High Methoxyl Pectin–Tomato Paste Edible Films Formed Under Different Drying Temperatures

Abstract

Pectin–tomato paste edible films with potential antioxidant activity were studied. Initially, the films were formed by drying at 40 °C in the presence and absence of glycerol. The effect of drying temperature on several physicochemical, mechanical, and optical properties of glycerol films formed after drying at 40, 50, and 60 °C was investigated. Finally, films formed at different drying conditions (namely F40, F50, and F60) sharing the same antioxidant activity (44.28–45.53%) were studied in terms of their surface pH; solubility; folding endurance; antimicrobial, dynamic mechanical, and barrier properties; contact angle; and FT-IR. Their thickness, weight, opacity, strength, stiffness, and antioxidant activity (AA) [a*] increased with increasing tomato paste content, whereas [L*] decreased. The moisture content was statistically affected by both the presence of glycerol and the drying temperature. AA decreased as drying temperature increased. Overall, the thickness varied from 45 to 182.31 μm, weight from 0.27 to 1.24 g, moisture content from 20.74 to 56.66%, stress from 189 to 959 kPa, Young’s modulus from 86 to 382 kPa, and AA from 16.9 to 53%. In the last step, F60 was less hydrophilic, had a greater density, and better barrier properties, whereas F50 was stiffer and the least strong. Our findings provide information regarding the selection of an optimum drying temperature for pectin-based films with antioxidant properties.

1. Introduction

Given the worldwide demand for replacing plastics with renewable and biodegradable materials, packaging, apart from searching for greener alternatives, evolves even further in the field of food protection. A great example are edible films. Apart from being environmentally friendly, they can also offer some mechanical protection and act as barriers between food and the environment [1].

Their main constituents are biopolymers, like polysaccharides and proteins, which are natural, non-toxic, edible, and have a low cost. Plasticisers, like sorbitol or glycerol, are also present in order to improve the mechanical properties of the films. As a result, the films are transparent and odourless and thus, do not mask the product’s sensory properties. There are, however, cases that other ingredients like fruits or vegetables are incorporated in during film formulation [2]. Their implementation imparts sensory properties to the film which can be desirable in specific products. Fruits and vegetables are added as purees, extracts, juices, or food processing wastes. Due to their content in antioxidants and nutrients, their incorporation into edible film formulations is of particular interest for food packaging as it can reduce oxidation or even increase the nutritional value of food [3]. This is further supported by the focus of food packaging on “active packaging systems”, namely materials that prolong the shelf-life of products and enhance food preservation by interacting with the food product [4].

Among the most consumed food products worldwide, that has an antioxidant content of carotenoids, vitamin E, vitamin C, and phenolics, is tomato paste [5]. Tomato paste is obtained from the tomato fruit (Lycopersicum esculentum); its consumption is connected to a decreased risk of several chronic diseases (e.g., [6]). The main carotenoids of tomatoes are lycopene, which represents 80% of the total carotenoids and is responsible for the red colour of the tomato; β-carotene, which provides protection against UV/visible light; and lutein. The main phenolics include quercetin, rutin, and kaempferol glycosylated derivatives and naringenin glycosylated derivatives [7,8].

In the present study, edible films with potential antioxidant properties were formed from high methoxyl pectin solutions. High methoxyl pectin is well known as a dietary fibre, whereas it has been utilized in food packaging in the formation of edible films [9]. Several works on edible pectin-based films suggest that their physical properties can be effectively improved by the addition of various bioactive materials, like antioxidants, with the films showing significant antibacterial and antioxidant properties and barrier capacity, thus enhancing shelf life and maintaining the quality of food products [10,11].

The antioxidant source in our study was tomato paste. Initially, films were prepared in the presence of four tomato paste concentrations (1, 2, 5, and 10%wt) with and without glycerol after drying at 40 °C for 40 h. As the long drying times needed for film formation via the casting method are a limiting factor, the use of higher drying temperatures is suggested in order to increase the evaporation rates and thus decrease drying time. However, different drying conditions affect the functional performance of the biopolymer matrix, resulting in variations in the mechanical and barrier properties of the films [12]. Thus, in order to explore the effect of drying conditions on films’ properties, films with glycerol were also formed following drying at 50 °C for 20 h and 60 °C for 10 h. Several physicochemical, mechanical, and optical properties of the films were studied. As a final step, and in order to further investigate films formed at different drying conditions but sharing the same high antioxidant activity, three films (one per experimental condition) were studied in terms of their surface pH, solubility, folding endurance, antimicrobial properties, water and oxygen barrier properties, water contact angle, and FT-IR. Dynamic mechanical analysis was also performed.

2. Materials and Methods

2.1. Materials

Tomato paste (double concentrated, 30°Brix) was from Kyknos S.A. (Nafplio, Greece) and bought from a local supermarket. According to its manufacture, it contains 0.2% fat, 16.1% carbohydrates (of which 15% are sugars), 3.5% fibres, 4.2% proteins, and 0.08% salt. High methoxyl pectin with a DM of 70.7% (HMP, 76282) from apple was obtained from Sigma (Steinheim, Germany), whereas glycerol (4094.2500) was obtained from Merck (Darmstadt, Germany). Distilled water was used throughout the experiments.

