Natural Additives in Smart Food Packaging: A
Comprehensive Review of their Roles in Active and Intelligent Packaging Systems
Ujjwala Patel1,2, Bhanushree Gupta2*
1School of
Studies in Chemistry, Pt. Ravishnakar Shukla University, Raipur (C.G.), India,
492010.
2Center
for Basic Sciences, Pt. Ravishnakar Shukla University, Raipur (C.G.), India,
492010.
Abstract
Natural additives encompass compounds derived from
animals, minerals, plants and occasionally microorganisms. Integrating these
additives into food packaging extends shelf life, enhances sensory
characteristics and fortifies the safety of food articles. The incorporation of
food additives plays a pivotal role in preserving flavor, nutritional value and texture, thereby ensuring their
widespread acceptance. In recent years, consumers have exhibited a preference
for food products devoid of additives. When faced with limited options,
individuals tend to opt for foods containing natural additives rather than
synthetic ones. The utilization of naturally sourced additives is prioritized
over synthetic counterparts due to their minimal to nonexistent toxicity, as
synthetic additives can present significant risks to human health. Natural
additives have demonstrated efficacy in various capacities, including as
antimicrobials, antioxidants, pH indicators, and enhancers of the mechanical
and physical properties of food packaging.
This review provides a brief overview of natural additives, their origins and
their applications in real-world scenarios.
Keywords: Natural
additives, antimicrobials, antioxidants, active packaging, intelligent
packaging.
1. Introduction
Food packaging plays a crucial role in preserving and
ensuring the quality of fresh food items intended for export, storage, and
eventual consumption. The rising preferences for
demand for fresh, clean, minimally processed, high-quality and ready-to-eat
products reflects the changing lifestyle of modern consumers. For centuries,
the food sector has significantly relied on the incorporation of additives to
prolong the longevity of food items. As defined by the Codex Alimentarius
(WHO/FAO 2018), food additives are chemical substances
that are not typically consumed as food on their own, nor are they commonly
used as standard ingredients, but are intentionally added to food to achieve
specific technological or functional purposes (Albuquerque et al., 2021). The Joint FAO/WHO Expert Committee on Food
Additives (JECFA) along with the Food Safety and Standards Authority of India
(FSSAI) are key international and national organizations responsible for
guaranteeing food safety and overseeing the production, importation,
distribution and storage of food products.
Synthetic additives despite being economical, pose various
health risks like allergies, heart disease, nausea, obesity and cancer, due to
which consumers have started opting for natural additives for food
preservation. These natural additives offer numerous advantages like enhanced
nutritional value, potential health benefits, they can preserve food etc.,
therefore can be used as antioxidants, preservatives, gelling and thickening
agent, colorants, sweeteners in food industry.
Growing demands for food quality and safety have
accelerated the use of smart packaging. In smart packaging, active coatings and
chemical or physical sensors are integrated into packaging materials to prevent
spoilage, enhance longevity and monitor changes that affect food quality. Active
packaging and intelligent packaging (types
of Smart packaging technologies) are capable of tracking and regulating
factors like microbial
proliferation, temperature, gas exchange, water loss and pH variation,
making them increasing viable for widespread application within the food sector
(Yousefi et al., 2019).
Active packaging, in agreement with the guidelines provided by the European Union in Commission Regulation (EUGCR) No
450/2009, involves packaging materials that have the ability to absorb or
release active substances like antimicrobials, antioxidants, organoleptic
(flavoring) agents and tissue improvers either directly into the food or its
immediate surroundings (Yousefi et al., 2019). These materials
restrain the proliferation of microorganisms, limit the exchange of gases (such
as CO2, O2, NH3 and ethylene) between the food and its surroundings, prevent
the loss of aroma, flavors, and colors from the food, and enhance its
nutritional content (Brockgreitens & Abbas, 2016). Intelligent packaging films, in accordance with the
European Union Guidance to Commission Regulation (EUGCR) No. 1935/2004 (Yousefi et al., 2019), consist of materials which do
not interact with food products directly but are capable of monitoring the
state of the internal and external surroundings of the packaged food product. Intelligent
packaging allows for the real-time
communication of the packaged
food's condition to consumers, underscoring the critical role of consumer
participation in food packaging (Wyrwa & Barska, 2017). Biosensor-based
intelligent packaging films incorporated with natural additives act as
indicators of pH change, protein deterioration etc. by changing the colour of
the packaging.
