Pullulan-Apple Fiber Biocomposite Films: Optical, Mechanical, Barrier, Antioxidant and Antibacterial Properties
Pullulan-Apple Fiber Biocomposite Films: Optical, Mechanical, Barrier, Antioxidant and Antibacterial Properties
Pullulan-Apple Fiber Biocomposite Films: Optical, Mechanical, Barrier, Antioxidant and Antibacterial Properties
Article
Pullulan–Apple Fiber Biocomposite Films: Optical,
Mechanical, Barrier, Antioxidant and Antibacterial Properties
Ângelo Luís 1,2,*, Ana Ramos 3,4 and Fernanda Domingues 1,3
1 Health Sciences Research Centre (CICS‐UBI), University of Beira Interior, Avenida Infante D. Henrique,
6200‐506 Covilhã, Portugal; fdomingues@ubi.pt
2 Pharmaco‐Toxicology Laboratory, UBIMedical, University of Beira Interior, Estrada Municipal 506,
6200‐284 Covilhã, Portugal
3 Chemistry Department, Sciences Faculty, University da Beira Interior, Rua Marquês d’Ávila e Bolama,
6201‐001 Covilhã, Portugal; ammr@ubi.pt
4 Fiber Materials and Environmental Technologies Research Unit (FibEnTech), University of Beira Interior,
Rua Marquês d’Ávila e Bolama, 6201‐001 Covilhã, Portugal
* Correspondence: angelo.luis@ubi.pt
Abstract: More than 150 million tons of synthetic plastics are produced worldwide from
petrochemical‐based materials, many of these plastics being used to produce single‐use consumer
products like food packaging. The main goal of this work was to research the production and
characterization of pullulan–apple fiber biocomposite films as a new food packaging material. The
optical, mechanical, and barrier properties of the developed biocomposite films were evaluated.
Furthermore, the antioxidant and antibacterial activities of the biocomposite films were additionally
studied. The results show that the Tensile Index and Elastic Modulus of the pullulan–apple fiber
Citation: Luís, Â.; Ramos, A.; films were significantly higher (p‐value < 0.05) when compared to the pullulan films. Regarding the
Domingues, F. Pullulan–Apple Fiber water vapor permeability, no significant differences (p‐value < 0.05) were observed in water vapor
Biocomposite Films: Optical, transmission rate (WVTR) when the apple fiber was incorporated into the biocomposite films. A
Mechanical, Barrier, Antioxidant significant increase (p‐value < 0.05) of water contact angle in both sides of the films was observed
and Antibacterial Properties. when the apple fiber was incorporated into pullulan, indicating an increase in the hydrophobicity
Polymers 2021, 13, 870.
of the developed biocomposite films. It is worth noting the hydrophobicity of the (rough) upper
https://doi.org/10.3390/
side of the pullulan–apple fiber films, which present a water contact angle of 109.75°. It was possible
polym13060870
to verify the microbial growth inhibition around the pullulan–apple fiber films for all the tested
bacteria.
Academic Editor: Debora Puglia
Received: 16 February 2021
Keywords: pullulan; apple fiber; biocomposites; films; antioxidant activity; antibacterial properties
Accepted: 8 March 2021
Published: 11 March 2021
Publisher’s Note: MDPI stays 1. Introduction
neutral with regard to jurisdictional Currently, more than 150 million tons of synthetic plastics are produced worldwide
claims in published maps and
from petrochemical‐based materials, such as polyolefins and polyesters. Many of these
institutional affiliations.
plastics are used to produce single‐use consumer products like food packaging [1]. This
causes a serious environmental problem, as most of those plastics do not degrade when
disposed of in the environment at the end of their life cycle, being recalcitrant to
environmental attack [1]. Several studies have focused on finding alternatives for
Copyright: © 2021 by the authors.
conventional petroleum‐derived plastics using biopolymers or low‐cost biomass with
Licensee MDPI, Basel, Switzerland.
This article is an open access article
comparable properties [2]. Biopolymers from agricultural resources, such as starch,
distributed under the terms and
cellulose, proteins, and pectin [3], which could be degraded in a few months, can be
conditions of the Creative Commons applied to the sustainable production of packaging materials since they are
Attribution (CC BY) license biodegradable, non‐toxic, and recyclable [4].
(http://creativecommons.org/licenses Contamination of processed foods by foodborne pathogens is an important safety
/by/4.0/). issue for consumers and food processors. To address this question, food‐compatible,
Polymers 2021, 13, 870. https://doi.org/10.3390/polym13060870 www.mdpi.com/journal/polymers
Polymers 2021, 13, 870 2 of 15
plant‐derived antimicrobials could reduce surface contamination of food products [5].
Antimicrobial edible films produced from plant‐derived foods could offer an additional
level of protection against microbial spoilage of food, as well as meet the increasing
consumer demands for safe and natural foods [6]. The incorporation of natural
antioxidants obtained from fruit and plant extracts to enhance the shelf‐life of foods has
also become a widespread strategy [7].
Despite not being an exceptional dietary fiber source, apples may provide several
health benefits related to the synergy of their fiber fractions with other nutrients [8].
Dietary fibers found in apples are of superior quality than those obtained from cereals
since they present higher solubility and concentration in additional health‐promoting
bioactive compounds [8].
Several studies have depicted the application of apple by‐products as new raw
materials for the development of biodegradable materials such as edible films, which also
have antioxidant activity due to the polyphenols present in apples [4]. However, in earlier
studies [4,8], films made solely by apple by‐products showed a weak film‐forming
capacity, making their employment unfeasible in food packaging production [4,8]. Due to
this, other biopolymers are required to be mixed with apple‐derived products to enhance
their capability to form films [4].
