Introduction

A significant quantity of non-biodegradable plastic is discarded into the natural surroundings, resulting in serious environmental contamination. The development of biodegradable edible films has emerged as a means of reducing the use of synthetic or non-biodegradable plastic and preserving the environment [1]. Carbohydrates, protein, and lipids have been used to fabricate edible films individually or in combination with each other. Thermal stability is an important property for edible films, as it ensures that the film maintains its structural integrity and does not break down or degrade when exposed to high temperatures (Bhatia, Al-Harrasi, Shah, Altoubi, et al., [2]). To ensure the effectiveness of edible films as packaging materials, it is necessary to consider their thermal stability during the development and formulation of the films. There are different methods used to evaluate the thermal integrity of edible films such as Differential Scanning Calorimetry (DSC), Thermogravimetric Analysis (TGA), Dynamic Mechanical Analysis (DMA), and Thermomechanical Analysis (TMA). Thermo-physical properties such as thermal conductivity, diffusivity, glass transition behaviour, crystallization temperature, melting temperature, decomposition temperature, heat deformation temperature and heat expansion rate play an important role in determining the stability of the food packaging material. Ideally food packaging material must have poor thermal conductivity like plastics to restrict the heat transfer from packaging material to food [3].

Food packaging materials like plastics do not contain free electrons when compared with metallic packaging material, and thus restrict the transfer of heat from the environment to packed food products. Due to the recent advancements in food packaging materials, biomaterials derived from starch and other sources showed drastic improvement in mechanical strength and color attributes. Nevertheless, most of the biopolymers used in food packaging lack thermal insulation properties like synthesized plastic [4]. Most of the plastic materials used in the food packaging industry show intermediate thermal conductivity (0.15–0.4 W/mK) [5]. The addition of bioactive compounds to edible films can affect their thermal properties [6]. This review focuses on the thermal properties of edible films and various thermal analysis techniques available for analyzing biopolymeric films. Furthermore, the current review article explores the interrelation between film properties and thermal stability such as film crystallinity, morphological attributes, chemical arrangement, nano-reinforcements and interaction with other ingredients.

Methods to Determine the Thermal Stability of Edible Films

The thermal properties of edible films determine their stability during storage and transport. Temperature changes can cause the film to lose its mechanical and barrier properties, leading to spoilage of the food product. The thermal properties of edible films are also important for optimizing processing conditions. The commonly used methods to analyze the thermal properties of edible films are TGA, DSC, DMA, and TMA.

Thermal Gravimetric Analysis (TGA)

TGA is a valuable technique for assessing the thermal stability of films at higher temperatures (around 600 °C). This is one of the main features of packaging materials as they may be exposed to higher temperatures in packaging procedures such as sealing. In addition, TGA also helps in determining the effect of any additive or process condition or any other variable on the blank polymeric material especially polymeric degradation [7]. To differentiate the thermal stability of different films in terms of their decomposition or weight loss it is important to record their decomposition stages at a constant heating rate under an inert atmosphere by using TGA. Generally, the first weight loss during TGA analysis of edible films is observed due to the presence of free and bound water vapors whereas the second, as well as third stages of weight loss, corresponding to the decomposition of plasticizers such as glycerol and polymer [8]. It is important to compare the percentage of weight loss in each stage of control (blank polymer films) with composite films to understand the effect of secondary polymer or any additives on the thermal stability of the films. Additionally, the thermal attributes of the films such as initial degradation (Tonset), maximum decomposition (Tmax) and the residual mass left at the highest temperature (Tmrs) during TGA analysis must be evaluated [9]. A number of analyses can be carried out on the data obtained from the TGA using various methods. Some of the factors that affect this data are sample mass, heating rate, gas flow rate and nature, and the mathematical procedure that is applied to validate the data [10]. Reaction rate describes the derivative of the conversion, (α) with respect to time. The TG curves can be used to calculate the value of (α) in terms of mass loss using the following equation [10].

$${\upalpha }=\frac{{m}_{o}- m}{{m}_{o}- {m}_{\infty }}$$
(1)

In this equation, m represents the measured mass of a given sample at temperature T, \({m}_{o}\) indicates the initial mass while \({m}_{\infty }\) is the mass at the end of the non-isothermal TGA analysis.

