3.1. Response surface method
One of the design concepts of experiments is the response surface method (RSM). This method is useful for analyzing experiments in which one or more independent variables (as responses) are affected by many variables and the goal is to optimize the response. One of the advantages of using this method and Design Expert software, in addition to reducing the number of experiments, is the possibility of providing a mathematical relationship between the independent variable and the dependent variables. In addition, in this method, in addition to numerical variables, it is possible to study the effect of qualitative variables. In this study, the response surface statistical method was used to investigate the effect of copper oxide, zinc oxide and polyaniline nanoparticles on the physicochemical properties of the film. Using the response surface method, three-dimensional curves and mathematical relationships between the independent variables in the film (copper oxide nanoparticles, zinc oxide and polyaniline) and the physicochemical properties of the film was analyzed. Table 2 shows the mathematical relationships and regression coefficients and the adjusted regression coefficients between the independent variables and the answers obtained.
Table 2
Mathematical models and regression coefficients between independent and dependent variables
Response
|
Equation
|
R2
|
AdjR2
|
Thickness (µm)
|
= 0.1592 + 0.0186*ZnO + 0.0269*CuO − 0.005*ZnO*CuO
|
0.93
|
0.90
|
Water uptake (%)
|
= 2.6194 − 0.3977*ZnO + 0.0488*CuO
|
0.88
|
0.85
|
Moisture content (%)
|
= 12.1936–1.26092*ZnO − 1.12203*CuO + 0.2044*ZnO*CuO + 0.2980*ZnO^2 + 0.3536*CuO^2
|
0.84
|
0.75
|
Solubility (%)
|
= 7.50897 − 0.4666*ZnO-0.43111*CuO
|
0.91
|
0.90
|
WVP
|
= 2.4691e-06-4.6533e-07*ZnO-4.1545e-07*CuO + 1.1772e-07*ZnO*CuO-3.2409e-08*ZnO^2-4.2794e-08*CuO^2
|
0.99
|
0.98
|
Antioxidant activity (%)
|
= 16.4711 + 3.3394*ZnO + 3.4327*CuO-0.94333*ZnO*CuO
|
0.92
|
0.89
|
Electrical resistance (MΩ)
|
= 8456.95 + 8155.52*ZnO + 7786.63*CuO-1655.56*ZnO*CuO-1739.62*ZnO^2-2290.73*CuO^2
|
0.88
|
0.85
|
3-2- Thickness, WU, MC, water solubility and WVP
One of the most important problems in using biodegradable polymers in food packaging, especially watery food products (such as juices and concentrates), is the high sensitivity of biodegradable polymers to water. Most of these biodegradable polymers lose their mechanical and physical properties in the presence of water molecules. Polylactic acid is one of the few biodegradable polymers that has a very good waterproof property that makes this polymer suitable for use in the packaging of food products. The thickness of the packaging films is one of the factors that affect other film properties such as water absorption, moisture content, water solubility, water vapor permeability and mechanical properties, so check and control this factor in films is important. The aqueous properties of packaging films affect the ability of these films to package food products. Many food products are affected by microbial and bacterial spoilage in wet conditions. If the packaging film has a high solubility in water (such as gelatin films and carboxymethylcellulose films), they will not be useful in the packaging of watery food products such as juices because they will dissolve and disappear in the food product. The packaging film must effectively control the penetration of water vapor into the packaging of moisture-sensitive foods to minimize microbial spoilage.
Figure 2 shows the effect of the amount of copper oxide and zinc oxide nanoparticles on the thickness and aqueous properties of polylactic acid/polyaniline films. As it is known, the addition of copper oxide and zinc oxide nanoparticles has a significant effect on increasing the film thickness and the highest thickness is related to the film contains 3% copper oxide and 3% zinc oxide. The increase in thickness in the presence of nanoparticles is due to the fact that these particles increase the solid matter in the film and lead to an increase in the thickness of the film. Among the aqueous properties of polylactic acid/polyaniline films, water vapor permeability is reduced in the presence of both copper oxide and zinc oxide nanoparticles, which is probably due to the fact that these nanoparticles reduce the voids between polymer chains. Therefore, water vapor molecules take up more space to pass through the film and the permeability to water vapor is reduced. Copper oxide and zinc oxide nanoparticles also reduce solubility, moisture content and moisture absorption. Moisture absorption of polylactic acid films (due to the hydrophobic nature of this polymer) is probably due to the presence of possible pores in the surface and width of the film that cause water molecules to penetrate into these voids. However, with the addition of copper oxide and zinc oxide nanoparticles to the film, the pores are filled and water molecules will not be able to penetrate in the film structure. In 2020, Asadi and Pirsa made a polylactic acid film modified with lycopene pigment and titanium oxide nanoparticles and studied its thickness and aqueous properties. The results of their research are in good agreement with the results of the present studies (Asadi & Pirsa, 2020). In 2012, Delpouve et al. investigated the water vapor barrier properties of polylactic acid films. Their research results confirm the results of the present study (Delpouve et al., 2012).
