Improved Postharvest Techniques for Fruit Coatings

*Chalermchai Wongs-Aree, Hanh Thi Nguyen and Sompoch Noichinda*

## **Abstract**

Fruits, particularly tropical fruits, have a high moisture content, distinct morphological characteristics, and physiological changes, all of which contribute to their high rate of perishability. Nonetheless, their organoleptic and nutritional qualities make them one of the most important horticultural products. Fruit coating, which imitates natural packaging, is a postharvest solution that is practical and cost-effective for a variety of applications, including on-shelf display, transportation, and storage in support of the supply chain of fruits and vegetables. Gas and moisture permeability, microbiological resistance, and esthetic enhancement are the coating functions. Using modified materials and procedures, edible coatings for fresh and freshly cut fruits are currently being developed. Edible coatings infused with essential oils or volatiles may help to prevent disease resistance while also providing consumers with a fragrant preference. When considering how to advance fruit coating technology when agricultural wastes are the primary source of new coating materials, composite coatings, nanoparticles, encapsulation, and multiple-layer coatings all hold a great deal of promise. Future research may center on the optimal material for particular fruits during the logistics phase.

**Keywords:** edible coating, modified materials, modified techniques, quality maintenance, hypoxia, tropical fruits

### **1. Introduction**

Fruits, especially tropical fruits, are highly perishable and quickly lose their quality after being picked. Fruit postharvest loss in tropical areas could be up to 50%. The high level of physiological changes that occur in fruit is mostly attributable to the high level of moisture content [1]. The amount of water that evaporates from fruit is a significant factor in determining both the quality of the fruit and the economic quantity losses [2]. As a result, packaging that includes coating has the potential to successfully limit the loss of water from the fruit that has been stored. On the other hand, standard packaging might not be appropriate for certain fruits or environments. For instance, the sharp spines of a durian can easily puncture the plastic film, or the headspace in the package can accumulate condensed water and cause the fruit to rot. Edible coatings are a simple kind of fruit packaging that is applied to individual fruits in order to facilitate easier postharvest handling [3]. However, for fruit coating to be successful, it is necessary to consider not just different coating processes but also the characteristics of the fruit itself.

#### **2. Background related to fruit-based coatings**

The extracellular barrier of cuticles protects the plant's surface and interior from environmental stresses. The cuticle, composed of polysaccharides and a lipid matrix termed "cutin" [4], can dramatically transform the epidermis's outer cell wall. Many factors are essential determinants of shelf life and storage capacity, including the cuticle's moderating effect on water transpiration, fruit dehydration, and vulnerability to rots, pests, and diseases. Fruit cuticles have been proven to be highly responsive to environmental factors, and their quality changes even after harvest. When a fruit matures, the cuticle grows thicker [5]. Cutin polymers can be formed from long-chain fatty acids (C20-C34) but are typically made up of esterified and oxygenated C16 and C18 fatty acids [6]. It may also contain trace amounts of glycerol, phenylpropanoids, primary and secondary alkanes, alcohols, aldehydes, ketones, etc. [7]. Fatty acids are essential for the biosynthesis of cutin and wax, which occurs primarily in chloroplasts [8]. The components of wax extracted from the fruit surface of guavas comprise fatty acids and primary alcohols as the major components, followed by sterols, *n*-alkanes, and aldehydes. Interestingly, the wax also contains triterpenoids, a natural pesticide against insects and pathogenic fungi [9]. Thus, understanding the components and function of the cuticle should be useful in improving the coating materials used in fresh fruit markets.

Fresh fruits are perishable due to their quick deterioration due to dehydration, softening, discoloration, microbiological decay, disorder, and loss of nutrients [1]. These properties result from the metabolic processes occurring within the fruit, which are accelerated mainly by an improper relative humidity and gas composition in the storage atmosphere. High water loss and rapid metabolic changes associated with ripening and senescence are the primary causes of fresh fruit deterioration after harvest [10–13]. Artificial coatings mimic the protective properties of the fruit's natural cuticle and can postpone ripening and senescence by acting as a barrier to water and gases. Currently, edible coatings that are safe for food or handling provide a sustainable and effective solution for preserving the high quality of fruit throughout the postharvest value chain [3]. It is generally accepted that edible coatings as a type of MAP (modified atmosphere packaging) add value to fruits by shielding them from contamination, improving their esthetics, and preventing the loss of flavorful volatiles during preparation and storage (**Figure 1**). For the success of a fruit coating business, a thorough understanding of fruit characteristics, nature, respiration, and transpiration behavior is essential.