The thermal stability of the film containing a single polymer is considered as control and its decomposition stages are usually compared with composite films to understand the effect of additives such as polymer, bioactive compound, plasticizer, surfactant, etc. on the control film (Table 1). Several examples of control film containing single polymers, those are widely used in edible films are highlighted in Table 1.

Table 1 Thermal decomposition of films containing single polymer
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DSC (Differential Scanning Colorimetry) Analysis

DSC is generally used to examine characteristic temperatures, corresponding phase change enthalpies, crystallization behaviour and most importantly glass transition by identifying the change in heat capacity over the two periods [24]. Recently more sensitive techniques like modulated DSC (MDSC) have been employed to enhance the efficiency and precision of intricate thermal events in the study of edible films. Methods such as TMA, DMA and oscillation techniques are not employed commonly as they possess greater sensitivity [25]. Recently MDSC analysis was used to determine very low Tg temperatures ranging from about − 22 to about − 39 °C [26]. DSC thermograms of films made up of natural semi-crystalline polymers help in the determination of glass transition temperatures (Tg), melting temperatures (Tm), crystallization temperatures, crystallinity, and enthalpies (ΔH) [27]. Enthalpies of transitions can be determined using the DSC curve by integrating the peak corresponding to a particular thermal event [28, 29]. For a specific transition, the enthalpy can be described using Eq. 2:

$$\Delta H{\rm{ }} = {\rm{ }}KA$$
(2)

In this equation, ΔH indicated the enthalpy of transition, K represents the calorimetric constant, and A is the area under the peak.

Glass transition of polymeric material is related to the change of molecular mobility of the polymer due to rise in the temperature. This physical shift of polymeric material from glassy to an amorphous state impacts the molecular motion of polymeric material and thus controls thermal, mechanical and barrier properties [30]. It was suggested that the changes in the Tg, Tm and ΔH are the potential markers in determining the compatibility between several biopolymers [31]. DSC overlay of edible films generally presents both endothermic (water evaporation, melting, polymer degradation and glass transitions) and exothermic peaks (crystallization) (Fig. 1).

Fig. 1
figure 1

General DSC overlay of edible films (tm: melting temperature; tc: crystallization temperature; tg: glass transition)

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DSC is probably not a suitable approach for the thermal stability assessment of composite or complex materials such as edible films made of two polymers with bioactive components. This is due to the formation of multiple exothermic or endothermic peaks in both heating and cooling cycles while analyzing composite materials [32]. This makes the thermogram complicated in terms of distinguishing the Tg peak, as the endothermic relaxation enthalpy is descriptive for polymeric materials in the glassy state that experiences natural ageing. The endothermic peak in the DSC thermogram also represents melting temperature (Tm), and one or more glass transitions, that further complicate the thermogram in identifying peaks corresponding to Tm and Tg. Crystallization is an exothermic process and the degree of crystallinity of the edible films can be calculated from the area of the exothermic peak (further conformed from XRD of the same sample) [33]. In the majority of the composite edible film studies, the DSC thermogram of control (generally thermogram of single polymer without additive) was compared to the thermogram of composite material to understand the changes in the thermal stability of composite material (Table 2).

