Pectin-Based Films Loaded with Hydroponic Nopal Mucilages: Development and Physicochemical Characterization

: Nopal is a potential source of mucilage that can be used in di ﬀ erent food applications. One of its potential use is the development of ﬁlms and coatings where it can act as a packaging material but also as a source of bioactive compounds. Therefore, this work aimed to develop and characterize pectin-based ﬁlms loaded with mucilage extracted from two species of nopal, Copena F1 (Cop) and Villanueva (Vi). The obtained mucilages were denominated as materials without ﬁbre (Copwtf and Viwtf) and with ﬁbre (Copwf and Viwf), according to the ﬁbre’s size. Films were produced with pectin (2% w / v ), mucilage (2.5% w / v ) and glycerol (0.5% w / v ) by the casting method. The addition of mucilages was shown to inﬂuence the visual appearance, optical properties and morphology of the ﬁlms. The presence of mucilage also changed the moisture content, water contact angle and water vapour permeability of the ﬁlms. The pectin-based ﬁlms without mucilage presented the best mechanical properties. Fourier-transform infrared (FTIR) spectroscopy showed similar signals in terms of frequency and intensity, for all the ﬁlms, not showing any chemical modiﬁcation. Results show that the mucilage obtained from di ﬀ erent nopal fractions can be used in pectin-based ﬁlms foreseeing their use as ﬁlms or coatings in food applications.


Introduction
Films and coatings industries have generated a remarkable growth in recent years, specifically in the preservation of perishable food products. This growth is followed by the increasing interest in replacing synthetic polymers currently used by bio-based polymers and biodegradable polymers [1][2][3][4][5].
There are already several studies about films and coatings produced by bio-based polymers, such as polysaccharides, proteins and lipids, where they have been used on the preservation of food products. The performance of these films and coatings is directly influenced by chemical characteristics, production method, concentration and environmental conditions in which it is produced [6]. Another of the challenges is to use bio-based and biodegradable polymers obtained from subproducts and/or agricultural waste, in order to reduce food waste and make agriculture more sustainable.
Recently, in Mexico, some studies were carried out using nopal cactus (Opuntia ficus-indica), subproducts generated by the plant, as a source of bioactive compounds for food and nutrition [7].

Film Preparation
Pectin (2.0% w/v) was dissolved in distilled water with constant stirring during 1 h at 23 • C, after total dissolution 2.5% (w/v) of mucilage powders (Copwf, Viwf, Copwtf and Viwtf) were mixed and stirred a further 2 h. Afterwards, glycerol was added in the final concentration of 0.5% (w/v) and stirred until all mixture was dissolved. This formulation (i.e., concentrations and mixing procedure) was selected based on previous studies where the aim was to add the maximum amount of mucilage powders to the pectin matrix and maintain good film processability. Then, 15 mL of film-formation solution was cast into polystyrene Petri dishes and dried at 45 • C during 24 h. Films were conditioned at 54% ± 1% relative humidity (RH), and 20 ± 1 • C by placing them in a desiccator container a saturated solution of Mg(NO 3 ) 2 H 2 O. Film with 2.0% (w/v) pectin and 0.5% (w/v) glycerol was called the control film.

Optical Properties
Optical measurements were performed according to the reported by Martins et al. [30] using a Minolta colourimeter (Minolta CR 400, Tokyo, Japan). The CIELab scale was used to determine L*, a* and b* parameters. Standard values considered were those of a white background (L* = 84.67, a* = −0.55, and b* = 0.68). The opacity of the mucilage films indicated the amount of light that passed through the materials and were determined according to the Hunterlab method, as the relationship between the opacity of each sample on a white standard. Six measurements were made of each sample, and three samples of each treatment were tested.

Scanning Electron Microscopy (SEM)
The surface (air dried side) morphologies of films were examined using scanning electron microscopy (SEM) (Quanta FEG 650, FEI, Hillsboro, OR, USA) with an accelerating voltage of 5 kV. Before analysis, all samples were mounted on aluminium stubs using carbon adhesive tape and sputter-coated with gold.