2.2. Preparation of Film-Forming Solutions and Films

Films were formed with 0.5%wt HM pectin and different tomato paste concentrations (1, 2, 5, and 10%wt). Initially, the films were prepared in the presence and absence of glycerol after drying at 40 °C for 40 h. Then, films with glycerol were also prepared by drying at 50 °C for 20 h and 60 °C for 10 h. In all cases, glycerol’s concentration was 0.15%wt.

Film-forming solutions (FFSs) were prepared by dissolving the appropriate amount of HMP in distilled water at 90 °C under magnetic stirring for 10 min. Following pectin’s complete dissolution, the required amount of tomato paste was added. When present, glycerol was then added. The solutions were left to cool to room temperature and then brought to the correct total weight by the addition of water or continued evaporation, as appropriate. Following their preparation, 30 g of each FFS was poured onto sterile glass Petri dishes (90 mm diameter) and dried in an oven (Memmert, Schwabach, Germany). Three Petri dishes were studied per formulation. Finally, the dried films were peeled off and kept in a desiccator with silica gel until analysis.

2.3. Film Characterisation

2.3.1. Physicochemical and Optical Properties

Thickness, density, moisture content, opacity, and colour parameters ([L*] and [a*]) were determined as described by Zioga et al. [9], whereas the weight of the films was described by Zioga et al. [13]. Solubility was determined by soaking square pieces (20 mm × 20 mm) of each film in 30 mL of distilled water for 24 h at room temperature (25 °C) and subsequently drying them at 105 °C for 18 h [14]. For all properties, three films per formulation were measured.

2.3.2. Antioxidant Activity (AA)

At room temperature and for 24 h, 0.125 g of film was extracted by 15 mL of distilled water. The free radical scavenging activity of the films was evaluated by using the radical 2,2-diphenyl-1-picrylhydrazyl (DPPH). Firstly, the DPPH solution was prepared (0.1 mM in 80% aqueous ethanol). Then, 0.5 mL of each film extract was added in a glass tube, along with 1.5 mL of distilled water and 2 mL of the DPPH solution. The reaction mixture was stirred using a Vortex and kept in the dark for 30 min. Absorbance was measured in a double-beam UV–Vis spectrophotometer (UV1800, Shimadzu Europa GmbH, Duisburg, Germany) at 517 nm, and measurements were carried out in triplicate. For the control sample, 2 mL of distilled water was mixed with 2 mL of the DPPH solution. The antioxidant activity (AA, %) was calculated by the following equation:

$$\mathrm{A}\mathrm{A}(\%)={\displaystyle \frac{{\mathrm{A}}_0-{\mathrm{A}}_1}{{\mathrm{A}}_0}}\ast 100$$

(1)

where A₀ is the absorbance of the control sample and A₁ is the absorbance of each extract. The lower the absorbance of the extracts, the higher the antioxidant activity.

2.3.3. Mechanical Properties and Dynamic Mechanical Analysis

The stress at the break point, which correlates to the film’s strength, and Young’s modulus, which correlates to the film’s stiffness, were measured using an Instron Universal machine (Instron 1011, Norwood, MA, USA) equipped with a 50 N load cell and a cylindrical probe (3 mm diameter) [14]. Measurements were conducted at nine random points of the films, which were measured on the fifth day of storage. Three films per formulation were measured.

Dynamic mechanical analysis (DMA) of the films (15 mm × 6.5 mm × 0.03 mm) was performed in a dynamic mechanical analyser (242E Artemis, Netzsch, Germany) operating in tensile mode. Measurements were carried out from 30 to 80 °C at a frequency of 1 Hz with a heating rate of 2 °C/min. Curves of storage modulus (E′) and damping factor (tanδ) versus temperature were used to evaluate the dynamic mechanical behaviour of the samples.

2.3.4. Surface pH

The films were covered with distilled water on a plate and their surface pH was determined by placing pH paper on the surface of the swollen film. The readings of three films per formulation were recorded.

2.3.5. Folding Endurance

The folding endurance of each film was determined by folding the film manually and repeatedly at the same place until it was broken or folded up to 100 times. Three films per formulation were measured.

2.3.6. Water Vapour Permeability (WVP)

Water vapour permeability (WVP) was determined by the gravitational method [13]. Briefly, circular-shaped films (30 mm diameter) were fixed on top of glass cups containing silica gel (RH = 0%) that were placed in desiccators containing distilled water (RH = 100%). Cup weights (w) were measured at specific time intervals (4, 24, 30, and 48 h). Water vapour permeability (WVP) was calculated by the following equation:

$$\mathrm{WVP}={\displaystyle \frac{\mathrm{w}}{\mathrm{t}}}\left({\displaystyle \frac{\mathrm{x}}{\mathrm{A}\ast \Delta \mathrm{P}}}\right)$$

(2)

where x is the average thickness of the film (μm), A is the permeation area (7065 cm²), and ΔP is the difference between the partial vapour pressure difference across the films (2642 Pa, at 22 °C). The term (w/t) is the rate of weight change. Each of the tests was repeated three times (unit of measurement = 10⁻⁸ g mm cm⁻² h⁻¹ Pa⁻¹).

The water vapor transmission rate (WVTR) was calculated by dividing the slope of the weight loss vs. time plot with the exposed film area.