#### **2.1 Fruit types**

Three distinct fruit categories can be identified based on their floral structure: simple fruit, aggregation fruit, and multiple fruit. Some fruits, like the papaya, the rose apple, and the chili, have an internal cavity despite having a variety of morphologies. In addition, some fruits produce an inner supplementary tissue called aril (mangosteen, durian). Fruits that are either an aggregate (sugar apple, strawberry) or multiple fruit (pineapple, jackfruit) are made up of many fruitlets that have fused

**Figure 1.** *Fruit coating on mature green "Nam Dok Mai" mango during air drying.*

together. Variations in fruitlet maturation might cause uncertain ripening of the entire fruit [14]. Fruit coating could improve the ripening quality of the uncertain ripening by maintaining moisture and gas diffusion in the whole fruit.

### **2.2 Fruit structure/fruit parts**

Dermal tissues (peel), cortical tissues (pulp), vascular bundles (veins), seeds, and intercellular spaces make up the anatomy of most fruits. The peel consists primarily of parenchyma, of which some are transformed into guard cells (**Figure 2A**) and the lenticular opening channel (**Figure 2B**). Fruit stomata and lenticels are responsible for gas and water vapor exchanges, which could be an issue for fruit coating. In rambutan (*Nephilium lappaceum* L.), the epidermal tissues, spinterns, include up to 100–200 apertures/mm2 of stomata [10] that are constantly open [15]. Thus, stomata on spinterns that connect up to 20 groups of vascular bundles in the mesocarp are primarily responsible for moisture loss from rambutan fruit (**Figure 2A**). Replacement of water lost by spinterns with water from the skin [16]. The aril is still edible, despite water loss from the pericarp, producing skin withering, spintern drying, and pericarp browning (**Figure 3**).

Vascular bundles (phloem and xylem tissues) are gathered at the fruit's stylar end (peduncle/pedicel) that connects the fruit to the mother plant. In tangerine fruit, the vascular bundles start from the stylar end through the albedo mesocarp flavedo under the peel (**Figure 4**). The maturing fruit's peel is typically covered with the cuticle. Consequently, the cut peduncle is where the fruit quickly loses its moisture to the air after being harvested. Fruit edible flesh could be derived from the ovary wall, some accessory organs (receptacle and petal), or the unique tissue of "aril" covering the seed, which comprises a majority of parenchymal cells carrying high water content, stored chemicals, and biological metabolisms. Furthermore, in some fruits, there is a fruit cavity (papaya, rose apple) or seed cavity (mango, apple) that is a gas container in the fruit. The fruit's intercellular spaces and cavity (**Figure 2B**) play crucial roles in the fruit's ability to transpire and respire. The gaseous atmosphere in the intercellular

#### **Figure 2.**

*Anatomical structure of cross-sectioned rambutan fruit showing spintern stomata and vascular bundle networks (A) and cross-sectioned papaya fruit showing lenticel and pericarp tissues (B).*

#### **Figure 3.**

*Fruit pericarps and arils of ordinary fruit (left) and dehydrated fruit (right).*

spaces and cavity can be modified depending on the respiration rate and metabolism of the cells. The portions of intercellular spaces in several tropical fruits are shown in **Table 1**. Moreover, in fruit cavities, such as papaya, the different varieties exhibit varying volumes of cavities [17]. Intercellular spaces are important for cellular modification under hypoxic conditions [18]. Thus, the higher portion of intercellular air spaces is more tolerant to hypoxic conditions caused by fruit coatings. For example, apples and rose apples are tolerant to hypoxia from fruit coatings, in contrast to tangerines and guavas, which contain tiny intercellular spaces. Excessive CO2 from respiration could accumulate in the room to reduce cellular toxicity, or O2 demand can be taken from the portions to delay hypoxic cellular conditions. Coating material types and concentrations must be carefully considered. An off-flavor may result if a wax coating effectively prevents water loss from fruits with few intercellular spaces, such as tangerines. Thus, the modulation of the proper coating formula for each fruit depends on the fruit characteristics of each cultivar.