Table 2 Endothermic and exothermic peaks in DSC thermogram for the neat polymers
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Dynamic Mechanical Analysis (DMA)

The DMA technique is extensively employed in the characterization of a material’s properties with respect to various parameters such as temperature, time, frequency, stress, atmosphere, or a combination of these parameters [52]. Polymers exhibit two distinct responses to the energy of motion, namely elastic response, which is crucial for shape recovery, and viscous response, which plays a vital role in dispersing mechanical energy and preventing breakage. The viscoelastic properties, which refer to the responses of a material, are analyzed through the utilization of dynamic mechanical analysis (DMA) while subjecting the material to low levels of mechanical force. The viscoelastic behaviour of polymers is dependent upon the variables of temperature and time [53]. DMA instruments are equipped with regulated heating and cooling mechanisms to investigate the impact of temperature on the stiffness and resiliency of polymers [54]. The dynamic mechanical analysis method is a versatile approach that can be used to simultaneously characterize the rheological and thermal properties of an extensive range of different types of samples [55]. DMA can provide a comprehensive understanding of the thermal behavior and stability of edible films by measuring their mechanical response as a function of temperature [53]. This information aids in the development, formulation, and selection of edible films with improved thermal stability for various food packaging and preservation applications.

Thermomechanical Analysis (TMA)

Thermomechanical analysis also referred to as TMA, is one of the most important tools in the field of material science, particularly when it comes to thermal analysis. The alteration in the dimensions of the sample as a function of temperature is the basis of the TMA technique [56]. TMA is a useful technique for obtaining significant insights into the thermal expansion, glass transition temperature (Tg), softening points, composition, and phase changes of materials with varying geometries by subjecting the materials to a constant force as a function of temperature [57]. TMA can be employed to study the thermal behaviour of thin films, coatings, and surface treatments. It allows for the analysis of thermal expansion properties and the determination of the stress-strain behaviour of these thin layers. TMA can be used in optimizing the design and performance of coatings and thin films for various applications. While Dynamic Mechanical Thermal Analysis (DMTA) measures the viscoelastic properties by applying an oscillatory force. Together, TMA and DMTA techniques offer a comprehensive understanding of a material’s thermal and mechanical properties, crucial for applications like biopolymeric films in packaging.

Effect of Bioactive Compounds on the Thermal Behaviour of Edible Films

The thermal properties of a film are indicative of its ability to withstand varying temperatures and can have a significant impact on its compatibility to be employed as a food packaging material [58]. The thermal behaviour and stability of biopolymers can vary significantly depending on the specific type of biopolymer and its source. The thermal characteristics of films are frequently assessed to ascertain key parameters such as the glass transition temperature, melting point temperature and degradation temperature. These parameters serve as a viable metric for a uniform multi-polymer mixture and may be regarded as a standard for evaluating the capacity to combine polymer/biopolymer blends and withstand fluctuations in temperature [59]. Adding bioactive compounds to edible films or preparing a composite material with the addition of two or more biopolymers can have various effects on the thermal properties such as thermal stability, melting temperatures and glass transition temperatures of the resultant films [60].

Thermal properties of the pearl millet starch (PMS) and carrageenan gum (CG) based edible films were examined using DSC. The authors reported that the melting temperature (Tm) of the PMS films increased from 98 °C to 118 °C with the addition of CG due to the strong intermolecular interaction between starch and carrageenan [61]. In another study, Zhou et al. [62] studied the thermal characteristics of cassava starch-based (CS) films loaded with varying concentrations of cinnamon essential oil (CEO) using TGA. The results demonstrated that the addition of CEO in the CS-based edible films increased the thermal stability of the films due to the higher compatibility between the film-forming components including polymer and oil [62]. The impact of bioactive compounds on the thermal characteristics of edible films can vary based on factors such as the type, concentration, and compatibility of the bioactive compound with the film matrix.

The thermal properties of carrageenan, guar gum, and glycerol-based edible films containing varying concentrations (0.2%, 0.4%, and 0.6%) of lemongrass essential oil (LGO) were investigated using DSC [63]. The results of the DSC analysis of the prepared films revealed that Tg increased with increasing the concentration of oil from 0.2 to 0.6%. Tg of the edible film was determined to be 221.78ºC, 237.91ºC, and 241.26ºC for formulations containing 0.2%, 0.4%, and 0.6% oil, respectively [63]. In a previous study, DSC analysis was carried out for the edible film based on arrowroot starch and gelatin composite incorporated with cranberry powder [64]. The thermal analysis revealed that increasing the concentration of cranberry in the films caused a decrease in the Tg range. Incorporating fruit pulps into the polymeric matrix of the film has been found to release sugars that act as plasticizers. This plasticizing effect modifies the polymer interactions, increasing the free volume, enhancing chain motility, and reducing the Tg of the film [65, 66]. Furthermore, the incorporation of different bioactive compounds and their effect on the thermal properties of edible films have been shown in Table 3.

