A visual bi-layer indicator based on roselle anthocyanins with high hydrophobic property for monitoring griskin freshness

Deep-learning-assisted chemo-responsive alizarin red S-based hydrogel sensor for the rapid freshness sensing of aquatic product

Rapid detection of freshness particularly in aquatic products demands efficient sensing strategies. Here, a novel deep-learning-assisted chemo-responsive alizarin red S-based hydrogel sensing platform was established for rapid freshness assay of aquatic products. Four natural pigments that undergo observable color change to pH were investigated with the presence or absence of water-soluble aquatic proteins, and alizarin red S exhibited ideal responsiveness. The chromogenic hydrogels were fabricated by crosslinking of aldehyde-based sodium alginate and polyethyleneimine-hyaluronic acid, and the pore number of the chromogenic hydrogels were altered by changing contents of aldehyde-based sodium alginate, also resulting enhanced sensitivity in freshness detection. The photosensitive adhesive hydrogel prepared with lipoic acid modified fish skin gelatin made the chromogenic hydrogel more firmly fixed on the surface of aquatic products. The physical and chemical characteristics of the composite hydrogels were analyzed systematically. The composite hydrogels were utilized for detection of fish and shrimp under different storage conditions and excellent sensitivity was displayed. Besides, the deep convolutional neural network model was also conducted to evaluate the detection results. After optimization, established method exhibited high accuracy to 84 % for determining abovementioned freshness levels. This approach provides a promising platform for the advanced non-destructive sensitive detection of freshness detection products.

Two different fabrication method of high-hydrophobic Bi-layer indicators based on sodium alginate-polyvinyl alcohol/chitosan-zein incorporated with alizarin@ZIF-8 for beef freshness visualization

Intelligent films are essential for real-time monitoring of food spoilage. However, the application is often limited by challenges such as hydrophilicity and color instability. In this study, two high hydrophobic bilayer indicators based on sodium alginate-polyvinyl alcohol/chitosan-zein with alizarin grafted onto ZIF-8 carrier (AL@ZIF-8) were fabricated using the casting method and electrospinning, respectively. Molecular docking simulations confirmed the successful grafting of AL@ZIF-8 onto the film substrates. The performance of films was compared and the effectiveness in monitoring freshness indicators was evaluated. The cast film had a precisely controlled thickness due to self-assembly performance, resulting in superior mechanical properties (TS = 10.64 MPa, EB = 43.764 %). In comparison, the nanofiber films demonstrated higher UV-blocking ability (ΔE < 4), faster spoilage response, and greater safety (Cell viability >80 %). Importantly, the nanofiber films exhibited higher sensitivity in monitoring beef freshness, with a color change pattern strongly correlating with spoilage (R2 = 0.8976).

Preparation of photodynamic-controlled release packaging for pork preservation and its visualization

The current study aimed to synthesize a ZIF-8 metal-organic framework loaded with the Rose Bengal (ZIF-8@RB) as the photodynamic sterilization ingredient to address the uncontrolled release of active ingredients in packaging films. The photodynamic controlled release packaging film was then prepared using a PVDF polymer matrix via uniaxial electrospinning. The microstructure, particle size, excitation wavelength, and singlet oxygen yield of ZIF-8@RB were examined. The results showed that the particle size of ZIF-8@RB was approximately 232.80 nm and a polydispersity index (PDI) of 0.403. The significant reduction in survival rates of E. coli (0.83 %) and S. aureus (0.45 %) following light exposure indicated the antibacterial efficacy. Besides, the photodynamic sterilization condition can be triggered through the color of the homemade pork freshness sensor. Conclusively, the packaging film considerably extended the storage time of pork to 3 days at 4 °C, and the freshness sensor dynamically displayed the preservation process.

Oriented structure design of pectin/Ag nanosheets film with improved barrier and long-term antimicrobial properties for edible fungi preservation

Improving the antimicrobial control, barrier properties, and mechanical performance of bio-based food packaging materials is crucial for advancing their practical applications. In this study, oriented pectin/Ag nanosheet composite films were fabricated using a uniaxial stretching method. By adjusting the stretching ratio, the horizontal alignment of Ag nanosheets and pectin chains was promoted, resulting in increased crystallinity and orientation of the composite films. The stretching orientation improved the tensile strength and anti-UV capability of the composite films. In particular, the gas permeability was further reduced. The Pec/PAg-S30 % composite films, with a 30 % stretching ratio, exhibited more than a 70 % improvement in water vapour and oxygen barrier properties compared to pure pectin films. Additionally, the stretching orientation effect slowed and reduced Ag+ release, contributing to the long-term antimicrobial effect of the composite films. The films demonstrated excellent biosafety and effectively delayed the browning and spoilage of Agaricus bisporus.

Updated insights into steady-modified anthocyanin food packaging: Novel strategies, characterization, application and future challenges

Rising attentions on food safety and quality as well as disadvantages of conventional plastic food packaging motivates extensive study in anthocyanin-based food packaging. However, anthocyanins are susceptible to environmental conditions, resulting in easily-degradable properties of anthocyanin-based food packaging. Therefore, steady-modified anthocyanin-based food packaging are highly demanded for further deeper application. Based on this, thorough insights into steady-modified anthocyanin-based food packaging are provided in the current review. The degradation phenomenon and factors affecting stability of anthocyanin-based film during long-term storage were investigated. Novel steady-modification strategies to improve film stability were systemically summarized. Also, their effects on film physical (structure/mechanical/hydrophobic) properties and functional (pH-responsive, antioxidant and antibacterial) properties were explored. Meanwhile, application cases of steady-modified anthocyanin-based film regarding freshness monitoring and quality maintenance were comprehensively discussed. Finally, major challenges and future prospects were also proposed for further development.

