# Changes in the structure and barrier properties induced by corona atmospheric plasma process applied on wet gelatin layers for packaging film applications

Clément Poulain, Claire-Hélène Brachais, Anna Krystianiak, Olivier Heintz, Marie-Laure Léonard, Nasreddine Benbettaieb, Frédéric Debeaufort

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## ► To cite this version:

Clément Poulain, Claire-Hélène Brachais, Anna Krystianiak, Olivier Heintz, Marie-Laure Léonard, et al. Changes in the structure and barrier properties induced by corona atmospheric plasma process applied on wet gelatin layers for packaging film applications. Food Hydrocolloids, 2025, 160 (3), pp.110858. 10.1016/j.foodhyd.2024.110858. hal-05106100

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# Changes in the structure and barrier properties induced by corona atmospheric plasma process applied on wet gelatin layers for packaging film applications

Clément Poulain<sup>a,b</sup>, Claire-Hélène Brachais<sup>a,c</sup>, Anna Krystianiak<sup>c</sup>, Olivier Heintz<sup>c</sup>, Marie-Laure Léonard<sup>a</sup>, Nasreddine Benbettaieb<sup>b,d</sup>, Frédéric Debeaufort<sup>b,d,*</sup>

## ARTICLE INFO

**Keywords:**

Gelatin layer

Surface treatment

Corona plasma

Permeability

XPS

Biopackaging

## ABSTRACT

Fish gelatin is a by-product of the agri-food sector, derived from fish processing waste that can be used as or along a packaging material in direct contact with food while maintaining biodegradable properties. The aim of this work was to apply the corona discharge process on fish gelatin hydrogels and to analyse the properties of resulting films after drying. The understanding of changes in the physical or chemical structure of the gelatin layer was focused on at molecular level. Changes in the surface free energy due to corona treatment displayed an increase of the hydrophobicity at highest power tested. Results demonstrated the plasma's species crossed the gelatin layer and induces weak changes. Indeed, the permeability to various gases was maintained even at high power, confirming that corona discharge systems are suitable devices for treating wet hydrogels without significant degradation of the gelatin layer.

## 1. Introduction

With the widespread introduction of plastics at an industrial scale, starting with Bakelite and Formica, plastics have become an essential part of virtually every sector of industry since the 80's (Plastics, 2023). Easy to shape, versatile and often inexpensive, plastics are mass-produced. However, this democratisation has some major drawbacks. Plastics are used extensively in the production of disposable products. In addition to the environmental impact caused by the use of fossil resources in their manufacturing stage, these products present end-of-life issues due to their durability (Swetha et al., 2023). With more than 460 million tonnes of plastics produced worldwide in 2021 (OCDE, 2022), the majority of oil-based plastics cannot be recycled or reused because they are unclean, too thin or manufactured in multiple layers. Too often incinerated, landfilled or dumped in the wild, the use of these products is causing a major ecological and health crisis (Jung et al., 2023).

Biosourced and biodegradable polymers offer an attractive alternative to these materials (Jung et al., 2023). Often made from biomass,

which can be derived from agro-industrial wastes, these plastics are designed to degrade completely in an industrial composter (Lim & Thian, 2022). However, biopolymers encounter both economical and industrial boundaries due to their relative high costs (compare to oil-based materials) and new specific transformation tools requirements as current resources are geared towards materials derived from petroleum.

These new categories of polymers also suffer from lower functional properties. For example, the food industry uses PLA as a packaging film. Still, its barrier properties remain weak compared to more traditional polymers such as polyethylene, PET or complex films, particularly in moist environment (Swetha et al., 2023). To address these drawbacks, PLA can be directly modified by changing the formulation of the material using additives such as fillers. Another approach is to coat PLA with another biodegradable non-toxic material that will also compost at the end of its useful life (Jung et al., 2023).

Several types of compostable coatings have been considered, using polysaccharides like cellulose, starch and chitosan (Basiak et al., 2019; Cui et al., 2022; Li et al., 2020), proteins such as gelatin, gluten or zein

<footnote><sup>a</sup> Université Bourgogne Franche-Comté, ESIREM, Avenue Alain Savary, 21078, Dijon, Cedex, France</footnote>

<footnote><sup>b</sup> Université Bourgogne Franche-Comté, Institut Agro, Université Bourgogne, INRAE, UMR PAM 1517, 21000, Dijon, France</footnote>

<footnote><sup>c</sup> Université Bourgogne Franche-Comté, CNRS, UMR 6303, Laboratoire Interdisciplinaire Carnot de Bourgogne (ICB), 9 Avenue Alain Savary, 21078, Dijon, France</footnote>

<footnote><sup>d</sup> Université de Bourgogne, IUT-Dijon-Auxerre, Department of BioEngineering, 7 blvd Docteur Petitjean, F20178, Dijon, France</footnote>

<footnote>* Corresponding author. Université Bourgogne Franche-Comté, Institut Agro, Université Bourgogne, INRAE, UMR PAM 1517, 21000, Dijon, France.
E-mail address: frederic.debeaufort@u-bourgogne.fr (F. Debeaufort).</footnote>
(Chen et al., 2019; Cho et al., 2010; Etxabide, Leceta, et al., 2016) and even some ester-loaded synthetic polymers, which can confer degradation properties. Such coatings aim to protect, preserve and contain the packaged food (Mendes & Pedersen, 2021). In addition, the packaging must comply with necessary regulatory and commercial aspects regarding traceability, price and appearance. As an example, PLA with a coating should not be too visibly altered and retain a pleasant, appealing appearance.

Gelatin is a group of proteins derived from the acid or alkaline hydrolysis of collagen, a building block of skin and bone (Alipal et al., 2021; Nur Hanani et al., 2014). This compound is abundant as a co-product and by-product of the leather and food industries. However, porcine and bovine gelatin, the most widely used and studied, can lead to religious and health problems if coming into contact with food (Etxabide, Leceta, et al., 2016). Fish gelatin is therefore preferred for packaging applications with the benefit of not competing with food production as a reused marine by-product (Alfaro et al., 2015; Hosseini et al., 2016; Limpisophon & Schleining, 2017; Sow & Yang, 2015). However, fish gelatin could bring undesired odour to the packaging material that should be tuned. Within the frame of a circular economy approach, a coating of fish gelatin alone or incorporating bio-active compounds could therefore be considered for use in barrier or active packaging (Debeaufort et al., 2022), competing with petroleum-based materials. However like many compostable compounds, gelatin is very sensitive to moisture (Etxabide, Leceta, et al., 2016; Park et al., 2021). Therefore, the coating must be modified to change its properties in response to humid environment exposition. Indeed, some studies focused on chemical or physical crosslinking of gelatin for reducing both solubility and mass transfers through gelatin films but very few consider friendly environmental techniques, except using natural compound for a green chemical crosslinking (Albert et al., 1986; Etxabide, Urdanpilleta, et al., 2016; Kchaou et al., 2019; Peng et al., 2021; Zhang et al., 2010). For instance, using phenolic compounds could help to crosslink gelatin, but it often needs high temperature or another physical processes like UV irradiation to improve reaction.

