The Use of Nanocellulose in Edible Coatings for the Preservation of Perishable Fruits and Vegetables

Abstract

The usage of edible coatings (ECs) represents an emerging approach for extending the shelf life of highly perishable foods, such as fresh and fresh-cut fruits and vegetables. This review addresses, in particular, the use of reinforcing agents in film-forming solutions to tailor the physicochemical, mechanical and antimicrobial properties of composite coatings. In this scenario, this review summarizes the available data on the various forms of nanocellulose (NC) typically used in ECs, focusing on the impact of their origin and chemical or physical treatments on their structural properties (morphology and shape, dimension and crystallinity) and their functionality. Moreover, this review also describes the deposition techniques of composite ECs, with details on the food engineering principles in the application methods and formulation optimization. The critical analysis of the recent advances in NC-based ECs contributes to a better understanding of the impact of the incorporation of complex nanoparticles in polymeric matrices on the enhancement of coating properties, as well as on the increase of shelf life and the quality of fruits and vegetables.

Full text

Coatings 2021, 11, 990. https://doi.org/10.3390/coatings11080990 www.mdpi.com/journal/coatings
Review
The Use of Nanocellulose in Edible Coatings for the
Preservation of Perishable Fruits and Vegetables
Annachiara Pirozzi 1, Giovanna Ferrari 1,2 and Francesco Donsì 1,*
1 Department of Industrial Engineering, University of Salerno, Via Giovanni Paolo II, 132,
84084 Fisciano, Italy; apirozzi@unisa.it (A.P.); gferrari@unisa.it (G.F.)
2 ProdAl Scarl, University of Salerno, Via Giovanni Paolo II, 132, 84084 Fisciano, Italy
* Correspondence: fdonsi@unisa.it
Abstract: The usage of edible coatings (ECs) represents an emerging approach for extending the
shelf life of highly perishable foods, such as fresh and fresh-cut fruits and vegetables. This review
addresses, in particular, the use of reinforcing agents in film-forming solutions to tailor the physi-
cochemical, mechanical and antimicrobial properties of composite coatings. In this scenario, this
review summarizes the available data on the various forms of nanocellulose (NC) typically used in
ECs, focusing on the impact of their origin and chemical or physical treatments on their structural
properties (morphology and shape, dimension and crystallinity) and their functionality. Moreover,
this review also describes the deposition techniques of composite ECs, with details on the food
engineering principles in the application methods and formulation optimization. The critical
analysis of the recent advances in NC-based ECs contributes to a better understanding of the im-
pact of the incorporation of complex nanoparticles in polymeric matrices on the enhancement of
coating properties, as well as on the increase of shelf life and the quality of fruits and vegetables.
Keywords: edible coating; shelf life; quality; barrier; cellulose; nanocomposite;
reinforcing agent; nanofiller
1. Introduction
Traditional commercial food packaging materials, such as glass, aluminum, tin, and
petroleum-based polymers, are widely used for the protection of goods from physical
damage, external contamination or deterioration [1]. To limit the environmental pollu-
tion caused by non-degradable plastic packaging, the use of biocompatible macromole-
cules seems a promising strategy for a more sustainable packaging. In this frame, edible
coatings (ECs) represent a consolidated technology to improve the postharvest quality of
fruits and vegetables by slowing down respiration rate, water loss and oxidation pro-
cesses [2], as well as helping to maintain the physiological properties.
ECs consist of a thin layers of proteins, polysaccharides or lipids, and are applied di-
rectly to the surface of the food in a liquid form with different techniques, forming a mi-
cro-layer film on the surface of the food [3]. ECs acts as primary (closest to food) packaging.
Thus, the main advantage over traditional synthetic packaging is that ECs can be con-
sumed with the food, with no package to dispose of [4], reducing the cost and complexity
of packaging systems designed to protect fresh perishable foods. Moreover, also if they are
not eaten by the consumers, ECs could still contribute to the reduction of environmental
pollution, because they are produced exclusively from renewable, edible ingredients and
therefore degrade more readily than polymeric materials [5]. In addition, because of the
additional protection they offer, ECs enable also the simplification of the secondary pack-
aging (next layer of packaging), making recycling more accessible [6]. However, the per-
formance of most ECs is insufficient to meet practical applications, in particular with ref-
erence to conjugating restricted thickness with adequate mechanical and barrier properties.
Citation: Pirozzi, A.; Ferrari, G.;
Donsì, F. The Use of Nanocellulose
in Edible Coatings for the
Preservation of Perishable Fruits and
Vegetables. Coatings 2021, 11, 990.
https://doi.org/10.3390/
coatings11080990
Academic Editor: Jaejoon Han
Received: 28 July 2021
Accepted: 17 August 2021
Published: 19 August 2021
Publisher’s Note: MDPI stays
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Copyright: © 2021 by the authors.
Licensee MDPI, Basel, Switzerland.
This article is an open access article
distributed under the terms and
conditions of the Creative Commons
Attribution (CC BY) license
(http://creativecommons.org/licenses
/by/4.0/).

Coatings 2021, 11, 990 2 of 27
Therefore, recent studies have focused on the implementation of different types of filler to
improve coating properties [7]. The preservation action of ECs can be enhanced through
the incorporation of a wide range of bioactive compounds, like aroma compounds, essen-
tial oils, antioxidants, pigments and ions [8–10], which contribute to slowing down the
browning reactions of fresh and fresh-cut fruits and vegetables, as well as decreasing mi-
crobial growth, thereby leading to the shelf-life extension of the products [11]. Moreover,
the synergistic interaction between reinforcement agents and the polymeric material,
through hydrogen bonding or ionic complexation, enables the ECs mechanical properties
to be increased and permeability of moisture and gases to be reduced [12].
Among the different reinforcing agents, nanofillers or additives with their size lying
in the range of 100 nm have attracted increasing attention. In this scenario, nanocellulose
(NC) emerged as a promising material for tailoring ECs properties in food preservation.
For their nano-reinforcing effect in many different polymer matrices, three types of cel-
lulose are mainly used, such as cellulose nanocrystals (CNC), cellulose nanofibrils (CNF),
and bacterial nanocellulose (BNC).
This review describes the engineering and applicative aspects related to the use of
NC in composite ECs to prevent the quality decay of perishable fruits and vegetables
during storage. In this regard, after a brief overview of ECs deposition techniques and the
optimization of their formulation, this review illustrates the property enhancement of
ECs reinforced with NC and other active compounds as well.
2. Edible Coating Deposition and Optimization
2.1. Methods of Coating Application to Food Products
ECs can be applied to food products using different techniques (Figure 1), which are
described below.
The selection of the most suitable deposition technique of the EC layer is generally
based on (a) the characteristics of the foods to be coated, and in particular surface hy-
drophobicity and roughness, (b) the physical properties of the coating, such as viscosity,
density and surface tension [13], as well as (c) the intended effect of the coating, (d) the
available drying technique, (e) the intended industrial application, and (f) the cost.
Figure 1. Schematic representation of the main coating deposition methods: (a) spraying method,
(b) electrospraying method, (c) dipping method, (d) spreading method, (e) layer-by-layer coating
deposition, and (f) cross-linked coating.
Coating solution Coating solution
Coating solution
Coating
solution
--
-
++
+
Negatively
charged coating
solution
Positively
charged coating
solution
----
--
- -
------
--
- -
--
+
+
+
+
+
+
+
+
+
+
First
coating solution Second
coating solution
a b
cd
e
f