2.3.7. Antimicrobial (AM) Activity

The agar diffusion method was used to test the AM activity of the films. The cultures of Bacillus subtilis and Escherichia coli were aseptically overlaid onto hard nutrient agar plates using a cotton swab. Film discs with a diameter of 5–6 mm were placed on the inoculated plates using sterilized forceps. The plates were then incubated at 37 °C for 48 h in the incubator and visually examined for inhibition zones around the discs, with the diameter (mm) of each inhibition zone being measured. Three repetitions per film were performed.

2.3.8. Oxygen Barrier Capacity

The oxygen barrier capacity was determined by the deoxidizer absorption method [15]. Vials were filled with 15 mL of corn oil, sealed and covered with the films, and then left at room temperature for 7 days. The film’s ability to act as an oxygen barrier was reflected using the oil’s peroxide value. Thus, the peroxide value (POV) of the oil was determined by the sodium thiosulfate titration method and calculated using the following equation:

$$\mathrm{POV}(\mathrm{meq}/\mathrm{kg})=\left[{\displaystyle \frac{\mathrm{C}\ast (\mathrm{V}-{\mathrm{V}}_0)}{\mathrm{m}}}\right]\ast 1000$$

(3)

where C is the normality of Na₂S₂O₃ (mol/L); V and V₀ are the volumes of Na₂S₂O₃ consumed by the sample and the control, respectively; and m represents the sample mass (g).

2.3.9. Water Contact Angle

The static contact angle of the films was measured by the Theta Flow Optical Tensiometer (Biolin Scientific, Gothenburg, Sweden) according to the ASTM D5946 method [16]. Films were placed on the equipment platform, and 4 μL of distilled water was dropped onto the surface of the film at room temperature. Measurements were performed at 10 different points.

2.3.10. Fourier Transform Infrared (FT-IR) Spectroscopy

An IROS-05 spectrophotometer equipped with diamond crystal (Ostec corporation group, Moscow, Russia) coupled to a Mercury–Cadmium–Telluride (MCT) detector was used for recording the FT-IR spectra of the samples in triplicate (three different sub-samples). The recording parameters of the spectra were as follows: 64 scans; resolution 4 cm⁻¹; and speed of the interferometer moving mirror 0.3164 mm·s⁻¹. The recorded spectra were manipulated by OMNIC (ver. 8.2.0.387; Thermo Fisher Scientific Inc., Waltham, MA, USA) software using “automatic smoothing” and “automatically baseline corrected” using the Savitzky–Golay algorithm (2nd order, 5-point window) and (2nd order polynomial fit), respectively. Then the average spectrum of each sample was calculated. For comparison reasons, apart from the films, three samples of tomato paste dried at 40, 50, and 60 °C for 3 min as well as high methoxyl pectin powder, were tested.

2.4. Statistical Analysis

One-way and multifactor analysis of variance (ANOVA) and least significant difference (LSD) tests were carried out on the data in order to determine significant differences among the samples. The significant level was p < 0.05 throughout this study. Analysis of the data was carried out with Statistica (Stat-Soft, Inc., Tulsa, OK, USA).

3. Results and Discussion

3.1. Films Formed in the Presence and Absence of Glycerol After Drying at 40 °C for 40 h

All films produced in the present study were peelable with a smooth surface with no bubbles or cracks.

The first part of the investigation focused on the effect of plasticisers on the film properties. Thus, films with HM pectin and an increasing tomato paste concentration were made both in the presence and absence of glycerol (0.15%wt). Nowadays, as edible films are considered a new alternative for food packaging, several properties related to their use were studied. Thickness, weight, moisture content, opacity, colour, stress at break, Young’s modulus, and antioxidant activity were investigated as they are related to the product’s acceptability by consumers as well as its integrity and self-life. Their corresponding values are presented in

Table 1. Thickness, weight, moisture content, opacity, stress at break, Young’s modulus, opacity, colour parameters, and antioxidant activity of films in the absence and presence of glycerol. Films were prepared by drying at 40 °C for 40 h.

HΜP (% wt)	Tomato Paste (% wt)	Glycerol (%wt)	Thickness (μm)	Weight (g)	Moisture Content (%)	Stress at Break (kPa)	Young’s Modulus (kPa)	Opacity (Area T%)	[L*]	[a*]	Antioxidant Activity (AA%)
0.5	1	0	57.73 a ± 5.18	0.27 a ± 0.00	56.66 a ± 2.57	202.1 a ± 63.6	85.94 a ± 5.24	25484 a ± 1194	81.07 a ± 2.57	7.97 a ± 2.33	33.65 a ± 1.14
	2	0	81.15 b ± 9.39	0.38 b ± 0.01	44.26 b ± 3.12	227.7 a ± 55.1	114.35 a,b ± 10.19	12125 b ± 87	72.16 b ± 3.46	17.47 b ± 2.19	35.18 a,b ± 0.14
	5	0	106.00 c ± 7.84	0.61 c ± 0.01	32.73 c ± 1.55	234.2 a ± 46.7	123.51 b ± 13.64	6273 c ± 127	60.34 c ± 2.30	27.03 c ± 2.34	36.40 b ± 0.27
	10	0	154.23 d ± 11.70	1.10 d ± 0.01	23.39 d ± 3.74	908.6 b ± 64.0	317.86 c ± 40.10	4201 d ± 212	48.26 d ± 2.33	36.46 d± 1.41	53.17 c ± 0.49
0.5	1	0.15	50.83 a ± 5.15	0.28 a ± 0.01	21.21 a ± 1.98	397.9 a ± 41.0	107.74 a ± 11.02	18395 a ± 438	81.88 a ± 1.30	6.41 a ± 0.81	30.81 a ± 0.76
	2	0.15	71.25 b ± 8.76	0.37 b ± 0.00	20.84 a ± 0.06	525.8 b ± 64.2	152.86 b ± 16.50	12649 b ± 494	74.08 b ± 2.47	12.67 b ± 1.72	41.97 b ± 1.60
	5	0.15	130.83 c ± 16.63	0.68 c ± 0.03	24.02 b ± 0.06	752.6 c ± 43.3	192.48 c ± 3.51	7563 c ± 165	60.41 c ± 2.69	26.48 c ± 1.48	45.53 c ± 1.37
	10	0.15	182.31 d ± 14.95	1.24 d ± 0.04	25.75 b ± 0.12	866.5 d ± 61.0	198.27 c ± 7.52	4866 d ± 346	45.70 d ± 1.47	34.56 d ± 1.92	48.93 d ± 0.28

