1. Introduction
Films based on polysaccharides arouse more and more interest both in the world of science and in the food industry [
1,
2]. Their use can contribute to reducing environmental pollution, limiting the consumption of non-renewable energy sources, or increasing food safety. The use of materials based on polysaccharides brings several benefits resulting from their properties. They are characterised by non-toxicity, biocompatibility, bioadhesiveness, and biodegradability [
3,
4]. However, despite the many advantages of using polysaccharides alone, there is a growing demand for the development of materials based on natural polymers with functional properties, in particular as carriers of bioactive substances.
Food spoilage is a major public health concern. The study of new technologies to stop or delay food spoilage is a key aspect in nutritional science. Food packaging is designed to protect the contents against various environmental factors, including moisture, light, oxygen, microorganisms, dust, and mechanical stress. Due to growing consumer demand for low-processed food and ready meals, as well as the globalisation of the food industry, there is a need for solutions that ensure the freshness and optimal quality of food over long periods of time, which drives the development of innovative packaging [
5]. Increasingly sought-after alternatives are packing materials of natural origin, which could help to reduce the harmful effects on the environment [
6]. All these factors are driving the development of innovative packaging to ensure food safety, extended shelf life, and reduced amounts of non-biodegradable waste generated in the process [
7,
8]. With the expansion of the food packaging market, apart from food quality, increasing attention has been paid to the issue of the environmental impact of the materials used. The use of sustainable solutions for both active ingredients and the packaging matrix, including the use of renewable and biodegradable materials such as biopolymers, has become a priority [
9,
10]. Packaging has to be active and smart. Polymer nanocomposites have shown great potential in this respect [
11,
12,
13].
Nanotechnology is a field of science that offers practical applications for a variety of sectors, including medicine, biology, pharmacology, agriculture, and food technology [
2,
14,
15]. Nanostructures are designed to encapsulate a wide range of substances to improve the properties and usability of materials while at the same time minimising the negative impact on human health or the environment [
16]. Recently, the topic of the application of nanotechnology in medicine and food technology has been generating a fair amount of interest. In-depth research has been conducted to examine the compatibility and ability of nanoparticles to act as controlled and targeted release systems for the delivery of various ingredients. Polymer nanoparticles can modulate the pharmacokinetic properties of active substances due to their subcellular size and affect the compatibility and biodegradability of the polymers used to produce the nanoparticles [
17].
Based on their internal structure, polymer nanoparticles can be divided into nanospheres or nanocapsules, which range in size from 10 to 1000 nm. Nanospheres are characterised by a homogeneous, solid matrix structure with one or more target substances uniformly impregnated. Nanocapsules in turn have a core region (either hollow or filled) as well as a shell-like material covering the encapsulated structure [
16]. In recent years, due to their core–shell microstructure, polymer nanocapsules have been gaining interest in terms of their potential application for the delivery of bioactive ingredients. The core of the nanocapsule allows the effective increase in the loading capacity of the bioactive ingredient while at the same time reducing the polymer matrix content of the nanoparticles [
17]. An advantage offered by the process of encapsulation is the targeted release of bioactive ingredients in food products [
18]. Some of the most widely used carriers for encapsulation are liposomes and micelles. The basic constituents of liposomes are lipids and fatty acids, which occur naturally in cell membranes and are biocompatible and biodegradable [
19]. Liposomes are colloidal, vesicular structures composed of one or more lipid layers surrounding a varying number of aqueous compartments. The properties of liposomes depend on the composition of lipids, surface charge, and method of preparation. Liposomes can act as carriers for drugs or other macromolecules delivered to the human and animal body, as they simplify the drug delivery process to a specific site. They also form an attractive solution for the food industry as they can be used to encapsulate unstable compounds (e.g., antimicrobials, flavourings, and bioactive substances) to increase their stability [
20]. Phospholipid molecules are the main building blocks of liposomes. The most commonly used phospholipids are soy lecithin, egg lecithin, marine lecithin, and milk phospholipids. The main component of phospholipids is glycerol, acting as a backbone for the polar end and fatty acids. Liposome technology is used for a variety of applications, including the production of functional foods, nutraceuticals, cosmetic products, and pharmaceuticals [
21]. Micelles, on the other hand, have a unique core–shell structure and can be used for solubilisation, enveloping hydrophobic compounds and improving their bioavailability [
22]. The most popular of these are polymeric micelles, which are self-organising systems in amphiphilic polymers. Their structural properties ensure optimal solubilisation of hydrophobic agents encapsulated in a lipophilic core [
23].
