The effect of using natural plant‐based waxes in coating/film materials on postharvest quality of fruits and vegetables

Natural plant-based wax coatings/films function as a gas, moisture, oxygen, and light barrier, inhibit the loss of volatile aroma components, and promote the migration of antimicrobial and antioxidant components into the fruit; thus, they extend the shelf life of fruit and vegetables and improve quality properties like moisture and firmness. Fruits and vegetables (F&V) have a short shelf life after harvest and are prone to biochemical and physicochemical deterioration since they are very perishable. Despite their high water and nutrient contents, they may lose their nutritional, functional, and sensory quality due to elevated transpiration and respiration, as well as other biochemical changes (Devi et al., 2022). Inadequate packaging, poor storage, and handling procedures cause postharvest losses. In recent years, several studies have focused on the postharvest management of F&V (Abhirami et al., 2020). As a thin layer on the surface of F&V, edible coatings/films act as a barrier against oxygen, moisture, and solute movement, reducing the amount of water loss, the pace of respiration, and the level of oxidative reaction (Devi et al., 2022) (Figure 1). Natural plant-based wax coating/film. The term “edible coatings” refers to thin, primary packaging layers made from edible materials that can be applied directly to the products. Typically, they are applied by spraying, dipping, or brushing the coating/film solution directly on the outer layer of F&V (Abhirami et al., 2020). The majority of composite films or coatings combine a hydrophilic structural matrix with a hydrophobic lipid component and improve the functionality of pure hydrocolloid films, particularly in terms of their moisture barrier qualities. Emulsions or bilayers are both options for obtaining composite materials. The second layer of the bilayer composite structure is lipids on top of the polysaccharide or protein layer. However, the lipid is dispersed into the biopolymer emulsion matrix (Galus & Kadzińska, 2015). Esters of high-molecular-weight monohydroxy alcohols and high-molecular-weight carboxylic acids are combined to form waxes. Natural waxes can be derived from a variety of sources (animal, plant, or mineral origin) (Aranda-Ledesma et al., 2022), and they have numerous industrial uses including the food and cosmetic industries (Pashova, 2023). Natural plant-based wax coating/film (NPWCF) is edible and does not have a negative impact on the environment. On the other hand, antibacterial, antioxidant, and anti-browning components in NPWCF provide additional benefits for food quality and safety as well as health (Pashova, 2023). NPWCF, among other lipid-based coatings, demonstrates important barrier properties that prevent F&V against moisture loss (Abhirami et al., 2020; Devi et al., 2023). However, compared to polysaccharides, proteins, or resin materials, they are less efficient as gas barriers (Miranda et al., 2022). The thickness of the layer, the price of the ingredients, and the lack of regulations about the formulation and applications are difficulties encountered during their applications (Pashova, 2023). Additionally, NPWCF can be replaced with waxes of animal origin to prepare vegan formulations. Plant waxes, such as carnauba, sunflower, rice bran, sugarcane, and candelilla, have been investigated for prospective applications due to their distinctive structural features and physicochemical properties (Aranda-Ledesma et al., 2022). At concentrations as low as 0.5%–4% weight percent, they create a sophisticated three-dimensional network that traps liquid oil into a gel-like matrix (Wijarnprecha et al., 2018). In recent years, NPWCF, especially carnauba and candelilla waxes, have been successfully used for many horticultural crops such as papaya, tomato, citrus, apple, lemon, mandarin, pomegranate, cucumber, and brussels sprouts, and the results obtained after storage compared to the control samples are summarized in Table 1. NPWCF contributes to maintaining the product's quality and giving it a more appealing appearance. Candelilla wax and carnauba wax are approved for use by the FDA as generally recognized as safe (GRAS) (FDA, 2023). Carnauba wax, candelilla wax, and rice bran wax have the potential for different applications in foods, medicines, polymers, cosmetics, and leather goods. Abhirami et al. (2020) revealed that coating tomatoes with rice bran wax, a secondary by-product of rice processing, may block lenticels and stomata and result in slower weight loss. Other plant-based waxes that are being investigated for NPWCF during the postharvest of F&V are carnauba wax and candelilla wax. Interestingly, Dassanayake et al. (2009) discovered that, in contrast to 2% for candelilla wax and 4% for carnauba wax, a minimum of 0.5% rice bran was required for oil gelation. The superior gelation ability of rice bran has been linked to the presence of high aspect ratio rice bran wax needles (Wijarnprecha et al., 2018). The dark brown resinous substance in defatted rice bran wax is the cause of impurities, and the melting point of crude wax (75–79°C) increases when it is purified by removing the resinous matter (80–83°C). Saturated esters of C22 and C24 fatty acids and C24 to C40 aliphatic alcohols made up the majority of rice bran wax. C24 and C30 were the most abundant fatty acids and fatty alcohols, respectively. There were also trace levels of branched and odd-carbon number fatty alcohols in the alcohol part of the wax esters (Vali et al., 2005). On the other hand, beeswax (62–66°C) has lower melting points compared to carnauba and rice bran wax (∼80°C) (Devi et al., 2022; Vali et al., 2005). Brazilian palm tree