Figure 2
3.3. Antioxidant and electrical properties
The antioxidant properties are one of the most important properties of active films that are considered in the packaging of oxidation-sensitive food products. Active antioxidant films can easily increase the shelf life of food products such as oils by absorbing oxidizing agents. Antioxidants such as essential oils and metal oxide nanoparticles by contact with oxidizing agents quench them and protect the food from the oxidation process.
Figure 3 shows the effect of the amount of copper oxide and zinc oxide nanoparticles on the antioxidant properties and electrical resistance of polylactic acid/polyaniline films. As it is known, both copper oxide and zinc oxide nanoparticles have greatly increased the antioxidant properties of the film, and in films where copper and zinc oxide nanoparticles are present simultaneously, these two nanoparticles have a synergistic effect and the highest antioxidant property are observed. The antioxidant properties of various nanoparticles such as titanium oxide, copper oxide and zinc oxide have been confirmed by various researchers. The antioxidant properties are investigated as the ability to absorb DPPH free radicals. The adsorption of these free radicals occurs either through oxidation/reduction or electrostatic reactions by antioxidant agents or these radicals are physically adsorbed on the agents. In the case of copper oxide and zinc oxide nanoparticles, it can be said that the deactivation of free radicals can be due to both electrostatic interactions between nanoparticles and DPPH radicals and can occur due to the physical adsorption of free radicals on the surface of nanoparticles. Das et al. (2013) investigated the antioxidant properties of metal oxide (copper oxide) nanoparticles. The results of their research confirm the results of this research (Das et al., 2013).
Electric conductive films can be used as smart films in food packaging. The electrical conductivity of these films changes in contact with chemical gases produced in the food product or due to mechanical pressure created in the food product. By observing the changes in electrical conductivity and making the relationship between storage conditions (temperature and storage time), chemical properties of the food and the electrical properties of the conductive film, mathematical models can be obtained. These mathematical models can help to detect the shelf life, expiration date and chemical conditions of the product. The primary electrical properties of conductive films can affect the sensitivity of these films to environmental changes. Therefore, it is very important to study the initial electrical properties of these films. The pure polytlactic acid film had no electrical conductivity while polylactic acid/polyaniline/copper oxide/zinc oxide film had a good electrical conductivity (500 kΩ) which showed polyaniline induced a very good electrical property of the film. As can be seen from the curves of the effect of copper oxide and zinc oxide nanoparticles on the electrical resistance of films, these nanoparticles did not have a significant effect on the electrical property of the film, which indicates that by placing polyaniline between polylactic acid chains, significant electrical conductivity acid is created in the film that the placement of copper oxide and zinc oxide nanoparticles on polyaniline nanoparticles does not prevent the transfer of electrical charges. Electrical conductivity in conductive polymers, such as polyaniline, is due to the mobile carriers that are obtained by π electronic system. By removing electrons from the valence band (positive charge), or by adding electrons to the conduction band (negative charge), an electric charge is induced in the polymer chain and causes an important change in the position of the atoms at the charge site that finally changes electrical conductivity. In 2018, Pirsa et al. used polypyrrole to produce smart electrical conductive films based on bacterial cellulose film. They confirmed the establishment of good electrical conductivity in biodegradable films in the presence of polypyrrole. The results of their research are completely consistent with the results of the present study (Pirsa et al., 2018).
