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Food Reviews International
ISSN: (Print) (Online) Journal homepage: https://www.tandfonline.com/loi/lfri20
A Review on Potential Use of Gelatin-based Film
as Active and Smart Biodegradable Films for Food
Packaging Application
N.S. Said, Nazlin K. Howell & N.M Sarbon
To cite this article: N.S. Said, Nazlin K. Howell & N.M Sarbon (2021): A Review on Potential Use
of Gelatin-based Film as Active and Smart Biodegradable Films for Food Packaging Application,
Food Reviews International, DOI: 10.1080/87559129.2021.1929298
To link to this article: https://doi.org/10.1080/87559129.2021.1929298
Published online: 02 Jun 2021.
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A Review on Potential Use of Gelatin-based Film as Active and
Smart Biodegradable Films for Food Packaging Application
N.S. Said
a
, Nazlin K. Howell
b
, and N.M Sarbon
a
a
School of Food Science and Technology, Universiti Malaysia Terengganu, Kuala Nerus, Malaysia;
b
Department of
Health and Medical Sciences, University of Surrey, Guildford, UK
ABSTRACT
This review discusses the potential use of gelatin-based lm as active and
smart edible lms in creating new innovative food packaging technologies as
a substitution towards utilization of synthetic packaging. Nowadays, gelatin-
based lm has emerged as one of the most extensively studied in food
packaging sector as it exhibits good mechanical and barrier properties
while being environmentally friendly, biodegradable and having low produc-
tion cost. The production of gelatin biodegradable lm has the potential to
help industry in reducing water usage, solid wastage, electricity and emis-
sions. In addition, gelatin-based lm also possessed good matrix and com-
patibility which enable for it to act as a medium for incorporation with
antimicrobial and antioxidant agents into the lm to carry out their specic
functions for enhancing safety, stability, functionality and shelf-life of food
products. This paper detailed out information on active and smart-edible
lms derived from gelatin with expectation to raise and promote awareness
regarding food safety and healthy lifestyle towards the consumer while
maintaining the quality and prolonged the shelf life of food product.
KEYWORDS
Gelatin; biodegradable film;
active packaging; smart
packaging; gelatin-based
film
Introduction
Recently, plastics produced from non-renewable resources such as petrochemicals have been exten-
sively utilized in the packaging sector due to their low material cost and convenience attributes.
[1]
However, there are some major drawbacks as synthetic packaging materials are non-biodegradable
and raise critical environmental concerns among consumers. Furthermore, the insufficient future
supplies of fossil fuels and concerns regarding the depletion of natural resources, have driven efforts to
explore alternative biopolymers to replace synthetic polymers as packaging materials.
[2]
This has
engaged researchers’ interest to develop alternative biodegradable packaging from renewable materials
as a substitute for petroleum-based polymers.
[1]
Thus, this research has increased the global market for
biodegradable polymers, which is expected to rise at an average annual growth rate of 12.6% to
206 million pounds by 2020.
[3]
Biodegradable materials are associated with the use of renewable raw materials, such as proteins
and polysaccharides extracted from agricultural, marine, and animal sources. According to Machado
et al.
[4]
biodegradable polymers are defined as materials that are able to decompose and allow
anaerobic digestion that yield into carbon dioxide, methane, water, inorganic compounds, or biomass
to occur by the open environment and microorganisms. The speed of biodegradation depends on
temperature, humidity, number and type of microbes. In industrial composting bioplastics are
converted into biomass, water and CO
2
in about some days or even months depends on the raw
materials.
[5,6]
Biodegradable films or coatings from renewable natural raw materials, such as
CONTACT N.M Sarbon norizah@umt.edu.my School of Food Science and Technology, Universiti Malaysia Terengganu, Kuala
Nerus 21300, Malaysia
FOOD REVIEWS INTERNATIONAL
https://doi.org/10.1080/87559129.2021.1929298
© 2021 Taylor & Francis
polysaccharides (starch and chitin), lipids (waxes and parafin), proteins (collagen and gelatin) or the
combination of these components have gained interest mainly due to their environmentally friendly
traits and their potential substitution for some petrochemicals in the food packaging industry.
[7–9]
Gelatin is one of the most promising biopolymers that is widely studied owing to its good film-
forming capacity and ability to act as an outer packaging layer in protecting food from exposure to
light, temperature and oxygen.
[10,11]
Gelatin is a protein obtained by hydrolyzing the collagen that can
be found in connective tissues, skin and bones of animals. Gelatin were commonly extracted from
mammalian sources such as pig skin, bovine hide and cattle bones.
[12]
However, a major drawback is
the high hygroscopic nature of gelatin films that makes it swell or dissolve when it is in contact with the
surface of foodstuffs with a high moisture content.
[10]
Several approaches have been investigated to overcome these drawbacks by incorporating other
natural agents or derivatives into gelatin-based films to improve or extend the film’s application. The
incorporation of active agents such as natural extracts and essential oils can help in increasing the
hydrophobic phase of polymer and reduce the film’s tendency toward water uptakes capacity. In
addition, some natural extracts are able to reduce water vapor permeability by forming more compact
structure with polymer matrix derived from internal crosslinking.
[13]
The application of gelatin-based
films can be classified into two main categories namely active and smart edible films. ‘Active packa-
ging’ has been defined as a mode of packaging that is intended to extend shelf-life or to enhance safety
or sensory properties in order to maintain food quality. While, ‘smart packaging’ refers to the
packaging which monitors the condition of packaged foods and provides information about product
quality to manufacturers, retailers or consumers.
[14]
This paper provides an overview on the potential
application of gelatin-based films as active and smart biodegradable film as summarized in Fig. 1.
Gelatin
Gelatin is a protein obtained by hydrolyzing the collagen that can found in bones and skin of animals.
Physical and chemical properties of the gelatin produced are greatly affected by the sources, age of
animal, collagen type, and extraction method used.
[15]
The global gelatin production was 348.9 kilo
Figure 1. Schematic diagram showing the application of active and smart biodegradable films in the food industry.
2N. S. SAID ET AL.
tons in 2011 and is expected to reach 450.7 kilo tons in 2018, growing at a compound annual growth
rate (CAGR) of 3.73% from 2012 to 2018.
[16]
Gelatin can be classified into two types which determined
by gelatin pretreatment during the extraction process. Type A gelatin with an isoionic point of 6–9 is
obtained from acid-treated precursor, whereas type B gelatin with an isoionic point of 5 is derived
from an alkali-treated precursor. The quality of gelatin is determined by gel strength and viscosity
extracted gelatin properties. Gel strength which is also known as ‘bloom’ value, is an indicator of the
strength and stiffness of the gelatin. It reflected the average molecular weight of gelatin constituents
that was commonly between 30 and 300 bloom (<150 for low, 150–220 for medium and 220–300 for
high bloom). Higher bloom value indicates greater gelatin strength. A different bloom value for gelatin
is applied based on the type of product required and its function.
[17]
On the other hand, gel strength, gelatin concentration, pH and temperature are directly affecting
the viscosity of gelatin.
[18]
Gelatin is nearly tasteless, odorless substance which was commonly
produced in a granulated or powdered form. Although it has hygroscopic properties, but its water
absorbing capacity depends on the relative humidity at which it is dried and stored. Extreme pH and
high temperature can denature the gelatin and change its properties by disrupting three-dimensional
structures of gelatin and forming a random coil which resulted in lower viscosity and elastic modulus
of gelatin. Therefore, the processing conditions of gelatin need to be carefully controlled to get a high
gel strength.
