Films and edible coatings containing antioxidants – a review

The incorporation of natural antioxidants into fi lms and edible coatings can modify their structure, improving their functionality and applicability in foods, such as in fresh-cut fruits. This paper reviews the more recent literature on the incorporation of antioxidants from several sources into fi lms and edible coatings, for application in fruits and vegetables. The use of synthetic antioxidants in foods has been avoided due to their possible toxic effects. Instead, a wide range of natural antioxidants (such as essential oils and plant extracts, as well as pure compounds, like ascorbic acid and α-tocopherol) have been incorporated into edible fi lms and coatings to improve their bioactive properties. Films and coatings containing added antioxidants help to preserve or enhance the sensory properties of foods and add value to the food products by increasing their shelf life.


Introduction
The greatest hurdle of the food industry is the limited shelf life of food products, a consequence of oxidation reactions such as degradation, enzymatic browning, and oxidative rancidity (SOLIVA-FORTUNY and MARTÍN-BELLOSO, 2003).One approach to reduce food deterioration is to use edible fi lms and coatings.
Edible fi lms or coatings constitute thin layers of material that are suitable for consumption and which act as a barrier against different agents (water vapor, oxygen, and moisture).They help to improve the quality and extend the shelf life of fresh and processed foods.The addition of active compounds, such as antioxidants, to these fi lms and coatings can enhance their functional properties and make them potentially applicable in food preservation (SÁNCHEZ-GONZÁLEZ et al., 2011).Indeed, antioxidants can bind free radicals to protect materials against oxidation processes, regardless of the action mechanism (POKORNÝ, 2007a).
Results presented by the aforementioned authors have suggested that incorporation of antibrowning agents into edible coatings maintains the quality properties of the food.Nevertheless, the overall quality and the antioxidant activity resulting from this incorporation have not been widely studied.
This work aimed to review the information available on the use of edible fi lms and coatings as carriers of antioxidant compounds to improve the quality, safety, and functionality of fruits.It will identify the state-of-the-art of this innovative approach to food technology as well as discuss perspectives in this area.

Antioxidants: compounds, action mechanisms, and assays
Antioxidants comprise substances that can protect materials (not only foods) against autoxidation irrespective of the action mechanism (POKORNÝ, 2007a).These compounds can be classifi ed as primary or secondary antioxidants, depending on the action mechanism.Some http://bjft.ital.sp.gov.brBraz.J. Food Technol active compounds.An interesting alternative that may confer functional properties to such materials is to add the antioxidant as a pure compound, like ascorbic acid, citric acid, resveratrol, or tocopherol.These are generally the compounds of choice, because they constitute antioxidant models, supplement the diet, and protect the sensory and nutritive quality of the food itself (LEÓN and ROJAS, 2007).
The literature contains little information on how incorporation of compounds like resveratrol, ascorbic acid, α-tocopherol, butylated hydroxytoluene (BHT), and butylated hydroxyanisole (BHA), among others, affects fi lm properties.However, their antioxidant activity (and in some cases their antimicrobial properties) has been extensively studied by physicochemical methods.For instance, ascorbic acid avoids enzymatic browning of fruits by reducing the o-quinones originating from the action of polyphenoloxidase enzymes.Unfortunately, after complete ascorbic acid oxidation to dehydroascorbic acid, quinones can accumulate again and undergo browning (ROJAS-GRAÜ et al., 2008).
Table 1 shows recent studies about films and coatings containing pure antioxidant compounds and highlights the implications of adding antioxidants to the materials.In most of the cases, the antioxidant capacities of the fi lms are proportional to the concentration of the active compound in the fi lm, without notable activity loss during fi lm formation and conditioning (PASTOR et al., 2011;NORONHA, 2012).Nonetheless, very diverse effects emerge upon addition of these compounds into a polymeric matrix, as verified by microstructural, mechanical, barrier, and optical properties, as well as antioxidant capacity (BASTOS et al., 2009;DE'NOBILI et al., 2013;JIMÉNEZ et al., 2013).On the other hand, effects like the cross-linking between the active compound and the polymer could also arise, to improve fi lm properties.Acids, such as ascorbic and citric acids, and polymer chains, among others, reduce the oxygen permeability in fi lms, which could protect the material against oxidation (ATARÉS et al., 2011;FABRA et al., 2011;HAN and KROCHTA, 2007).
It is crucial to evaluate compound stability in the fi lms during storage.One way is to determine the percentage of antioxidant retention in the fi lm under adverse light, relative humidity, and temperature conditions.Some works have verifi ed that ascorbic acid is 100% retained after fi lm casting; however, the degradation the FRAP ferric ion reduction to ferrous iron.The ABTS assay, it is applicable in both aqueous and lipid phases.ABTS discoloration provides information on the antioxidant activity of the natural products.The discoloration can be measured on the basis of the reduction of the radical cation, as the percentage inhibition of the absorbance at 734 nm (MOON and SHIBAMOTO, 2009).
The rapid ORAC assay provides results that often coincide with the total phenols as determined by the Folin-Ciocalteu reagent (BERGER et al., 2011).The phenolic content can function as an indicator of the antioxidant capacity; it fi nds application in the preliminary screening of any product intended as a natural source of antioxidants in functional foods (VIUDA-MARTOS et al., 2011).On the other hand, high phenolic content can indicate polyphenol oxidase activity, which underlies oxidative processes, such as fruit browning.
Enzymatic reactions that change the color of products impact the commercialization of fresh-cut fruits.In addition, cutting the fresh fruit can modify it in undesirable ways, to alter the fl avor and smell as well as the fi rmness of fruit tissues (MARTÍN-BELLOSO et al., 2007).These changes originate from the enzymatic browning that occurs after peeling and cutting of the fruit in the presence of oxygen, a result of the polyphenol oxidase activity mentioned above.To tackle this problem, it is necessary to employ a browning inhibitor; e.g., an antioxidant, to prevent development of a brown coloration (ZAMBRANO-ZARAGOZA et al., 2013).

