Recent Postharvest Technologies in the Banana Supply Chain

Abstract

Postharvest technology plays a vital role in preserving fruits. In particular, banana is a climacteric fruit with a short shelf-life; thus, good postharvest practice through the supply chain is essential to maintaining its quality. Prior to applying proper postharvest technology, it is important to understand the physiological changes in banana during the ripening process. This review describes the physiological characteristics of banana after harvesting as well as the technologies being studied and adopted practically to achieve postharvest goals. The discussion herein highlights the critical quality control points in the banana supply chain, e.g., storage, artificial ripening, and transportation. By assembling the current knowledge, this review could contribute on quality improvement through applying the proper postharvest technologies.

1. Introduction

Banana is a popular crop in world agricultural production and trade. According to the FAO [1], banana is consumed at a rate of around 13 kg person⁻¹ year⁻¹ in Japan, India, China, Russia, the USA, and Europe. Furthermore, banana production and trade volume by producing countries has increased rapidly over recent decades; moreover, the consumption of banana is predicted to continue to increase as the world population grows. The countries that currently produce the most bananas include India, China, Brazil, Ecuador, the Philippines, and Indonesia.

More than 1,000 banana varieties currently exist and almost all of these are for grown as edible produce. The most commercialized banana variety is “Cavendish” (Musa acuminata, AAA group, Cavendish subgroup) which is estimated to account for around 40–50% of global banana production. Moreover, all exported banana varieties are “Cavendish” because it is better suited to international trade than other varieties because of its high production yields. “Cavendish” plantations are also more tolerant against environmental challenges such as storms and able to retrieve more from natural disaster [1].

Statistics show that the global export of banana reached a new high record, ~21 million tons, in 2019, which was an increase of 10% from the previous year. Ecuador and the Philippines are two of the leading banana exporters. In Ecuador, the total production of banana increased from 6.18 to 6.48 million tons, of which approximately 97% was for export; such a high export rate was maintained (as of 2017–2019). In Philippines, the total banana production was slightly smaller than Ecuador and kept constant around 6 million tons for these three years, meanwhile, the export rate increased drastically from 45% (2.70 million tons) to 73% (4.33 million tons). However, only 15% of total banana production in the world is currently traded in the global market, whereas the remaining bananas are used for local consumption. The global import volume of banana was reported as ~19 million tons in 2019. The European Union and the USA are two of the leading importing countries; they account for 1% and 3% of imports, respectively [1].

The supply chain of banana includes production, processing, and distribution to customers. During the COVID-19 pandemic, there were disruptions and contractions in supply worldwide; the demand for bananas also increased due to the adverse effects of the physical distancing measures. However, banana is perishable product that requires timely and appropriate postharvest handling as well as an uninterrupted cold chain. A delay in the supply chain can have detrimental effects on bananas during transportation due to shrinkage; this affects the quality of the produce and reduces the value of shipments received by importing countries. For instance, quarantine measures have recently been strengthened in several countries at ports and borders, which retard trade activity. In addition, many local markets were closed during the pandemic; thus, distribution from producers to local and national outlets was disrupted [2].

To respond appropriately to uncertain situations like the pandemic, possessing knowledge on the postharvest characteristics of banana is important for maintaining quality and extending shelf-life, which are the main goals of postharvest technology. In addition, postharvest technology plays an important role in preventing the deterioration of fruit. A brief review of recent postharvest technology that can be appropriately applied to banana is provided here. Specifically, the adopted postharvest technologies in the banana supply chain are presented in this review.

2. Physiological characteristics of banana after harvest

2.1 Ripening

2.1.1 Climacteric rise in respiration

Respiration is a crucial factor in preserving the quality of fruits and vegetables, especially in banana. Since banana is a climacteric fruit, its respiration rate increases substantially during ripening in contrast to the respiration rate of non-climacteric fruits. Respiration involves the breakdown of carbohydrates, proteins, and fats into simple final products with ATP released as energy. Sugar and organic acids are two major respiratory substrates found in all fruits; they are largely insulated within the vacuoles. The composition of sugar and organic acids contributes to the formation of flavor in fruit and affects its taste [3]. Commonly O₂ is used and CO₂ is produced in a respiration process; this leads to senescence depending on the rate of respiration. The more the respiration rate increases, the faster the fruit deteriorates in quality [4].