Pullulan is a water‐soluble extracellular polysaccharide produced by Aureobasidium
pullulans in cultures of sugar and starch [9]. The linear polymer mostly consists of
maltotriose units connected to each other by α‐(1,6) glycosidic bonds [10]. This single
linkage pattern gives pullulan distinct physical properties to form transparent, strong,
low‐permeability, and water‐soluble films [11]. Despite the advantageous properties of
pullulan, its use in food applications is frequently hindered due to its high cost. Therefore,
pullulan is frequently blended with other biopolymers to reduce the cost, as well as to
improve the material properties [11].
The main goal of this work was, therefore, to research the production and
characterization of pullulan–apple fiber biocomposite films as a new food packaging
material. The optical, mechanical, barrier, antioxidant, and antibacterial properties of the
biocomposite films developed were further evaluated.
2. Materials and Methods
2.1. Reagents
Pullulan (CAS Number: 9057‐02‐7), with a molar mass of 574.57 g/mol ((C23H42O16)n),
was supplied by TCI Europe (Zwijndrecht, Belgium). Apple fiber, with a mean particle
size of 150 μm, was obtained from My Protein (a THG company) (Voyager House,
Manchester; UK). Glycerol (anhydrous) (CAS Number: 56‐81‐5) was purchased from
Merck (Darmstadt, Germany).
2.2. Biocomposite Films Preparation
Initially, 2 g of pullulan (2%, w/v) were mixed with 1 g of apple fiber (1%, w/v) at
room temperature in 100 mL of distilled water. This mixture was stirred using a magnetic
stirrer (Heidolph, Schwabach, Germany) (250 rpm) for 15 min. Then, 20% glycerol (w/w,
relative to the biopolymers) was included as a plasticizer in the polymers mixture and
stirred at room temperature for 5 min, and at 50 °C for another 30 min. Finally, this
mixture was homogenized for 5 min at 10,000 rpm using a disperser IKA T25 Digital Ultra‐
Turrax (Staufen, Germany). The biocomposite films were obtained by casting 16 mL of the
polymer mixture in polystyrene Petri dishes (9 cm of diameter), which were subsequently
dried for around 3 h at 60 °C in a forced‐air laboratory oven. Pullulan films not including
apple fiber were also produced, to be used as a control. Finally, the dried films were
detached from the Petri dishes and kept under controlled relative humidity (RH) (50 ± 5%)
and temperature (23 ± 2 °C) [1,2,4,12,13].
Polymers 2021, 13, 870 3 of 15
2.3. Surface Morphology of the Films
The surface morphology of the biocomposite films was observed by optical
microscopy using a Nikon Labophot‐2 microscope (Nikon, Tokyo, Japan) equipped with
a Leica MC190 HD camera and (Leica, Wetzlar, Germany) controlled by the LAS v4.13
software (https://imillermicroscopes.com/pages/software‐download).
2.4. Infrared Spectra
A Nicolet iS10 smart iTRBasic Thermo Fisher Scientific (Waltham, MA, USA) was
used to obtain the Fourier‐Transform Infrared Spectroscopy (FTIR) spectra of the
biocomposite films between 600 and 4000 cm−1, acquiring 120 scans with 4 cm−1 of
resolution [12].
2.5. Thermal Analysis
A calorimeter Netzsch DSC 204 (GWP, Munich, Germany) was used to obtain the
Differential Scanning Calorimetry (DSC) thermograms of the biocomposite films,
functioning with the following conditions: an inert atmosphere, a heating rate of 5 °C/min,
and a temperature ranging from 25 to 500 °C. Before the study, samples of the films were
maintained at 105 °C for 24 h to totally evaporate the water, also being obtained the
respective baselines [12].
2.6. Grammage, Thickness, Mechanical, and Optical Properties
The biocomposite films’ grammage was determined by the quotient between their
mass and area (g/m2), according to the ISO 536:1995. An Adamel Lhomargy Model MI 20
micrometer (TMI, Veenendaal, Netherlands) was used to measure the thickness (μm) of
the films, considering different random assessments, following the ISO 534:2011 [12,13].
The mechanical properties of the biocomposite films (Peak Elongation (%), Tensile
Index (N m/g), Tensile Strength (N/m), and Elastic Modulus (MPa)) were determined
using a Thwing‐Albert Instrument Co. (West Berlin, NJ, USA) tensile tester, adjusting the
crosshead at 10 mm/min, and the initial grip at 50 mm, following the ISO 1924/1 [12,13].
The optical properties of the biocomposite films (color coordinates and transparency)
were measured using a Color Touch 2 spectrophotometer (Technidyne, New Albany, MS,
USA). The determinations were achieved considering several arbitrary positions of the
films using the illuminant D65 (daylight with a UV component) and an observation angle
of 10°. Color coordinates L*, a*, and b* (lightness; redness/±red‐green;
yellowness/±yellow‐blue) were obtained [12,13].
2.7. Barrier Properties
2.7.1. Oil Permeability
Firstly, 5 mL of edible vegetal oil (obtained from sunflower seeds and composed of
70% linoleic acid, 10% linolenic acid, and 10% oleic acid) (Vitaquell, Hamburg, Germany)
were put into test tubes, which were then sealed with the biocomposite films. The tubes
were turned upside down on the surface of a filter paper previously dried at 105 °C for 24
h and weighed. The oil permeability (OP) (g mm/m2 day) was calculated with the weight
difference of the filter paper, the thickness of the biocomposite films, the effective contact
area, and the storage period (24 h) according to the following equation:
∆
𝑂𝑃 , (1)
where ΔW is the weight difference of the filter paper (g), e corresponds to the thickness of
the film (mm), A is the contact area (m2), and T is the storage period (days) [14].
