1. Introduction
Packaging is an important step in protecting the sensory, nutritional and hygienic properties during storage and marketing of food. Packaging is the wrapping of goods or putting them in containers with protective materials to preserve the goods, enhance their performance and fulfill their informational function. In order to maintain food quality and safety in the period between production and consumption: Glass, paper, cardboard, cardboard, aluminium and various plastics can be used as packaging materials. All of these packaging materials lead to chemical migration into food at different rates depending on their properties (Pascall et al., 2014). By migration, harmful substances from packaging material get into food and therefore require an alternative packaging material that is as safe as the food.
Packaging material is divided into two groups: synthetic and natural. Synthetic packaging is usually petrochemical-based and although it is effective in preserving the product and is mostly preferred in the industry, it is recommended to reduce the use of this packaging, due to pollution and migration issues (Zuhal et al., 2018). Edible wraps derived from natural resources have been offered as an alternative to these wraps and can be used in various foods such as fruits and vegetables, dried nuts, meat and meat products, cereals and dairy (Otoni et al., 2017). Fruits and vegetables can spoil quickly, as the water content of these products is high, leading to respiration and perspiration. Therefore, the packaging of these is important.
2. History of Biodegradable Films
The history of the use of edible films and coatings may seem recent as it has been in the past 50 years, but the use of edible films as a coating dates back to the 12th and 13th centuries. Waxes were applied to oranges and lemons in China to retard water loss during transport and storage in the 12th century. In the early 15th century, the first edible film formations were made in Japan through the use of soy milk proteins after pan cooking and further air drying called yuba films (Erkmen et al., 2018). Like waxing, spiking fruit, vegetables, meat and fish was common in 16th century in England to prevent moisture loss. In the 19th century, the first US patent was granted for gelatin films to protect several meat products (Choi et al., 2001). Sucrose and sugar derivatives have been used as a protective coating on nuts to prevent oxidative rancidity by limiting gas transport through edible coatings. Commercial wax and lipid coatings were applied to fruits and vegetables in the 1930s, while allowing natura l respiration and limiting dehydration during shipment. At the beginning of the 20th century, during the First and Second World Wars, the high demand for textile products from protein-based agricultural materials for the manufacture of textile products for soldiers such as uniforms, blankets, etc. increased the production rate of commercialized protein on a wool substitute basis. Casein, peanut protein, soy and corn zein have been used to make commercial textiles, buttons, boxes and umbrella handles. The increased demand leads to a search for cheaper materials such as petroleum-based products for the manufacture of food packaging (Chakravartula et al., 2019). In 2005-2006, the cost of packaging materials increased, due to the dramatic increase in the cost of petroleum. Currently, from 1990 to present, more than 90 patents and 220 scientific research papers have been published sharing the same concerns for food packaging; Limiting water vapour transmission and carrying functional properties in the films such as antioxidants, antimicrobials, vitamins and flavour compounds. Today, increasing consumer quality demands for fresh and safe food materials and healthy packaging alternatives. This has led to the production of commercial edible films that are prepared for various food packaging systems.
3. Biodegradable Film
Biodegradable packaging includes the use of biopolymer materials such as polysaccharides, such as cellulose derivatives, starches, alginates, pectin, chitosan, carrageenan, gums and fibres; Proteins such as soybean protein, wheat gluten, corn zein, gelatin, sunflower protein, whey, casein and keratin, lipids such as waxes, triglycerides, acetylated monoglycerides, free fatty acids, sucrose esters and shellac resin. Bio plastics, as they are commonly known, are considered environmentally friendly and non-toxic to all life forms, due to their source of production (Mellinas et al., 2016). It also creates the carbon footprint. Due to their degradability, these compounds require much less energy for recycling.
3.1 Edible films
Edible film is defined as a thin layer of material that can be eaten and provides a barrier to moisture, oxygen and solute movement for the food (Bourtoom et al., 2008). The material can be a complete food coating or can be placed as a continuous layer between food components. Edible films can be formed as food coatings and free-standing films and have the potential to be used with food as a gas aroma barrier. Edible films and coatings have received considerable attention in recent years for their advantages over synthetic films.
