Authors J.M. Vieira, M.L. Flores-López, D.J. de Rodríguez, M.C. Sousa, A.A. Vicente, J.T. Martins
Blueberries (Vaccinium corymbosum) are currently one of the most valuable fruits worldwide due to its organoleptic and nutritional properties. However, from the moment that blueberries are harvested they are very susceptible to structural, nutritional, and biochemical changes. These postharvest changes can be accelerated principally, by water loss and action of microorganisms, mostly by fungal outbreaks (e.g., Botrytis cinerea). In recent years, edible films and coatings have been considered one technology with great potential to improve safety of food and to protect it from the influence of external environmental factors, thus increasing its shelf life. This type of coatings can be a biodegradable alternative to the use of plastic packages, since they can create a protective barrier, semi-permeable to gases and water vapor, and could reduce microbiological proliferation. One of the main food applications of edible coatings is on fruit surface, such as strawberry, grapes, tropical fruits, among others. The purpose is to create a more efficient system for fruit storage, aiming to reduce the degradation of qualitative aspects in the postharvest period and lower loss rates to extend shelf-life. Also, the properties of the coatings can be enhanced using functional ingredients incorporated within such as anti-browning and antimicrobial agents, nutraceuticals, volatile precursors, and colors. Other ingredients, such as preservatives, antioxidants, and firming agents can be added to coatings to improve microbial stability, appearance, and texture of coated product. To improve the efficiency and stability of edible coatings/films it is essential to find adequate materials. Coatings/films can be produced using a wide variety of products, such as polysaccharides, proteins, lipids or resins, alone or, more often, in combination. Chitosan (1,4-linked 2-amino-2-deoxy-?-D-glucan) is one of the most widely used natural compounds in the edible coating production. Due to its characteristics such as high antimicrobial activity, biocompatibility, biodegradability and non-toxic profile, this polysaccharide has been studied for application in different areas, with primary emphasis on food and pharmaceutical industries, but also in medicine, agriculture, and environment. Chitosan coatings are an excellent carrier of other functional substances, such as antimicrobials and antioxidants. Aloe vera (Aloe barbadensis Miller) is a member of the family Liliaceae. It is one of the most biologically active plants, since it is a rich source of antimicrobial and antioxidant agents, such as phenolic compounds. Therefore, A. vera is widely used in food, pharmaceutical and cosmetic industries. The main feature of the A. vera fractions (pulp and/or liquid fraction) is their high-water content (above 90%), having a complex chemical composition. Some compounds in A. vera have been identified as bioactive, such as carbohydrate polymers (mostly acemannan), soluble sugars, organic acids, fibers, proteins, phenolic compounds, vitamins, minerals, amino acids, and mineral salts. Recent studies have demonstrated the effectiveness of A. vera extracts (pulp and/or liquid fraction) against numerous forms of diseases in fruits and vegetables caused by fungi. The main reason to separate the two fractions is due to the difference in bioactive compounds (and concentration) present in each fraction, and thus their biological activity can be different. Recently, coatings based on A. vera pulp have been applied on fruits to maintain quality and reduce microorganism proliferation of strawberries and table grapes. However, as far as we know there is no studies on the application of A. vera liquid fraction as coatings on fruits or incorporated in polysaccharide coatings such as chitosan. The objectives of this work were: (1) to evaluate antifungal and antioxidant activities in vitro of A. vera fractions (pulp and liquid), (2) to choose the best chitosan-based formulation to be applied on blueberries, and (3) to evaluate the postharvest quality of cold-stored blueberries coated with chitosan-based coating containing A. vera fractions. Postharvest losses, which result in a decrease in fruit quantity and quality after harvest, are a severe problem. Poor handling and storage, as well as insufficient packaging and microbial and fungal diseases, are likely to cause losses. Several approaches and technologies have been developed in recent decades to control postharvest losses. An edible coating is one of the modified atmosphere methods that have shown promising results for preserving fruits quality. Now you must be thinking what is modified atmosphere methods. The main premise of this procedure is to alter the atmosphere surrounding the fruits by eliminating or changing the levels of gases necessary for respiration, such as oxygen, carbon dioxide, and ethylene. The use of a changed atmosphere has been performed from the dawn of civilization, for example in China, Greece, and other early civilizations, where fruits were sealed in clay pots with fresh leaves and grass. This created an unfavorable environment, which slowed the ripening process by altering the internal gas composition and decreasing the metabolic activity of the fruits. As a result, senescence was delayed, and microbial development was inhibited. Now lets come to our main topic, which is edible coatings, in edible coating methods, to stop the ripening process of fruits or vegetables, biological or chemical components are used as a coating layer on the product’s surface to limit gaseous exchange. Edible coating as A thin covering put to the surface of fruit to establish a barrier between the fruit and the environment that may be eaten as part of the overall product. The edible coating can be applied by dipping or spraying the coating solution on the surface of fresh/fresh-cut fruits. By functioning as a barrier to gas exchange, an excellent edible coating provides a partial barrier to water movement, reducing moisture loss from the fruit surface and modifies the atmosphere