Application of antioxidant fi lms and coatings
One way to control fruit browning is to immerse the sample in antioxidant solutions after peeling or cutting.This methodology relies on modifi ed atmosphere packaging and storage at low temperature, to increase product shelf life (BALDWIN et al., 1995).Films and edible coatings can also enhance the shelf life of fresh-cut fruits.Further addition of antioxidants to the formulation of fi lms and coatings can improve the preservative function, inhibit browning, and reduce the undesirable effects of nutrients oxidation (PASTOR et al., 2011;BONILLA et al., 2013).
Before adding antioxidants to fi lms and coatings, it is necessary to evaluate not only their antioxidant capacity, but also how they infl uence (i) the properties of the materials into which they are being incorporated, such as the retention power, and (ii) the characteristics of the food product, like flavor, color, and chemical modifi cations.Tables 1, 2 and 3 present the composition, added antioxidant, method of measurement, and the main results of several studies on antioxidant fi lms and coatings.

Pure compounds
New trends in edible fi lms and coatings have aimed to develop their functionality through incorporation of  -Ascorbic acid -Spectrophotometric method The initially determined AA concentration was 3.00 g of AA/100 g of fi lm, which accounted for 100% AA recovery after casting.AA was the least stable in 80% methylated pectin, with higher retention in 50% and 70 % methylated pectin networks.

Total phenolsspectrophotometric method
Antioxidant activity -ABTS assay In fi lms containing 1.5 and 3.0 mg.cm -2 phenolic, the total released GA was 1.6-and 1.9-fold higher than total released CAT, respectively.The trolox equivalent antioxidant capacity of total GA released from fi lms containing 1.5 and 3.0 mg cm -2 phenolic compounds was 3.6-and 4.1-fold higher than those of total CAT released from the corresponding fi lms, respectively.

Arcan and Yemenicioğlu (2011)
Calcium alginate-Capsul Ascorbic acid -Ascorbic acid -Titration method The antioxidant model was stable for fi ve months when incorporated in these fi lms stored at refrigeration in the dark and when stored at room temperature it was maintained for three months, thus suggesting that the fi lm protected the antioxidant effi ciently, mainly from the adverse conditions of light.-Antioxidant activity -ABTS assay All the fi lms containing cinnamon showed higher antioxidant activity.The higher the cinnamon content in the dry fi lm the greater the antioxidant power.OA addition did not signifi cantly affect the antioxidant activity of cinnamon in the fi lms.However, there was greater retention of the compounds from the cinnamon essential oil during fi lm formation and handling when OA was present in the formulation.GSE, the phenols content was 17 times greater than that of the control.The results also showed that ZEO incorporation into GSE formulated fi lms, signifi cantly decreased the TP of the fi lm (P < 0.05).Furthermore, the results revealed that chitosan+ ZEO and, to a greater extent, chitosan+ GSE contained more phenolics capable of quenching free radicals, to give more stable products.Total phenolsspectrophotometric method Antioxidant activity -DPPH assay HPMC coatings prevented weight loss and browning of moscatel table grapes during cold storage, while improving their gloss and microbial safety and controlling the increase in oxygen consumption.Nevertheless, the incorporation of propolis did not signifi cantly affect grape quality preservation during storage.Total phenolsspectrophotometric method A pure oxygen pretreatment combined with a chitosan coating that included rosemary afforded the lowest rates of browning, softening, and sensory degradation in the pear wedges after 3 days of storage at 20 °C.In addition, the combined treatment effectively reduced membrane permeability, vitamin C loss and weight loss by maintaining low pH and high L and h values in the fresh-cut pears.