Polymers 2021, 13, 870 4 of 15
2.7.2. Water Vapor Permeability
The water vapor permeability (WVP) (g/Pa day m) and the water vapor transmission
rate (WVTR) (g/m2 day) were evaluated following the ASTM E96‐00 standard procedure.
For that, equilibrated test cups containing a desiccant (15 g of anhydrous CaCl2 (Sigma‐
Aldrich, MO, USA), previously dried at 105 °C) were sealed with the biocomposite films.
Then, the cups were put in a container at 23 ± 2 °C and 50 ± 5% RH, the weight differences
being checked every 2 h for 48 h. The slope of a linear regression of the weight increase
versus time was used to determine the gradient [15]. The WVTR and WVP were calculated
using Equations (2) and (3):
𝑊𝑉𝑇𝑅 , (2)
where ∆m is the weight change of the test cups (g), T is the test time (h), and A is the test
area (m2).
where p is the water vapor pressure at 23 °C (Pa), RH1 is the RH of the container (50%),
RH2 is the RH inside the cups (0%), and e is the thickness (m) of the biocomposite films.
2.8. Contact Angle and Surface Free Energy
The values of the contact angles of the biocomposite films were measured by the
sessile drop method using an OCAH 200 model from DataPhysics Instruments
(Filderstadt, Germany), which allowed for image acquisition and data analysis at the same
time [12,13]. The surface free energy (total, dispersive, and polar components) of the
biocomposite films was calculated by determining the contact angles using three reference
liquids (distilled water, diiodomethane, and ethylene glycol) [16]. The surface free energy
components of these liquids were obtained directly from the software [17]. Contact angles
were taken from six measurements for each liquid and each sample, considering random
positions of the biocomposite films, the Owens‐Wendt approach being employed in the
obtention of the surface free energies of the biocomposite films [18].
2.9. Antioxidant Activity
2.9.1. DPPH Free Radical Scavenging Assay
For this assay, 2.9 mL of a DPPH (2,2‐diphenyl‐1‐picrylhydrazyl) (Sigma‐Aldrich,
Missouri, USA) solution (0.1 mM in methanol) were mixed with 3 disks of the
biocomposite films (6 mm in diameter). Then, the absorbances of these mixtures were
measured every 30 min for 5 h at 517 nm against methanol (Sigma‐Aldrich, Missouri,
USA) as a blank. The mixture of 2.9 mL of the DPPH solution with 100 μL of methanol
was used as the control sample. The antioxidant activity of the biocomposite films was
calculated using the following equation [13,19]:
where Acontrol is the absorbance of the control, and Asample is the absorbance of the samples
(films).
2.9.2. β‐Carotene Bleaching Test
Firstly, 50 μL of a β‐carotene solution (20 mg/mL in chloroform (Sigma‐Aldrich, MO,
USA)) were mixed with linoleic acid (40 μL), Tween 40 (400 μL), and chloroform (1 mL),
the chloroform being evaporated under vacuum. Then, oxygenated distilled water (100
mL) was added to the residue, forming an emulsion. Subsequently, 5 mL of the emulsion
were mixed with 3 disks of the biocomposite films (6 mm in diameter) in test tubes.
Finally, the tubes were placed in a 50 °C environment for 1 h. The absorbances of these
Polymers 2021, 13, 870 5 of 15
samples were measured at 470 nm against a blank containing an emulsion prepared
without the β‐carotene. The mixture of 5 mL of the emulsion with 300 μL of methanol was
used as the control sample. The antioxidant activity of the biocomposite films was
calculated as the percentage of inhibition of β‐carotene oxidation using the following
equation [13,19]:
where At=1h is the absorbance of the samples (films) or the control at the final time of
reaction, and At=0h is the absorbance of the control at the initial time of reaction.
2.10. Antibacterial Properties
The antibacterial activity of the biocomposite films against seven foodborne
pathogens (Staphylococcus aureus ATCC 25923, Listeria monocytogenes LMG 16779,
Enterococcus faecalis ATCC 29212, Bacillus cereus ATCC 11778, Salmonella Typhimurium
ATCC 13311, Escherichia coli ATCC 25922, and Pseudomonas aeruginosa ATCC 27853) was
evaluated by solid assay. Stock cultures of the bacterial species were maintained at −80 °C
with 20% (v/v) glycerol. All the bacterial species were cultivated in brain–heart infusion
agar (BHI) for 24 h before the antibacterial tests [19]. For the solid assay, several microbial
colonies were suspended in a sterile saline solution (NaCl (Sigma‐Aldrich, MO, USA);
0.85%; w/v), adjusting the suspension of the inoculums to 0.5 McFarland (≈1.5 × 108
colony‐forming units (CFU)/mL). Disks of the biocomposite films (6 mm in diameter)
were cut under asepsis. Then, the prepared disks were placed on the inoculated Müeller‐
Hinton agar (MHA) or BHI plates. Lastly, the plates were incubated for 18 h at 37 °C. After
the incubation period, they were visually examined for inhibition zones and their
diameters were measured with a pachymeter [12,13]. Additionally, the plates were
observed by optical microscopy, as described above, to verify the microbial growth
inhibition and the integrity of the films after the incubation period. The results were
achieved by three independent assays.
2.11. Biodegradability
To evaluate the biodegradability of the biocomposite films, a soil burial degradation
test was performed. Small strips of the biocomposite films (2 × 7 cm) were interred in
organic soil at a depth of 10 cm and maintained at 50 ± 5% RH and 23 ± 2 °C for 10 days.