leaves are used to make carnauba wax, which has excellent water barrier qualities and a highly hydrophobic nature (Devi et al., 2022; Oliveira Filho et al., 2022, 2023). Esters (80%), aliphatic acids, aromatic acids, free alcohols, hydrocarbons, free-hydrocarboxylic acids, and triterpene diols are the main components of carnauba wax (Devi et al., 2022). The main types of esters found in carnauba wax are cinnamic acid diesters and aliphatic esters, thus, 10% of the esters are mono-carboxylic acids (C28) and α, ω-diols (C30), while the 90% of the esters are ω-hydroxy acids (C26) and mono alcohols (C32) (Devi et al., 2022). Among natural waxes, carnauba wax has the greatest melting point (79.2–84.2°C) and is the hardest due to its low solubility and early drying nature (Devi et al., 2022). It is frequently used to raise the melting point of wax mixtures as a hardener. As an FDA-approved food ingredient, carnauba wax is used to glaze some foods (Pashova, 2023). On the other hand, E. antisyphilitica Zucc., which is only found in the Chihuahuan Desert in northern Mexico and the southern United States of America, is the source of candelilla wax. n-Alkanes (hentriacontanes) are the primary component of candelilla wax, following high-molecular-weight esters (29–33 carbons), alcohols and sterols (20%–29%), free acids (7%–9%), and resins (12%–14%, mostly triterpenoid esters) (Aranda-Ledesma et al., 2022). Candelilla wax has a melting point of 56.84 and 79.0°C, which is similar to beeswax and contains more unsaponifiable materials (such as sterols, pigments, mineral oils, and hydrocarbons) than other plant waxes, so it has fewer esters overall (Aranda-Ledesma et al., 2022). It is a hard, brittle wax that is insoluble in water. Candelilla wax has been described as having much lower water vapor permeability than carnauba and beeswax (Kowalczyk & Baraniak, 2014). The high efficiency of NPWCF on F&V quality including appearance and other sensory aspects, is a crucial factor for the consumer's choice of buying, and it is achieved by mixing different components. The use of different biopolymers such as chitosan, carboxymethylcellulose, proteins, or different bioactive additives in the coating matrix has also been studied (Table 1). 1-Methylcyclopropene (Chen et al., 2020), γ-aminobutyric acid (Nazoori et al., 2022), essential oils in carnauba wax (Gutiérrez-Pacheco et al., 2020; Oliveira Filho et al., 2022, 2023) and Flourensia cernua extract in candelilla wax (Ruiz-Martínez et al., 2020) improved the effectiveness of NPWCF and increased the phenolic content and antioxidant potential. Besides, the antifungal and antibacterial effects of essential oils in NPWCF have also been examined by different groups (Gutiérrez-Pacheco et al., 2020; Oliveira Filho et al., 2022, 2023; Ruiz-Martínez et al., 2020; Sanchez-Tamayo et al., 2024; Valle-Ortiz et al., 2019). There are more studies on carnauba or candelilla wax composite films and coatings blended with several biopolymers (proteins, polysaccharides, lipids) and other natural waxes. Carnauba and candelilla wax provide hydrophobicity to the structure in these blends and improve some physical properties (water vapor barrier, film opacity, and mechanical resistance) (Aranda-Ledesma et al., 2022; Devi et al., 2022). There are also studies on the comparison of different coating materials with NPWCF. Gutiérrez-Pacheco et al. (2020) demonstrated that chitosan coatings were most successful at reducing the microbial load on the treated fresh cucumber, whereas carnauba wax coatings were most effective at preventing weight loss. Nonetheless, the combined effect of NPWCF and their application may not be suitable for every F&V. For example, Kowalczyk et al. (2019) emphasized that cellulose and candelilla wax reduced the storage life of the diseased Brussels sprouts and encouraged fungal development. High water and nutrient contents cause respiration, transpiration, and metabolic alterations that result in losses of sensory, functional, and nutritional quality in F&V. The use of innovative NPWCF may address the drawbacks of many conventional food preservation techniques since they have intrinsic functional qualities including antibacterial, antioxidant, anti-browning, flavoring, etc. (Devi et al., 2022). Eventually, they reduce the evaporation rate of moisture, gases (such as carbon dioxide and oxygen), and volatile ingredients (Pashova, 2023). Water loss is a key cause of postharvest degradation in fruits and vegetables. Fruit water loss is frequently confused with mass loss and weight loss. Apart from resulting in a decrease in marketable weight, excessive water loss also causes browning, loss of fruit texture and flavor, hastened senescence, membrane disintegration, and susceptibility to chilling injury (Lufu et al., 2020). Senescence affects skin permeability, which leads to a rise in water loss. Fruit loses weight and firmness through transpiration as a result of respiration that occurs in the stomata of the epidermis, which uses up sugar and water reserves (Lufu et al., 2020; Oliveira Filho et al., 2023). This causes them to shrink and dry up. Typically, the water loss profile starts with a higher rate of water loss and gets lower during storage (Lufu et al., 2020). NPWCF applications can reduce these effects. Less weight loss during storage was reported with NPWCF on F&V (Table 1). The respiration rate of F&V after harvest is the main contributor to deterioration (Abhirami et al., 2020). By lowering the rate of fruit respiration, ethylene generation, and organic acid concentration, boosting the antioxidant system, and maintaining cellular integrity, the NPWCF preserves fruit quality (Duan et al., 2023). It has been reported that fruit respiration rate decreased directly proportional