Under practical conditions, banana is harvested at the green maturity stage when the respiration rate is low and ethylene production is almost undetectable. Hailu et al. [3] termed this stage “green life” or the “pre-climacteric period.” After harvesting, the respiration rate of green Cavendish bananas is about 20 mg CO₂ kg⁻¹ h⁻¹ when storage for a couple of days at 20 °C. At the climacteric peak of four days storage, the respiration rate significantly increases up to 250 mg CO₂ kg⁻¹ h⁻¹ after which it declines gradually in the post-climacteric stage [5]. As well as the transpiration process, the respiration process reduces the total weight of banana. Consequently, in marketing systems, the quantity of bananas produced is reduced by the respiration process, which can result in reduced commodity value. Therefore, to preserve the quality of banana, respiration should be always considered when applying postharvest technology.

2.1.2 Ethylene synthesis and reaction

The biosynthetic pathway of ethylene is associated with ripening and senescence (Fig. 1) [6]. Fundamental ethylene biosynthesis usually includes three major steps. First, the amino acid methionine, a precursor of ethylene in higher plants, is catalyzed by S-adenosyl methionine (SAM) synthase at the expense of one molecule of ATP per molecule. Second, SAM is converted to 1-aminocyclopropane 1-carboxylic acid (ACC) by ACC synthase (ACS). Because of oxygen, ACC is converted to ethylene by ACC oxidase (ACO) [7, 8]. Next, receptors (ETRs, ERSs, EIN4) perceive ethylene, which are negative regulators located at the endoplasmic reticulum. In the absence of ethylene, Constitutive Triple Response (CTR1) is activated through inactivation of Ethylene Insensitive 2 (EIN2). Therefore, ethylene response is suppressed by CTR1. The primary transcription factor involves Ethylene Insensitive 3 (EIN3)/Ethylene Insensitive 3 like 1 (EIL1) to activate a transcriptional cascade. As a result, Ethylene Response Factor (ERF) regulates genes underlying ripening process of banana [9].

Some genes related to ethylene biosynthesis and related pathways in banana, including ACS, ACO, ethylene receptor, a CTR1 orthologue, and ethylene insensitive 3-like genes, have previously been investigated [8, 9, 10]. The promoters of major ripening genes in banana fruits are MaACS1 and MaACO1. MaACS1 was reported to be phosphorylated by a Ser/Thr protein kinase during fruit ripening, which increases the stability of the protein and induces ethylene production [11, 12]. Importantly, Xiao et al. [13] reported the role of the Ethylene Response Factor (ERF) Transcription Factor (TF) in the transcriptional regulation of ethylene synthesis through regulating MaACS1 and MaACO1. It was proved that MaERF9 activated the promoter of MaACS1, while MaERF11 repressed the promoters of MaACS1 and MaACO1. As a result, MaERFs play an important role in banana fruit ripening via transcriptional regulation of or interaction with ethylene biosynthesis genes.

The ethylene production rate increases according to the stage of maturity, mechanical injuries, disease, and increasing temperature up to 30 °C. In contrast, low temperature, reduced O₂, and increased CO₂ levels around the commodity suppress ethylene production [14]. Both endogenous ethylene and exogenous ethylene can alter the quality changes to the color, texture, sweetness, aroma, volatile production, and nutritional value of banana.

Figure 1: Ethylene biosynthesis pathway and signal transduction

2.1.3 Biochemical and physical changes of banana during ripening

After harvesting, the banana continues to ripen, and the biochemical characteristics of the fruit drastically change. These biochemical changes are summarized in Table 1. The most changed biochemical compound during ripening is starch which is abundant in banana. At the unripe stage, the starch level in banana is high; levels decrease to a deficiency during ripening due to the conversion of starch into soluble sugars. Simultaneously, the levels of soluble sugars, such as sucrose, glucose, and fructose, which are the main detectable compounds in ripened banana, increase. Initially, sucrose is the predominant soluble sugar at the onset of ripening. Subsequently, fructose followed by glucose become the main soluble sugars [15, 16].