Afterwards, the strips were collected, cleaned with distilled water, and dried in an oven
(50 °C) for 24 h. The weight loss (WL) was determined using the following equation
[15,20]:
𝑊𝐿 % 100, (6)
where W corresponds to the weight of the samples before and after the soil burial
degradation test.
2.12. Statistical Analysis
Overall, the results are shown as the mean ± standard deviation (SD). The IBM SPSS
Statistics v25 software (https://www.ibm.com/analytics/spss‐statistics‐software) was used
to analyze the raw values, employing the Student’s T‐test (assuming the normal
distribution of the continuous variables). Differences among means are considered to be
significant if the p‐value is < 0.05 (a confidence level of 95%).
Polymers 2021, 13, 870 6 of 15
3. Results and Discussion
3.1. Appearance and Surface Morphology of the Films
The pullulan films appeared to be colorless, transparent, and homogenous (Figure
1a), presenting a smooth surface with compact structural integrity (Figure 1c). On the
other hand, the pullulan–apple fiber films were yellowish, although transparent (Figure
1b), with the apple fiber uniformly dispersed in the pullulan matrix. In addition, these
films had a rough surface on the upper side of the film (Figure 1d). In both types of the
developed films, there were no pinholes formation, which will particularly affect their
barrier properties.
(a) Pullulan (b) Pullulan and Apple Fiber
(c) Pullulan (d) Pullulan and Apple Fiber
Figure 1. Appearance of the pullulan film (a), pullulan and apple fiber film (b), and surface morphology (optical
microscopy, magnification: 400 ×) of the pullulan film (c), pullulan and apple fiber film (d).
3.2. FTIR and DSC Assays
Figure 2a shows the FTIR spectra of the apple fiber (maximum absorbance = 0.23),
which present a similar profile to that of the biocomposite films (Figure 2b). Apples are
particularly rich in pectin, a type of soluble fiber, also a polysaccharide‐like pullulan.
The FTIR spectrum of the pullulan film (Figure 2b) shows the infrared bands
corresponding to CH vibrations at 2930 cm−1 and the stretching vibrations of the OH
groups of the polymer at 3310 cm−1. Furthermore, the CO vibrations of the glycosidic and
etheric bounds of the pullulan molecule were detected at 929 cm−1, 995 cm−1, 1078 cm−1,
and 1148 cm−1 (maximum absorbance = 0.48) [12,21].
The FTIR spectrum of the pullulan–apple fiber film (Figure 2b) showed slight
differences in the zone of 1800 to 1200 cm−1; however, a considerable decrease in the
absorbance was noticed at 2930 cm−1, 1078 cm−1, and 995 cm−1, indicating that the apple
fiber was successfully incorporated into the pullulan matrix, suggesting its interaction
Polymers 2021, 13, 870 7 of 15
with the pullulan molecules via hydrogen bonds, which also corroborates the visual
aspect of the biocomposite films and their surface morphology, as described above.
The DSC results (Figure 3) revealed an increase in the glass transition temperature
(Tg) of the films when the apple fiber was incorporated into pullulan, the value of Tg being
154.5 °C [9]. This Tg increase suggests that the incorporation of apple fiber in the pullulan
matrix changed the structure of the biocomposite films and, therefore, their mechanical
properties [9]. To overcome this change in the mechanical properties, a higher percentage
of glycerol (20%, w/w), used as a plasticizer, was added in the formulation of the
biocomposite films, aiming to reduce the ratio of the crystalline to the amorphous region
[22]. However, in previous work, we developed pullulan films incorporating rockrose
essential oil, and only 15% (w/w) of glycerol was used, taking into account the potential
plasticizer effect of the essential oil [12].
Figure 2. FTIR results of the raw apple fiber (a) and the biocomposite films (b).
Polymers 2021, 13, 870 8 of 15
Figure 3. Differential Scanning Calorimetry (DSC) results of the biocomposite films.
3.3. Grammage, Thickness, Mechanical, and Optical Properties
The grammage and thickness of the biocomposite films increased significantly (p‐
value < 0.05) with the apple fiber incorporation into the pullulan (Table 1). This increase
was particularly substantial for the thickness, which, in the pullulan films, was 35.59 μm,
and in the films with apple fiber was 155.87 μm. This result is a consequence of the rough
surface of the upper side of the biocomposite films. It was previously described that plant‐
based fibers may present a certain degree of inconsistency [23], which also explains the
roughness observed in the pullulan–apple fiber films.
Table 1. Grammage, thickness, mechanical, and optical properties of the biocomposite films.
The mechanical properties of the biocomposite films must be carefully evaluated, as
they are important when considering the potential application of these films in food
packaging [24]. In the present study, the Peak Elongation, the Tensile Index, the Tensile
Strength, and the Elastic Modulus of the films were determined (Table 1). The Peak
Elongation of the pullulan–apple fiber films was similar to the one of the pullulan films
incorporated with rockrose essential oil (2.34%) previously developed [12], despite being
significantly lower (p‐value < 0.05) than the observed for the pullulan films (Table 1). The
high value of Peak Elongation obtained for the pullulan films may be explained by the
high percentage of glycerol used as a plasticizer. The high volumes of plasticizer
decreased the cohesive forces amongst polymer chains, with the replacement of strong
interactions with hydrogen bonds. Moreover, the hygroscopic behavior of glycerol
enhanced the water absorption, which is also a film plasticizer. Consequently, the films
Polymers 2021, 13, 870 9 of 15
became less rigid and more flexible [25]. It was also verified that the Tensile Index of the
pullulan–apple fiber films was significantly higher (p‐value < 0.05) when compared to the
pullulan films (Table 1), indicating that the Tensile Strength of the pullulan–apple fiber
films was also significantly higher (p‐value < 0.05) (Table 1). Tensile Strength is the
evaluation of the maximum strength of a film to withstand applied tensile stress [26].