Thickness
A digital micrometre (No. 293-340, Mitutoyo, Kanagawa, Japan) was used to measure the film thickness measurements. Five thickness measurements were randomly taken on each film sample and the average was used to calculate water vapour permeability and tensile strength. The MC was expressed as the percentage of water removed from the initial mass sample. MC was determined gravimetrically by drying film samples (about 50 mg) at 105 • C in an oven with forced air circulation for 24 h. The experiments were performed on each film sample in triplicate.
The solubility was carried out in quadruplicate, based on the method proposed by Casariego et al. [31]. After drying at 105 • C during 24 h, the films (disks with 2 cm in diameter) were weighed and immersed into 50 mL of Mili-Q water and placed in an orbital shaker under stirring for 24 h at 20 • C. The films were inspected by the naked eye every hour to determine the time until complete solubility. The films were considered soluble after the total disintegration of the films in the solution.

Water Contact Angle (WCA)
The water contact angle (θ) was determined by the sessile drop technique using a DSA 100E drop shape analysis system (Kruss Gmbh, Hamburg, Germany). A water droplet (2 uL) was deposited in different points of the film surface (air-dried side), and afterwards a digital camera connected to DSA 3 drop shape image analysis software recorded drop images. The image produced was used to calculate the contact angle via the Laplace-Young fitting method. Three replicated measurements of θ (upper surface of the film) were obtained.

Water Vapour Permeability (WVP)
Water vapour permeability (WVP) was determined gravimetrically using the ASTM E96-92 method [32] described by Casariego et al. [31]. Three samples were cut from each film. Each film sample was sealed on a permeation cell containing distilled water (100% RH; 2.337 × 10 −3 Pa vapour pressure at 20 • C), and placed in a desiccator containing silica gel (0% RH; 20 • C). The cells were weighted at 2 h intervals during 10 h. The slope of weight loss versus time was obtained by linear regression. The measured (WVP) of the films was determined as follows: where WVTR in the measured water vapour transmission rate through a film, L is the mean film thickness (m), and ∆P is the partial water vapour pressure difference (Pa) across the two sides of the film. The experiments were performed in triplicate for each mucilage film.

Mechanical Properties
Tensile strength (TS), Young's modulus (YM) and elongation-at-break (EB) were determined with a texture analyzer (TA-HD plus, Stable Micro System, Godalming, UK) using ASTM D 882-91 [33] described by Ricardo et al. [34]. Films (120 mm × 20 mm) were cut from each preconditioned (54% RH). The initial grip separation and crosshead speed were set to 100 mm and 5 mm·min −1 , respectively. At least seven replications of each film were performed.

X-ray Diffraction (XRD)
The X-ray diffractometer X Pert PRO MRD from Malvern Panalytical Ltd. (Royston, UK) was used for XRD analysis during the different time-points. X-ray scans were done at room temperature (~22 • C) in the range of 10 • to 50 • (2θ degrees), using a Cu source, X-ray tube (γ = 1.54056 Å) at 45 kV and 40 mA with θ set to −0.0372 degrees for fine calibration offset.
2.3.9. Fourier Transform Infrared (FTIR) Spectroscopy FTIR spectra of the films were recorded with a Bruker FT-IR VERTEX 80/80v (Boston, MA, USA) in attenuated total reflectance mode (ATR) with a platinum crystal accessory in the wavenumber range: Coatings 2020, 10, 467 5 of 14 4000-400 cm −1 , using 16 scans at a resolution of 4 cm −1 . Before analysis, an open bean background spectrum was recorded as a blank. All measurements were performed in triplicate.

Thermal Analysis
The thermal stability and degradation of the films were analyzed by the thermogravimetric analysis (TGA), performed with a simultaneous thermal analyzer and a differential scanning calorimeter (TGA/DSC 3+, Mettler Toledo, Palatine, IL, USA). Samples (≈5 mg) were heated from 20 to 500 • C at a heating rate of 10 • C min −1 under a nitrogen atmosphere to avoid thermo-oxidative reactions.

Statistical Analysis
Statistical analysis was performed using analysis of variance (ANOVA) with a 95% confidence level using Vassarstats (http://vassarstats.net).

Visual Appearance and Optical Properties
The films obtained were homogeneous with a uniform and compact structure, suggesting the proper incorporation of mucilages into pectin matrix, as can be seen in Figure 1. However, the fibres are visible in films with Copena F1 and Villanueva with fibre ( Figure 1C,E), while for films using mucilage without fibre is only visible a colour variation ( Figure 1A,D). This difference is explained by the different fibre sizes obtained after sieving, where those sieved in a sieve at Ø1.75 mm had a higher visual influence on the films. range: 4000-400 cm −1 , using 16 scans at a resolution of 4 cm −1 . Before analysis, an open bean background spectrum was recorded as a blank. All measurements were performed in triplicate.