Curcumin is a natural polyphenolic compound [
22] and the most abundant curcuminoid in the turmeric rhizome. It is used as a colouring and flavouring agent in the food industry [
24]. Curcumin exhibits antioxidant, anti-inflammatory, anticancer, and antimicrobial properties [
22]. It is characterised by low solubility in water, chemical instability, and low bioavailability, and for this reason it needs to be encapsulated in lipids, hydrogels, cyclodextrin compounds, liposomes, or biopolymer particles [
25].
Hibiscus is an ornamental flowering shrub, widely cultivated in East Asia. Its flowering period usually lasts five months and its delicate flowers come in a wide range of colours, shapes, and sizes. Hibiscus has high medicinal and industrial value due to its biological properties and nutritional composition [
26]. It is a source of such bioactive compounds as polyphenols, carotenoids, ascorbic acid, and tannins, the content of which varies depending on the part of the plant used, as well as climate, plant maturity, and differences in genotypes [
27]. It is an industrial plant, cultivated mainly for its antioxidant and antimicrobial properties [
28]. Numerous studies have confirmed that it contains potent bioactive compounds with limited stability. For this reason, encapsulation techniques are used in order to increase their stability [
29].
The study presented in this paper aimed to obtain chitosan–starch films acting as carriers of micellar nanostructures composed of a core from an ethanol extract of turmeric or hibiscus and an outer lipid layer. The physicochemical and functional properties and storage stability of the films obtained were subsequently examined.
2. Results and Discussion
Polysaccharide films containing micellar structures with hibiscus and turmeric extracts were successfully obtained by drying the appropriate gels at 25 °C (±2 °C), which is discussed in detail in the methodological part.
Figure 1 shows the polysaccharide films in daylight (
Figure 1A–C) and in UV (365 nm) radiation (
Figure 1D–F). It also shows the film without additives (
Figure 1A,D—control) and films containing extracts of hibiscus (
Figure 1B,E) and turmeric (
Figure 1C,F).
The morphology, size, and shape of the obtained particles, as well as the structural and optical properties of the obtained composites, were characterised by SEM, FTIR, UV-VIS, and photoluminescence spectroscopy. The functional properties of the obtained films were tested (solubility, degree of swelling, water content, water absorption and water vapour permeability, thickness, transparency, thermal and mechanical properties, contact angle, and colour). Storage tests of the obtained composites were carried out.
2.1. Scanning Electron Microscopy (SEM)
Scanning electron microscopy (SEM) was used for the visualisation of the surface and morphology of the film, as well as to characterise the resulting micellar nano-/microstructures.
Figure 2 shows a microscopic photograph of the control film. The surface of the film is homogeneous, slightly corrugated, with no obvious cracks or pores. This suggests good structural integrity. The plasticiser enhanced the coherence and integrity of the surface structure, resulting in a homogeneous, flexible matrix [
30].
Figure 3,
Figure 4 and
Figure 5 show microscopic images of films with nano-/microstructures. In all of the films below, one can observe the presence of spherical nano-/microstructures, distributed in the polysaccharide matrix, ranging in size from 500 to 1500 nm. During electron microscopy imaging, the resulting capsules were bursting in the vacuum bombarded by electrons, which enabled the analysis of their internal structure. The shape of the obtained spherical structures varied slightly depending on the extract used.
Figure 5 shows the complex structure of the obtained micelles. In both samples, the capsules consist of a core, a lipid layer (lecithin), and an envelope (polysaccharides). The sizes of the micelle cores with turmeric extracts were significantly smaller than those with hibiscus.
2.2. UV-VIS Spectroscopy
Figure 6 shows the absorption spectra of the control film and the obtained composites. The control film exhibits no absorbance of radiation in the visible range, while the composites containing nanostructures with extracts show intense absorption bands at wavelengths 330, 286, 243, and 226 nm.
Pure curcumin has characteristic absorption bands at 250 nm and 427 nm, which can be attributed to low-energy π-π* excitation [
31]. Hibiscus extracts have two characteristic bands, one of about 328 nm and the other of about 540 nm [
32]. The changes in the absorption spectra can be attributed to the functionalisation of the polysaccharide composite with emulsions. Encapsulation causes an increase in absorbance within the 230 to 280 nm range [
6,
33]. The significant increase in absorbance of the films with micelles indicates that the latter absorb light, which allows for their potential use for the production of the packaging of selected food products in order to protect them from the adverse effects of light radiation.
