1. Introduction

Biopolymers are organic polymers and according with its origin are classified, mainly, into three types: biomass-derived polymers (e.g., proteins and polysaccharides including chitosan, starch and cellulose); synthetic polymers derived from oil or biomass monomers such as polylactic acid (PLA), polycaprolactones (PCL) and polyvinyl alcohol (PVA); polymers produced by natural microorganisms or genetically modified (e.g., polyhydroxyalcanoates and bacterial cellulose) [1,6,8,9,10]. In general, bioplastics present some advantages relative to common plastics: namely, they use less energy and are biodegradable, safe and emit less greenhouse gases; provide longer shelf life and are suitable for the production of compost; the chemical structure is more diversified, making possible the customization of the properties of the final package according to the type of food; and possibility of nanoparticles incorporation and adding or improving important properties such as thermic stability, gas impermeability/barrier including to oxygen, antimicrobial properties and strength [4,9]. Regardless of these advantages, bioplastics have some issues concerning its mechanical and barriers properties, which require the addition of additives or other synthetic polymers (e.g., via hydroxypropylation) improving the referred properties [1,7]. Therefore, biopolymers may not be 100% made of renewable sources as some are blends of natural polymers with synthetic polymers or incorporate additives that improve the functional properties of the final package [1].

Biopolymers are used to prepare edible films and coatings, materials with thickness below 0.3 mm that are produced through a blend of biopolymers with different additives dispersed in an aqueous phase, and are safe to eat [5,11,12]. Edible films and coatings are easily manufactured, economically viable [13] and provide mechanical protection and improvement of food quality through moisture regulation and internal equilibrium between gases and solutes [5,12,13]. In addition to that, these materials can protect against UV radiation, fungi and bacterial contamination [5]. Edible films and coatings have the capacity to carry bioactive compounds, such as phenolic compounds, vitamins, nutraceuticals and probiotics, which improve food quality and, at the same time, provide additional health properties to the consumer after food consumption [5,14]. All these characteristics contribute to shelf-life increase [5,12] and a reduction in the use of additives [15]. In general, edible films and coatings are classified into the following: oil/water emulsions and colloidal dispersions [5]. They can be applied by dipping, spreading, spraying and wrapping (Figure 1) [5,12,13]. They can also be classified according to the type of application: edible coatings applied by dipping, spreading or spraying and edible films produced by compression molding, solvent casting or extrusion and that are used to wrap food [5,12].

Figure 1. Schematic representation of coating techniques: (a) spraying; (b) dipping; (c) spreading and (d) wrapping (adapted from [16]).

In the food industry, food is submitted to several treatments (e.g., heat treatment, pH reduction, salting and drying) in order to prevent fungal and bacterial growth contamination, and synthetic antioxidants, namely butylated hydroxyanisole (BHA) and butylated hydroxytoluene (BHT), are frequently added to prevent lipid oxidation [14,17]. However, nowadays, consumers tend to prefer food products that are minimally processed and that are healthier [14,18,19]. In this line, the scientific community evaluated the possibility of incorporating bioactive compounds (antioxidants, antimicrobials, flavours and probiotics) in food packaging, mainly in edible films and coatings to prevent microbiological contaminations and/or oxidative processes [14,18].

2. Gums Used in Food Packaging

Gums are complex polysaccharides of high molecular weight composed of glucose, fructose, mannose and other sugars [22], and they have the ability to form gels or mucilages [23]. They are also called hydrocolloids because of their solubility in water [23,24]. Gums are chemically inert, biocompatible, non-toxic, odourless and widely available in the nature [24]. Due to their chemical structure and properties, gums are used in the industry, namely in cosmetic [25], pharmaceutical [26] and food industry [24,25,26,27] as gel, thickener, moisturizer, emulsifier, stabilizer [23], coating agents and packaging films [27]. Natural gums are derived from plants, bacteria and animals [23]. In the vegetable kingdom, gums provide protection against microbial or mechanical injury [24,25] and can be found in plant seed endosperm (e.g., guar gum) [24,25]; plant exudates [24,25], shrubs or trees; and algae extracts (agar) [23]. The gums xanthan, pullulan, curdlan, gellan and cellulose are microbial gums, and according to their physicochemical properties, they are used to form gels (e.g., curdlan and gellan), thickeners agents (xanthan and pullulan) and film solutions (e.g., cellulose, pullulan and gellan) [22]. Examples of tree gums extruded are Arabic gum, tragacanth gum, ghatti gum, Persian gum, karaya gum and cashew gum [27]. Locust bean, guar gum (Figure 2), tara gum, basil gum, flaxseed gum, psyllium gum and Barhang gum are from seeds [27]. Konjac mannan is a tuber gum [27].