Table 3 The incorporation of bioactive compounds and their effect on the thermal properties of the edible films
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Crystallinity and Thermal Stability of the Films

Ideally, edible films must be highly crystalline to gain more thermal stability and mechanical flexibility [60]. However, most of the natural polymers used in the preparation of edible films are semi-crystalline and approaches employed to increase polymer crystallinity generally increase the brittleness of the edible films. In addition, crystallizing polymers on an industrial scale can be very difficult. Approaches like co-crystallization to enhance the crystallinity and thermal stability of polymeric films while maintaining their mechanical flexibility could be reliable but have not been used so far. Thus, it is important to draw the relationship between the crystallinity and the thermal behaviour of the film [75]. There are multiple factors such as functional groups, molecular weight, branch degree, cross-linking, and degree of crystallinity of the polymers that impact the thermal stability of the edible films as shown in Fig. 2.

Fig. 2
figure 2

A representation of the effect of additives on the crystallinity of the edible films and effecting the thermal stability

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XRD (X-ray diffraction analysis) is an analytical technique used to determine the degree of crystallinity of materials. Important attributes such as the nature of crystal lattice (polymorphic), crystal structure, polymorphism, preferred crystal orientations (texture), crystal defects, percentage of crystalline material, and most importantly crystal size can be determined using XRD [76]. The perentage of crystallinity can be calculated using the Eq. 3 [77].

$$\text{C}\text{r}\text{y}\text{s}\text{t}\text{a}\text{l}\text{l}\text{i}\text{n}\text{i}\text{t}\text{y} \left(\text{\%}\right) = \frac{\text{a}\text{r}\text{e}\text{a} \text{u}\text{n}\text{d}\text{e}\text{r} \text{p}\text{e}\text{a}\text{k}\text{s}}{\text{T}\text{o}\text{t}\text{a}\text{l} \text{a}\text{r}\text{e}\text{a}}$$
(3)

With the latest advancements in XRD, small-angle, ultra-small-angle X-ray and neutron scattering approaches can be used to offer structural data on the nanometer-to-micron length scale [78]. X-ray diffractogram of amorphous-crystalline structures is usually represented by strong characteristic peaks related to the crystalline zone where narrowness and broadness determine the percentage of crystallinity [79]. The most important factor that determines the crystalline behaviours is the polymer’s inherent properties such as its type, source, and composition (Table 4) that ultimately impact the thermal stability of the films.

Table 4 Polymer inherent compositional attributes that affect the crystallinity of the films
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Effect of Morphology and Surface Property on the Thermal Behaviour of Edible Films

The thermal resistance of the films is also influenced by their morphology [62]. The morphology of the edible films is generally determined by scanning electron microscopy. SEM coupled with EDS offer not only morphological insights but also provide the elemental composition of the material which is essential to assess the heat insulation of the material. Also, atomic force microscopy helps in determining surface roughness of the polymeric films by calculating roughness means square values which is again an essential parameter to correlate with heat conduction. Morphological attributes such as compactness, density, porosity, cracks, uniformity, roughness and particles impact the heat conduction of the material and thus impact the overall thermal stability of the edible films. Similarly, surface hydrophobicity and hydrophilicity also impact the thermal stability of films [88]. Surface hydrophilicity or wettability of edible films can be determined by using a goniometer to measure the contact angle of a droplet on a surface [89]. Hydrophilic material attracts more water vapors over the surface of the films resulting in variation in thermal conduction.