This review outlines the significance of natural
additives in food packaging due to their
chemical, physical, and biological attributes. It explores their use as
antioxidants, antimicrobials and for improving mechanical properties in active packaging. Furthermore, it examines their potential as
indicators or sensing materials for pH, gas, humidity, and other factors in
intelligent packaging.
2. Types of natural
additives
Natural additives can be derived from various sources,
including plants, animals, minerals, and microorganisms. Here are some common
types of natural additives based on their sources.
Plant-based additives include herbs and spices like
garlic (antimicrobial), rosemary extract (used as an antioxidant) and turmeric
(colorant). Fruits such as citrus extracts (used as flavorings), apple extract (antioxidant). Animal-based
additives include gelatin, derived from animal collagen and used as a gelling
agent in foods, whey is used in dairy products and as a source of protein in
various foods.
2.1 Fruit-based additive:
Natural additives
sourced from fruits, their extracts,
and by-products are deemed safe for
human consumption. With consumers increasingly prioritizing dietary health,
there's a demand for foods that are both safe and natural. In some cases,
potential fruit waste or extracts may serve as natural additives, offering an
alternative to synthetic options.
(a)
Citrus Fruits: Citrus industries produce a large quantity of
by-products. Citrus fibers and their wash water are commonly applied in
emulsified meat processing due to their antioxidative potential. They
effectively suppress lipid oxidation and lower residual nitrite levels,
consequently mitigating the generation of carcinogenic nitrosamines (Viuda-Martos et al., 2010). Citrus essential
oils are commonly used as flavorings in the food industry (Cohen et al., 2019) and citrus peel pigment is
a popular ingredient in beverages (Barman et al., 2020). Dried citrus peels are
used as natural flavor enhancer and expertly mixed into marmalades, teas, ice
creams and other food items, owing to their aromatic flavor and scent
properties (Tekgül & Baysal, 2018;
Wedamulla et al., 2022).
(b) Pomegranate:
Pomegranate (Punica grantam L.), also
known as "super fruit" due to its exceptionally high nutritional
content. Its edible and non-edible part is equally beneficial for human health
as it contains bioactive compounds and phytochemicals that possess antioxidant
and antimicrobial characteristics, similar to chemical preservatives and
synthetic antioxidants (Giri et al., 2023). Chitosan coating with 1%
pomegranate peel extract proved most effective in extending apricot shelf life,
reducing weight loss and decay while maintaining antioxidant activity,
firmness, and ascorbic acid content compared to untreated fruit (Giri et al., 2023). Pomegranate peel extract
or powder enhances the fiber content and boosts oxidative and microbiological
stability in food products; while maintaining sensory quality It is also widely
used in the fields of pharmaceuticals and cosmetics (Chen et al., 2020). Colletotrichum gloeosporioides is known to cause deterioration in
capsicums. However, a significant improvement in color, firmness, and reduction
in physiological loss was observed
when capsicums were coated with pomegranate peel extract (Nair et al., 2018). The utilization of
pomegranate peel extract as a natural antioxidant in sardine fish oil at
different concentrations was compared to a synthetic
antioxidant (BHA) for oxidative stability. The highest concentration of
pomegranate peel extract demonstrated better stability in comparison to BHA (Sarojini A et al., 2019).
(c)
Banana: The banana peel contains a wide range of polyphenols,
such as flavonoids, anthocyanins, dopamine, catecholamines, phenolic acids etc.
These compounds are known for their potent antioxidant properties, especially
in antioxidant processes like radical scavenging ability tests (González-Montelongo et al., 2010). In a
study, incorporation of banana peel extract to yogurt increased its shelf life
and nutrition value by preserving the total phenolic content during the storage
period (Kabir et al., 2021).
(d) Guava: The
bioactivity of guava seeds is diverse, and they are packed with essential
nutrients like protein, phenolics, vitamins, lipids, carotenoids and dietary
fiber. The biomedical and food
sectors have recognized the promising applications of guava seed extract. Nevertheless, the presence of
antinutritional factors limits their utilization in the food industry (Kumar et al., 2022). Rich in antioxidants such
as condensed flavonoids, tannins, flavonols, and other polyphenols (Liu et al., 2018), the guava seed extracts,
particularly the ethanol: water extracts, exhibit significant radical
scavenging ability (antioxidant activity). These extracts have been found to
possess a wide array of biological properties like neuroprotective, anticancer
effects, immunomodulatory, antimicrobial, anti-inflammatory etc. (Kumar et al., 2022).