Figure 3
3.4. SEM and FTIR
Figure 4 shows the SEM images and FTIR spectra of polylactic acid films and their composites with polyaniline, copper oxide and zinc oxide. As can be seen from the SEM images, the pure polylactic acid film has a smooth surface with surface cracks in some places. In the polylactic acid/polyaniline composite film, the presence of polyaniline grains on the polymer surface in the dimensions of 50 to 120 nm is observed. In this film, the polyaniline beads are interconnected like rosary beads. The presence of polyaniline on the surface of polylactic acid can affect the thickness, water vapor permeability, and mechanical properties. The surface morphology of PLA/PAn/CuO, PLA/PAn/ZnO and PLA/PAn/CuO/ZnO films are very similar. In these films, the dispersion of copper oxide and zinc oxide particles in the dimensions of 20 to 50 nm on polyaniline beads is clear. PLA/PAn/CuO films have a more regular structure PLA/PAn/ZnO and PLA/PAn/CuO/ZnO, which is probably due to the fact that copper oxide has a more regular structure than zinc oxide. Wang et al. (2020) investigated the structure of polylactic acid film and its composites with polyaniline. The results of the present study are consistent with the results of Wang et al in terms of surface morphology and SEM images (Wang et al., 2020b). Tang et al. (2020) investigated the structure of polylactic acid film and its composites with zinc oxide nanoparticles. The results of Tang et al. in terms of surface morphology and dispersion of nanoparticles on the polymer surface confirm the results of the present study (Tang et al., 2020).
To study the chemical structure of the prepared films, their FTIR spectra were studied. In the pure polylactic acid spectrum, the peak of 3494 is related to the tensile vibration of the -OH groups. Peak at 2931 is related to interatomic vibrations in the R-CO-OH and C = C-OH groups. Peak at 2082 is related to C = O vibrations. Peak at 1719 is related to the vibrations of the acidic functional groups (RCOOH). Peak at 1453 shows the tensile vibrations of the CH3 and CH2 groups. The high peak at 1052 also confirms the C-O tensile vibrations in acidic groups. Peak at 753 also shows off-screen C-H vibrations. In the spectra of polylactic acid composite films with polyaniline, copper oxide and zinc oxide, all peaks related to the spectrum of pure polylactic acid are observed, with the difference that the peaks belonging to different functional groups shift to different wave numbers and some new peaks have been generated in these spectra, indicating that electrostatic interactions between polylactic acid, polyaniline, copper oxide, and zinc oxide have been established. These changes also confirm the presence of polyaniline, copper oxide and zinc oxide in the structure of the polylactic acid film. For example, peak at 3494 (corresponding to -OH vibrations) in the polylactic acid/polyaniline spectrum shifted to 3436 (corresponding to -OH and -NH vibrations). This almost extreme shift indicates that polyaniline strongly affects the chemical structure of polylactic acid. Also in the polylactic acid/polyaniline spectrum, a new peak is created in the 1478 cm-1, which is related to N-O vibrations, which indicates the interaction between N related to polyaniline and O related to polylactic acid. Also on this spectrum, peak at 681 is related to RNH2 and R2NH vibrations. In the polylactic acid/polyaniline/copper oxide spectrum, the peak intensities of the OH and NH vibrations (at 3435 cm-1) are significantly reduced, indicating that the CuO covers the OH and NH groups. In general, nanoparticles of copper oxide and zinc oxide significantly shifted the peaks belonging to the functional groups of polylactic acid/polyaniline composite film to different wavenumbers. These changes in the wavenumbers and the intensity of the corresponding peaks indicate the significant impact of these nanoparticles on the chemical structure of the polylactic acid/polyaniline composite film. Wong et al. (2020) investigated the structure of polylactic acid film and it’s composite with polyaniline using the FTIR spectrum. The results of their study on the peaks of the functional groups of polylactic acid and polyaniline confirm the results of the present study (Wong et al., 2020). Therias et al. investigated the effect of zinc oxide nanoparticles on the FTIR spectrum of polylactic acid and reported that the nanoparticles had a significant effect on the wave number and peak intensities of the polylactic acid functional groups. The results of the FTIR spectra of the present study are in good agreement with the results of Therias et al. (Therias et al., 2012).
