Recent Advances in pH-Responsive Freshness Indicators Using Natural Food Colorants to Monitor Food Freshness

Optically pH-Sensing in smart wound dressings towards real-time monitoring of wound states: A review

BACKGROUND

Over the recent years, the investigations on wound dressings have been undergoing significant evolution, and now smart dressings with the function of the real-time monitoring of the wound states have been recognized as one of the most advanced treatment modalities. Among a variety of wound-related biomarkers, pH represents a promising candidate for in situ supervising the wound healing status. In this regard, a variety of optically pH sensing agents have been widely incorporated into different types of wound dressings.

RESULTS

Herein, we first presented an overview of the advanced wound dressings, especially those commonly used in wound pH sensing. Then, a comprehensive summary of the optical pH sensing agents that could be incorporated into the wound dressings for detecting the pH alteration on the wound bed was described in detail. These materials were classified into colorimetric dyes (i.e., synthetic and plant-based dyes) and fluorescent probes (i.e., small-molecular fluorescein and fluorescent nanomaterials). Each type of pH sensing agent was fully discussed with advantages and limitations for monitoring the wound pH alteration, as well as typical examples of practical applications. To well interpret messages produced by the color-coding dressings, the approaches for defining and communicating color were also summarized, and a proof-of-concept, the smartphone-based remote supervision was particularly highlighted.

SIGNIFICANCE

This review provides a comprehensive overview of the utilization of optically pH sensing in advanced wound dressings for the real-time monitoring of the wound states. It was expected to be an informative source for the exploitation of novel diagnostic dressings for wound management, and also a reference the for application of these materials in the biosensing of other physiological or pathological fluids.

Transcriptome Sequencing Reveals Survival Strategies and Pathogenic Potential of Vibrio parahaemolyticus Under Gastric Acid Stress

As a common food-borne pathogen, Vibrio parahaemolyticus comes into direct or indirect contact with gastric acid after ingestion. However, the mechanisms by which Vibrio parahaemolyticus passes through the gastric acid barrier, recovers, and causes pathogenicity remain unclear. In this study, static in vitro digestion simulation experiments showed that some strains can pass through the gastric acid barrier by utilizing microacid tolerance mechanisms and altering their survival state. Food digestion simulation experiments showed that food matrices could help bacteria escape gastric acid stress, with significantly different survival rates observed for bacteria in various food matrices after exposure to gastric acid. Interestingly, surviving Vibrio parahaemolyticus showed a significantly shorter growth lag time (LT) during recovery. Transcriptome sequencing (RNA-seq) analyses indicated that the bacteria adapted to gastric acid stress by regulating the two-component system through stress proteins secreted via the ribosomal pathway. Pathogenic Vibrio parahaemolyticus that successfully passes through the gastric acid barrier potentially exhibits enhanced pathogenicity during recovery due to the significant upregulation of virulence genes such as tdh and yscF. This study provides a scientific basis for revealing the tolerance mechanisms of food-borne pathogens represented by Vibrio parahaemolyticus in the human body.

Advanced chitosan hybrid dye labels for dynamic monitoring of shrimp and milk freshness

This study presents the development of intelligent screen-printed labels for real-time food freshness monitoring. Using chitosan grafted with rosolic acid (RA) and immobilized on montmorillonite (MMT) through cationic exchange, a hybrid dye was synthesized and applied in screen-printing inks. The hybrid structure was characterized by XRD, TGA, and UV-vis, confirming improved thermal stability and maintained halochromic properties. SEM analysis showed consistent ink deposition on filter paper, while water contact angle (WCA) measurements demonstrated enhanced surface hydrophobicity due to the MMT. The labels exhibited clear pH-sensitive color transitions from yellow to purplish red (pH 2.0-12.0) and rapid ammonia sensitivity, with ΔE values exceeding 45.0 within 10 min. The labels also demonstrated excellent reversibility, storage stability, leaching resistance, and cytocompatibility. Practical tests on shrimp and milk confirmed the labels' ability to accurately monitor freshness through visible color changes. These findings highlight the potential of hybrid labels as effective, scalable freshness indicators for intelligent food packaging.