Various other fruits exhibit great antioxidant
capacity and high polyphenol concentration such as kiwi, longan mango, papaya,
red dragon and sapodilla (Mahattanatawee et al., 2006). Bergamot peel
possesses natural antibacterial properties that are highly effective against
gram-negative bacteria (Mandalari et al., 2007). Ethanolic extract of
Grape seed exhibited antioxidant and antibacterial properties due to the
presence of phenolic acids, flavanoids and aromatic compounds (Faustino et al., 2019). Indisputably, these
chemical compounds emerge as a compelling choice
for incorporation as food additives,
serving to preserve and enhance the quality of food products, while
simultaneously prevent oxidation.
2.2 Vitamins:
Among the thirteen
recognized vitamins, only five find application in food technology. Ascorbic acid, known as vitamin C, stands out as
the most versatile, with extensive
potential limited only by innovation. Carotenoids (provitamins A) and
dl-α-tocopherol (vitamin E) follow closely behind, albeit in more specific
applications. Nicotinic acid and its amide, sometimes called vitamin PP, along
with riboflavin and its phosphate sodium salt (vitamin B2), have lesser but
notable significance in this field. Figure 1 shows the structures of different
vitamins.
(a) Carotenoids (provitamins A): originate from photosynthetic organisms such as
cyanobacteria, algae and plants. Essential sources of carotenoids include
fruits and vegetables like carrots, mango, spinach, apricot, and broccoli and
are used to provide fascinating colors to chewing gums, sugar-coated dragees,
cakes, ice creams, sorbets, fruit-based drinks etc. Carotenoids display strong
antioxidant activity, by effectively scavenging various reactive oxygen
species, because of components such as lycopene, norbixin, and zeaxanthin (Srivastava, 2021). Studies have also revealed
that fucoxanthin, a carotenoid, possesses antimicrobial potential against
several microorganisms including Streptococcus
spp., Enterococcus spp. (Karpiński & Adamczak, 2019).
(b) dl- α -Tocopherol
(Vitamin E): Vitamin E is a
colorless, yellow, and nearly odorless oil having viscous consistency. It is
insoluble in water but soluble in ethyl alcohol. Tocopherols are present in
different oils and fatty foods as a
natural food antioxidant. When paired with ascorbyl palmitate, they show a
remarkable synergistic impact, boosting the efficiency of both substances and minimizing the needed amount (Counsell, 1993). Vitamins C and E are
found naturally in many foods, and when combined, they have the potential to be
more widely accepted than other antioxidant blends.
(c) Vitamin C:
The presence of oxygen triggers enzymatic browning through the action of polyphenol oxidase. Additionally,
non-enzymic browning can occur when ascorbic acid undergoes oxidation. The
overall quality of fresh-cut fruits and vegetables is negatively impacted by the
combination of low ascorbic acid levels and high phenolase enzyme activity (González-Aguilar et al., 2000). Berries and
citrus fruits do not experience enzymic browning because of their reduced substrate levels and
elevated concentrations of ascorbic acid. The inclusion of ascorbic acid offers
significant advantages to frozen and canned apricots, aiding in the retention
of color and flavor (Adkison et al., 2018). The incorporation of
ascorbic acid helps in maintaining the optimal quality of alcoholic beverages as it regulates the taste and color, which can be affected by high
levels of dissolved oxygen.
(d) Niacin: The
main purpose of niacin in the food industry is its application in meat
processing, typically combined with ascorbic acid to include both components (Counsell, 1993).
(e)
Riboflavin (Vitamin B2): Riboflavin is also used as a food coloring agent. It
gives a beautiful yellow hue to boiled sweets, sugar-coated products, fondant,
icings etc. (Counsell, 1993).