The fabrication and intelligent evaluation for meat freshness of colorimetric hydrogels using zein and sodium alginate loading anthocyanin and curcumin: Stability and sensitivity to pH and volatile amines

Intelligent packaging designed to detect food spoilage is receiving increasing attention, and pH-sensitive colorimetric hydrogels show great potential for monitoring food spoilage. The pH-sensitive colorimetric hydrogels incorporating dual indicators of anthocyanin (BA) and curcumin (CUR) were fabricated via the interactions of zein and sodium alginate (SA) to assess meat freshness. The effects of the addition ratios of BA and CUR on zein/sodium alginate hydrogels were characterized by morphological observation, structural analysis and cumulative release profiles, and the sensitivity of the colorimetric hydrogels was also evaluated. The zein/sodium alginate hydrogel (zein/SA/Mix2), which incorporated the mixture of BA and CUR at a ratio of 70:1, exhibited the smallest particle size (1152.67 nm) and displayed a more homogeneous and dense gel structure compared with other treatments. FTIR and XRD results indicated that the interactions between sodium alginate and zein were primarily governed by hydrogen bonds and electrostatic forces, and the zein/SA/Mix2 hydrogel exhibited the weakest peak intensity at 3422 cm-1 and at 2θ = 28.25°, indicating the highest degree of crosslinking among these treatments. The zein/SA/Mix2 hydrogel rapidly responded to volatile amines within 2 min, and the release rates of BA and CUR remained below 26 % and 5 % in 95 % ethanol solution within 96 h, respectively, indicating its high stability and sensitivity. During the storage of air-dried goose meat and chilled chicken meat, the zein/SA/Mix2 hydrogel transitioned from yellow to green, and finally to dark brown, effectively distinguishing meat freshness, which was further confirmed by partial least squares regression analysis.

Preparation of Hydrophobic Purple Sweet Potato-Based Intelligent Packaging Films by Stearic Acid Coating and Heat Pressing Treatments

The high hydrophilicity of biopolymer-anthocyanin intelligent packaging films seriously limits their applications in high-humidity environments. Here, a surface hydrophobization technique was adopted to overcome the hydrophilicity of purple sweet potato (PSP)-based intelligent packaging films through stearic acid (SA) coating combined with heat pressing treatments. The structural characteristics, physical properties, and color changeability of the films were investigated. After SA coating treatment, the surface of the films was loosely covered by thick SA layers. As compared with the untreated PSP films, the SA-coated films displayed lower transparency, mechanical property, moisture content, surface wettability, anthocyanin leaching potential, and color changeability. When the SA-coated films were further heat-pressed, the SA-coated layers were closely bound to the films. The heat-pressed films had a higher transparency, mechanical property, and water vapor blocking ability than the SA-coated films. Notably, the color and color changeability of the heat-pressed films were affected by the heat pressing temperature. The films heat-pressed at 100 °C showed a vivid purple color and elevated color changeability, whereas the films heat-pressed at 150 °C showed a brown color and lost color changeability. This study demonstrates that SA coating combined with heat pressing is effective in constructing surface-hydrophobized intelligent packaging films.

Review of Recent Advances in Intelligent and Antibacterial Packaging for Meat Quality and Safety

Intelligent and antimicrobial packaging technologies are transforming meat preservation by enhancing food safety, enabling real-time quality monitoring, and extending shelf life. This review critically examines advancements in intelligent systems, including radio frequency identification (RFID), gas sensors, time-temperature indicators (TTIs), and colorimetric indicators for continuous freshness assessment. A key focus is natural compound-based chromogenic indicators, which establish visual spoilage detection via distinct color transitions. Concurrently, antimicrobial systems integrating inorganic compounds, organic bioactive agents, and natural antimicrobials effectively inhibit microbial growth. Strategic incorporation of these agents into polymeric matrices enhances meat safety, supported by standardized evaluation protocols for regulatory compliance and quality assurance. Future research should prioritize optimizing sensitivity, cost-efficiency, and sustainability, alongside developing biodegradable materials to balance food safety with reduced environmental impact, advancing sustainable food supply chains.

Application of Smart Packaging in Fruit and Vegetable Preservation: A Review

The application of smart packaging technology in fruit and vegetable preservation has shown significant potential with the ongoing advancement of science and technology. Smart packaging leverages advanced sensors, smart materials, and Internet of Things (IoT) technologies to monitor and regulate the storage environment of fruits and vegetables in real time. This approach effectively extends shelf life, enhances food safety, and reduces food waste. The principle behind smart packaging involves real-time monitoring of environmental factors, such as temperature, humidity, and gas concentrations, with precise adjustments based on data analysis to ensure optimal storage conditions for fruits and vegetables. Smart packaging technologies encompass various functions, including antibacterial action, humidity regulation, and gas control. These functions enable the packaging to automatically adjust its internal environment according to the specific requirements of different fruits and vegetables, thereby slowing the growth of bacteria and mold, prolonging freshness, and retaining nutritional content. Despite its advantages, the widespread adoption of smart packaging technology faces several challenges, including high costs, limited material diversity and reliability, lack of standardization, and consumer acceptance. However, as technology matures, costs decrease, and degradable smart packaging materials are developed, smart packaging is expected to play a more prominent role in fruit and vegetable preservation. Future developments are likely to focus on material innovation, deeper integration of IoT and big data, and the promotion of environmentally sustainable packaging solutions, all of which will drive the fruit and vegetable preservation industry toward greater efficiency, intelligence, and sustainability.

Advances in 3D and 4D Printing of Gel-Based Foods: Mechanisms, Applications, and Future Directions

This review examines recent advancements in gel-based 3D and 4D food-printing technologies, with a focus on their applications in personalized nutrition and functional foods. It emphasizes the critical role of tunable rheological and mechanical properties in gels such as starch, protein, and Pickering emulsions, which are essential for successful printing. The review further explores 4D food printing, highlighting stimuli-responsive mechanisms, including color changes and deformation induced by external factors like temperature and pH. These innovations enhance both the sensory and functional properties of printed foods, advancing opportunities for personalization. Key findings from recent studies are presented, demonstrating the potential of various gels to address dietary challenges, such as dysphagia, and to enable precise nutritional customization. The review integrates cutting-edge research, identifies emerging trends and challenges, and underscores the pivotal role of gel-based materials in producing high-quality 3D-printed foods. Additionally, it highlights the potential of Pickering emulsions and lipid gels for expanding functionality and structural diversity. Overall, this work provides a comprehensive foundation for advancing future research and practical applications in gel-based 3D and 4D food printing.

