Effect of Native Mucilage on the Mechanical Properties of Pectin-Based and Alginate-Based Polymeric Films

Abstract

Although the benefits of polymeric blend films have been well-established, mucilages’ interaction with other commercial bio-based polymers remains greatly unknown. Pectin and alginate were used to develop such polymeric blend films, with the addition of both ‘Algerian’ and ‘Morado’ cultivar freeze-dried, native mucilage precipitate. Mucilage additions at 0.25% and 1% (w/w), together with glycerol at 60% (w/w), influenced the mechanical properties of the 5% (w/w) pectin–mucilage and 5% (w/w) alginate–mucilage blend films differently. ‘Morado’ mucilage positively influenced the pectin film tensile strength (TS) and puncture force (PF) measurements, increasing the overall film strength compared to the homopolymeric pectin films. Calcium chloride (CaCl₂) treatment and increasing mucilage concentration further enhanced the composite pectin–mucilage film strength. Contrasting results were obtained for alginate blend films compared to those of pectin. Studying the scanning electron microscope (SEM) images of the microstructures of CaCl₂-treated pectin–mucilage and alginate–mucilage films confirmed superior microstructural film networks for pectin–mucilage films, which resulted in enhanced film strength, where the lack of polymer compatibility, as seen with alginate–mucilage, resulted in decreased film strength. These results indicate that native mucilage should be considered when developing such blend polymeric films, as it has the potential to enhance the films’ strength and elasticity.

1. Introduction

In recent years, the development of bio-based polymeric films from natural biodegradable materials has been strongly considered, as the use of synthetic, plastic-based packaging presents major environmental and human health concerns [1,2,3]. Polysaccharides, such as pectin and alginate, have specifically been identified for their desirable functional properties, and have proven advantageous in formulating bio-based polymer packaging as they produce films displaying satisfying mechanical properties [1,4]. Film mechanical properties are considered essential to maintain the structural integrity of packaging, providing adequate protection during transportation, handling and storage of food products [1,2,4].

Specifically, charged bio-based polymers, such as pectin and alginate, have been considered for the development of edible films due to their ability to form stable polymeric networks with various functional and non-functional components, overcoming the limitations of homopolymer films [4,5,6,7].

It is generally well-established that polysaccharide-based packaging displays satisfactory physicochemical properties; however, their mechanical properties (strength and elasticity) present more of a challenge. In addressing these polysaccharide-based packaging concerns, blend bio-based polymer films have been actively investigated to minimize the limitations associated with homopolymeric films [7,8,9].

Blending multiple polymers together to form a film network has proven advantageous, allowing for enhanced film properties which would not likely have been observed from the individual polymers alone. Interactions between different polymers that enhance certain functional properties are often referred to as ‘synergetic’ interactions [7,8,9,10]. These synergistic interactions have been responsible for increased bio-based polymer film strength, increased film stability and often additional unexpected synergistic effects between different polymers. Research has further shown that, although indications of possible synergistic interactions may not be obvious between two polymers, one polymer may still hold the potential to enhance the functional properties of another polymer [10].

As has been observed for homopolymeric film formation, certain film properties can be altered by adding cross-linking agents and polymer concentration used to prepare film-forming solution. Film preparation methods, calcium treatment methods and the addition of plasticizers must all be considered to influence blend film properties and, thus, their application. As with homopolymeric films, adding plasticizers, such as glycerol, is essential to developing composite polysaccharide-based films, ensuring films of uniform microstructures that display adequate barrier, mechanical and thermal properties [11,12,13].

Blend film success has resulted in improved structural properties, consequently offering enhanced protection of food [7,8,14]. Additionally, the blending of different components in film development has also been shown to further enable the incorporation of antimicrobial-promoting components into bio-based films, broadening their range of application and success as a packaging solution in the food industry [15,16]. Although certain natural ingredients have been used in the formulation of composite bio-based polymer films, limitations regarding their physical strength and elasticity, costs and availability have resulted in researchers focusing on identifying novel, plant-derived ingredients to address the shortcomings of natural packaging. Cactus pear mucilage (Opuntia. spp.) has recently been considered as a component in the development of blend bio-based polymer films [17,18,19]. Although limited, research on the potential synergistic effects of mucilages has been reported. Due to native mucilages’ unique, branched structural nature and the possibility of containing various charged fractions that display gelling potential, it has shown the potential to interact with other polymers, such as pectin [11,18,20].

Due to the functional properties associated with the mucilage polymer, composite mucilage films have specifically been shown to enhance certain mechanical properties of well-established starches and gums used to develop composite bio-based polymer films [11,17].

Although the benefits of composite bio-based polymer films have been well-established in the literature, limited research is available on mucilages’ interaction with other commercial polymers. It has been suggested that compatible chemical synergetic interactions between ingredients could result in enhanced film mechanical properties, essential for developing low-cost, biodegradable packaging [7,11,18,19]. In order to develop viable, cost-effective polysaccharide-based packaging, it is essential to better understand how native mucilage will interact with other well-established polymers, which have already shown the potential to be used in the development of packaging with bio-based polymers. This study aimed to determine the influence that different Opuntia ficus-indica cultivars’ native mucilage precipitates would have on the physical properties of pectin-based and alginate-based blended bio-based polymer films. Considering the influence of mucilage concentration and the addition of calcium chloride (CaCl₂) as a cross-linker would further aid in determining synergistic interactions that may enhance film mechanical properties, understanding mucilage as a functional bio-based polymer and its potential to address certain limitations associated with pectin and alginate polymer films.

