimp spoilage. Generally, the shrimps with TVC values lower than 5 log CFU/g are considered a fresh state, between 5 and 6 log CFU/g are considered a sub-fresh state, and higher than 6 log CFU/g are considered a spoiled state [19,41]. The initial TVC of fresh Penaeus vannamei was 2.4 lg CFU/g, followed by increasing to 2.7, 5.2, 5.3, and 6.1 lg CFU/g at 1 d, 2 d, 3 d, and 4 d storage, respectively (Figure 5e), meaning that the shrimps were completely spoiled at the fourth day. The results of TVC were consistent with pH and TVB-N changes, indicating a fresh state in 0–1 d, a sub-fresh state in 2–3 d, and spoiled on the fourth day of Penaeus vannamei under the 4 °C storage.
As shown in Figure 5b,f, the color of the electrospun nanofiber film was initially pink and turned purple on the first day, which was mainly due to the electrospun nanofiber films absorbing moisture from the environment [15]. On the 2nd and 3rd days, the color of GA turned purplish black, GZA 3:1, GZA 2:1, and GZA turned grayish purple. On the 4th day, the colors of GA, GZA 3:1, GZA 2:1, and GZA 1:1 turned blue. The results indicated that the sub-fresh and spoiled state of Penaeus vannamei could be discriminated via the color of GA, GZA 3:1, GZA 2:1, and GZA 1:1 by the naked eye. However, the Penaeus vannamei with the fresh and sub-fresh state only could be distinguished via the color of GZA 3:1, and not easy to distinguish the color of GA, GZA 2:1, and GZA 1:1. It can be speculated that the ratio of zein has an influence on the color response of films, possibly attributed to the increased hydrophobicity of films, leading to a prevent of converting the ammonia vapors into NH4+ and OH− ions by water (adhere to the nanofiber films) and further influence the color reaction of PSPA [42]. These results demonstrate that GZA 3:1 can visually discriminate the fresh, sub-fresh, and spoiled states of Penaeus vannamei and has the potential for monitoring shrimp freshness.
The cluster analysis (K-means algorithm) was conducted to further verify the freshness monitoring function of GA, GZA 3:1, GZA 2:1, and GZA 1:1. As shown in Figure 5g–j, according to the TVB-N values (horizontal coordinate), the stored Penaeus vannamei was separate into three group clusters representing three freshness states. The ΔE value (vertical coordinate) was the color changes of electrospun nanofiber film corresponding to different Penaeus vannamei freshness states [15]. From a vertical coordinate perspective, the ΔE values of Penaeus vannamei with the same freshness state were hoped to be clustered together and did not overlap with different fresh states. The results showed that GA, GZA 2:1, and GZA 1:1 were not satisfied with the freshness indicator, which ΔE value of the fresh and sub-fresh state of Penaeus vannamei overlapped (Figure 5i,j). Compared to GA, GZA 2:1, and GZA 1:1, the fresh state of Penaeus vannamei was easier to distinguish for GZA 3:1. Thus, GZA 3:1 was a satisfied freshness monitoring film for Penaeus vannamei, which ΔE values clustered together with the same freshness state and did not overlap with different freshness states (Figure 5h).
In summary, GZA 3:1 was a promising freshness monitoring film with a proven hydrophobicity and effectively distinguishing the states of fresh, sub-fresh, and spoiled 4 °C stored Penaeus vannamei.

4. Conclusions

In this study, the electrospun nanofiber films with different gelatin and zein ratios (GA, GZA 3:1, GZA 2:1, and GZA 1:1) loaded with PSPA were prepared for freshness, indicating aquatic products. The gelatin, zein, and PSPA had good compatibility and were interacted by hydrogen bonds. Adding zein reduced WS and SR and increased the thickness and WCA of the films, indicating that zein could enhance the water resistance of the electrospun nanofiber films. All the electrospun nanofiber films showed good color response to pH and ammonia, reversibility, and stability. Furthermore, the color changes of GZA 3:1 were consistent with the freshness states of packaged Penaeus vannamei and could be distinguished with the naked eye. Therefore, GZA 3:1 can be used as a potential freshness monitoring film for real-time nondestructive monitoring of Penaeus vannamei freshness states.

Author Contributions

C.W. and A.H. Conceptualization, Software, Methodology, Validation, Data curation, Writing—original draft; J.F., S.L. and S.W. Methodology, Validation, Formal analysis, Investigation. D.S., Q.M. and Z.Y. Resources, Formal analysis, Visualization. Y.L. Conceptualization, Methodology, Writing—review & editing, Funding acquisition, Project administration. W.J. Conceptualization, Writing—review & editing, Supervision, Project administration, Funding acquisition. All authors have read and agreed to the published version of the manuscript.

Funding

This research was supported by The Open Project from Zhejiang Institute of Freshwater Fisheries (ZJK202213), the Zhejiang Provincial Natural Science Foundation of China (LTGN23C200013).

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

Data is contained within the article.

Conflicts of Interest

The authors declare no conflicts of interest.