Furthermore, the Elastic Modulus of the pullulan–apple fiber films was significantly
higher (p‐value < 0.05) than that of the pullulan films (Table 1).
Concerning the optical properties of the biocomposite films (Table 1), it was observed
that when apple fiber was incorporated, there was a significant increase (p‐value < 0.05)
in color coordinates (L*, a*, and b*), which is consistent with their yellowish color.
Moreover, a significant decrease (p‐value < 0.05) in the transparency was verified in the
pullulan–apple fiber films (Table 1), as it was also possible to visually observe.
3.4. Barrier Properties
The barrier properties of the biocomposite films were evaluated in terms of their
permeability to water vapor and oil (Table 2) since good barrier properties are a steady‐
state indicator of biodegradable packaging materials [27].
Regarding the water vapor permeability, it was verified that no significant
differences (p‐value < 0.05) were observed in the WVTR when the apple fiber was
incorporated into the biocomposite films (Table 2). However, a significant increase (p‐
value < 0.05) in the WVP was noticed (Table 2), which is related to the higher thickness of
the pullulan–apple fiber films, as mentioned before. Previous works demonstrated that
an increase in the thickness of films could result in an increase of WVP, since the resistance
of the films to water vapor is decreased with the increase of the thickness. Therefore, a
stagnant air layer is created, characterized by high partial pressure of water vapor on the
inner film surface [14]. Similar results were observed for the OP of the biocomposite films
developed (Table 2). It was assumed that glycerol, used as a plasticizer, increased the free
volume between the pullulan molecules, which made oil permeate easily [14,27].
Table 2. Barrier properties of the biocomposite films.
3.5. Contact Angles and Surface Free Energy
Surface wettability, which is frequently described by the water contact angle [28],
was determined in the biocomposite films using a sessile drop method. Based on the
Owens‐Wendt approach, the surface free energy parameters of the films were calculated
by the mean contact angles of water, diiodomethane, and ethylene glycol (Table 3). A
significant increase (p‐value < 0.05) of water contact angle in both sides of the films was
verified when apple fiber was incorporated into the pullulan (Table 3), indicating an
increase in the hydrophobicity of the developed biocomposite films (water contact angle
is higher than 90° [29]), which is an important finding since food products often present
high amounts of water. It is worth noting the hydrophobicity of the (rough) upper side of
the pullulan–apple fiber films, presenting a water contact angle of 109.75°. It is well known
that the roughness of the surfaces is directly related to the increase of the water contact
angle, the so‐called “lotus effect” [30], which we also previously explored [15]. Other
authors reported that one of the most common design strategies for making an anti‐
wetting surface is to implement the surface roughness [31], which was clearly observed in
the pullulan–apple fiber films now developed. While the intrinsic wettability of a material
Polymers 2021, 13, 870 10 of 15
depends primarily on its surface free energy, the role of surface roughness becomes
critical when fabricating anti‐wetting surfaces [31].
It was observed that the total surface free energy (the sum of the polar and dispersive
components of surface energy) of both types of films was not significantly different (p‐
value > 0.05) (Table 3) despite presenting significantly different (p‐value < 0.05) surface
free energy components (polar and dispersive) (Table 3), which will affect their interfacial
interactions with liquids and thereby would show different wettability [31].
Table 3. Contact angles and surface free energy of the biocomposite films.
Pullulan Pullulan and Apple Fiber
Properties 1 Lower Side Upper Side p‐Value
Lower Side a Upper Side b
(smooth) c (rough) d
< 0.001 ac*
Water contact angle (°) 66.17 ± 1.79 64.99 ± 3.11 99.69 ± 2.11 109.75 ± 4.83
< 0.001 bd*
0.001 ac*
Diiodomethane contact angle (°) 31.42 ± 1.32 37.05 ± 0.43 44.72 ± 1.87 41.82 ± 2.02
0.049 bd*
0.001 ac*
Ethylene glycol contact angle (°) 59.06 ± 1.95 49.93 ± 0.78 44.72 ± 1.87 41.82 ± 2.02
0.011 bd*
0.361 ac
Total surface free energy, ɤT (mN/m) 40.92 ± 2.04 43.10 ± 2.15 42.43 ± 1.39 41.33 ± 0.52
0.288 bd
0.001 ac*
Polar component, ɤP (mN/m) 27.62 ± 1.37 31.25 ± 1.55 1.13 ± 0.33 2.95 ± 0.43
< 0.001 bd*
< 0.001 ac*
Dispersive component, ɤD (mN/m) 13.28 ± 0.65 11.84 ± 0.58 41.29 ± 1.37 38.38 ± 0.29
< 0.001 bd*
1 Results shown as mean ± SD; superscript letters (a, b, c, d) identify the pairs of samples under
statistical comparison; * Significant result (p‐value < 0.05).
3.6. Antioxidant Activity
The antioxidant capacity of the biocomposite films was examined by quantifying the
constant release of antioxidant compounds into the reaction mixture, evaluated by the
DPPH free radical scavenging method, as shown in Figure 4.
Figure 4. Antioxidant capacity of the biocomposite films measured by DPPH assay.
The obtained results indicated that the DPPH scavenging capacity was clearly
associated (R2 = 0.9642) with the time of reaction in the case of the pullulan–apple fiber
films after the 2 h of reaction (before that, there was no antioxidant activity), while the
pullulan films presented no capacity to scavenge the DPPH free radicals (Figure 4). These
results may be explained by the presence of several polyphenolic compounds in apple
Polymers 2021, 13, 870 11 of 15
fiber that were able to be released from the biocomposite films and to scavenge free
radicals, as described previously by other authors [8,32,33].