Xiao et al. (2010)
Films and edible coatings containing antioxidants -a review EÇA K. S. et al.
http://bjft.ital.sp.gov.brBraz.J. Food Technol toxicity issues, and possible changes in the organoleptic properties of the food -have limited their use in food preservation.A strategy to solve this problem has been to incorporate essential oils into edible fi lms and coatings.It is possible to minimize the required doses by encapsulating them into the polymer matrix, which limits their volatilization, controls their release (thereby reducing the negative impact of these ingredients), and preserves the quality and safety attributes of fresh-cut fruits and vegetables (SÁNCHEZ-GONZÁLEZ et al., 2011;BONILLA et al., 2013;RUIZ-NAVAJAS et al., 2013).
Table 2 lists many publications on essential oils incorporated into coatings or fi lms prepared from biopolymers of several sources.Tongnuanchan et al. (2013) studied the antioxidant properties of the film prepared from fi sh skin gelatin incorporated with essential oils from roots (ginger, turmeric, and plai), to show that these fi lms display higher antioxidant activity than the control fi lm.Perdones et al. (2014) found that chitosan films containing cinnamon leaf essential oil exhibit higher antioxidant activity.Ruiz-Navajas et al. ( 2013) also produced chitosan fi lms and used the DPPH and FRAP methods to demonstrate that films containing Thymus piperella essential oil present higher antioxidant activity than fi lms containing Thymus moroderi essential oil.The antioxidant activity thus depends on the type of essential oils and results from the structural features of the molecules, mainly the reactivity of the hydroxyl groups present in the compounds.Concentration, temperature, light, substrate type, physical state of the system, and microcomponents acting as pro-oxidants or synergists also impact the antioxidant action.Furthermore, various antioxidants can interact with the fi lm matrix in different ways, to release the free antioxidant in the essential oils through diverse mechanisms (ČÍŽ et al., 2010;TONGNUANCHAN et al., 2013).Ponce et al. (2008) demonstrated that butternut squash containing chitosan coatings enriched with oleoresins improves the antioxidant protection of the fresh-cut squash, preventing the browning reactions and the consequent quality loss in fruits and vegetables without adversely affecting their sensory acceptability.However, the addition of antioxidants to fi lms does not always enhance the antioxidant properties.Indeed, Atarés et al. (2010) described that incorporation of cinnamon and ginger essential oils into sodium caseinate fi lms does not elicit any antioxidant effect as compared with the sodium caseinate fi lm without essential oils, even though cinnamon essential oil alone possesses high antioxidant potential.
Besides their high antioxidant capacity, essential oils can also improve the water barrier properties of the fi lm because they display the hydrophobic nature characteristic of lipids (ATARÉS et al., 2010).Several of this acid increases with higher relative humidity (LEÓN and ROJAS, 2007;DE'NOBILI et al., 2013).In contrast, Bastos et al. (2009) compared the influence of light and temperature, to conclude that both parameters are important during ascorbic acid degradation.Degradation becomes signifi cantly faster upon a temperature rise of only 15 °C, whereas light impacts this reaction only slightly.
Results from different studies have pointed out that functional edible fi lms containing added pure antioxidant are potentially applicable in food products that are sensitive to oxidative processes, to prolong their shelf life.Investigations have focused on improving several coatings and render them carriers of pure compounds.Such coatings have proven to effi ciently maintain the quality properties of different foods (Table 1), but most of the employed antioxidants can still undergo rapid degradation due to oxidative processes (PIERUCCI et al., 2004).
Among the biopolymers used to formulate coatings, alginate, hydroxypropylmethylcellulose, pectin, and gellan are interesting options: they are odorless, tasteless, and biodegradable (KROCHTA and DE MULDER-JOHNSON, 1997).In the case of fruits and vegetables, the edible coatings carry antibrowning agents (SOLIVA-FORTUNY and MARTÍN-BELLOSO, 2003;CHIUMARELLI et al., 2010).Robles-Sánchez et al. (2013) and Oms-Oliu et al. (2008), who worked with minimally processed fruits, observed that the addition of antioxidants signifi cantly impacts the overall quality of fresh-cut fruits.These compounds effectively reduce bioactive compounds loss (ascorbic acid, polyphenols), to keep the natural color of the fruits and increase their antioxidant potential.