2.3. ATR-FTIR Spectroscopy
Figure 7 shows the FTIR spectra of the control film and the obtained composites. The presence of a broad band at 3270 cm
−1 in the spectra of the control film and the obtained composites indicates stretching of the OH groups and their overlap with stretching NH groups in the same region. The band at 1556 cm
−1 indicates the bending of the NH groups (amide II). The band near 1644 cm
−1 corresponds to carbonyl group stretching vibrations (amide I). The multiple bands in the 1149–950 cm
−1 range correspond to asymmetric vibrations of C-O-C bridge bonds, asymmetric vibrations of rings, and stretching vibrations of the (C-O) bond. In the spectra of all samples, a band is observed at 2925 and 2850 cm
−1 from the -CH
2- and C-H groups in the polysaccharide molecules.
Essentially, the control film and composite spectra are very similar and no significant displacements of bands are observed. Only the differences in the intensity of the absorbance of individual components probably result from intermolecular interactions between the components with different film thicknesses and water content. In the spectra, one can only observe the appearance of a new band at 1743 cm
−1, corresponding to the carbonyl functional group of triglycerides from the olive oil present in the composites [
34]. One can also observe an increase in the intensity of the bands at 2925 cm
−1 and 2854 cm
−1 due to the overlapping of symmetric and asymmetric stretching vibration bands of the -CH
2 and CH
3 groups from the olive oil [
35] with the corresponding bands from the polysaccharides.
2.4. Transparency and Colour
Opacity is a property which determines the degree of light impermeability of a material. The higher the value, the lower the transparency of the film and the higher the resistance to UV rays. The nanostructures’ films show lower transparency than the control sample, but as shown by statistical analysis, the difference is insignificant. Light-sensitive foods require packaging with strong UV-blocking properties. However, if the product is to be visible through the packaging, the transparency of the film should be sufficiently high so that the consumer can see the product. The results obtained in this study indicate a high transparency of the film, in comparison with other films described in the literature: based on starch [
36], starch and chitosan [
37], as well as sodium alginate and chitosan [
38].
Table 1 shows the results of the film’s colour measurement. The colour of the material is determined by three parameters. The parameter L* represents lightness (contribution of black or white varying between 0 and 100), where the maximum value indicates the lightest colour and the lowest value is the darkest [
6,
39]. The parameter L* ranged from 89.49 to 98.72. This means that all films were relatively bright; however, the addition of micellar nanostructures affected the darkening of the surface of the films compared to the control sample. The parameter a* represents the proportion of green or red (negative or positive) in the analysed colour [
40,
41]. In the control sample and with the addition of curcumin, it indicated the predominance of green shades (the results took a negative value). In contrast, the film with the addition of hibiscus extract showed a predominance of red tones (the value of the parameter a* had a positive value). The parameter b* represents the proportion of blue or yellow (negative or positive) in the analysed colour. As can be seen from the presented data (
Table 1), all samples were characterised by the predominance of the yellow shade (the values were positive). The highest values the of b* parameter were characterised by the film with the addition of turmeric extract, because it is a yellow pigment present in the spice turmeric (
Curcuma longa) [
42], often called golden yellow, a naturally occurring polyphenolic compound [
43,
44]. Our previous studies have confirmed that the yellow pigment of turmeric extract has a decisive effect on the final colour of the composites, regardless of whether the capsules are introduced into a chitosan–alginate matrix [
6] or, as in this work, a chitosan–starch matrix. The turmeric extract changes colour under the influence of pH and thus can become an indicator of product freshness [
6]. In an acidic environment, films with curcumin show a light-yellow colour, while in an alkaline environment, the colour is intensely red [
45,
46]. These properties can be effectively used in smart packaging, as curcumin is a good indicator of spoilage in, for example, shrimp, which produce volatile amines during decomposition, causing the pH to change to alkaline and the colour of the film to become red [
6,
47,
48,
49]. In an alkaline environment, curcumin’s diketone groups are converted to a keto-enolic form, prompting a spectral shift with a colour transition from yellow to orange/red [
45,
50]. Thanks to this ability, curcumin can be successfully used as a colourimetric indicator to monitor food spoilage.