Development of antioxidant films based on anthocyanin microcapsules extracted from purple corn cob and incorporated into a chitosan matrix

Biodegradable food packaging films were prepared from chitosan incorporated with microencapsulated anthocyanins powder (MAP) that was extracted from purple corn cob using the casting method. Anthocyanins extracts were microencapsulated with maltodextrin, gum arabic, and soy protein using a spray-drying method. The film based on chitosan and MAP (CHt@MAP) was prepared through citric acid cross-linking and plasticization with glycerol. The structural analysis of the CHt@MAP film revealed a semicrystalline structure by X-ray diffraction. The interactions were mainly via electrostatic and hydrogen bonding, as confirmed by Fourier-transform infrared. Based on scanning electron microscopy, the morphology of the films revealed evidence of the presence of MAP on the surface and cross-section. The microcapsules inside the films produced an increase in thickness (0.18-0.21 mm), lower water vapor permeability (12.4-8.5 × 10-10 g m-1s-1Pa-1), and reduced elongation at break (217 % to 165 %), as well as tensile strength (1.3 to 0.45 MPa) compared to the chitosan film. Furthermore, the antioxidant activity of CHt@MAP film was high, with a radical scavenging activity of 56 %. It also exhibited a strong barrier to UV and visible light. The results indicate that the CHt@MAP film preserves the shelf life of blueberries at room temperature and could be used as an active packaging film for foods.

A ratiometric fluorescent electrospun film with high amine sensitivity and stability for visual monitoring of livestock meat freshness

Ratiometric fluorescent films with high amine sensitivity and stability were developed to monitor the freshness of beef and pork. Fluorescein isothiocyanate (FITC) and red carbon quantum dots (R-CQD) were used as the amine-responsive indicator and internal reference, respectively. The electrospun films prepared by immobilizing FITC and R-CQD complex (F-R) into polyvinylidene fluoride (PVDF) under 35 %, 55 % and 75 % of relative humidity (RH) were named F-R@PVDF-1, F-R@PVDF-2 and F-R@PVDF-3, respectively. In comparison, the F-R@PVDF-2 film exhibited the highest sensitivity to trimethylamine (TMA), demonstrating a limit of detection (LOD) value of 1.59 μM, and meanwhile high stability during storage with ΔE value of 1.99 after 14 days of storage at 4 °C. The F-R@PVDF-2 film also showed a significant fluorescent red-to-brown color change during meat freshness monitoring at 4 °C. Conclusively, this study reported a new ratiometric fluorescent film that can be used to track the freshness of meats in food packaging.

Fabrication and characterization of double-layer active intelligent film based on chitosan, polyvinyl alcohol, grape skin anthocyanin and selenium nanoparticle

This study involved the fabrication of double-layer intelligent films using chitosan (CS), polyvinyl alcohol (PVA), grape skin anthocyanin (GSA), gellan gum (GG) and selenium nanoparticles (SeNPs). The CS/PVA/GSA layer functioned as the internal indicator layer, and the GG/SeNPs layer acted as the external layer for antioxidant and antimicrobial purposes. SEM, FTIR, XRD, and TGA results confirmed the successful fabrication of double-layer films as well as the presence of hydrogen bonding interaction between the two layers. The tensile strength of double-layer films (8.06 MPa-9.61 MPa) fallen between that of single-layer CS/PVA/GSA film (12.51 MPa) and GG/SeNPs film (1.50 MPa-7.67 MPa). The double-layer films demonstrated good UV-blocking abilities, as well as outstanding antioxidant (ABTS scavenging rate can be up to ∼80 %) and antimicrobial properties. Compared with single-layer CS/PVA/GSA film, the double-layer film incorporated with 6.6 wt% SeNPs (CPG/GS2 film) possessed a more rapid and stronger response towards NH3/acetic acid as well as enhanced storage stability. Furthermore, the CPG/GS2 film can increase the shelf life of strawberries at 25 °C by 4 days, and its visible color change showed strong correlation with the weight loss rate (R2 = 0.99) and hardness (R2 = 0.98) of strawberries.

Rapid-response nanofiber films against ammonia based on black wolfberry anthocyanins, polyvinyl alcohol and sodium alginate for intelligent packaging

To develop novel intelligent indicator films, the mixture of anthocyanin (BWA), polyvinyl alcohol (PVA) and sodium alginate (SA) were spun into PVA/SA/BWA nanofiber films with BWA concentration of 0 %, 5 %, 10 %, and 15 % (based on PVA and SA) via electrospinning technology. The results showed that the BWA was sensitive to pH and was controlled release from films. With increasing BWA concentration, the fiber diameter, tensile strength, and elongation at break gradually decreased, while water contact angle, thickness, moisture content, and antioxidant properties gradually increased. The electrospinning films exhibited high sensitivity to ammonia with rapid color changes in 1 s and excellent color reversibility and color stability within 21 d. The application for shrimp packaging showed that the colorimetric response of the films was closely related to the changes in pH, total volatile basic nitrogen (TVB-N), and total viable count (TVC) of shrimp. This suggests that the prepared films are promising in application for intelligent packaging.

Impact of environmental storage conditions on properties and stability of a smart bilayer film

This study aimed to investigate the behavior of smart bilayer films under various temperature and relative humidity (RH). Smart bilayer films were fabricated using sodium alginate with incorporated butterfly pea anthocyanin and agar containing catechin-lysozyme. Cellulose nanospheres were added at concentrations of 0% and 10% w/w of the film and subjected to test at 4 °C and 25 °C, considering different RHs (0%, 50%, and 80%). The results showed that RH had a greater impact on the mechanical properties than temperature, leading to a decrease in tensile strength and an increase in elongation at break with higher RH. The films displayed increased strength but reduced flexibility at low temperatures. Oxygen permeability was negatively affected by increasing RH, while water vapor barrier properties were better at 25 °C than at 4 °C. In terms of color stability, the temperature played a more important role, with both types of smart bilayer films retaining their color stability throughout 14-day storage at 4 °C, even maintaining their ability to change color with pH. However, the films stored at 25 °C exhibited lower color stability and showed potential for color change with varying pH levels, but with lower intensity. The findings of this study demonstrate the significant impact of temperature and RH on the functional properties of smart bilayer films, with and without the addition of cellulose nanospheres. Such smart bilayer films have great potential for various applications, particularly in food packaging, where maintaining color, mechanical, and barrier properties under varying environmental conditions is crucial.