Coatings 2021, 11, 990 3 of 27
2.1.1. Spraying Method
Spraying (Figure 1a) is a conventional method for applying low-viscosity coating
solutions on food surfaces, and is generally based on high-pressure atomization in the
60–80 psi (4.1–5.5 bar) range to produce fine droplets, which are deposited on the food
products until a homogeneous and uniform layer is formed [14]. The thickness of the
coating layer can be controlled through the atomization conditions and is generally as-
sociated with the hydrodynamic diameter of the droplet. Good atomization conditions
are associated to the size distribution of the sprayed droplets, which depends on the main
operating parameters, including the atomizer features (spray gun type, operating pres-
sure, and nozzle temperature), and the main operating conditions, such as air and liquid
flow rate, the humidity of incoming air and the polymer solution [15]. The volumes of
coating solution required per unit mass of product to be coated are lower for spraying
than for other processes. For all these reasons, spraying is especially recommended when
a high-quality product is desired for large-scale productions. However, the thickness of
the coating layer that can be obtained by spraying is limited by the lower limit of the
hydrodynamic diameter of the droplets that can be obtained by atomization, which is of
around 20 µm, whereas in electrospraying processes (discussed in the following section),
an aerosol with droplets below 100 nm can be generated [13,16,17]. The final coating
quality in spraying processes depends also on the drying method and parameters, such
as drying time, temperature, and relative humidity of the drying medium [16].
Among the advantages of spraying methods in coating application there are the
absence of contamination of the coating solution, the possibility of continuous produc-
tion automation and their suitability to be applied to heat-sensitive compounds [17].
To the best of our knowledge, no studies have been reported about the spraying
deposition technique of NC-reinforced ECs on foods. Shanmugam and coworkers se-
lected the spraying technique to deposit NC films by spraying on the surface of imper-
meable substrate, because they found that the spraying time was independent of NC
suspension concentration [18]. However, to improve the smoothness of the obtained NC
film, they preliminarily subjected the NC suspension to a high-pressure homogenization
treatment to reduce fiber diameter and length. Other researchers reported that spraying a
NC suspension on solid surfaces represent a valid alternative technique to vacuum fil-
tration to obtain high-quality thin films. Therefore, spraying offers significant advantages
in specialized applications, such as contouring or contour coating (the coating solution
freely falls across the substrate and forms a curtain, which follows the substrate contours)
and contactless coating (no mechanical contact takes place between the base substrate
and the distributor of the coating solution), especially because a high NC content can be
usedthan in vacuu-filtration, hence reducing the amount of water to be removed by
drying [19,20].
2.1.2. Electrospraying Method
A novel technique for coating application to food surfaces is electrospraying (Figure 1b),
where the atomization of the coating solutions is carried out in a high-intensity electric
field, which enables the formation of micrometric and sub-micrometric charged droplets
with an extremely narrow size distribution [21]. The application of high voltage to the
coating solution at the tip of an emitter causes the formation of a Taylor cone, with the
accumulation of charge near the surface of the nascent droplet, and the destabilization of
the liquid surface, which disrupts into multiple fine charged droplets [22,23]. Therefore,
in comparison with spraying, which produces uncharged droplets, electrospraying offers
the additional advantage of further promoting the adhesion to the food surface, due to
electrostatic interactions [24]. During electrospraying, the droplet size, deposition rate,
and layer thickness can be controlled by optimizing the main process parameters, such as
coating solution flow rate and properties, namely conductivity and viscosity [25,26].

Coatings 2021, 11, 990 4 of 27
It must be remarked that, because of the low flow rate of the coating solution at each
emitter (because of the sequential formation of individual, submicrometric droplets) and
of the requirement for specialized personnel to work with the high-voltage generator,
scaling-up of electrospraying is more expensive than spraying (multiple emitters are
needed). In particular the electrospraying technique appears to be suitable especially for
the fabrication of thin NC-based composite materials with oriented fibers, for tunable
properties in an electric or magnetic fields (for example, using magnetic cellulose) [27].
2.1.3. Dipping Technique
The dipping method (Figure 1c) involves three important stages [28]:
1. Immersion and holding (dwell time). The substrate is immersed into the coating
solution, followed by a holding time to allow the substrate to interact for a sufficient
dwell time with the coating solution to complete wetting.
2. Deposition and drainage. By pulling the substrate upward, a thin layer of the coat-
ing solution is entrained, causing film deposition. In this stage, excess liquid drains
from the surface of the substrate.
3. Evaporation and/or drying. The excess diluent leaves the food surface by evapora-
tion at room temperature or drying with heated air, thus achieving a thin film of the
coating solution.
Previous studies have shown that the thickness and morphology of the coatings
deposited by dipping on fruits, vegetables, meat, and fish significantly depends on im-
mersion time, withdrawal speed, dip-coating cycles, density, viscosity and surface ten-
sion of the coating solution, substrate surface characteristics and drying conditions of the
coating solutions [29]. The dipping method is especially advantageous when the coating
has to be applied to products with a complex and rough surface, which cannot be uni-
formly reached by spraying methods. Dipping, however, suffers also from several
drawbacks. First of all, dipping generally forms a thick coating layer, which may cause
excessive reduction of product respiration and damage of food surface [30,31] and deg-
radation of its functionality, reducing the storage characteristics [32]. Secondly, issues
related to the contamination of the coating solution with bacterial load or dirtiness from
processed fruit represents an important issue to consider in industrial scale-up. Finally,
in general, large volumes of coating solutions are needed per unit mass of product to be
coated, to ensure proper dipping conditions. Therefore, dipping is best suited to
small-scale or batch processes.
The traditional way of ECs application is by dipping directly in a liquid form,
forming a micro-layer film on the surface of the fresh food. However, from literature,
only a few studies have used this technique for the deposition of NC-based coatings. For
example, Herrera and coworkers verified the potential of NC-based coatings, applied by
dipping, as an ecological bio-based option for developing barrier applications on pa-
per-based packaging [33].
2.1.4. Spreading Method
The spreading method (Figure 1d), also known as brushing, is suitable for
high-viscosity coating solutions that are spread directly onto the material surface and
then dried. The main parameters used to characterize the spreading of the coating solu-
tion of the food surface are, generally, the wetting degree and the spreading rate [34]. The
degree of spreading/wettability of a surface by a particular liquid is commonly evaluated
by contact angle measurements, which are described in Section 4.1.3. The efficient coating
deposition by spreading is affected by several factors, such as the substrate properties,
and in particular surface roughness and geometry, liquid properties, such as viscosity,
surface tension and density, and drying conditions, including temperature and relative
humidity [35].