Fatty acids can be categorized as saturated fatty acids (SFAs), monounsaturated fatty acids, and polyunsaturated fatty acids. The major fatty acids in banana are palmitic (16:0), stearic (18:0), oleic (18:1), linoleic (18:2), and linolenic (18:1) acids [17, 18, 19]. Phenolics are important secondary metabolites in banana peel and are contained at a higher level compared with those found in other fruits [20]. The phenolic compounds in banana peel are categorized into four subgroups, as summarized by Vu et al.: hydroxycinnamic acids, flavonols, flavan-3-ols, and catecholamines [21]. Bioactive amines are also contained in banana and their levels decline during ripening. Dopamine and noradrenaline are susceptible to enzymatic browning; these substances are responsible for such reactions in bananas [15]. The peel color of banana generally turns from green to yellow because of the degradation of chlorophylls [22, 23]. Overall, these changes contribute to the appearance and quality of ripened banana.

Table 1: Estimated biochemical compounds in bananas

Compound (units)	Banana varieties / Origins	Contents		Ref.
		Green stage	Yellow stage
Carbohydrate in pulp
Starch (g/100 g)	Prata Banana / Brazil	15.7	3.4	[15]
	Cavendish	13	2	[16]
Fructose (g/100 g)	Prata Banana / Brazil	0.52	6.27	[15]
	Cavendish	1.5	7	[16]
Glucose (g/100 g)	Prata Banana / Brazil	0.35	4.63	[15]
	Cavendish	1.5	7	[16]
Sucrose (g/100 g)	Prata Banana / Brazil	0.39	3.4	[15]
	Cavendish	1	3	[16]
Fatty acid in pulp
Palmitic acid (16:0) (mg/100 g)	Kolikutttu / Sri Lanka	-	90.88	[17]
	Musa sp / Brazil	-	242.6	[18]
	Montham / India	-	103	[19]
Stearic acid (18:0) (mg/100 g)	Kolikutttu / Sri Lanka	-	6.84	[17]
	Musa sp / Brazil	-	16.9	[18]
	Montham / India	-	6.8	[19]
Oleic acid (18:1) (mg/100 g)	Kolikutttu / Sri Lanka	-	13.15	[17]
	Musa sp / Brazil	-	26.5	[18]
	Montham / India	-	8.73	[19]
Linoleic acid (18:2) (mg/100 g)	Kolikutttu / Sri Lanka	-	51.07	[17]
	Musa sp / Brazil	-	145.2	[18]
α-linolenic acids (18:3) (mg/100 g)	Kolikutttu / Sri Lanka	-	39.71	[17]
	Musa sp / Brazil	-	186.4	[18]
Vitamins and Minerals in pulp
Vitamin A (μg/100 g)	Kolikutttu / Sri Lanka	-	14.37	[17]
Vitamin D2 (μg/100 g)	Kolikutttu / Sri Lanka	-	0.4	[17]
Vitamin E (μg/100 g)	Kolikutttu / Sri Lanka	-	81.44	[17]
Vitamin K1 (μg/100 g)	Kolikutttu / Sri Lanka	-	1.81	[17]
Vitamin B1 (μg/100 g)	Kolikutttu / Sri Lanka	-	51.18	[17]
Vitamin B2 (μg/100 g)	Kolikutttu / Sri Lanka	-	83.9	[17]
Vitamin C (mg/100 g)	Kolikutttu / Sri Lanka	-	14.1	[17]
Pottasium (K) (mg/100 g)	Kolikutttu / Sri Lanka	-	554	[17]
Calcium (Ca) (mg/100 g)	Kolikutttu / Sri Lanka	-	45.5	[17]
Magnesium (Mg) (mg/100 g)	Kolikutttu / Sri Lanka	-	38	[17]
Phenolic compounds in pulp
Gallic acid equivalent (mg/g)	Musa AAB group / Sri Lanka	-	39.69	[20]
The total amount of amines in pulp (mg/100g)	Prata Banana / Brazil	3.52	3.12	[15]
Pigments in peel
Chlorophyll a (mg/cm2)	Musa AAB group / Sri Lanka	7.74	0.6	[22]
Chlorophyll b (mg/cm2)	Musa AAB group / Sri Lanka	4.9	1.1	[22]
β-carotene (µg/100 g)	Dwarf Brazilian / Hawaii	-	73	[23]
	Williams / Hawaii	-	42.8	[23]
α-carotene (µg/100 g)	Dwarf Brazilian / Hawaii	-	92.6	[23]
	Williams / Hawaii	-	60	[23]

2.2 Chilling injury

Low temperature storage is the most effective technology by which to prolong shelf-life and maintain fruit quality [24]. However, chilling injury (CI) occurs when banana is stored at temperatures <13 °C [25]. CI symptoms, which include subepidermal discoloration, occurrence of extensive brown spots on the peel, loss of original aroma and flavor, inhibition of starch–sugar conversion, and reduction of aroma formation, develop gradually in chilled banana. Furthermore, CI in banana triggers microbial decay, which leads to a decline in fruit quality and economic loss [26]. CI can occur at any stage in the supply chain system including harvesting, transportation, and preservation. The severity of CI depends on developmental stage, temperature, and duration of exposure [27].