An antibacterial film using κ-carrageenan loaded with benzyl isothiocyanate nanoemulsion: Characterization and application in beef preservation

Benzyl isothiocyanate (BITC) is a naturally active bacteriostatic substance and κ-carrageenan (KC) is a good film-forming substrate. In the present study, a nanoemulsion incorporating BITC was fabricated with a particle size of 224.1 nm and an encapsulation efficiency of 69.2 %. Subsequently, the acquired BITC nanoemulsion (BITC-NE) was incorporated into the KC-based film, and the light transmittance of the prepared composite films was lower than that of the pure KC film. Fourier transform infrared spectroscopy and scanning electron microscopy revealed that BITC-NE was compatible with the KC matrix. BITC-NE incorporation enhanced the tensile strength of the KC-based films by 33.7 %, decreased the elongation at break by 33.8 %, decreased the water vapor permeability by 60.1 %, increased the maximum thermal degradation temperature by 48.8 %, and decreased the oxygen permeability by 42 % (p < 0.05). Furthermore, the composite films showed enhanced antimicrobial activity against Staphylococcus aureus, Salmonella typhimurium, and Pseudomonas fluorescens. The developed KC-based composite films were applied to wrap raw beef, which significantly delayed the increase in total viable count, total volatile base nitrogen content, and thiobarbituric acid reactive substances, and prolonged the shelf-life of the raw beef by up to 10 days. These results indicated that the composite films prepared by incorporating BITC nanoemulsions into KC matrices have great antimicrobial application potential.

A review on improving the sensitivity and color stability of naturally sourced pH-sensitive indicator films

Naturally sourced pH-sensitive indicator films are of interest for real-time monitoring of food freshness through color changes because of their safety. Therefore, natural pigments for indicator films are required. However, pigment stability is affected by environmental factors, which can in turn affect the sensitivity and color stability of the pH-sensitive indicator film. First, natural pigments (anthocyanin, betalain, curcumin, alizarin, and shikonin) commonly used in pH-sensitive indicator films are presented. Subsequently, the mechanisms behind the change in pigment color under different pH environments and their applications in monitoring food freshness are also described. Third, influence factors, such as the sources, types, and pH sensitivity of pigments, as well as environmental parameters (light, temperature, humidity, and oxygen) of sensitivity and color stability, are analyzed. Finally, methods for improving the pH-sensitive indicator film are explored, encapsulation of natural pigments, incorporation of a hydrophobic film-forming matrix or function material, and protective layer have been shown to enhance the color stability of indicator films, the addition of copigments or mental ions, blending of different natural pigments, and the utilization of electrospinning have been proved to increase the color sensitivity of indicator films. This review could provide theoretical support for the development of naturally sourced pH-sensitive indicator films with high stability and sensitivity and facilitate the development in the field of monitoring food freshness.

Development of smart colorimetric indicators for tracking kimchi freshness by loading aronia extract in agar, κ-carrageenan, and cellulose nanofiber films

Color indicator films incorporating aronia extract powder (AEP) and biopolymers like agar, carrageenan, and cellulose nanofiber (CNF) were developed to monitor kimchi freshness. AEP-containing films showed strong UV-barrier properties, and reduced light transmittance by 99.12 % for agar, 98.86 % for carrageenan, and 98.67 % for CNF-based films. All AEP-films exhibited high sensitivity to pH changes and vapor exposure to ammonia and acetic acid. Color change notably influenced by the polymer type, particularly evident with ammonia vapor exposure, especially in the AEP/carrageenan film. The chemical structure and thermal stability of the biopolymers remained unchanged after AEP-addition. Tensile strength increased by 24.2 % for AEP/CNF but decreased by 19.4 % for AEP/agar and 24.3 % for AEP/carrageenan films. AEP-containing films displayed strong antioxidant activity, with 99 % free radical scavenging in ABTS and ~ 80 % in DPPH assays. Alkalized AEP-indicator films were more effective in detecting color changes during kimchi packaging tests. Among the labels, alkalized AEP/agar film showed the most obvious color change from green-gray (fresh kimchi, pH 5.5, acidity 0.48 %) to pale brown (optimal fermentation, pH 4.6, acidity 0.70 %), and pale violet-brown (over-fermented, pH 3.80, acidity 1.35 %). Alkalized AEP-indicator films offer promising real-time detection of packed fermented foods like kimchi.

Sodium-alginate-based indicator film containing a hydrophobic nanosilica layer for monitoring fish freshness

In this study, hydrophobic sodium alginate/anthocyanin/cellulose nanocrystal indicator films were fabricated by incorporating nanosilica (NS) as a waterproofing layer. The concentrations and formation methods (spraying (S), coating (C), and impregnation (I)) of the NS layer (denoted as NSS, NSC, NSI, respectively) were optimized. The results indicated that the optimum concentration of the NS layer was 5 % at a water contact angle (WCA) 110.5°. Further, Fourier transform infrared spectra showed the presence of SiOSi and SiCH3 groups in the NSS, NSC, and NSI films, and X-ray diffraction spectra indicated that original structures of these films were disordered. Moreover, the surface morphology, mechanical properties, and light transmission were affected by the NS layer, and the optimal layer was found to be NSI. After 10 days of storage at 100 % humidity, the NSI film exhibited low water vapor adsorption (37.22 g) and permeability (0.1484 g/m·s·Pa·10-11) and a high WCA (110.2°). In addition, the NSI film exhibited a visible color shift with an increasing pH of the buffer solution. A monitoring test of fish freshness showed that the NSI film displayed a distinctive color change corresponding to fish spoilage during 14 days of storage. This indicates that NSI has high potential in indicator film applications.

Recent advancements in chemosensors for the detection of food spoilage

The need for reliable sensors has become a major requirement to confirm the quality and safety of food commodities. Chemosensors are promising sensing tools to identify contaminants and food spoilage to ensure food safety. Chemosensing materials are evolving and becoming potential mechanisms to enable onsite and real-time monitoring of food safety. This review summarizes the information about the basic four types of chemosensors (colorimetric, optical, electrochemical, and piezoelectric) employed in the food sector, the latest advancements in the development of chemo-sensing mechanisms, and their food applications, with special emphasis on the future outlook of them. In this review, we discuss the novel chemosensors developed from the year 2018 to 2022 to detect spoilage in some common types of food like fish, meat, milk, cheese and soy sauce. This work will provide a fundamental step toward further development and innovations of chemosensors targeting different arenas in the food industry.