Values with different superscripts for each parameter within the same glycerol concentration are significantly different (p < 0.05).

. Thickness varied from 50.83 to 182.31 μm, weight from 0.27 to 1.24 g, and moisture content from 20.84 to 56.66%. According to the performed statistics, both factors (“tomato paste concentration” and “glycerol concentration”) as well as their interaction were significant for all three properties. Both thickness and weight increased with the tomato paste concentration regardless of the presence of glycerol. Films with 1%wt tomato paste had statistically the same weight and thickness both in the presence and absence of glycerol (~2.27 g and ~57 μm, respectively). However, the two greater tomato paste concentrations led to greater weights and thickness when glycerol was present. Moisture content decreased as the concentration of tomato paste in the formulation increased for films with no glycerol (~57 to ~23%). When glycerol was present, the two lower tomato paste concentrations shared statistically the same moisture content (~21%), which was lower than that of the two higher concentrations that had a moisture content of ~25%. In addition, glycerol films with 2, 5, and 10% tomato paste concentrations shared statistically the same value as the film with 10% tomato paste and no glycerol.

The increase in thickness and weight with increased tomato paste concentration both in the presence and absence of glycerol results from the increased solid content of the formulation (e.g., [17,18]). Regarding the effect of glycerol on weight and thickness, the literature reports an increase in relation to the addition of glycerol or plasticisers in general (e.g., [19,20]). This was attributed to an increase in molecular volume due to the plasticisers disrupting the intermolecular linkages between the polymer chains and thus restructuring the polymer’s organisation to a more expanded structure. In the present study, this was demonstrated at the two higher tomato paste concentrations, suggesting that the constituents of tomato paste, i.e., carotenoids, phenolics, sugars, carbohydrates, and proteins, are also participating in interactions.

Similarly for moisture content, a positive effect of plasticiser on the moisture content of various polysaccharide edible films due to their hydrophilic nature is reported in the literature (e.g., [19,20,21]). In our case, the glycerol films had a lower moisture content than those without glycerol. Moreover, the presence of tomato paste had a negative effect on moisture content for the films without glycerol but a positive one on glycerol films. As the concentration of the films’ constituents and their properties, i.e., their hydrophilic/hydrophobic nature or the presence of functional groups in the biopolymers (e.g., hydroxyl and carboxyl groups for pectin), affect the interactions with each other and water as well as the possible hindering of the chain–chain association of the polysaccharides (e.g., [18,22]), it seems that our findings are governed by a competing effect between tomato paste and glycerol. Due to the composition of tomato paste, a number of other possible interactions, like biopolymer–biopolymer, biopolymer–phenolics, and biopolymer–carotenoids, can occur, thus reducing the available hydroxyl groups of the polysaccharides for interaction with water molecules [23]. Moreover, the presence of hydrophobic lycopene and its interactions with the hydroxyl groups of the biopolymers can also be a contributing factor to moisture content [24].

The film’s strength was determined by puncture experiments from which the stress at break was calculated. According to Table 1, the films became stronger with tomato paste concentration when glycerol was present (398–866 kPa). For films without glycerol, tomato paste concentrations of 1, 2, and 5%wt shared statistically the same stress at break (~221 kPa), with the 10%wt concentration presenting once again the stronger film (909 kPa). From the same puncture tests, Young’s modulus, which relates to the stiffness of the films, was also determined and is presented in

Table 2. Thickness, weight, moisture content, stress at break, and Young’s modulus of films with 0.15% glycerol prepared under different drying conditions.