2. Materials and Methods

2.1. Materials

Pectin powder from apples (Sigma-Aldrich, Cape Town, South Africa) with a 50%–75% esterification and ≤10% moisture content, sodium alginate powder (Sigma-Aldrich, South Africa) with both glucuronic and mannuronic acid content, glycerol, >99% purity (Merck, Johannesburg, South Africa), acetic acid, 100% and NaOH with a minimum of 98% purity (Merck, Johannesburg, South Africa) were used.

2.2. Mucilage Precipitation and Freeze-Drying

Freeze-dried mucilage powders prepared from liquid mucilage precipitate from ‘Algerian’ and ‘Morado’ cultivars of Opuntia ficus-indica, with ≤10% moisture content, were used. The native mucilage precipitations were prepared following a patented extraction procedure described by Du Toit and De Wit [21]. This well-established procedure is suited for this research as it is cost-effective and easily replicated, with the resultant precipitate being well characterized by previous studies [22,23]. Mature cladodes (~24 months old) were selected from cactus pear plants harvested from an orchard at the University of the Free State (29.1076° S, 26.1925° E). The density of the cactus pear orchard was 666 cactus pear/ha without irrigation. Liquid mucilage precipitate was obtained by cubing whole cladodes and then microwaving the cubes for 4 min at 900 W, promoting the mucilage precipitation. Heated cubes were then macerated, and the pulp was centrifuged for 15 min at 8000 rpm at 4 °C to effectively separate the mucilage precipitate from the solids using a Beckman^® Centrifuge (2315, Brea, CA, USA). Freeze-dried mucilage powders were then prepared from the liquid mucilage precipitate. The freeze-drying process involved moisture removal under constant vacuum at low temperatures (−30 °C to −40 °C) until a weight loss of 95% was observed for the samples. A standard mortar and pestle were used to mill the freeze-dried samples until they were a fine, consistent powder with ≤10% moisture content.

2.3. Film Solution Preparation

All film-forming solutions were prepared using standardized methods, with some modifications, as established by Espino-Díaz et al. [24] and Gheribi et al. [25]. Desired amounts of polymer powder were dispersed into distilled water containing glycerol. Glycerol was used as a plasticizer at 60% inclusion (w/w, based on the polymer weight).

Using a magnetic stirrer (Freed Electric-Model MH-4, Rehovot, Israel) at ambient temperature (~25 °C), the film-forming solutions were homogenized and rehydrated for no less than 30 min, followed by mechanical mixing, using a Stick Blender (Mellerwave-Model 85200, Cape Town, South Africa) for 10 s, ensuring homogeneity. Reducing the potential volume and structural differences caused by excess entrapped air [24], film-forming solutions were degassed under vacuum sealing (Genesis Vacuum Sealer, Verimark (Pty) Ltd., Pretoria, South Africa).

2.4. Film Development

Using a batch process approach, best suited for laboratory-scale procedures, the film casting method was employed using standardized methods, with some modifications, to form the films for this current work [4,26,27,28]. A basic approach regarding the casting method involves evenly spreading a prepared film-forming solution onto a non-stick surface and allowing the films to dry. After this, the films are removed from the non-stick surface for evaluation [27]. In the current research, films were formed by spreading 70 g of prepared film-forming solution into 140 mm diameter Petri dishes, which were then placed into a ventilated oven (EcoTherm-Model 920, 1000 W, Labotec, Johannesburg, South Africa) at 40 °C for 24 h. Before film evaluation, films were stored and equilibrated in a closed container at ~52% RH, at room temperature (~25 °C), for 24 h.

2.5. Film Calcium Treatment

The various films were treated with calcium by preparing a 10% (w/w) stock CaCl₂ solution with distilled water. Once the required amounts of film-forming solution had been cast into the Petri dishes (as deceived in Section 2.4), the stock CaCl₂ solution was carefully poured into the Petri dishes containing the liquid film-forming solution to cover the films completely. A CaCl₂ reaction time of 5 min was allowed, whereafter, the excess calcium solution was removed from the Petri dishes, and the resultant films were gently damped with a fresh paper towel to remove any excess CaCl₂ solution. Thereafter, the Petri dishes containing the various CaCl₂-treated films were oven-dried and conditioned, awaiting evaluation.