Atmospheric corona is one possible process for modifying gelatin in film form (Božović et al., 2023). This technique is widely used in industry to change surface properties (roughness, adhesion, wettability) and is considered as a relatively energy efficient and non-toxic process (Pankaj et al., 2015). The gelatin network has been shown to cross-link under plasma treatment, thus improving the water resistance properties (Liguori et al., 2016). In addition, several studies have shown that the use of corona destroys or prevents the growth of bacteria and other harmful pathogens on the treated surface (Pankaj et al., 2014; Sani et al., 2023).

This process generates a non-thermal plasma under atmospheric pressure and temperature using ambient air. Plasma here refers to an ionised gas. Cold Air Plasma (CAP) is created within ambient air and just above room temperature (at 40 °C on average and at maximum 60 °C) instead of traditional plasma formed largely over 1000 °C. Cold air plasma is made by a pulse electric discharge between electrodes, producing amounts of ions, radical species, UV light and minor heat (Goldman et al., 1985).

Compare to plasma torch, jet equipment or other plasma applicators different than corona (discharge barrier plasma), the corona process usually applies locally less energy per surface area and has a limited temperature impact (Niemira, 2012; Pankaj & Thomas, 2016).

Moreover, by improving the functional properties of gelatin coatings, the use of CAP can be a useful tool on packaging for providing surface decontamination (not studied here) without the need of any solvents nor harmful chemicals. Overall, the multiple functions of corona treatment could replace some of the more power-hungry processes in the production line, thereby reducing the energy costs of production. This study will focus on studying the impact of the corona process on wet gelatin gel properties, in order to optimise the properties required for food-grade packaging or at least not degrading the gelatin properties used

as a carrier for active compounds. In this aim, the structure of fish gelatin films will be studied and the modifications caused by the plasma on the wet system, which have been minimally or not documented at all so far, will be analysed.

## 2. Experimental

### 2.1. Materials

A food grade fish gelatin powder, used as a film forming agent, was purchased from Louis Francois (Marne-La-Vallée, France). The degree of bloom of gelatin is 200 and its viscosity is 3.5–4.5 mPa s at a concentration of 6.6% in water and 60 °C according to supplier.

Glycerol of food grade (99.5 % purity, Fluka-Fisher Scientific SAS, Illkirch, France) was added to the film forming solution to plasticise the film and make it easy to handle. Glycerol was chosen for several reasons: it is a natural compound already found in many food products and is non-toxic, making it safe for food packaging. This plasticiser is also commonly used with biopolymers in many studies, enabling direct comparisons with many other papers (Rivero et al., 2010; Tyuftin & Kerry, 2021).

Potassium acetate, potassium nitrate, magnesium nitrate hexahydrate, sodium bromide, sodium chloride, potassium chloride and diammonium phosphate salts (min 99% purity, Sigma Aldrich, St Quentin Fallavier, France) were used to fix the relative humidity of the atmospheres used for some characterisation such as sorption isotherm and permeability. Ethylene-glycol and diiodomethane used for goniometry characterisation were at least 99 % purity (Sigma Aldrich, St Quentin Fallavier, France).

### 2.2. Films preparation

A fish gelatin solution was prepared in distilled water. The powder (15 % w/w of distilled water) was dissolved and stirred at 50 °C during 30 min before adding 30 % (w/w of dry gelatin) of glycerol, stirred for 10 min prior casting. The content of 30% glycerol to the gelatine weight was chosen aiming to maximise the molecular mobility and, thus, the reactions induced by the corona plasma treatment in a plasticized network.

The films were casted into strips onto clean polystyrene plates using a spreader set to 750 µm (TLC Spreader, Desaga, Germany) under ambient conditions. When the gelatin is sufficiently viscous and gelled to be handled (<5 min) the still wet film-forming hydrogels were treated with Cold Air Plasma (CAP treatment for short).

An Arcotec CAP generator was used (Arcotec GmbH, Monsheim, Germany). A large potential difference is applied between two electrodes of different shapes, as shown in Fig. 1. The corona equipment characteristics were 230 V, 50 Hz, 4.5 A, and a maximum of 1000 W for the power input. The adjustable output electrode had the following characteristics: 3–20 kV, 25–50 kHz, 200 mm wide electrode manual roller, and a 460 × 460 mm² counter electrode plate. The distance between the sample surface to treat and the electrode roller is fixed to 1

<figure><img src="image_5.png" /><figcaption>Fig. 1. Schematic representation of the cold air plasma generator.</figcaption></figure>
mm. The treatment is carried out under a fume hood for safety reasons (release of ozone) at ambient conditions (25 ± 2 °C and 40 ± 5 %RH).

The high intensity of the electric field will locally produce electric arcs in the air in contact with the roll electrode, ionising the chemical species present. The device's geometry determines its use and the shape of the plasma obtained: in this case a crown shape, hence the name corona. Treatment is carried out by multiple corona passes at a defined and constant speed of 2 cm/s and a power of 150 W.

Equipment's used in many studies (jet or torch, microwave, radio-frequency; or gas discharge in confined chamber) applied approximately the same range of electrical power (from 100 to 400 W) but the exposure times are much longer from 10 to 60 min (Laroque et al., 2022; Pankaj et al., 2018), compared to Corona for which we applied a few seconds as described in Table 1. Consequently the energy per surface area on the wet gelatin layer is at least one or two orders of magnitude lower with the Corona process we used.

The nomenclature of film according to the Corona discharge treatment is given in Table 1. Sample name is related to the Corona power, film composition were kept the same.

All films were left to dry during 24 h in controlled conditions (25 ± 2 °C and 40 ± 5 %RH). When dried, the film surfaces were annotated to track which side was exposed to plasma and peeled off the polystyrene plates.

Before all characterization measurements, all film samples were equilibrated at 25 ± 2 °C and 50 % relative humidity for at least 3 days to reach equilibrium. For all characterisations, the film thickness was measured prior analysis at 5 positions of the sample and average, using a digital caliper (Hans Schmidt and Co GmbH, FD100, Waldkraiburg, Germany) which accuracy is 1 µm in the 0–100 µm and 10 µm in the 100 µm-12.5 mm, except for permeability tests where thickness have been measured after permeation.

## 2.3. Film characterizations

### 2.3.1. Water sorption of the gelatin films

The gelatin film contains a proportion of so-called structural water (water remaining between the gelatin chains and the glycerol due to intermolecular interactions during film drying). As gelatin is a highly hygroscopic material, this water content can change depending on the environmental conditions (Debeaufort et al., 2022). The water content of films is a critical point for their use as packaging as it influences not only the mechanical properties (plasticizing effect of water on the network) but also the affinities for certain compounds (influence on permeability).

The sorption or desorption of water as a result of relative humidity can be followed by monitoring the mass over time according to the Wolf et al. method (Wolf et al., 1985). This was done using hermetically sealed chambers containing mini-fans. These were activated every 2 h for 15 min to ensure a homogeneous atmosphere. Each chamber contained the different film samples with 5 duplicates. The different atmospheric conditions tested were 22, 44, 52, 58, 75, 84 and 92%RH (in a room regulated at 25 ± 2 °C), fixed by saturated salt solutions of potassium acetate, potassium nitrate, magnesium nitrate hexahydrate, sodium bromide, sodium chloride, potassium chloride and diammonium

<caption>**Table 1**</caption>

Sample name and characteristics of the cold air plasma treatment of each samples with error – the number in the sample name refers to the Corona power applied on samples.