The films with the addition of hibiscus extract were characterised by a predominance of red and yellow colour, as it contains a higher quantity of pigments, mainly anthocyanins, which are a natural pigment soluble in water. This pigment changes colour when exposed to different values of pH [
51,
52]. For example, chitosan/PVA composites containing anthocyanin extracted from purple cabbage were used to indicate the alteration of milk quality from the colour change of the system [
53]. In other studies, black carrot [
54] and purple sweet potato [
55] anthocyanins were applied for monitoring fish freshness.
These observations may therefore suggest that both hibiscus and turmeric extracts have potential uses as natural pH indicators.
2.5. Mechanical Properties
The thickness of the obtained films and their mechanical properties are shown in
Table 2. Based on the data, it can be seen that the film with hibiscus extract was the thickest (more than twice as thick as the control film). High values of this parameter were also obtained for the film with turmeric extract. Although the same amount of film-forming solution was poured onto the trays, the differences in thickness were statistically significant. This may have been due to the solid content enrichment in the samples [
37].
The incorporation of micellar nanostructures into the polysaccharide matrix weakened the strength (TS) of the film, and it also improved its elongation at break (EAB). An increase in the elongation of the chitosan/alginate structure by 99% was observed in the case when turmeric extract was added and by 131% when hibiscus extract was incorporated. The extensibility of films incorporating hibiscus and turmeric extract s were higher than some synthetic materials: polyester (PE), polyvinylidene chloride (PVDC), and low-density polyethylene (LDPE) reported by the Shiku et al. [
56].
The strength of the film, on the other hand, decreased 4.9–6.4 times. According to Kumar et al. 2017 [
57], this may have been due to the reduction in polymer network continuity and cohesion caused by the introduction of nanoparticles into the matrix.
2.6. Solubility and Water Absorption
Table 3 shows the results of the measurements of water content, solubility, and swelling degree of the control film and the films with micellar nanostructures added containing turmeric and hibiscus extracts. The films with nanoparticles added had a higher water content (by approx. 49 and 67%, respectively) compared to the control sample. This indicates that the water encapsulated in the micelles does not evaporate upon drying, confirming the stability of the resulting spherical structures. On the other hand, the solubility of the film with the active substance introduced decreased by 22.6–25.9%, and it was at a similar level of 14.31–14.93% for both samples. It can, therefore, be concluded that the addition of the active substance improved the barrier properties of the film. A low degree of film solubility is a desirable property when the film is to be used for storing food or medicines containing a significant amount of water. Otherwise, it could cause the penetration of film particles into the packaged products. For this reason, the solubility of film components is an important feature that determines its application [
37,
38]. Statistically significant differences in the degree of swelling between control films and films with nanostructures have also been observed, which may suggest the blocking of certain active groups for water absorption [
37].
2.7. Water Vapour Barrier Properties
Table 4 shows the water vapour barrier properties of the tested films. Two-factor analysis showed that both RH and film type affected the WVTR and WVP values. In the case of WVTR, the impact of film type was only significantly statistically significant under storage conditions with the lowest relative humidity (RH = 55%), in which case this value was about twice as low as for films with micelles. High relative humidity of the ambient air increased the WVTR significantly relative to RH = 55%. However, it was observed that the value of this parameter did not change as a result of the modification of the film composition.
WVP is a parameter that takes into account the impact of film thickness on water vapour permeability. The addition of micelles to the film increased the WVP value relative to the control sample regardless of storage conditions. The modified films did not differ in terms of WVP value at low RH. In the case of storage under high RH conditions, the film with hibiscus extract exhibited the highest water vapour permeability, and this was correlated with film thickness. It can be observed that the higher RH values did not change the WVP values for individual films, which may suggest that maximum values were reached.
When it comes to water absorption and swelling of the samples, the results obtained can be explained by a change in the water sensitivity of the sample. This was confirmed in works in which, in addition to research related to barrier properties, sorption isotherms were determined [
58]. These authors showed that, at low RH values, the process of water vapour sorption through the film played a key role, while values of partial pressure of water vapour above 1300 Pa increased the force driving water vapour through the film barrier. Wiles et al. [
59] observed that, at a high partial pressure of water vapour (above 2000 Pa), an increase in moisture sorption in chitosan films can lead to swelling and changes in the structure of the biopolymer. It leads to an increase in the amount of absorbed moisture and loosening of the microstructure of the film. The consequence of this phenomenon is an increase in the stream of water vapour flowing through the matrix as well as a change in their mechanical properties [
60,
61]. Therefore, when considering the application properties of films based on biopolymers, their water sensitivity should be analysed. The literature data show that chitosan films have lower water vapour permeability in relation to other biopolymer films [
62]; therefore, even though the incorporation of micellar nanostructures into the film matrices reduced their barrier properties, they are still an interesting application solution as biodegradable packaging.