Moreover, the pullulan–apple fiber films were able to hinder the lipid peroxidation,
assessed by a β‐carotene bleaching test (Table 4). The presence of antioxidants associated
with apple fiber, like polyphenols, may be responsible for this protective effect against
oxidation [8].
Overall, these findings evidence that not only can apple fiber be incorporated into
pullulan films, reducing the quantity of pullulan needed to produce films, but also that
when the apple fiber was added to the films, they acquired antioxidant properties, which
could be beneficial to extending the shelf‐life of packaged foods.
Table 4. Antioxidant activity (β‐carotene bleaching test) and weight loss (biodegradability) of the
biocomposite films.
Properties 1 Pullulan Pullulan and Apple Fiber p‐Value
β‐carotene bleaching test Inhibition (%) 0.00 ± 0.00 0.96 ± 0.06 0.001 *
Biodegradability Weight loss (%) 100.00 ± 0.00 100.00 ± 0.00 > 0.05
1
Results shown as mean ± SD; * Significant result (p‐value < 0.05).
3.7. Antibacterial Properties
The antibacterial activity of the biocomposite films was evaluated by solid assay,
with the inhibition zones diameters being measured (Table 5). It was observed that the
pullulan–apple fiber films were able to cause bacterial cell retraction at the contact area
with a clear inhibition zone for the tested Gram‐positive and Gram‐negative bacteria,
contrariwise to what was noted for the pullulan films. Once more, the functionalization
of the biocomposite films was achieved by the incorporation of apple fiber into the
pullulan matrix, probably due to the bioactive compounds, like polyphenols, present in
the apple fiber. Other authors developed polysaccharide‐based edible coatings enriched
with apple fiber and concluded that its incorporation into the formulations had a marked
effect in reducing psychrophilic and mesophilic counts [8].
Table 5. Antibacterial properties of the biocomposite films.
Pullulan and Apple Fiber
Diameters of Inhibition Zones 1 Pullulan Lower Side Upper Side p‐Value
(smooth) (rough)
Staphylococcus aureus ATCC 25923 0.00 ± 0.00 (‐) 6.00 ± 0.00 (+) 6.00 ± 0.00 (+) < 0.001 *
Listeria monocytogenes LMG 16779 0.00 ± 0.00 (‐) 6.00 ± 0.00 (+) 6.00 ± 0.00 (+) < 0.001 *
Enterococcus faecalis ATCC 29212 0.00 ± 0.00 (‐) 6.00 ± 0.00 (+) 6.00 ± 0.00 (+) < 0.001 *
Bacillus cereus ATCC 11778 0.00 ± 0.00 (‐) 6.00 ± 0.00 (+) 6.00 ± 0.00 (+) < 0.001 *
Salmonella typhimurium ATCC 13311 0.00 ± 0.00 (‐) 6.00 ± 0.00 (+) 6.00 ± 0.00 (+) < 0.001 *
Escherichia coli ATCC 25922 0.00 ± 0.00 (‐) 6.00 ± 0.00 (+) 6.00 ± 0.00 (+) < 0.001 *
Pseudomonas aeruginosa ATCC 27853 0.00 ± 0.00 (‐) 6.00 ± 0.00 (+) 6.00 ± 0.00 (+) < 0.001 *
1 Results shown as mean ± SD; (‐) bacterial growth on the top of the films; (+) bacterial cell
retraction at the contact area with a clear inhibition zone; * Significant result (p‐value < 0.05).
The results of antibacterial activity were also confirmed by optical microscopy. By
observing the images presented in Table 6, it was possible to verify the microbial growth
inhibition around the pullulan–apple fiber films for all the tested bacteria. Furthermore,
the antibacterial effect was similar on both sides of the biocomposite films. Additionally,
the images showed that the pullulan–apple fiber films maintain their integrity over the
incubation period, contrary to what was observed for the pullulan films that dissolved
along the incubation period. The obtained results suggest the relevance of the
biocomposite films developed in this work for future applications in food packaging.
Polymers 2021, 13, 870 12 of 15
Table 6. Optical microscopy images of antibacterial activity (Magnification: 400×).
Pullulan and Apple Fiber
Pullulan
Lower Side (smooth) Upper Side (rough)
Staphylococcus aureus ATCC 25923
Listeria monocytogenes LMG 16779
Colonies
Film
Enterococcus faecalis ATCC 29212
Colonies
Bacillus cereus ATCC 11778
Colonies
Film
Salmonella Typhimurium ATCC 13311
Polymers 2021, 13, 870 13 of 15
Escherichia coli ATCC 25922
Film Film
Pseudomonas aeruginosa ATCC 27853
Colonies
Colonies
3.8. Biodegradability
A biodegradability test is a very valuable tool to help understand the environmental
compatibility of developed materials. The soil biodegradability of food packaging
materials was already stated. Particularly, the food packaging films produced with
renewable biopolymers exhibited appealing soil biodegradable behaviors [34].
The biodegradability test of the developed biocomposite films was examined by a
soil burial degradation test over 10 days (Table 4). It was verified that both types of films
were completely degraded after 10 days, showing a weight loss of 100%. The addition of
the apple fiber to the pullulan did not alter the biodegradability of the biocomposite films,
making them appropriate for being put back in the environment without causing adverse
effects, as other authors have also reported [35].