Figure 1. Structure of different vitamins. (A) Vitamin
C, (B) dl-α-tocopherol, (C) Carotenoids, (D) Riboflavin, (E) Niacin
2.3 Herbs & spices - based additives:
According to the U.S. Food and Drug
Administration (FDA), spice is defined as an "aromatic vegetable substance
in its entirety, whether whole, fragmented, or ground, predominantly applied
for seasoning food rather than for nutritional purposes" (Sung et al., 2012). The realm of spices incorporates
an extensive assortment of plant components, which include berries (like
peppercorns), bark (such as cinnamon), stems (like coriander), fruits (like
cumin, red chili, and black pepper), flowers (such as clove), roots (such as
ginger), bulbs (such as garlic and onion), aromatic seeds (like ajawain) and
various other parts of plants. However, alternative parts of the plant, frequently in a dehydrated
state, possess the ability to function as a spice. Spices and herbs possess
antioxidant properties in different formats, including aqueous or methanolic
extracts, resins, whole forms, essential oils and oleoresins. When added to
food, they can act as antifungal and antimicrobial agents, as well as sweeteners
and colorants. Spices exhibit
a diverse range of activities, effectively targeting various types of bacteria (both Gram-positive and
Gram-negative), molds, yeasts, and viruses. This efficacy is attributed to the
presence of phytochemical constituents, particularly phenolic compounds, which
possess the capability to obstruct oxidative rancidity and inhibit the
formation of undesirable flavors in certain food items (Vallverdú-Queralt et al., 2014).
As per the findings of a study, out of the 22 commonly
employed spice extracts, turmeric and nutmeg extracts were found to possess the
greatest efficacy in combating plant pathogens from the Colletotrichum
genus (Gottardi et al., 2016). The addition of
sumac, garlic, marjoram, cardamom, fennel, coriander, and thyme to bread
resulted in an enhanced antioxidant capacity due to their natural antioxidant
properties (El-Ola et al., 2014). In vitro, the CO2 extract of ginger showed comparable effectiveness
to BHT in inhibiting lipid peroxidation (Gottardi et al., 2016). The individual and
combined antioxidant activities of rosemary, sumac and sage extracts in peanut
oil were examined, revealing that the blend of sage and sumac exhibited the
highest effectiveness based on peroxide value (Özcan, 2003). Cellulose-based pouches with
cinnamon oil, clove oil, and BHA were used to
assess soybean oil peroxide values. Cinnamon oil showed
higher antioxidant properties than BHA, suggesting potential for plant extracts
as food packaging antioxidants (Phoopuritham et al., 2012).
Natural sweeteners improve the taste of various food
items like milk, jams, juices, confectionery, and beverages. However, they are
seen as sugar substitutes that contribute to obesity due to their high calorie
content (Carocho et al., 2015). Glycyrrhizin (E958),
derived from the liquorice plant Glycyrrhiza glabra L., is 50 times
sweeter than sucrose. This triterpene glycoside not only provides potent
sweetness but also serves as a foaming agent and flavor enhancer (Barclay
et al., n.d.). Neohesperidin dihydrochalcone (E959), a
powerful sweetener derived from citrus compounds neohesperidin or naringin,
boasts sweetness 1500 times more
intense than sucrose, making it a potent alternative (El-Samragy, 2012).
Colorants are crucial in the food industry, as color
is one of the first things
consumers notice. These colorants help in combating color changes due to light,
humidity, air, processing and
storage, thereby, boosting sensory appeal. Annatto, derived
from the Bixa orellana L. tree, is
rich in carotenoids bixin and norbixin, which impart a characteristic
yellow to orange hue. Recognized as a
natural food colorant (E160b), it is widely used in products such as dairy
items, baked goods, meat, snacks and beverages (Carocho et al., 2015). A diverse range of carotenoids, including, β-carotene,
fucoxanthin, lutein, astaxanthin and lycopene etc, extracted from algae,
insects and plants, serve as versatile additives in various food items such as
sauces, beverages, marinades, spice blends, coatings etc. Alfalfa-derived
chlorophyll a and b are commonly
employed as natural color additives in various food items and drinks (MacDougall, 2002).
Curcuma longa,
one of the most common and widely used household spice, and is also used as a
colorant [E100] in savory snack products, fine bakery wares, coatings etc. (MacDougall, 2002).
