Entertainment, protection, communication, and convenience are the main functions of packaging ^[1]. The packaging has provided the basis for the initial development of food preservation systems. However, traditional packaging cannot provide real-time information about the freshness or quality of the food. Thus, intelligent packaging has been developed to adapt to this need of the consumer. Intelligent packaging is referred to by the academic community as “a packaging system that is capable of carrying out intelligent functions (such as detecting, sensing, recording, tracing, communicating, and applying scientific logic) to facilitate decision-making to extend shelf life, enhance safety, improve quality, provide information, and warn about possible problems” ^[2]. Intelligent packaging monitors the quality/safety condition of a food product and can provide early warning to the consumer or food manufacturer. An intelligent packaging system contains small smart devices that are capable of acquiring, storing, and transferring information about the functions and properties of the packaged food. Intelligent packaging includes time–temperature indicators, gas detectors, and freshness and/or ripening indicators ^[3]. Color sensitivity indicators can reflect changes in food quality in real-time based on changes in pH value leading to color changes.

During meat storage, protein decomposition will produce a large number of volatile organic amines such as trimethylamine, which will cause a rise in pH value in the packaging. Therefore, many researchers use pH-sensitive materials to prepare color-sensitive intelligent packaging that can reflect the freshness of meat. However, most of the synthetic pH-sensitive materials have toxicity, posing potential safety hazards when used as food packaging. Currently, the development of active packaging containing anthocyanins occupies an important position in the field of food engineering ^[4]. Anthocyanins are non-toxic and harmless natural pigments and can show different colors with the change in pH value ^[5].

2. Barrier Properties

The shelf life of packaged food products is also influenced by the barrier properties of polymer films. The key to maintaining food quality through packaging film is to prevent molecular transfer between food and the environment [8]^[6]. By measuring these characteristics, we can know the permeability of molecules such as O₂ or CO₂, water vapor, organic vapor, or liquid through thin films [9]^[7].

2.1. Water Vapour Permeability (WVP)

Water vapor permeability represents the barrier property of film against water vapor and is the most extensively studied property of food packaging films because of the important role of water in deteriorating reactions, keeping the freshness, or preventing dehydration [9]^[7]. Chen et al.’s research [10]^[8] showed that the incorporation of red cabbage anthocyanins (RCAs) into chitosan (CS)/oxidized chitin nanocrystals (OCN) composites significantly decreased the WVP, from 1.89 × 10⁻¹⁰ to 1.56 × 10⁻¹⁰ gm⁻¹s⁻¹Pa⁻¹. On the contrary, Yan et al. [11]^[9] presented that the addition of Kadsura coccinea extract with anthocyanins (KC) significantly increased the WVP of chitosan (CS), gelatin (GL), and sodium alginate (SA) films. The incorporation of dragon fruit skin extract with anthocyanins (DFSE) increased the WVP of gelatin films [12]^[10]. Similar findings were reported by Wen et al. [13]^[11], Naghdi et al. [14]^[12], and Roy et al. [15]^[13].

2.2. Oxygen Permeability (OP)

The oxygen resistance of thin films is determined by the strength of their oxygen permeability ^[4]. Oxygen permeability is one of the essential factors to maintain food quality and safety [10]^[8]. Research showed that the incorporation of red cabbage anthocyanins (RCAs) into chitosan (CS)/oxidized-chitin nanocrystals (OCN) composites significantly declined the oxygen permeability values from 1.81 to 1.49 cm³ m⁻²atm⁻¹ [10]^[8]. They believe that the hydrogen bonds formed between RCAs and CS/OCN composite materials, as well as the large aromatic rings in the RCA’s skeleton structure, make the microstructure network of the composite membrane very dense, resulting in a lower affinity for water molecules, leading to these changes. In addition, the cross-network effect in the composite membrane is also affected by the unique molecular geometry of the RCA phenol skeleton, which reduces oxygen permeability by limiting the movement of oxygen molecules.