4. Conclusions
In this work, the incorporation of apple fiber in pullulan was shown to be a novel
biopolymer basis to produce edible and bioactive films. The biocomposite films
developed appeared to be resistant, flexible, and hydrophobic, together with their good
visual aspect. Besides that, they were capable of scavenging free radicals, inhibiting lipid
peroxidation, and inhibiting the growth of known foodborne pathogens.
The obtained results indicate that the pullulan–apple fiber biocomposite films
developed in this work present a strong potential and are a promising material to develop
new biodegradable alternatives to package foods, avoiding the use of traditional plastics.
Author Contributions: Conceptualization, Â.L. and F.D.; methodology, Â.L. and A.R.; formal
analysis, A.R. and F.D.; resources, Â.L., A.R. and F.D.; data curation, Â.L.; writing—original draft
preparation, Â.L.; writing—review and editing, A.R. and F.D. All authors have contributed
significantly to this version of the manuscript and have read and agreed to its publication. All
authors have read and agreed to the published version of the manuscript.
Funding: Ângelo Luís recognizes the contract of Scientific Employment (Microbiology) financed
by Fundação para a Ciência e a Tecnologia (FCT) under the scope of DL 57/2016. This work was
partially supported by CICS‐UBI, which is financed by National Funds from FCT and by FEDER
under the scope of PORTUGAL 2020 CENTRO 2020, within the project UIDB/00709/2020.
Acknowledgments: Authors acknowledge Ana Paula Gomes (Laboratório de Microscopia
Eletrónica, UBI) for her assistance in DSC.
Institutional Review Board Statement: Not applicable.
Polymers 2021, 13, 870 14 of 15
Informed Consent Statement: Not applicable.
Data Availability Statement: The data presented in this study are available on request from the
corresponding author.
Conflicts of Interest: The authors declare no conflict of interest.
References
1. Chiellini, E.; Cinelli, P.; Imam, S.H.; Mao, L. Composite films based on biorelated agro‐industrial waste and poly(vinyl alcohol).
Preparation and mechanical properties characterization. Biomacromolecules, 2001, 2, 1029–1037.
2. Shin, S.H.; Kim, S.J.; Lee, S. H.; Park, K.M.; Han, J. Apple Peel and Carboxymethylcellulose‐Based Nanocomposite Films
Containing Different Nanoclays. J. Food Sci., 2014, 79, E342–E353.
3. Gustafsson, J.; Landberg, M.; Bátori, V.; Åkesson, D.; Taherzadeh, M.J.; Zamani, A. Development of bio‐based films and 3D
objects from apple pomace. Polymers, 2019, 11, 289.
4. Choi, I.; Chang, Y.; Shin, S.H.; Joo, E.; Song, H.J.; Eom, H.; Han, J. Development of biopolymer composite films using a
microfluidization technique for carboxymethylcellulose and apple skin particles. Int. J. Mol. Sci., 2017, 18, 1278.
5. Ravishankar, S.; Zhu, L.; Olsen, C.W.; McHugh, T.H.; Friedman, M. Edible apple film wraps containing plant antimicrobials
inactivate foodborne pathogens on meat and poultry products. J. Food Sci., 2009, 74, M440–M445.
6. Mild, R.M.; Joens, L.A.; Friedman, M.; Olsen, C.W.; McHugh, T.H.; Law, B.; Ravishankar, S. Antimicrobial edible apple films
inactivate antibiotic resistant and susceptible Campylobacter jejuni strains on chicken breast. J. Food Sci., 2011, 76, M163–M168.
7. Du, W.X., Avena‐Bustillos, R.J.; Woods, R.; Breksa, A.P.; McHugh, T.H.; Friedman, M.; Levin, C.E.; Mandrell, R. Sensory
evaluation of baked chicken wrapped with antimicrobial apple and tomato edible films formulated with cinnamaldehyde and
carvacrol. J. Agric. Food Chem., 2012, 60, 7799–7804.
8. Moreira, M.R.; Cassani, L.; Martín‐Belloso, O.; Soliva‐Fortuny, R. Effects of polysaccharide‐based edible coatings enriched with
dietary fiber on quality attributes of fresh‐cut apples. J. Food Sci. Technol., 2015, 52, 7795–7805.
9. Hassannia‐Kolaee, M.; Khodaiyan, F.; Pourahmad, R.; Shahabi‐Ghahfarrokhi, I. Development of ecofriendly bionanocomposite:
Whey protein isolate/pullulan films with nano‐SiO2. Int. J. Biol. Macromol., 2016, 86, 139–144.
10. Niu, B.; Shao, P.; Chen, H.; Sun, P. Structural and physiochemical characterization of novel hydrophobic packaging films based
on pullulan derivatives for fruits preservation. Carbohydr. Polym., 2019, 208, 276–284.
11. Tong, Q.; Xiao, Q.; Lim, L.T. Effects of glycerol, sorbitol, xylitol and fructose plasticisers on mechanical and moisture barrier
properties of pullulan‐alginate‐carboxymethylcellulose blend films. Int. J. Food Sci. Technol., 2013, 48, 870–878.
12. Luís, Â.; Ramos, A.; Domingues, F. Pullulan Films Containing Rockrose Essential Oil for Potential Food Packaging Applications.
Antibiotics, 2020, 9, 681.
13. Luís, Â.; Gallardo, E.; Ramos, A.; Domingues, F. Design and characterization of bioactive bilayer films: Release kinetics of
isopropyl palmitate. Antibiotics, 2020, 9, 443.
14. Yan, Q.; Hou, H.; Guo, P.; Dong, H. Effects of extrusion and glycerol content on properties of oxidized and acetylated corn
starch‐based films. Carbohydr. Polym., 2012, 87, 707–712.
