pH-responsive Films Research Articles

Improved barrier, mechanical and antioxidant properties of pH-responsive film by incorporating dialdehyde starch and anthocyanins into rice protein/sodium alginate

This study aimed to develop rice protein/sodium alginate (RP/SA) pH-responsive films with enhanced properties by incorporating dialdehyde starch (DAS) and Lycium ruthenicum anthocyanins (LRA) and the film was applied to monitor the fruit freshness. The effect of the DAS and LRA content on the morphology, mechanical, water barrier, oxygen barrier, ultraviolet-blocking and thermal stability properties of RP/SA-based films was investigated. The best water vapor permeability of the film was 3.33 g·mm/(m2·d·kPa) with 10 % DAS. With increasing the DAS content from 0 % to 20 %, the tensile strength of the film was increased by 66.3 % compared with that of the film without DAS, while the thermal stability, oxygen barrier and ultraviolet light barrier properties were significantly enhanced. These results may be due to the formation of a compact structure observed by scanning electron microscopy, and stronger hydrogen bonding interactions and lower binding energy in the RP/SA/DAS/LRA system obtained by Fourier transform infrared spectroscopy, X-ray diffraction and molecular dynamics simulation. The RP/SA/DAS10/LRA5 film had 51.6 times higher antioxidant property than the film without LRA and exhibited significant color variations with changes in fresh-cut cantaloupe quality. The developed RP/SA-based films with improved properties are potential intelligent food packaging materials.

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.

Antimicrobial photodynamic inactivation pH-responsive films based on gelatin/chitosan incorporated with aloe-emodin

Using novel active food packaging has gradually become a daily necessity in terms of impeding microbial contamination. Here, an antimicrobial photodynamic inactivation (PDI) pH-responsive film is developed by incorporating aloe-emodin (AE) into a vehicle of gelatin/chitosan (GC). Besides enhancement in hydrophobicity, the well-dispersed crystals of AE in the GC matrix by hydrogen bonding can upgrade the film's mechanical strength and barrier. The matrix is capable of regulating the release of AE in response to acidic stimuli by a combination mechanism of diffusion and polymer relaxation. Being benefitted from the inherent bioactivity of AE and the PDI activity under visible light irradiation (i.e., 456 nm), the target film of GC-AE2 has excellent antibacterial effect towards Staphylococcus aureus and Escherichia coli, showing bacterial viability of 9.93 ± 1.33 % and 14.85 ± 1.16 %, respectively. Furthermore, the film can effectively thwart Botrytis cinerea infection in cherry tomatoes, demonstrating its potential in preventing the microbial spoilage of postharvest fruits.

Fluorescence and pectinase double-triggered chitosan/pectin/calcium propionate/curcumin-β-cyclodextrin complex film for pork freshness monitoring and maintenance

An intelligent and active food packaging film based on chitosan (CS), pectin (P), calcium propionate (CP), and curcumin-β-cyclodextrin complex (Cur-β-CD) was prepared. The CS/P/CP/Cur-β-CD film exhibited improved hydrophobicity (74.78 ± 0.53°), water vapor (4.55 ± 0.16 × 10−11 g·(m·s·Pa)−1), and oxygen (1.50 ± 0.06 × 10−12 g·(m·s·Pa)−1) barrier properties, as well as antioxidant (72.34 ± 3.79 % for DPPH and 86.05 ± 0.14 % for ABTS) and antibacterial (79.41 ± 2.89 % for E. coli and 83.82 ± 3.96 % for S. aureus) activities. The release of CP and Cur could be triggered by pectinase, with their cumulative release reaching 92.62 ± 1.20 % and 42.24 ± 1.15 %, respectively. The CS/P/CP/Cur-β-CD film showed delayed alterations in surface color, pH value, total volatile bases nitrogen, total viable counts, thiobarbituric acid reactive substance, hardness, and springiness of pork. Additionally, the fluorescence intensity of the film gradually decreased. In conclusion, we have developed a pH-responsive film with pectinase-triggered release function, providing a new concept for the design of multi-signal responsive intelligent food packaging.