Eco-Friendly and Smart Electrospun Food Packaging Films Based on Polyvinyl Alcohol and Sumac Extract: Physicochemical, Mechanical, Antibacterial, and Antioxidant Properties

With the increasing concern over environmental pollution caused by synthetic packaging, there is a growing demand for sustainable, biodegradable, and functional materials in the food industry. In this study, the antioxidant, antimicrobial, physicochemical, and mechanical properties of electrospun edible films based on sumac extract and polyvinyl alcohol were investigated. The films demonstrated a clear colorimetric response to pH changes, shifting from red in acidic to yellow in alkaline conditions, making them suitable for food packaging and freshness monitoring. The film containing 30% sumac extract (P-SE 30%) exhibited strong antimicrobial activity against Escherichia coli (17.01 mm) and Staphylococcus aureus (18.02 mm), along with acceptable antioxidant activity (46.32%). The film with 10% sumac extract showed the best mechanical strength (0.034 MPa). Moreover, moisture content (4.3%) and water vapor permeability (9.49 g mm/m2 Pa) were significantly reduced. Also, the physicochemical properties (SEM, FT-IR, X-ray, thickness, Opacity, and mechanical) of electrospun films were improved compared to the control sample. In general, this study demonstrates the potential of electrospun films reinforced with sumac extract as a smart food packaging solution for enhancing food safety.

A Fluorescence/Colorimetric Synergistic-Enhanced Type-I Heterostructured MOF@QDs for Both Multi-Depth Food-Freshness Prediction and Extra Preservation

Spoiled food has significantly impacted the global economy and public health, which increases worldwide concern about monitoring and preserving food freshness. Herein, a multi-functional type-I heterojunction (Eu@ZMC) is designed by europium metal-organic framework (EuMOF), zinc oxide quantum dots (ZnO QDs), and chlorogenic acid (CGA). Eu@ZMC achieves ratiometric fluorescent/colorimetric sensing of pH and biogenic amines to detect freshness. Besides, a paper-based platform (PEu@ZMC) is prepared and can detect histamine with the LOD of 0.0142 and 0.0136 µg mL-1 in fluorescent and colorimetric modes, respectively. An advanced OR/NOT-gate logic device is further constructed to distinguish freshness into three levels (fresh, less fresh, and spoiled). This dual-mode sensor is synergistic-enhanced by the energy transfer triggered by ZnO QDs-promoted colorimetry and the type-I heterostructure of fluorescent EuMOF and ZnO QDs. The release of low-toxic zinc ions inhibits various bacterial growth, including Salmonella typhimurium. According to raw fish evaluation, Eu@ZMC not only effectively monitors spoilage externally and internally aligning with a commercial kit, but also reduces spoilage speed, which cannot be achieved through the classical detection strategy. This original work provides a simple, convenient, and reliable method for multi-depth and real-time visual food monitoring with extract freshness preservation, contributing to economic benefits and human health assurance.

Enhancing sensitivity and stability of natural pigments in pH-responsive freshness indicators: A review

Natural pigments are an indicator component in the freshness indicator, which is advantageous due to their safety, renewability, and low cost. However, freshness indicator with natural pigments as pH indicators has the problems of low stability and the color rendering domain could not effectively cover the shelf life of food. This paper describes the types and structures of natural pigments commonly used in freshness indicators and their color change mechanisms under different pH conditions. Also, the preparation methods of natural pigments freshness indicators are reviewed. Based on the current limitations and shortcomings faced by natural pigments freshness indicators, this paper highlights optimization strategies to enhance their sensitivity and stability, including modification, co-pigmentation, natural pigments mixing, encapsulation, and metal-ion complexation. The exploitation of these optimization strategies can help develop natural pigment-based intelligent packaging with superior performance to meet the food industry's needs for quality and safety monitoring.

Development of a Novel Colorimetric pH Biosensor Based on A-Motif Structures for Rapid Food Freshness Monitoring and Spoilage Detection

Accurate methods for assessing food freshness through colorimetric pH response play a critical role in determining food spoilage and ensuring food quality standards. This study introduces a novel unlabeled DNA sequence, poly-dA20, designed to exploit the colorimetric properties of both the single strand and the fold-back A-motif structure in conjunction with gold nanoparticles (AuNPs) under varying pH conditions. When exposed to storage temperatures of 4 °C and 25 °C, the color variations in the AuNP solution, influenced by pH level changes in mutton and sea bass samples' different storage periods, are easily discernible to the naked eye within a minute. The ratio of UV absorption values at 527 nm and 700 nm (A527/A700) demonstrates a strong linear correlation with both the storage duration and pH of the food samples. Furthermore, a comprehensive analysis combining the total volatile basic nitrogen (TVB-N) value with the A527/A700 ratio is employed for precise assessment of food freshness. The innovative pH-responsive sensing strategy not only provides a new approach for on-site food freshness and spoilage detection systems but also serves as a valuable tool for pH-related biological detection in clinical diagnostic applications.