Thermal Behavior of Common Edible film Constituents

The low thermal conductivity of biopolymer films compared to synthetic films is one of the major technical barriers that limit its application at the commercial scale [90]. Biopolymer films are relatively more sensitive to heat than synthetic films and thus offer low thermal resistance. Therefore, improving the thermal conductivity of biopolymers and their respective composites is of great interest. To engineer high thermal conductive films using biopolymers more chemical insights are required such as polymer chain structural arrangements (backbone and side chains) and interchain coupling (hydrogen bonding and crosslinking) in the films [91]. Heat conduction expresses the transfer of heat across a material directed by a temperature gradient. In contrast to metals, heat conduction in polymeric material occurs via lattice vibrations (phonons) [92]. Normally, an amorphous polymeric structure leads to a reduction in the normal free path of phonons, which decreases the overall heat conductivity [93].

Synthetic polymers with highly conducting or insulating features generally form toxic residues when exposed to higher temperatures. Therefore, biopolymers-based films have been considered over synthetic polymers to prevent the immigration of toxic components into food. Film composition and process conditions are critical parameters in determining the thermal stability of the films. Some films are designed in a way to bear thermal stress e.g., thermoplastic starch is fabricated by using edible starch plasticized with food-grade plasticizers to improve the overall thermal stability than native starch-based films. The type and quantity of plasticizer used in the development of thermoplastic starch-based films greatly impact their thermal stability [94].

There are various sources of starch, however, some sources provide a high yield of starch with its high amylose content. Ulluco starch-based edible films have been reported for their better thermal stability profile [95]. The chemical composition of the films in terms of the ratio of polysaccharides, proteins and lipids also impacts the thermal stability of the films. Generally, the thermal stability of polysaccharides is poor as these macromolecules are not conducive to their successive molding processing. Using organic or synthetic crosslinkers to improve the crosslinking is considered as reliable approach to improve the thermal stability of polysaccharide-based films [91]. Although polysaccharide aerogels possess low thermal conductivity, its equivalent to silica aerogels and synthetic polystyrene foams. Thus, the incorporation of polysaccharide-based aerogels can be used to prepare films with relatively high thermal stability [96]. Likewise blending between polysaccharide and protein offers more thermal resistant films e.g., blending of collagen–alginic acid improved the thermal stability by 15 °C after alginic acid addition in neat collagen (native) materials [97].

Chitosan shows great sensitivity to several types of degradation, including thermal degradation, which is dependent on the degree of deacetylation. It is reported that commercial chitin thermally decomposed at lower temperatures than highly deacetylated chitosan [98]. The blending of neat chitosan polymer with naturally isolated polymeric fraction can also result in an improvement in the thermal stability of the resultant films. Previous reports suggested that chitosan-corn cob biocomposite films showed improvement in thermal stability by 15 °C [99].

Similarly, the combination of chitosan with thermoplastic polymers, like poly(butylene succinate), poly (-butylene terephthalate adipate), and poly(butylene succinate adipate) is an alternate approach to increase the heat resistance of the films [100]. Another example of improvement in thermal stability was observed when chitin– bentonite blend film was made [101]. The collagen-based film usually offers good barrier properties against oxygen and water vapor however offers a rough surface and poor thermal resistance. Acid hydrolysis of collagen generally allows destabilization of its triple helix, resulting in the formation of soluble gelatin [102]. In gelatin-based films, triple helix content, molecular weight and amino acid composition are the critical factors that impact the thermal stability of the resultant films [102]. Agar-based films are highly brittle and possess poor thermal properties [103]. Sodium alginate is often used as a common ingredient among several edible films based on recent research. The composition, sequence, and proportion of alginate monomers M immediately impact the properties of alginate-based films. Domination of G-monomers results in strong films whereas dominance of M-monomers leads to more elastic films. This is mainly dependent on its interaction with metal cations, such as calcium ions as such interaction between monomers and metal ions resulted in the formation of a stable and ordered 3-D “egg box” model grid. Due to the selectivity of the monomers, especially G monomer, it is important to add a suitable crosslinker which can help in the formation of a stable complex to improve the thermal stability of the film [104].