Table 1 summarizes some of the gums used as food coatings or films and their properties as edible food packaging. In a general manner, most of the edible coatings of gums have antimicrobial properties, inhibiting its growth, preventing weight and moisture loss, providing firmness and preserving the organoleptic properties of the food, which, in this manner, contributes to quality maintenance and shelf-life extension. According with the data compiled in Table 1, xanthan, Arabic and guar gums are the most used gums in producing edible packaging. Moreover, dipping and immersion are the most commonly used food application methods.

3. Starches Used in Food Packaging

Starch is produced by plants, some strains of fungi and algae and is used for energy storage [63]. It comprises amylose and amylopectin polymers (Figure 3) [7,64,65] made up of α-D-glucose monomers [7,64].

In the food industry, starch is used in food packaging as coatings or films produced mainly by extrusion and casting [2]. In the manufacture of starch edible coatings, amylose is preferred over amylopectin [66]. Starch based films are low cost, abundant, tasteless, colourless and odourless with very good oxygen barrier properties (due to its compact structure and low solubility) contributing to the improvement of food preservation and, consequently, its shelf-life extension [65,66,67]. In addition, starch is biodegradable and edible [67]. However, starch presents some disadvantages due to its hydrophilic nature caused by hydroxyl groups and formation of hydrogen bonds among starch molecules [66]. The hydrophilic nature of starch-based films is responsible for moisture absorption, resulting in swelling of the starch matrix, disruption of H bonds and increase in free volume within the starch-based film matrix. These events cause the weakening of moisture and gases barriers and low water vapour barrier capacity [7,68]. In addition, starch films have poor mechanical properties, resulting in the use of additives, including plasticizers (e.g., glycerol and sorbitol) [68] and hydrophobic substances [67] in order to improve these properties [64]. Although starch is relatively cheap, the incorporation of others substances to improve the film matrix makes starch films more expensive [64].

Table 2 summarizes the uses of starch-based edible films and its benefits in food packaging. Among the described starch-based coatings and films compiled in Table 2, cassava starch is the most commonly used. Starch-based coatings and films are mainly applied in highly perishable foods such as fruits and vegetables.

4. Methods of Incorporation Phenolic Compounds in Gums and Starch-Based Coatings and Films

The incorporation of phenolic compounds must consider final packaging characteristics, the bioactive substances used and their application in foods. To produce active films, the incorporation of bioactive compounds can be made by mixing them directly with films production or by encapsulation resulting in nanoparticles formation [88]. The usual methods for starch films production are by casting, pressing and extrusion [88]. The solvent casting method is the most used for the incorporation of phenolic compounds in polysaccharide films or coatings [89]. With respect to the formation of film, the polymeric solution is spread on the intended surface and posteriorly air dried in a ventilated oven. After solvent evaporation, the film is removed from the surface maintaining the surface form. However, the removal of film surface can cause it to wrinkle and tear. The physico-chemical properties of films depend on the composition of casting solution, wet casting thickness, temperature and relative humidity conditions [89]. The extrusion method is based on the thermoplastic properties of biopolymers. The biopolymer solution contains a plasticizer that is heated above its glass transition temperature under reduced water conditions. Posteriorly, the films are submitted to compression, casting and extrusion, resulting in the formation of more elastic films [89]. The applications of coatings, thickness and their ability to adhere to food surface are important characteristics for food quality preservation. Therefore, coatings can be applied by spraying, dipping and spreading [89].