2.6. Film Evaluation

The tensile and puncture tests, used to determine the films’ mechanical properties, were performed at room temperature (~25 °C) in a controlled environment. The tensile tests involved completing the tensile strength and elongation at break tests, using a Texture Analyser CT3™ (Brookfield AMETEK^®, Westville, South Africa) with the accompanying Roller Cam Accessory grips (TA-RCA). The distance between the grips was set at 50 mm. Tests were completed by consulting the ASTM International standard methods (ASTM-D882 [29]), as described by Harper [30]. The test speed was set at 0.80 mm·s⁻¹, with a test distance of 35 mm used for all measurements. Films cut into 25 mm × 80 mm rectangular strips were tested for each treatment. The film’s tensile strength represents the maximum stress (force/area) a film can withstand against an applied force before it tears [4]. Film tensile strength is calculated by dividing the maximum load (N) by the initial cross-sectional area of the film, expressed in MPa [24,25,31]. The film elongation at break percentage was also determined. A film’s elongation at break is considered a measure of the film’s maximum capacity to extend before breaking [4]. The elongation at break is determined by dividing the film’s length difference at rupture by the initial sample length [32]. Puncture tests were also completed using the texture analyzer with Probe TA44 (4 mm diameter probe). The test speed was set at 0.80 mm·s⁻¹. The puncture force and distance to puncture were determined using the generated force–distance graph. Puncture force is considered the maximum force required to puncture (tear) the film, measured in Newtons (N) [33]. A total of twelve films were tested per treatment. Film thickness was measured on the conditioned films using a digital micrometer (Grip, Johannesburg, South Africa) by measuring the thickness of the films at six different positions throughout the film.

Polymer films were subjected to scanning electron microscopy (SEM) imaging. Imaging was carried out using a JEOL Ltd. (JSM-7800F, Tokyo, Japan) scanning electron microscope. Images used in this research were representative of the overall microstructures of each treatment (representative of the different repeats) as determined by a group of five (5) researchers with experience in microscopy, one (1) being a trained, independent expert in microscopy.

2.7. Experimental Design and Statistical Analysis

The addition of both ‘Algerian’ and ‘Morado’ freeze-dried native mucilage to pectin-based and alginate-based ‘dry’ films was considered. Blends of 5% pectin (or alginate) + 0.25% mucilage and 5% pectin (or alginate) + 1% mucilage solutions were prepared. Pectin and alginate 5% (w/w) solutions were set as controls, and the resultant film’s mechanical properties were investigated. Mucilage inclusions (at 0.25% and 1%) were selected according to previous work by Harper [30], who also investigated various polymer inclusions into alginate-based films at these concentrations to best assess their impact on blend bio-based polymer films. The calcium treatment’s impact on the various films’ mechanical properties was also considered using a 10% (w/w) CaCl₂ stock solution.

Blend films displaying the most altered mechanical properties caused by adding mucilage were selected for SEM evaluation of their surface morphology. Evaluating the morphological alterations induced by the addition of mucilage would further assist in characterizing the structural impact associated with pectin + mucilage and alginate + mucilage films, in comparison to the homopolymeric pectin and alginate films. The SEM images of CaCl₂-treated pectin + ‘Algerian’ and pectin + ‘Morado’ films, with 1% mucilage additions, were compared to the control homopolymeric pectin films, elaborated at a concentration of 5% (w/w). Similarly, the CaCl₂-treated alginate + ‘Algerian’ and alginate + ‘Morado’ films, with 1% mucilage inclusions, were compared to the control, homopolymeric, alginate films elaborated at a concentration of 5% (w/w).

Results were recorded using Microsoft Excel (2016). Upon completion of the various trials, the data were subjected to statistical analysis (ANOVA) using a one-way analysis of variance (NCSS Statistical Software package, version 11.0.20). The Tukey–Kramer multiple comparison test (α = 0.05) was used to identify significant differences between the treatment means (NCSS Statistical Software package, version 11.0.20).

3. Results

3.1. Mechanical Properties of Blend Pectin–Mucilage and Alginate–Mucilage Films

The influence that the ‘Algerian’ and ‘Morado’ mucilage had on the mechanical properties of pectin–mucilage and alginate–mucilage blend films are presented in Figure 1, Figure 2, Figure 3 and Figure 4 and

Table 1. Mechanical properties of homopolymeric pectin and blend pectin–mucilage films with 0.25% or 1% ‘Algerian’ and ‘Morado’ mucilage additions, with or without calcium treatment.

Pectin-Based Homopolymeric and Bend Films	Puncture Force (N)	Distance to Puncture (mm)	Film Thickness (mm)
Pectin 5% (Control)	31.51 ± 3.05 bc	3.99 ± 0.30 bc	0.273 ± 0.046 abc
Pectin 5% + CaCl2	35.99 ± 5.38 cd	4.67 ± 0.34 cd	0.284 ± 0.030 abc
Pectin 5% + ‘Algerian’ 0.25%	38.37 ± 3.12 de	3.12 ± 0.47 a	0.250 ± 0.019 abc
Pectin 5% + ‘Morado’ 0.25%	38.81 ± 2.91 de	4.06 ± 0.72 bc	0.213 ± 0.037 a
(Pectin 5% + ‘Algerian’ 0.25%) + CaCl2	46.51 ± 2.23 f	5.11 ± 0.26 d	0.277 ± 0.054 abc
(Pectin 5% + ‘Morado’ 0.25%) + CaCl2	47.58 ± 3.13 f	4.62 ± 0.31 cd	0.278 ± 0.031 abc
Pectin 5% + ‘Algerian’ 1%	28.00 ± 1.07 b	2.96 ± 0.35 a	0.292 ± 0.057 bc
Pectin 5% + ‘Morado’ 1%	30.89 ± 2.37 bc	4.22 ± 0.73 bc	0.233 ± 0.020 ab
(Pectin 5% + ‘Algerian’ 1%) + CaCl2	32.97 ± 3.62 bcd	4.38 ± 0.50 cd	0.305 ± 0.025 c
(Pectin 5% + ‘Morado’ 1%) + CaCl2	42.58 ± 2.26 ef	3.42 ± 0.22 ab	0.276 ± 0.035 abc
Significance level	p < 0.001	p < 0.001	p = 0.003