Thermal Analysis
The thermal stability and degradation of the films were analyzed by the thermogravimetric analysis (TGA), performed with a simultaneous thermal analyzer and a differential scanning calorimeter (TGA/DSC 3+, Mettler Toledo, Palatine, IL, USA). Samples (≈5 mg) were heated from 20 to 500 °C at a heating rate of 10 °C min −1 under a nitrogen atmosphere to avoid thermo-oxidative reactions.

Statistical Analysis
Statistical analysis was performed using analysis of variance (ANOVA) with a 95% confidence level using Vassarstats (http://vassarstats.net).

Visual Appearance and Optical Properties
The films obtained were homogeneous with a uniform and compact structure, suggesting the proper incorporation of mucilages into pectin matrix, as can be seen in Figure 1. However, the fibres are visible in films with Copena F1 and Villanueva with fibre ( Figure 1C,E), while for films using mucilage without fibre is only visible a colour variation ( Figure 1A,D). This difference is explained by the different fibre sizes obtained after sieving, where those sieved in a sieve at Ø1.75 mm had a higher visual influence on the films. The presence of mucilage has a significant influence on the optical properties of the films. Table 1 presents the CIELab parameters and opacity values for the produced films. The films Copwtf (6.21%) and Viwtf (6.39%) showed the highest opacity values. On the other hand, films with fibres (Copwf and Viwf) reached values of 3.23% and 3.92% presenting lower (p < 0.05) opacity values than the control films (4.38%). These results show that the addition of mucilage with higher size fibres decreases the opacity values. The presence of mucilage has a significant influence on the optical properties of the films. Table 1 presents the CIELab parameters and opacity values for the produced films. The films Copwtf (6.21%) and Viwtf (6.39%) showed the highest opacity values. On the other hand, films with fibres (Copwf and Viwf) reached values of 3.23% and 3.92% presenting lower (p < 0.05) opacity values than the control films (4.38%). These results show that the addition of mucilage with higher size fibres decreases the opacity values.  22). This variation is in agreement with the opacity values of the films. The results show that the sieving process, besides sieving the different sized fibres, also influenced the film composition and thus influenced the opacity values. The difference in the mucilage structures after sieving can be related to their final composition, where the ratio between materials is changed.
The Viwtf film presents a high yellow profile (b* = 28.43) that is followed by the brown colour observed in Figure 1, while for the other films is observed a slight beige tone due resulting in values ranged between 13.86 and 18.09. The red coordinates (a*) were more pronounced in Ctrl and Viwtf films (15.50, 3.29, respectively).
The values obtained are in agreement with Luo et al. [27] and Gheribi et al. [15] that determined the luminosity of the films with mucilages and obtained values ranged between 86.72 to 93.05. By contrast, Oliveira et al. [35] estimated L* values from 21.94 to 24.02 of films with mucilages of Pereskia aculeata Miller, lower than the values presented in this work. Figure 2 shows the SEM images of surfaces of the films that appeared without pores and cracks. The Ctrl film presented a smooth and continuous surface that is explained by a good homogeneous mixture attributed to the polymer-plasticizer compatibility. Control films present the highest luminosity (L*) values (99. 19), while the lowest values were obtained for the films of Viwtf (69. 22). This variation is in agreement with the opacity values of the films. The results show that the sieving process, besides sieving the different sized fibres, also influenced the film composition and thus influenced the opacity values. The difference in the mucilage structures after sieving can be related to their final composition, where the ratio between materials is changed.