Enhancing seafood freshness monitoring: Integrating color change of a food-safe on-package colorimetric sensor with mathematical models, microbiological, and chemical analyses

The study assessed a developed food-safe on-package label as a real-time spoilage indicator for fish fillets. This colorimetric sensor is sensitive to Total Volatile Base Nitrogen (TVB-N) levels, providing a correct indication of fish freshness and spoilage. This study evaluates and predicts the shelf-life and effectiveness of an on-package colorimetric indicator. The sensor, using black rice (BC) dye with polyvinyl alcohol (PVOH), polyethylene glycol (PEG), and citric acid (CA) as binders and crosslinking agents, is applied to PET films. The food-safe pH indicator, prepared via lab-scale flexography printing, is durable in humid environments, making it suitable for practical packaging scenarios. The sensor visibly monitored fish spoilage at 4 °C for 9 days. Quality assessment included tracking ΔRGB (total color difference), chemical (TVB-N, pH), and microbiological analyses. Results indicate that the fish samples are fresh up to 4 days of storage at 4 °C; the total viable count (TVC), Pseudomonas growth, TVB-N contents and pH reached: 5.2 (log CFU/ml), 4.31(log CFU/ml), 26.22 (mg N/100 gr sample) and 7.48, respectively. Integrating colorimetric sensor data with mathematical modeling can predict spoilage trends over time. Integrated system offers a smart approach to accurately predicting shelf-life, aiding in optimizing storage conditions, minimizing food waste, and delivering fresh, high-quality fish products to consumers.

The latest research progress on polysaccharides-based biosensors for food packaging: A review

In recent years, polysaccharide-based biosensors have emerged as promising technologies for intelligent food packaging, offering innovative solutions to enhance food quality and safety. This review highlights advancements in designing, developing, and applying these biosensors, particularly those utilizing polysaccharides such as chitosan, cellulose and alginate. Engineered with nanomaterials like ZnO, silver, and carbon nano-tubes demonstrated high sensitivity in real-time monitoring of food spoilage indicators, including pH changes, volatile nitrogen compounds and microbial activity. We discuss the electrochemical properties of these biosensors, highlighting how the integration of electrochemical methods significantly improves their detection capabilities within packaging environments, leading to sensor sensitivity enhancement, greater accuracy, and spoilage detection, ultimately extending the shelf life of perishable food products. Additionally, the review addresses the practical challenges of industrial implementation and explores future research directions for optimizing sensor functionality and scalability. The findings underscore the potential of polysaccharide-based intelligent packaging as a sustainable and effective alternative to conventional methods, paving the way for broader commercial adoption.

Intelligent Biopolymer-Based Films: Promising New Solutions for Food Packaging Applications

The development of biopolymer-based films represents a promising direction in the packaging industry that responds to stringent needs for sustainability, reducing the ecological impact. Traditional fossil-derived polymers present major concerns because of their long decomposition time and their significant contribution to the pollution of the environment. On the contrary, biopolymers such as chitosan, PVA, and PLA offer viable alternatives. This study aimed to obtain an innovative pH indicator for smart packaging using a synthetic non-toxic anthocyanin analogue dye incorporated in bio-based films to indicate meat freshness and quality. The pH-responsive color-changing properties of the dye make it suitable for developing intelligent films to monitor food freshness. The obtained polymeric films were characterized by FT-IR and UV-VIS spectroscopy, and their thermal properties were assessed using thermogravimetric methods. Moisture content, swelling capacity, and water solubility of the polymeric films were also evaluated. The sensitivity of the biopolymer-flavylium composite films to pH variations was studied in the pH range of 2 to 12 and noticeable color variations were observed, allowing the monitoring of the meat's quality damage through pH changes. The pH-responsive films were applied directly on the surface or in the proximity of pork and chicken meat samples, to evaluate their colorimetric response to fresh and spoilt meat. This study can be the starting point for creating more durable packaging solutions leading to a circular economy.