High-sensitivity intelligent packaging films harnessing rose anthocyanins and hydrophilic silica aerogel for visual food freshness monitoring

Abstract The enhancement of the sensitivity for anthocyanin-based indicator films in food freshness monitoring in real time is important for application. In this study, hydrophilic silica aerogel (SiO2NA) was incorporated into corn starch (CS)/chitosan (CH)/rose anthocyanins (RACNs)-encapsulated potato amylopectin nanoparticles (APNPs) composite film to increase the sensitivity for shrimp freshness detection. The microstructure of films revealed that the gas absorption capacity was improved by amorphous SiO2NA via hydrogen interactions. The pore size (1.74–5.60 times), pore volume (3.92–5.60 times), and specific surface area (2.21–2.34 times) of films increased with the addition of SiO2NA. The sensing of NH3 and pH and the reversibility of films were also reinforced. Meanwhile, the pH-responsive films containing SiO2NA changed visibly in color from purple–red to orange–gray and finally to gray, enabling effective monitoring of shrimp freshness during storage at 4 °C. Thus, anthocyanin-based indicator films with improved sensitivity by adding SiO2NA were successfully designed for monitoring shrimp freshness.

Development of pH-responsive intelligent and active films based on pectin incorporating Schiff base (Phenylalanine/syringaldehyde) for monitoring and preservation of fruits

This study aimed to develop pectin-based films by incorporating Schiff base compounds (SPS) synthesized by phenylalanine and syringaldehyde. The SEM images showed good compatibility between SPS and pectin matrix. The interaction of SPS and pectin matrix was analyzed by FTIR and XRD. Results indicated that the cross-linking effects between SPS and pectin matrix improved the thermal stability, water resistance and light shielding ability of the films. The incorporation of SPS in the films scavenged more than 80% of DPPH and ABTS free radicals, exhibited sustained antimicrobial ability against S. aureus, E. coli and B. cinerea, and showed significant color changes as pH-responsive films. Especially, the intelligent active coating/films inhibited the quality deterioration of cherry tomatoes and fresh-cut mangoes, and monitored the freshness of fresh-cut mangoes during storage. Therefore, the SPS/PE films have a potential application in maintaining fruit quality and monitoring the freshness of fresh-cut fruit.

Intelligent active films of sodium alginate and konjac glucomannan mixed by Lycium ruthenicum anthocyanins and tea polyphenols for milk preservation and freshness monitoring

To achieve real-time monitoring of food freshness, a pH-responsive film based on sodium alginate-konjac glucomannan loaded with Lycium ruthenicum anthocyanins (LRA) was prepared, with the addition of tea polyphenols (TP) to enhance the stability of LRA. The surface structure of the films was observed by AFM. The results of FTIR and molecular docking simulation showed that LRA and TP were bound to polysaccharide by hydrogen bonds. The mechanical properties, barrier properties, and antioxidant/antibacterial properties of the films were significantly improved and the films showed obvious color response to pH. Notably, the AFM images showed TP and LRA could lead to more severe damage to the bacterial structure. The results of molecular docking simulation suggested that TP and LRA could act on different components of the bacterial cell wall, indicating their synergistic mechanism in antimicrobial activity. Moreover, the stability of LRA was improved due to the interactions of TP and polysaccharides with LRA. The aggregates formed by TP and LRA were clearly observed by AFM. Finally, the film showed excellent preservation and freshness monitoring effect in milk. In conclusion, TP-LRA-SA-KGM intelligent film exhibited excellent performance and represented a promising novel food packaging material with potential applications.