Multifunctional alginate films blended with polyphenol-rich extract from unconventional edible sources: Bioactive properties, UV-light protection, and food freshness monitoring

The development of active and smart packaging from non-conventional food sources is an ecological trend to ensure safe food supply in the food chain. The study aimed to develop multifunctional films based on alginate blended with different concentrations of purple onion peel (POPE) and butterfly-pea flower extract (BFE). The addition of the extracts increased the opacity of the films by 80 %, indicating greater UV-light barrier ability. The tensile strength and elongation at break of the films increased by 70 % and 30 %, while water vapor permeability decreased by 15 %. The interaction between the extract and the alginate positively modified the structure of the films, increasing the melting temperature of the films (112-131 °C). Mixing both extracts in the matrix generated materials with antioxidant activity, antimicrobial capabilities, and sensitivity to freshness factors (gases, pH, and temperature) superior to films added with a single extract, suggesting better active and intelligent performances. The films protected the color of food products against the effects of UV-light, being strongly capable of colorimetrically checking the deterioration of protein-rich products. Therefore, alginate films blended with POPE and BFE have a promising potential for developing smart materials, preserving, and monitoring the food quality.

Functional pH-Sensitive Film Containing Purple Sweet Potato Anthocyanins for Pork Freshness Monitoring and Cherry Preservation

An antioxidative and pH-sensitive multifunctional film, incorporating anthocyanin-rich purple sweet potato extract (PPE) was fabricated from polyvinyl alcohol (PVA) and sodium alginate (SA)/sodium carboxymethyl cellulose (CMC-Na). The film was composed of 6:4 PVA:SA/CMC-Na (mass ratio, SA:CMC-Na at 1:1) with added PPE, and changed color with changes in pH, and also had useful UV-blocking, antioxidant, mechanical, and water vapor barrier properties, which enable its use as a food coating film. In addition, the incorporation of 300 mg PPE increased the biodegradability of the film in soil from 52.47 ± 1.12% to 64.29 ± 1.75% at 17 days. The pH sensitivity of the film enabled its successful use for the evaluation of pork freshness. Cherries coated with the film had an extended shelf life from 3-4 to 7-9 days, during storage at 25 °C. Consequently, the multifunctional film can be applied to packaging for real-time pH/freshness monitoring and for effectively preserving the freshness of meat and fruit.

On-site colorimetric food spoilage monitoring with smartphone embedded machine learning

Real-time and on-site food spoilage monitoring is still a challenging issue to prevent food poisoning. At the onset of food spoilage, microbial and enzymatic activities lead to the formation of volatile amines. Monitoring of these amines with conventional methods requires sophisticated, costly, labor-intensive, and time consuming analysis. Here, anthocyanins rich red cabbage extract (ARCE) based colorimetric sensing system was developed with the incorporation of embedded machine learning in a smartphone application for real-time food spoilage monitoring. FG-UV-CD100 films were first fabricated by crosslinking ARCE-doped fish gelatin (FG) with carbon dots (CDs) under UV light. The color change of FG-UV-CD100 films with varying ammonia vapor concentrations was captured in different light sources with smartphones of various brands, and a comprehensive dataset was created to train machine learning (ML) classifiers to be robust and adaptable to ambient conditions, resulting in 98.8% classification accuracy. Meanwhile, the ML classifier was embedded into our Android application, SmartFood++, enabling analysis in about 0.1 s without internet access, unlike its counterpart using cloud operation via internet. The proposed system was also tested on a real fish sample with 99.6% accuracy, demonstrating that it has a great advantage as a potent tool for on-site real-time monitoring of food spoilage by non-specialized personnel.

The highly stable indicator film incorporating roselle anthocyanin co-pigmented with oxalic acid: Preparation, characterization and freshness monitoring application

A novel stable PVA/HPMC/roselle anthocyanin (RAE) indicator film co-pigmented with oxalic acid (OA) was prepared, its properties, application effects and stability enhancement mechanism were investigated correspondingly. The structural characterization revealed that more stable network was formed due to the co-pigmentation facilitated generation of molecular interactions. Meanwhile, the co-pigmentation improved film mechanical and hydrophobic properties compared to both PVA/HPMC/RAE newly prepared (PHRN) or stored (PHRS) film, expressing as higher tensile strength values (12.25% and 14.44% higher than PHRN and PHRS), lower water solubility (7.22% and 10.09% lower than PHRN and PHRS) and water vapor permeability values (33.20% and 21.05% lower than PHRN and PHRS) of PVA/HPMC/RAE/OA newly prepared (PHON) or stored (PHOS) film. Compared with the PHRS film, the PHOS film still presented more distinguishable color variations when being applied to monitor shrimp freshness, owing to the stabilization behaviors of co-pigmentation in anthocyanin conformation. Hence, the co-pigmentation was an effective strategy to enhance film stability, physical and pH-responsive properties after long term storage, leading to better film monitoring effects when applied in real-time freshness monitoring.

Simultaneously realizing enhancement of sensitivity for freshness monitoring and multinomial properties of carrageenan/konjac glucomannan/blueberry anthocyanin-based intelligent film by diatomite

Here, highly sensitive blueberry anthocyanin (BBA)-induced intelligent indicating films were fabricated by incorporating a novel composite ingredient, diatomite (DA), into a matrix of konjac glucomannan (KGM), carrageenan (CAR) and BBA. We systematically investigated the effects of introducing DA and BBA on the structure, physical properties, colorimetric response, and practical application of the KGM/CAR film. Our findings revealed that the DA particles and BBA were well-distributed in the KGM/CAR matrix through hydrogen bonding interactions. This distribution significantly improved tensile strength, surface hydrophobicity, thermal stability, and barrier properties of the KGM/CAR film. Notably, the KGM/CAR-based intelligent film loaded with 6 % DA exhibited the most optimal properties. Furthermore, DA exhibited a hierarchical porous structure, enabling the KGM/CAR film to detect volatile amines with heightened sensitivity. When applied to monitor shrimp spoilage in transparent plastic packaging, the color of the composite film underwent remarkable changes from bright pink to bluish violet. These color changes correlated well with the total volatile basic nitrogen (TVB-N) and pH changes in the shrimp, as determined by standard laboratory procedures. Our work presents a promising approach to the development of high-performance and intelligent food packaging materials. These materials hold great potential for practical applications in the field of food packaging.