Scanning Electron Microscopy (SEM)
The Viwtf film presents a high yellow profile (b* = 28.43) that is followed by the brown colour observed in Figure 1, while for the other films is observed a slight beige tone due resulting in values ranged between 13.86 and 18.09. The red coordinates (a*) were more pronounced in Ctrl and Viwtf films (15.50, 3.29, respectively) The values obtained are in agreement with Luo et al. [27] and Gheribi et al. [15] that determined the luminosity of the films with mucilages and obtained values ranged between 86.72 to 93.05. By contrast, Oliveira et al. [35] estimated L* values from 21.94 to 24.02 of films with mucilages of Pereskia aculeata Miller, lower than the values presented in this work. Figure 2 shows the SEM images of surfaces of the films that appeared without pores and cracks. The Ctrl film presented a smooth and continuous surface that is explained by a good homogeneous mixture attributed to the polymer-plasticizer compatibility. The films without fibre present a similar aspect. In the case of Copwtf ( Figure 2B), a more heterogeneous structure is observed. However, it is clear for both cases, that the sieving process has The films without fibre present a similar aspect. In the case of Copwtf ( Figure 2B), a more heterogeneous structure is observed. However, it is clear for both cases, that the sieving process has increased the soluble material presented in the mucilage, added to the films. This behaviour can be explained by the difference in the mucilage structures after sieving. In the case of films with mucilages with a sieving process using a higher size (Ø1.75 mm), the structures look smaller and with more space, which can be explained by the larger size of the fibres ( Figure 2C,E). This is in agreement with opacity values where a more compact structure decreased the light that passes by the films and, therefore, the films of Copwf and Viwf presented the lowest opacity values. Likewise, Lira-Vargas et al. [36] state that films made with mucilages of Opuntia spp. are lumpy, independent of the variety used, since they contain dispersed granules making their structure discontinuous.

Thickness, Moisture Content (MC), Water Contact Angle (WCA) and Water Solubility (WS)
The films' thickness is presented in Table 2 and shows that the incorporation of mucilages increased the thickness of most of the films. The Ctrl film presented a value of 0.08 mm that is statistically similar to the values obtained for Viwtf, however, for Viwf, Copwf and Copwtf, there is a significant increase (p < 0.05) in the values. This increase is more pronounced for the films with Copena mucilages, and is justified by the increasing amount of solids of films with mucilage. Also, the presence of fibres influenced the thickness values leading to a statistically significant (p < 0.05) increase for Copwf films values (0.22 mm) when compared with Copwtf films values (0.17 mm). Table 2 shows that the control film presents the lower MC values and that the presence of mucilages in the films leads to an increase of MC. This behaviour can be explained by hydrophilic groups of mucilage that present more affinity to water molecules. The films produced with Cop, without and with fibre, presented similar values (12.41% and 12.44%, respectively), not showing any effect of the presence of the fibres on the MC values. Concerning the films with Vi mucilages without and with fibres, the behaviour is different. Viwtf films showed a higher moisture content (13.18 %) since it probably had a higher affinity with highly hydrophilic compounds in their matrix resultant of the presence of the fibres. Sadhegi et al. [26] also produced films with mucilage extracted from Lallemantia iberica seeds at a concentration of 1.2% and glycerol (35%) and reported a MC value of 19.86%, which are higher than the values obtained in this work. Beigomi et al. [25] produced mucilage films obtained from Dracocephalum moldavica seeds using glycerol concentrations varying from 10% to 40% and obtained MC values of 16.40% to 27.06%, respectively. The variations observed can be explained by plasticizers and the different mucilages used, but also by the different concentrations of the polymers used during their formulation.
The CA values are shown in Table 2. The Ctrl film showed the highest water contact angle values (87.22 • ) when compared with films with mucilage. The high degree of esterification of methoxyl of pectin can leads to a hydrophobic surface. The mucilages added in the films significantly decreased the contact angle of the films. This behaviour can be explained by the presence of polar groups present in the mucilage powder. Films without and with fibre presented similar values of water contact angle (37.12 • -39.88 • ), showing that the presence of the fibres does not affect the surface hydrophobicity of the films.
Jouki et al. [22] reported values of 48.53 • and 76.65 • in films with mucilages and with the addition of glycerol in concentrations ranging between 25% and 50%.
Regarding WS, results showed that the control film and films with mucilage are entirely soluble in water. However, it was observed that the films without fibre were soluble after 1 h, and those with fibres only reach total solubility after 3 h. González-Sandoval et al. [13] obtained solubility values ranged from 81.3% to 91.0% for films with mucilages from Opuntia spp., while Beigomi et al. [25] obtained solubility values ranged between 46.40% and 54.63% for films with mucilages extracted from Dracocephalum moldavica. These differences can be explained by the differences in the polymer matrix when different types of mucilages are used. Table 2 shows that the WVP values of the control films and films with mucilages are statistically different (p < 0.05). The incorporation of mucilages led to an increase of WVP, being the films without fibre those with higher WVP values. Results showed that the presence of the fibres (Viwf and Copwf) resulted in films with lower WVP when compared with films without fibre (Viwf and Copwf). The results show that the size of the fibres and composition of mucilage can influence the polymeric network decreasing the water vapour permeability.