Cellulose Nanofiber/Polyvinyl Alcohol-Based pH-Responsive Films Containing Anthocyanin and Carbon Dots

Cellulose Nanofiber/Polyvinyl Alcohol-Based pH-Responsive Films Containing Anthocyanin and Carbon Dots

PH-Responsive Polymer Films Based on Click Polymerization for Food Freshness Monitoring: Non-Toxic, Non-Leaking, and Antibacterial.

With the requirements for food safety and quality, there has been increasing attention on intelligent food packaging, especially pH-responsive intelligent packaging. However, the toxicity of indicators and the vulnerability of composite films to leakage tend to change the composition of food and endanger human health. In this study, 2-allyoxy-1-hydroxy-anthraquinone (AhAQ), a pH-responsive plant dye that was modified from alizarin (AI), was grafted onto the pH-responsive intelligent film (AhAQF) via click polymerization. The obtained AhAQF film shows color change in response to ammonia vapor and exhibits adequate reversibility after treatment with volatile acetic acid. The obtained AhAQF exhibits zero leakage, owing to the covalent immobilization of AhAQ. Thus, the prepared pH-responsive films are non-toxic and antibacterial and show promising application prospects in visual food intelligent packaging and gas-sensitive labels.

Development of a multifunctional food packaging for meat products by incorporating carboxylated cellulose nanocrystal and beetroot extract into sodium alginate films.

Consumers' pursuit for safe meat products is challenging to develop smart food packaging with proper mechanical properties and multifunctional properties. Therefore, this work attempted to introduce carboxylated cellulose nanocrystal (C-CNC) and beetroot extract (BTE) into sodium alginate (SA) matrix films to enhance their mechanical properties and endow them with antioxidant properties and pH-responsive capacity. The rheological results showed the C-CNC and BTE were consistently dispersed in the SA matrix. The incorporation of C-CNC made the surface and cross-section of the films rough but still dense, thus significantly improving the mechanical properties of the films. The integration of BTE provided antioxidant properties and pH responsiveness without significantly changing the thermal stability of the film. The highest tensile strength (55.74±4.52MPa) and strongest antioxidant capacities were achieved for the SA-based film with BTE and 10wt% C-CNC. Additionally, the films possessed higher UV-light barrier properties after incorporating BTE and C-CNC. More notably, the pH-responsive films discolored when TVB-N value exceeded 18.0mg/100g during storage of pork at 4°C and 20°C, respectively. Therefore, the SA-based film with enhanced mechanical and functional properties has a high potential for quality detection in smart food packaging applications.

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.

Intelligent starch/chitosan-based film incorporated by anthocyanin-encapsulated amylopectin nanoparticles with high stability for food freshness monitoring

Corn starch (CS)/chitosan (CH) composite film attained higher stability by incorporating rose anthocyanins (RACNs)-encapsulated potato amylopectin nanoparticles (APNPs) as pH indicator applied for shrimp freshness detection. Apart from the physical-chemical characterization of nanoparticles, the effects of different rose anthocyanins-encapsulated by potato amylopectin nanoparticles (ANPs) contents on the mechanical strength, barrier properties and indicative performance of the films were explored. The results illustrated that the mean size of the ANPs was 192.7 nm and their zeta potential was maintained at −6.70 mV, which had the encapsulation efficiency of up to 89.7% and presented as dendritic. At the same time, the addition of ANPs formed new hydrogen bonds with matrix and a tighter network structure, improving film mechanical properties and its barrier to water vapor and light. The pH-responsive films also exhibited sensitivity to ammonia of 28.67% in 30 min and reversible pH-sensing in five alternating CH3COOH–NH3 exposure cycles. During 14-day storage, CS/CH/ANPs films showed higher stability than those with free RACNs at both 4 °C and 25 °C, whose color stability appeared positively correlated with ANPs. Meanwhile, pH-responsive films with ANPs exhibited obvious color changes from pink-colorless-yellow at 4 °C to reflect the deterioration of shrimp, foreshadowing their excellent potential in smart packaging for seafood freshness monitoring.