[1]
Gelatin has been globally utilized in photographic, cosmetic and pharmaceutical
industries due to its gel-forming properties. In addition, gelatin also has numerous uses in food
industry such as emulsifiers, foaming agents, colloid stabilizers, biodegradable film-forming materials
and microencapsulating agents.
[15]
Gelatin are able to be conceived from varied types of collagen
sources such as from porcine,
[19]
bovine,
[20]
fish,
[21]
poultry,
[8]
insects
[22]
and etc.
In regards to gelatin safety concern associated with TSE (transmissible spongiform encephalo-
pathy) and bovine spongiform encephalopathy (BSE), FDA
[23]
has clarified that gelatin is not
considered a prohibited cattle material if it is manufactured using the customary industry processes
specified. Moreover, European Food Safety Authority
[24]
clarified that BSE risk of bone-derived gelatin
was small and skull and vertebrae from bovine animals older than 12 months should not be used in the
production of gelatin. Meanwhile, the effect of gelatin films towards intended food products have
shown better organoleptic properties of food compared to the untreated ones.
[25,26]
Thus, this has
proved gelatin films can help to minimize the oxidative effects, pro-longing the product shelf-life while
maintaining quality of the intended food product.
Sources of gelatin
Gelatin from mammalian sources
The most abundant gelatin sources are mainly derived from mammals especially bovine and pig which
accounts for around 46% for pig skin, 29.4% for bovine hide and 23.1% for pork and cattle bones.
[15]
Bovine and porcine skin gelatins have become widespread across food manufacturing industry due to
its high availability. Generally, gelatin from bovine skin is known as type B gelatin which produced
from alkaline treatment while porcine skin gelatin is known as type A gelatin which produced from
acidic treatment
[20]
with isoelectric point of pH 4.8–5.5 and pH 7–9.4, respectively.
[27]
Bloom values
from pig skin and bovine skin gelatin were reported in the range of 130–308 g
[28,29]
and
227–350 g
[30,31]
respectively. While the viscosity values from pigskin gelatin was in the range of 6.37
to 7.28 cP
[28]
and viscosity value of bovine skin gelatin was reported at 3.90 cP.
[31]
The amino acid
composition of porcine gelatin was found higher in glycine, proline and arginine contents as
compared to bovine gelatin.
[20]
Mammalian’s gelatin is more popular compared to other sources
due to their superior gel qualities (gel strength and viscosity) and has good film forming properties.
They were, however, have major drawbacks and issue regarding religious concerns and Halal issue as
they cannot be used or consumed by Muslims, Jews, or Hindus on various grounds.
[32]
Moreover, due
to the potential risk in spreading harmful pathogens derived from bovine spongiform encephalopathy
(BSE) which also known as mad cow diseases and food and mouth disease (FMD), alternative gelatin
FOOD REVIEWS INTERNATIONAL 3
sources for porcine and bovine gelatin substitution have been given priority and takes into
consideration.
[33]
Thus, the utilization of alternatives gelatin from various sources has become greatly
advantageous towards food industry plus there were rapidly growing interest on the global market for
halal certified food.
[32]
New gelatin sources such as from marine species like fish,
[12]
giant squid
skin
[34]
and eel skin
[21,35]
poultry sources like chicken skin, feet, and bone
[8,36]
and duck feet
[37]
has
increased in order to replace mammalian resources.
Gelatin from marine sources
Due to problem arise from production of mammalian gelatin, a numerous study of gelatin extraction
from fish or marine species are continually growing and received great interest from many researchers.
A mild acid pre-treatment is commonly used for type-A gelatin extraction from fish skins.
[38]
Fish
gelatin typically has lower bloom value ranging from 0–270 g as compared to the bloom values for
mammalian gelatin, which have bloom values of 130–308 g. Cho et al.
[39]
stated that the gel strength of
yellowfin tuna skin gelatin that was recorded at 426 g bloom resulted in higher value than bovine and
porcine gelatins which were reported at 216 g bloom and 295 g bloom, respectively. Marine gelatins
might exhibit wide range of bloom values due to their differences in proline and hydroxyproline
content in collagens from different types of species and also associated with the habitat’s temperature.
The range of viscosity values (cP) reported for gelatin skin of different freshwater fish species are from
1.87 to 3.63 cP.
[40]
Variation of the viscosity value may be due to different fish species, environment
and extraction method used. Generally, fish gelatins have lower concentrations of imino acids (proline
and hydroxyproline) as compared to mammalian gelatins. A study by Ninan et al.
[30]
revealed that
warm-water fish gelatins such as from bigeye-tuna and tilapia species have a higher imino acid content
as compared to cold-water fish gelatin like from cod, whiting and halibut species. Muyonga et al.
[41]
reported that the proline and hydroxyproline contents for warm-water fish and cold-water fish were
approximately 22–25% and 17%, respectively. Based on a study conducted by Sila et al.
[35]
the amino
acid profile of the gelatin prepared from European eel (Anguilla anguilla) skin resulted high propor-
tion of glycine and imino acid residues. Overall, fish gelatin exhibits good film properties while being
transparent, almost colorless, water soluble and highly extensible films.
[42]
Gelatin from poultry sources
New gelatin sources such as poultry skin, feet, and bone has risen attention as a substitution to
mammalian resources.
[36]
The poultry species used include species from duck, chicken, and turkey.
Avian gelatin was reported to possess amino acids, secondary structure, and molecular weight that
nearly similar with mammalian gelatin.
[37]
Sarbon et al.
[8]
reported that gel strength from chicken
gelatin has significantly higher bloom value (355 g) as compared to bovine gelatin (229 g). Meanwhile,
Nik Aisyah et al.
[43]
reported that duck feet gelatin with various acids treatment had higher bloom
strength ranged between 225.53 and 334.17 g than commercial bovine gelatin that exhibited bloom
value at 216.63 g. High bloom strength contribute to high melting temperature and higher viscosity
due to higher proportion of cross-linked component of ß and α chain. Sarbon
[44]
mentioned that
chicken gelatin exhibits higher viscosity value (150 ml/g) as compared to bovine gelatin (127 ml/g).
Furthermore, it also stated that chicken skin gelatin possessed amino acids such as glycine (33.70%),
proline (13.42%), hydroxyproline (12.13%) and alanine (10.08%) which contributed to the higher gel
strength and stability. In addition, the imino acids (proline and hydroxyproline) value of chicken skin
gelatin was reported higher than bovine gelatin (12.66 and 10.67%, respectively).
[8]
Gelatin from
poultry sources exhibit good film forming properties as they showed high bloom value with high
imino acids content.
[8,43,45]
Film forming properties of gelatin
Gelatin has been widely studied on account of its film-forming ability especially in films production
due to its outstanding filmogenic properties. Furthermore, it can be produced at low prices while
4N. S. SAID ET AL.
yielding its uniqueness properties as outer film to secure food from moisture loss and exposure
towards light and oxygen.
[46]
Gelatin also possessed the ability to form physical gels and formed as
thermo-reversible gels. The gelatin gel formation is obtained from structural re-arrangement of
protein formed by breaking the triple helix structures of collagen into single-strand molecules.