HΜ Pectin (%wt)	Tomato Paste (%wt)	Drying Conditions	Thickness (μm)	Weight (g)	Moisture Content (%)	Stress at Break (kPa)	Young’s Modulus (kPa)
0.5	1	40 °C–40 h	50.83 a ± 5.15	0.28 a ± 0.01	21.21 a ± 1.98	397.9 a ± 41.0	107.74 a ± 11.02
	2		71.25 b ± 8.76	0.37 b ± 0.00	20.84 a ± 0.06	525.8 b ± 64.2	152.86 b ± 16.50
	5		130.83 c ± 16.63	0.68 c ± 0.03	24.02 b ± 0.06	752.6 c ± 43.3	192.48 c ± 3.51
	10		182.31 d ± 14.95	1.24 d ± 0.04	25.75 b ± 0.12	866.5 d ± 61.0	198.27 c ± 7.52
0.5	1	50 °C–20 h	53.13 a ± 8.73	0.30 a ± 0.01	21.15 a ± 0.27	189.1 a ± 59.2	215.09 a ± 29.04
	2		60.63 b ± 7.93	0.39 b ± 0.01	22.92 b ± 0.18	310.8 b ± 68.5	289.17 b ± 31.08
	5		107.35 c ± 9.54	0.70 c ± 0.02	24.67 c ± 0.27	417.6 c ± 35.3	357.47 c ± 25.89
	10		144.69 d ± 10.87	1.18 d ± 0.01	24.71 c ± 0.90	659.4 d ± 58.5	382.09 c ± 42.60
0.5	1	60 °C–10 h	45.00 a ± 8.42	0.31 a ± 0.04	20.87 a ± 0.62	441.2 a ± 80.5	154.82 a ± 22.74
	2		75.45 a ± 9.61	0.36 a ± 0.01	20.74 a ± 1.03	563.8 a ± 67.7	182.36 a ± 32.70
	5		108.85 b ± 7.12	0.64 b ± 0.02	21.02 a ± 0.08	725.1 b ± 57.5	222.92 b ± 22.58
	10		166.82 c ± 7.83	1.13 c ± 0.04	20.87 a ± 0.62	959.2 c ± 59.1	267.39 c ± 32.06

Values with different superscripts for each parameter within the same drying conditions are significantly different (p < 0.05).

. Its values ranged from 86 to 318 kPa, and for both the presence and absence of glycerol, they increased with the tomato paste concentration. Statistics showed that for stress, both factors and their interaction were significant for its measured values, whereas for Young’s modulus, only the “glycerol concentration” factor was not significant.

According to the literature, the plasticisers directly affect the mechanical properties by reducing the interactions between neighbouring polymer chains, which leads to a decrease in strength and an increase in elasticity [25]. In the present study, a positive effect of tomato paste concentration on the strength and stiffness of the films was observed, while the presence of glycerol led to stronger films but did not affect their stiffness. It seems that the composition and concentration of the tomato concentration are the dominant factors for the mechanical properties, supported by the low glycerol concentration used for film formation. The high solid concentration and possible protein–polysaccharide interactions affect the formation of film network structure [26].

The optical properties of an edible packaging film are very important as they can affect consumers’ acceptance of the product. Table 1 presents the corresponding values for opacity and [L*] and [a*] colour parameters. According to the performed statistics, both factors and their interaction were significant for opacity and [a*] values, whereas only the “tomato paste concentration” factor was significant for [L*]. The opacity values varied from 4201 to 25,485 and increased with tomato paste concentration regardless of the presence of glycerol. Films with 2 and 10%wt tomato paste concentrations shared statistically the same opacity both in the presence and absence of glycerol. Regarding colour, the films became darker and redder as the concentration of tomato paste became greater. Lightness varied from 46 to 81 and redness from 6.4 to ~36.5. Statistics have shown that the presence of glycerol did not have an effect on the values of all colour parameters when measured for the same tomato paste concentration.

Overall, the increase in tomato paste concentration increased their red hue due to the increased concentrations of lycopene, the carotenoid responsible for the red colour of ripe tomatoes [27]. Additionally, it had a negative effect on the lightness and transparency of the films. As it is generally believed that plasticisers increase the [L*] value of polysaccharide films (e.g., [21,28]), it is the base material that contributes to colour. Thus, once again, the carotenoids present in the paste are lowering the brightness and imparting the light barrier properties to the films [29].

Antioxidant activity (AA) was another measured parameter. Statistics showed that both factors and their interaction were significant for AA. As expected, due to the well-established antioxidant activity of tomato products attributed to lycopene, antioxidant activity increased when more tomato paste was incorporated into the formulation of the films for both 0 and 0.15%wt glycerol concentrations; it varied from 31 to 53%.

3.2. Films Formed in the Presence of Glycerol Under Different Drying Temperatures

The second step of the present work involved the investigation of the effect of drying temperature on the physicochemical, mechanical, and optical properties of the pectin–tomato paste films. For this set of experiments, all films were formed in the presence of glycerol (0.15%wt). Three different drying conditions were applied, and the collected values of thickness, weight, moisture content, stress at break, Young’s modulus, opacity, colour parameters, and AA are shown in Table 2 and

Table 3. Antioxidant activity and optical properties of films with 0.15% glycerol prepared under different drying conditions.