15. Luís, Â.; Domingues, F.; Ramos, A. Production of Hydrophobic Zein‐Based Films Bioinspired by The Lotus Leaf Surface:
Characterization and Bioactive Properties. Microorganisms, 2019, 7, 267.
16. Luís, Â.; Silva, F.; Sousa, S.; Duarte, A.P.; Domingues, F. Antistaphylococcal and biofilm inhibitory activities of gallic, caffeic,
and chlorogenic acids. Biofouling, 2014, 30, 69–79.
17. Good, R.; van Oss, C. The modern theory of contact angles and the hydrogen bond components of surface energies, in Modern
approaches to wettability applications, M. Schrader and G. Loeb, Eds. New York: Plenum Press, 1991, 1–27.
18. Silva, Â.; Duarte, A.; Sousa, S.; Ramos, A.; Domingues, F.C. Characterization and antimicrobial activity of cellulose derivatives
films incorporated with a resveratrol inclusion complex. LWT–Food Sci. Technol., 2016, 73, 481–489.
19. Luís, Â.; Pereira, L.; Domingues, F.; Ramos, A. Development of a carboxymethyl xylan film containing licorice essential oil with
antioxidant properties to inhibit the growth of foodborne pathogens. LWT‐Food Sci. Technol., 2019, 111, 218–225.
20. Kumar, D.; Kumar, P.; Pandey, J. Binary grafted chitosan film: Synthesis, characterization, antibacterial activity and prospects
for food packaging. Int. J. Biol. Macromol., 2018, 115, 341–348.
21. Hezarkhani, M.; Yilmaz, E. Pullulan modification via poly(N‐vinylimidazole) grafting. Int. J. Biol. Macromol., 2019, 123, 149–156.
22. Ramos, Ó.L.; Fernandes, J.C.; Silva, S.I.; Pintado, M.E.; Malcata, F.X. Edible Films and Coatings from Whey Proteins: A Review
on Formulation, and on Mechanical and Bioactive Properties. Crit. Rev. Food Sci. Nutr., 2012, 52, 533–552.
23. Mahmud, S.; Hasan, K.M.F.; Jahid, M.A.; Mohiuddin, K.; Zhang, R.; Zhu, J. Comprehensive review on plant fiber‐reinforced
polymeric biocomposites. J. Mater. Sci., 2021, 56, 7231–7264.
Polymers 2021, 13, 870 15 of 15
24. Chu, Y.; Xu, T.; Gao, C.; Liu, X.; Zhang, N.; Feng, X.; Liu, X.; Shen, X.; Tang, X. Evaluations of physicochemical and biological
properties of pullulan‐based films incorporated with cinnamon essential oil and Tween 80. Int. J. Biol. Macromol., 2019, 122, 388–
394.
25. Díaz, O.; Ferreiro, T.; Rodríguez‐Otero, J.L.; Cobos, Á. Characterization of chickpea (Cicer arietinum L.) flour films: Effects of pH
and plasticizer concentration. Int. J. Mol. Sci., 2019, 20, 1246.
26. Liang, J.; Yan, H.; Zhang, J.; Dai, W.; Gao, X.; Zhou, Y.; Wan, X.; Puligundla, P. Preparation and characterization of antioxidant
edible chitosan films incorporated with epigallocatechin gallate nanocapsules. Carbohydr. Polym., 2017, 171, 300–306.
27. Sun, H.; Shao, X.; Jiang, R.; Shen, Z.; Ma, Z. Mechanical and barrier properties of corn distarch phosphate‐zein bilayer films by
thermocompression. Int. J. Biol. Macromol., 2018, 118, 2076–2081.
28. Chen, G.; Ali, F.; Dong, S.; Yin, Z.; Li, S.; Chen, Y. Preparation, characterization and functional evaluation of chitosan‐based
films with zein coatings produced by cold plasma. Carbohydr. Polym., 2018, 202, 39–46.
29. Dong, F.; Padua, G.W.; Wang, Y. Controlled formation of hydrophobic surfaces by self‐assembly of an amphiphilic natural
protein from aqueous solutions. Soft Matter, 2013, 9, 5933–5941.
30. Wang, G.; Guo, Z.; Liu, W. Interfacial effects of superhydrophobic plant surfaces: A Review. J. Bionic Eng., 2014, 11, 325–345.
31. Song, K.; Lee, J.; Choi, S.O.; Kim, J. Interaction of surface energy components between solid and liquid on wettability, and its
application to textile anti‐wetting finish. Polymers, 2019, 11, 498.
32. Reis, S.F.; Rai, D.K.; Abu‐Ghannam, N. Apple pomace as a potential ingredient for the development of new functional foods.
Int. J. Food Sci. Technol., 2014, 49, 1743–1750.
33. Serra, A.T.; Matias, A.A.; Frade, R.F.M.; Duarte, R.O.; Feliciano, R.P.; Bronze, M.R.; Figueira, M.E.; Carvalho, A.; Duarte, C.M.M.
Characterization of traditional and exotic apple varieties from Portugal. Part 2–Antioxidant and antiproliferative activities. J.
Funct. Foods, 2010, 2, 46–53.
34. Yadav, M.; Liu, Y.; Chiu, F. Fabrication of Cellulose Nanocrystal/Silver/Alginate Bionanocomposite Films with Enhanced
Mechanical and Barrier Properties for Food Packaging Application. Nanomaterials, 2019, 9, 1523.
35. Nouraddini, M.; Esmaiili, M.; Mohtarami, F. Development and characterization of edible films based on eggplant flour and corn
starch. Int. J. Biol. Macromol., 2018, 120, 1639–1645.