Recent advances on natural colorants-based intelligent colorimetric food freshness indicators: fabrication, multifunctional applications and optimization strategies

With the increasing concerns of food safety and public health, tremendous efforts have been concentrated on the development of effective, reliable, nondestructive methods to evaluate the freshness level of different kinds of food. Natural colorants-based intelligent colorimetric indicators which are typically constructed with natural colorants and polymer matrices has been regarded as an innovative approach to notify the customers and retailers of the food quality during the storage and transportation procedure in real-time. This review briefly elucidates the mechanism of natural colorants used for intelligent colorimetric indicators and fabrication methodologies of natural colorants-based food freshness indicators. Subsequently, their multifunctional applications in intelligent food packaging systems like antioxidant packaging, antimicrobial packaging, biodegradable packaging, UV-blocking packaging and inkless packaging are well introduced. This paper also summarizes several optimizing strategies for the practical application of this advanced technology from different perspectives. Strategies like adopting a hydrophobic matrix, constructing double-layer film and encapsulation have been developed to improve the stability of the indicators. Co-pigmentation, metal ion complexation, pigment-mixing and using substrates with high surface area are proved to be effective to enhance the sensitivity of the indicators. Approaches include multi-index evaluation, machine learning and smartphone-assisted evaluation have been proven to improve the accuracy of the intelligent food freshness indicators. Finally, future research opportunities and challenges are proposed. Based on the fundamental understanding of natural colorants-based intelligent colorimetric food freshness indicators, and the latest research and findings from literature, this review article will help to develop better, lower cost and more reliable food freshness evaluation technique for modern food industry.

A visual bi-layer indicator based on mulberry anthocyanins with high stability for monitoring Chinese mitten crab freshness

In this study, a bilayer film (BIF) was fabricated to improve the stability of an anthocyanin-based freshness indicator film. The sensor layer consists of gellan gum (GG) and mulberry anthocyanin (MAE) for freshness indication. The oxygen barrier layer was constructed from chitosan (CS), polyvinyl alcohol (PVA), sodium alginate (SA), and pullulan (Pu) to the protection of MAE from oxidation. The highest antioxidant activity of BIF was 91.28 %. BIF was used to monitor the Chinese mitten crab freshness. The total volatile basic nitrogen (TVB-N) level was increased to 31.23 mg/100 g on day 8, and the color of the indicator presented a visible change from pink to dark green. The acquired results revealed a good correlation between TVB-N, pH, and color change of the indicator. The research indicated that the BIF was applied for freshness monitoring of Chinese mitten crab and displayed significant color changes that would be effective in commercial environments.

Fabrication and characterization of polyvinyl alcohol/sodium alginate/zein/ chitosan bilayer film for dynamic visualization of pork quality

The development of real-time and convenient meat freshness indication technology is crucial to ensure food safety. A novel antibacterial visualized intelligent film was designed based on polyvinyl alcohol (PA), sodium alginate (SA), zein (ZN), chitosan (CS), alizarin (AL) and vanillin (VA) using layer-by-layer assembly (LBL) method for real-time and in situ monitoring of pork freshness. The fabricated film had various advantageous properties, including an excellent hydrophobicity with a water contact angle (WCA) of 91.59°, improved color stability, excellent water barrier properties and increased mechanical performance (TS = 42.86 MPa). The fabricated film also demonstrated effective antibacterial properties with a bacteriostatic circle diameter of 13.6 mm for Escherichia coli. Moreover, the film can perceive and visualize the antibacterial effect through color changes, enabling dynamic visual monitoring of the antibacterial effect. A good correlation (R² = 0.9188) between the color changes (ΔE) and total viable count (TVC) of pork was documented. Conclusively, fabricated multifunctional film improves the accuracy and versatility of freshness indication and had great potential for food preservation and freshness monitoring. The outcomes of this research provides a new perspective for the design and development of multifunctional intelligent films.

Novel color stability and colorimetry-enhanced intelligent CO2 indicators by metal complexation of anthocyanins for monitoring chicken freshness

This study aims to improve the color stability of anthocyanins and develop a CO2-sensitive indicator based on black goji anthocyanin (BGA) extract. Although the BGA extracts showed distinct color changes, such as red-purple-blue, their intrinsic color diminished after 24 h. A metal complexation method was used for the high color stability of BGA. BGA extracts were chelated with various concentrations of Al3+ [0 - 20% (w/w)]. It showed high color stability and strong intensity in a dose-dependent manner. A CO2-sensitive indicator sachet was developed using hydroxypropyl methylcellulose hydrogel, based on 5% (w/w) Al3+-BGA complexes. The indicator was applied to the chicken breast and detected its spoilage after 3 days with its changing color to greyish blue, due to the microbial growth to 7.00 log CFU/g. These results demonstrated the possibility of chelated anthocyanin complexes as indicating dyes and the ability to monitor the food quality changes through noticeable color changes.

Colorimetric porous microspheres of natural sodium alginate for chilled pork visual monitoring

Chilled meat is subject to deterioration by various factors during storage and distribution. Therefore, it is very important to monitor the quality of meat in real time. This study aims at preparing a natural, low-cost indicating microsphere to visualize the freshness of meat by the combination of sodium alginate (SA) and chitosan with 0-10 wt% anthocyanins derived from chokeberry as a colorant using ionic gelation method. Size-controlled porous SA microspheres with were further constructed by freeze-drying and their physicochemical properties were characterized by SEM, FTIR, DSC, and XRD. Results showed that microspheres with 1 wt% anthocyanin showed good responsiveness to different concentrations of ammonia and were able to effectively identify the freshness of chilled meat by color change. Principal component analysis showed that the color difference of the porous microspheres was highly significantly correlated with pH, TVB-N, total plate count and thiobarbituric acid active substance (p < 0.01), suggesting a visible satisfactory capability of the microspheres to identify the spoilage in pork. Principal component analysis showed that the color difference of the porous microspheres was highly significantly correlated with pH, TVB-N, total plate count and thiobarbituric acid active substance (P < 0.01), suggesting a visible satisfactory capability of the microspheres to identify the spoilage in pork.

A visual bi-layer sensor based on Agar/TiO2/butterfly bean flower anthocyanin/κ-carrageenan with photostability for monitoring Penaeus chinensis freshness

Visual indicator bi-layer films were manufactured incorporating κ-carrageenan, butterfly pea flower anthocyanin, varying Nano‑titanium dioxide (TiO₂) content and agar for Penaeus chinensis (Chinese white shrimp) freshness detection. The κ-carrageenan-anthocyanin (CA) layer served as indicator, while the TiO₂-agar (TA) layer functioned as the protective layer to improve the photostability of film. The bi-layer structure was characterized by scanning electron microscopy (SEM). The TA2-CA film had the best tensile strength with a value of 17.8 MPa and the lowest water vapor permeability (WVP) value of bi-layer films was 2.98 × 10^-7 g.m^-1.h^-1.pa^-1. The bi-layer film protected anthocyanin against exudation when immersed in aqueous solution of varying pH. The TiO₂ particles filled the pores of the protective layer, increasing the opacity from 1.61 up to 4.49 significantly improving the photostability with a consequent slight color change under illumination of UV/visible light. Under UV irradiation, the TA2-CA film had no significant color change with a ΔE value of 4.23. Finally, the TA2-CA films showed an obvious color change from blue to yellow green in the early stages of Penaeus chinensis putrefaction (≤48 h) then the color change and Penaeus chinensis freshness were well correlated (R² = 0.8739).