Cellulose-based films are commonly used in the form of edible films. Cellulose generally shows degradation at the temperature of 350 °C [105]. On the other side cellulose nanocrystals or nanocellulose are highly crystalline materials with improved thermal properties [106]. The incorporation of nanocellulose having a low thermal expansion coefficient can improve the thermal stability of the films [106]. Similarly, highly thermal-resistant proteins such as zein can also improve the thermal stability of the edible films [107]. Poly(lactic acid) is also known for its good thermal properties which are dependent on D and L ratio (two mesoforms). Its barrier and mechanical strength properties are comparable to polystyrene. Thus, its suitable blend can improve the thermal resistance of the other polymer films [108]. Several polyhydroxyalkanoates have been reported for their comparable thermal properties like PE, PS, and PP [108]. The thermal resistance of the biopolymer-based films can be improved with the incorporation of graphene, clays, cellulose nanocrystals (nanofillers), and metal oxides [108]. The degradation temperature of films can be extended by using such components as these substances are effective in reducing biocomposite thermal degradation [109]. In addition to the chemical nature, thermal and irradiation treatments can also improve the thermal resistance of the films. Coupling thermal treatment with irradiation can also be utilized to fabricate the films with better thermal stability [110]. Exposure of films to radiation can also result in films with denser networks via rearrangement of chain improvement increasing the thermal stability of the films. For an instance thermal stability of composite gelatin-nutshell fiber was increased after a 40 kGy electron beam irradiation dose [111]. Previous research also demonstrated the use of ultrasonication in developing transparent cellulose film from ginger nanofiber with improved thermal stability [112].

Nanoreinforcements in Edible Films and Their Effect on Thermal Stability

Transformation of the macroscale materials to nanoscale (1–100 nm) allows significant manipulation of the physical and chemical attributes of the material. Food nanoscience is one of the most advanced areas that permits nano reinforcement of biobased materials to offer material with better properties [113]. Due to the suitable size, the nano reinforcements offer a relatively larger surface area per mass of additive than the micro-reinforcements, which allows better interaction with the polymer matrix [114]. Thus, it could be expected that the resultant materials could present improved thermal properties. This nano-fortification of biopolymers can be succeeded with minimal filling volume whereas macro fortification traditionally requires higher filler volume to attain a similar effect. This approach results in a decline in the weight of the packaging material with improved thermal and other properties in the similar polymer matrix. These nanofillers for edible films are classified as organic (clay minerals, polysaccharides) and inorganic materials (metal or metal oxides) [115]. Due to their thermal stability, more stress is retained on inorganic fillers mainly iron oxide (Fe3O4), silver (Ag), gold (Au), titanium oxide (TiO2), zinc oxide (ZnO), copper oxide (CuO), cerium dioxide hydroxides, aluminium oxides, calcium carbonate and carbon-based materials [116, 117].

Nanoclays (e.g., halloysite, montmorillonite, bentonite, kaolinite, sepiolite, and Laponite®) have been used to reinforce the polymers in food packaging materials [118]. The addition of these nanomaterials in the edible films can improve their thermal resistance. Different types of thermodynamically possible composites can be achieved when a polymer and the clay are amalgamated (Tactoids, Intercalated, Exfoliated). This arrangement impacts the thermal stability of the whole formulation [119]. Films synthesized from a single material often offer poor thermal stability [120]. Nanocellulose, nanostarch, and nanochitosan-based nano-reinforcements of edible films have been reported to improve the thermal resistance of biopolymers. Cellulose nanocrystals from bacterial cellulose produced by Gluconacetobacter xylinus were loaded into a gelatin matrix to prepare edible nanocomposites with improved thermal properties [121]. Sessini et al. [122] extracted starch nanocrystals from waxy cornstarch granules through acid hydrolysis and used these particles to increase the mechanical attributes and thermal stability of glycerol-plasticized potato starch edible films [122]. Starch nanocrystals have been used in edible films to improve crystallinity and thermal profile [123]. These starch nanocrystals have been recently synthesized to improve the thermal resistance of starch films [124]. Chitosan nanoparticles have been used to improve the thermal stability of pectin-based nanocomposite edible films and starch/guar gum-based coating materials [125,126,127].