Figure 4
3.5. XRD, TGA and DTA
Figure 5 shows the XRD, TGA and DTA spectra of polylactic acid film and its composites. The XRD spectrum of pure polylactic acid film shows a wide peak at 2θ of 18 to 20 degrees, indicating the amorphous structure of polylactic acid film. In the spectrum of polylactic acid/polyaniline films, three sharp and well-defined peaks are observed in 2θ of 15, 20 and 25 degrees. These three sharp peaks are mounted on a wide peak corresponding to polylactic acid. Sharp peaks are related to the presence of polyaniline in the film structure. These three peaks also indicate that polyaniline (in the form of emeraldine) has a crystalline structure and these peaks correspond to crystal plates 113, 121 and 322, respectively. In the spectra of polylactic acid/polyaniline/copper oxide, polylactic acid/polyaniline/zinc oxide and polylactic acid/polyaniline/copper oxide/zinc oxide, the wide peak corresponds to the amorphous structure of polylactic acid has been disappeared and peaks related to the crystalline structure of polyaniline, copper oxide and zinc oxide are clearly visible. Peaks related to copper oxide are seen in 2θ of 32, 36, 38, 44 and 48 degrees, which indicates the presence of copper oxide nanoparticles in the structure of composite films. Peaks related to zinc oxide can be seen in 2θ of 32, 37, 42, 53, 63 and 65 degrees, which confirms the presence of crystalline zinc oxide nanoparticles in the structure of composite films (Ahmad, 2019). Almost all peaks related to polyaniline, copper oxide and zinc oxide can be clearly seen in the polylactic acid/polyaniline/copper oxide/zinc composite film. This film shows the highest number of peaks. The general conclusion of XRD spectra is that polyaniline, copper oxide and oxide have improved polylactic acid crystalline structure. Padmapriya et al. (2018) studied the structure of polyaniline by XRD spectrum. They report that polyaniline has a crystalline/semi-crystalline structure, which confirms the results of the present study (Padmapriya et al., 2018). de Souza et al. (2018) investigated the structure of polyaniline-copper oxide by XRD technique. The results of their report confirm the results of the present study in terms of peaks appearing for copper oxide and polyaniline-copper oxide composites (de Souza et al., 2018).
Examination of the TGA spectra of all films shows that in all films the thermal decomposition of the film occurs in two stages. The first stage of film decomposition occurs at a temperature of approximately 70 to 150°C. The rate of thermal decomposition of the films in the first stage is 5 to 10% of the film weight. The thermal decomposition of the first stage is probably due to the evaporation of chloroform molecules trapped in the film structure because chloroform is used as a solvent for lactic acid monomers in the film preparation stage. Also, the thermal decomposition of the first stage can be due to evaporation or thermal decomposition of thermally unstable materials in the film structure. The second stage of thermal decomposition occurs at a temperature of 200 to 350°C. The second stage of thermal decomposition is related to the structural decomposition of polylactic acid and polyaniline films. By comparing different composites, it was found that the polylactic acid/polyaniline composite film (decomposition at 344) has higher thermal stability than the pure polylactic acid film (decomposition at 269). The reason for the higher stability of polylactic acid/polyaniline composite films than pure polylactic acid films is due to this fact that strong interactions between polyaniline and polylactic acid occur during polyaniline synthesis.
Examination of TGA and DTA results also showed that the addition of copper oxide and zinc oxide nanoparticles to the polylactic acid/polyaniline composite reduced the thermal stability, which is probably due to the fact that copper oxide and zinc oxide nanoparticles are located between polylactic acid- polyaniline polymer chains and reduce the electrostatic interactions between the polymer chains and thus reduce the thermal resistance. Therefore, polylactic acid/polyaniline film, which simultaneously contains copper oxide and zinc oxide nanoparticles, has the lowest thermal stability compared to polylactic acid/polyaniline film, which shows the synergistic effect of copper oxide and zinc oxide nanoparticles in reducing thermal resistance of film. Also, by examining the percentage of film decomposition in the second stage, it was observed that by adding copper oxide and zinc oxide nanoparticles to the film, the amount of thermal decomposition of the film is reduced. Given that the nanoparticles of copper oxide and zinc oxide have very high decomposition temperatures, this result is acceptable and logical. Wang et al. (2020) investigated the effect of polyaniline on the thermal stability of polylactic acid films using TGA spectroscopy. They reported that polyaniline increases the thermal stability of polylactic acid, which confirms the results of the present study (Wang et al., 2020b). Marra et al. (2017) investigated the effect of titanium oxide and zinc oxide nanoparticles on the thermal resistance of polylactic acid. They concluded that the nanoparticles somewhat reduce the thermal stability of the film. The results of Marra et al. (2017) confirm the results of the present study (Marra et al., 2017).