Coatings 2021, 11, 990 5 of 27
Brushing is generally carried out by specialized operators and, therefore, the quality
of the spread coating and layer uniformity is strongly affected by the human factor. For
all these considerations, spreading is more suitable for small-scale productions.
In literature, no applications have been reported for the spreading method for
NC-based ECs deposition on food products. Nevertheless, the coating deposition by
spreading seem to be a promising technique, when considering the numerous studies
that have highlighted the effectiveness of NCs-reinforced films production through the
conceptually similar film casting technique [36–42].
2.1.5. Layer-by-Layer Deposition
The application of ECs is often limited by the difficult adhesion of the coating solu-
tion to the product surface, especially in the case of fresh-cut fruits, characterized by high
hydrophilic surfaces [16]. In the layer-by-layer (LbL) deposition method (Figure 1e), ad-
hesion to the food surface is enhanced by electrostatic interaction of the food surface with
charged polyelectrolytes. The electrostatic interactions are also exploited to form coatings
made of two or more layers of nanometric dimensions, which are physically or chemi-
cally bonded to each other [43,44], enabling the efficient control of physicochemical
properties and functionality of ECs.
The LbL electrostatic deposition technique generally relies on the combination of
oppositely charged polyelectrolytes, through alternate dipping of the food product in
different coating solutions. The alternate dipping process is repeated as many times as
many coating layers are desired. The amount of adsorbed polyelectrolyte onto the food
surface during each dipping depends on the ionic strength, pH, and charge densities of
each coating solution.
LbL deposition methods do not only work with oppositely charged polyelectrolytes
but also with macromolecules capable of developing hydrogen-bond, hydrophobic or
covalent interactions through mutually interacting binding sites. Table 1 reports some
examples of LbL coatings applied to fresh products exploiting the electrostatic interac-
tions of the different biopolymer layers, with a special focus on the use of NC as anionic
polyelectrolyte.
Table 1. Applications of electrostatic layer-by-layer (LbL) coating deposition methods for different food products.
Product
Polyelectrolytes
Results
References
Anionic
Cationic
-
Nanocellulose
Nanochitin
Reinforcing film agent with excellent
gas-barrier properties, highly trans-
parent, unfavorable to bacterial adhe-
sion and thermally recyclable, thus
promising for advanced food packag-
ing applications.
[45]
-
Nanocellulose
Chitosan, cationic starch and
collagen
Ability to finely tailor the nanoarchi-
tecture of the film providing ways high
performance free-standing films or
coatings with advanced properties.
[46]
-
Nanocellulose
Chitosan
Promising nanocomposite film with
high oxygen barrier in transparent
flexible packaging materials and semi
rigid tridimensional objects.
[47]
-
Nanocellulose
Polyethyleneimine
Thin films with unique mechanical
properties and the morphology of a
“porous matchsticks pile”, which
brings about strong antireflective
properties.
[48]
Fresh-cut apples
Carboxymethylcellulose
Chitosan
Polyelectrolyte multilayer (PEM) film
[49]

Coatings 2021, 11, 990 6 of 27
sodium salt (NaCMC)
shown good browning, weight loss,
and metabolic activity inhibition abil-
ity.
Mandarin fruits
Carboxymethylcellulose
(CMC)
Chitosan
The LbL polysaccharides-based coating
notably improved the physiological
quality of mandarins and their firm-
ness.
[50]
Fresh-cut mangoes
Sodium alginate
Chitosan
Nanomultilayer coating by electrostatic
self-assembly improved the microbio-
logical and physicochemical quality of
during storage time.
[51]
Citrus fruit
Carboxymethyl cellulose
(CMC)
Chitosan
CMC/chitosan electrostatic bilayer EC
greatly enhanced fruit glossiness and
appearance but was not very effective
in preventing weight loss.
[52]
Fresh-cut melons
Sodium alginate
Chitosan
LbL electrostatic deposition of ECs had
benefits on food firmness, gas ex-
change, and microbiological protection
[53]
Mango fruits
Polystyrene sulfonate so-
dium salt (PSS)
Poly diallyl dimethylammo-
nium chloride (PDADMAC)
PDADMAC/PSS films based-coated
fruit shown significantly improved the
hydrophilicity of the outer surface.
[54]
LbL has drawn considerable attention because of its ability to control the thickness
of the coating at the nanoscale, for the extensive choice of materials [55] and the possibil-
ity to embed antimicrobial components into the polymer matrix to construct the antimi-
crobial composites [56–58]. However, it must be remarked that, for industrial applica-
tions, its use is limited by the complex coating deposition procedure, based on alternating
the use of different coating solutions, often with the need for intermediate washing
phases to remove excess coating solutions.
2.1.6. Cross-Linking Technique
The cross-linking technique can be described as the procedure of linking the poly-
mer chains by covalent and non-covalent bonds. Cross-linked coatings (Figure 1f) are
generally produced by deposition of the coating solution on the food surface by spraying,
dipping, or spreading, followed by the deposition of a cross-linking agent, for the for-
mation of a more compact and resistant coating. Cross-linked coatings offer significant
advantages especially in the reduced migration of external molecules into the coatings
[59,60], as well as in improving mechanical strength, chemical resistance, and the thermal
stability of the coating [61]. The most common crosslinking agents are symmetrical bi-
functional compounds with reactive groups with specificity for functional groups pre-
sent on the matrix macromolecules [62]. Cross-linking is especially useful for biopolymer
materials, such as those derived from proteins or polysaccharides, although it is more
commonly applied to proteins than to polysaccharides since proteins have more func-
tional groups [63]. The typical cross-linking process involves the use of a wide range of
cross-linking agents (Table 2).
Table 2. Cross-linking agents commonly used in different types of edible coating.
Cross-Linking Agent
Biopolymers
References
Glutaraldehyde
Gelatin
Cellulosic derivatives
Chitosan
[64]
[65]
[66,67]
Epichlorohydrin
Starch
[68,69]
Ca2+ ions
Alginate
Pectin
[70–72]
[70]

Coatings 2021, 11, 990 7 of 27
Whey protein
[73]
Sodium benzoate
Starch
[74]
Citric acid
Starch
Cellulosic derivatives
[75,76]
[77,78]
Boric acid
Cellulose
[79]
Tannic acid
Chitosan
Gelatin
[80]
[81]
Ferulic acid
Gelatin
[81]
The cross-linking technique can be applied also to the preparation of NC-based
coatings, through the prior modification of NC. For example, nanocomposite films were
prepared with corn nano-starch as the biopolymeric matrix and modified-CNCs as the
reinforcement. The CNCs were modified through a two-step method, in which they were
initially crosslinked with citric acid, and subsequently amidated with chitosan. The
modified-CNCs loaded nanostarch-based nanocomposite film contributed to conferring
(i) a stronger network structure through intra- and intermolecular hydrogen bonds and
mutual entanglements with the starch matrix, (ii) an increase in tensile strength and wa-
ter contact angle value, (iii) a decrease in water vapor permeability, (iv) a better antimi-
crobial activity against E. coli and S. aureus bacteria, when compared with the pure corn
nano-starch film [39].
2.2. Optimization of Film-Forming Formulation
The efficiency of coating deposition is reported to depend primarily on the nature of
the coating ingredients and their relative, optimal concentrations [82]. The optimization
of the operating parameters for EC deposition can be supported by multivariate statistic
techniques, such as response surface methodology (RSM). RSM is a collection of mathe-
matical and statistical tools based on the fit of a polynomial equation to the experimental
data. It is intended to replicate the observed experimental behavior and help to derive
statistical conclusions, to enable the reduction in the number of experimental runs nor-
mally required to assess the optimal values of multiple variables (multivariate analysis),
especially in the case of significant variable interactions [83].
In the process of optimization of coating formulations, response variables are related
to independent variables by a second-order polynomial equation (Equation (1)):
  

   



  


(1)
where Xi is independent variables, β0 the intercept; βi, βii, βij are regression coefficients
linear, quadratic, and interaction terms, respectively, and k is the number of variables.
Different response variables were used in coating optimization. For example, RSM
was applied to optimize gelatin/chitosan solutions’ concentration in film in terms of
finding the maximum elasticity by minimizing Young’s modulus [84]. It is also used, for
example, to optimize the sodium alginate and calcium chloride concentrations and dip-
ping time to minimize the coating thickness [85]. In another study, the process variables
for the preparation of edible composite films from pearl millet starch and carrageenan
gum blends were optimized as a function of coating quality, evaluated in terms of
thickness, water vapor permeability, solubility, and tensile strength [86]. The response
surface methodology was used also to estimate the effects of the independent variables,
such as alginate, glycerol, and citric acid concentrations on the surface solid density of
coated papaya [87]. Color, water content, water-solubility, puncture strength, percentage
of elongation, and water vapor permeability of coatings were also evaluated. RSM was
also used to determine the relationships that both turmeric oil volume and coating
thickness have with the antimicrobial agent’s migration rate, the microbial inhibition
zone and the degree of weight loss during biodegradation [88].