Among inorganic materials, nanoscale hydroxyapatite (an inorganic component of bones and teeth) has been reported to improve the thermal properties of bovine skin-derived type A gelatin films [128]. Similarly, magnesium hydroxide nanoplates loaded in a pectin matrix improved thermal properties [129].

Effect of Crosslinkers on Thermal Stability of Edible Films

The cross-linking approach is one of the conventional and reliable approaches to chemically modify a polymer, resulting in a material with improved thermal resistance. This approach makes the films more resistant to heat and at the same time offers a superior level of dimensional stability, mechanical strength, and chemical and solvent resistance. Crosslinking improves the polymer’s thermal stability by reducing the molecular rotation and vibration that occurs during thermal excitation [130]. The thermal properties of the films are significantly impacted by the extent of crosslinking, the regularity of the resultant material, and the percentage of crystallinity. The addition of natural (tannic, gallic, caffeic, ferulic) acids, or synthetic crosslinkers (formaldehyde and glutaraldehyde, carbodiimides, polyepoxy compounds, acylazide etc.) can improve the thermal stability of the edible films [91, 131, 132]. Additionally, some other natural agents such as genipin, flavonoids (catechin, flavone, or quercetin) carboxylic acids (malic acid, citric acid, and succinic acid) and xylose also showed crosslinking effects on the films [133,134,135][132].

The addition of crosslinkers can produce larger molecular aggregates resulting in an enhanced film-forming solution with high viscosity characteristics. These crosslinkers increase the covalent bonding, to establish stronger intermolecular covalent bonds, to attain closer molecular packing and reduced polymer mobility [136]. These crosslinkers can efficiently improve intramolecular cross-linking (in polymer) or intermolecular cross-linking (between polymers). Carbodiimide is also utilized as a potential crosslinkers in the food packaging industry [91]. The addition of gallic acid improved the thermal stability of gelatin and casein-based composite films [133].

The incorporation of pectin /sodium alginate via crosslinking with citric acid and tartaric acid slightly improved the thermal stability of the composite films [137]. The inclusion of citric acid in carboxymethyl chitosan/poly(vinyl alcohol) improved the thermal stability of the films [138]. Xylose has also improved the thermal stability of grasshopper protein/soy protein isolate/cinnamaldehyde films [132]. The thermal stability of the casein films was improved by crosslinking with tannic acid [139]. Crosslinking can also be achieved by ionic gelation (metallic ions), electrostatic interactions (opposite charge to form PEC), and a self-assembly process. However, the amount and type of crosslinker and type of polymer matrix determine the extent of the crosslinking reaction. Figure 3 shows the effect of incorporating crosslinkers in the formation of edible films.

Fig. 3
figure 3

Effect of the crosslinkers on the thermal stability of the films

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Final Remarks

Due to renewable, biodegradable, and relatively high safety profile, natural polymeric films are considered potential replacements for conventional plastic-based packaging. However, these biobased films failed to offer high mechanical and thermal stability equivalent to synthetic food packaging materials. The low thermal resistance of the biopolymeric films and their unsatisfactory thermal stability profile always hinder their reachability to the market. Advance understanding of thermal analytical tools and approaches to cater more information from thermograms of biopolymeric films could help in determining their thermal stability efficiently. Additionally, the thermal degradation profile of neat biopolymeric films offers useful information about native polymer thermal stability that helps in designing the composite formulation with relatively high thermal resistant films. The design and development of stable heat-sealable biopolymeric films need more emphasis on their fluctuating crystallinity, morphology, thickness, and other parameters under unfavourable temperature conditions. Composition, especially the incorporation of additives in the case of active packaging films plays a vital role in regulating the thermal profile of the biopolymeric films. With the growing interest in sustainable and eco-friendly packaging solutions, the insights presented in this review can have a significant impact on the development of future food packaging technologies that prioritize both functionality and environmental considerations.