Preparation of Langmuir-Blodgett Films from Quinoxalines Exhibiting Aggregation-Induced Emission and Their Acidochromism

The development of aggregation-induced emission (AIE)-exhibiting compounds heavily relies on our evolving comprehension of their behavior at interfaces, an understanding that still remains notably limited. In this study, we explored the preparation of two-dimensional (2D) sensing films from 2,3-diphenylquinoxaline-based diazapolyoxa- and polyazamacrocycles displaying AIE via the Langmuir-Blodgett (LB) technique. This systematic investigation highlights the key role of the heteroatom-containing tether of 2,3-diphenylquinoxalines in the successful fabrication of Langmuir layers at the air-water interface and the transfer of AIE-emitting supramolecular aggregates onto solid supports. Using both diazapolyoxa- and polyazamacrocycles, we prepared AIE-exhibiting monolayer films containing emissive supramolecular aggregates on silica, mica, and quartz glass and characterized them using ultraviolet-visible (UV-vis) and photoluminescence (PL) spectroscopies, atomic force microscopy (AFM) imaging, and fluorescence microscopy. We also obtained multilayer AIE-emitting films through the LB technique, albeit with increased complexity. Remarkably, by employing the smallest macrocycle N2C3Q, we successfully prepared LB films suitable for the visual detection of acidic vapors. This sensing material, which contains a much lesser amount of organic dye compared with traditional drop-cast films, can be regenerated and utilized for real-life sample analysis, such as monitoring the presence of ammonia in the air and the freshness of meat.

Nanofillers in Novel Food Packaging Systems and Their Toxicity Issues

Background: Environmental concerns about petroleum-based plastic packaging materials and the growing demand for food have inspired researchers and the food industry to develop food packaging with better food preservation and biodegradability. Nanocomposites consisting of nanofillers, and synthetic/biopolymers can be applied to improve the physiochemical and antimicrobial properties and sustainability of food packaging. Scope and approach: This review summarized the recent advances in nanofiller and their applications in improved food packaging systems (e.g., nanoclay, carbon nanotubes), active food packaging (e.g., silver nanoparticles (Ag NPs), zinc oxide nanoparticles (ZnO NPs)), intelligent food packaging, and degradable packaging (e.g., titanium dioxide nanoparticles (e.g., TiO2 NPs)). Additionally, the migration processes and related assessment methods for nanofillers were considered, as well as the use of nanofillers to reduce migration. The potential cytotoxicity and ecotoxicity of nanofillers were also reviewed. Key findings: The incorporation of nanofillers may increase Young's modulus (YM) while decreasing the elongation at break (EAB) (y = -1.55x + 1.38, R2 = 0.128, r = -0.358, p = 0.018) and decreasing the water vapor (WVP) and oxygen permeability (OP) (y = 0.30x - 0.57, R2 = 0.039, r = 0.197, p = 0.065). Meanwhile, the addition of metal-based NPs could also extend the shelf-life of food products by lowering lipid oxidation by an average of approx. 350.74% and weight loss by approx. 28.39% during the longest storage period, and significantly increasing antibacterial efficacy against S. aureus compared to the neat polymer films (p = 0.034). Moreover, the migration process of nanofillers may be negligible but still requires further research. Additionally, the ecotoxicity of nanofillers is unclear, as the final distribution of nanocomposites in the environment is unknown. Conclusions: Nanotechnology helps to overcome the challenges associated with traditional packaging materials. Strong regulatory frameworks and safety standards are needed to ensure the appropriate use of nanocomposites. There is also a need to explore how to realize the economic and technical requirements for large-scale implementation of nanocomposite technologies.

Biopolymer-based intelligent packaging integrated with natural colourimetric sensors for food safety and sustainability

Increasing concerns about global food safety and security demands innovative solutions, particularly in food packaging technologies. This review paper investigates the advanced integration of natural colourimetric sensors with biopolymer-based packaging materials, with a focus on developments over the past 5 years. These sensors change colour in response to environmental stimuli such as oxygen, temperature, pH and relative humidity, intuitively indicating food freshness and safety. The paper emphasizes the recent advancements in using natural colourants, such as alizarin, anthocyanins, betacyanins, chlorophyll, curcumin and shikonin. When combined with either natural or synthetic biopolymers, these colourants contribute to a sustainable and eco-friendly approach to food packaging. Such technological advances could notably decrease the incidence of foodborne illnesses by signaling potential spoilage or contamination, while also addressing food wastage by providing clear indications of edibility. Although challenges remain in sensor longevity and widespread adoption, the prospects for biopolymer-based food packaging with embedded natural colourimetric sensors are promising.