Coatings 2021, 11, 990 8 of 27
The use of NC as a reinforcement to improve the performance of ECs, as extensively
described in Section 4, requires the optimization of NC concentration, to avoids its ag-
gregation in the film-forming solutions. The optimization of NC, polymer and plasticizer
concentrations through RSM has been previously carried out by investigating their com-
bined effect on the mechanical properties of the resulting coating (Young modulus, ten-
sile strength at break and strain analysis at break) [89].
3. Classification and Properties of Nanocellulose
Cellulose, which is the most abundant carbohydrate polymer on earth, is character-
ized by noteworthy structure and properties. This renewable natural biopolymer, to-
gether with the materials deriving from it, has attracted considerable interest, especially
for application in environmentally friendly and biocompatible products and in foods
[90,91]. Cellulose exhibits a unique molecular structure, consisting of a linear homopol-
ysaccharide composed of glucose monomers, linked together by β-1-4-glycosidic bonds,
which confers unique properties, such as hydrophilicity, chirality, degradability, and
broad chemical variability initiated by the high donor reactivity of the OH groups.
Moreover, cellulose isolation and modification, especially through advanced nanotech-
nology tools, enabled further promotion of its techno-functional attributes [92]. Owing to
their hierarchical order in a supramolecular structure and organization given by the hy-
drogen bond network between hydroxyl groups, nanoparticles can be efficiently isolated
from cellulose [93] via mechanical and chemical methods, or through their combination.
The various types of cellulose nanoparticle (also known as nanocellulose, NC) can be
classified based on their shape, dimension, function, and preparation method, which in
turn primarily depend on the cellulose origin, the isolation and processing conditions as
well as the eventual pre- or post-treatment [94,95]. The physicochemical characteristics of
cellulose at the nanoscale, such as high specific surface area and aspect ratio, high crys-
tallinity, purity, excellent mechanical properties, and low thermal expansion and density
[95–101], open new prospects for NC use in several fields, including biomedical, envi-
ronmental, and energy applications [102]. The cellulosic materials having at least one
dimension in the nanometer range, based on structure and particle diameters [103], is
usually classified into cellulose nanocrystals (CNC), cellulose nanofibers (CNF) and
bacterial nanocellulose (BNC). CNC and CNF can be extracted through a top-down pro-
cess, whereas BNC is synthesized through a bottom-up approach [104].
3.1. Cellulose Nanocrystals (CNC)
CNC are renewable bio-based nanoparticles with a rod-like shape and at least one
dimension below 100 nm [105]. CNC are usually isolated from crystalline cellulose mi-
crofibrils upon treatment with acid at high temperature [106]. CNC have many attractive
characteristics, such as high mechanical strength, high aspect ratio (having mean diame-
ters of 2–20 nm and lengths of 100–500 nm), lightweight, biodegradability, good bio-
compatibility, and potential for surface chemical modifications [107–109]. These distinc-
tive features make CNC a promising material for numerous applications, especially in
packaging materials, biomedical engineering, food emulsions, biosensors, hydrogel sys-
tems, and water purification [110]. Table 3 reports the most recent advances in the ap-
plication of CNC isolated from different raw materials and with different morphological
and shape characteristics.
Table 3. Recent advances in the production process, physicochemical characteristics and application of CNC extracted
from different sources (literature data for years 2020 and 2021).
Source
Production
Process
Morphology/Shape
Dimensions
Crystallinity
Applications
References
Pine
Acid hydrolysis
Spherical
morphology
50–100 nm
diameter
55%
–
[111]

Coatings 2021, 11, 990 9 of 27
Teak
Rod-like surface
topographies
50–60 nm
diameter
52%
Sugarcane bagasse
Rod-like structure
20–60 nm in
diameter
45%
Eucalyptus pulp
Acid hydrolysis
Rod-like structure
130–250 nm in
length and 15–
30 nm in
diameter
–
Starch based composite
film
[112]
Waste cotton fibers
Ultrasound-assist
ed acid
hydrolysis
Short rod shape
200–500 nm
length and 10–
15 nm diameter
86%
PLLA/PDLA composites
films
[113]
Commercial
microcrystalline
cellulose
Alkali hydrolysis
followed by
ultrasound-assist
ed acid
hydrolysis
Spherical shape
30–60 nm in
diameter
81%
Stabilizer for Pickering
emulsions
[114]
Water hyacinth stem
fiber
Acid hydrolysis
Spherical-like
particles
20–50 nm in
diameter
72%
Reinforcement for
polyvinyl alcohol
(PVA)-gelatin
nanocomposite
[115]
Commercial
microcrystalline
cellulose
Acid hydrolysis
Spherical shape
126–134 nm
length and 3–
11 nm diameter
77%–83%
Pickering emulsion
stabilizers and surface
cleaning agents
[116]
Enteromorpha Ulva
prolifera green
seaweed
Acid hydrolysis
–
–
–
Reinforcement for
chitosan-ulvan hydrogel
[117]
Cellulose-rich cotton
fibers
Alkali hydrolysis
followed by acid
hydrolysis
Bundles of rod-like
particles
60 nm in lenght
89%
Reinforcement for
chitosan-ulvan hydrogel
[118]
Cotton
Ultrasound-assist
ed acid
hydrolysis
Spherical rod-like
shape
50 nm in
diameter
81%
–
[119]
Commercial
cellulose
Acid hydrolysis
Ribbon-like structure
173 ± 6.3 nm in
length and 10 ±
0.4 nm in
diameter
81%
Reinforcement for
waterborne
polyurethanes
[120]
Commercial
cellulose
Acid hydrolysis
Rod-like particles
128 ± 55 nm in
length and 14 ±
4 nm in
diameter
84%
Tunable nanomaterial
for pervaporation
membranes based on a
hydrophobic
poly(styrene)-poly(buta
diene)-poly(styrene)
(SBS) matrix
[121]
Paper powders
Acid hydrolysis
Rod-like particles
100 nm in
length and 7
nm in diameter
65%
Reinforcement for
polyurethane (PU)
nanocomposites for
medical applications
[122]
Sawdust
Ultrasound
pre-treatment
followed by aid
hydrolysis
Dot-like shape
6 nm in
diameter
–
Polyamide thin-film
composite membranes
for enhanced water
recovery
[123]
Jute fibers
Acid hydrolysis
followed by
alkali hydrolysis
Rod-like structure
400–1200 nm
length and 40–
90 nm diameter
–
Reinforcement for
pSiDm hydrogel to treat
waste effluent
[124]
Palm fibre
Acid hydrolysis
Rod-like shapes
–
84%
Potential filling agent
[125]