Figure 5
3.6. Antibacterial properties
Antibacterial active films used in food packaging delay food spoilage by inhibiting bacterial growth and increase food shelf life. Figure 6 shows the growth inhibition zone of two types of bacteria, Gram-negative (Escherichia coli) and Gram-positive (Staphylococcus aureus) in the presence of polylactic acid film and its composites. As it turns out, the pure polylactic acid film does not have any specific antibacterial properties against any of the bacteria. The addition of all three materials polyaniline, copper oxide and zinc oxide have created antibacterial properties in the polylactic acid film. The highest halo of non-growth is observed in the polylactic acid film containing all three substances polyaniline, copper oxide and zinc oxide, which shows that these three substances have strengthened the effect of each other. Also, the antibacterial properties of films containing polyaniline, copper oxide and zinc oxide against Gram-positive bacteria (Staphylococcus aureus) were higher than Gram-negative bacteria (Escherichia coli), which is due to the structure of the bacterial cell wall. The physical and chemical structure of Gram-negative bacteria are more complex, and antibacterial agents are less able to penetrate into the cells of Gram-negative bacteria and therefore have less ability to inactivate them. Gram-positive bacteria, on the other hand, have a simpler cell wall and antibacterial agents easily penetrate and inactivate the bacteria. Compared to Gram-positive bacteria, Gram-negative bacteria are more resistant to antibiotics because of their impermeable wall. The antibacterial property of polyaniline is probably due to the fact that the surface of polyaniline is filled with positive and negative electric charges as well as electrical cavities, which can establish electrostatic interactions with the relative electric charge of the bacterial surface and disrupt bacterial activity. Kucekova et al. (2014) investigated and confirmed the antibacterial properties of polyaniline. Their research results confirm the results of the present study (Kucekova et al., 2014). Widiarti et al. (2017) synthesized ZnO-CuO nanoparticles and investigated their antimicrobial properties. Their research results confirm the results of the present study (Widiarti et al., 2017).
Figure 6
3.7. Active and smart packaging of orange juice with polylactic acid film
3.7.1. The effect of active film on chemical quality control of orange juice
Figure 7 shows the chemical factors (pH, ascorbic acid and browning index) of orange juice packaged with polylactic acid films during 56 days of storage. As can be seen from the curves, the pH of orange juice (packaged with all 5 types of polymers) increased relative during storage. The increase in pH is probably due to the growth of aerobic mesophilic microorganisms and the degradation of vitamin C during storage. The highest pH changes are related to the sample packaged with pure PLA and the lowest pH changes are related to the sample packed with PLA/PAn/CuO/ZnO. This result shows that nanoparticles of copper oxide, zinc oxide and polyaniline have caused the chemical stability of orange juice. In other words, nanoparticles of copper oxide, zinc oxide and polyaniline have controlled the factors that produce OH- ions during storage. Usually the amount of ascorbic acid in orange juice is in the range of 26–84 mg/100 g of extract (Plaza et al., 2006). In the case of ascorbic acid, the amount of ascorbic acid decreases with increasing storage time, which reduces the nutritional value of the juice. The reason for the decrease in ascorbic acid during storage is probably related to the oxidation process of this acid. Also, by examining ascorbic acid curve, it was found that the type of polymer used for packaging does not have a significant effect on the stability of ascorbic acid. By studying the effect of polyethylene bags on the shelf life of orange juice, Fellers reported that the decrease in ascorbic acid of orange juice was due to oxidation reactions during storage (Fellers, 1998).