High-Stability Bi-Layer Films Incorporated with Liposomes @Anthocyanin/Carrageenan/Agar for Shrimp Freshness Monitoring

High-stability bi-layer films were prepared by incorporating anthocyanin-loaded liposomes into carrageenan and agar (A-CBAL) for non-destructive shrimp freshness monitoring. The encapsulation efficiency of the anthocyanin-loaded liposomes increased from 36.06% to 46.99% with an increasing ratio of lecithin. The water vapor transmission (WVP) of the A-CBAL films, with a value of 2.32 × 10^-7 g · m^-1 · h^-1 · pa^-1, was lower than that of the film with free anthocyanins (A-CBA). The exudation rate of the A-CBA film reached 100% at pH 7 and pH 9 after 50 min, while the A-CBAL films slowed down to a value lower than 45%. The encapsulation of anthocyanins slightly decreased the ammonia sensitivity. Finally, the bi-layer films with liposomes successfully monitored shrimp freshness with visible color changes to the naked eye. These results indicated that films with anthocyanin-loaded liposomes have potential applications in high-humidity environments.

A novel pH-sensitive antibacterial bilayer film for intelligent packaging

Intelligent single-layer packaging is widely used in food monitoring and storage. However, most single-layer intelligent packaging has poor mechanical strength and water barrier properties. In this study, a bilayer intelligent detector film based on polyvinyl alcohol-chitosan (PVA-CS)/nano-ZnO/sodium alginate (SA) combined with anthocyanin extract (cyanidin chloride) was prepared using a layer-by-layer solution casting assembly technique. The effects of different levels of anthocyanin extracts on the physical and functional properties of the films, including microstructure, mechanical property, barrier property, pH sensitivity, and antibacterial property, were investigated. The results show that the bilayers exhibit excellent physical properties, lower water vapor permeability, better light transmission and UV-blocking properties, a broader pH sensitivity (ΔE > 10), and good antibacterial activity. In short, the bilayer films studied are superior to the single-layer films in terms of their packaging potential for products with low moisture content, offering new directions for active intelligent packaging and biodegradable materials for the food industry.

Graphical Abstract

On-package indicator films based on natural pigments and polysaccharides for monitoring food quality: a review

Deterioration of food quality and freshness is mainly due to microbial growth and enzyme activity. Chilled fresh food, especially meat and seafood, as well as pasteurized products, rapidly lose quality and freshness during packing, distribution and storage. Real-time food quality monitoring using on-package indicator films can help consumers make informed purchasing decisions. Interest in the use of intelligent packaging systems for monitoring safety and food quality has increased in recent years. Polysaccharide-based films can be developed into on-package indicator films due to their excellent film-forming properties and biodegradability. Another important component is the use of colorants with visible color changes at various pH levels. Currently, natural pigments are receiving increased attention because of their safety and environmental friendliness. This review highlights the recent findings regarding the role of natural pigments, the effects of incorporating natural pigments and polysaccharides on properties of indicator film, current application and limitations of on-package indicator films based on polysaccharides in some foods, problems and improvement of physical properties and color conversion of indicator film containing natural pigments, and development of polysaccharide-based pH-responsive films. © 2022 Society of Chemical Industry.

Colorimetric Indicators Based on Anthocyanin Polymer Composites: A Review

This review explores the colorimetric indicators based on anthocyanin polymer composites fabricated in the last decade, in order to provide a comprehensive overview of their morphological and compositional characteristics and their efficacy in their various application fields. Notably, the structural properties of the developed materials and the effect on their performance will be thoroughly and critically discussed in order to highlight their important role. Finally, yet importantly, the current challenges and the future perspectives of the use of anthocyanins as components of colorimetric indicator platforms will be highlighted, in order to stimulate the exploration of new anthocyanin sources and the in-depth investigation of all the possibilities that they can offer. This can pave the way for the development of high-end materials and the expansion of their use to new application fields.

On-package ratiometric fluorescent sensing label based on AIE polymers for real-time and visual detection of fish freshness

Freshness is an important parameter of fish quality. This study aims to develop a ratiometric fluorescent sensing label that is responsive to volatile amines, affording real-time and visual detection of fish freshness. For developing of the sensing label, an aggregation-induced emissive (AIE) polymer was prepared from the stimuli-responsive polymer polymethacrylic acid and the AIE molecule tetraphenylethylene and coated on to filter paper with rhodamine B as an internal reference. By exploiting the ratiometric response, the freshness of fish could be identified clearly and easily according to the color of on-package label, which changes from pink (fresh) to dark blue. The difference was linearly correlated with the total volatile basic nitrogen (TVB-N, R² = 0.995 and 0.994 at 25 °C and 4 °C, respectively) in the range of 15-25 mg/100 g for the salmon samples, which indicated that the sensing label feasibly and non-destructively quantified TVB-N.

Smart films fabricated from natural pigments for measurement of total volatile basic nitrogen (TVB-N) content of meat for freshness evaluation: A systematic review

Major databases were searched from January 2012 to August 2021 and 54 eligible studies were included in the meta-analysis to estimate the overall mean of total volatile basic nitrogen (TVB-N) in meat. The mean of TVB-N was 24.96 mg/100 g (95 % CI:23.10-26.82). The pooled estimate of naphthoquinone, curcumin, anthocyanins, alizarin and betalains were 25.98 mg/100 g (95 %CI:19.63-32.33), 30.03 mg/100 g (95 %CI: 24.15-35.91), 24.92 mg/100 g (95 %CI: 22.55-27.30), 23.37 mg/100 g (95 %CI:19.42-27.33) and 19.50 mg/100 g (95 %CI:17.87-21.12), respectively. Meanwhile, subgroups based on meat types showed that smart film was most used in aquatic products at 27.19 mg/100 g (95 %CI:24.97-29.42), followed by red meat at 19.69 mg/100 g (95 %CI:17.44-21.94). Furthermore, 4 °C was the most storage temperature used for testing the performance of smart films at 25.48 mg/100 g (95 %CI:23.05-27.90), followed by storage at 25 °C of 25.65 mg/100 g (95 %CI:22.17-29.13). Substantial heterogeneity was found across the eligible studies (I² = 99 %, p = 0.00). The results of the trim-and-fill method demonstrated publication bias was well controlled.