Browning is a primary CI symptom in banana (Fig. 2); however, it cannot be detected immediately at low temperature. The obvious symptoms appear on the peel when banana fruit is exposed to high temperature conditions after low temperature storage. Peel browning in banana is induced by the reaction of phenolics with polyphenol oxidase and/or peroxidase. Currently, data on differences in chilling sensitivity according to the maturity stage of banana is not available. Further studies on the CI phenomena across maturity stages are therefore necessary to improve the practical handling of bananas.

The mechanism of CI in banana might involve a similar reaction to that in other chilling sensitive fruits. In banana, the CI mechanism starts with the phase transition of cell membrane lipids from a flexible liquid-crystalline to a solid-gel structure due to chilling stress. The loss of cell membrane integrity is then caused by the degradation of cellular membrane lipids [28]. Increasing of electrolyte leakage and malondialdehyde (MDA) contents can be quantitative indicators of chilling injury symptoms of bananas. Membrane permeability can be determined by electrolyte leakage while MDA is one of the final products of polyunsaturated fatty acids peroxidation [29]. Moreover, reactive oxygen species (ROS) are specifically involved in membrane structure breakdown as they oxidize biomolecules such as proteins and lipids. ROS production is enhanced by low temperature during storage. This process triggers oxidative stress, which leads to lipid peroxidation, protein degradation, DNA damage, and ultimately cell death. Stress-induced ROS can be eliminated using an enzymatic antioxidant system including scavengers such as superoxide dismutase (SOD), ascorbate peroxidase (APX), glutathione peroxidase (GPX), glutathione S-transferase (GST), and catalase (CAT). ROS are also eliminated by antioxidant substances such as ascorbic acid (AsA), glutathione (GSH), phenolic compounds, α-tocopherols, carotenoids, and flavonoids [30]. Therefore, levels of ROS are increased enormously when plants suffer stress or the scavenging system does not function normally. Induced cellular antioxidants are an indicator of the primary defense system of banana fruit tissue against chilling stress [31, 32].

The sensitivity of both the peel and pulp in banana in response to chilling temperature was previously reported [33, 34]. They analyzed the temperature dependence of electrolyte leakage from banana tissues using an Arrhenius plot; they found breaking points at 8.9 °C and 3 °C in peel and pulp, respectively. In contrast, other studies have been conducted from the perspective of fatty acid changes because they are strongly correlated with physical phase transition of membrane lipids. The ratio of unsaturated fatty acid (USFA) against saturated fatty acid (SFA) decreased more substantially in peel than it did in pulp during storage under chilled conditions [34].

Figure 2: The appearance of normal ripen banana (A) and chilling injured banana (B)

3. Postharvest technologies for banana

3.1 Artificial ripening technology

3.1.1 The importance of artificial ripening

For practical reasons, banana is harvested for the commercial market at the mature green stage or pre-climacteric stage. Naturally ripened banana is not appropriate for commercial purposes because fruits become over-ripened during distribution due to the nature of climacteric fruit; this results in a significant economical loss for traders. Therefore, artificial ripening of banana is a postharvest technology that is considered important for minimizing losses during transportation, achieving timely distribution, and fulfilling consumer expectations [9].

3.1.2 Ripening room

The design of ripening room conditions should be optimized to tightly control the banana ripening process. The ripening room should be equipped with refrigeration, heating, and air circulation systems. The room should also be sealable to prevent the loss of ripening inducing agents such as ethylene gas (C₂H₄). During the ripening process, a large quantity of heat is released from banana; therefore, sufficient refrigeration capacity should be installed to control temperature. In subtropical areas, a heating system is needed to maintain room temperature during the cold season. In addition, the ripening room must have an adequate air circulation system; good circulation and a proper air flow pattern produces uniform banana ripening.