<table><thead><tr><th>Sample name</th><th>CTRL</th><th>C150</th><th>C300</th><th>C600</th><th>C900</th></tr></thead><tbody><tr><td>Corona power (W)</td><td>0</td><td>150</td><td>300</td><td>600</td><td>900</td></tr><tr><td>Passes of Corona applicator</td><td>0</td><td>1</td><td>2</td><td>4</td><td>6</td></tr><tr><td>Exposition time [s]</td><td>0</td><td>3 ± 0.5</td><td>6 ± 1</td><td>12 ± 1</td><td>18 ± 2</td></tr><tr><td>Corona energy exposure of film surface [J/cm²]</td><td>0</td><td>4.5 ± 0.4</td><td>9 ± 1</td><td>18 ± 2</td><td>27 ± 3</td></tr></tbody></table>

phosphate, respectively.

For this test films were cut (1 × 2 cm²) and placed in labelled glass vials of known mass. The sample weights were measured daily using a precision balance (Sartorius BP211D) until the films were balanced. When all sample weights were constant (sorption equilibrium), the vials were placed in an oven at 102 °C for 2 h to determine the film dry matter and water content at equilibrium.

Using a dedicated MATLAB programme, which combines data processing and solver, it is possible to fit the sorption data to the GAB (Guggenheim-Anderson-De Boer) model (Anderson, 1946). The program process for the fitting of the isotherms with the GAB model is designed and schemed in Appendix 1 of the supplementary data. This extension of the BET (Brunauer-Emmett-Teller) (Brunauer et al., 1938) model is based on a triplet of parameters: M, C and K (Equation (1)).

$$W = \frac{M*C*K*Aw}{(1 - [K*Aw])* (1 - [K*Aw] + [C*K*Aw])} \quad (1)$$

where W is the film's water content (gram water/gram dry basis), Aw is the water activity and M, C and K are the parameters to be determined. M refers to the water content of the first water layer adsorbed on the solid surface. The parameter C represents the adsorption energy of the water monolayer, while K relates to the adsorption energy of the subsequent water multilayers (Blahovec, 2004).

In the absence of additional equations, it is necessary to use other forms of this formula. What's more, solutions must make physical sense, imposing additional constraints. The missing information for the solver is provided by a first fit of the data with a polynomial and several related calculations. These form the proper starting hypothesis for the solver. To ensure that the system is stable (no deviation during the calculation process), a second solver is used immediately after a second modification of the starting hypothesis.

### 2.3.2. Oxygen and water vapour permeabilities

The oxygen permeability (OP) of the films was measured using the standard manometric method ISO 15.105 (International Standard Organisation, 2007). A Brugger GTT permeameter (Brugger Feinmechanik GmbH, Munich, Germany) was used and consist in a two-chamber cell, one is initially under primary vacuum and the opposite chamber is swept with a dry or wet oxygen flow.

The gelatin film acts as a barrier between the two chambers. The relative humidity and temperature were regulated during the experiment (50 %RH and 25 °C). By monitoring the pressure over time, the Brugger equipment determined the transfer rate of the gas (GTR) through the film when the steady state was reached. GTR was then converted to OP expressed as cm³.m/m².s.Pa. The OP was calculated from the average thickness of the film and considering the GTR and pressure differential. The film thickness was measured after OP determination using an automatic digital caliper.

Indeed, thickness is determined after permeability measurement to prevent any potential damage due to the metallic gauge of the caliper. Moreover, the real thickness of the film at the steady state of permeation, considering the possible swelling of the film during permeation due to moisture, has been measured at the end of permeation experiment. The gravimetric cup method described in ISO 2528 (2017) and adapted to hydrophilic biopolymers by Debeaufort et al. (1993) was used to measure the water vapour permeability (WVP) of the films (Debeaufort et al., 1993; International Standard Organisation, 2017). The chosen relative humidity differential was 33–75 %RH at 25 ± 1 °C. Permeation cells were placed in a closed and ventilated chamber (Binder, KBF240, Germany) regulated at 25 °C, as the relative humidity is set at 33% RH. A saturated saline solution of NaCl is placed in the permeation cell to obtain an atmosphere of 75% RH. A film sealed with a silicone ring is placed over each cell ensuring airtightness between the film and the cell. The water vapour diffuses through the film due to the humidity differential, leading to a loss of the cell mass. The water vapour permeability,
expressed in g.m⁻².s.Pa, is calculated at the steady state of permeation from the cell mass variation over time, the water vapour partial pressure differential and the surface area of the film. To avoid damaging the samples, the thickness was measured only after the water permeability experiments.

The data collected from the sorption isotherms can then be processed to obtain the average water content of films at the steady state of permeation subjected to a given linear water activity gradient across the film. By keeping the same 33–75 %RH gradient for the calculations and assuming that the water activity (linked to the chemical potential of water) profile is linear through the thickness, the calculation provides access to the water content profile through the thickness of the gelatin films. This involves calculating the theoretical water content at which a given water activity occurs in a film and then fitting this to the experimental data. The average water content in the film is then calculated by integration along the thickness using the polynomial that best fits the data.

### 2.3.3. ATR-FTIR spectrometry

Fourier-transform infrared spectroscopy (FTIR) spectra of the fish gelatin films were acquired using an IR Affinity-FTIR spectrophotometer (Spectrum 65; PerkinElmer, Haguenau, France) equipped with an Attenuated Total Reflectance (ATR) attachment with a Ge crystal. Measurements (side exposed to air during the drying process in contact with the Ge crystal) were done in the range from 4000 to 600 cm⁻¹, at room temperature (25 °C), using a resolution of 2 cm⁻¹, and 32 scans. The Spectrum Suite software was used to analyse the measured spectra, and the results were represented as transmittance. Data results have been analysed and interpreted according to previous paper (Božović et al., 2023; Nandiyanto et al., 2019).

### 2.3.4. Surface characterisations

The goniometry method was used to determine the contact angle of liquid droplets at the surface of films using the sessile drop method (Good, 1992; van Oss, 2007). The surface free energy of the solid phase was calculated from the contact angles (van Oss, 2007; Young, 1805; ZISMAN, 1964).

The contact angles were analysed onto the side exposed to air during the drying process (side treated by CAP) using a DSA30S goniometer (Drop Shape Analyzer 30S, Kruss GmbH, Germany), equipped with an image analysis software (Advance, Kruss GmbH, Germany), in a room with controlled temperature and humidity (25 °C, 50% RH) (Debeaufort et al., 2022).

Measurements were performed for four liquids: water, glycerol, ethylene-glycol and diiodomethane. A drop of tested liquid (~2 µL) was automatically deposited on the gelatin film surface (exposed to CAP if treated). The geometric parameters of the drop were measured every second over 2 min, the most important being the contact angle between the baseline of the drop and the tangent at the drop boundary and the droplet volume over time. From the contact angle, volume and critical surface tension of liquids able to wet the film surface, the surface free energy (and its polar and dispersive components) of films and the water absorption rate have been calculated according to procedures previously described (Božović et al., 2023; Hambleton et al., 2012; Owens & Wendt, 1969; ZISMAN, 1964).