2.8. Water Contact Angle
The values of the static contact angles of the films against deionised water are listed in
Table 5. Due to the fact that the produced films were characterised by different surface structures on both sides, two series of contact angle measurements were carried out, i.e., on the matte surface and on the glossy surface. The values of the contact angles provide information on the wettability of the tested material with a specific liquid. The terms hydrophilicity and hydrophobicity refer to the affinity towards and repellency against water, respectively. In general, they are defined based on the three-phase contact angle. (θ): θ < 90° indicates a hydrophilic surface, and θ > 90° indicated a hydrophobic surface [
63]. It should be noted, however, that the contact angle is the combined effect of the affinity of the liquid towards the surface and the physicochemical heterogeneity of the surface [
63]. In addition, the contact angle is sensitive to environmental conditions, sample preparation and measurement methods [
64]. Thus, it may not be possible to compare the literature data on surface phenomena in the same systems.
The presence of nanoparticles containing hibiscus or turmeric extract in starch–chitosan films resulted in increased hydrophilicity of the films, which was reflected in lower values of the contact angles. This finding is in accordance with the water vapour barrier properties reflected in the WVP values of the films (
Table 4). Stanisławska et al. [
6] also found that the sodium alginate–chitosan film with curcumin nanocapsules exhibited higher hydrophilicity compared to the control film. In the present study, it was also observed that the matte (slightly rough) surface of the film was slightly less hydrophilic than the glossy (smooth) surface. It can be assumed that the roughness of the matte surface of the film resulted in partial wetting of the surface, with air inclusions between the droplet and solid elements of the film surfaces. Moreover, the different degree of hydrophilicity of both sides of the films could result from the different concentration of nanocapsules in the successive layers of the films, depending on the methods of their production. When the waterdrops deposited on the films were imaged for about 20 s, it was observed that they were absorbed by the films but not expanded on the film surfaces. This proves the strong hydrophilic properties of the films and is in line with the results of measurements of the water vapour barrier properties.
2.9. Differential Scanning Calorimetry (DSC)
Table 6 and
Table 7 show the results of the DSC analysis of the examined films. Three characteristic phenomena were identified. The first is related to the glass transition phenomenon (
Table 7). As can be seen in
Figure 8, this transformation is fairly complex. On the one hand, it may be related to the history of the sample and the relaxation phenomenon, and on the other, it may be related to the overlapping of the glass transition phenomenon of the main film components—chitosan and starch. The transition occurs in typical ranges for these substances.
According to Dong et al. [
65], the glass transition temperature of chitosan is in the range of 140–150 °C, which is consistent with the values obtained in this study. However, the above-mentioned values are significantly higher than those reported by Jiang et al. [
66] obtained for chitosan films, which can in turn be attributed to the presence of starch. As can be seen in
Table 6, no significant effect of film modification on the values characterising this phenomenon was found, which also suggests that it is exclusively related to the main components. Another transformation is related to the phenomenon of melting of crystalline structures (
Table 6), one of the important characteristics of polysaccharides. Starch consists of two polymers—linear amylose and branched amylopectin. According to Dome et al. [
67] the complex of these polysaccharides forms grains/structures that are partially crystalline. The grains contain alternating semi-crystalline and amorphous areas, and the degree of crystallinity depends on the origin and composition of the starch-containing raw material. In turn, in the case of chitin and chitosan, the crystallinity depends on the proportion of different monomers present in chitosan. In this case, the crystallinity is influenced, among others, by the deacetylation process, which may lead to a decrease in chitosan crystallinity, which may be due to the breaking of extensive intermolecular hydrogen bonds present in chitin. In general, chitin has a higher crystallinity than chitosan [
68]. The crystallinity of starch–chitosan systems depends on the share of individual polymers. In the study of Lopez et al. [
69], the addition of chitosan to corn starch did not have a statistically significant effect on the values of characteristic temperatures, despite a noticeable downward trend. The authors attributed this to the interaction between starch and chitosan molecules, which interrupts the rearrangement of the starch polymer chain. In this work, a statistically significant impact of the modification was found on the values of temperature and enthalpy. The addition of hibiscus had no effect on these parameters and this sample did not differ from the control sample (despite the visible decrease). In turn, the addition of curcumin caused a decrease in the values of all the quantities characterising the melting peak. The decrease in the characteristic temperatures may be caused by the modification of the crystal lattice after the addition of hibiscus/curcumin, which leads to a weakening of the interactions between polysaccharide molecules and, as a result, a reduction in the amount of energy necessary to break them down [
69]. In turn, the decrease in enthalpy can be attributed to increased water retention in the system. According to Lopez et al. [
69] and Tongdeesoontorn et al. [
70], increased interactions between hydrocolloids and starch result in increased water retention and greater water mobility during heating, increased kinetic energy, and decreased enthalpy. This can be confirmed by the increased water content in the tested samples (
Table 3). The last peak should be attributed to the phenomenon of polymer breakdown. According to Mathew et al. [
71], the exothermic peak starting at 250 °C in chitosan- and starch-based films can be attributed to polymer breakdown, including dehydration of saccharide rings, depolymerisation, and breakdown of acetylated and deacetylated chitosan units. In this case, a change in the peak temperature value following modification was found in both cases analysed, which may suggest compromised thermal stability of the modified polymers.