A critical review on the stability of natural food pigments and stabilization techniques

This comprehensive review article delves into the complex world of natural edible pigments, with a primary focus on their stability and the factors that influence them. The study primarily explores four classes of pigments: anthocyanins, betalains, chlorophylls and carotenoids by investigating both their intrinsic and extrinsic stability factors. The review examines factors affecting the stability of anthocyanins which act as intrinsic factors like their structure, intermolecular and intramolecular interactions, copigmentation, and self-association as well as extrinsic factors such as temperature, light exposure, metal ions, and enzymatic activities. The scrutiny extends to betalains which are nitrogen-based pigments, and delves into intrinsic factors like chemical composition and glycosylation, as well as extrinsic factors like temperature, light exposure, and oxygen levels affecting for their stability. Carotenoids are analyzed concerning their intrinsic and extrinsic stability factors. The article emphasizes the role of chemical structure, isomerization, and copigmentation as intrinsic factors and discusses how light, temperature, oxygen, and moisture levels influence carotenoid stability. The impacts of food processing methods on carotenoid preservation are explored by offering guidance on maximizing retention and nutritional value. Chlorophyll is examined for its sensitivity to external factors like light, temperature, oxygen exposure, pH, metal ions, enzymatic actions, and the food matrix composition. In conclusion, this review article provides a comprehensive exploration of the stability of natural edible pigments, highlighting the intricate interplay of intrinsic and extrinsic factors. In addition, it is important to note that all the references cited in this review article are within the past five years, ensuring the most up-to-date and relevant sources have been considered in the analysis.

Advanced technologies in biodegradable packaging using intelligent sensing to fight food waste

The limitation of conventional packaging in demonstrating accurate and real-time food expiration dates leads to food waste and foodborne diseases. Real-time food quality monitoring via intelligent packaging could be an effective solution to reduce food waste and foodborne illnesses. This review focuses on recent technological advances incorporated into food packaging for monitoring food spoilage, with a major focus on paper-based sensors and their combination with smartphone. This review paper offers a comprehensive exploration of advanced macromolecular technologies in biodegradable packaging, a general overview of paper-based probes and their incorporation into food packaging coupled with intelligent sensing mechanisms for monitoring food freshness. Given the escalating global concerns surrounding food waste, our manuscript serves as a pivotal resource, consolidating current research findings and highlighting the transformative potential of these innovative packaging solutions. We also highlight the current intelligent paper-based food freshness sensors and their various advantages and limitations. Examples of implementation of paper-based sensors/probes for food storage and their accuracy are presented. Finally, we examined how intelligent packaging can be an alternative to reduce food waste. Several technologies discussed here have good potential to be used in food packaging for real-time food monitoring, especially when combined with smartphone diagnosis.

Unraveling the Physicochemical Properties and Bacterial Communities in Rabbit Meat during Chilled Storage

The freshness and bacterial communities of fresh and salted rabbit meat during 8 days of refrigerated storage at 4 °C were evaluated. The results showed that the addition of 2% salt significantly changed the color of meat, of which the lightness (L*), redness (a*), and yellowness (b*) were lower than that of fresh meat over time. The pH of all samples increased during storage, and meat with salt addition had lower values in comparison to fresh samples over time. The total volatile base nitrogen (TVB-N) concentration increased rapidly in salt-treated meat but was significantly (p < 0.05) lower than that in meat without salt added before 6 days. Over time, the content of thiobarbituric acid reactive substances (TBARS) showed a progressive trend, but a rapid increase occurred in salted meat. High-throughput sequencing showed that the microflora of each sample had a positive trend in alpha diversity and a negative trend in beta diversity. Bacterial taxonomic analysis indicated that the initial microbial flora for chilled rabbit meat was dominated by Shigaella, Bacteroides, and Lactococcus, and the population of Brochothrix and Psychrobacter increased over time and became the dominant spoilage bacterium. In particular, the addition of salt significantly reduced the abundance of Psychrobacter and Brochothrix. These findings might provide valuable information regarding the quality monitoring of rabbit meat during chilled storage.