HΜ Pectin (%wt)	Tomato Paste (%wt)	Glycerol (%wt)	Drying Conditions	Opacity (Area T%)	[L*]	[a*]	Antioxidant Activity (AA%)
0.5	1	0.15	40 °C–40 h	18,395 a ± 438	81.88 a ± 1.30	6.41 a ± 0.81	30.81 a ± 0.76
	2	0.15		12,649 b ± 494	74.08 b ± 2.47	12.67 b ± 1.72	41.97 b ± 1.60
	5	0.15		7563 c ± 165	60.41 c ± 2.69	26.48 c ± 1.48	45.53 c ± 1.37
	10	0.15		4866 d ± 346	45.70 d ± 1.47	34.56 d ± 1.92	48.93 d ± 0.28
0.5	1	0.15	50 °C–20 h	23,950 a ± 556	81.06 a ± 2.08	6.83 a ± 1.16	27.53 a ± 1.72
	2	0.15		15,258 b ± 210	74.03 b ± 2.06	12.84 b ± 2.01	34.44 b ± 0.17
	5	0.15		9643 c ± 192	59.51 c ± 1.53	25.71 c ± 1.41	44.51 c ± 0.20
	10	0.15		6226 d ± 321	49.44 d ± 1.19	33.27 d ± 0.63	52.06 d ± 0.81
0.5	1	0.15	60 °C–10 h	19,968 a ± 978	81.40 a ± 2.40	7.01 a ± 1.76	16.93 a ± 0.90
	2	0.15		12,220 b ± 947	73.89 b ± 2.22	13.37 b ± 1.86	24.14 a ± 0.35
	5	0.15		9018 c ± 288	59.16 c ± 1.66	26.48 c ± 1.64	38.10 b ± 0.45
	10	0.15		4648 d ± 109	43.69 d ± 2.12	35.57 d ± 0.65	44.28 c ± 0.46

Values with different superscripts for each parameter within the same drying conditions are significantly different (p < 0.05).

. As expected due to the greater total solid content, thickness and weight increased with tomato paste concentration within the same drying temperature. Overall, thickness varied from 45 to 182 μm and weight from 0.28 to 1.24 g (Table 2). Based on the statistical analysis, only the “drying conditions” factor was not important for weight, whereas both factors (“tomato paste concentration” and “drying conditions”) and their interaction were significant for thickness. The moisture content for all drying conditions ranged from 20.7 to 25.8% (Table 2). For the maximum drying temperature (60 °C), all films shared statistically the same moisture content (~20.9%), probably due to the quicker evaporation of water at this high temperature. For the remaining two temperatures, the films with 5 and 10% tomato paste presented statistically the same and higher moisture content (~24.9 and ~24.7% for 40 and 50 °C, respectively) than those of the films with the two lower concentrations, probably due to the increased interactions of the tomato paste constituents and water. Moreover, studies suggest glycerol losses by evaporation during film drying for temperatures greater than 50 °C, which reduce the film’s hydroscopicity and thus its moisture content (e.g., [30]).

Stress at break and thus film strength increased with tomato paste concentration for all the drying temperatures (Table 2). Overall, strength varied from 189 to 959 kPa. Statistics showed that the “drying conditions” × “tomato paste concentration” interaction was not significant for stress at break. The stiffness of the films, as expressed by Young’s modulus, also increased with tomato paste concentration for all the drying temperatures (Table 2), and according to the statistics, both factors and their interaction were significant for its values (107.7–382 kPa).

The formation of films by drying involves a simultaneous heat and mass transfer process, with drying temperature being a critical factor for their properties [31]. Regarding its effect on the mechanical properties, reports in the literature contrary these results. In several cases, e.g., chitosan films dried at 45, 55, 65, 75, and 85 °C in an oven [32] and carboxymethyl cellulose/gelatin films dried at 2–8, 23, and 50 °C [33], higher drying temperature led to a decrease in tensile strength. At the same time, other studies, e.g., alginate films dried at 25, 57, and 90 °C in an oven [34], soy protein isolate films by the heat conduction drying method [35], and konjac flour films dried at ambient temperature and 30, 40, and 50 °C [36], reported an increase in tensile strength with increasing temperature. These contrary results could be attributed to the differences in the molecular structures of the biopolymers and microstructures of the films prepared at different drying temperatures. For example, in the case of proteins, temperature affects the structure and its interactions [35]. In the present study, within the same formulation, the increase from 40 to 50 °C led to less strong and less flexible films, whereas a further increase in the temperature to 60 °C resulted in stronger and stiffer films. Bearing in mind our findings and comments on moisture content, it seems that at higher temperatures, a tighter and more compact network is formed [36,37]. Moreover, the presence of hydrophobic compounds, like the lycopene of the tomato paste, in a hydrophilic matrix can affect the chain interactions of the polymer matrix, resulting in decreased polymer–polymer interactions that promote the formation of discontinuities in the structure and thus reduce the tensile strength and elasticity of the films [38].

The optical properties and AA of the films were determined next (Table 3). According to the performed statistics, the “drying conditions” X “tomato paste concentration” interaction was not significant for [a*], whereas both factors and their interaction were significant for [L*], opacity, and AA. The presence of tomato paste was, once again, the crucial factor for opacity, colour, and antioxidant activity. Thus, within the same drying conditions, films became darker, redder, more opaque, and had a greater antioxidant activity as the tomato paste concentration increased (Table 3). [L*] varied from 43.7 to 81.9, [a*] from 6.4 to 35.6, and the opacity values ranged from 4648 to 23,950. AA varied from 16.9 to 52%. Within films with the same tomato paste content, antioxidant activity increased with temperature decrease.

According to our findings, the increasing drying temperature led to changes in both colour and AA, suggesting changes in lycopene content. As the literature reports, time and temperature conditions are affecting lycopene content as both parameters affect its stability (e.g., [39]). There are several factors affecting its content during heat treatment, like the degradation of all-trans and cis-isomer lycopene, isomerization from all-trans to cis-isomer lycopene, the more efficient extraction of lycopene from the tomato matrix, and oxidation [40]. Apart from carotenoids and lycopene, phenolics are also present in tomato paste, with a reduction in their content being reported when tomato purée was heat-treated [41]. As such, the reduction in tomato paste phenolics can also contribute to the AA values reported here.