Essential oils
Consumers have been demanding the use of fewer chemicals in minimally processed fruits and vegetables.Hence, the search for naturally occurring substances that can act as alternative antioxidants is essential.Antioxidants can prevent sensorial and nutritional quality loss and improve lipids stability, to lengthen the shelf life of food products (PONCE et al., 2008).
Essential oils are aromatic, natural antioxidant, and antimicrobial substances extracted from vegetables by physical means.They consist of a complex mixture of natural compounds; most of them contain a mixture of terpenes, terpenoids, phenolic acids, and other aromatic and aliphatic compounds, but their composition may vary depending on their origin.Because essential oils can lower lipid oxidation, their presence in food products could extend the shelf life (TONGNUANCHAN et al., 2013;PERDONES et al., 2014).
Essential oils exhibit great antioxidant potential and are classified as Generally Recognized as Safe (GRAS).However, some of their features -intense aroma, with gelatin,to reduce free hydrogen.On the other hand, works have revealed that extract incorporation in fi lms may generate a heterogeneous surface with numerous small pores (NORAJIT et al., 2010), which could account for the high water vapor permeability of the incorporated fi lms.
In the same way that natural extracts can be successfully incorporated into biodegradable fi lms, the use of edible coatings in fruits and vegetables could improve food quality and shelf life.However, light may degrade the active compound during storage and deteriorate optical properties like luminosity.Nevertheless, some papers have shown that this technology can better control weight loss and respiration rates, allowing for longer storage time as compared with samples without coating (PASTOR et al., 2011;SUPAPVANICH et al., 2012;DAS et al., 2013).

Conclusion
Coatings and fi lms containing antioxidant agents constitute a natural and biodegradable alternative to chemical preservatives, by acting as protective barriers and extending foods shelf life.The addition of antioxidant compounds to edible fi lms and coatings can increase food safety and quality by inhibiting deterioration reactions of the food materials.
Determination of the antioxidant capacity helps to evaluate the antioxidant potential status of the food tissue, which is a function of the type and amount of bioactive compounds present in the material.Research has indicated that applying edible coating containing antioxidants to fresh-cut fruits effectively reduces browning while increasing the antioxidant capacity of the coated or packed food.authors have reported that the antioxidant power of a biodegradable fi lm containing essential oils is proportional to the amount of added essential oils; in other words, the antioxidant activity rises with increasing essential oil concentration in the fi lm (GÓMEZ-ESTACA et al., 2009b;MORADI et al., 2012;SHOJAEE-ALIABADI et al., 2013;TONGNUANCHAN et al., 2013;JOUKI et al., 2014).

Extracts
Because synthetic antioxidants have raised some safety concerns and regulatory agencies have restricted their use as food additives, researchers have targeted fi lms containing antioxidant agents from natural sources such as natural extracts (MURCIA and MARTÍNEZ-TOMÉ, 2001;DE'NOBILI et al., 2013).These extracts should also contribute to nutritional and quality aspects without impacting the food product integrity (GUILBERT et al., 1996).
Extracts like tea extracts (DAS et al., 2013;LI et al., 2014), fruit and vegetables extracts (AKHTAR et al., 2012;SUPAPVANICH et al., 2012), ginseng extract (NORAJIT et al., 2010), plant extracts (GÓMEZ-ESTACA et al., 2009b), andpropolis (PASTOR et al., 2011) possess excellent antioxidant activity, can retard lipid oxidation, and improve the quality and shelf life of various food model systems in different ways (Table 3).The antioxidant activity of these extracts results mainly from phenolic compounds and their synergistic, antagonistic, and additive effects (KROCHTA and DE MULDER-JOHNSON, 1997).However, despite their strong scavenging activity and ability to protect food products, they are still less active than synthetic antioxidants.
Many authors have looked into the different functionalities of antioxidant extracts.Recently, fruit and vegetable extracts have been considered for application as natural bioactive additives for their coloring potential, pharmaceutical activities, and bioactivity, regarding aspects of hygiene, nutrition, and environmental consciousness (AKHTAR et al., 2012).Several studies on antioxidant and antiradical extracts that confer color to fi lms have been published (GÓMEZ-ESTACA et al., 2009a, b;NORAJIT et al., 2010;AKHTAR et al., 2012;BITENCOURT, 2013;LI et al., 2014).In addition, it has been well documented that extracts exhibit coloring and antioxidant properties that, in some cases, enable good control against photo-oxidation through reduced light transmission, especially UV radiation (PASTOR et al., 2013;NORAJIT et al., 2010;LI et al., 2014).
In general, the physical properties of the fi lm, like moisture content and water solubility, remain unaltered upon the addition of extracts, because the extract and the fi lm matrix interact well.Li et al. (2014) analyzed gelatinbased fi lm incorporated with tea extracts through FTIR and verifi ed that the extracts establish hydrogen bonds