[47]
Gelatin-based film are highly affected by its rheological properties which depends on viscoelasticity,
viscosity and processing temperature of the film. During preparation of gelatin films, gelatin might
undergo different transitions according to the formation of gelatin and solution.
[48]
However, the physical properties of gelatin films are depending on the characterization of raw
materials and the extraction method used which derived from the different processing conditions and
animal species. Furthermore, gelatin properties are also affected by physical parameters that involved
in film processing such as the addition of substances or ingredients into the film processing method
such as the inclusion of plasticizers,
[49]
polymers
[50,51]
and cross-linkers.
[10]
The formation of edible
films can be executed through casting or extrusion process. Commonly, casting method have been
extensively reported in film formation process. This method involved the dissolving of biopolymer
which then being incorporated with either plasticizers or additives to obtain a film-forming solution.
Later, the film-forming solution will be cast onto plates and the solution will be dried off.
[52]
The
advantages in using casting method for film preparation over other processes includes mold simplicity,
lower production cost, greater film thickness uniformity, excellent dimensional stability and free from
pinholes, gels and residual stress. It also offers superior optical characteristics with a high degree of
flatness and dimensional stability over flat-sheet extrusion.
[53]
Casted film also resulted in isotropic
orientation (mechanical and optical) as film is not stretched during manufacture and absence of
typical extrusion process lubricants. In addition, due to the usage of low heat which is inherent in the
solvent casting processing, it provides an extended service life to the film. Despite the advantages,
casting film however are not feasible to be use in commercial scale as it requires long drying time, has
restriction in shape formation and amount of film created and involve many variables such as drying
temperature and relative humidity level.
[53]
Moreover, the polymer must be soluble in a volatile
solvent or water which could raise concern on the usage of toxic solvents in certain film production
industry.
Since this casting technique is feasible only on a laboratory scale, thus, alternative processes have to
be used on a large-scale production such as through extrusion process. Extrusion is a mechanical
process where polymers in the form of granules, pellets powders, beads, flakes, pellets, or combinations
of these forms is fed into an extruder and melted.
[54]
The material flow process starts by feeding the
mass through the extruder, flow through the die, and exit from the die and subsequent downstream
processing. During material processing, the mass is transformed mainly by the shear force, pressure,
cooling rate, shaping, and residence time. The extrusion channel is commonly divided into three parts
zones: i) the feeding zone, ii) the kneading zone, and iii) the heating zone at the final part/exit from the
machine.
[53,55]
There are several advantages of manufacturing film using the film extrusion process
including short processing time with low energy consumption compared to casting method which is
more efficient to be used in food packaging industry. This method does not involve any solvents to
solubilize the polymer and has wider range of processing conditions with better control of feed
residence time and degree of mixing. Moreover, it has the ability to manipulate the mechanical
properties of the final product based on the process conditions and types of polymer used.
[53]
Extrusion methods able to produce a wide range of forms and manufacture a variety of single or multi-
layer films with a range of film thickness and width that are not offered by the solvent casting method.
The primary drawback of hot-melt extrusion is that it subjects the film ingredients to high tempera-
tures, which could cause thermal degradation, create voids in the film and affect its uniformity,
strength and appearance. Besides, this machinery setup requires high amount of initial and main-
tenance cost.
[53]
Sealing ability is one of the most important characteristics for the polymerization of films to extend
their applications in food packaging sector to be further form as sachets, pouches or bags. Heat sealing
ability is commonly influenced by temperature, pressure, heating and cooling time during the
FOOD REVIEWS INTERNATIONAL 5
process.
[56,57]
Heat sealing properties of polymers also depend on surface chemistry of the materials.
Thermal adhesion of the film occurs around its onset temperature and generally determined by using
differential scanning calorimetry (DSC). Upon heat sealing, gelatin films are pressed together between
two hot metal bars for a sufficient amount of time and immediately cooled to allow for melting of the
polymer at high temperatures.
[57]
The process is achieved when the molecular interaction between
individual layers is enough to destroy the interface, leading to the formation of a new homogenous
layer and facilitate an interfacial interaction between the melted films.
[57,58]
Many studies have shown
that the addition of plasticizer, surfactants, natural extracts and nanoparticles have lowered the film’s
melting point due to the reduction in moisture content and flexibility of the films, thus lowering the
required energy for disruption and required lower seal and hot-tack initiation temperatures compared
to single gelatin films.
[56,59–61]
In recent decades, nanotechnology has become increasingly important as an appealing technology
for the food packaging industry. Nano-applications including nanoencapsulation, nanocomposite,
nanoemulsions, edible nano-coatings and nano-coating materials have recently emerged to be the
potential alternatives to conventional nanoparticle applications in active food packaging.
[62–64,147]
Nanoencapsulation techniques is commonly used for entrapment of essential oils to enhance their
water dispersibility, chemical stability, bioavailability, and bioactivity during processing and help in
improving film’s physicochemical properties.
[62,65]
On the other hand, nanomaterials are often used to
improve the properties of food packaging due to their antimicrobial, UV protection activity, and
possibility of oxidation prevention which are essential for packaging materials. Nanomaterials are
defined as insoluble materials with an inner composition in the range from 1 to 100 nm.
Nanomaterials can be applied by different technologies such through mechanical mixing methods
(high-speed mixer, extruder), electromagnetic fields, microwave, ultrasound, electrospinning and
electrospray methods.
[62–64,66]
In addition, gelatin also has been commonly utilized in biomedical
and pharmaceutical applications due to its favorable biodegradability, biocompatibility, and low
antigenicity. One of the common method is by using nanovesicles technique in which nanovesicles
(active pharmaceutical ingredients, lipids and plasticizer) are encapsulated into gelatin hydrogels.
Gelatin hydrogels can protect them from rapid clearance and can enhance their membrane integrity
and mechanical stability. The hydrogels’ physical, mechanical, and biological properties can be
improved and tuned by nanofunctionalization with nanovesicles to form controlled release composite
hydrogel delivery systems that have been used for many biomedical applications.
[67]
However, up to
present moment, there is still no study conducted on gelatin film packaging using the nanovesicles
technique.
Gelatin-based edible film also known to exhibit transparent and excellent in gas (O
2
and CO
2
)
barrier property. However, they also possessed low water-vapor barrier and mechanical property as
compared to synthetic films.
[68]
Thus, many studies have been conducted in order to improve and
modified the functionality of protein-based film and coating as food packaging which includes the
addition of different substance or agents such as cross linkers, plasticizers and additives with anti-
oxidant and antimicrobial properties. The incorporation of certain additives into packaging systems
that intended to maintain or extend the quality of product or shelf life is referred as active packaging.
Gelatin-based active lms
It is widely published that; gelatin exhibits good film-forming capacity which can be utilized to
improve the shelf-life of food products by acting as an outer film that inhibits direct exposure of
light and oxygen into the food material.
[12]
Accordingly, the application of gelatin-based films can be
extended by incorporating different additives and agents in the film, that help to enhance physical,
mechanical and biological properties of food products. The research on gelatin-based films that
function as active packaging has gained wide interest around the world. Active packaging technologies
involve physical, chemical or biological actions by specific components incorporated inside the
polymer which alter the interactions within a package, products and/or the headspace inside the
6N. S. SAID ET AL.
food packaging.