The mean values of 10 treatments and their standard deviations (±) are displayed. Means with different superscripts in the same column differ significantly.

and

Table 2. Mechanical properties of homopolymeric alginate and blend alginate–mucilage films at 0.25% and 1% ‘Algerian’ or ‘Morado’ mucilage additions, with or without calcium chloride (CaCl₂) treatment.

Alginate-Based Homopolymeric and Bend Films	Puncture Force (N)	Distance to Puncture (mm)	Film Thickness (mm)
Alginate 5% (Control)	72.11 ± 7.51 cd	5.55 ± 0.50 de	0.244 ± 0.011 ab
Alginate 5% + CaCl2	83.10 ± 9.35 d	6.24 ± 0.56 e	0.281 ± 0.008 bc
Alginate 5% + ‘Algerian’ 0.25%	64.13 ± 7.71 bcd	5.47 ± 0.25 de	0.267 ± 0.034 abc
Alginate 5% + ‘Morado’ 0.25%	48.21 ± 10.62 ab	4.97 ± 0.91 cd	0.229 ± 0.051 a
(Alginate 5% + ‘Algerian’ 0.25%) + CaCl2	70.52 ± 15.57 cd	4.82 ± 0.87 bcd	0.256 ± 0.013 abc
(Alginate 5% + ‘Morado’ 0.25%) + CaCl2	59.07 ± 8.78 abc	3.84 ± 0.51 abc	0.289 ± 0.014 bc
Alginate 5% + ‘Algerian’ 1%	42.85 ± 6.47 a	3.93 ± 0.67 abc	0.297 ± 0.034 c
Alginate 5% + ‘Morado’ 1%	42.16 ± 5.70 a	3.39 ± 0.41 a	0.268 ± 0.014 abc
(Alginate 5% + ‘Algerian’ 1%) + CaCl2	54.18 ± 12.03 abc	3.89 ± 0.63 abc	0.371 ± 0.038 d
(Alginate 5% + ‘Morado’ 1%) + CaCl2	54.69 ± 11.62 abc	3.79 ± 0.59 ab	0.357 ± 0.014 d
Significance level	p < 0.001	p < 0.001	p < 0.001

The mean values of 10 treatments and their standard deviations (±) are displayed. Means with different superscripts in the same column differ significantly.

. Both the tensile tests and puncture tests were completed for all films investigated.

3.1.1. Blend Pectin–Mucilage Films

The influence that ‘Algerian’ and ‘Morado’ mucilage had on the pectin-based film tensile strength (TS) is presented in Figure 1.

Calcium had no significant influence (p > 0.05) on the homopolymeric pectin film tensile strength (TS) (Figure 1). The addition of 0.25% ‘Algerian’ mucilage inclusions had no significant effect (p > 0.05) on the film TS; however, ‘Morado’ at the same concentration, significantly increased (p < 0.05) the TS of the blend films, when compared to the control homopolymeric pectin films. ‘Morado’ mucilage further showed increased CaCl₂ sensitivity when compared to the ‘Algerian’ mucilage at 0.25% inclusions, significantly increasing (p < 0.05) the film TS values when compared to the control pectin-CaCl₂ films and pectin + ‘Algerian’ films treated with calcium (Figure 1). Calcium sensitivity had been related to structural differences, polymer concentration and, specifically, the presence of carboxyl groups in the polymers [34,35].

At a 1% mucilage concentration, blend pectin films showed minimal differences in TS values when compared to blend pectin films with 0.25% mucilage inclusions (Figure 1). However, CaCl₂ treatment of these films was shown to significantly increase (p < 0.05) the film TS values when compared to their non-CaCl₂ treated counterparts. CaCl₂-treated blend films displayed the overall highest TS values, reporting values of 9.23 MPa and 10.82 MPa, respectively.

Overall, although both mucilage cultivars showed CaCl₂ sensitivity at 1% inclusions, blend pectin + ‘Algerian’ mucilage films showed decreased sensitivity towards CaCl₂ treatments compared to pectin + ‘Morado’ mucilage films. ‘Morado’ mucilage also increased the pectin films TS to a greater degree than ‘Algerian’ mucilage, even for non-CaCl₂-treated films (Figure 1).

Evaluating the blended pectin–mucilage film, elongation at break percentage (%E) was also determined. While TS is a measure of the cohesion and compatibility between polymer chains, %E more accurately measures a film’s capacity to extend before breaking [4,7]. The %E values for pectin films and pectin–mucilage films are presented in Figure 2.