Novel intelligent films containing anthocyanin and phycocyanin for nondestructively tracing fish spoilage

Potential pH-responsive films were developed based on incorporating anthocyanin, phycocyanin, and a mixture of anthocyanin-phycocyanin (at a ratio of 3:1 v/v) within composite gelatin (1.5 g / 100 g film solution)/soybean polysaccharide (2.5 g / 100 g film solution) matrices. FTIR and SEM assessments certified the incorporation of the pigments into the film matrices. All the colorimetric films were highly sensitive when exposed to headspace ammonia, except the phycocyanin-incorporated film. Moreover, the fabricated intelligent films were conducted to trace the spoilage process of grass carp. An obvious relationship between the color (L*, a*, b*, ΔE) of indicators and pH, TVB-N, bacterial growth, ATP-related compounds, and K-value of the fish was observed. The film incorporated with mixed Ipomoea nil extract and phycocyanin possessed the best color variations (dark orange, pink, dark blue) with the degree of fish spoilage. Therefore, this intelligent film can be considered a promising sensor for monitoring fish freshness non-destructively.

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.

An active and pH-responsive film developed by sodium carboxymethyl cellulose/polyvinyl alcohol doped with rose anthocyanin extracts

Many anthocyanins were used in active and pH-responsive packaging. The purpose of the study was to prepare an active and pH-responsive sensitive film based on sodium carboxymethyl cellulose/polyvinyl alcohol (CPVA) by a casting process, which contained rose anthocyanin extracts (RAEs) to monitor the freshness of pork. The concentration of RAEs had an important influence on the physicochemical property of RAEs-CPVA films, especially excellent anti-oxidation and light barrier properties. Importantly, the 160-RAEs-CPVA film had a strong response to pH, showing different color at different pHs. Furthermore, when monitoring the freshness of pork stored at 25 °C, the light green color of the 160-RAEs-CPVA film indicated that the freshness of the pork was higher, while the dark green and orange appearance indicated that the pork was spoiled. Therefore, 160-RAEs-CPVA film can be used as a smart indicator for freshness monitoring of pork.

Ionically Self-Assembled Monolayer pH Sensitive Films-Fabrication and their Long Term Response to Varying Temperature Conditions

This study presents the long time effects of varying temperature conditions on pH-responsive films deposited on glass slides. The films were fabricated from Brilliant Yellow and poly (allylamine hydrochloride) through ionically self-assembled monolayer technique using an automated slide strainer. The absorbance of the films was monitored and the effect of varying temperature on the optical properties of the films was studied. We found that as the films are maintained at increasing temperatures their absorbance slightly decreased. As the temperature increased the percent change decreased reaching a plateau. Films kept at low temperatures of 3.24 °C and below freezing (-9.02 °C) had a small increase in absorbance. Finally, we monitored the absorbance of films kept at room temperature over a long time (128 days) and found that the films showed decreased absorbance by 19%.

Developing a simultaneously antioxidant and pH-responsive κ-carrageenan/hydroxypropyl methylcellulose film blended with Prunus maackii extract

к-Carrageenan and hydroxypropyl methylcellulose were used as matrix materials in which the extract of Prunus maackii pomace (EPm) was added to prepare a simultaneously antioxidant and pH-responsive film. EPm effectively increased the flexibility, barrier properties, antioxidant activity and pH-responsiveness of the film. When EPm content was increased to 8%, the elongation at break, heat-sealing strength and oxygen permeability of the film could reach 43.20 ± 2.07%, 1096.39 ± 51.29 N/m and 2.23 ± 0.02 cm3 mm m−2 day−1 atm−1, respectively. Ultraviolet-visible spectra of EPm and the films color changed significantly in the range of pH 2.0–12.0. The κC/Hm/EPmy film obviously slowed the oxidation of lard compared with commercial PE film, and exhibited visible changes from red to purple when TVB-N in the pork reached 13.18 mg/100 g which was close to the freshness limit of 15.00 mg/100 g. Therefore, the κC/Hm/EPmy film can be promisingly used for extending oil shelf life and indicating the protein-rich food quality.