[69,70]
Further, recent advances in gelatin-based active packaging include the incor-
poration of specific additives that have potential to release active agents that can impart antimicrobial,
antifungal, antioxidant and other biocatalytic activity.
[71–76]
The release rates of active compounds to the intended food product depend on the nature of active
compound (molecular weight, chemical structure, polarity, solubility in the media) and film-forming
material (pore size, packing density and solubility) with food or food simulants, along with external
factor (relative humidity, temperature and time of contact). There are different mechanisms release of
active compounds from biopolymer packaging which in direct contact with food or food simulants
such as melting, diffusion, swelling, degradation, or particle fracture. Melting mechanism involves the
melting of the structural matrix to release the bioactive compounds. Meanwhile, diffusion release is
when active compound diffuses through the micro or macro-porous structure of the polymer matrix
and is transported away from the film surface into the food. On the other hand, swelling release is
applied when active compound has a low coefficient of diffusion which requires the polymer matrix to
be placed in a compatible liquid or wet food and swells when fluid enters its matrix that allows the
diffusion coefficient of the active compound to diffuse out of the film. Lastly, degradation or particle
fracture release involves degradation, cleavage or deformation of polymer.
[77,78]
The diffusion rate of
small molecules in the polymers can be investigate by using molecular dynamics simulation (MDS),
a computer simulation technique based on classical molecular mechanics, which artificially simulates
the diffusion process.
Antimicrobial agents in gelatin-based active lm
Food spoilage is commonly caused by microbial contamination that leads to metabolic processes
which cause deterioration of foods resulting in inedible, undesirable or unacceptable products that are
unfit for human consumption due to changes in sensory and safety characteristics.
[79]
Therefore, an
antimicrobial packaging system is a promising method which incorporates the antimicrobial agent
into a polymer film in order to restrain the growth and activities of targeted microorganisms which
otherwise cause contamination to the foods; the polymer film has antimicrobial properties in addition
to its main goal to prolong the shelf life and freshness of food products.
[10,80]
The antimicrobial packaging systems involve several methods such as the addition of a sachet or
pad into the package in which the volatile bioactive substance is emitted during further storage. The
other method is by direct diffusion of bioactive agents into the packaging or diffusion of coating
bioactive agents on the surface of the packaging material that acts as a carrier system for antimicrobial
agents. The antimicrobial packaging systems can also be implemented by utilizing antimicrobial
macromolecules with film-forming properties or edible matrices.
[81]
However, due to diverse physiol-
ogies, antimicrobial agents have different mechanism of action on different types of pathogenic
microorganisms.
[82]
Currently, a wide variety of antimicrobial agents incorporated into gelatin-
based film have been proposed and studied such as essential oils,
[71]
metal ions,
[72]
polymer,
[73]
natural
extracts,
[83]
organic acids,
[84]
bacteriocins
[85]
and enzymes.
[86]
Essential oils as antimicrobial agents in gelatin-based active film
Essentials oils are one of the natural compounds that have been extensively used in gelatin-based
biodegradable film. For instance, the addition of essential oils into the biodegradable film for
antimicrobial purpose include those extracted from clove,
[71]
oregano,
[87]
ziziphora
clinopodioides,
[25]
peppermint and citronella,
[88]
cinnamon,
[89]
bergamot and lemongrass,
[90]
rosemary
[91]
and garlic.
[92]
In addition, α-phellandrene, limonene, d-limonene, carvacrol, thymol, α/β-
thujone, camphor, carvone, menthol and menthone are among the major compounds that are present
at relatively higher concentrations in essential oils.
[93]
However, direct incorporation of these essential
oils may result in non-uniform film-forming solution and exhibit some phase separation. Hence, the
preparation of the film-forming dispersions requires the emulsification of the oil phase in the aqueous
phases prior to the casting for film formation.
[94]
The mixtures of two immiscible liquids; essential oils
FOOD REVIEWS INTERNATIONAL 7
and water are commonly stabilized by using emulsifier such as Tween-20 and Tween-80. These
emulsifiers were used to break lipid molecules into smaller droplet particles and form an emulsion,
which caused the films solution to look very smooth and homogenized.
[95]
The film-forming solution
can be further homogenized using rotor-stator and high-pressure homogenizers to reduce destabiliza-
tion phenomena (flocculation, coalescence, creaming) during the drying step of the film.
[94]
The other
possible concern is the significant losses of volatile compounds which could occur during the drying
stage of the film. The approach that can be made is by using micro- and nanoencapsulation technique
which could be a promising alternative to increase the stability and improve the effectiveness of films
enriched with essential oils.
Several studies on the mechanism of essential oils as antimicrobial agents that are incorporated into
gelatin-based film, derived from marines’ sources have been reported.
[88,89]
The addition of pepper-
mint and citronella essential oils into fish skin gelatin films have been observed to exert more effective
antimicrobial inhibition towards gram-positive (Staphylococcus aureus) compared to gram-negative
bacteria (Escherichia coli).
[88]
This may be due to the presence of lipopolysaccharides in the outer cell
wall in gram-negative bacteria, which act as protective membranes against the phenolic components
from essential oils.
[96]
Other studies have illustrated similar antimicrobial properties with fish skin
gelatin-based film containing cinnamon essential oils that also exhibited good inhibitory properties
towards gram-positive (Staphylococcus aureus) and gram-negative bacteria (Escherichia coli).
Additionally, fish skin/cinnamon gelatin film had more effective antifungal activity towards
Aspergillus niger, Rhizopus oryzae, and Paecilomyces variotii than resistance to bacterial growth.
[89]
Besides, a similar inhibitory effect towards gram-positive bacteria (Staphylococcus aureus and Listeria
monocytogenes) was observed in fish gelatin-based film derived from skin of unicorn leatherjacket with
the addition of lemongrass and bergamot essential oils.
[90]
However, only unicorn leatherjacket skin
gelatin/lemongrass oil films showed inhibitory properties against gram-negative bacteria (Escherichia
coli and Salmonella typhimurium). Both essentials’ oils showed better inhibition activity towards
gram-positive bacteria as compared to gram-negative bacteria. Higher resistance rendered by gram-
negative bacteria were attributed to the presence of lipopolysaccharide as an additional external barrier
that surrounds the cell wall of gram-negative bacteria, which restricts the diffusion of hydrophobic
compounds through the membranes.
[90]
Further, studies on antimicrobial activity in gelatin film derived from bovine hide have also been
reported by Gómez-Estaca et al.
[71]
The bovine hide gelatin films incorporated with clove oil and had
a high inhibitory effect against different types of gram-positive (Lactobacillus acidophilus and Listeria
innocua) and gram-negative bacteria (Pseudomonas uorescens and Escherichia coli).
[71]
Clove essen-
tial oils contain eugenol as the major component, followed by β-caryophyllene, caryophyllene oxide
and eugenyl acetate; these compounds help to destroy the cell wall and membranes of bacteria which
result in the loss of intracellular materials that lead to death of bacteria.
[97]
In addition, several studies on antimicrobial properties of composite film of gelatin in combination
with other polymers such as chitosan, along with essential oils as antimicrobial agents, have also been
reported.