The chemical state and composition of the film surfaces were studied by X-ray Photoelectron Spectroscopy (XPS) on an XPS PHI 5000 VersaProbe I (Physical Electronics, Chanhassen, USA) with a monochromatized Al Kα1 source (1486.7 eV). The area of measurement was a circular disk of 200 µm in diameter. The signal comes from the first 10 nm of the sample. First a wide scan (survey) was used to detect the element on the surfaces in which a high-resolution scan was then used to selectively measure the signal of particular elements, providing information on the composition and bonding state/chemical environment for the main interesting peaks (XPS, 2023).

Only the CTRL, C300 et C900 samples were analysed on each side

under ultra-high vacuum. The samples used had just been gelled and dried before being vacuum pumped. Special care was taken to avoid any form of surface contamination. Plasma treatment was performed during gelation.

The side exposed to air (and CAP) during the drying/gelling process was recorded as R, opposed to the side exposed to polystyrene casting-support plates and ‘protected’ from air (labelled as PS).

All film samples had equivalent drying process as the relative humidity was the same for all the film making. However, the water content at equilibrium drying could differ from samples due to corona process possible composition modification. Yet, the films are considered perfectly dry due to the high vacuum used for XPS (around 10-8 Pa). Their water content is only related to the water involved in the structure and not available for reaction or evaporation.

Data were analysed using Casa XPS software (Version 2.3.25) (Fairley et al., 2021). The binding energies were calibrated from the C 1s (C–C and C–H bonds) peak supposed to be at 284.8 eV. After calibration and by assuming a Shirley background the area of each signal was computed to obtain surface composition.

For a better interpretation of the impact of the corona discharge treatment, all interested peaks (carbon C1s and oxygen O1s), when possible and useful, have been decomposed using the narrow scan data. Curve fitting of the C1s peaks were made using mixed Gaussian–Lorentzian curves (70% of Lorentzian character). All components were set with the same procedure assuming same width at half-height of the peak and optimized by reducing the Root Mean Square Error.

## 2.4. Statistical analyses

All the data were at least triplicated and the statistical analysis (one-way ANOVA followed by Tukey test) was done using SPSS 13.0 (SPSS Inc., Chicago, IL). A standard deviation (p-value <0.05) at the 95% confidence level was used to compare data. The GAB model fit was evaluated, and the worst error found (10%) was used during data evaluation.

# 3. Results and discussion

As a first step, functional properties of interest for packaging were measured. Thus oxygen and water vapour permeabilities were determined on gelatin films treated with different levels of CAP. These data were then combined with water sorption experiments to compare the average water content of films subjected to the same humidity gradient used for water vapour permeability experiments. In a second step these same data were also used to calculate sorption parameters according to the GAB model using a MATLab programme.

Finally, more detailed analyses were carried out to understand the physical-chemical and chemical modifications induced by CAP on the structure. The surface was analysed using X-ray Photoelectron Spectroscopy and goniometry to determine the chemical environment on the surface and the associated free energies.

This research strategy will allow us to qualify the possible changes in structure and properties of CAP-treated gelatin films simulating a coating applied onto biodegradable polymer such as PLA.

Except for film at 900 W exposure, no eye visible difference in their aspect (colour, transparency, shininess) have been observed. However, some brown spots are visible on the 900 W treated film, many at the edge. The burned zones at the edge of wet gelatine layer could correspond to only the heat effect, probably due to the quite small distance between the electrode and film surface (1 mm) as no sparks was observed. If electrical sparking occurred due to conductivity of the high water content of the gelatine layer, no visible sparks nor related noise were observed. Moreover, the bottom PS plate between the wet gelatine layer and the ground electrode was 5 mm thick, enough to theoretically prevent electrical conduction and thus sparking discharges. Burning only occurred at a power of 900 W for which the temperature increase
was noticeable (higher than 60 °C, but quite difficult to measure accurately).

## 3.1. Water sorption on the gelatin films

The equilibrium in the sorption of water vapour by the films was achieved over fairly long periods (3–5 weeks). As the films were initially conditioned around 50% RH, a loss of mass (of water) was observed for samples stored at humidities below ambient, and the opposite for those exposed to higher relative humidities. Once equilibrium was reached, sorption isotherms could be plotted, representing the water content (to dry basis) at equilibrium as a function of relative humidity. Fig. 2a displays the moisture sorption isotherm of the control film (CTRL) and those of all the other films (C150 to C900) are given as appendix 2 in the supplementary data. The vertical dotted lines indicate the water activity values used for moisture differential used for the water vapour permeation, discussed in next section.

The shape of the isotherms obtained with the sorption analysis, type II according to the IUPAC international classification of sorption isotherms, is characteristic of hygroscopic products (Sing, 1985). The shape of the isotherm is very similar to those obtained by other authors for gelatin (Aguirre-Alvarez et al., 2013; de Moraes Crizel et al., 2018; Hazaveh et al., 2015), having values very similar in the water activity range studied. The differences observed from the literature in the water content for a given RH remained less than 4%, and is mainly due to glycerol content that varies according to the authors. Indeed, the glycerol content used by the previously cited authors is different of the content we used. It is well-known that glycerol content favours water sorption (and thus water content) at the higher humidities in edible hydrocolloid-based films. In fact, due to its plasticizing efficacy, glycerol induces a swelling of the denser zones of the gelatin network, making accessible some absorption sites for water (Basiak et al., 2018; Izzi et al., 2023; Kokoszka et al., 2010; Lau & Sarbon, 2022). Glycerol is a hygroscopic substance often used as moisturizer and thus this property also favors the moisture absorption (Fisher et al., 2020; Morita et al., 2022).

The isotherms were fitted with polynomials to estimate the average water content through the gelatin films during moisture permeation at the steady state of transfer, by assuming a linear profile of water activity (Fig. 2b and appendix 3 of Supplementary data) cross the film thickness.

Using solvers along with modelling via MATLAB, it is possible to apply the GAB model to the data acquired. The results for the control film is given in Fig. 2a (and in appendix 2 of supplementary data for the other film samples) and in Table 2. The fitting with the program process wasdesigned to obtained mathematically and physically consistent

<caption>**Table 2**</caption>

GAB parameter values calculated by MATLAB (considering the normalised RMSE, and the R² of the fit to model) and W<sub>eq</sub> the average water content of films during permeation under a 33–75%RH gradient.

<table><thead><tr><th>Sample</th><th>CTRL</th><th>C150</th><th>C300</th><th>C600</th><th>C900</th></tr></thead><tbody><tr><td>M [g/g<sub>d.m</sub>]</td><td>0.087<sup>a</sup></td><td>0.083<sup>a</sup></td><td>0.088<sup>a</sup></td><td>0.071<sup>b</sup></td><td>0.057<sup>c</sup></td></tr><tr><td>C</td><td>4.4<sup>a</sup></td><td>2.1<sup>b</sup></td><td>1.35<sup>c</sup></td><td>5.7<sup>d</sup></td><td>4.4<sup>a</sup></td></tr><tr><td>K</td><td>0.93<sup>a</sup></td><td>0.94<sup>a</sup></td><td>0.95<sup>a</sup></td><td>0.975<sup>a</sup></td><td>1.01<sup>a</sup></td></tr><tr><td>R<sup>2</sup></td><td>0.992</td><td>0.949</td><td>0.962</td><td>0.994</td><td>0.949</td></tr><tr><td>W<sub>eq</sub> [g/g<sub>d.m</sub>]</td><td>0.144</td><td>0.124</td><td>0.118</td><td>0.135</td><td>0.114</td></tr></tbody></table>

<sup>a,b,c,d</sup> values in the same row are not significantly different at p-level = 0.05.

results. Indeed, the values calculated for the untreated film made of gelatin and glycerol (CTRL) are not different to those calculated by Diaz et al. (Díaz et al., 2011) for bovine gelatin containing various content of glycerol, and are very similar to the M, C, K values obtained for pure gelatin by Aguirre Alvarez (Aguirre-Alvarez et al., 2013).