Treating the homopolymeric pectin films with calcium significantly increased (p < 0.05) the %E values (Figure 2). The addition of 0.25% ‘Algerian’ mucilage was shown to have a negative influence on the blend film elastic properties, as it significantly decreased (p < 0.05) the %E values. ‘Morado’ mucilage had a less negative influence on the blend films’ %E values.

Treating the 0.25% blend films with CaCl₂ showed a trend of increased %E values compared to their non-CaCl₂ counterpart films, although not significant (p > 0.05). However, the CaCl₂-treated blend films showed significantly lower (p < 0.05) %E values than the pectin–CaCl₂ films (Figure 2). A 1% mucilage concentration was shown to significantly decrease (p < 0.05) the films %E values, compared to the respective non-CaCl₂-and CaC₂-treated control, homopolymeric pectin films (Figure 2). Interestingly, when comparing these findings to the TS values of these blend films, 1% mucilage additions were shown to enhance the blended pectin–mucilage film TS. Conversely, at this same concentration (1%), mucilage was shown to negatively influence the films’ %E values (Figure 1 and Figure 2). Da Silva et al. [4] suggested that, when considering the films’ TS and %E values, usually higher TS values were related to lower %E values in films. The authors also proposed that both these properties should be analyzed simultaneously in order to better understand the mechanical profiles of the different films due to these correlations [4].

Puncture tests were also determined on the blend pectin–mucilage films, as displayed in Table 1. The puncture tests represent the puncture force (PF) and distance to puncture (DTP) measurements. The various films’ thicknesses were also considered (Table 1).

The addition of 0.25% mucilage resulted in significantly increasing (p < 0.05) the blend film PF values when compared to the control homopolymeric pectin films (Table 1). However, no film strength enhancements were observed at 1% mucilage inclusions (Table 1). The homopolymeric pectin films showed minimal sensitivity towards CaCl₂ treatments, as no significant differences (p > 0.05) were observed between the control pectin films and the pectin–CaCl₂ films (Table 1). Similarly, the TS evaluation also showed CaCl₂ treatments to have a minimal impact on the control pectin film strength (Figure 1). CaCl₂ treatment of the 0.25% mucilage blend films was shown to increase the PF values when compared to the homopolymeric pectin-CaCl₂ films (Table 1). Similar trends were also reported for the film TS values (Figure 1). At 1% mucilage inclusions, the superior interactions between the ‘Morado’ mucilage polymer and pectin polymers was further observed, as treating the blend ‘Morado’ films with CaCl₂ significantly increased (p < 0.05) the film PF values, when compared to the 1% ‘Algerian’ blend mucilage films and homopolymeric pectin–CaCl₂ films (Table 1).

The films’ DTP measurements were also investigated (Table 1). The blend ‘Morado’ film DTP values were not significantly influenced (p > 0.05) at either 0.25% or 1% mucilage inclusions when compared to the control, homopolymeric pectin films (Table 1). Superior compatibility was again observed between the pectin and ‘Morado’ polymers as the blend ‘Algerian’ films showed significantly decreased (p < 0.05) DTP values when compared to the control homopolymeric pectin films (Table 1). These findings were also observed for the %E evaluation of the pectin–mucilage blend films, confirming the superior synergy between pectin and ‘Morado’, as ‘Algerian’ mucilage negatively influenced the pectin film elasticity at both 0.25% and 1% inclusions (DTP and %E) (Figure 2 and Table 1). Lastly, the film thickness was also considered (Table 1). Only minimal differences were reported between the different film thicknesses, indicating a plausible degree of polymer synergy and positive interactions, especially for ‘Morado’ and pectin blend films.

3.1.2. Blend Alginate–Mucilage Films

Treating the homopolymeric alginate films with calcium reported significant increases (p < 0.05) in the films’ TS values. TS increased from 17.13 MPa to 19.63 MPa for the homopolymeric alginate films and alginate–CaCl₂ films, respectively (Figure 3). It has been established in the literature that the addition of calcium results in an increase in alginate film strength [32,36]. At 0.25% mucilage inclusions, blend alginate film TS values significantly decreased (p < 0.05) in comparison to the control homopolymeric alginate films (Figure 3). These decreases in the film TS values were greater with the addition of ‘Morado’ mucilage than for ‘Algerian’ mucilage, in contradiction of the findings observed for blend pectin + ‘Morado’ films, where ‘Morado’ mucilage had a positive influence on the blend film TS values, (Figure 1). The negative influence mucilage had on the blend alginate films TS was further highlighted when investigating the influence CaCl₂ had on the alginate films. Blend alginate films treated with CaCl₂ showed significantly lower (p < 0.05) TS values than the alginate–CaCl₂ films (Figure 3).

At a 1% mucilage concentration, regardless of CaCl₂ treatment, blend alginate films were shown to have a further negative influence on the film TS, when compared to the homopolymeric alginate films (Figure 3). Scognamiglio et al. [17] suggested that, due to the structural nature of mucilage and its expected uneven distribution in a film, it would likely interfere with the cross-linking of polymers. It is therefore hypothesized that the dissimilar structures expected between mucilage and alginate could impact the alginate film TS and ability to cross-link effectively, consequently reducing the blend film TS.