Coatings 2021, 11, 990 10 of 27
3.2. Cellulose Nanofibers (CNF)
CNF are characterized by very different structures and properties than CNC, thus,
defining different application areas [94]. CNF are composed of stretched bundles (ag-
gregates) of elementary nanofibrils constructed from alternating crystalline and amor-
phous domains. Unlike CNC, the nanofibrils can contain a considerable non-crystalline
fraction, with their crystallinity typically in the range of 50%–65%. CNF have lateral size
of several tens of nanometers and length of few microns and, therefore, the aspect ratio of
CNF is relatively large [7]. CNF have been isolated through different mechanical disin-
tegration methods, such as high-pressure homogenization, ultrasonication, microfluidi-
zation, grinding, cryo-crushing, ball milling, and extrusion [101] or mechanical treatment
in combination with chemical or enzymatic hydrolysis. Owing to their high aspect ratio
and entanglement, cellulose nanofibers have the potential to be used in many different
areas (see Table 4), particularly as strong reinforcement in development of nanocompo-
sites [126,127].
Table 4. Recent advances in the production process, physicochemical characteristics and application of CNF extracted
from different sources (literature data for years 2020 and 2021).
Source
Production Process
Morphology/Shape
Dimensions
Crystallinity
Applications
References
Waste cotton fibers
Ultrasound-assiste
d acid hydrolysis
Fibrous
15–20 nm in width
and 1000–3000 nm
in length
79%
PLLA/PDLA composites
films
[113]
Sugarcane bagasse
(NH4)2HPO4
phosphorylation
and mechanical
high-speed
blending
Fiber bundles
18 ± 9 μm in width
and 458 ± 130 μm
in length
69%
Gel
[128]
Bleached pulp paper
Enzymatic
pre-treatment and
then a
high-pressure
homogenization
step
Fiber bundles
28.1 nm in diameter
and 4.9 µm length
–
Stabilization of the
emulsion of Alkenyl
Succinic Anhydride in
water
[129]
Birch fibers
Microfluidizer
assisted
TEMPO-mediated
oxidation
–
–
–
Reinforcement for
hydrogels
[130]
Recycled
milk-container board
Deep eutectic
solvent treated and
mechanical
grinding
–
2–80 nm in
diameter
–
Filter material for aerosol
filtration
[131]
Rice straw
Alkaline
hydrolysis,
bleaching and
TEMPO-mediated
oxidation
Homogeneous
fibril structure
5–10 μm diameter
and 10–40 nm
width
–
Composite membrane to
increase electrochemical
performance of
supercapacitor
[132]
Wood pulp sheets
(NH4)2HPO4
phosphorylation
and mechanical
ultra-fine grinder
Soft fiber structure
10–20 μm in
diameter
–
Cellulose-based film for
flame-retardant packaging
materials
[133]
Bamboo pulp sheets
Low lignin-containing
bamboo pulp sheets
Bamboo powder
Rod-like structure
Commercial
microcrystalline
cellulose
Ultrasonic
treatment following
sulfuric acid
hydrolysis
Beads-on-a-string
cellulose nanofibril
10–30 μm width
and 40–50 μm
length
77%
Gelatin composite
hydrogels
[127]
Licorice residues
Alkali and
enzymatic
hydrolysis
followed by
high-pressure
homogenization
Nanofiber structure
130 nm in diameter
and 8 µm in lenght
–
Nanocomposite film
[134]

Coatings 2021, 11, 990 11 of 27
Commercial chitosan
powder
High-pressure
homogenization
assisted
TEMPO-mediated
oxidation
204 nm in diameter
and 13 µm in
lenght
Maize stalk waste
residues
Mechanical
grinding assisted
chemical
treatments
Highly entangled
fibres network and
web like structure
35.48 ± 12.60 nm in
diameter
71%
Reinforcement material for
biopolymer films for food
packaging applications
[135]
3.3. Bacterial Nanocellulose (BNC)
BNC has the same chemical structure as plant cellulose, i.e., is a linear hompolymer
of repeating subunits β(1,4)-D-glucose with the molecular formula (C6H10O5)n. Compared
to plant cellulose, BNC is chemically pure since it is free from hemicellulose, pectin and
lignin. The synthesis of BNC occurs via cellulose synthase enzyme at cytoplasmic mem-
brane level by several microbial genera belonging to Acetobacter, Achromobacter, Bacillus,
Sarcina, Aerobacter, Agrobacterium, Escherichia, Azotobacter, Rhizobium, Enterobacter,
Klebsiella, Salmonella [136–138]. Due to the standardized high molecular structure and
inherent nanostructure, BNC possesses multifunctionality and good mechanical proper-
ties [139]. It is generally characterized by good hydrophilicity, high water-holding ca-
pacity, slow water release rate, high degree of crystallinity, and ultrafine fiber network
[102,140,141]. The properties of BNC depend not only on its species of origin but also on
the used substrate, cultivation mode and cultural parameters. In addition to its multiple
unique features, BNC also belongs to the category of generally recognized as safe (GRAS)
products, and, therefore, it is widely used in food industry, biomedical, and pharmaceu-
tical, as summarized in Table 5.
Table 5. Recent advances in the production process, physicochemical characteristics and application of CNF extracted
from different bacterial sources and substrates (literature data for years 2020 and 2021).
Source
Production
Process
Morphology/Shape
Dimensions
Crystallinity
Applications
References
Bacterial
cellulose
pellicles
Acid hydrolysis
and ultrasonic
treatment
Rod or
needle-shaped
nanocrystals
15–56 nm in width
and 259–1142 nm
in length
83%
Nisin-loaded BCNs as
antimicrobial agents in
active food packaging
[140]
Pellicle-shaped
bacterial
cellulose
Mechanically
defibrillation and
acid hydrolysis
Rod-type crystal
morphology
20–30 nm in
diameter
-
Reinforcement for sericin
film
[141]
Bacterial
cellulose
2,2,6,6-tetramethyl
piperidine-nitroge
n-oxide (TEMPO)
oxidation
Fibrils bundles
70–100 nm in
width
-
O/W Pickering emulsion
stabilizer
[142]
Bacterial
cellulose
pellicles from
organic waste
and kombucha
Fermentation
using glycerol as
carbon source
3D structure of
cellulose fibrils
100–2000 nm in
length and 5 nm in
width
64%–80%
Composites
[143]
Bacterial
cellulose
2,2,6,6-tetramethyl
piperidine-1-oxyl
radical (TEMPO)
oxidation
Nanofibrils
5–10 nm in width
-
Pickering emulsion
system stabilizer
[144]
Bacterial
cellulose
pellicles from
grape pomace
Fermentation
using carbon and
nitrogen source
Ribbon-shaped
cellulose nanofibers
and nanofiber
aggregates
18–57 nm in width
and micrometers in
length
68%–85%
Nanoadditives for oil
well cement cement
[145]
Bacterial
cellulose
High-pressure
homogenization
Nanofibrils
97 nm in width
and 6 nm in height
-
Pickering emulsion
stabilizer
[146]

Coatings 2021, 11, 990 12 of 27
treatment
SCOBY, black
tea
Fermentation
Nanofibers
20–100 nm in
diameter
73%–79%
Reinforcement for
chitosan
nano-biocomposite films
[147]
Bacterial
cellulose
Alkaline
treatment
Tangled fibers
50.73–140.25 nm in
diameter
84%–88%
Small-caliber vascular
grafts
[148]
Bacterial
cellulose
Fermentation in
static culture
Ribbon-shaped
fibrils
70–80 nm in width
-
Reinforcement for film
with carbon dots
[149]
4. Characterization of Nanocellulose (NC)-Reinforced Coatings
This section provides a brief review of the analytical measurements that are rou-
tinely used to assess the successful coating deposition, as well as its reproducibility, for
meeting the required specification for industrial applications.
4.1. Physical-Chemical Properties
4.1.1. Thickness Determination
The coatings thickness represents an important factor when selecting or optimizing
a deposition process for a particular application [150]. In addition to determining the
acceptability of the coating process, it affects also the coating functionality, particularly
permeability to water and gases [151].
The coating thickness is a function of the coating solution properties, such as poly-
mer concentration (see Table 6), density, viscosity, and surface tension, as well as of the
operating parameters of deposition, such as, for example, the surface withdrawal speed
for dipping deposition [152]. It can be determined by peeling the coating from the surface
of the coated product and proceeding to the direct measurement of the film thickness
using a micrometer screw gauge, simply known also as a micrometer. When peeling is
difficult, for example in the case of the very thin coating layers obtained by the LbL
method, in situ techniques can be applied, such as confocal Raman microspectrometry
(CRM), surface-enhanced Raman scattering (SERS), and Fourier transform (FT)-Raman
spectrometry [153,154].
Table 6. Typical values of tensile strength and percent elongation at break of commonly used edi-
ble coatings and films, as a function of the film-forming material, its concentration, and resulting
thickness.
Film-Forming
Material
Concentration
(% w/w)
Thickness
(µm)
Mechanical Properties
References
Tensile Strength
(MPa)
Elongation at Break
(%)
Agar
1–3
31.2–70.2
14.3–37.4
12.4–31.8
[155]
Starch
5
200
1.41–8.03
12.97–56.25
[156]
Alginate
1.5
26.2–38.9
44–52
12.1–16.4
[157]
Cellulose
5
500
25
7
[158]
Chitosan
1.5
14.4–16.2
47.8–58.2
27.7–36.1
[159]
Carrageenan
2.5
51.6–64.8
40
20
[160]
Gums
10
-
3.5
60–80
[161]
Pectin
3
36
42–82
12–28
[162]
Proteins
-
-
3.3–3.9
160-213
[163]
In general, the addition of nanocellulose into nanocomposite coatings results in a
slight increase in thickness, mainly related to the higher solid content in the coating so-
lutions and the interruption of the original polymeric structure by NC, as extensively
shown in Table 7. Therefore, the effect of NC incorporation on coating thickness can be
correlated well with the concentration of NC in the formulation [36–38,42,164].