Development of nanofiber indicator with high sensitivity for pork preservation and freshness monitoring

A visual Polyvinylidene Fluoride (PVDF) fibrous film incorporated with Roselle anthocyanin (RS) and Cinnamon essential oil (CEO) (PRC film) was designed via electrospinning technology for pork preservation and freshness monitoring. The PRC film presented well structural integrity and stability in buffer solutions without leaking out RS. And PCR film had well hydrophobic and high permeability with water contact angle (WCA) of 109.52° and water vapor permeability (WVP) of 2.63 × 10-7 g m-1h-1Pa-1. Importantly, PRC film exhibited good antibacterial activity with the inhibition diameter at 29.0 mm and 27.1 mm which against Escherichia coli and staphylococcus aureus, respectively. Finally, the PRC film was employed as a colorimetric sensor for monitoring pork freshness. It presented visible color changes from pink to blue and effectively prolonged the pork shelf-life by 2 days at 4 °C. These results indicate a great potential in intelligent and active packaging.

High- sensitivity bilayer nanofiber film based on polyvinyl alcohol/sodium alginate/polyvinylidene fluoride for pork spoilage visual monitoring and preservation

A colorimetric bilayer film for pork freshness detection and preservation was developed using electrospinning technique. The bilayer film consisted of a layer with polyvinyl alcohol - sodium alginate - alizarin as sensor layer and a layer with polyvinylidene fluoride - vanillin as antibacterial layer. The water contact angle of bilayer film was larger than the single colorimetric layer. The color sensitivity to the ammonia of the bilayer film was higher, with an ΔE value of 47.99. The film could display color shifts from yellow to purple with the naked eye is critical for checking pork freshness. In addition, the bilayer film exhibited sensitive antibacterial activity, with an inhibition zone against S. aureus (8.3 mm) and E. coli (14.7 mm), respectively. Finally, the bilayer film was applied to freshness monitoring of pork. The film displayed significant color changes and prolonged the pork shelf life by 24 h at 25 °C.

Ixiolirion tataricum anthocyanins-loaded biocellulose label: Characterization and application for food freshness monitoring

A new intelligent pH-sensitive colorimetric label was fabricated by immobilizing Ixiolirion tataricum anthocyanins (ITA) into biocellulose (bacterial nanocellulose; BNC) film and was then studied to determine how it can be used as a label for monitoring freshness/spoilage of shrimp during storage at 4 °C. The formation of new interactions between ITA and BNC film and disruption of crystalline structure of BNC after anthocyanins immobilization were approved by FT-IR and XRD analyses, respectively. According to FE-SEM observations, the porosity of the BNC network decreased after ITA incorporation. The fabricated BNC-ITA label showed a distinct color change from violet to green over the pH range of 4-12. The pH, total volatile basic nitrogen (TVB-N), total psychrophiles count (TPC), and the quantity of biogenic amines (histamine, cadaverine, putrescine, and tyramine) in the shrimp samples and their correlation with color changes on the label were measured over a 4-day storage period. Consistent with changes in levels of TVB-N, TPC, pH, and biogenic amines, a visually distinguishable color change occurred on the BNC-ITA label as blue (fresh), dark green (medium fresh), and kelly green (spoiled). This research showed that ITA as a novel pH-sensitive dye is a promising candidate for developing pH labels for seafood intelligent packaging.

Facile fabrication of sandwich-like anthocyanin/chitosan/lemongrass essential oil films via 3D printing for intelligent evaluation of pork freshness

In this study, chitosan (CH), mulberry anthocyanin (MA), and lemongrass essential oils (LEO) were used as an interlayer using a 3D printer. Further, cassava starch (CS) was used as a protective layer to form indicator films. The indicator films containing LEO showed significant antioxidant and antibacterial properties, and the release rate of LEO increased with a rise in pH. When chilled pork spoiled, the color of the indicator films changed from red to gray-blue, and the RGB values could be automatically analyzed by a smartphone application to determine pork freshness. These films hold implications as easy-to-use indicators of meat freshness, with great potential for monitoring food spoilage, as part of an intelligent packaging system.

Preparation of pH-indicator films based on soy protein isolate/bromothymol blue and methyl red for monitoring fresh-cut apple freshness

Intelligent pH-indicator films based on soy protein isolate (SPI) were prepared using pH-sensitive dyes (bromothymol blue and methyl red). The addition of mixed indicators imparts pH-indicator films with an appreciable microstructure, acceptable water resistance, and favorable optical properties. The incorporation of the mixed indicators did not lead to significant improvement in the mechanical properties of films due to weak ionic cross-linking by hydrogen bonding between the SPI macromolecules and low-molecular-weight indicators. Fourier-transform infrared spectroscopy indicated hydrogen bond-mediated intermolecular interactions, and scanning electron microscopy showed that BB/MR were well dispersed in the SPI film. The indicator addition hindered the sorption and passage of water molecules. The water vapor permeability, moisture sorption, moisture content, and total soluble matter were 4.32 to 6.12 ×10^-12  g·cm/cm² ·s·Pa, 36.70% to 73.33%, 25.28% to 44.11%, and 8.21% to 25.56%, respectively. Also, the addition of indicators reduced UV light transmittance with minimal effect on the transparency of the film. The presence of indicators enhanced the pH sensitivity, seen as a visible color reaction at different pHs (total color difference, ΔE > 5). When the pH-indicator film containing 8 ml/100 ml final film emulsions was used to monitor the fresh-cut apple freshness, a green color for fresh status was observed, which turned blue after 60 h. Collectively, our findings suggested that indicator-containing SPI films have the potential for monitoring the freshness of fruits. PRACTICAL APPLICATION: pH-indicator films can help consumers to identify the freshness of packaged food by a change in the color of the packaging material, which is easily visible to the unaided eye without the need for opening the packaging. This protects consumers' interests.