Smart packaging films based on corn starch/polyvinyl alcohol containing nano SIM-1 for monitoring food freshness

There is at present an acute need for the construction of biopolymer-based smart packaging material that can be applied for the real-time visual monitoring of food freshness. Herein, a nano-sized substituted imidazolate material (SIM-1) with ammonia-sensitive and antibacterial ability was effectively manufactured and then anchored within corn starch/polyvinyl alcohol (CS/PVA) blend to construct biopolymeric smart active packaging material. The structure, physical and functional performances of CS/PVA-based films with different content of SIM-1 (0.5, 1.0 and 2.0 wt% on CS/PVA basis) were then explored in detail. Results revealed that the incorporated SIM-1 nanocrystals were equally anchored within the CS/PVA matrix owing to the establishment of potent hydrogen-bonding interactions, which produced an obvious improvement in the compatibility of CS/PVA blend film, as well as its mechanical strength, water/oxygen barrier and UV-screening performances. The constructed CS/PVA/SIM-1 blend films further demonstrated superior long-term color stability property, ammonia-sensitive and antibacterial functions. Furthermore, the CS/PVA/SIM-1 blend films were utilized for effectively monitoring the deterioration of shrimp via observable color alteration. The above findings suggested the potential applications of CS/PVA/SIM-1 blend films in smart active packaging.

Multi-Sensing Platform Based on 2D Monoelement Germanane

Covalently functionalized germanane is a novel type of fluorescent probe that can be employed in material science and analytical sensing. Here, a fluorometric sensing platform based on methyl-functionalized germanane (CH3 Ge) is developed for gas (humidity and ammonia) sensing, pH (1-9) sensing, and anti-counterfeiting. Luminescence (red-orange) is seen when a gas molecule intercalates into the interlayer space of CH3 Ge and the luminescence disappears upon deintercalation. This allows for direct detection of gas absorption via fluorometric measurements of the CH3 Ge. Structural and optical properties of CH3 Ge with intercalated gas molecules are investigated by density functional theory (DFT). To demonstrate real-time and on-the-spot testing, absorbed gas molecules are first precisely quantified by CH3 Ge using a smartphone camera with an installed color intensity processing application (APP). Further, CH3 Ge-paper-based sensor is integrated into real food packets (e.g., fish and milk) to monitor the shelf life of perishable foods. Finally, CH3 Ge-based rewritable paper is applied in water jet printing to illustrate the potential for secret communication with quick coloration and good reversibility by water evaporation.

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.

Physical-Vapor-Deposited Metal Oxide Thin Films for pH Sensing Applications: Last Decade of Research Progress

In the last several decades, metal oxide thin films have attracted significant attention for the development of various existing and emerging technological applications, including pH sensors. The mandate for consistent and precise pH sensing techniques has been increasing across various fields, including environmental monitoring, biotechnology, food and agricultural industries, and medical diagnostics. Metal oxide thin films grown using physical vapor deposition (PVD) with precise control over film thickness, composition, and morphology are beneficial for pH sensing applications such as enhancing pH sensitivity and stability, quicker response, repeatability, and compatibility with miniaturization. Various PVD techniques, including sputtering, evaporation, and ion beam deposition, used to fabricate thin films for tailoring materials' properties for the advanced design and development of high-performing pH sensors, have been explored worldwide by many research groups. In addition, various thin film materials have also been investigated, including metal oxides, nitrides, and nanostructured films, to make very robust pH sensing electrodes with higher pH sensing performance. The development of novel materials and structures has enabled higher sensitivity, improved selectivity, and enhanced durability in harsh pH environments. The last decade has witnessed significant advancements in PVD thin films for pH sensing applications. The combination of precise film deposition techniques, novel materials, and surface functionalization strategies has led to improved pH sensing performance, making PVD thin films a promising choice for future pH sensing technologies.

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.

Development of Indicator Film Based on Cassava Starch-Chitosan Incorporated with Red Dragon Fruit Peel Anthocyanins-Gambier Catechins to Detect Banana Ripeness

Banana ripeness is generally determined based on physical attributes, such as skin color; however, it is considered subjective because it depends on individual factors and lighting conditions. In addition, improper handling can cause mechanical damage to the fruit. Intelligent packaging in the form of indicator film incorporated with anthocyanins from red dragon fruit peel has been applied for shrimp freshness detection; however, this film has low color stability during storage, necessitating the addition of gambier catechins as a co-pigment to increase anthocyanin stability. Nevertheless, the characteristics of films that contain gambier catechins and their applications to bananas have not been studied yet; therefore, this study aims to develop and characterize indicator films that were incorporated with red dragon fruit peel anthocyanins and gambier catechins to detect banana ripeness. In this study, the indicator films were made via solvent casting. The films were characterized for their structural, mechanical, and physicochemical properties, and then applied to banana packaging. The results show that the film incorporated with anthocyanins and catechins in a ratio of 1:40 (w/w) resulted in better color stability, mechanical properties, light and water vapor barrier ability, and antioxidant activity. The application of the indicator films to banana packaging resulted in a change in color on the third day of storage. It can be concluded that these films could potentially be used as an indicator to monitor banana ripeness.