Water Vapour Permeability (WVP)
The values are in agreement with those presented by González-Sandoval et al. [13] that obtained values of 1.63 to 8.40 × 10 −11 g (m·s·Pa) −1 for films produced with mucilage from nopal. The main differences obtained by these authors are justified by the incorporation of additives and the different concentrations of mucilages used in their production.

Mechanical Properties
Young's modulus (YM), tensile strength (TS) and elongation-at-break (EB) of the films are presented in Table 3. The control film presented the higher values in all the properties, and are similar to some values of TS and YM values obtained for films with mucilage. The control film presents a very compact structure (as shown by SEM) and the incorporation of mucilage with fibres and other constituents interfere with their structure resulting in a decline of their mechanical properties. Among films with mucilage, the films with Viwf, Copwf and Copwtf showed similar YM values (6.56, 4.83 and 5.13 MPa, respectively) and EB values lower than 10%. However, the influence of fibres in the films with mucilage from Cop is clear, where the presence of the fibre decreases the values of TS and EB to 3.34 ± 1.71 MPa and 0.89% ± 0.20%, respectively. The values are in agreement with the work of Espino-Díaz et al. [29] that used mucilage with plasticizer and obtained values of 0.95 MPa to TS and 14.99% to EB. Gheribi et al. [15] produced films of mucilage with glycerol and other polyols, and also found a TS higher than 1 MPa, however they presented EB values greater than 60%. The EB values obtained by Gheribi et al. [15] can be explained by the amount of plasticizer used, where high amounts of plasticizer lead typically to high EB values. Figure 3 shows the XRD spectra of the films. In general, all the films with mucilage exhibited the same peaks and in a similar position. However, the films with Copwtf and Viwtf showed the highest intensity compared to films with fibres.

X-ray Diffraction (XRD)
The diffractogram of the film control shows one small peak in 2θ = 15.5 • . This signal is related to pectin and its crystalline structure. Pectin frequently presents two signals at 2θ, 12 • and 21.6 • , and they can appear as a shoulder or peak that indicate if the sample has an amorphous or crystalline structure, respectively [37]. However, in this case, the mixture with a plasticizer (glycerol) changed the position of the first peak and made the second signal disappear [26]. However, the first peak increased with added mucilage with fibres.
In the case of the Copwtf and Viwtf, films showed two intensity peaks, approximately, in 2θ = 24.5 • and 30 • , and two small peaks in 36 • and 38 • 2θ. The peaks 2θ = 24.5 • and 38 • can be related to calcium oxalates (C 2 CaO 4 H 2 O) that is the major crystalline component found in nopal powders. Some works also related the peak in 2θ = 15.5 • with the presence of the calcium oxalates. Therefore, this can explain the increase of intensity of the peak in 2θ = 15.5 • when mucilage was added [9,38]. Concerning the peaks 2θ = 30 • and 36 • , these were related to the presence of calcium carbonate as calcite (CaCO 3 ) and potassium peroxydiphosphate (K 4 P 2 O 8 ), respectively. The peak in 2θ = 30 • can also be related to the presence of calcium hydride (CaH 2 ). The presence of the calcium carbonate can be one advantage for the application of these films as coatings and in the edible films industry because calcium is a good component for the human body. Therefore, this fact can be important for using of the nopal mucilage in the food packaging [39]. Figure 3 shows the XRD spectra of the films. In general, all the films with mucilage exhibited the same peaks and in a similar position. However, the films with Copwtf and Viwtf showed the highest intensity compared to films with fibres.