2.4 Peptides:
Bacteria and their antimicrobial compounds play a
crucial role in preventing food spoilage, thereby improving food safety and
prolonging shelf life. Research has demonstrated the efficacy of antimicrobial
peptides in combating food-borne pathogens, showing potential for enhancing
food preservation. These peptides can be utilized independently or in
conjunction with other antimicrobial agents, polymeric nanoparticles, and
essential oils to increase the longevity of food products (Rai et al., 2016). Antimicrobial peptide, Warnericin RB4,
derived from Staphylococcus warneri, is used for the preservation
of acidic soft beverages (Rai et al., 2016). Also, Magainins, Parasin and Buforin, obtained from Amphibians
showcase good antibacterial activity (Rai et al., 2016; Tiwari et al., 2009).
Currently, the primary antimicrobial peptide utilized
for food preservation is Nisin, which is commercially available as nisaplinTM
in lyophilized form (Rai et al., 2016). Nisin serves as a
preservative inhibiting the growth of heat-resistant, spore-forming
microorganisms in fruit and vegetable juices, and in canned vegetables like
baby corn, carrots, peas etc. (D’Amato & Sinigaglia, 2010). It also protects wine and beer from
acid-tolerant microorganisms like Leuconostoc,
Lactobacillus, Pediococcus (Rai et al., 2016). Recently, researchers have
explored microbial and enzymatic methods to produce antioxidant peptides from
different protein sources. These peptides act as food antioxidants, boosting
in-vivo antioxidant activity, reducing oxidative damage, and are sometimes
added to meat products (Lorenzo et al., 2018). Studies have identified
antioxidant peptides in Spanish dry-cured hams, like SAGNPN and GLAGA showing
both reducing power and antioxidant capabilities (Escudero et al., 2013).
3. Role of natural
additives in Active
Packaging systems
Active packaging systems, comprising films and
coatings, is applied to food products that utilize edible polymers sourced from
natural materials (cellulose, starches, lipids,
polysaccharides and proteins) as they preserve food’s freshness while
minimizing environmental impact. By incorporating active ingredients with
antioxidant and antimicrobial properties into the polymeric matrix, active
packaging enables controlled release into the
food, enhancing its quality. Smart packaging solutions extend food shelf life by safeguarding against microbial contamination and
internal deterioration during transportation and storage, offering relief.
Incorporating natural additives into biopolymeric matrices can address physical
limitations.
3.1 Biological Properties
of Natural Additives:
3.1.1 Antimicrobial
property
In recent years, the food industry has increasingly
favored natural antimicrobials over synthetic additives. Utilizing natural
additives like plant extracts and essential oils rich in phytochemicals
presents a potent strategy against
resistant microbes. Active food packaging incorporates antimicrobial materials
into polymeric films to prevent microbial growth on food surfaces.
Antimicrobial agents are either embedded in the packaging film or applied as a
coating, creating edible films that allow these agents to diffuse into the
food, providing protective effects (Brockgreitens & Abbas, 2016). Recently,
natural antimicrobial additives have been encapsulated using methods like spray
drying, nano-encapsulation, polymerization, etc. to enhance their controlled
release in food systems. Figure 2 represents some of the probable mechanisms
through which essential oils exhibit antimicrobial activity. Incorporating
cinnamon oil into PP film inhibited mold formation (Manso et al., 2015), while in PVA matrix, it repelled Plodia interpunctella larvae (Jo et al., 2015). Black plum extract and
apricot kernel along with chitosan, each displayed antimicrobial and
antioxidant property (Wang et al., 2020; Yang et al., 2022).
Figure 2.
Schematic representation of probable mechanisms of antimicrobial activity (Basavegowda & Baek, 2021).
3.1.2
Antioxidant property
Antioxidant compounds, naturally occurring during
nutrient metabolism and immune function, balance oxidation-reduction and
safeguard cellular components. During oxidative stress, they defend against
radical and non-radical species. Phytochemicals abundant in certain fruits,
vitamins, spices, and herbs serve as effective natural antioxidants.
(a) Fruits: The antioxidant activity of fruit purees and
extracts, rich in flavonoids, is widely acknowledged (Brewer, 2011). Anthocyanins, quercetin
glycosides, and caffeic acid, found in berries like cranberry, lingonberry, and
chokeberry, contribute significantly to their antioxidant properties (Jiang & Xiong, 2016). Alginate incorporated
Black chokeberry extract displayed significant antioxidant activity (Kim et al., 2018).