The influence that ‘Algerian’ and ‘Morado’ mucilage had on the blend alginate–mucilage film %E were also considered, as presented in Figure 4.

Treating the homopolymeric alginate films with CaCl₂ were shown to significantly increase (p < 0.05) the %E values when compared to the non-CaCl₂ treated films (Figure 4). Mucilage at 0.25% inclusions resulted in the blend alginate films reporting significant increases (p < 0.05) in the %E values compared to the control alginate films. However, mucilage had no significant influence (p > 0.05) on the %E values at 1% inclusion when compared to the control homopolymeric alginate films (Figure 4). These findings indicate that the addition of mucilage decreased the TS values of the blend alginate films (Figure 3), accompanied by increases in %E values (Figure 4). Correlations between TS and %E for films have also been reported by other authors, who suggested that this phenomenon could be a result of a varying degree of reinforcement between the polymers, which can be directly dependent on the degree of compatibility between the different polymers introduced into the films [4,36]. Specifically, at 0.25% concentration, blend ‘Morado’ films showed significantly greater (p < 0.05) film elasticity when compared to the blend ‘Algerian’ films (Figure 3 and Figure 4). The influence that CaCl₂ treatments had on the blend alginate–mucilage film %E was also considered (Figure 4). Although, 0.25% ‘Morado’ mucilage additions were shown to improve the elasticity of the blend ‘Morado’ film in comparison to that of blend ‘Algerian’ films. Resulting in a significant reduction (p < 0.05) in film %E values in comparison to the homopolymeric alginate-CaCl₂ films and CaCl₂-treated blend ‘Algerian’ films at 0.25% inclusions (Figure 4). These findings further highlighted clear differences observed between ‘Algerian’ and ‘Morado’ mucilage. As ‘Algerian’ mucilage were shown to have less of an influence on the film TS than ‘Morado’ mucilage, it was suggested that the ‘Algerian’ polymer displayed improved compatibility with alginate compared to ‘Morado’ mucilage. However, it is hypothesized that, without the addition of CaCl₂, the decreased reinforcement induced by ‘Morado’ on the blend ‘Morado’ films allowed for more interchain flexibility and interruption of the alginate polymer, resulting in increased %E values when compared to blend ‘Algerian’ films, which were not able to interfere with the alginate structure to such a great degree on account of the ‘Algerian’ polymer’s improved compatibility with alginate (Figure 4).

Blend ‘Algerian’ films with 0.25% mucilage showed no significant influence (p > 0.05) on the film PF values compared to the control, homopolymeric alginate films. Blend ‘Morado’ films showed significantly lower (p < 0.05) PF values than the control homopolymeric alginate films (Table 2). These trends were also observed for the TS evaluation of these alginate–mucilage blend films, where ‘Morado’ mucilage also was shown to clearly have a negative influence on the alginate film strength, with ‘Algerian’ mucilage having less of a negative impact on alginate film strength, compared to blend ‘Morado’ films (Figure 3 and Table 2). Increasing the mucilage concentration to 1% in the blend films was shown to consequently further reduce the film PF values when compared to the control, homopolymeric alginate films. These current findings emphasized the poor compatibility between alginate and mucilage, regarding film strength.

CaCl₂ treatment of the blend ‘Algerian’ films showed that ‘Algerian’ additions at 0.25% had no significant influence (p > 0.05) on the film’s CaCl₂ sensitivity when compared to the pure, homopolymeric alginate–CaCl₂ film PF values. However, blend ‘Morado’ films were shown to significantly decrease (p < 0.05) the film PF values when compared to the control alginate–CaCl₂ films (Table 2). Further increasing the concentrations of both ‘Algerian’ and ‘Morado’ mucilage into the blend films was shown to significantly interfere (p < 0.05) with the alginate cross-linking, reducing the film PF values when compared to the homopolymeric alginate–CaCl₂ films. Similar findings were reported for the CaCl₂ treatment of the blend alginate film TS evaluation, confirming the negative influence that native mucilage had on the mechanical strength of blend alginate films. Cultivar variations were shown to influence the mechanical strength of the blend films to different degrees. Specifically, ‘Algerian’ showed less of a negative influence on the film’s strength than ‘Morado’, which accounted for possible structural variations between the two cultivars’ mucilage, interfering with the alginate polymer cohesion to different degrees in the blend alginate films.

As to our knowledge, no literature is available on blend alginate–mucilage films, therefore, we compared these findings to the influence other polysaccharides had on blend alginate film mechanical properties. Wang et al. [37] reported similar trends regarding decreases observed in blend alginate film TS and PF with the addition of whey protein isolate (WPI). The authors accounted these decreases in film strength to the incompatibility between the alginate polymer and WPI, resulting in the WPI interfering with the alginate polymer’s interactions [37]. Harper reported on the influence of soy protein isolate (SPI) on alginate blend films [30]. The author found that the inclusion of SPI resulted in a decrease in the force required to puncture the films, suggested to be due to the SPI interrupting the alginate network due to their structural differences. Thus, the addition of mucilage to blend alginate films was shown to behave similarly to that of commercial starches and proteins.