The main advantage of edible films over traditional plastics is that they can be consumed with the packaged products. There is no packaging to dispose of and even if the films are not consumed, they could still help reduce environmental impact. The films are made exclusively from renewable, edible ingredients and are therefore expected to degrade more readily than polymeric materials. The films can improve the organoleptic properties of packaged foods as long as they contain d ifferent components such as bioactive compounds (Quiros et al., 2014). The films can be used for individual packaging of small portions of food, particularly products that are currently not individually wrapped for practical reasons, such as pears, beans, nuts and strawberries. In a related application, they can also be used on the surface of food to control the rate of diffusion of preservatives from the surface to the interior of the food. Another possible application for edible films could be their use in multi-layer food packaging materials together with non-edible films. In this case, the edible films would be the inner layers in direct contact with food materials. The production of edible films creates less waste and pollution; However, their permeability and mechanical properties are generally inferior to synthetic films. Edible films are thin layers of natural biopolymer applied to the surface of the food product with the main purpose of extending shelf life and improving product quality. Compared to synthetic polymers, the advantages of the edible films are edibility, biodegradability, biocompatibility and natural appearance. The edible films have the potential to provide a selective barrier to moisture, carbon dioxide and oxygen, as well as preventing flavour loss and improving mechanical and structural properties. By incorporating various anti-microbial agents into the film formulation, the growth of the microorganisms could be inhibited, reduced or delayed.
3.2.1. Classification of Edible films
Edible films can be made from the film-formable material. During manufacture, film materials must be dispersed and dissolved in a solvent such as water, alcohol, or a mixture of water and alcohol, or a mixture of other solvents. Plasticizers, antimicrobial agents, colourants or flavourings can be added. Adjusting the pH or heating the solutions can be done for the specific polymer to facilitate dispersion (Ramos et al., 2012). The film forming solution is then cast and dried at a desired temperature and relative humidity to obtain free-standing films.
In food applications, film solutions could be applied to food by various methods such as dipping, spraying, brushing and waving followed by drying. Components used to make edible films can be divided into three categories: hydrocolloids (such as proteins, polysaccharides and alginates), lipids (such as fatty acids, acylglycerol, waxes) and composites. (Suput et al., 2015)
3.3 Polysaccharides
Polysaccharides used for edible films or coatings include cellulose, starch derivatives, pectin derivatives, seaweed extracts (carrageenan, agar, alginate), exudate gums (acacia, guar), microbial fermentation gums, pullulan and chitosan. Polysaccharides are generally very hydrophilic, resulting in poor water vapour and gas barrier properties. Although, polysaccharide polymer coatings may not provide a good water vapour barrier, these coatings can act as sacrificial agents, delaying moisture loss from food products. (Gonzalez et al., 2011)
3.2.1. Polysaccharide Films
Polysaccharide films are made from starch, alginate, cellulose ethers, chitosan, carrageenan or pectin and impart hardness, crispness, compactness, thickening quality, viscosity, adhesiveness and gelling ability to a variety of films. These films exhibit excellent gas permeability properties, owing to the composition of the polymer chains, resulting in desirable modified atmospheres that improve product shelf life without creating anaerobic conditions (Khairunnisa et al., 2018). Additionally, polysaccharide films and coatings can be used to extend the shelf life of muscle foods by preventing dehydration, oxidative rancidity and surface browning, but their hydrophilic nature makes them poor barriers to water vapour. (Dhanapal et al., 2012)
4. Bioactive compounds
Bioactive compounds are non-nutritional components typically found in small amounts in food (Soni et al., 2018). In general, these compounds are found in millions of species of plants, animals, marine organisms and microorganisms and can be obtained through extraction and biotechnology processes. Extracted bioactive compounds can be incorporated to create new edible films, improving the shelf life, nutritional quality and consumer acceptability of these goods. The most commonly used bioactive compounds include antioxidants, antimicrobials, probiotics and flavourings, in addition to nutraceuticals substances are also used. (Salgado et al., 2015)
4.1. Essential oils
Essential oils are aromatic and volatile oil extracts. Most of them are derived from plant materials such as leaves, flowers, roots, buds and bark. They can be used as flavourings in foods. However, the direct intake of essential oils as a food preservative is often limited due to the strong taste. To avoid this problem, essential oils can be added to the edible films. The commonly used essential oils in bio-based sheet materials are cinnamon, clove, ginger, lemongrass, marjoram, oregano, sage, thyme, Eucalyptus globulus and Ziziphora clinopodioides. They have proven themselves against the various microorganisms. The antimicrobial activity of essential oils can be attributed to their main phenolic compounds such as thymol, eugenol, carvacrol or terpenoid compounds (-pinene, -pinene, 1, 8-cineole, menthol, linalool), present in concentrations as high as 85%. Different types of essential oils had differences in their main compounds, which had different abilities to bind to the membrane proteins of microb i al cells and alter membrane permeability. (Kris et al., 2002)
5. Product Development
It is useful to examine the methodology used in the development of an edible film from product conception through formulation, laboratory evaluation and finally pilot plant operation to full production. Careful progression through each of these stages of development is critical to efficiently producing high-quality films. The development of a product starts with a clearly formulated idea or concept. The challenge is to select the ingredients, solution preparation method and drying process that produce a film with the desired performance characteristics at an acceptable cost. Typically, this development follows a clearly defined sequence of activities.