[25,87]
The addition of oregano essential oil to composite film comprising cold water fish skin
gelatin/chitosan nanoparticles, were observed to exert more effective antimicrobial activity against
gram-negative (Salmonella enteritidis and Escherichia coli) than gram-positive bacteria
(Staphylococcus aureus and Listeria monocytogenes).
[87]
This may be due to the oregano oil constitu-
ents mainly composed of terpenoid fraction (carvacrol, thymol, and p-cymene).
[98]
In particular,
carvacrol was more effective against gram-negative rather than gram-positive bacteria due to its high
affinity for the outer membrane and ability to disintegrate the lipids cell wall causing lipopolysacchar-
ides to be released and increase the permeability of the cytoplasmic membrane.
[99,100]
In addition,
Kakaei and Shahbazi
[25]
have also reported antimicrobial properties of gelatin/chitosan composite film
containing ziziphora clinopodioides essential oil and grape seed extract. The results revealed broader
inhibition activity towards several group of gram-positive (Listeria monocytogenes and lactic acid
bacteria) and gram-negative bacteria (Pseudomonas spp., Shewanella putrefaciens, Enterobacteriaceae
and Pseudomonas uorescens).
[25]
The inhibitory effect towards bacteria was attributed to the presence
8N. S. SAID ET AL.
Table 1A. Active gelatin films incorporating antimicrobial agents.
Antimicrobial agent Gelatin film
Inhibition activity
References
Gram-positive
bacteria Gram-negative bacteria Fungal
Essential
oil
Peppermint and
citronella
Fish skin Staphylococcus
aureus
Escherichia coli - Yanwong and
Threepopnatkul
[88]
Cinnamon Fish skin
Staphylococcus
aureus
Escherichia
coli
Aspergillus
niger,
Rhizopus
oryzae, and
Paecilomyces
variotii
Wu et al.
[89]
Lemongrass Unicorn leatherjacket
skin
Staphylococcus aureus
and Listeria
monocytogenes
Escherichia coli
and
Salmonella
typhimurium
-
Ahmad et al.
[90]
Bergamot Unicorn leatherjacket
skin
Staphylococcus aureus
and Listeria
monocytogenes
- -
Ahmad et al.
[90]
Clove Bovine hide Lactobacillus
acidophilus
and Listeria
innocua
Pseudomonas fluorescens
and Escherichia coli
- Gómez-Estaca
[82]
Oregano Cold water
fish skin
Staphylococcus
aureus and
Listeria
monocytogenes Salmonella
enteritidis
and
Escherichia
coli
-
Hosseini et al.
[87]
Ziziphora
clinopodioides
oil/grape seed
extract
Gelatin/
chitosan
Listeria monocytogenes and
lactic acid bacteria
Pseudomonas
spp.,
Shewanella
putrefaciens,
Enterobacteriaceae and
Pseudomonas
fluorescens
- Kakaei and
Shahbazi
[25]
Metal
ions
Copper (II)-
exchanged
montmorillonite (Cu
2
+
MMt)
Bovine hide Listeria
monocytogenes
Escherichia coli - Martucci and Ruseckaite
[71]
Silver
nanoparticles
-Listeria monocytogenes Escherichia coli -
Kanmani and
Rhim
[99]
Zinc oxide (ZnO)
nanoparticles
-Staphylococcus
aureus
Vibrio cholerae and
Escherichia coli
- Umamaheswari
et al.
[101]
Zinc oxide (ZnO)
nanoparticles
Chicken skin Staphylococcus
aureus
Escherichia coli - Lee
[21]
Zinc oxide (ZnO)
nanoparticles
-Listeria monocytogenes Escherichia coli -
Shankar et al.
[102]
Silver- copper
nanoparticles
Fish skin Listeria monocytogenes Salmonella
typhimurium
Arfat et al.
[100]
Titanium dioxide (TiO
2
) nanocomposite Fish skin Staphylococcus
aureus
Escherichia coli - He et al.
[103]
Polymer Chitosan/
ethanolic
extracts
Pig skin Staphylococcus
aureus
Escherichia coli Bonilla and
Sobral
[104]
Chitosan Bovine hide - Escherichia coli Pereda et al.
[105]
0, 3, 5 and 10%
(based on dry
gelatin)
concentrations
of N-chitin
Bovine - - Aspergillus-niger Sahraee et al.
[106]
FOOD REVIEWS INTERNATIONAL 9
of ziziphora clinopodioides essential oil which contains a high concentration of phenolic compounds
including thymol and carvacrol.
[25]
Active gelatin films derived from various sources of gelatin
incorporated with essential oils as antimicrobial agents are presented in Table 1A.
The incorporation of essential oils in film formulation normally produces an increase in hetero-
geneity and a surface roughness thus decreasing the gloss and light transmission. Active marine gelatin
composite film incorporated with natural extracts and essential oils such as bergamot, lemongrass,
grape seed, kaffir lime, lemon, lime, ginger, turmeric, plai, and green tea showed lower light transmis-
sion value as compared to single gelatin films.
[90,107–109]
This effect is explained by the migration of oil
droplets or aggregates to the film surface during drying, decreasing the specular reflectance in the air–
film interface, and thus obstructed the transmission of light.
[107,108,110]
However, many findings found
that the incorporation of essential oils at small amounts did not cause the detrimental effect on smell
perception or unacceptability of the film.
[107,108]
The application of gelatin-based active films incor-
porated with natural extracts and essential oils have been reported to inhibit microbial growth and
delay oxidative reactions in chicken meat, minced pork, turkey meat, fish fillets as well as in shrimp
cracker.
[111–115]
A study by Ahmad et al.
[113]
mentioned that incorporation of lemongrass essential oils
into gelatin film enhanced the antimicrobial and antioxidative properties and extend the shelf-life of
the sea bass slices stored at refrigerated temperature. Meanwhile, another study reported by Khan
et al.
[114]
stated that edible gelatin composite films enriched with polyphenol loaded nanoemulsions
showed excellent results by increasing the shelf-life of fresh broiler meat up to 17 days in comparison
to control (10 days). Hence, this showed that gelatin film containing natural extracts is a potentially
promising film that could help delay oxidative reactions in real-time food product.
Metal ions as antimicrobial agents in gelatin-based active film
Metal ions and metal-based nanoparticles such as copper, silver, zinc, titanium, palladium and
cerium are demonstrated to be excellent antimicrobial agents and showed a wide spectrum of
antimicrobial activity against bacteria, viruses and fungi.
[72,116]
Metal nanoparticles exhibit bacter-
icidal effect that is attributed to their small size and high surface-to-volume ratio, which allows close
interaction with the bacterial membranes that leads to loss of membrane integrity, oxidative stress,
protein dysfunction and DNA degradation after interaction of metal nanoparticles with the target
cells.
[103]
Several studies have reported the incorporation of metal ions as antimicrobial agents into different
types of gelatin films derived from mammalian and marine sources. For instance, the bovine hides
gelatin films with incorporation of copper (II)-exchanged montmorillonite (Cu
2+
MMt) have shown
greater inhibition against gram-positive (Listeria monocytogenes) as compared to gram-negative
(Escherichia coli) pathogens.
[72]
The greater inhibition effect against gram-positive bacteria was
attributed to the thick layer of cell wall that provide negatively charged sites for cations to bind; this
caused Listeria monocytogenes to be more vulnerable to Cu
2+
MMt. Similar inhibition activity towards
gram-positive (Listeria monocytogenes) and gram-negative bacteria (Escherichia coli) were also
observed in active gelatin-based nanocomposite films containing silver nanoparticles (AgNPs).