Considering the various alginate film DTP measurements (Table 2), the inclusion of mucilage at 0.25% was shown to have no significant influence (p > 0.05) on the film DTP values when compared to the control alginate films. Increasing the mucilage concentration to 1% in the blend alginate–mucilage films showed a significant negative influence (p < 0.05) on the film DTP values compared to the control homopolymeric alginate films. Treating the blend alginate + mucilage films with CaCl₂ showed no significant effect (p > 0.05) at increasing the DTP values compared to their non-CaCl₂-treated blend alginate–mucilage counterparts. All CaCl₂-treated blend alginate–mucilage films (at 0.25% and 1%), however, reported significantly lower (p < 0.05) DTP values than the homopolymeric alginate–CaCl₂ films (Table 2). These findings confirmed that the addition of mucilage in the formulation of blend alginate–mucilage films was shown to negatively influence the alginate polymer cross-linking ability, resulting in a loss of film elasticity. However, from the %E measurements of the blend alginate–mucilage films, cultivar variations were more noticeable at 0.25%, when compared to the results obtained for the films DTP values (Figure 4 and Table 2). It could be expected that the tensile tests more accurately distinguish between smaller differences between films, whereas the puncture test provides valuable product performance insight from a commercial perspective [38]. Harper also reported trends of decreased film elasticity (PF and DTP) when SPI was incorporated into blend 5% alginate ‘dry’ films [30]. The author further showed that, due to structural differences between the SPI and the alginate polymer, both CaCl₂-treated and non-CaCl₂-treated blend alginate + SPI films were shown to have negatively influenced the films’ mechanical properties, related to the SPI interfering with the alginate polymer network [30]. Lastly, film thickness showed that the addition of mucilage and CaCl₂ resulted in trends of increased film thicknesses. Specifically, 1% inclusions of ‘Algerian’ mucilage resulted in increased film thickness when compared to the control alginate films. Furthermore, the addition of CaCl₂ at 1% mucilage inclusions resulted in significant increased (p < 0.05) film thickness of both ‘Algerian’ and ‘Morado’ mucilage. Similarly, Harper also reported increased film thickness with the addition of SPI and CaCl₂ as a result of structural interference [30].

3.2. Influence of Native Mucilage Additions on the Microstructures of Pectin and Alginate Blend Films Treated with CaCl₂

The SEM micrographs of homopolymeric pectin and alginate films, in comparison to blend pectin–mucilage and alginate–mucilage films treated with CaCl₂ are displayed in Figure 5. The CaCl₂-treated homopolymeric pectin and alginate films’ surface microstructures showed a degree of homogeneity. However, CaCl₂-treated pectin films had a decreased homogeneity and more roughness than CaCl₂-treated alginate films (Figure 5). When considering the influence native mucilage had on the pectin films’ microstructures, it was seen that both ‘Algerian’ and ‘Morado’ resulted in alterations of the microstructures when compared to the homopolymeric pectin films. Adding mucilage into pectin films appeared to result in a more organized and compact polymer network structure with fewer pores in comparison to the homopolymeric pectin films. Although both cultivars’ mucilage had a similar effect on the surface morphology of the pectin films, dark patches were observed for ‘Algerian’ mucilage-including pectin films, possibly indicative of poorer interactions between different components in the film matrix (Figure 5). Similarly, Luna-Sosa et al. [18] reported blend pectin–mucilage films that displayed a more dense and compact structure could be due to a better homogeneity and synergy observed between the different polymers used to develop the film network. Considering the blend alginate–mucilage films, mucilage reduced the film homogeneity, forming less structured and more lumpy film matrix surfaces compared to the homopolymeric alginate films (Figure 5).

4. Discussion

Comparing the influence mucilage had on the blend pectin–mucilage and alginate–mucilage films’ mechanical properties, considerable variations were observed between the two polymers’ interactions with mucilage. Although pectin and alginate are both known to be anionic polysaccharides, their chemical structures are known to differ considerably. Pectin is generally represented by a highly branched structure, where the alginate polymer is typically unbranched and linear, with varying degrees of charged groups associated with their structures [36,39,40]. Additionally, the homopolymeric pectin and alginate films displayed different degrees of CaCl₂ sensitivity. These findings were also supported by da Silva et al. [4], who concluded that, despite using similar CaCl₂ cross-linking procedures, pectin showed less-effective cross-linking than the alginate polymer, resulting in pectin films displaying lower TS values than alginate films. Variations between pectin and alginate films were further highlighted when observing these polymers’ interaction with mucilage and their resultant influence on the various film’s mechanical properties.