2.3. Light Barrier Property

UV–vis light barrier property of film is very important for light-sensitive food packaging [17]^[14]. The characteristic UV–vis light transmittance property of anthocyanins was introduced in some studies [11,18,19,20,21,22,23]^{[9][15][16][17][18][19][20]}. Membranes containing saffron anthocyanins (intelligent colorimetric membranes) have stronger UV barrier properties than methylcellulose and methylcellulose/chitosan nanofiber membranes (λ < 370 nm) and significantly reduced transparency [23]^[20]. Yan et al. [11]^[9] reported that Kadsura coccinea (KC) extract significantly (p < 0.05) declined the light transmittance due to the refracting and scattering. Huang et al.’s study also obtained similar results [18,21]^[15][18]. The reduction in light reflection and scattering is caused by the reduction, and the aromatic rings in anthocyanin phenolic compounds and their binding to the membrane are the reasons for this phenomenon [19]^[16]. Furthermore, the addition of TiO₂ can increase the UV-vis light barrier property due to the mutual polymerization between BPPE and TiO₂ [17]^[14]. By using high-opacity films, packaged food can be prevented from being exposed to visible light and ultraviolet radiation, thereby reducing nutritional loss and inhibiting oxidation processes [24]^[21].

3. Stability

The stability of packaging is a key factor in improving the quality and safety of food, extending its shelf life, and providing consumers with economical and convenient products [25,26,27]^[22][23][24]. Total color difference and relative color change in different situations were usually tested to monitor the color stability of the films. The characteristics of thin films are usually studied by plotting the thermal degradation curves of thin films using thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) [28]^[25]. Certain thermal and color stability are the basic requirements of intelligent food packaging materials.

4. Mechanical Properties

The suitable mechanical strength of the composite film is needful to guarantee the integrity and sustainability of the food. The strength of the packaging film is reflected by the numerical value of tensile strength (TS), and the flexibility of the packaging film is reflected by the numerical value of elongation at break (EAB). Therefore, TS and EAB are two important indicators of strength. TS and EAB are often used as mechanical criteria when specifying packaging films. TS is the amount of load or stress that can be handled by a composite film before it stretches and breaks. In addition, EAB is also known as the best potential indicator for reflecting membrane resistance to changes in membrane length [23]^[20]. Wang et al.’s study showed that polyvinyl alcohol/methylcellulose (PVA/MC) membranes loaded with 5% black wolfberry (BW) anthocyanins have excellent mechanical properties, with significantly higher elongation at break (145.2%) and tensile strength (18.0 MPa) than PVA/MC membranes loaded with 2.5% and 10% anthocyanins [30,36]^[26][27]. The mechanical properties of polyvinyl alcohol/methyl cellulose/5% Black Wolfberry anthocyanins (PVA/MC/BW-5%) provide enhanced tensile strength and flexibility and allow the transfer of stress to the cellulose chains because of their good dispersion and compatibility with the polymers. Yan et al.’s [11]^[9] research showed that 15 wt % anthocyanin-rich Kadsura coccinea extract (KC) can significantly increase the tensile strength and elongation at the break of chitosan (CS), gelatin (GL), and sodium alginate (SA) film due to a good interaction of molecular chains between KC molecules and the composite matrix. On the contrary, RCAs changed the mechanical properties, resulting in a decrease in TS of the colorimetric film and an increase in EAB [30,37]^[26][28]. They believe that the state of hydrogen bonds within the polymer chain is enhanced by the plasticization and interaction of RCAs, which enhances the mobility of molecules and disrupts the integrity network [34]^[29]. Rezaie et al. [38]^[30] presented that the addition of violet basil (Ocimum basilicum L.) anthocyanin into arabic gum-carboxy methyl cellulose composite film decreased the EAB. This may be related to the content and composition of anthocyanins. The tensile strength of the membrane solution with added anthocyanins increased from 19 to 23.64 MPa, but when the addition amount exceeded 60 mg/100 g, the tensile strength gradually decreased with the increase in anthocyanin content. Therefore, the mechanical strength of anthocyanin-loaded membranes is influenced by the molecular interaction between anthocyanins and polymers, the type of polymer, the type of anthocyanins, and the concentration of anthocyanins. Electrostatic heavy pulses and hydrogen are key interactions related to the binding of anthocyanins and membrane components. The preparation and storage conditions of the film also affect the mechanical properties of pH-sensitive base films (Table 1).