(b) Vitamins: Vitamins A, C (ascorbic acid), and E, often
consumed as supplements, provide protection against reactive oxygen species
(ROS), acting as antioxidants. Vitamin C exhibits antioxidant effects by
scavenging hydroxyl and superoxide radicals, neutralizing free radicals, and
interrupting lipid peroxidation chains (Rock et al., 1996).
(c) Herbs and spices: Spices and herbs (such as fennel,
cardamom, garlic, coriander, ginger, cinnamon, thyme, oregano, rosemary etc.)
exhibit antioxidant activity due to their rich concentration of phenolic
compounds, which terminate free radical chain reactions by donating hydrogen
and electrons. These compounds include
flavonoids (e.g., quercetin, catechin), volatile oils (e.g.,
eugenol, menthol), phenolic acids (e.g., gallic acid, caffeic acid) (Brewer, 2011).
(d) Peptides: The peptides in protein hydrolysates
produced from the enzymatic hydrolysis of plant proteins like corn, potato, soy, and
buckwheat, as well as animal-derived
proteins such as casein, gelatin, and whey, display antioxidant activity owing
to their effective radical scavenging and metal ion-binding capabilities (Jiang & Xiong, 2016).
3.2
Physical Properties of Natural
Additives:
3.2.1 Optical properties
Food packaging films possess optical properties that
are significant in ensuring the aesthetic and functional integrity of packaged
food products. When food items and other consumables are exposed to UV
radiation for sterilization or illumination in food retail environments, it can
lead to photo-oxidation in light-sensitive food materials. Biopolymer-based
packaging films (chitosan and gelatin), demonstrate significant light
transmittance due to absence of active groups capable of absorbing light photons
and establishing a light barrier. The incorporation of natural additives
abundant in phenolic compounds alongside these biopolymers results in
diminished light permeability of packaging films ans a heightened ability to
absorb UV radiation (Carla et al., 2017).
Chitosan films exhibit UV transmittance between 0-88%
(200-400 nm) and 88-93% in the visible range (Carla et al., 2017). Additives like
cinnamaldehyde alter the film's color, serving as a visible indicator of
chemical change (Seydim & Sarikus, 2006). Natural additives
like TP, rich in phenolic compounds, improve UV barrier properties in active
packaging films (Dou et al., 2018). Adding 0.4-2% TP to a
gelatin-sodium alginate film reduced UV transmittance from 59.02% to 2.22% at
280 nm and from 90.29% to 88.33% at 600 nm.
3.2.2 Barrier properties
External factors such as UV radiation, light, oxygen,
moisture, water exchange between food and the environment hampers the quality
of packaged food, therefore barrier properties of packaging film are crucial
for shielding food. Packaging films with barrier properties help to preserve
active compounds and inhibit microorganism growth by reducing oxygen and water
vapor penetration. Recent studies highlight the potential of chitosan films
with natural additives, showing enhanced physical, mechanical and biological
properties, making them promising alternatives to plastic in food packaging.
However, modifications of chitosan films, degree of deacetylation, molecular
weight, production and drying methods, addition of plasticizers and bioactive agents, significantly influence
their barrier properties (Ashrafi et al., 2018). Incorporating propolis
extract into chitosan films reduced water vapor and oxygen permeability, which
further decreased with higher additive concentrations (Siripatrawan & Vitchayakitti, 2016). Some
studies suggest that film prepared using curcumin incorporated with gelatin,
polylactic acid, bacteria cellulose exhibited great barrier properties (oxygen, water vapour, UV) (Roy & Rhim, 2020; Said & Sarbon, 2020; Xie et al.,
2020).
3.2.3
Mechanical properties
The mechanical properties of food packaging are vital
for safeguarding the quality, safety and longevity of packaged food from
mechanical stress. Elongation at break (EB), thickness and tensile strength
(TS) are the key indicators that define mechanical properties in active
packaging films. These attributes are closely linked to the internal structure
and interaction forces within the film matrix, dictating their performance
under stress during application.
In a study, green tea and black tea extract were
blended in chitosan to prepare a composite film, the thickness of the film was
increased from 96.2 to 131.8 micron and 98.4 to 131.0 micron respectively as
the concentration of the extracts was increased from 25% to 100% (Peng et al., 2013). Recent studies suggest
that addition of plant extracts and nanocellulose having higher solid content
increase active packaging film’s thickness by altering the inherent crystal
structure of the film matrix,
thereby expanding the spatial distance
of the film substrate (Mir et al., 2018). In another studies, addition
of 25% green tea extract to chitosan film and 66% TP to calcium alginate
hydrogel film increased its thickness from 71.6 to 94.2 micron and 32.5 to 46.2
micron, respectively (Biao et al., 2019; Peng et al., 2013).