Due to the possible synergistic effects observed between the pectin polymer and mucilage polymers, increases in the film TS and PF were observed, when compared to the homopolymeric pectin films. Specifically, low amounts of mucilage (0.25%) were shown to positively impact the pectin film strength (Figure 1 and Table 1). As treating the homopolymeric pectin films with CaCl₂ were shown to have only a minimal impact on the pectin–CaCl₂ film’s strength, the significant increases (p > 0.05) observed for the blend pectin–mucilage film TS and PF was a clear indication of the enhanced functionality provided to the pectin films by the addition of mucilage. ‘Morado’ mucilage, in some instances, showed superior compatibility with pectin compared to that of ‘Algerian’ mucilage, in its ability to enhance the pectin film strength. The differences observed between ‘Algerian’ and ‘Morado’ mucilage could be related to structural differences between the mucilages, due to the variations in the availability of carboxyl groups in their polymer structures to react with the CaCl₂ [41]. Specifically, it is well-known that when CaCl₂ reacts with the carboxyl groups associated with a polymer’s chemical structure, it has the potential to increase a film’s TS.

These increases in TS values observed for the blend pectin films containing ‘Morado’ mucilage are indicative of synergistic interactions between the pectin and ‘Morado’ mucilage polymers, referred to as polymer compatibility or, more accurately, bio-compatibility. The compatibility of polymers, resulting in increases in TS values, can account for the reinforcement effect between the different polymers and has been directly related to structural compatibility observed between different polymers [17,42]. Both pectin and mucilage are known to be highly branched, less-structured polymers which could also result in their structural entanglement and compatibility and the consequential increases observed for the blend film TS when compared to the homopolymeric pectin films [43,44].

Acting in an opposite manner to pectin–mucilage films, alginate–mucilage films showed less compatibility, resulting in decreasing the alginate films’ strength in most instances. A possible explanation for these decreases in alginate film TS, with the addition of mucilage, is provided by Scognamiglio [17]. The authors suggested that, as mucilage is a less-structured polymer, its addition with more-structured polymers would act oppositely to the reinforcement observed between similar polymers, reducing a film’s TS [17]. These structural differences between pectin and alginate would directly account for their different interactions with the mucilage in the current research, as the mechanical properties displayed by bio-based polymer films have been directly linked to their chemical structure. Specifically, differences observed between molecular size, intra and intermolecular associations and molecular weight of the polymers would have a direct influence on the film strength and elasticity [24,45]. Mucilage’s expected high molecular weight and branched nature would have acted oppositely to the reinforcement observed between pectin and mucilage, interfering with the cohesion and structural interactions of alginate, and weakening the film matrix. In recent research done by De Farias et al. [46], the authors showed that the addition of cladode powder from Opuntia ficus-indica in the development of ‘dry’ blend cassava starch films negatively influenced the film mechanical properties. These decreases in the film strength and elasticity accounted for large molecular structure associated with mucilage, which disrupted the cassava matrix by decreasing the interactions between the cassava starch [46]. The addition of mucilage to the blend alginate films produced similar results, with further variation observed between the different mucilage cultivars, which were shown to influence the films’ mechanical properties to different degrees.

TS is considered an important measurement in the determination of the film’s strength and, ultimately, industrial applications, as it is essential to ensure the structural protection of food during transportation, storage and handling [11,36]. The variations between the mucilage cultivars’ functional properties investigated in the current research showed ‘Algerian’ mucilage to have less of an influence on the TS values for both pectin and alginate blend films, with ‘Morado’ showing improved compatibility and CaCl₂ sensitivity with pectin, consequently having more of a negative effect on the alginate blend film strength, with and without the addition of CaCl₂. Therefore, as ‘Morado’ displayed improved compatibility between pectin, it showed increased film strength and elasticity when compared to the blend pectin + ‘Algerian’ films. However, ‘Morado’ mucilage was shown to have a higher degree of incompatibility when introduced into alginate films than that of ‘Algerian’ mucilage. Therefore, it is hypothesized that the chemical structure of mucilage from the ‘Morado’ cultivar could be expected to differ from that of the ‘Algerian’ cultivar, with the same structural advantages offered by ‘Morado’ mucilage to pectin, consequently disadvantaging the alginate film polymer matrix.

The mechanical tests indicated that pectin and mucilage showed superior compatibility, and the SEM micrographs confirmed this, as the physical interactions between pectin and mucilage could be observed, and the negative influence mucilage had on the alginate polymer films could also be confirmed. Therefore, polymer compatibility, resulting in synergistic interactions between different polymers in blend film development, must strongly be considered, as further correlation between increased film strength being related to decreased film elasticity (and visa-versa) proved to have a consequential impact on the film’s mechanical properties, ultimately determining a film’s application and protection offered to the food product if used as biodegradable packaging.

5. Conclusions

The mechanical properties of films are viewed as extremely important, as they can be directly linked to the various applications and the product performance expected from biodegradable packaging to resist damage and allow for transport and storage. As pectin and alginate are well-known polymers used in the development of biodegradable packaging and films, knowledge of their interactions with novel bio-based polymers, such as native mucilage precipitate, would prove beneficial when exploring the possibility to improve or enhance blend films, therefore broadening their applications as biodegradable packaging. These findings confirmed that native mucilage precipitate has the potential to enhance certain vital mechanical properties of bio-based polymer films, such as in the case of blend pectin–mucilage films, which would not have been possible with the individual polymers alone. It is hypothesized that a lack of polymer compatibility, such as in the case observed for blend alginate–mucilage films, would be responsible for reducing the resultant film’s mechanical properties, specifically film strength.