2.10. Microbial Storage Stability
No pathogenic bacteria such as Salmonella sp., Listeria sp., and S. aureus were found in the analysed food products, both fresh and stored.
Based on the results obtained, it can be concluded that the films with hibiscus or curcumin extract had no significant effect on the total bacterial count during storage of cottage cheese (
Table 8). Perhaps in this case the concentration of curcumin extract in the tested film was too low to obtain an antibacterial effect. It should be noted that a statistically significant effect was noticed for the foil with curcumin during storage of salmon (
Table 8). Admittedly, the total bacterial count increased by one order of magnitude in relation to the number of bacteria determined in the fresh product but was significantly lower when compared to the stored product in the other analysed cases. The total count of
E. coli bacteria after the storage period was in this case similar to that determined for the fresh product but significantly lower compared to the other variants of the experiment, too (
Table 8). The results obtained for the film with curcumin are consistent with our previous studies [
6] and with other reports in the literature on the subject [
72,
73,
74,
75,
76]. Curcumin has been shown to have strong antimicrobial properties against both Gram-positive and Gram-negative bacteria. It has been proven to effectively inhibit biofilm formation by bacteria, including
Escherichia coli, through interfering with bacterial Quorum Sensing (QS). Curcumin has also been found to exhibit a photodynamic effect by producing cytotoxic reactive oxygen species (ROS), in addition to inhibiting bacterial DNA replication and altering gene expression. It can also damage the bacterial cell membrane, reduce microbial motility, and inhibit cell division and bacterial proliferation [
77,
78]. It has also been shown that nanoparticles with curcumin exhibit potent antimicrobial properties against S. aureus in periarticular joint infections [
79].
2.11. Fluorescence Spectroscopy
Photoluminescence spectra were measured to verify the optical properties of the obtained films, as well as possible sensitivity to changes during the storage of food (
Figure 9).
Figure 9A shows the emission spectra of a starch–chitosan film (control sample) and films containing nanostructures with hibiscus and turmeric extracts. The presence of nanostructures has a significant impact on the emission intensity of the films obtained. An intense band can be observed on the spectra with a maximum at 463 and 501 nm for films with hibiscus and turmeric, respectively. Storage of cheese and fish in control films causes minimal changes in emission properties (
Figure 9B). A fairly significant difference is observed in the case of cheese storage, which may be due to the formation of acidic products that cause an increase in chitosan emission intensity.
In the case of films containing extracts, the emission changes depend on the nature of the food product. For composites containing turmeric extracts (
Figure 9C), in addition to the change in emission intensity, one can also observe a shift in the emission maximum from 501 nm to 492 nm in the case of cheese storage and 497 nm in the case of fish storage. The changes in intensity may be due to either the presence of acidic or basic compounds formed on the micelle structure or structural changes of the turmeric extract (keto-enol tautomerism).
For composites containing hibiscus extracts (
Figure 9D), a significant increase in emission intensity is observed following fish storage (from 4800 to 6950) and cheese storage (up to 7650). Anthocyanins present in the hibiscus extract within the nanostructures are responsible for the emissions. In general, anthocyanins exhibit better solubility in water, so changes in emissions may also result from changes in water content. The intensity of emissions depends on both the concentration and the pH value, as well as the presence of other biological components that can also affect the acid–base balance. The results of the emission tests conducted suggest that the bionanocomposites obtained are sensitive to changes occurring during storage of cheese and fish and could therefore serve as sensors of the freshness of the aforementioned products.