*Improved Postharvest Techniques for Fruit Coatings DOI: http://dx.doi.org/10.5772/intechopen.110099*

#### **Figure 4.**

*Fruit structure of tangerine fruit when cross-sectioned (left) and longitudinally sectioned (right).*


#### **Table 1.**

*Percentages of intercellular spaces in some tropical fruits (own data).*

#### **2.3 Maturity, respiration, and ethylene production characters**

Fruits are classified as climacteric or non-climacteric based on their respiratory and ethylene production patterns during maturation. The success of fruit coatings depends on the fruit's maturation, transpiration, and respiration rates. After fruit setting and during fruit growth due to cell division, the respiration rate is at its highest; it then gradually decreases to its lowest point during the early stage of fruit maturation. During ripening, respiration and ethylene production rates in climacteric fruits rise sharply, peak, and drop off, while both rates are not apparent in non-climacteric fruits. Storage temperatures play important roles in the respiration (**Table 2**) and ethylene production rates of fruits (**Table 3**) [19]. When fruit is coated with exogenous waxes, natural gases, and humidity, exchanges between the fruit and its respective microclimates are disrupted. The respiration, ethylene production, and gas permeability of the coated fruit will alter the gas concentrations within the cellular fruit, decreasing O2 and increasing CO2 and C2H4. By measuring the concentration of gases in the fruit's intercellular spaces and/or cavity, the fruit gets into an equilibrium of internal gases (**Figure 5**). Ethylene and CO2 accumulation at different levels in "Solo" papaya during on-tree ripening [20] are shown in **Table 4**.

Many tropical fruits, such as mangosteen, longan, longkong, rambutan, durian, and salah, contain two parts: the pericarp (peel) and aril (flesh), which individually develop during fruit maturation. The aril of some tropical fruits is derived either from the funiculus (durian and lychee) or integument (mangosteen and rambutan) of the seed [14]. During maturation and ripening, the pericarp and the aril mature


#### **Table 2.**

*Respiration rates of some tropical fruits at 13°C and 25°C (adopted from Kosiyachinda and Tansiriyakul [19]).*


#### **Table 3.**

*Ethylene production rates of some tropical fruits at 13°C and 25°C (adopted from Kosiyachinda and Tansiriyakul [19]).*

independently at different levels. Fruits like the mango, whose flesh develops from the ovary wall, soften, change color, and release their natural aromas as they ripen regularly and consistently. However, the pericarp (dusk) of the durian fruit releases most of the climacteric ethylene during whole fruit ripening, which in turn causes the aril to ripen. Double climacteric blooming occurs in several cultivars (fruit ripening and dehiscence). The respiration and ethylene production rates of durian pulp during ripening are much lower than those from the pericarp [21]. The endogenous ethylene produced in the husk is required for whole fruit and pulp ripening [22]. Thus, a coating of fresh-cut durian must undergo proper ripening before husk removal because unripe pulp often fails to ripen regularly [23]. Interestingly, stages of maturity are crucial for fruit coating. In "Nam Dok Mai" mango, for example, the mature green fruit was induced to get anaerobic conditions and produce off flavor at a high concentration of composite coating, but the treated fruit was typical when the ripe fruit was coated at the same concentration [24].

### **3. Basic materials for fruit coatings**

After harvesting, coating fruits with shellac or fat-related substances was a frequent procedure in the past. Later, fat-based plant extracts from bananas, pineapple leaves,

*Improved Postharvest Techniques for Fruit Coatings DOI: http://dx.doi.org/10.5772/intechopen.110099*

#### **Figure 5.**

*Dynamic and metabolism changes in coated fruits and the surroundings.*


#### **Table 4.**

*Internal CO2 and C2H4 concentrations in the fruit cavity of "solo" papaya during fruit ripening (adopted from Akamine and Goo [20]).*

and carnauba trees were utilized. In contrast, coating materials often consist of a single complex molecule that is effective at preventing water loss but has poor gas exchange. Therefore, it is vital to understand the composition before searching for an acceptable covering. There are three major chemical properties of edible coatings.