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Coatings 15 00079 g001
Figure 1. SEM images and diameter distribution of (a) GA, (b) GZA 3:1, (c) GZA 2:1, and (d) GZA 1:1.
Figure 1. SEM images and diameter distribution of (a) GA, (b) GZA 3:1, (c) GZA 2:1, and (d) GZA 1:1.
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Figure 2. (a) FTIR spectra, (b) XRD patterns, (c) thermogravimetric, and (d) differential thermal of electrospun nanofiber films.
Figure 2. (a) FTIR spectra, (b) XRD patterns, (c) thermogravimetric, and (d) differential thermal of electrospun nanofiber films.
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Coatings 15 00079 g003
Figure 3. (a) Color change of films to volatile ammonia (0.8 mol/L) for 1 min; (b) ΔE of films in the ammonia-air circulation.
Figure 3. (a) Color change of films to volatile ammonia (0.8 mol/L) for 1 min; (b) ΔE of films in the ammonia-air circulation.
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Coatings 15 00079 g004
Figure 4. ΔE values of GA, GZA 3:1, GZA 2:1, and GZA 1:1 during storage at (a) 4 °C and (b) 25 °C for 20 days; (c) Color response and (d) ΔE values of electrospun nanofiber films to ammonia. GA, GZA 3:1, GZA 2:1, and GZA 1:1 indicate freshly prepared films. 4 °C GA, 4 °C GZA 3:1, 4 °C GZA 2:1 and 4 °C GZA 1:1 indicate films stored at 4 °C for 20 days. 25 °C GA, 25 °C GZA 3:1, 25 °C GZA 2:1 and 25 °C GZA 1:1 indicate films stored at 25 °C for 20 days.
Figure 4. ΔE values of GA, GZA 3:1, GZA 2:1, and GZA 1:1 during storage at (a) 4 °C and (b) 25 °C for 20 days; (c) Color response and (d) ΔE values of electrospun nanofiber films to ammonia. GA, GZA 3:1, GZA 2:1, and GZA 1:1 indicate freshly prepared films. 4 °C GA, 4 °C GZA 3:1, 4 °C GZA 2:1 and 4 °C GZA 1:1 indicate films stored at 4 °C for 20 days. 25 °C GA, 25 °C GZA 3:1, 25 °C GZA 2:1 and 25 °C GZA 1:1 indicate films stored at 25 °C for 20 days.
Coatings 15 00079 g004
Coatings 15 00079 g005aCoatings 15 00079 g005b
Figure 5. (a) The application photos of the experiments; (b) Colors of films; (c) pH values, (d) TVB-N contents and (e) TVC values of Penaeus vannamei during storage; (f) ∆E values of GA, GZA 3:1, GZA 2:1 and GZA 1:1 for freshness monitoring of Penaeus vannamei; The cluster analysis diagrams of freshness discrimination by ∆E values of (g) GA, (h) GZA 3:1, (i) GZA 2:1 and (j) GZA 1:1.
Figure 5. (a) The application photos of the experiments; (b) Colors of films; (c) pH values, (d) TVB-N contents and (e) TVC values of Penaeus vannamei during storage; (f) ∆E values of GA, GZA 3:1, GZA 2:1 and GZA 1:1 for freshness monitoring of Penaeus vannamei; The cluster analysis diagrams of freshness discrimination by ∆E values of (g) GA, (h) GZA 3:1, (i) GZA 2:1 and (j) GZA 1:1.
Coatings 15 00079 g005aCoatings 15 00079 g005b
Table 1. Physical properties of GA, GZA 3:1, GZA 2:1, and GZA 1:1.
Table 1. Physical properties of GA, GZA 3:1, GZA 2:1, and GZA 1:1.
Physical PropertiesElectrospun Nanofiber Films
GAGZA 3:1GZA 2:1GZA 1:1
Thickness (µm)89.6 ± 3.6 b179.5 ± 51.3 b149.1 ± 43.1 b470.2 ± 59.1 a
WCA (◦)0.0 ± 0.0 d68.7 ± 0.4 c Coatings 15 00079 i00182.9 ± 1.6 b Coatings 15 00079 i002113.3 ± 2.4 a Coatings 15 00079 i003
SR (%)ND204.9 ± 4.9 a181.5 ± 9.1 b100.7 ± 12.9 c
WS (%)100.0 ± 0.0 a58.3 ± 0.8 b51.9 ± 0.0 c30.0 ± 5.1 d
Different letters in the same line represent significant differences (p < 0.05).
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MDPI and ACS Style

Wang, C.; Huang, A.; Fang, J.; Li, S.; Wu, S.; Sun, D.; Ma, Q.; Yu, Z.; Liu, Y.; Jiang, W. Development of Gelatin/Zein Electrospun Nanofiber Films Containing Purple Sweet Potato Anthocyanin for Real-Time Freshness Monitoring of Aquatic Products. Coatings 2025, 15, 79. https://doi.org/10.3390/coatings15010079

AMA Style

Wang C, Huang A, Fang J, Li S, Wu S, Sun D, Ma Q, Yu Z, Liu Y, Jiang W. Development of Gelatin/Zein Electrospun Nanofiber Films Containing Purple Sweet Potato Anthocyanin for Real-Time Freshness Monitoring of Aquatic Products. Coatings. 2025; 15(1):79. https://doi.org/10.3390/coatings15010079

Chicago/Turabian Style

Wang, Chenyu, Aixia Huang, Jiaxuan Fang, Shuangdie Li, Siyu Wu, Di Sun, Qingbao Ma, Zhongjie Yu, Yu Liu, and Wei Jiang. 2025. "Development of Gelatin/Zein Electrospun Nanofiber Films Containing Purple Sweet Potato Anthocyanin for Real-Time Freshness Monitoring of Aquatic Products" Coatings 15, no. 1: 79. https://doi.org/10.3390/coatings15010079

APA Style

Wang, C., Huang, A., Fang, J., Li, S., Wu, S., Sun, D., Ma, Q., Yu, Z., Liu, Y., & Jiang, W. (2025). Development of Gelatin/Zein Electrospun Nanofiber Films Containing Purple Sweet Potato Anthocyanin for Real-Time Freshness Monitoring of Aquatic Products. Coatings, 15(1), 79. https://doi.org/10.3390/coatings15010079

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