5. pH-Sensitive

5.1. Sources of Anthocyanins and pH-Sensitive

The pH-sensitivity property is the most important property of anthocyanins in intelligent packaging. The pH sensitivity of anthocyanins from different plant extracts was different. Kan et al. [22]^[19] extracted and determined the total anthocyanin content and pH sensitivity from 14 plants by the same methods. They showed different color-changing profiles with pH increasing due to the different anthocyanin content and composition in the extract in 14 plants. Rawdkuen et al. [75]^[31] extracted anthocyanins from red cabbage, sweet potato, rose eggplant, butterfly pea, fruit shell, bamboo, and red dragon fruit, and then prepared gelatin-based intelligent films. According to the experimental results, anthocyanins extracted from butterfly peas have the highest pH sensitivity. Butterfly pea, purple potato, red cabbage, blueberry, black wolfberry, lycium ruthenicum, mulberry, roselle and saffron petal are the most anthocyanin sources of the published research articles of pH-sensitive colorimetric indicator film. The result showed that the pH-sensitivity property varies in different sources of anthocyanin solution. The color changes and pH sensitivity of anthocyanins-rich solutions are closely related to the content and composition of anthocyanins [22,58]^[19][32]. The anthocyanin source greatly influences the pH sensitivity of the film due to the different anthocyanin content and composition [49]^[33]. In order to develop a visual freshness indicator film and explore its feasibility in the monitoring of clam freshness, Wang Xin et al. prepared five intelligent indicator films with pH-sensitive blueberry anthocyanidin as the indicator and chitosan as the matrix through compound gelatin, nisin and rosemary essential oil, and studied their pH sensitivity, color responsiveness, microstructure, barrier performance, mechanical properties, water content, water solubility antioxidant and antibacterial properties. Results show that the color reaction of blueberry anthocyanidin solution was obvious in the pH range of 3–12. As the membrane components increase, the roughness of the membrane microstructure increases, while the water vapor barrier performance gradually decreases. The addition of nisin and rosemary essential oil significantly enhanced its antioxidant and antibacterial abilities. The chitosan/nisin/rosemary essential oil blueberry anthocyanidin (CSNR–ATH) film has excellent ultraviolet barrier performance and low water solubility. The CSNR-ATH film can sensitively reflect the changes in the freshness of clams during refrigeration. The composite indicator film has changed from light green to yellow-green. It was found that the chitosan-based blueberry anthocyanidin intelligent indicator film provided a new choice for the fresh-keeping monitoring of clams [76]^[34]. Shiyang F et al. developed a food-grade milk freshness indicator label that can be soaked in liquid, using ethyl cellulose as a polymer matrix and blueberry anthocyanins as pH-sensitive infectious substances, to monitor the freshness of milk. The results showed that when the amount of anthocyanins added was 10% of the mass fraction of ethyl cellulose, the indicator label displayed light purple in fresh milk and pink in spoiled milk. This soaking indicator has good application value and development prospects in indicating the freshness of milk and also proves the broad application prospects of anthocyanin pH sensitivity in food [77]^[35].

5.2. Extraction of Anthocyanins and pH-Sensitive

The extraction of anthocyanins is the premise of obtaining pH-sensitive films. Anthocyanins are unstable and easily affected by changes in pH, oxidation, and high temperatures. In addition to obtaining more anthocyanins to the maximum extent, the extraction must ensure the activity of anthocyanins. Solvent extraction is the most common method. The techniques commonly include maceration, digestion, decoction, percolation and filtration. These techniques are based on the use of different types of solvents and/or heat. Methanol, ethanol, water, acetone or mixtures thereof are the common solvents used to extract anthocyanins. Generally, a mixture of acidified organic solvent or acidified water is used during extraction procedures because it can help stabilize the flavylium cation, which is stable in highly acidic conditions (pH~3). Compared with conventional extraction methods, new and promising extraction techniques have been introduced over the years. These techniques are more environmentally friendly and have important industrial focuses, as they aim to improve extraction efficiency and yield. However, they have not been employed on a massive scale yet. Among these extraction methods, the most applied techniques to extract anthocyanins are ultrasound-assisted extraction (UAE), microwave-assisted extraction (MAE), supercritical fluid extraction (SFE), high-pressure liquid extraction (HPLE), pulsed electric fields (PEFE), high voltage electrical discharge (HVED) and enzyme assisted extraction (EAE).