Food browning is the process by which a food turns brown during a series of chemical reactions. The pathways involved in the browning process are specifically divided into two main categories: the enzymatic and non-enzymatic browning pathways. Enzymatic browning is one of the most important reactions that occurs in most fruits, vegetables as well as seafood. These processes affect the taste, color and value of these foods. In general, this type of browning is a chemical reaction between a polyphenol oxidase, catechol oxidase, and other enzymes that converts natural phenols to melanin and benzoquinone. Performing the process of enzymatic browning (also called food oxidation) requires available oxygen. This process begins with the oxidation of phenols by the enzyme polyphenol oxidase to quinone. Due to its high nucleophilic nature, quinone has a great potential for accepting protein. The quinones produced then participate in a series of polymer reactions, eventually leading to the formation of brown pigments on the surface of the food. In non-enzymatic browning, as in the case of enzymatic browning, a brown pigment is produced in the food. The two main forms of non-enzymatic browning are caramelization and Maillard reaction. The rate of both reactions is a function of the amount of water in the sample. By studying the browning index curve, it was found that in all 5 types of packaging, browning has increased with increasing storage time of orange juice. However, the increase in browning index in PLA/PAn/CuO/ZnO packaging is less than other packages. The highest increase in browning index during storage time is related to pure PLA packaging. As mentioned, enzymatic browning is an oxidation chemical reaction and since in PLA/PAn/CuO/ZnO polymer, all three nanoparticles of copper oxide, zinc oxide and polyaniline have antioxidant properties, so PLA/PAn/CuO/ZnO with the highest antioxidant properties has reduced the oxidation process and reduced the browning speed of orange juice. In general, by examining the chemical properties of orange juice packaged with various polylactic acid composites, it was found that the highest quality control occurred in orange juice packed with PLA/PAn/CuO/ZnO. In a similar study, Kumar et al. (2019) used ZnO-modified agar active film in green grape packaging. They showed that the use of antioxidant active film controls the chemical quality of the product, the results of which confirm the results of the present study (Kumar et al., 2019). Polat et al. (2018) prepared polypropylene film containing various nanoparticles and used it for packaging and quality control of lemon juice. Their results showed that the active film controls the chemical quality of fruit juice, which confirms the results of the present study (Polat et al., 2018).
Figure 7
3.7.2. The effect of active film on microbial quality control of orange juice
Figure 8 shows the microbial factors (yeast-mold, total aerobic bacteria and acidophilus bacteria) of orange juice packed with polylactic acid films during 56 days of storage. Examination of the yeast-mold curve showed that the type of polymer used for packaging did not have a significant effect on yeast-mold. Molds and yeasts were more compatible with orange juice and cold storage conditions than bacteria. This result was consistent with the studies of Emamifar et al (Emamifar et al., 2010). However, by examining the curves of total aerobic bacteria and acidophilus bacteria, it has been determined that with increasing storage time, the levels of total aerobic bacteria and acidophilus bacteria increase. Examining the effect of polymer type on the growth of total aerobic bacteria and acidophilus bacteria, it was found that the growth rate of total aerobic bacteria and acidophilus bacteria in the presence of PLA/PAn/CuO/ZnO polymer is the lowest. This result indicates that the antibacterial nanoparticles of copper oxide and zinc oxide are highly effective in controlling the microbial quality of orange juice. Because polyaniline also has antibacterial properties, in PLA/PAn/CuO/ZnO film, polyaniline has a synergistic effect with copper oxide and zinc oxide nanoparticles and has shown the greatest effect in controlling the microbial quality of orange juice. Jin and Niemira (2011) used antibacterial modified polylactic acid film for packaging and microbial quality control of apples. They show that the active film has the ability to control the growth of Escherichia coli and Salmonella bacteria. Their research results confirm the results of the present study (Jin & Niemira, 2011). Lee et al. (2004) investigated the effect of antimicrobial packaging on the rate of microbial spoilage of milk and orange juice. They have confirmed the effect of antibacterial packaging on microbial quality control of orange juice (Lee et al., 2004). Fernández et al. (2009) studied the antimicrobial effects of zinc oxide, copper oxide and magnesium oxide and reported that these three metal oxides have good antimicrobial power against a wide range of microorganisms. They have also reported that the reason for the antimicrobial effect of metal nanoparticles is the production of free radicals in the microbial environment and consequently damage to cell membranes and their eventual destruction (Fernández et al., 2009). Llorens et al. (2012) also investigated the antimicrobial properties of cellulose/copper oxide composites and concluded that copper oxide nanoparticles showed strong antifungal activity in pineapple juice; it reduced the load of mold and yeast up to 4 logarithmic cycles. However, it showed weaker antifungal activity in melon juice, which is probably related to the neutral pH of melon juice, which prevents the exchange of copper ions (Llorens et al., 2012). Ram et al. (2013) investigated the effect of packaging containing oxidized nanoparticles on the shelf life of fresh mandarin oranges. Packaging containing oxidized nanoparticles was suitable for maintaining the microbial quality during 6 to 30 days after packaging (Ram et al., 2013).