The C values decrease with cold air plasma for reasonable powers of corona treatment (up to 600 W), indicating a modification of the films and a slight increase of the hydrophilicity. For higher CAP powers, the C value rises indicating that more energy is required for water binding, that is also confirmed from goniometry measurements (discussed in part 3.4). For the C600 and C900 films, the both M and C values decrease with the corona power increase, which may indicate that the surface is becoming more hydrophobic or has been degraded at the highest power applied. Indeed, the water content of the monolayer, corresponding to the structural water or bound water is reduced. This behaviour is often attributed to proteins degradation by the heat that induces condensation and/or Maillard reactions (Nilsuwan et al., 2020). Indeed, protein denaturation often provokes a hydrophobicity increase and thus a lower solubility in aqueous liquids. Moreover, few authors displayed that Maillard reaction, in a very low extend, could occur between proteins (particularly gelatin) and glycerol in dry conditions and high temperatures, even in absence of sugars. This therefore favours crosslinking in gelatin network that makes it less sensitive to moisture (Cerny & Guntz-Dubini, 2006; Du conseille et al., 2015; Kchaou et al., 2019; Smarrito-Menozzi et al., 2013). Moreover, crosslinking from Maillard in proteins significantly reduces the water sorption by gelatin films (Etxabide, Urdanpilleta, et al., 2016; Gómez-Guillén et al., 2011).

However, at 900 W, the values return to their original state. One possible explanation for this behavior is that corona plasma exposure may have changed the number of surface polar sites due to etching (could access to original structure), which seems primarily related to the molecular structure of the different layers of the hydrogel. However, this

<figure><img src="image_6.png" /><figcaption>**Fig. 2.** (A) Sorption isotherm of the CTRL film; (B) water activity linear gradient (in blue) and water content profile (in orange) for the CTRL film at the steady state of permeation for the 33–75 RH differential, calculated using the model fit.</figcaption></figure>

<figure><img src="image_7.png" /></figure>
hypothesis was not confirmed from XPS analysis.

A more in-depth analysis of the surface is therefore required.

## 3.2. Permeability of fish gelatin to oxygen and water vapour

The permeability results are displayed in Fig. 3a and b, where oxygen permeability decreases with the corona power up to around 600 W, then increases again towards the value of the non-treated films.

From this result, the performance of gelatin films against oxygen seems to be improved (but not-significant) for low corona powers (C300 and C600) as displayed in Fig. 3a. Indeed, a non-significant decrease of 25% of oxygen permeability was measured. We can therefore assume that the corona treatment changes the film's structure by inducing cross-linking or a change in affinity with the various reactive species induced by the plasma (Liguori et al., 2016).

At higher powers, the benefits are lost, which probably indicates film degradation: the film is subjected to slight average heating at the whole surface exposed (about 40 °C on average) by the plasma but locally (at the edge), we observed some burn spots (brown colour) at 900 W due to excess heating points, as described previously. Therefore, we could suspect that a possible Maillard or condensation reaction may then occur, favoured by the presence of glycerol. This is already described in the literature (Cerny & Guntz-Dubini, 2006; Duconseille et al., 2015) but is most probably due to a heat denaturation of the protein that starts for temperature from 40 °C (Gómez-Guillén et al., 2011; Kchaou et al., 2019).

Using gelatin as a coating could enhance the barrier properties of PLA even if CAP does not significantly change performance against oxygen (Božović et al., 2023; Debeaufort et al., 2022). Indeed at 50% relative humidity the oxygen permeability of PLA is almost 18,000 $cm^3 m/m^2 \cdot s \cdot Pa$ compared with the one of gelatin at 570 $cm^3 m/m^2 \cdot s \cdot Pa$, achieving properties close to those of HDPE. These values demonstrate that using gelatin as a coating could be beneficial on PLA regarding oxygen transfer as it lowers the permeability of the whole system (Nilsuwan et al., 2020).

Water vapour permeability for a 33–75 %RH differential as a function of plasma treatment is shown in Fig. 3b. Gelatin, even treated, has higher WVP than PLA due to its higher affinity for liquid water and water vapour than PLA (Debeaufort et al., 2022). However, the results are inconclusive as the permeabilities are only slightly modified by cold air plasma, possibly due to the water plasticization as often stated in the literature (Nilsuwan et al., 2020).

The benefits of cold plasma treatment remain moderate, and further analysis needs to be carried out at lower power levels. The changes in permeability properties are simply observed at this stage and not fully

explained. However, there is no significant loss of barrier properties with plasma treatment, suggesting that the gelatin plasticization by water is not significantly affected by the corona treatment. Indeed, no significant changes were observed in the water content of the films $W_{eq}$ at the stationary permeation state (Fig. 2b for the CTRL film, and appendix 3 in the supplementary data for films C150 to C900) computed from the isotherm fits. The effect of plasticization of the films by water during permeation probably also induces swelling of the films.

## 3.3. Structural analysis by FTIR

All samples were analysed at the side exposed to air and CAP treatment facing the Ge crystal of the FTIR-ATR module (Nandiyanto et al., 2019).

The structures of gelatin films are primarily discussed based on their protein amide groups (Sow & Yang, 2015). The bands of interest in this study are amides A, B, I, II, and III. Due to water and glycerol content, signals from amide A and B are overlapped by -OH bands, therefore focusing the spectra between 2000 $cm^{-1}$ and 600 $cm^{-1}$.

No major changes were observed between the untreated films and the plasma treated samples. As shown in Fig. 4, no new peaks appear along the treatment. This indicates that no new covalent bonds were formed by the CAP or if new groups or covalent bonds have been generated, they are in too low amount to be observable with ATR mode, or are not localised closed to the surface observed. Without a clear peak corresponding to a chemically stable bond (during process) to use as a reference, all quantitative analyses were avoided.

Fish gelatin structure can be influenced by changes in non-covalent bonds/intermolecular interactions, such as van der Waals interactions and hydrogen bonding. The study of chemical shift of amines I, II, and III bands compared to the CTRL sample revealed nothing significant, as most bands were broad and the minimum not changing more than 4 $cm^{-1}$ (Table 3).

In other words, no significant or visible by FTIR-ATR structural changes were observed in the film after CAP treatment. This result is therefore confirmed by the XRD measurement (data not displayed), since no changes are registered in the ratio between amorphous and structured gelatin, with no modifications in the position of the gelatin peaks within the diffractogram.