around the fruit. The edible coating components are biodegradable and non-toxic. Biopolymer matrixes (like polysaccharides such as Pullulan, chitosan, carrageenan, starch, alginate, cellulose, pectin, gellan gum, and xanthan gum), And proteins (like collagen gelatin, gluten, mung bean protein, corn zein, soy protein, & casein), lipids(like bee wax, paraffin wax, carnauba wax, polyethylene wax, candelilla wax, rice bran wax, ouricouri wax, and jojoba oil), and composite materials are used to create edible materials, the application of biopolymers based edible coating on food products acts as a barrier layer against gas diffusion, water migration, aroma changes and different solute exchange. General Methods for edible coating 1. Dipping: edible coatings can be applied to goods by dipping them in coating solutions and letting the excess coating drain as it dries and solidifies. Dipping has long been a popular method for coating fruits, vegetables, and meats. The Florida citrus sector was the first to use dipping, with the fruits being submerged in an emulsion coating tank. The fruit was subsequently transported to a drier in a controlled environment. 2. Dripping: This is the most cost-effective form of coating application. It also has the capability of immediately applying the coating to the commodity surface or to the brushes. However, because of the relatively large droplet sizes, uniform coverage can only be accomplished when the commodity is tumbling over numerous brushes saturated with the coatings. Coating fruits and vegetables with drippings is a frequent practice. 3. Foaming: Some emulsion coatings are applied with foam. The coating is foamed with a foaming agent, or compressed air is blasted into the applicator tank. To break the foam and ensure consistent dispersion, a lot of tumbling activity is required. The agitated foam is applied to commodities using rollers, fabric flaps, or brushes to distribute the emulsion over the commodity’s surface. Because this form of emulsion includes minimal water, it dries rapidly, yet poor coverage is a common issue. 4. Fluidized bed coating: is a method for applying a very thin layer to dried particles with a very low density or small size. It was created as a pharmaceutical coating technology, but it is currently being used more frequently in the food business. It can be used to improve the effect of functional ingredients and additives like processing aids, preservatives, fortifiers, flavors and other additives, as well as to make them easier to handle. Aesthetics, taste, and colour have all enhanced. Fluidized-bed processes are often used to cover bakery items. 5. Panning: Panning is commonly used to coat candies, nuts, and some processed fruits with a smooth, regular surface obtained through the pan’s polishing action. A stainless-steel pan is encased and perforated along the side panels in this technology. A pump distributes the coating to spray guns positioned across the pan. Panning is a time-consuming operation in which the speed of the pan changes depending on the size of the centre. 15 rpm is required for extra-large nuts. 6. Spraying: it is useful when a thin and homogeneous coating is required for specific surfaces. With the invention of high-pressure spray applicators and air-atomizing devices, this is the most preferred approach for covering whole fruits and vegetables. Spray applications are also appropriate for putting films to a specific side or when a dual application is required for cross-linking, as in the case of alginate coating. 7. ELECTROSTATIC COATING: it is a method that uses charged particles to cover a surface more efficiently. Formal spraying methods are used to project a powdered particle or atomized liquid towards a conductive surface, which is subsequently propelled toward the surface by a high electrostatic charge. For culinary applications, the exact performance of liquid electrostatic coating techniques is unknown. In some applications, such as the impregnation of bread with edible vegetable oil and the coating of confectionery and chocolate items, these coatings have shown considerable promise. The success of an edible coating for meeting the specific needs of food strongly depends on “Its barrier property to gases, especially oxygen and water vapour, Its adhesion to the surface, Uniformity of coverage of coating and Sensory quality of the coated food products. Some advantages of edible coating: Improved retention of colour, acids, sugars, and flavour components, Reduced weight loss, Maintenance of quality during shipping and storage, Reduction of storage disorders, Improved consumer appeal, Extended shelf life, Addition of the value of the natural polymer material, Reduction of synthetic packaging. Edible coatings are used to improve the quality of fresh foods, with a focus on some applications with attractive prospects, regulatory considerations, and application constraints. There are numerous technologies available today that can be utilized to improve the quality and safety of fresh produce. However, among other strategies for preserving the freshness of fresh and cut fruits and vegetables, the application of new edible coatings stands out. Many studies have focused on developing and characterizing low-cost coating materials, optimizing coating application processes, and the effects and implications of coating treatments on various food matrices. However, in most situations, commercializing edible coatings will necessitate additional research and consideration of key concerns such as consumer acceptance and regulatory considerations. One major advantage of using edible coatings is that several active compounds can be incorporated into the polymer matrix and consumed with the food. In this way, incorporation of active ingredients into edible coatings for fresh and minimally processed fruits and vegetables is feasible through nanotechnological solutions. However, current applications of nano systems to fresh products are still scarce. More research about the effect of nanotechnologies on quality and nutritional aspects of fresh and minimally processed fruits and vegetables should be conducted.