6. Applications
Commercial applications for edible films continue to emerge at such a rapid pace that no comprehensive list can be current and complete. Improvements in film formulation and manufacturing processes contribute to the expanded utility of these films. Most applications fall into a few broad categories as shown in Table 2.
6.1. Method of application of edible film
Large losses in quality and quantity of fresh fruit occur between harvest and consumption. Savings achieved by reducing post-harvest fruit losses are considered “hidden harvest”. With a better understanding of the fresh fruit respiration process, several techniques have been developed that successfully extend shelf life. Controlled atmosphere storage and modified atmosphere storage have been used to preserve fruits by reducing their quality changes and quantity losses during storage. Edible film packaging of fresh fruit can provide an alternative to modified atmosphere storage by reducing quality change and volume loss through modification and control of the internal atmosphere of each fruit.
6.2. Solvent casting
Solvent casting is the most commonly used technique to form edible hydrocolloid films. Water or water-ethanol solutions or dispersions of the edible materials are spread on a suitable substrate and later dried. During drying of the films, solvent evaporation leads to a reduction in the solubility of the polymer, until the polymer chains align themselves to form films.
The choice of substrate is important to obtain films that can be easily peeled off without damage after the solvent has evaporated. Generally, the films are air dried in a ventilated oven for several hours. Optimum moisture content (510% w/v) is desirable in the dried film. Film structures depend on the drying conditions (temperature and relative humidity), the wet cast thickness as well as the composition of the casting solution (Kaya et al., 2018).
6.3. Biodegradability
Biodegradability can be defined as a technique involving the degradation of materials by microorganisms or other biological agents. Organic compounds can be broken down aerobically (with oxygen) or anaerobically (without oxygen). Bio-surfactants are produced by microbes to speed up the degradation process. Different materials require different time intervals for degradation. (Saleha et al., 2013)
6.4. Shelf life
The shelf life determines the time in which a product/goods retains its freshness and does not become unusable. It is often affected by parameters such as exposure to light, heat, humidity, gas transmission, mechanical stress and microbial contamination. Food quality deteriorates after the expiration date but may still be safe for consumption. Barrier packaging plays an important role in extending shelf life (Gupta et al., 2010). Packaging with low moisture vapour transmission rates often reduces the amount of moisture transferred to the product, thereby preventing spoilage.
7. Current State and Recent Advances
There are numerous applications for edible films and coatings in the food industry. These include (1) oxygen sensitive foods like nuts to extend shelf life and reduce packaging; (2) nuts to prevent oil migration into surrounding food ingredients (e.g. nuts in chocolate); (3) fragile foods, such as breakfast cereals and freeze-dried foods, to improve integrity and reduce loss due to breakage; (4) fresh fruits and vegetables, whole and pre-cut to extend product shelf life by reducing moisture loss, respiration and colour changes; (5) moisture-sensitive foods or inclusions (e.g., nuts, cookies, and/or candy in ice cream) to provide a moisture barrier to keep products and inclusions crisp; (6) low-fat and non-fat snack foods (e.g., chips) to help condiments stick to products; (7) frozen foods to prevent oxidation, as well as to prevent moisture, flavour, or colour migration; Film separation layers for heterogeneous foods; and film pouches for dry food ingredients. Within these applications, the use of edible films and coatings to carry active ingredients stands out as a promising application for active food packaging.
7.1. Future Trends
The development of new technologies to improve the carrier properties of edible films and coatings is an important topic for future research. Currently, the use of such edible films and coatings is limited. One of the main obstacles is cost, which limits its application to high-value products. Besides cost, other limiting factors for the commercial use of edible films and coatings are the lack of materials with the desired functionalities, the capital costs of installing new film production or coating equipment, the difficulty of the production process and the strictness of regulations. Despite these limitations, the food industry is looking for edible films and coatings that can be used on a wide range of foods, add value to their products, extend product shelf life and/or reduce packaging. However, more studies are needed to develop new edible films and coatings that contain active ingredients to understand the interactions between the components used in their manufacture. When flavourings and active ingredients (eg. antimicrobials, antioxidants and nutraceuticals) are added to edible films and coatings, mechanical properties can be dramatically compromised. Studies on this topic are still very limited and more information is needed to understand this behaviour.
8. Conclusion
In general, edible films should provide secure protection for food materials during storage. Determining the shelf life of perishable food needs special consideration when coating food materials with edible films, due to the altered gas and moisture exchange between the inner and outer layers of food surfaces. Generally, accepted quality parameters and the nature of edible film components are effective factors that extend the shelf life of coated food materials. To determine the appropriate coating material and its application, each food material requires preliminary research and development of its unique properties of edible film materials. Therefore, edible films and their general property profiles should be studied in detail during the manufacture of food materials that have an extended shelf life when coated with edible films.
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