Coatings 2021, 11, 990 13 of 27
Table 7. Effect of nanocellulose (NC) on thickness and mechanical properties of edible coatings and films.
Film-Forming
Material
Cellulose
Thickness
(µm)
Mechanical Properties
References
Type
Concentration
(% w/w)
Tensile Strength
(MPa)
Elongation at Break
(%)
Chitosan
CNF
1.5
14.5–21.2
-
-
[165]
Tapioca, potato, corn
CNF
0
2.99
0.047
6.67
[166]
10
6.33
0.055
22.67
20
5.71
0.056
30.51
Faba bean protein
isolate
CNC
0
-
4.3
105.0
[167]
1
4.2
61.3
3
3.8
48.1
5
5.3
48.2
7
6.5
46.3
Cassava starch
Microcrystalline
cellulose
0
-
7.15 ± 0.6
22.75 ± 2.34
[168]
0.14
8.19 ± 0.9
19.23 ± 2.25
0.3
9.91 ± 0.7
5.85 ± 1.43
0.6
10.99 ± 0.5
1.31 ± 0.25
Okara soluble dietary
fiber and pectin
Sodium
carboxymethyl
cellulose
0.5
123 ± 70
6.567 ± 0.33
16.67 ± 0.35
[169]
Konjac glucomannan
BNC
0
39 ± 6
46.43
6.34
[170]
1
40 ± 12
50.36
8.58
2
41 ± 0
69.29
9.44
3
41 ± 15
74.05
8.18
4
42 ± 10
82.01
5.70
Cassia-gum
Carboxylated
CNC
0
89 ± 5
18.53
28.87
[36]
2
90 ± 3
24.77
31.88
4
93 ± 2
32.85
34.75
6
98 ± 4
28.75
36.51
Polyvinyl alcohol
NC
1
-
6.42 ± 0.59
89.99 ± 11.77
[171]
3
9.47 ± 1.62
106.94 ± 7.04
5
11.17 ± 1.08
117.52 ± 10.28
κ-carrageenan
CNC
0
20
38.33 ± 3.79
21.50 ± 3.72
[172]
1
30
38.43 ± 5.94
22.93 ± 1.50
3
40
39.83 ± 0.38
23.83 ± 2.71
5
25
40.07 ± 2.80
24.33 ± 3.00
7
25
52.73 ± 0.70
28.27 ± 2.39
9
35
39.10 ± 1.04
25.83 ± 2.61
k-CA biopolymer
CNC
0
80
49.0
27.5
[40]
1
59.2
23.1
3
66.6
20.7
5
80.9
18.9
8
85.1
15.4
Whey protein
CNC
0
-
1.30
47
[41]
1
1.65
35
2
2.04
33
3
2.10
34
4
2.29
35
5
2.30
35
10
2.70
25
15
3.15
24
Corn nanostarch
CNC
0
300
3.41 ± 0.17
-
[39]
0.2
5.99 ± 0.30
0.4
7.28 ± 0.36

Coatings 2021, 11, 990 14 of 27
0.6
8.61 ± 0.43
0.8
11.25 ± 0.56
1
7.78 ± 0.39
Agar
BNC
0
-
22.10 ± 0.64
10.76 ± 2.30
[173]
0.045
27.95 ± 1.42
14.50 ± 0.88
0.075
31.26 ± 2.26
27.47 ± 1.08
0.12
34.20 ± 1.35
21.53 ± 1.62
0.15
44.51 ± 1.86
13.02 ± 1.70
Whey protein
CNC
0
-
2.30 ± 0.35
46.07 ± 23.25
[174]
2
3.41 ± 0.87
20.82 ± 9.85
5
3.49 ± 0.91
26.54 ± 9.12
8
4.93 ± 0.49
17.63 ± 3.93
Chitosan
BNC
0
90
21.07 ± 1.64
33.84 ± 2.51
[37]
2
100
27.03 ± 1.46
29.71 ± 2.15
4
100
41.32 ± 2.20
23.76 ± 1.52
6
110
34.75 ± 1.02
25.11 ± 2.93
4.1.2. Mechanical Properties
ECs must resist breakage and abrasion during food handling. Moreover, they must
also exhibit adequate flexibility to adapt to possible food deformation during storage
without breaking, while still protecting the food. The mechanical properties of films and
coatings are generally characterized through two main parameters, tensile strength, and
percent elongation at break, determined as specified in the standards of the American
Society for Testing and Materials (ASTM). The mechanical properties of ECs depend on the
type of film-forming material, its concentration, and production technique [175] (Table 6).
Moreover, different studies demonstrated that the incorporation of NC in the
film-forming material enhanced the coating mechanical properties (Table 7), by altering
the internal structure and intensifying the interaction forces [176,177].
4.1.3. Surface Wettability
The effectiveness of ECs for food protection depends on the uniformity of wetting
and spreading on the surface of the fresh produce and, after drying, depends on their
adhesion, cohesion, and durability [178]. The effective spreading of a coating solution on
the surface of food depends on the wettability of the coating solutions on the food surface
and can be correlated with the resulting coating thickness and consequent biological
properties and shelf life of the coated product.
When a drop of liquid is placed on a solid surface, the liquid is subjected to the
balance between adhesive and cohesive forces, where adhesive forces cause the liquid to
spread over the solid surface, while cohesive forces cause it to shrink [32]. The wetting of
the liquid on the solid surface can be evaluated through contact angle measurements,
which assess the mechanical equilibrium of the drop under the action of three interfacial
tension forces, at solid–vapor, solid–liquid, and liquid–vapor interfaces, according to the
equilibrium relation known as Young’s equation [179]. The ideal case of a contact angle
value equal to 0° corresponds to a hydrophilic solid surface where total wetting condi-
tions can be attained by an aqueous solution. A contact angle value comprising between
0° and 180° suggests the occurrence of partial wetting, which is higher for the contact
angle below 90°. The ideal case of a contact angle equal to 180° corresponds to a hydro-
phobic solid surface, where no wetting conditions occur when in contact with an aqueous
medium. The contact angle can be measured directly on the food surface through the
sessile drop method [180,181] or atomic force microscopy (AFM) [182].