Figure 8
3.7.3. Ability of smart film in identifying the storage time of orange juice
The purpose of food packaging is to increase the shelf life of food by preventing bacterial spoilage or loss of nutrients. Smart and active packaging systems produced with nanotechnology will be able to respond to environmental conditions such as changes in temperature and humidity. Smart packaging systems are systems that can warn of product quality changes during storage. Smart packaging has sensors that determine the freshness of the material. Smart food packaging can detect that its contents are spoiling and alert the customer. As the process of food spoilage begins, this active packaging will release preservatives such as antimicrobials, condiments, dyes or supplements into the food. Figure 9 shows the calibration curve of the relationship between shelf life and changes in electrical resistance of polyaniline-containing films. In orange juice packaging, different gases are produced according to biological and chemical activities. These gases put pressure on the polymers used for packaging at the bottle cap. Due to the pressure caused by these gases, the electrical resistance of the film changes. Over time, the amount of gas produced increases, and naturally the pressure applied to the film increases and the rate of change in electrical resistance also increases. The results of the electrical resistance of the packaging films showed that there is a linear-exponential relationship between the changes in the electrical resistance of the film and the storage time of orange juice. By examining the electrical resistance of the film in each day of storage, it is possible to estimate the storage time of orange juice and the exact expiration time of orange juice. By comparing the sensitivity of different films to storage time, it was found that PLA/PAn/ZnO film has the highest sensitivity to chemical changes during storage of orange juice. In a similar study, Pirsa and Shamusi used polypyrrole-modified bacterial cellulose film for intelligent packaging of chicken thigh meat. They reported that there is a good linear relationship between storage time and temperature and changes in electrical resistance, which by examining this relationship the expiration date of chicken thighs meat can be estimated (Pirsa & Shamusi, 2019). Asdagh and Pirsa (2020) used smart and active pectin/beta-carotene film to package local butter. They reported that active and intelligent film increased the durability of the local butter, also could estimate the corruption of local butter (Asdagh & Pirsa, 2020).
To evaluate the performance of smart films in identifying the shelf life of orange juice (expiration date), orange juice samples were prepared and packaged with 4 films and their electrical resistance was measured randomly on different days (real time). By placing the electrical resistance value in the mathematical models obtained in Fig. 9, the storage time was calculated (estimated time). From the following relationships, the error and accuracy of the films in calculating the storage time were calculated. The results verified the acceptable performance of smart films in determining the shelf life of two samples of orange juice (Table 3). The results of Table 3 show that all 4 types of smart films with an accuracy of over 90% are able to detect the storage time of orange juice.
Table 3
Error and accuracy of smart film performance in determining the shelf life of two samples of orange juice
Sample
|
Film type
|
Real storage time
|
Estimated storage time
|
Error of detection (%)
|
Accuracy of detection (%)
|
1
|
PLA/PAn
|
20
|
22
|
10
|
90
|
PLA/PAn/CuO
|
20
|
21.5
|
7.5
|
92.5
|
PLA/PAn/ZnO
|
20
|
21.8
|
9
|
91
|
PLA/PAn/CuO/ZnO
|
20
|
21
|
5
|
95
|
2
|
PLA/PAn
|
45
|
42.5
|
5.5
|
94.5
|
PLA/PAn/CuO
|
45
|
43.1
|
4.2
|
95.8
|
PLA/PAn/ZnO
|
45
|
46.7
|
3.7
|
96.3
|
PLA/PAn/CuO/ZnO
|
45
|
45.8
|
1.7
|
98.3
|
$$\text{E}\text{r}\text{r}\text{o}\text{r} \text{o}\text{f} \text{d}\text{e}\text{t}\text{e}\text{c}\text{t}\text{i}\text{o}\text{n} \left(\text{\%}\right)=\frac{Real time-Estimated time}{Real time}\times 100$$
8
$$\text{A}\text{c}\text{c}\text{u}\text{r}\text{a}\text{c}\text{y} \text{o}\text{f} \text{s}\text{t}\text{o}\text{r}\text{a}\text{g}\text{e} \text{t}\text{i}\text{m}\text{e} \text{d}\text{e}\text{t}\text{e}\text{c}\text{t}\text{i}\text{o}\text{n} \left(\text{\%}\right)=100-\text{E}\text{r}\text{r}\text{o}\text{r} \text{o}\text{f} \text{d}\text{e}\text{t}\text{e}\text{c}\text{t}\text{i}\text{o}\text{n}$$
9
Figure 9