## 3.4. Surface analysis by goniometry

Goniometry is a simple and quick method used to determine physicochemical properties of smooth surfaces. Droplets of liquid are deposited on gelatin films treated with cold air plasma. The main

<figure><img src="image_8.png" /><figcaption>**Fig. 3.** Influence of CAP power: (A) on the oxygen permeability at 50 %RH; and (B) on the water vapour permeability at 33–75% RH differential of gelatin films compared to commercial films: high density polyethylene (HDPE), low density polyethylene (LDPE) and polylactic acid PLA.</figcaption></figure>

<figure><img src="image_9.png" /></figure>
<figure><img src="image_10.png" /><figcaption>Fig. 4. Normalised results of ATR-FTIR on the fish gelatin films depending of the CAP treatment (maximum transmittance sets at 100%).</figcaption></figure>

<table><caption>**Table 3**
Location and assignment of the amide peaks identified in FTIR spectra.</caption><thead><tr><th rowspan="2">Film sample</th><th colspan="5">Amide function in gelatin network</th></tr><tr><th>A</th><th>B</th><th>I</th><th>II</th><th>III</th></tr></thead><tbody><tr><td>CTRL</td><td>3293 <sup>a</sup></td><td>2945 <sup>a</sup></td><td>1650 <sup>a</sup></td><td>1552 <sup>a</sup></td><td>1242 <sup>a</sup></td></tr><tr><td>C150</td><td>3293 <sup>a</sup></td><td>2944 <sup>a</sup></td><td>1649 <sup>a</sup></td><td>1552 <sup>a</sup></td><td>1241 <sup>a</sup></td></tr><tr><td>C300</td><td>3293 <sup>a</sup></td><td>2945 <sup>a</sup></td><td>1649 <sup>a</sup></td><td>1552 <sup>a</sup></td><td>1239 <sup>a</sup></td></tr><tr><td>C600</td><td>3293 <sup>a</sup></td><td>2933 <sup>b</sup></td><td>1649 <sup>a</sup></td><td>1551 <sup>a</sup></td><td>1242 <sup>a</sup></td></tr><tr><td>C900</td><td>3293 <sup>a</sup></td><td>2943 <sup>a</sup></td><td>1649 <sup>a</sup></td><td>1552 <sup>a</sup></td><td>1243 <sup>a</sup></td></tr></tbody></table>

goniometry results are summarised in Table 4.

The initial water contact angle values are higher than 100°, which indicate apparently a very hydrophobic surface. There is also no clear trend in the data of the initial contact angle according the CAP power treatment. However, WCA at 60 s, displays a fast change in the WCA, indicating a film surface change, probably a swelling and fast absorption, as often observed for such kind of films (Liu et al., 2023; Yao et al.,

<table><caption>**Table 4**
Water contact angle (WCA) with the gelatin film when the drop is formed and after 60 s, critical surface tension (CST) and surface free energy components $\gamma_s^P$ (polar) and $\gamma_s^D$ (dispersive) of the films studied.</caption><thead><tr><th>Corona Power [W]</th><th>WCA [°] at t = 0s</th><th>WCA [°] at t = 60s</th><th>CST (R<sup>2</sup>) [mN/m]</th><th>γ<sub>s</sub><sup>P</sup> [mN/m]</th><th>γ<sub>s</sub><sup>D</sup> [mN/m]</th><th>γ<sub>s</sub> [mN/m]</th></tr></thead><tbody><tr><td>CTRL</td><td>108.2 <sup>a</sup></td><td>103.2 <sup>a</sup></td><td>42.03 (0.80)</td><td>1.00</td><td>52.66 <sup>a</sup></td><td>53.66 <sup>a</sup></td></tr><tr><td>C150</td><td>107.3 <sup>a</sup></td><td>78.6 <sup>a</sup></td><td>40.88 (0.88)</td><td>0.46 <sup>b</sup></td><td>39.65 <sup>a</sup></td><td>40.11 <sup>a</sup></td></tr><tr><td>C300</td><td>107.4 <sup>a</sup></td><td>91.7 <sup>a</sup></td><td>41.28 (0.99)</td><td>0.59 <sup>b</sup></td><td>48.03 <sup>a</sup></td><td>48.62 <sup>a</sup></td></tr><tr><td>C600</td><td>107.6 <sup>a</sup></td><td>99.7 <sup>a</sup></td><td>36.42 (0.81)</td><td>0.07 <sup>a</sup></td><td>41.35 <sup>b</sup></td><td>41.41 <sup>a</sup></td></tr><tr><td>C900</td><td>99.2 <sup>a</sup></td><td>91.3 <sup>a</sup></td><td>38.62 (0.92)</td><td>0.04 <sup>a</sup></td><td>41.95 <sup>b</sup></td><td>41.99 <sup>a</sup></td></tr></tbody></table>

2019). In a same fashion and despite a decrease of the angle over time, no statistically significant changes occur in how the film absorbed water. The water droplets placed on the surface of the film were completely absorbed over time, causing the film to swell. This swelling made the goniometer measurements with water somewhat unreliable, as the baseline had to be adjusted afterwards to follow the actual evolution of the system. This swelling was not present with the other liquids studied, even when using glycerol which was the plasticiser of the system. This confirm the most efficient plasticizing effect of water. The previous assumption on the measured hydrophobic behaviour of the surface is therefore due to swelling during the measurements.

Critical surface tensions (CST) are calculated using the contact angles at the time of initial deposition of the drop on the substrate, knowing the surface free energies of the liquids used. The same angles are used to calculate the polar and dispersive components. Critical surface tension determines the surface tension of a liquid able to immediately wet the surface of a solid. As given in Table 4 the CAP treatment did not change significantly the exposed fish gelatin film surface. The samples are thus highly wettable by liquids having surface tension close to 40 mN/m, like acetone, toluene, ethanol or n-decane.

The energy of adhesion was calculated from the surface free energy values (polar and dispersive components) measured on PLA films and on the gelatin layer according to Dupré equation (Ebnesajjad & Landrock, 2015) and compared to the literature (Bozović et al., 2023). The work of adhesion between PLA film and the wet gelatin layer without corona treatment (CTRL) is 76.6 ± 12.1 mN m⁻¹, between PLA and 300W corona treated gelatin layer is 72.1 ± 12.2 mN m⁻¹. When the gelatin layer is corona-treated after drying (dry gelatin layer) the work of adhesion with PLA was 69.7 ± 1.8 mN m⁻¹, calculated from Bozović et al., 2023. The corona applied on either wet or dried gelatin seems not significantly modify its adhesion on PLA film. In industrial applications, specific minimum values are often considered indicative of suitable adhesion properties. Indeed, values higher than 70 mN m⁻¹ are expected for latex painting whereas values around 65 mN m⁻¹ are more suitable for organic coatings to be applied onto metals can for example (George et al., 2023; Hegemann, 2014). To coat paper and polymer films, values of 60 mN m⁻¹ are usually targeted. The values we obtained are in line

<footnote>a,b values in the same column are not significantly different at p-level = 0.05.</footnote>

<footnote>a,b,c values in the same column are not significantly different at p-level = 0.05.</footnote>
with the adhesion energy requirement (Madeira et al., 2018; Zhao & Kwon, 2011).