The irreversible oxidation of lycopene leads to its fragmentation, producing acetone, methyl heptenone, laevulinic aldehyde, and probably glyoxal. As a result, apparent colour loss may occur [40]. Meanwhile, the redness and other colour changes can be attributed to other physical and chemical properties of the films like water loss and/or the Maillard reaction products (brown pigments) [39]. Overall, in our case, the applied drying temperatures are certainly affecting the lycopene content, but still the heating did not cause great damages to the tomato pigments, and the observed colour changes are not very dramatic.

3.3. Films Formed at Different Drying Conditions Sharing the Same Antioxidant Activity

As the main goal of this study was the formation of films with antioxidant activity, three films, one per drying condition, sharing the same AA (44.28–45.53%), were selected for further studying. They were named F40, F50, and F60 for forming after drying at 40, 50, and 60 °C, respectively. F40 and F50 were formed in the presence of 5% tomato paste, whereas F60 was formed of 10% tomato paste.

In order to investigate possible interactions between the constituents of the film matrices, FT-IR analysis was performed. Initially, the FT-IR spectra (4000–840 cm⁻¹) of tomato paste samples dried at 40, 50, and 60 °C for 3 min were recorded and are presented in Figure 1. The band at 3288–3272 cm⁻¹ is correlated to O-H and N-H stretching [42,43] and the one at 2921–2914 cm⁻¹ to -CH- stretching [43]. The next band found in the spectra extends from 1750 to 1490 cm⁻¹ and is cantered between 1590 and 1572 cm⁻¹. In this spectral region, there are several overlapping bands, i.e., the >C=O, COO-, C=C stretching and the bending of water and C-H of organic acids [42,43,44,45,46]. This band is narrower for the tomato paste heated at 60 °C, possibly due to a greater degree of dehydration of the tomato paste. The absorbance at 1404–1402 cm⁻¹ has been assigned to C-H bending [43] and the one at 1025–1022 cm⁻¹ to the stretching of C-O and C-C, the deformation of C-O-C and C-O-H (sugars) [43], and the absorbances of lycopene (957 cm⁻¹) and β-carotene (968 cm⁻¹) [47].

The FT-IR (4000–840 cm⁻¹) spectra of high methoxyl pectin (HMP) as well as of the three films (F40, F50, and F60) are presented in Figure 2. In the HMP spectrum, bands are observed at 3323, 2943, 1742, 1607, 1436, 1364, 1228, 1097, 1013, and 920 cm⁻¹. These bands correlate by previous studies [48,49] to the following vibrations: O-H (stretching); C-H (stretching); >C=O (in esters); COO- (asymmetric stretching) and water bending; COO- (symmetric stretching), stretching and bending of C-OH; CH (bending); -OH (bending); combination of C-O stretching and -OH bending; C-C and C-O stretching; and CH₃ (rocking vibration). Comparing the HMP and the films’ spectra, the bands at 1364 and 920 cm⁻¹ are covered by the 1410–1408 and 1015–1012 cm⁻¹ bands, respectively. In addition, a shift in the 3323 cm⁻¹ band to shorter wavenumbers (3296–3253 cm⁻¹) was observed. This shift was greater in the spectrum of the F60 film, which has twice the tomato paste content compared to the other two films, suggesting the development of additional hydrogen bonds in the film. When comparing the spectra of the tomato paste samples with the corresponding ones of the films, it has been noticed that the band at 1590–1572 cm⁻¹ was shifted to 1612–1608 cm⁻¹. This shift may be due to the COO⁻¹ of the added pectins. Overall, this spectroscopic study shows that intermolecular forces develop between tomato paste and pectin.

Several physicochemical and barrier properties of the films were studied, and the corresponding values are presented in

Table 4. Density, folding endurance, pH, contact angle, oxygen, and water barrier properties of tomato films with same antioxidant activity, formed under varying drying conditions. All films were formed in the presence of 0.5% HM pectin and 0.15% glycerol.

Sample Name	Tomato Paste (%wt)	Drying Conditions	Density (g/cm3)	Folding Endurance	pH	Contact Angle	Oxygen Barrier Properties (POV; meq/kg)	WVTR (10−4 g/h cm2)	WVP (10−8 g mm/h cm2 Pa)
F40	5	40 °C–40 h	1.60 a ± 0.18	No break	3.5 a ± 0.4	25.65 a ± 1.99	3.05 a ± 0.11	8.12 a ± 0.49	4.00 a ± 0.24
F50	5	50 °C–20 h	1.54 a ± 0.08	No break	3.5 a ± 0.4	29.93 b ± 2.61	3.86 b ± 0.10	11.29 b ± 0.57	6.03 a ± 0.30
F60	10	60 °C–10 h	2.02 b ± 0.19	No break	3.5 a ± 0.4	34.54 c ± 1.63	1.82 c ± 0.10	8.04 a ± 0.41	5.01 a ± 0.25

Values with different superscripts for each property are significantly different (p < 0.05).