Incorporating 25% pomegranate peel extract
into moong bean protein-based edible film yielded EB (172.96
± 12.05%) and the highest
TS (5.84 ± 0.19 MPa).
The peel's complex fibers,
polysaccharides, starch and pectin enhance film flexibility, molecular
mobility, and strength (Moghadam et al., 2020). Biopolymers like
gelatin (Giménez et al., 2013), carrageenan (Rhim, 2013) etc. when blended with Agar
(extracted from marine red algae like Gelidium and Gracilaria spp.),
produces packaging films with enhanced mechanical and physical properties.
Also, Konjac glucomannan/chitosan infused mulberry anthocyanins extract films
were observed to have significant mechanical properties (Sun et al., 2020).
4. Role of natural
additives in intelligent packaging systems
Intelligent
packaging integrates sensory compounds into polymeric materials to monitor
factors like humidity, microbial contamination, gas exchange, UV exposure,
temperature and pH changes. These compounds trigger color changes to indicate
food freshness, often using natural dyes as indicators.
4.1 Gas
indicators: Various gases interacting
with food can cause oxidative acidity, microbial growth and discoloration.
Optical gas sensors detect gases like ethylene, oxygen, hydrogen sulfide and volatile amines (Pirsa et al., 2022) [94] ensuring food
quality. pH-sensitive dye detectors, like methyl red/cellulose membrane and
curcumin/bacterial cellulose membrane, detect volatile amines in fish, meat and
poultry visible through discoloration (Pankaj et al., 2016; Pirsa et al., 2015). Evolution of gases like
ammonia and CO2 are monitored by pH-sensitive
natural pigments like Anthocyanins and chlorophyll (Prester, 2011).
4.2
Time-temperature indicator (TTI): In
cold chain management, food quality is heavily impacted by its temperature
history. TTIs are affixed to food packages to track temperature's cumulative effect on quality, undergoing irreversible
discoloration in response to
chemical, mechanical or enzymatic changes (Soltani Firouz et al., 2021) Chitosan films
integrated with anthocyanin and chlorophyll act as natural temperature
variation indicator (MacIel et al., 2012) [100].
4.3 pH
indicator: Anthocyanin and curcumin,
natural compounds present in flowers, fruits, berries, and vegetables, serve as
natural pH indicators (Choi et al., 2017; Silva-Pereira et al., 2015).
The protonation or deprotonation of these compounds triggers a modification in
their electronic structure, causing a shift in color. Notably, phytocompounds
such as chlorophyll and β-carotene display color changes as a result of their
significant sensitivity to oxidative species (Brasil Silva et al., 2017; Pénicaud et al., 2011). Black tea and green tea extracts
containing pH-sensitive phytochemicals like chlorophyll, catechins and
theaflavins are incorporated into an intelligent furcellaran-gelatin film to
monitor fish freshness in real-timece(Jamróz et al., 2019). pH-indicator films,
biodegradable cassava starch films infused with basil
extracts, blueberry residue and green tea extracts, utilize chlorophyll and carotenoids from
these extracts to undergo color changes in response to pH variations (Andretta et al., 2019; Medina-Jaramillo et al., 2017).
Some common natural dyes used as pH-indicators are briefly discussed below:
Figure 3. Halochromism of natural pigments. (A)
Curcumin, (B) Anthocyanin, (C) Betalains
(a)
Curcumin: Curcumin is a curcuminoid
derivative obtained from the rhizomes of Curcuma
longa. It exhibits antimicrobial and antioxidant properties, UV protection
and mechanical strength enhancement, along with noticeable colour change. Its
crystal structure consists a seven-carbon chain, α, β-unsaturated β-diketone
moiety, linked to two aromatic rings containing ortho-methoxy phenolic -OH
groups (Sahne et al., 2017).