Active starch and gum-based films and coatings were prepared mainly by casting method. Glycerol was the most used plasticizer in the preparation of active coating and films [90,91,92,93,94]. In general, the percentage of bioactive compound or extract in starch and gum-based coatings is low, ranging between 0.03 and 1.20%, and all the extracts and isolated bioactive compounds tested acted at the level of lipidic oxidation and microbiological contamination, preventing food spoilage and increasing its shelf-life.

5. Phenolic Compounds Incorporated in Gums and Starches for Preparing Active Films

Antimicrobial activity of polyphenolic compounds is generally related with the induction of the leakage of cellular content; blocking DNA/RNA/protein synthesis; and, consequently, their function; modifications on membrane potential; disruption of membrane structure and function and induction of cytoplasmatic acidification; and coagulation of cytoplasmatic constituents [112]. On the other hand, most polyphenols present antioxidant properties as they can act against oxidative species such as Reactive Oxygen Species (ROS) through radical scavenging and also prevent ROS generation by iron chelation, presenting antioxidant properties [113]. In addition to its biological properties, many polyphenols have the capacity to enhance physico-chemical properties of the film or coating packages such as enhancement of its water/vapour barriers, tensile strength, colour, solubility and organoleptic characteristics [100]. Essential oils also have antimicrobial activity against bacteria and fungi. When added to films or coatings, due to its volatile compounds content, essential oils can also modify the organoleptic characteristics of foods (e.g., unpleasant odour and flavour). However, comparatively to plant extracts, the production of essential oil has lower yields and, consequently, less economic value. Thus, essential oils applications in food packaging are limited [112].

There are several studies concerning the incorporation of polyphenols in films containing polysaccharides. Cassava starch films with green tea and aqueous palm oil extracts were developed and applied in butter. The active edible starch films presented antioxidant activity decreasing the peroxide index, but in high concentrations of green tea extract polyphenols can act as pro-oxidants. Moreover, green tea and palm oil extracts reduced tensile strength and water vapour barriers [90].

Mango peel powder, a by-product, was added to starch edible films and applied on apple slices storage at a temperature of 4 °C. The phenolic compounds present in mango peel powder were responsible for higher antioxidant activity. The mango peel extract also increased the resistance of edible films, maintaining the film structure as almost intact [114].

In order to preserve acidic foods and drinks (e.g., wine and juices), chitosan/genipin active edible films were developed. The authors fractionated the wine extract in three fractions and produced three active edible chitosan films: chitosan films with phenolic compounds mixture (PCM), chitosan films with anthocyanins (AN) and chitosan edible films with phenolic acids (PA). The qualitative analysis of phenolic acids fraction showed the presence of gallic ((6), Figure 5), p-coumaric ((3), Figure 5), trans-caftaric ((7), Figure 5), p-hydroxybenzoic ((5), Figure 5), caffeic ((1), Figure 5) and chlorogenic acids ((4), Figure 5), (+)-catechin ((23), Figure 5) and (−)-epicatechin ((24), Figure 5). The anthocyanins fraction was mainly comprised malvidin-3-glucoside ((19), Figure 5). PCM and AN active edible chitosan/genipin films showed lower solubility and, therefore, lower weight loss. Concerning antioxidant activity, PA chitosan films were the most active followed by anthocyanins and phenolic compounds mixture. Thus, this active chitosan film has potential as an acidic food preservative [115].