X-Ray Diffraction (XRD)
The diffractogram of the film control shows one small peak in 2θ = 15.5°. This signal is related to pectin and its crystalline structure. Pectin frequently presents two signals at 2θ, 12° and 21.6°, and they can appear as a shoulder or peak that indicate if the sample has an amorphous or crystalline structure, respectively [37]. However, in this case, the mixture with a plasticizer (glycerol) changed the position of the first peak and made the second signal disappear [26]. However, the first peak increased with added mucilage with fibres.
In the case of the Copwtf and Viwtf, films showed two intensity peaks, approximately, in 2θ = 24.5° and 30°, and two small peaks in 36° and 38° 2θ. The peaks 2θ = 24.5° and 38° can be related to calcium oxalates (C2CaO4H2O) that is the major crystalline component found in nopal powders. Some works also related the peak in 2θ = 15.5° with the presence of the calcium oxalates. Therefore, this can explain the increase of intensity of the peak in 2θ = 15.5° when mucilage was added [9,38]. Concerning the peaks 2θ = 30° and 36°, these were related to the presence of calcium carbonate as calcite (CaCO3) and potassium peroxydiphosphate (K4P2O8), respectively. The peak in 2θ = 30° can also be related to the presence of calcium hydride (CaH2). The presence of the calcium carbonate can be one advantage for the application of these films as coatings and in the edible films industry because calcium is a good component for the human body. Therefore, this fact can be important for using of the nopal mucilage in the food packaging [39].  Figure 4 shows FTIR spectra of the control film and films with mucilage. In general, all the films exhibited similar behaviour even with control film. The incorporation of mucilage of Copena and Villanueva in film formulation did not change the spectrum when compared to the control film. Moreover, the intensity of bands was also unchanged, so it can be suggested that no chemical interactions occur between pectin, glycerol and mucilage. However, it is important to mention that the distribution of fibres and some components can be heterogeneously distributed in the film matrix and it is possible that some changes cannot be identified by the ATR-FTIR methodology.  Figure 4 shows FTIR spectra of the control film and films with mucilage. In general, all the films exhibited similar behaviour even with control film. The incorporation of mucilage of Copena and Villanueva in film formulation did not change the spectrum when compared to the control film. Moreover, the intensity of bands was also unchanged, so it can be suggested that no chemical interactions occur between pectin, glycerol and mucilage. However, it is important to mention that the distribution of fibres and some components can be heterogeneously distributed in the film matrix and it is possible that some changes cannot be identified by the ATR-FTIR methodology.

Fourier-Transform Infrared Spectroscopy
The broad band in the range 3000-3500 cm −1 relates to hydrogen bonding (−OH) of the polysaccharide chains, glycerol and water molecules [40]. Another band was displayed in the range 2900-2700 cm −1 , due to methoxyl groups (CH 3 O−) present in mucilage and the C-H stretch of pectin structure [39]. The spectra also exhibited the typical response of pectin at 1735 and 1605 cm −1 corresponds to the carbonyl group (C=O) and carboxylate groups (COO−), respectively [41].
The alpha configuration of sugar in the mucilage was located at the wavelength of 724 cm −1 , which coincides with the reports of Cai et al. [42].
Coatings 2020, 10, x FOR PEER REVIEW 10 of 14 The broad band in the range 3000-3500 cm −1 relates to hydrogen bonding (-OH) of the polysaccharide chains, glycerol and water molecules [40]. Another band was displayed in the range 2900-2700 cm −1 , due to methoxyl groups (CH3O-) present in mucilage and the C-H stretch of pectin structure [39]. The spectra also exhibited the typical response of pectin at 1735 and 1605 cm −1 corresponds to the carbonyl group (C=O) and carboxylate groups (COO−), respectively [41].
The alpha configuration of sugar in the mucilage was located at the wavelength of 724 cm −1 , which coincides with the reports of Cai et al. [42].