In acidic pH, curcumin
mainly exists in its bis-keto form, having low water solubility and yellow
colour. With pH modification from neutral to basic conditions, the phenolic
group deprotonates, leading to rapid decomposition. As the pH increases
further, a red color emerges due to
trans-6-(4-hydroxy-3-methoxyphenyl)-2,4-dioxo-5-hexenal formation as primary,
along with feruloyl methane, vanillin
and ferulic acid as secondary degradation products (Noureddin et al., 2019; Typek et al., 2019).
The halochromism of curcumin is given in Figure 3 (A).
(b) Anthocyanin: Anthocyanins, made of aglycone anthocyanidins, mainly
flavylium 3- glucosides or 2-phenyl benzopyrylium cation, undergo pH-dependent
structural changes leading to color alteration (Moradi et al., 2019). Anthocyanins display
intense red color as flavylium cations in highly acidic conditions, fading as
pH rises due to rapid hydration to carbinol pseudobase and chalone.
Deprotonation forms quinonoidal anhydrobase, turning blue at pH 7 and deep blue
at pH 8. As pH increases further, anthocyanin substituent groups degrade,
forming yellow chalcone (Yong et al., 2019).
Anthocyanin extracts
from purple sweet potato in agar and potato starch (pH 2.0-10.0) and from red radish in gelatin-gellan gum (pH
2.0-12.0) functioned as pH sensors (Choi et al., 2017; Zhai et al., 2018).
Additionally, roselle anthocyanins in starch/polyvinyl alcohol films were
utilized for fish freshness monitoring. The halochromism of anthocyanin is
shown in Figure 3 (B).
(c) Betalains: Betalains are water-soluble, red-purple-crimson-coloured pigments
(due to nitrogen in their
basic structure), also known as chromoalkaloids, found in beet and pitaya
plants (Polturak & Aharoni, 2019). There are two
main groups of betalains: yellow/orange betaxanthins, which consist of
condensation products of betalamic acid and amino acids or amines and
red/purple betacyanins, which include betalamic acid (the chromophore) and
cyclo-3,4- dihydroxyphenylalanine (cyclo-DOPA) (Villaño et al., 2015). In acidic to neutral
pH (3-7), betalains remain stable, displaying intense red color. As pH rises to
8-9 and then to 10-12, color shifts to orange
and yellow respectively, and betacyanins degrade into cyclo-DOPA
5-O-(malonyl)-β- glucoside and yellow betalamic acid in alkaline solutions (Herbach et al., 2006). Betalains, susceptible
to temperature, alkaline pH (7), water activity, light, enzymes and oxygen,
serve to monitor the freshness of shrimp and fish (Ardiyansyah et al., 2018). The halochromism of
betalain is given in Figure 3 (C).
Conclusion
The harmful effects of synthetic preservatives and
lack of food safety have raised significant concerns among not only the
research community but also the general public. Consequently, there has been a notable shift in attention towards the potential role of natural additives in preserving food. Over the past few
years, there have been numerous instances where natural additives derived from plants,
herbs, spices, fruits and vegetables, have shown promising antimicrobial and
antioxidant properties for use in active and intelligent packaging systems.
These additives from various sources often serve as functional fillers, while
biopolymers like pectin, whey, chitosan and
soy, serve as the foundation for the polymeric matrix. Although the impact of natural additives on enhancing
the physical and mechanical properties of packaging films has been extensively
researched, their practical
application remains somewhat limited. In intelligent packaging, natural
additives-based sensors and indicators, especially pH sensitive natural colorants like anthocyanins, curcumin, and
chlorophyll are used for tracking the freshness of food materials. However,
their potential as time- temperature indicators, gas indicators and humidity
indicators, remains widely understudied. Despite this, the food industry can
greatly benefit from the application of natural additives due to their
non-toxic nature, environmental friendliness, and compatibility with packaging
films made from polymeric or biopolymeric matrices. Active and intelligent
packaging holds promise for the future, aligning well with food safety
strategies by enhancing safety levels, shelf-life and providing transparency to
consumers. However, Bridging the gap between their laboratory potential and
commercial application is a key challenge for the food packaging industry.
Acknowledgement
We are thankful to the Head of Department, School of
Studies in Chemistry and Director, Center for Basic Sciences, Pt. Ravishankar
Shukla University, Raipur for providing the research facilities. The authors
are grateful to Joint CSIR- UGC for providing financial assistance (NTA Ref.
No. 231610057455).