As an example, photos of the banana ripening room in operation are shown in Fig. 3 (A) and (B). The maximum processing capacity of the ripening room shown in the photo is 40 tons per chamber. The banana boxes are stacked with each hand-held hole and ventilation apertures exactly aligned, and the wall side of the stack is placed along a sheet to prevent air leakage. An airflow direction of this system is illustrated in Fig. 3 (C). The air with C₂H₄ is suctioned by fans on the ceiling, subsequently blows out from the top of both side walls, then passes into each banana box through the hand-hold hole. The static pressure generated by the air circulation fans provides uniform airflow through the ventilation apertures of the box. In a typical case, the temperature of the chamber is set at 20 °C on the first day of processing and reduced to 15 °C over four or five days. The temperature is controlled based on the experience of the operator depending on the desired ripeness. The degree of ripening is supplementarily monitored by the concentration of carbon dioxide in the chamber, which is generated by bananas’ respiration.

Figure 3: The exterior and interior of the banana ripening room. (A): Ripening chambers, (B): Ripening room condition (Pictures were taken by authors at Unifrutti Japan Corporation, Tokai, Ota-ku, Tokyo on 30 January 2020), (C): An illustration of air flow direction in the ripening room

3.1.3 Ethylene gas treatment

Ethylene (C₂H₄) gas is commonly used for artificial ripening of bananas. The artificial ripening technique is conducted in a ripening room equipped with ventilation and exhaust systems under an optimum temperature, relative humidity, and with C₂H₄. Banana is fumigated with C₂H₄ gas at a concentration of 10–150 ppm for 2–3 days [35]. After fumigation, the fruit is brought to the optimum ripening temperature. Low level C₂H₄ concentrations, such as 0.1 ppm, can accelerate the ripening of bananas. Another study reported that exposure to C₂H₄ at 100 ppm for 12 h induces endogenous ethylene immediately and raises CO₂ production as in the climacteric respiratory process [14].

The C₂H₄ fumigation technique cannot be applied practically in small ripening houses because of the high cost. Recently, microbubble technology that can produce tiny bubbles (10–50 μm in diameter) has also been used to accelerate ripening. This technology can assist the gas to dissolve into a solvent effectively. It has been reported that the color of banana peel changes after efficient treatment [36].

3.1.4 Acetylene treatment

Calcium carbide (CaC₂) can be hydrolyzed into acetylene (C₂H₂), which acts as a C₂H₄ analog and stimulates fruit ripening. It has been shown that C₂H₂ emitted from CaC₂ can enhance banana ripening. C₂H₂ can produce optimally ripened bananas with a uniform yellow color, good flavor, medium starch content, and comparatively soft texture after 120 h of treatment [37]. Compared to C₂H₄, C₂H₂ has lower biological activity; therefore, it was reported that the concentration should be >2,800 ppm to enhance the ripening of bananas [9]. Another study mentioned that no change was observed in bananas when they were exposed to 10 ppm of C₂H₂. In contrast, exposure to 1,000 ppm of C₂H₂ produced indistinguishable color and soluble solids content, which was the same effect as that of ethylene treatment [38]. In addition, no significant difference in sensory attributes was detected between bananas treated with C₂H₄ and C₂H₂ at 1,000 ppm when both treated bananas were compared at the same stage of ripeness [9].

3.1.5 Ethephon treatment

Ethephon (or 2-chloroethyl phosphonic acid) can be used as an alternative ripening agent. Ethephon, which acts as a C₂H₄ releasing compound within the fruit, is widely applied, much like C₂H₄ gas. Studies have reported that ethephon successfully induces ripening in banana [39]. However, the application of this chemical is limited due to the hazards of the compound [40].

Recently, a study revealed that ethephon at 1,000 ppm showed similar efficiency in enhancing banana ripening; it produced a four-fold increase of ripening acceleration compared with natural ripening. Additional results showed that ethephon exposure could produce entirely yellow fruit within the first two days, whereas natural ripening to this stage took eight days. This agent also significantly accelerated physiological changes such as flesh firmness, pulp moisture, TSS, and pH. Moreover, biochemical compounds such as total sugar, vitamin C, and pigments (chlorophylls and carotenoids) were also induced by this ripening treatment. Phenolics, flavonoids, and antioxidant activity in banana flesh were also significantly increase by ethephon [20].