to the amount of natural wax. NPWCF changed the internal atmosphere of fruits by altering the concentration of O2 and CO2 in addition to producing a gas exchange barrier between the fruit tissue and the environment (Galus & Kadzińska, 2015). On the other hand, since the ripening process takes longer with NPWCF, the pH is higher as less organic acid is produced with the conversion of fruit sugars during storage (Oliveira Filho et al., 2022, 2023). With the effect of NPWCF, a delay in ripening and gas exchange (lower O2 consumption and CO2 production) was observed, and ethylene production and enzymatic activity were reduced (Oliveira Filho et al., 2022). Reduced fruit firmness is caused by turgor pressure and tissue water loss (Devi et al., 2023). It is a natural result that firmness decreases during storage, but with the NPWCF application, this decrease is limited and occurs gradually. Also, an increase in total soluble solids as a result of the decomposition of cell wall carbohydrates such as hemicellulose, starch, and pectin, the conversion of organic acids, and frangible cell wall structures are observed in postharvest handling (Abhirami et al., 2020). A thin film of natural wax forms a strong bond with the surface of the fruit, covers the stomata, lenticels, and micropores completely or in part, and acts as a barrier that has a semipermeable, uniform, and stable structure. It reduces the rate of respiration, transpiration, and oxidation (Devi et al., 2022). A sophisticated defense mechanism protects plants from oxidative damage. Both enzymatic and non-enzymatic components (antioxidants) make up the defensive system. Fresh fruits and vegetables are good sources of antioxidants, and their consumption helps keeping the body healthy, especially when battling free radicals, which harm cells and cause chronic ailments (Meitha et al., 2020). It has been revealed that the activities of catalase and peroxidase, two key components of the cellular defense mechanism against oxidative stress, were improved by the NPWCF of apples providing a ROS scavenging effect and preventing fruit degradation (Chen et al., 2020). In addition to enzymatic components, non-enzymatic components such as α-tocopherol, carotenoids, flavonoids, and proline also aid in the breakdown of ROS in plants. Lipid-soluble substances called carotenoids shield the cell's photosynthetic apparatus, which functions as an antioxidant. By reacting with excited chlorophyll molecules, they remove the excess excitation energy through the xanthophyll cycle and thus prevent the creation of oxygen singlets (Meitha et al., 2020). The amount of important bioactive compounds, especially carotenoid and lycopene content, was better preserved during storage with NPWCF application (Abhirami et al., 2020; Baswal et al., 2020). Further, Duan et al. (2023) implied that commercial natural wax can delay citrus fruit color by delaying the pathway involved in the biosynthesis of carotenoids. It has been noted that ascorbic acid, which is also an important parameter in terms of the nutritional value of F&V, is protected by NPWCF (Baswal et al., 2020; Chen et al., 2020; Duan et al., 2023). The positive effects of NPWCF are not directly correlated with an increase in wax concentration. They are most likely associated with the physicochemical interactions between the surface of F&V and the NPWCF. The negative effects of excessive natural wax application have also been noted (Abhirami et al., 2020). Abhirami et al. (2020) stated that the greater wax concentration obstructed the respiratory cells, anaerobic conditions, and the accumulation of CO2. Natural waxes have the potential to be employed as a technique to maintain the overall quality and customer acceptability of F&V during postharvest handling, and they serve as a kind of modified atmosphere packaging system by acting as a semipermeable barrier to O2 and CO2. Further, there is a need for new or improved, natural methods of preventing fruits from ripening and rotting. Protective effects of NPWCF in terms of delaying ripening, lowering both the loss of fresh mass and the emergence of illnesses during storage, have been reported in the literature. To extend the postharvest life of F&V, NPWCF could be employed as a palatable and environmentally friendly alternative to conventional or synthetic wax-based coatings. It is generally more common to use them in combination with different biopolymers rather than using them alone. They give more hydrophobicity and brittleness to the mixture they are added to, improve barrier properties, and increase the melting point. The effects of using phenolic extracts or essential oils with different antioxidant and antibacterial properties can be investigated in the future. In particular, phenolic additives obtained from agricultural wastes or by-products could be interesting, leading to ingredients that cause minimal environmental pollution and provide additional benefits through upcycling. On the other hand, it is known that phenolic additives act as cross-linking agents in different biopolymers, which could be further investigated. Besides, the effects of different classes of phenolics in wax-based coatings or films, especially those with hydrophobic properties, can be investigated in future studies. Deniz Günal-Köroğlu: Conceptualization; writing – original draft preparation. Esra Capanoglu: Writing – review and editing; supervision. All authors have read and agreed to the published version of the manuscript. This research received no specific grant from any funding agency in the public, commercial, or not-for-profit sectors. There are no conflicts to declare. None declared. Data sharing is not applicable to this article as no datasets were generated or analyzed during the current study.