6. Intelligent and Active Starch/Gums Films with Phenolic Compounds

While active polysaccharides films have an active function for enhancing food quality and preventing food spoilage, intelligent polysaccharides films monitor food conditions inside packaging, providing information about food safety and quality [125]. Unlike active packaging, intelligent packaging does not release its compounds into food [126]. Bio-based smart packaging present environmental and economic advantages and allow real-time monitorization, assuring food safety and quality [125]. Intelligent packaging can be classified as indicators, sensors and data carrier. Indicators provide visual, qualitative or quantitative information about food to colour change or dye diffusion. Changes in pH and temperature are responsible for food packaging colour change and can be identified by indicators such as freshness, microbial spoilage, gases, integrity and time-temperature indicators (TTI). The most common indicators are based in pH changes due to metabolite production such as n-butyrate, L-lactic acid, D-lactate, acetic acid and volatile amines [126]. The second type of intelligent packaging is sensors, which detect and register information about biological reactions. The most common sensors are biosensors that use biological or organic material that recognize the respective analyte. Some biosensors are currently commercialized to detect food pathogens. Food Sentinel System (SIRA Technologies Inc.) is a biosensor used to detect the presence of pathogens through the formation of a dark colouration band. ToxinGuard^® (Toxin Alert, Canada) is a biosensor that detects specific microorganisms such as Salmonela spp., E. coli, Listeria spp. and Campylobacter spp. The third type of smart packaging includes data carriers. Data carriers do not provide quantitative or qualitative information about food products. They are responsible for identification, automation, anti-theft prevention and traceability of food products. Foods can be identified through radiofrequency identification labels (RFID), bar and QR codes [126].

Polyphenolic compounds and others present in plant extracts (e.g., chlorophyll and carotenoids) have a proper colour that, due to pH variations, change to another colour [125]. However, the type of phenolic compounds chosen is important for intelligent film packaging development due to their physico-chemical properties and pH-sensitive capacity. Among phenolic compounds, anthocyanins and curcumin are more suitable for smart packaging development [127].

Polysaccharide films are a good alternative to common packaging, being healthier, eco-friendly and economical. In order to enhance food quality and safety, phenolic compounds could be used as an alternative to decrease or substitute synthetic preservatives, as well food indicators. However, polysaccharide matrices present some disadvantages relative to common packaging such as instable tensile strength, water-vapour barriers, solubility and others. The addition of phenolic compounds, in some cases, improves the physico-chemical properties of film matrix. In addition, in order to enhance film resistance, plasticizers are added. Polyphenols are obtained from natural sources but it is also from by-products (from agricultural wastes, for example) [130], where the latter is more economic. The diversified structures of polyphenols allow them to have two fundamental roles in edible packaging: used in active edible packaging due to their biological properties, namely antioxidant and antimicrobial; and in intelligent packaging, such as pH indicators, controlling and monitoring the quality and safety of food. Moreover, polyphenols have the capability to modulate the physico-chemical properties of starch and gums films. In some cases, they improve the mechanical characteristics of films as well barriers properties. Most phenolic compounds have no toxicity in human health [135]. On the other hand, phenolic compounds are unstable with respect to variations in pH, temperature and UV light exposure; thus, care must be taken when preparing extracts and incorporating them into films. The studies described in this review also show that some polyphenols can act as pro-oxidants when they are above certain concentrations.

8. Conclusions

Natural polysaccharides are good alternatives to conventional packaging as they are generally more economical, environmentally friendly and biodegradable (at least to some extent). These polysaccharides form an edible matrix capable of coating food, thereby reducing the use of synthetic and non-biodegradable packaging. Given the demand for healthier and less processed foods, the addition of natural compounds, such as phenolic compounds, can reduce or replace synthetic additives, preventing lipid oxidation and contamination by microorganisms, improving the preservation of food quality and extending the expiration date. In addition to these biological properties, phenolic compounds also modulate the physical and chemical properties of films, which can improve or worsen them. More studies are needed in order to improve the physical properties of edible films or coatings. Active films or coatings containing polyphenols are an alternative with less environmental impact and also healthier for human consumption.

9. Future Perspectives

More studies are needed to improve the physico-chemical characteristics of films without plasticizer addition. Due to their diversity, phenolic compounds must be studied in order to improve the physico-chemical structure of polysaccharide films and achieve better biological properties. Polyphenols present in plant extracts are mostly glycosylated. Some studies mentioned here refer to genins as active compounds in the tested films. Thus, it is important to assess whether glycosylation and its degree interfere with the biological properties (antioxidant and antimicrobial) and physicochemical characteristics of the films.

This entry is adapted from the peer-reviewed paper 10.3390/coatings11111393