Recent Advance of Intelligent Packaging Aided by Artificial Intelligence for Monitoring Food Freshness

Food safety is a pressing concern for human society, as it directly impacts people's lives, while food freshness serves as one of the most crucial indicators in ensuring food safety. There exist diverse techniques for monitoring food freshness, among which intelligent packaging based on artificial intelligence technology boasts the advantages of low cost, high efficiency, fast speed and wide applicability; however, it is currently underutilized. By analyzing the current research status of intelligent packaging both domestically and internationally, this paper provides a clear classification of intelligent packaging technology. Additionally, it outlines the advantages and disadvantages of using intelligent packaging technology for food freshness detection methods, while summarizing the latest research progress in applying artificial intelligence-based technologies to food freshness detection through intelligent packaging. Finally, the author points out the limitations of the current research, and anticipates future developments in artificial intelligence technology for assisting freshness detection in intelligent packaging. This will provide valuable insights for the future development of intelligent packaging in the field of food freshness detection.

Coffee Silverskin Cellulose-Based Composite Film with Natural Pigments for Food Packaging: Physicochemical and Sensory Abilities

To promote a circular economy, the use of agricultural by-products as food packaging material has steadily increased. However, designing food packaging films that meet consumers' preferences and requirements is still a challenge. In this work, cellulose extracted from coffee silverskin (a by-product of coffee roasting) and chitosan were combined with different natural pigments (curcumin, phycocyanin, and lycopene) to generate a variety of composite films with different colors for food packaging. The physicochemical and sensory properties of the films were evaluated. The cellulose/chitosan film showed favorable mechanical properties and water sensitivity. Addition of natural pigments resulted in different film colors, and significantly affected the optical properties and improved the UV-barrier, swelling degree, and water vapor permeability (WVP), but there were also slight decreases in the mechanical properties. The various colored films can influence the perceived features and evoke different emotions from consumers, resulting in films receiving different attraction and liking scores. This work provides a comprehensive evaluation strategy for coffee silverskin cellulose-based composite films with incorporated pigments, and a new perspective on the consideration of the hedonic ratings of consumers regarding bio-based films when designing food packaging.

Consumer-oriented smart dynamic detection of fresh food quality: recent advances and future prospects

Since fresh foods include a significant amount of water, fat, and protein, it is more likely to become infected by microorganisms causing a major loss of quality. Traditional detection techniques are less able to meet customer expectations owing to the limitations of high cost, slow response time, and inability to permit dynamic monitoring. Intelligent non-destructive detection technologies have emerged in recent years, which offer the advantages of small size and fast response at low cost. However, dynamic monitoring of fresh food quality based on intelligent detection technologies on the consumer side has not been rigorously evaluated yet. This paper discussed the application of intelligent detection technologies based on the consumer side in the dynamic monitoring of fresh food freshness, microorganisms, food additives, and pesticide residues. Furthermore, the application of intelligent detection technologies combined with smartphones for quality monitoring and detection of fresh foods is evaluated. Moreover, the challenges and development trends of intelligent fresh food quality detection technologies are also discussed. Intelligent detection technologies based on the consumer side are designed to detect in real-time the quality of fresh food through visual color changes in combination with smartphones. This paper provides ideas and recommendations for the application of intelligent detection technologies based on the consumer side in food quality detection/monitoring and future research trends.

Preparation of pH-Responsive Films from Polyvinyl Alcohol/Agar Containing Cochineal for Monitoring the Freshness of Pork

This study reported the production of pH-responsive films based on 8 wt% polyvinyl alcohol solution/0.2 wt% agar solution incorporated with cochineal-loaded starch particles (CSN) (2, 4, 6 and 8 wt% on agar basis) by a casting process. Results revealed that CSN presented obvious color changes over the pH range of 2-12. FTIR, XRD spectra and SEM micrographs presented that the incorporation of CSN formed new hydrogen bonds with a matrix and a tighter network structure. A certain improvement was observed in the color stability, swelling index and functional properties (antimicrobial and antioxidant activities) but water solubility, water vapor permeability and water contact angle of the pH-responsive films were decreased by the addition of CSN. The release of cochineal was a rate-limiting step following the Korsmeyer-Peppas model. The agar/polyvinyl alcohol film containing 6% CSN (PVA/GG-6) exhibited the best sensitivity for ammonia detection and its limit of detection was 35.4 ppm (part per million) for ammonia. The application trials showed that the PVA/GG-6 film presented different color changes for pork freshness. Hence, these pH-responsive films can be used as potential packaging materials for tracking the freshness of protein-rich fresh food in a non-destructive way.