Coatings 2021, 11, 990 15 of 27
4.1.4. Barrier Properties
The efficiency of ECs strongly depends also on their barrier properties to the per-
meation of gas, water vapor, aroma, and oil. The barrier properties can be considered as a
function of the chemical composition and structure of the coating-forming polymers, the
characteristics of the product, and the storage conditions [16]. The mass transport prop-
erties through the coating can be described by three principal mechanisms [183]:
1. Diffusion. It is the rate of movement of a permeant molecule through the tangled
polymer matrix, based, for example, on the size of the permeant molecule and the
structure of the polymer matrix. Molecular diffusion through a film generally obeys
Fick’s first law in one dimension, as described by Equation (2):
   
  

(2)
where J is the molecular diffusion of the permeant molecule, D is its diffusion coef-
ficient and C its concentration, l is the thickness of the edible film, and subscripts 1
and 2 refer to the internal and external sides of the coating.
2. Solubility. This is the partitioning behavior of a permeant molecule between the
surface of the polymer and the surrounding headspace. The solubility coefficient C
can be defined by Henry’s law, as shown in Equation (3):
    
(3)
where S is the solubility coefficient of the permeant molecule, and P is the envi-
ronmental pressure.
3. Permeability. This is the rate of transport of a permeant molecule through the pol-
ymeric layer as a result of the combined effects of diffusion (D) and solubility (S).
Therefore, the permeability coefficient (Π), which characterizes the intrinsic per-
meability of the edible film, can be described as shown in Equation (4):
    
(4)
With the assumptions that the diffusion occurs in a steady-state, and the diffusivity
coefficient is constant, the molecular flux (J) can be expressed through Equation (5):
   


(5)
where Δm is the amount of vapor or gases diffusing through a film of area (A), during a
finite time (Δt). The application of Henry’s law allows expression of the driving force in
terms of partial pressure (ΔP). Rearrangement of terms yields the following equation in
terms of permeability:
   


(6)
Then, the permeabilities of O2, CO2, and water vapor can be calculated by the fol-
lowing equation [184].
  
 

(7)
Factors affecting a polymer’s structure have a direct effect on segmental mobility
and, therefore, influence its mass transport properties [183]. Several polymer properties
influence permeability: chemical structure, method of polymer preparation, polymer
processing conditions, free volume, crystallinity, polarity, tacticity, cross-linking and
grafting, orientation, presence of additives, and use of polymer blends [185].
The incorporation of NC in coating solutions is generally reported to significantly
affect the barrier properties of the films. In some polymeric matrices, the transmission
rate of water vapor was reported to increase with NC addition, because of (i) the increase

Coatings 2021, 11, 990 16 of 27
of hydrophilicity within the polymer [38]; (ii) the change in polymer adsorption since the
crystallinity, internal structure and interaction forces are changed [42]; (iii) the higher
concentration of NC which causes its agglomeration in the film matrix [36]. However, in
other cases, the water–vapor barrier properties were reported to increase, because of the
increased surface–volume ratio and compactness of film network [37], due to the for-
mation of a network of hydrogen bridges between NC and the polymeric matrix, which
resulted in a winding path for the water molecules, hindering their propagation through
films [36,39]. The effect of NC incorporation in film-forming solution on the barrier
properties can, therefore, be related to the chemical nature of NC (chemical structure,
polarity, degree of crystallinity) and its concentration, as well as the hydrophilicity and
hydrophobicity of the film matrix.
4.1.5. Optical Properties
The appearance of the coated food products affects consumers’ acceptance. There-
fore, coating optical properties, such as color, gloss, and transparency, also need to be
optimized. The parameters that mainly affect the optical properties of the coating layer
can be reported in terms of its internal and surface microstructure. The intensity of light
reflected by the coated food can be determined in terms of the light directly reflected at
the interface between air and the coated food surface (specular reflection), and by the
light re-emitted out of the surface in all the directions after penetrating the coating of the
food and scattering internally (indirect reflection) [16].
The transparency of ECs depends on their internal structure, which is affected by the
film-forming compositions and concentrations, particle size distribution and rearrange-
ment during drying, due to destabilization phenomena such as creaming, aggregation
and/or coalescence [186]. The incorporation of NC as reinforcement material commonly
decreases the transparency of coatings, hence, causing higher opacity than control films
[41]. This is due to the strong interaction between NC and the polymeric matrix, as well
as to the light dispersion effect from added NC [187]. The transparency of films can be
measured through the Kubelka–Munk theoretical model [188]. This theory models the
reflected and transmitted spectrum of a colored layer based on a material-dependent
scattering and absorption function, with the following assumptions [189]:
1. A translucent colorant layer on the top of an opaque background;
2. Within the colorant layer, both absorption and scattering occur;
3. The light within the colorant layer is completely diffuse.
The gloss of films is affected by their microstructure and depends in particular on
the type and concentration of surfactant, diameter and particle size distribution of the
dispersed phase, relative humidity, storage time and surface roughness [186,190–192].
Nevertheless, other factors such as the angle of incident light or the intrinsic properties
(refractive index) of the material also affect the film gloss [193].
The color can be evaluated through a colorimeter or a spectrophotometer. The pa-
rameters denoting luminosity (L*), red-green hue (a*) and yellow-blue hue (b*) are the
edible film color values in the CIElab color space. The main color parameters used in
evaluating the optical properties of the films [194] are reported in the following:
• Color difference ΔE (Equation (8)):
  
(8)
• Chrome C (Equation (9)):
  󰇛󰇜󰇛󰇜
(9)
• Hue angle H (Equation (10)):

Coatings 2021, 11, 990 17 of 27
   

(10)
• Whiteness index WI (Equation (11)):
 󰇛  󰇜󰇛󰇜󰇛󰇜
(11)
where ΔL, Δa, and Δb are the differences of L*, a*, and b* with the standard color of a
white disk (L*0, a*0, and b*0, respectively).
4.1.6. Microstructure
Properties of ECs depend on several factors, such as the ratio of crystalline to
amorphous zones, polymeric chain mobility, and specific interactions between functional
groups of polymers and the permeant substance within amorphous zones. Common
techniques used to elucidate the coating microstructure include scanning electron mi-
croscopy (SEM), Fourier transform infrared spectroscopy (FTIR), X-ray diffraction, dif-
ferential scanning calorimetry (DSC), thermo-mechanical analysis (TMA), and dynamic
mechanical analysis (DMA) [195]. SEM may be useful to evaluate film homogeneity,
layer structure, the morphology of pores and cracks, surface smoothness and thickness
[196]. FTIR may be used to evaluate the extent of interactions between the different film
components [197]. X-ray diffraction may provide an estimate of the amor-
phous-crystalline structure of film polymers and to track recrystallization during storage
[198]. The evolution of the crystalline structure in the coating matrix during storage can
be evaluated by DSC. Both DSC and TMA techniques are commonly used to estimate the
glass transition temperature, which is strongly dependent on both the film composition
and moisture content and, therefore, can be correlated with the stability of a polymeric
film [199].
Generally, films containing NC as a reinforcement additive presented characteristics
of being homogenous, continuous, having a smooth surface without pores or granules
and bubble-free, indicating good NC dispersion in the polymeric matrix. However, when
the concentration of NC was increased, the roughness of the cross-section of the films
also increased [172,187,200–203]. This can be ascribed to the aggregation of nanoparticles
due to their high hydroxyl content [200,204], to the hydrogen bonds and electrostatic in-
teractions between NC and polymers, tightening the network, resulting in smaller pores,
and making the materials less homogeneous and more opaque [202]. The microstructure
change is reflected by the reduction of barrier properties, since the formation of paths
may facilitate the passage of water vapor, as previously reported. The compatibility of
the materials was attributed to factors such as (i) chemical similarities between starch and
cellulose, (ii) interaction of hydrogen bonds between NC and the matrix and (iii) effect of
NC nanometric size [187].
4.2. Antimicrobial Properties and Shelf-Life Extension
ECs are usually applied on highly perishable products, such as fresh and fresh-cut
fruits and vegetables, to extend the shelf life and to preserve their quality and minimize
losses through controlling physiological, biochemical or oxidation processes. To enhance
their efficiency and functionality, ECs can be loaded with different bioactive compounds
(as illustrated in Figure 2) to develop specific functionalities, such as antimicrobial, an-
ti-browning, antioxidant, coloring, and flavoring, or even nutritive actions [205].
The addition of antimicrobial agents to the coating solution is reported to develop a
synergistic action with the physical barrier of the coating. Moreover, the incorporation in
the coating layer might also enable the controlled release of the antimicrobial molecules
on the food surface [16,206–208], contributing to improving the shelf-life of the product,
by inhibiting the growth of bacterial and fungal cells over an extended time. In contrast,
the direct use of antimicrobials in the food is reported to cause immediate microbial in-