#### **3.1 Polysaccharides**

The polysaccharide-based coating comes from modified or extracted polysaccharides from natural products, typically including many starches from plants (corn, wheat, rice, cassava, and potato), glucomannan, galactomannan, inulin, plant cellulose and pectin, plant gum, alginate from brown seaweeds, pullulan from the fungus *Aureobasidium pullulans*, and chitosan from shrimp. The edible film made of polysaccharides is transparent, strong mechanically, and impervious to lipids. From those

materials, chitosan, a plant disease elicitor, has been studied for coating many fruits. With stored longans (*Dimocarpus longan* Lour. cv. "Daw"), chitosan coating at 1.0% and 1.5% revealed a delay in pericarp browning and an effective retardant of disease growth with less than 4% disease incidence at 4°C [25]. Gac fruit (*Momordica cochinchinensis* Spreng) at the yellow stage coated with 0.5%, and 1.0% chitosan delayed fungal infection and enhanced fruit appearance [26]. Chitosan coatings have been successfully used to maintain the quality of many tropical fruits such as pineapple [27], banana [28], and papaya [29].

The polysaccharide-based coating is useful for coating freshly cut fruits because it can prevent fat remnants from leaking out. However, the constraint of the polysaccharide coatings is a lack of control over water loss and O2 and CO2 exchanges [30]. Moreover, if the logistics of handling fresh produce involve a high-temperature fluctuation, or if the fresh produce is immediately removed from the cold storage and placed at room temperature, the coating may be in a reversible phase, resulting in the peeling of the coating from condensed water droplets on the surface. Researchers have attempted to create an edible polysaccharide coating or film using antioxidants from plants. Typically, this is done to enhance the physical and chemical properties of the film and coating for safe consumption.

#### **3.2 Proteins/oligo peptides**

Proteins from plants, animals, and microorganisms are biopolymers that can be utilized to make films with tunable physical and functional properties when mixed with plasticizers or other components. The proteins such as gelatin, casein, collagen, whey proteins, egg white, etc. have been studied. Covalent unions (side chain cross-linking) and electrostatic or ionic interactions between protein chains contribute to coating formation [31]. Protein films' mechanical and hydrophobic barrier properties, and hence their suitability for food packaging applications, are strongly influenced by the final chain contacts and bonds. The major bonding processes are controlled by production conditions such as pH, salt addition, heating, enzyme action, drying, and reactions to food-grade chemicals [32]. Protein films have been enhanced with antimicrobials and antioxidants, among other active compounds. Some protein extracts have been studied for fruit coatings. Park et al. [33] used a corn-zein coating to delay fruit ripening in tomato fruit, while Avena-Bustillos et al. [34] used a casein (milk) protein coating to reduce weight loss in zucchini. Furthermore, zein and gelatin coatings could delay ripening in mangoes stored at 32°C [35].

Although the protein-based coating belongs to the hydrophilic group, the covalent cross-links, and electrostatic networks boost the coating's structural stability, limiting the coating's reversible phase when droplets condense on the surface during logistics.

#### **3.3 Lipids**

Fatty acid derivatives, as a significant component of the natural cuticle, play an important role in preventing water loss and gas exchanges in fruits. The properties of flexibility, hydrophobicity, and cohesion that edible films require are provided by lipids [36]. The quality of fruits can be maintained by edible coatings made of lipids, which are effective barriers against moisture, O2, and CO2, but not C2H4. Edible coatings such as carnauba wax, shellac, bee wax, and some plant oils based on lipids can cover fruits and vegetables. Films and edible coatings made from fat

*Improved Postharvest Techniques for Fruit Coatings DOI: http://dx.doi.org/10.5772/intechopen.110099*

have gained appeal due to their functional and nutritional benefits. Shellac and carnauba have long been applied to postharvest fruits. Ten percent shellac coating prevented fresh weight loss and disease infection, and reduced the respiration and ethylene production rates of gac fruit at 25°C storage, while 15% shellac coating led to a high accumulation of acetaldehyde since day nine [37]. Shellac and carnauba emulsions were coated on "Nova" mandarins (*Citrus reticulata*) at 20°C storage. The carnauba waxes resulted in minor weight loss compared to the uncoated control and shellac coating, but shellac-coated fruit showed the highest fruit shine. The highest levels of CO2 and the lowest level of O2 were found in shellac-coated fruit, resulting in the highest ethanol content in the juice due to induced anaerobic respiration [38].

Many lipid-based coatings can provide the best water transpiration prevention due mainly to their strong hydrophobic properties, but for fresh produce, the switch from aerobic to anaerobic respiration caused by too low O2 and/or high CO2 in the coated fruit must be considered. Although long fatty acid derivatives are the most abundant in natural cuticles, attachment to sterols, terpenoids, polysaccharides, and phenolics may modify the complex structure, increasing gas exchange permeability.