pH-sensitive smart indicators based on cellulose and different natural pigments for tracing kimchi ripening stages

Five natural pigments including water-soluble [butterfly pea (BP), red cabbage (RC), and aronia (AR)] and alcohol-soluble [shikonin (SK) and alizarin (ALZ)] were extracted, characterized, and loaded onto cellulose for preparing pH-sensitive indicators. The indicators were tested their color response efficiency, gas sensitivity, response to lactic acid, color release, and antioxidant activity. Cellulose-water soluble indicators showed more obvious color changes than alcohol-soluble indicators in lactic acid solution and pH solutions (1-13). All cellulose-pigment indicators exhibited prominent sensitivity to ammonia compared to acidic vapor. Antioxidant activity and release behavior of the indicators were influenced by pigment type and simulants. Kimchi packaging test was carried out using original and alkalized indicators. The alkalized indicators were more effective in showing visible color changes during kimchi storage than the original indicators, and cellulose-ALZ displayed the most distinct color change from violet (fresh kimchi, pH 5.6, acidity 0.45 %) to gray (optimum fermented kimchi, pH 4.7, acidity 0.72 %), and to yellow (over fermented kimchi, pH 3.8, acidity 1.38 %) which followed by BP, AR, RC, SK respectively. The findings of the study suggest that the alkalization method could be used to show noticeable color changes in a narrow pH range for application with acidic foods.

Edible pH sensitive polysaccharide-anthocyanin complex films for meat freshness monitoring

One of the innovative methods for real-time determination of food freshness is the application of pH-indicator sensors, where the color change can be used for the visual detection of acidic/basic volatile compounds formed during product storage due to microbial growth. The aim of the present study is to develop a pH-responsive freshness indicator based on anthocyanins from chokeberry (Aronia melanocarpa Elliot) and black carrot (Daucus carota ssp. sativus var. atrorubens Alef.), incorporated into an alginate/pectin/arabic gum composite film. The resulting films show distinct color changes as the pH varies. The color changes from red (pH 2.0 - 3.0) through pink and pale pink (pH 4.0, 5.0 and 6.0) to purple and blue (pH 7.0 - 8.0). The most distinct is the color transition between pH 6.0 and 7.0 for the black carrot extract and the chokeberry: black carrot mixture (1:3). The applicability of the developed pH-indicator films was demonstrated in chicken meat by tracking the changes during its storage at 4°C for 7 d. The observed results show a distinct color change from pink (day 1-3) to violet and blue on day 7. The developed pH- sensitive films have potential for use in a smart packaging system as a sensor for meat freshness monitoring.

A Facile Strategy for Development of pH-Sensing Indicator Films Based on Red Cabbage Puree and Polyvinyl Alcohol for Monitoring Fish Freshness

This study aimed to develop a novel pH-sensing biopolymer film based on red cabbage puree (RCP) incorporated with polyvinyl alcohol (PVA), which was utilized for monitoring fish freshness during storage at 25 °C. A homogenized RCP suspension with a mean particle size of 12.86 ± 0.03 μm and a total anthocyanin concentration of 292.17 ± 2.65 mg/L was directly used as a film-forming substance and anthocyanin source to blend with PVA, showing visual changes in color and ultraviolet-visible spectra within a pH of 2–12. Rheological and microstructural studies certified the strong interactions and good compatibility between the RCP and PVA, resulting in better mechanical properties and water resistance of the composite film than those of a pure RCP film, but without affecting its pH sensitivity. When used for fish freshness monitoring at 25 °C, the developed RCP/PVA film presented visible color differences from purple to yellow, which corresponded to the spoilage threshold of the total volatile basic nitrogen and the total viable count in fish samples. The study highlights that anthocyanin-rich purees of fruits and vegetables, in this case red cabbage puree, can be fully utilized to develop eco-friendly pH-sensing indicator films for intelligent food packaging.