Environment friendly qualitatively responsive ethyl cellulose films as smart food packaging

The present study assessed the applicability of pH indicators in the polymeric films that can be used as smart packaging in the food industries. The pH responsive films using Ethyl cellulose (EC) and azo indicators [i.e., Methyl Orange (MO) and Methyl Red (MR)] have been developed, which have shown remarkable sensitivity towards the pH variation. After activation in different pH range, the colour variation was measured for each film with the CIE Lab methodology. A significant L * [the parameter L * represents the lightness of colours from 0 (dark) to 100 (light), in CIELAB units] variation of EC-MO was seen ranging from 59 (at 0% acid) to 32 (at 60% acid) while for EC-MR, the variation in L * parameter was seen ranging from 89 (at 0% acid) to 32 (at 50% acid). These qualitatively responsive films were further tested for their water absorption capacity and mechanical properties. The water absorption capacity of the EC film incorporated with indicators were observed to be low as compared to the EC-standard film. The incorporation of MO resulted in a regular increase in water absorption capacity range from 34.08–47.11 while the MR incorporated films showed an irregular increase from 41.13–42.13, during a 24 hrs interval. The film with MO also showed a good mechanical property when tested by a micro UTM. The peak load was observed at around 2.7 N. Necking was more observant in the EC film incorporated with MO as compared to other samples, thus showing good plasticity.

Optimized pH-responsive film based on a eutectic mixture-plasticized chitosan

Chitosan (CS, 2g/100mL)/curcumin 1g/100mL in acetic acid aqueous solution were used to prepare films to be used as food indicator. Microcrystalline cellulose (MCC) and a eutectic mixture (DES) were incorporated as reinforcing and plasticizing agents, respectively. The MCC content (133 mas%) and DES composition (7.93 mass%), based on CS dry mass, were optimized. The properties of the DES-plasticized film were compared to those for the unplasticized and glycerol (G)-plasticized CS films. The DES-plasticized film presented initial temperature of thermal decomposition at 267.7°C, dynamic glass transition at 149.3°C, water vapor permeability of 7.21×10−10gs−1m−1Pa−1, water solubility of 3.07% and stress at break of 20.1MPa. The incorporation of MCC contributed to increase the crystallinity of the composite films. Colorimetric tests, carried out in aqueous environments for the DES-plasticized film, revealed accentuated color changes, mainly at pHs higher than pH 8.

Self-assembled pH-responsive films prepared from mussel anchoring threads.

The byssus is a series of collagen-rich fibers securing mussels to surfaces. The complex but elegant heterogeneous assembly of the various proteins in the threads is responsible for their remarkable mechanical properties combining strength and extensibility. Along with the well-known biocompatibility and biodegradability attributed to collagen-based materials, these mechanical properties are highly desirable to produce biomaterials for soft tissue engineering and drug delivery applications. In order to replicate the byssus natural features and properties, we prepared a soluble byssus protein hydrolyzate (BPH) that can generate water-insoluble self-standing films. Atomic force and scanning electron microscopy revealed the presence of self-assembled collagen-like fibrils at the surface of the films. Infrared spectroscopy analysis of the film formation showed that insolubility is caused by the self-assembly of polypeptides from the hydrolyzate into antiparallel β-sheets, aggregated β-strands and collagen triple-helix structures. The mechanical properties and water swelling measurements on the films can be reversibly pH-modulated by modifying the electrostatic interactions between the ∼30 mol% of charged amino acids. Optimal mechanical properties and minimum swelling are obtained at the isoelectric point (pH 4.5). Higher or lower pH treatment reversibly decreases their stiffness and strength and increases their swelling ratio. Altogether, our results show that byssus proteins are an interesting sustainable feedstock for preparing new solid-state pH-tuneable biomaterials.