[101]
The inhibition effect was considered to be due to the interaction of AgNPs with phosphorous and
sulfur containing compounds of proteins and DNA which then prevent DNA replication and lead to
cell death.
[101]
Moreover, as study by Arfat et al.
[102]
the addition of silver-copper nanoparticles into fish skin
gelatin film have shown antimicrobial effects towards gram-positive (Listeria monocytogenes) and
gram-negative bacteria (Salmonella typhimurium). The mechanisms of inhibition by silver-copper
nanoparticles against both bacteria may be due to the release of silver or copper ions that penetrate
into pits and gaps within bacterial membrane. The intrusion may lead to disruption of metabolic
processes and cause rupture of bacterial cell walls, loss of DNA replication ability and cell death.
[117]
However, another study using fish skin gelatin film incorporating titanium dioxide (TiO
2
) nanopar-
ticles reported more effective antibacterial activity against Escherichia coli as compared to
Staphylococcus aureus. The study also attributed the inhibition activity and to cell lysis when irradiated
10 N. S. SAID ET AL.
TiO
2
particles came in contact with microbes thereby lethally affecting the genome and other
intracellular molecules which led to inactivation of microorganisms.
[106]
In addition, the chicken skin gelatin film with addition of 1–5% zinc oxide (ZnO) nanoparticles
have shown inhibitory effect against gram-positive bacteria (Staphylococcus aureus) and gram-
negative bacteria (Escherichia coli).
[104]
The results were in line with findings by Umamaheswari,
et al.
[105]
who reported that gelatin films incorporating zinc oxide (ZnO) nanoparticles showed good
inhibition activity towards similar gram-positive bacteria (Staphylococcus aureus), followed by gram-
negative bacteria (Vibrio cholerae and Escherichia coli). Meanwhile, the antibacterial activity against
similar types of gram-negative bacteria (Escherichia coli) were also observed in other gelatin-based
nanocomposite films incorporating zinc oxide nanoparticles.
[118]
Shankar et al.
[118]
also observed the
inhibition activity of gelatin-based films containing zinc oxide nanoparticles towards other types of
gram-positive (Listeria monocytogenes) foodborne pathogenic bacteria. The inhibition effect may be
due to the release of Zn
2+
ions by zinc oxide nanoparticles which could penetrate the bacteria cell walls
and react to the cytoplasmic content, leading to bacterial death.
[118]
Active gelatin film derived from
various sources of gelatin incorporated with metal ions as antimicrobial agent are presented in
Table 1A.
Polymer as antimicrobial agent in gelatin-based active film
In the past decade, polymers with antimicrobial properties have commanded the attention of
researchers internationally and resulted in a drastic increase in the number of FDA-approved anti-
microbial polymers. The activity of antimicrobial polymers is characterized as passive or active, based
on their modes of mechanism.
[119]
For instance, the polymer with antimicrobial agent in gelatin-based
active film might be from chitin
[120]
and chitosan.
[121]
A few studies on the incorporation of chitosan into mammalian gelatin-based films have been
reported to yield antimicrobial activity against selected microorganisms. For example, the incorpora-
tion of chitosan into pig skin gelatin-based composite film with added ethanolic extracts have been
reported to exhibit great inhibition towards gram-negative (Escherichia coli) and gram-positive
(Staphylococcus aureus) bacteria.
[121]
Noticeable antimicrobial activity was observed when chitosan
was mixed with gelatin in a 50:50 ratio in the blended film. The antimicrobial effect of chitosan was
reported to be due to the interaction between positively charged amino groups with negatively charged
microbial cell membranes, which led to the loss of intracellular components and cell death.
[121]
These
findings were supported by results obtained for bovine hide gelatin/chitosan composite films that also
exhibited antimicrobial activity towards gram-negative bacteria (Escherichia coli).
[122]
However,
Gómez-Estaca et al.
[71]
reported no antimicrobial activity in bovine hide gelatin/chitosan composite
films; these contradictory results were probably due to different methods used during film develop-
ment which influenced the antimicrobial capacity in relation to several microorganisms.
Moreover, bovine gelatin nanocomposite films containing 0 to 10% (based on dry gelatin) con-
centrations of N-chitin exhibited antifungal activity against Aspergillus-niger. The inhibition mechan-
ism was attributed to chitin penetration into the microbial cell membrane, binding with DNA and
inhibiting RNA and protein synthesis of the cell.
[120]
Many studies on gelatin multilayer film also have been conducted such as gelatin-dialdehyde starch
(DAS)/gelatin-sodium montmorillonite (MMt)/gelatin-dialdehyde starch (DAS)
[123]
and PLA/gelatin/
PLA.
[124,125]
The gelatin multilayer film laminated with PLA as outer sheet has improved the moisture
resistance and mechanical properties while reducing the water vapor and oxygen permeability.
[124]
In
addition, there were also studies on bilayer film consisted of gelatin film combined with another
polymer such as agar, polyethylene and chitosan have been reported.
[126–129]
The results from all these
studies demonstrated that the gelatin bilayer film able to strengthen film’s mechanical properties while
also exerting antimicrobial activity and suitable to apply as active packaging film.
Meanwhile, a non-gelling hydrocolloids such as guar gum, xanthan gum, tragacanth gum and
Persian gum are commonly used as a thickener of another hydrocolloid to obtain a synergistic increase
in viscosity. The incorporation of these gums into gelatin-based film have been reported to improve
FOOD REVIEWS INTERNATIONAL 11
the film’s physical and mechanical properties as compared to single gelatin film, gelatin/methylcellu-
lose composite films, gelatin/octenyl succinic anhydride modified starch composite films, gelatin/
water-soluble soy polysaccharides composite films and dually modified gelatin/sago starch/κ-
carrageenan film
[130–133]
Active gelatin films derived from various sources of gelatin incorporating
antimicrobial agents are presented in Table 1A.
Antioxidant agents in gelatin-based active lm
Antioxidants are defined as substances that can inhibit or delay oxidation; in food, polyunsaturated
lipids are subject to autoxidation which can result in the formation of free radicals, secondary and
tertiary products that can cause rancidity, deterioration and discoloration. Lipid-free radicals can
transfer to amino acids and proteins leading to further damage and formation of potential toxic
products.
[134]
Current studies have focused on improving or extending the functional properties of
biodegradable active films by incorporating different antioxidant compounds derived from natural
sources such as food and non-food plants, spices and food residues as alternatives to synthetic
antioxidants such as butylated hydroxyl toluene (BHT) and butylated hydroxyl anisole (BHA).
[10]
Synthetic antioxidants are reported to have safety issues. Plant and spices extracts have advantages
over synthetic antioxidants as they contain many phytochemicals such as cathequins, phenolic
diterpenes, flavonoids, tannins and phenolic acids that serve as potential sources of natural antiox-
idants that are reported to possess anti-inflammatory and anticancer activities.