. Regarding their water solubility, there are no values presented, as the films were completely disintegrated after their 24 h immersion in water, and there were no remaining film pieces. Their density varied from 1.54 to 2.02 g/cm³, with F50 showing the lower and F60 the higher value (Table 4). As a higher density relates to a more compact structure, the reported higher value for F60 can be attributed to additional hydrogen bonding occurring at 60 °C, as denoted by the FT-IR spectra (Figure 1). The results of this spectroscopic study can also contribute to the understanding of the mechanical properties of the films reported in Table 2. None of the films broke when manually folded 100 times, thus suggesting that they have a mechanical strength that allows them to withstand breaks during their handling [50]. All films shared statistically the same surface pH (3.5). Their acidic value results from the presence of tomato paste. Generally, tomato pastes have a pH between 4.2 and 4.6, in order to prevent the growth of some harmful microorganisms like Bacillus and Clostridium [51].

The quality of a film is affected by its contact angle, which expresses its ability to resist water absorption. Surfaces with water contact angles lower than 90° (θ  <  90°) are characterized as hydrophilic, whereas those with water contact angles higher than 90° (θ  >  90°) are characterized as hydrophobic [52]. In the present work, the contact angle of the films ranged from 25.65 to 34.54°, with F60 showing the higher and F40 the lower angle. In all cases, contact angle values were lower than 90°; thus, the films are hydrophilic. This can explain their high solubility. The possible glycerol losses with increasing temperature can also contribute to this finding [30].

Water vapour and oxygen barrier properties relate to the capacity of the films to reduce or hinder moisture or oxygen transfer, respectively, between the food and its external environment [32]. As seen in Table 4, F40 and F60 films share statistically the same WVP and WVTR (~8.08 × 10⁻⁴ g/h cm² Pa and ~4.5 × 10⁻⁸ g mm/h cm² Pa, respectively), which were lower than the corresponding values for F50 (11.29 × 10⁻⁴ g/h cm² Pa and 6.03 × 10⁻⁸ g mm/h cm² Pa, for WVP and WVTR, respectively). As oxygen causes oxidation, several food changes can be initiated, thus affecting the shelf life of the product. The oxygen barrier properties of the films were evaluated by the peroxide value (POV) of corn oil, and the corresponding values are shown in Table 4. The higher the POV values, the worst the oxygen barrier performance, as more oxygen is permeated through the film [12]. In the present study, POV ranged from 1.82 to 3.86 meq/kg, with F60 showing the best barrier performance and F50 the worst.

Overall, F60 showed better barrier properties among the films, which could relate to both its higher density and lower hydrophilicity (Table 4). Furthermore, the negative effect of increasing temperature on both water and oxygen barrier properties for F40 and F50 has also been reported in the literature for alginate films [30] and CMC–gelatin films, among others [33].

Due to the presence of the antioxidants and the AA they imparted to the films, the antimicrobial activity of the films was tested. Antimicrobial packaging materials have emerged as an effective solution to safeguard food from contamination by pathogenic microorganisms [6]. The Gram-positive bacteria B. subtilis and the Gram-negative bacteria E. coli were selected because both strains are commonly known as foodborne microbes. No inhibition zones were seen for all films and both bacteria, which is why no values on antimicrobial activity are presented. Thus, despite their antioxidant activity, all films were non-active against both Gram-positive and Gram-negative bacteria. In good agreement with our findings, Du et al. [53] also reported that films containing HM pectin and tomato puree did not inhibit the growth of E. coli.

Dynamic mechanical analysis of the films was performed, with Figure 3 showing the plot of E’ and tanδ as a function of temperature. E’ reflects the mechanical energy stored by samples during a loading cycle and relates to the stiffness of the material. As such, it is often correlated with Young’s modulus [54]. Its plot as a function of temperature is segmented into three phases: the glassy, the transition, and the rubbery phase [55]. tanδ is regarded as the mechanical damping factor and is defined as the ratio of loss and storage modulus (tanδ = E″/E′). A high tanδ value is indicative of a material having a high, non-elastic strain component, while a low value indicates high elasticity [56]. As seen from Figure 3, the F50 film showed the higher storage modulus, in good agreement with the reported Young’s modulus values (Table 2). Furthermore, the storage modulus values for F40 and F60 did not have great differences. The E’ values of all the films did not exhibit variation over the temperature range, indicating that the films were at the rubbery stage in the range of 30–80 °C [56].

4. Conclusions

The present work studied high methoxyl pectin edible films with potential antioxidant properties due to the presence of tomato paste. Moisture content and mechanical properties were greatly affected by the presence and absence of glycerol for films formed at 40 °C as well as for glycerol films formed after drying at 40, 50, and 60 °C. Increased tomato paste concentration led to increased thickness, weight, opacity, strength, stiffness, and antioxidant activity (AA) and also affected the colour of the films. The increase in drying temperature had a negative effect on AA. For the three selected films that shared the same AA but were formed at different drying temperatures, the higher drying temperature of 60 °C led to a film with a greater density and better water and oxygen barrier properties, which was less hydrophilic than the rest. However, the film formed at 50 °C was the stiffer and least strong among the films. Due to its properties and colour, the film formed at 60 °C can be used in ready-to-eat products like sushi. Overall, this work is a first step toward the selection of the optimal drying temperature for pectin-based films with antioxidant properties formed by the casting method. However, further experiments on the formulation of the films, in order to increase their hydrophobicity and improve their performance, seem worthy of performing.