Films
and edible coatings containing antioxidants -a review EÇA K. S. et al.
of fi lms containing only α-tocopherol was not statistically different at studied storage periods.However, incorporation of oleic acid decreased the antioxidant capacity partly because it promoted oxidation reactions, due to increased oxygen radical scavenging activity was similar to that exhibited by the freshly prepared α-tocopherol solution.However, the high α-tocopherol content conferred the fi lm an oily aspect.
The CAS content in the fi lm affected the initial radical-scavenging activity (RSA).During storage, RSA was satisfactorily stable.The surface RSA of NaCAS fi lms containing phenolic compounds increased with storage time due to plasticizing and perhaps alteration of the NaCAS and CAS networks.developed with 70% β-carotene nanoparticles showed low antioxidant capacity.However, this activity was signifi cantly (p ˂ 0.05) higher than that of the control treatment using methylcellulose Release and antioxidant activity analyses showed higher values for the samples containing the lecithin formulation.The fi lms had low values of α-tocopherol release in ethanol solution.The fi lm matrix was stable over the analysis time.For both assays, the fi lm control had no radical scavenging activity.The results showed that the DPPH and ABTS scavenging activity of the fi lms signifi cantly increased (p < 0.05) with NCs concentration.The 70% NCs fi lm exhibited the higher radical scavenging activity (DPPH -56.21%,ABTS -223.5 TEAC µMol/g).Noronha (2012)Films and edible coatings containing antioxidants -a review EÇA K. S. et al.http://bjft.ital.sp.gov.
of the fi lm containing thyme essential oil was higher than that of the fi lm containing basil essential oil.The compounds lost their antioxidant capacity during fi lm formation and the extraction procedure, probably due to their volatilization during fi lms exhibited some antioxidant activity, which was signifi cantly improved upon addition of the essential oils.The fi lms containing coriander and citronella oils considerably increased the DPPH radical-scavenging capacity.Pires et al. (2013)Fish skin gelatin Root essential oils of ginger, turmeric and plai -Antioxidant activity -DPPH and ABTS assays Films incorporated with turmeric and plai essential oils showed higher antioxidant activity than those incorporated with ginger, as attested by both DPPH and ABTS methods.FIC assays At all the assayed concentrations, the TMEO fi lms showed lower (p < 0.05) antioxidant activity than the TPEO fi lms, as attested by both the DPPH and FRAP methods.The same behavior was observed for total phenols content.that the DPPH-scavenging activity and total phenols content of the fi lms increased signifi cantly (P < 0.05) with larger SEO concentrations, an effect that was greatly improved upon addition of 3% (v/v) SEO.lms incorporated with 10 gL -1 lms exhibited some antioxidant activity, improved by addition of 0.25 mL of thyme oil/g of protein.lms effectively protected the sunfl ower oil against oxidation, probably due to their low permeability to oxygen at the low relative humidity of the surrounding atmosphere.activity increased signifi cantly (p < 0.05) as the added oil concentration rose.The coating treatments signifi cantly affected (p < 0.05) the total phenolic and fl avonoid content as well as the antioxidant capacity of fresh-cut peach.
The high AA content retention after 20 days of storage can be attributed to the effect of phenolic substances in the coating.core extract more effectively retarded fruit browning as compared with the other extracts.Regarding browning inhibition from using KG + PE, this treatment led to the lowest PPO and POD activities and the highest total phenols content.

Table 1 .
Coating and fi lms incorporated with pure compounds.

Table 2 .
Coating and fi lms incorporated with essential oil.

Table 3 .
Coatings and fi lms incorporated with extracts.