Using the known critical surface energies and the polar over dispersive components of each liquid used it is possible to obtain the different components of the surface according to (Owens & Wendt, 1969). As shown in Fig. 5 and considering the precision of this method the surface polarity barely changes with treatment power up to 300 Watts. However, the dispersive over polar ratio increases significantly when higher power is used on gelatin (Fig. 5) and confirms the sorption data discussion. This result is linked to the GAB components, where the hydrophobicity of these same films increased, indicating a possible degradation of proteins at high cold plasma powers. It would be risky to draw other conclusions from these measurements alone. Corona treatment may affect gelatin hydrogels, but this cannot be assessed only by goniometry.

Even if swelling problems are omitted, the method often gives vari-
able results because it assumes that the surface analysed is smooth and
homogeneous. However, corona treatment is known not only to change
surface free energies by incorporating various species, but also to modify
surface roughness. After plasma treatment, treated objects often become
rougher because the surface can be altered (scoured) by the process (Liu
et al., 2023; Pankaj et al., 2015; Yao et al., 2019). This impact of the
treatment implies that the goniometric measurement also takes into
account the different capillary forces on the surface of the films due to
roughness of surface. In other words, the physical effect of the corona on
the gelatin interferes with the measurement of the chemical effect of the
process.

Carbohydrates (cross-linking and glycosylation), lipids (oxidation), and proteins (secondary structure) are more significantly impacted by cold plasma at higher intensities and longer treatment times but these chemical changes depend on the composition of food and thus of the hydration level (Sruthi et al., 2022; Warne et al., 2021; Zhang et al., 2022). However, there is no studies directly related to the impact of the moisture level (water content or water activity) on the efficacy or impact of cold plasma application on packaging nor on foods. Most of the studies have been done on dry or semi-moist hydrocolloid-based products like powders or edible films or coatings(Teresa Fernández-Felipe et al., 2024). We can also assume an effect similar to that observed for plasma-activated water, as a reaction medium containing many reactive species, including H₂O₂ and O₃ (Zhao et al., 2020). Indeed, corona-treated gelatin gel (before drying) has a water content of 85% and a water activity greater than 0.99, i.e. a sufficiently mobile water for reactions. Indeed, particularly on gelatin-rich foods (meat, fish),

<figure><img src="image_11.png" /><figcaption>Fig. 5. Ratio between the dispersive and polar components of the gelatin films at the corona-exposed side as a function of corona power.</figcaption></figure>

plasma-activated water has been shown able to modify animal proteins (denaturation, oxidation, cross-linking) (Patra et al., 2022; Zhao et al., 2020).

Despite the repetition and care taken during handling, it is therefore
necessary to use a more in-depth analysis method to highlight the
chemical modification of the surface induced by the corona treatment.

3.5. Surface analysis by XPS

XPS measurements were conducted on the surfaces of CTRL, C300, and C900 films to detect the elements present on them (Fig. 6). The R surface (air side and corona expose side) and PS surface (casting support side) were analysed, and CAP treated surfaces were measured at two points: the theoretically densest plasma zone and near the edges, relating to the shape of the cylindrical electrode.

This procedure aimed to determine whether or not the radical spe-
cies diffuse into the hydrogel and whether any chemical changes occur
at the surface. The elemental analysis results are shown in Fig. 6, which
displays the overall spectrum. Table 5 gives the elementary composition
of the film surface according to the face and position. As expected for
proteins, bonds containing carbon (C1s), nitrogen (N1s) and oxygen
(O1s) are detected (hydrogen is not detectable in XPS). Sulphur S2p(1/
2) and sulphur S2p(3/2) signals are present to a lesser extent, together
with a few traces of silicon from Si2p (attributed to impurities).

There is a difference between the R and PS sides. The most striking
variation is the one obtained after C300 treatment, where the oxygen/
carbon ratio drops from an average of 0.25 to almost 0.1 (Table 5).

The analysis was then extended by decomposition of the carbon and oxygen XPS peaks. The sulphur signal was too noisy and weak to fit. The nitrogen signal was also mathematically impossible to decompose because the signals overlapped into a near symmetrical shape. appendix 4 of supplementary data displays for the CTRL films (face R) the deconvolution of the carbon and oxygen signals, and for nitrogen and sulphur, the raw signal that was not able to be deconvoluted.

Deconvolution of gelatin posed challenges for several reasons. First, there is an unsystematic inclusion or exclusion of some components in the treatment of the data in the literature (Major et al., 2020). In addition, the attribution and calibration of such peaks is not clear as many sources use different values for the same components (no global agreement on values, no standard) (“Table of Elements | Thermo Fisher Scientific - FR,” 2023; “XPS, AES, UPS and ESCA, laSurface.com,” 2023; “X-ray Photoelectron Spectroscopy (XPS) Reference Pages,” 2023; X-ray, 2023; XPS, 2023). The deformations of C1s due to oxygen and nitrogen overlap, making it impossible to distinguish these elements during C1s deconvolution with our data. Therefore, all decomposition components were set relatively to the C-C/C-H peak and any similar bond between carbon and nitrogen or oxygen was considered as one component. This is illustrated in Fig. 7 for the carbon signal for the air exposed face of the CTRL film.

In Table 6 an effect on the casting support is observed on the CTRL film with the C1s signal. The PS side, which is not exposed to air, contains fewer bonds with oxygen and nitrogen. The two faces, R and PS, therefore appear to be different in terms of their chemical environment without any plasma treatment.

A moderate CAP treatment, C300, greatly reinforces this surface ef-
fect (the difference rises from less than 10% to more than 30% for C-C/C-
H bonds) highlighted in appendix 5 of the supplementary data. The 300
W treatment also seems to result in a chemically uniform surface on the
R side, the side exposed to the corona discharge.

With a more extensive treatment, there is a difference between the
middle and the side of the C900 R surface. The 'edge' treatment is
chemically close to an untreated R surface. On the other hand, the area
that underwent the densest treatment differs from all the R faces studied,
potentially indicating degradation with the plasma, under the influence
of heat, radiation or species generated by the plasma. During the initial
tests at this power, the visual appearance of the films changed and areas
<figure><img src="image_12.png" /><figcaption>**Fig. 6.** Annotated XPS Spectrum of the CTRL fish gelatin film (no CAP treatment) on the side exposed to air during the drying process (KLL correspond to the Auger peak of the elements and was not considered in the study).</figcaption></figure>

<table><caption>**Table 5**
Elementary composition [atomic %] of gelatin film surfaces obtained by XPS.</caption><thead><tr><th>Sample\Component</th><th>C</th><th>N</th><th>O</th><th>S</th><th>Si</th><th>O/C</th><th>N/C</th></tr></thead><tbody><tr><td>CTRL Side R<sup>a</sup></td><td>68.0</td><td>14.6</td><td>17.0</td><td>0.4</td><td>ND</td><td>0.3</td><td>0.2</td></tr><tr><td>CTRL Side PS<sup>b</sup></td><td>70.1</td><td>12.8</td><td>16.2</td><td>0.4</td><td>0.4</td><td>0.2</td><td>0.2</td></tr><tr><td>C300 R<sup>a</sup> Middle<sup>γ</sup></td><td>65.7</td><td>14.1</td><td>18.4</td><td>0.3</td><td>1.5</td><td>0.3</td><td>0.2</td></tr><tr><td>C300 R<sup>a</sup> Edge<sup>δ</sup></td><td>67.7</td><td>14.4</td><td>17.5</td><td>0.4</td><td>ND</td><td>0.3</td><td>0.2</td></tr><tr><td>C300 PS<sup>b</sup></td><td>80.3</td><td>7.2</td><td>11.7</td><td>0.2</td><td>0.6</td><td>0.1</td><td>0.1</td></tr><tr><td>C900 R<sup>a</sup> Middle<sup>γ</sup></td><td>66.5</td><td>14.9</td><td>17.8</td><td>0.3</td><td>0.4</td><td>0.3</td><td>0.2</td></tr><tr><td>C900 R<sup>a</sup> Edge<sup>δ</sup></td><td>70.3</td><td>13.1</td><td>16.2</td><td>0.3</td><td>0.2</td><td>0.2</td><td>0.2</td></tr><tr><td>C900 PS<sup>b</sup></td><td>70.3</td><td>12.8</td><td>16.1</td><td>0.3</td><td>0.4</td><td>0.2</td><td>0.2</td></tr></tbody></table>

α Side R = air/CAP exposed side.