Coatings 2021, 11, 990 18 of 27
hibition, which is frequently followed by the recovery of injured cells [209]. Nowadays,
natural antimicrobial compounds represent a valid alternative to chemical preservative
agents, such as benzoic acid, propionic acid, sodium benzoate, sorbic acid, and potassium
sorbate [16], for preserving food quality, because they can be effective against both food
spoilage and foodborne pathogens [210], without constituting health concerns. The use of
natural antimicrobials as preservative agents has, therefore, attracted increasing interest
among consumers looking for clean food labels and more natural products. Several an-
timicrobial agents are present in nature, where they are produced mainly as secondary
metabolites in microorganisms, plants, and animals, as defense mechanisms against ex-
ogenous threats. The incorporation of such natural compounds into edible coatings ena-
bles the development of active coatings, which combine physical protection of the food
product (barrier effect) with significant antimicrobial activity.
Another class of compounds of interest for incorporation in ECs is represented by
antioxidants. They are used to enhance the protection of fresh products and to increase
their shelf-life as substances used to preserve food by retarding deterioration, rancidity,
or discoloration due to oxidation caused by free radicals [211]. In this case, the synergy
between the gas-barrier properties of the coating and the antioxidant activity is the key to
a successful decrease in oxidation processes in coated foods [212].
ECs can be loaded also with anti-browning agents, contributing to reducing the ex-
tent of enzymatic and non-enzymatic oxidation of phenolic compounds during the
shelf-life of fresh produce [213–215]. Anti-browning agents can be incorporated in
cross-linking solutions and applied after the adhesion of the edible coating solution on
the surface of fresh produce [216], for preservation during the entire storage period of
food color, which is a critical quality parameter.
The organoleptic properties of the coated products can also be improved if the
coating is loaded with flavoring or coloring agents, as well as with sweeteners, spices,
and seasonings [5,18,178], which are also reported to provide health benefits.
Figure 2. Natural compounds frequently used in edible active coatings.
Examples of NC addition in active systems are mainly reported in active films for
food packaging, where the role of NC is of stabilization and physical entrapping of the
active species. As shown in Table 8, the main effects of NC addition to films are related to
(i) ensuring high loading of the antimicrobial agents [217] because of the intrinsic high
surface area of NC, (ii) improving the controlled release characteristics of the bioactive
agents loaded in the biopolymer matrix, by affecting their permeation rate [218], and,

Coatings 2021, 11, 990 19 of 27
therefore, (iii) increasing the antioxidant properties of the film [219], when the payload
bioactives are antimicrobial agents [7].
Table 8. Recent advances in the antioxidant and antimicrobial properties of films reinforced with nanocellulose (literature
data for years 2018–2021).
Film-Forming Material
Additives
Effect of NC on Active Film
References
Reinforcing Agent
Active Agent
Sodium caseinate
(4% w/w)
Cellulose nanofibers
(2.5%–5% w/w)
Cinnamon bark essen-
tial oil-nanoemulsion
(5% w/w)
NC decreases the release rate of the
essential oil from sodium caseinate
matrix and also improves the anti-
oxidant properties of the film.
[218]
Soy protein
(5% w/v)
Microfibrillated cellu-
lose
(0%–0.6% w/v)
Clove essential oil
(2.5% w/v)
MFC’s presence favors the release of
the active compounds of CEO. A
higher concentration of MFC in-
creases the antioxidant properties as
well as the antimicrobial activity.
[219]
Mucilage
(50% v/v)
Cellulose nanofibers
(3%–6% w/v)
-
NCs incorporation successfully en-
hances the mechanical, hydropho-
bic, antioxidant and antimicrobial
properties of the mucilage compo-
site films.
[217]
Gelatin/agar
(2% w/v)
Cellulose nanofibers
(0.75% w/v)
Clove essential
oil-based Pickering
emulsion
(0, 0.02, 0.1, 0.2% w/v)
Composite film is transparent and
shows high UV-light barrier proper-
ties and water-resistant properties,
and improved antioxidant activity.
[220]
Poly (butylene
adipate-co-terephthalate)
(PBAT)
(15% w/w)
Cellulose nanofibers
(0.5, 1, 3% w/w)
Cinnamon essential oil
Films showed good thermal stabil-
ity, higher oil release, decreasing
water vapor permeability values and
preventing microbial attack through
the release of the essential oil.
[221]
5. Conclusions
This review summarized the recent advancements about the incorporation of
nanocellulose (NC) in a polymeric matrix to form edible coatings (ECs). Unlike NC used
alone, which forms a coating with poor resistance to water vapor, the reinforcement of
conventional coatings through the NC addition in the coating formulation is reported to
significantly improve the ECs’ properties. Remarkably, it was shown, through the critical
analysis of the literature, how the properties of nanocomposite coatings, based on NC
reinforcement, change depending on which type of NC (cellulose nanocrystals—CNC, or
cellulose nanofibrils—CNF) and concentration are used. Therefore, the ECs’ structural
and chemical properties can be tailored through formulation, in combination with the
selection of the optimal coating deposition technique.
Most of the studies to date have focused on the incorporation of CNC in coating
solutions, because their high-crystallinity structure may increase the mechanical re-
sistance (e.g., the tensile strength), as well as the barrier performance of the coatings.
However, CNC are reported to condense in the film-forming solutions, when their con-
centration is excessively high, with a consequent increase in water vapor permeability
and a decrease in elongation at break value in the resulting coatings. In contrast, the use
of CNF was reported to confer a more flexible structure to the coatings, due to their in-
dividual or aggregated softer and longer chains than CNC. The conflicting results re-
ported, to date, about the effect of CNF on the properties of nanocomposite coatings,
CNF has not been widely studied as a reinforcing agent. Bacterial nanocellulose (BNC)
has recently emerged as a potential additive in ECs, because of its purity, which is re-
ported to contribute to high tensile strength and mechanical flexibility to the coatings.

Coatings 2021, 11, 990 20 of 27
The main limitation to the use of BNC currently resides in the production process of
BNC-based composites, which needs to follow a bottom-up approach, with the need for
bacterial growth for BNC production to take place in the presence of a matrix biopoly-
mer. This restricts the possibility of changes in shape after fermentation, as well as gen-
erating high production costs.
Overall, the incorporation of NC in ECs represents a promising approach for im-
proving ECs’ mechanical and barrier properties, stability and eventual controlled release
of active agents, with a potential impact in the preservation of the quality and extension
of the shelf life of perishable fruits and vegetables with all-natural systems.
Author Contributions: Conceptualization, A.P. and F.D.; methodology, A.P.; investigation, A.P.;
resources, G.F.; writing—original draft preparation, A.P.; writing—review and editing, G.F. and
F.D.; supervision, F.D.; funding acquisition, G.F. All authors have read and agreed to the published
version of the manuscript.
Funding: This research was funded by the Italian Ministry of University (MUR) call PRIN 2017
with the project 2017LEPH3M “PANACEA: A technology PlAtform for the sustainable recovery
and advanced use of NAnostructured CEllulose from Agro-food residues”.
Institutional Review Board Statement: Not applicable.
Informed Consent Statement: Not applicable.
Conflicts of Interest: The authors declare no conflict of interest.
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