[74]
For examples,
natural extracts from essential oil, green tea, boldo leaf, grape seed, ginger and gingko leaf have
been studied for their excellent antioxidant properties, along with their impact on the gelatin film’s
properties such as mechanical strength, water vapor permeability (WVP), light transmission and film
transparency. The antioxidant activities commonly measured include free radical scavenging activities
by using the 2,2-azino-bis-3-ethylbenzothiazoline-6-sulfonic acid (ABTS), 2,2ʹ-diphenyl-1-picrylhy-
drazyl (DPPH) and ferric-reducing antioxidant power (FRAP) assays
[135]
as well as peroxide value and
thiobarbituric acid-related substances (TBARS).
The incorporation of natural extracts such as gingko leaf, green tea, grape seed (proanthocyani-
dins), and grape seed (polyphenols) into gelatin films derived from marines’ sources have been
reported to yield high DPPH radical scavenging activity, over than 80%.
[74,75]
The high antioxidant
capacity was attributed to flavonoids in gingko leaf extract; caffeic acid and epicatechin compound in
green tea extracts and epicatechin and catechin compounds found in grape seed extracts. However,
fish gelatin film incorporating ginger extract had the lowest antioxidant activity, 20% DPPH radical
scavenging activity as compared to other agents. The antioxidant activity of ginger extracts was due to
the presence of polyphenolic compounds including gingerols, shogaols, paradols and gingerdions.
[136]
Further, the incorporation of natural extracts such as oregano and lavender essential oils into
bovine hides gelatin film yielded lower DPPH radical scavenging activity compared to marine gelatin
films. The data showed that oregano and lavender essential oils exhibit DPPH radical scavenging
activity, 0.1–60% and 0–8.6%, respectively.
[137]
Moreover, the incorporation of Centella asiatica
extract into bovine and chicken skin gelatin-based film have also shown lower DPPH scavenging
activity than those reported for marine sources gelatin films, which were found to be between
34.70–47.51% and 55.42–68.67%, respectively.
[83,138]
The different of antioxidant activity observed
between each type of film might be due to different amino acid content in different gelatin sources
which also contribute the antioxidant activity besides the natural extract added.
In addition, the incorporation of natural extracts into the marine-sourced gelatin films has been
reported to improve the UV light barrier properties as the light transmission percentage was
decreased.
[75]
The resulting films incorporating natural extract also exhibited less than 5% opacity
which can be considered as transparent, and may not affect the visual appearance of the food product
when these films come into contact with the food surface.
[74]
However, the TS and EAB values
diminished with the addition of the natural extract into the gelatin-based film.
[75,137]
This was due
to the polyphenolic compounds present in these antioxidant extracts, that formed hydrogen and
12 N. S. SAID ET AL.
covalent bonds with amino and hydroxyl groups of gelatin polypeptide chain. These bonds may
weaken the protein–protein interactions that stabilize the protein network.
[75]
Moreover, the incor-
poration of natural extracts into the gelatin-based film also reduced the WVP as compared to the
gelatin film without any addition of the natural extracts. The WVP value was affected by the presence
natural extract due to its hydrophobic nature which affected the hydrophilic–hydrophobic balance of
the film.
[137]
Active gelatin films derived from various sources of gelatin, with addition of antioxidant
agent, are presented in Table 1B.
The incorporation of non-polar bioactive compounds (as essential oil) encapsulated into
conventional O/W emulsions, emulsions of pickering and nanoemulsions in gelatin-based
active lm
In general, gelatin-based films commonly exhibit poor water vapor barrier properties due to the
hydrophilic nature of gelatin and hydrophilic plasticizer incorporated into the film that tend to swell
easily when brought in contact with a food surface. This has limited their application and uses as
packaging materials in the food industry. Therefore, a new approach has been adopted to enhance the
water vapor barrier property of the resulting film. The addition of non-polar bioactive compounds
such as oils can enhance the film’s water vapor barrier property and improve water diffusion.
[139]
Proper viscosity of oil can benefit emulsification and the stability of emulsion. Under the same
emulsifying conditions, higher degree of oil is difficult to break into small droplets due to its higher
viscosity. Thus, lower degree of oil is preferred as it is easy to change into smaller droplets upon high-
speed shearing due to its lower viscosity. Smaller droplets would endow better homogeneous dis-
tribution of oil droplets to the resulting film, subsequently benefiting the enhancing of film
performances.
[140]
Table 1B. Active gelatin films incorporating antioxidant agents.
Antioxidant agent Gelatin film Mechanism on radical scavenging activity References
Gingko leaf extract Silver carp
skin
Showed high scavenging activity (over 80%).against DPPH
free radicals
Li et al.
[121]
Green tea extract Silver carp
skin
Li et al.
[121]
Grape seed extract
(proanthocyanidins)
Silver carp
skin
Li et al.
[121]
Grape seed extract
(polyphenols)
Silver carp
skin
Li et al.
[121]
Boldine extract Salmon López et al.
[119]
Ginger extract Silver carp
skin
Showed low scavenging activity (less than 20%) .against
DPPH free radicals
Li et al.
[121]
Oregano essential oils Bovine hide Exhibited DPPH radical scavenging activity (RSA %) at
0.1–60%
Martucci et al.
[134]
Lavender essential oils Bovine hide Exhibited DPPH radical scavenging activity (RSA %) at
0–8.6%
Martucci et al.
[134]
Centella asiatica (L.) urban
extract
Bovine Showed scavenging activity against DPPH radical at
34.70–47.51%
Rasid et al.
[73]
Centella asiatica (L.) urban
extract
Chicken
skin
Showed scavenging activity against DPPH radical at
55.42–68.67%
Suderman and
Sarbon
[74]
Table 1C. The incorporation of non-polar bioactive compounds (as essential oil) encapsulated into conventional O/W emulsions,
emulsions of pickering and nanoemulsions in active gelatin films.
Other agents Gelatin film Effect on film’s properties References
Corn oil Bovine Reduced water vapour activity of emulsion film
Enhanced the film’s barrier properties
Sahraee et al.
[106]
Palm oil Fish skin Decreased water vapor permeability and moisture content of the film Tongnuanchan et al.
[135]
FOOD REVIEWS INTERNATIONAL 13
The incorporation of non-polar bioactive compounds such as corn oil into mammalian gelatin-
based film derived from bovine sources has been reported to improve WVP and reduce the moisture
content of the resulting films. Increasing concentration levels of corn oil (0.1, 0.2, 0.3 g/g) incorporated
in gelatin films reduced the WVP value which were reported at 7.86, 7.81 and 7.68 × 10
−10
gms
−1
m
−2
Pa, respectively.
[141]
The results were in agreement with a study by Tongnuanchan et al.
[139]
which reported reduced values of moisture content and water vapor permeability for fish skin gelatin
films with increasing palm oil concentration. The results stated that WVP of fish skin gelatin films
containing palm oil at 25–100% (based on protein) noticeably decreased from 1.63 to 0.70 × 10
−11
gm
−1
s
−1
Pa
−1
as compared to control gelatin film without the incorporation of palm oil that was found
at 2.54 × 10
−11
gm
−1
s
−1
Pa
−1
.
The WVP and moisture content of resulting film were also reduced due to the increased hydro-
phobicity of the films. This caused fewer water molecules to be entrapped in the polymer matrix while
lowering water vapor diffusion and adsorption through the film that caused reduction in both
moisture content and WVP values.