Thermogravimetric Analysis (TGA)
TGA was used to evaluate the thermal stability of the mucilage films. All TGA and differential thermal analysis (DTA) curves ( Figure 5) exhibited similar behaviour of mass loss with two main phenomena. The first weight loss happened between 25 to 150 °C and was related to the evaporation of water molecules in the films. The second event happened in a range 150-325 °C and was associated the decomposition of polysaccharides, namely pectin and nopal mucilage, because in this range of temperature a higher loss of weight for all samples was observed. Finally, a quasi-constant loss was observed for high temperatures.
For control film, in the first phenomenon, was detected a higher weight loss (25%) when compared with the other films studied. This behaviour is related to the water evaporation present in the film matrix. For films with mucilages was observed a lower weight loss event. In the case of Copwtf and Viwtf they presented similar behaviour with weight loss values around 11% while for films with mucilage with fibre the values were below 7%.
In the second phenomenon, all the films presented a similar behaviour, with the loss weight of approx. 50% and a maximum temperature of decomposition detected between 213 and 218 °C. A third phenomenon was identified at around 310 °C for films with mucilage; this phenomenon corresponds to the decomposition of some nopal mucilage constituents [41]. The residual mass at the end of the analysis was 14%-16% for control film and 25%-30% for mucilage films, which is explained by the inorganic compounds present in the mucilage. These values are similar to those reported by El Achaby et al. [43], Mujtaba et al. [44] and Sucheta et al. [45].

Thermogravimetric Analysis (TGA)
TGA was used to evaluate the thermal stability of the mucilage films. All TGA and differential thermal analysis (DTA) curves ( Figure 5) exhibited similar behaviour of mass loss with two main phenomena. The first weight loss happened between 25 to 150 • C and was related to the evaporation of water molecules in the films. The second event happened in a range 150-325 • C and was associated the decomposition of polysaccharides, namely pectin and nopal mucilage, because in this range of temperature a higher loss of weight for all samples was observed. Finally, a quasi-constant loss was observed for high temperatures.
For control film, in the first phenomenon, was detected a higher weight loss (25%) when compared with the other films studied. This behaviour is related to the water evaporation present in the film matrix. For films with mucilages was observed a lower weight loss event. In the case of Copwtf and Viwtf they presented similar behaviour with weight loss values around 11% while for films with mucilage with fibre the values were below 7%.
In the second phenomenon, all the films presented a similar behaviour, with the loss weight of approx. 50% and a maximum temperature of decomposition detected between 213 and 218 • C. A third phenomenon was identified at around 310 • C for films with mucilage; this phenomenon corresponds to the decomposition of some nopal mucilage constituents [41]. The residual mass at the end of the analysis was 14%-16% for control film and 25%-30% for mucilage films, which is explained by the inorganic compounds present in the mucilage. These values are similar to those reported by El Achaby et al. [43], Mujtaba et al. [44] and Sucheta et al. [45]. Coatings 2020, 10, x FOR PEER REVIEW 11 of 14

Conclusions
Pectin-based films with nopal mucilage using two species: Copena F1 and Villanueva with and without fibres were successfully produced by the casting method. Results showed that mucilage (without and with fibres) from hydroponic nopal could be used to produce edible films with a good processability.
The presence of mucilage changed most of the films' properties. Overall, the pectin-based films without mucilage presented the best physical properties; however, in some cases, the values were close to those obtained for the films with mucilage. FTIR showed similar signals in terms of frequency and intensity for all the films and did not show any chemical modification. Taking into account all the measured properties, it can be said that the films where Villanueva mucilage with fibres (Viwf) were added were those with the best properties when compared with their counterparts to which mucilage was added.
This work provides a good view of how various sources of mucilage with and without fibres can influence films' behaviour. Taking into account the results obtained in this study, it can be said that the mucilage is suitable for incorporation in films and, therefore, can be used in the films' matrix as a source of bioactive compounds. Acknowledgements: They also thank Ms. Sara Magali Cruz Zamudio for the English revision of the manuscript.

Conflicts of Interest:
The authors declare no conflict of interest.

Conclusions
Pectin-based films with nopal mucilage using two species: Copena F1 and Villanueva with and without fibres were successfully produced by the casting method. Results showed that mucilage (without and with fibres) from hydroponic nopal could be used to produce edible films with a good processability.
The presence of mucilage changed most of the films' properties. Overall, the pectin-based films without mucilage presented the best physical properties; however, in some cases, the values were close to those obtained for the films with mucilage. FTIR showed similar signals in terms of frequency and intensity for all the films and did not show any chemical modification. Taking into account all the measured properties, it can be said that the films where Villanueva mucilage with fibres (Viwf) were added were those with the best properties when compared with their counterparts to which mucilage was added.
This work provides a good view of how various sources of mucilage with and without fibres can influence films' behaviour. Taking into account the results obtained in this study, it can be said that the mucilage is suitable for incorporation in films and, therefore, can be used in the films' matrix as a source of bioactive compounds.