β Side PS = protected from air during drying process.

γ Middle = treated area with a supposed denser plasma.

δ Edge = treated area with a supposed weaker plasma.

ND = not detected.

of the gelatin turned brown (burn spots), which was associated with a thermal degradation of protein (Gómez-Guillén et al., 2011; Kchaou et al., 2019). This confirms the un-homogeneous plasma generation along the electrode under certain conditions.

The additional differences between the faces induced by CAP can be explained by the diffusion of radical species into the gelatin. During treatment, the active species would diffuse to the interface between the hydrogel and the support. After treatment, these species could retro-diffuse through a mechanism similar to ageing (Van Deynse et al., 2014). One hypothesis would be that only the species close to the interface with the support could be protected from this ageing because they were not exposed to oxygen during CAP, in a gelled network with a mobility reducing over time.

## 4. Conclusions

The manuscript explores the improvement of wet gelatin films through exposure to cold air plasma (corona process) for use in biodegradable packaging. The film forming properties and the gelatin network were almost preserved or slightly improved after the corona treatment.

The surface properties (wetting, polarity, etc.) of the films were modified, tending towards more hydrophobic components at the surface exposed to air and plasma. The XPS measurements confirmed the need to differentiate the surfaces (exposed either to plasma and air, or in contact with the support and in oxygen deprived environment) of the gels during the measurements.

Therefore, the treatment has an apparent influence on the film surface characteristics and on the bulk structure due to the diffusion of the reactive species (radicals) within the wet hydrogel.

The use of CAP via corona treatment in combination with gelatin gels on existing biodegradable materials such as PLA and biobased coatings will help to open up new opportunities and increase their use.

However, a better understanding of the lifetime of the reactive species generated by the corona process, of the water activity or water content of the hydrocolloid gel, of the ageing would be necessary to determine their suitability for contact with food. An analysis of the process on the organoleptic and safety properties of such materials is also of key importance to consider for the final application particularly when bioactive compounds are encapsulated within the gelatine layer.

### CRediT authorship contribution statement

Clément Poulain: Writing – review & editing, Writing – original
<figure><img src="image_13.png" /><figcaption>Fig. 7. Curve fitting of C1s peaks of a fish gelatin film without CAP treatment, on the side exposed to air.</figcaption></figure>

<table><caption>**Table 6**
Decomposition of C1s and O1s peaks in XPS and possible groups obtained with CasaXPS software.</caption><thead><tr><th rowspan="2">Name<br>Component</th><th colspan="4">C1s</th><th colspan="2">O1s</th></tr><tr><th>C-C/C-H</th><th>C-O/C-N</th><th>C=O/C-N</th><th>O-C=O/N-C=O</th><th>C=O</th><th>C=O</th></tr></thead><tbody><tr><td>CTRL Side R<sup>a</sup></td><td>39.4</td><td>36.5</td><td>21.8</td><td>2.2</td><td>73.0</td><td>27.0</td></tr><tr><td>CTRL Side PS<sup>b</sup></td><td>47.0</td><td>32.8</td><td>16.7</td><td>3.5</td><td>69.2</td><td>30.8</td></tr><tr><td>C300 R<sup>a</sup><br>Middle<sup>7</sup></td><td>38.5</td><td>37.2</td><td>21.9</td><td>2.6</td><td>72.1</td><td>27.9</td></tr><tr><td>C300 R<sup>a</sup> Edge<sup>δ</sup></td><td>39.9</td><td>36.6</td><td>21.6</td><td>2.0</td><td>70.1</td><td>29.9</td></tr><tr><td>C300 PS<sup>b</sup></td><td>71.4</td><td>18.1</td><td>7.7</td><td>2.8</td><td>73.5</td><td>26.6</td></tr><tr><td>C900 R<sup>a</sup><br>Middle<sup>7</sup></td><td>48.2</td><td>31.3</td><td>18.3</td><td>2.2</td><td>69.7</td><td>30.3</td></tr><tr><td>C900 R<sup>a</sup> Edge<sup>δ</sup></td><td>38.4</td><td>37.2</td><td>22.0</td><td>2.4</td><td>67.5</td><td>32.5</td></tr><tr><td>C900 PS<sup>b</sup></td><td>45.9</td><td>32.9</td><td>17.6</td><td>3.7</td><td>68.7</td><td>31.3</td></tr></tbody></table>

α Side R = air/CAP exposed side β Side PS = protected from air during drying process γ Middle = treated area with a supposed denser plasma δ Edge = treated area with a supposed weaker plasma.

draft, Software, Methodology, Investigation, Formal analysis. **Claire-Hélène Brachais:** Writing – review & editing, Validation, Resources.
**Anna Krystianiak:** Writing – review & editing, Methodology, Investigation. **Olivier Heintz:** Writing – review & editing, Methodology, Investigation. **Marie-Laure Léonard:** Resources, Methodology. **Nasreddine Benbettaieb:** Writing – review & editing, Validation, Supervision, Resources, Funding acquisition, Formal analysis. **Frédéric Debeaufort:** Writing – review & editing, Validation, Supervision, Resources, Project administration, Methodology, Investigation, Funding acquisition, Data curation, Conceptualization.

## Fundings

This work was mainly supported by the Région Bourgogne Franche-Comté (ANER project ActiBioPack—ATP Grant No. 2020-Y-12730), by the PRIMA-ANR Horizon Europe MEdiet4All project (Section 2- Multi-topic 2022, Thematic Area 3-Food value chain, Topic 2.3.1–202-RIA, Grant No. ANR-23-P012-0008-04) and by the “Fonds Européens de Développement Régional (FEDER)” who invested in lab equipment’s, and by the own funds of the UMR PAM laboratory.

## Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

The author is an Editorial Board Member/Editor-in-Chief/Associate Editor/Guest Editor for *coatings, Foods, Polymers (MDPI)* and was not involved in the editorial review or the decision to publish this article.

## Acknowledgements

The authors thank the DIVVA platform (Institut Agro Dijon, Université de Bourgogne), and particularly Dr. Adrien Lerbret for the access to the different equipments (Goniometry, Permeability).

A special thanks to Bernadette Rollin and Agathe Pissis for their kind help and availability at any time.

## Appendix A. Supplementary data

Supplementary data to this article can be found online at https://doi.org/10.1016/j.foodhyd.2024.110858.
# Data availability

Data will be made available on request.

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