[139,141]
Active gelatin films derived from various sources of gelatin
with the non-polar bioactive compounds are presented in Table 1C.
pH sensing in gelatin-based smart lm
At present, pH-sensing methods using visual pH indicators are being extensively studied since pH
changes can determine or inform about the spoilage that occurs in various food products. The pH of
a food product is altered through the changes of organic acids concentration and volatile compounds
development resulted from growth and metabolism of microorganism.
[142]
The pH-sensing indicators
in smart packaging systems have several advantages including their excellent sensitivity for detection
of microbial spoilage as well as low cost and small size, which enables increased solubilization within
the film matrix. Visual pH indicators commonly consist of a solid support to immobilize the pH
sensitive dyes
[143]
from chemical reagents such as bromocresol purple, bromocresol green, bromo-
phenol blue, chlorophenol red and cresol red. However, these chemical reagents have potential
harmful effects on human health due to possible toxicity.
[143,144]
Thus, the utilization of natural
dyes as pH-sensitive indicators has received attention due to their potential use and substitute for
chemically derived dyes. Furthermore, natural dyes possess advantages as they exhibit low toxicity,
renewable and pollution-free properties.
[144]
The incorporation of natural indicators (curcumin and red cabbage (Brassica oleracea L.) extracts)
and synthetic acid-based indicators (methyl orange, neutral red and bromocresol green) into gelatin-
Table 2. Smart edible film incorporating pH sensing indicators.
Smart film pH sensing activity References
Natural
indicators
Curcumin Changed the film color from colorless to yellow at pH 6 and
orange-red at pH 11
Musso
et al.
[137]
Red cabbage (Brassica oleracea L.)
extracts
Changed the film color to pink at pH <4 and yellow at pH >11. Musso
et al.
[143]
Bauhinia blakeana Dunn Showed changes in film color from red to green Zhang
et al.
[141]
Black chokeberry (Aronia
Melanocarpa) pomace extract
Red color of chitosan-extract film turned to purplish-blue at pH
≥7
Halász and
Csóka
[144]
Anthocyanin Dark violet film color turned to pink at acidic pH, bluish-green
at neutral pH and violet at basic pH.
Yoshida
et al.
[146]
Red cabbage extract Had high red color intensity at pH 2–4 and high green color
intensity at pH >4
Silva-Pereira
et al.
[145]
Synthetic
indicators
Methyl orange acid-based
indicators
Film color changed at different pH in gaseous, liquid and
semisolid media
Musso
et al.
[142]
Neutral red acid-based indicators
Bromocresol green acid-based
indicators
14 N. S. SAID ET AL.
based film showed good response capacity against pH changes.
[142,145,146]
The addition of indicators
helps to color the gelatin-based film as they possess chain ionizable side groups that act as buffer
system that allows the films to change in color when it comes into contact with different pH values in
gaseous, liquid and semisolid medium. All films possess a good ability to sense the pH changes which
signify that these films can detect the changes that occur in either liquid, semisolid or gases that are
present in the headspace of the food containers, arising from the reactions of food spoilage.
[142]
Moreover, these films also exhibit a high potential to be utilized as smart packaging because they are
able inform the consumer regarding the quality and safety of the food product during storage and
distribution until consumed by the public.
[145]
The smart edible films incorporating pH sensing
indicators are presented in Table 2.
Advantages and disadvantages of gelatin-based lm compared to synthetic lm
packaging
A major advantage of gelatin-based film packaging is that they are biopolymer natural ingredients,
which is often required for ‘consumer-friendly’ labels. The sources of biopolymers are living organism
and they are often biodegradable.
[147]
Thus, gelatin biopolymers can be degraded or broken down
through the action of naturally occurring organisms, leaving behind organic by-products such as
carbon dioxide and water which have no detrimental effect on the environment. High biodegradation
rate for single gelatin films were reported within the range of 18 − 25% for 3 days observations.
[123–125]
Meanwhile, a study by that Neelam et al.
[148]
mentioned that polyethylene degrades 0.5% in 100 years
and with exposed to sunlight for 2 years its degradation is around 1%. In addition, the molecular and
functional properties of gelatin depend on the extraction, purification and processing treatments used
to produce functional ingredients. This provides food scientists to design and improve novel film
packaging according to their own purpose and liking. However, some of the drawbacks of using gelatin
as food packaging materials compared to the conventional non-biodegradable materials, especially
those that are petroleum-derived include poor mechanical and barrier properties. Gelatin are brittle,
have a low heat distortion temperature and humidity, low resistance to prolonged process operations
and low flexibility. Several studies on tensile strength value for mammalian gelatin-based film has been
reported within the range of 2.40–63.25 MPa.
[149–151]
The tensile strength of gelatin films is compar-
able to those conventional non-biodegradable packaging materials such as high-density polyethylene
(HDPE) and low-density polyethylene (LDPE) which have tensile strength values of17.3–44.8 and
8.2–31.4 MPa.
[152]
On the other hand, gelatin possess different variation in molecular and functional
properties which depend on its animal type and source. This vast variation can cause problems with
reproducibility during the manufacturing process. Thus, improved knowledge of the molecular basis
of gelatin film’s functionality in food packaging will aid in the design of foods and food processing
operations.
Current and future trends for gelatin-based active and smart edible lms
Many studies have been conducted to improve the gelatin-based film properties, while extending their
functionality as active and smart biodegradable films. Extensive studies such as controlled respiration
rate by using oxygen scavengers or ethylene absorber, enhancing water vapor permeability and
advance coating methods are needed for new and improved applications of gelatin-based active
films in the food packaging industry, aside from antimicrobial and antioxidant properties. In addition,
future investigations related to film development by using nanotechnology, are needed as they can
enhance food safety and prolong shelf-life. However, toxicity aspects, migration assays and risk
assessment have to be considered during the film development process. Novel gelatin-based active
films can be applied as food packaging in a variety of products such as bakery, meat, fruits, and
vegetables. There are only limited studies on smart biodegradable gelatin-based films. Thus, it is
suggested to broaden the study on development of gelatin-based smart biodegradable films to those
FOOD REVIEWS INTERNATIONAL 15
that have the capacity to sense pH or environmental changes or specific compounds generated by
products or mechanism of bacteria during handling of food packaging or storage. The development of
these gelatin smart films can be beneficial as quality or freshness indicators in many types of food
products such as seafood, meat, poultry and dairy products.
Conclusion
The development of active and smart gelatin-based films designed in innovative ways can create new
packaging materials and technologies. Many studies have been performed to improve the functional
properties of gelatin-based film by incorporating antimicrobial or antioxidant agents. Gelatin-based
films possess a good matrix for the incorporation of antimicrobial and antioxidant agents which can be
released to perform their specific functions in enhancing safety, stability, functionality and shelf-life of
food products. A wide number of studies also have been conducted to test the application of gelatin-
based film on variety types of food products which revealed that these active and smart biodegradable
films exhibit a longer and effective shelf-life to ensure food safety and quality. These films are
environmentally friendly, biodegradable, and convenient and have a low cost as they can be obtained
from many types of natural sources. However, further extensive studies are needed to improve
techniques or innovate methods for enhancing the functionality of the film as well as their extended
application in various sectors.
Author contributions
All authors contributed to the final manuscript. Said, N. S. collected the literature sources and wrote the manuscript.
Nazlin K. Howell and Sarbon, N. M. conceptualized the idea and performed critical reviews on manuscript.
ORCID
N.M Sarbon http://orcid.org/0000-0003-0904-1039
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