3.1.6 Current issues in ripening control

The artificial ripening process must be carefully considered to ensure that bananas are ideally yellow and ready to eat when they reach the markets or stores. Unfortunately, a lack of uniform ripening often occurs in many banana lots under practical conditions. Thus, ripening treatment should be enhanced to produce uniform lots. However, the use of ripening agents has hepatotoxic potential in human bodies, especially when ethephon is used [41]. Ethephon is an organophosphorus compound that is rapidly absorbed in the gut. In developing countries, the use of certain ripening agent doses sometimes is not considered well to accelerate the ripening process [42]. A better understanding, perception and action of the use of ripening agents’ doses should be considered more carefully by farmers and producers. Furthermore, postharvest strategies should be developed to mitigate the detrimental effects of ripening agents on the quality of bananas.

3.2 Storage technology

3.2.1 Edible coating

Recently, coatings that preserve the quality of bananas have been developed. Natural edible coatings are extensively used due to their ability to maintain color, organic acids, sugar, and flavor components during storage [43, 44]. The main component of the coating materials is film-forming polymers including natural carbohydrates, lipids, and proteins. Additional functions can be given to edible coatings by adding active additives such as antioxidants, antibacterial agents, flavors, and nutrients.

The use of starch-based edible coatings on bananas could control gas transfer rates, i.e., respiration and ethylene production. Coated banana fruit shows reduced weight loss and improved firmness. The efficacy of a chitosan coating was also investigated in “Cavendish” bananas; it was found to retard the ripening process. The effects of different degrees of deacetylation of chitosan have also been tested. A combination of 2% chitosan and 80% deacetylation produced a positive effect that reduced weight loss and maintain the vitamin C, and sensory properties of banana. In addition, coatings can be used as bio-fungicides to control postharvest anthracnose in banana; the use of 10% Arabic gum incorporated with 1% chitosan was found to suppress the disease in banana [45].

3.2.2 Gas modification for retarding ripening

Controlled atmosphere storage (CAS) is a technique used to preserve the quality of fruits by adjusting the proportion of O₂ and CO₂ in the storage environment, the levels of which are important for controlling banana ripening [46]. The proportion of O₂ and CO₂ in CAS has been reported for banana storage by several researchers. Although no general recommendations for optimum O₂ and CO₂ levels exist for CAS storage of bananas, most researchers favor a combination of 2% O₂ and 6%–8% CO₂ at an optimum storage temperature of 11 °C–15 °C. Exposing banana to 7.5% O₂ at 18 °C (normal ripening temperature) can entirely prevent the ethylene synthesis process during storage [47].

A combination of low O₂ and high CO₂, e.g., 2% O₂ and 4%–8% CO₂, has been shown to suppress the respiratory activity of banana at the pre-climacteric stage during storage [3]. Jedermann et al. [48] confirmed that 2% O₂ and 5% CO₂ could extend green life in storage vessels for 4 days at 15 °C and for 11 days at 18 °C.

Modified atmosphere packaging (MAP) is a packaging system for prolonging the shelf-life period of bananas. In this preservation technique, the surrounding air in the package changes depending on the permeability of the packaging film material and the respiration rate of fresh produce inside the package [49]. Similar to CAS, this technique can be used to minimize the physiological and microbial decay of perishable fresh produce using gas modification in the storage environment. MAP can be applied either by direct gas flushing, known as active MAP, or by respiration of the enclosed product, known as passive MAP. Importantly, the selection of packaging material is critical to achieving control of the desired atmospheric conditions within the package.

MAP comprising 75-μm-thick low-density polyethylene (LDPE) film as a packaging material can extend the storage life of bananas from 4 to 20 days at 25 °C [47]. Moreover, the MAP systems that uses a silicone membrane and diffusion channel systems can successfully establish a gas composition of 3.5% CO₂ and 3% O₂ within the package [50]. Recently, polypropylene and polyvinyl chloride film have been recommended as suitable packaging materials for banana at different storage temperatures according to the respiration rate and their permeability characteristics [51].

3.2.3 Alleviation of CI

CI has detrimental effects on both the quality and economic value of banana; therefore, several researchers have attempted to overcome the problem of CI in these fruits. Indeed, studies have examined the effectiveness of treatments such as melatonin and glycine betaine and the use of packaging material for reducing CI in bananas.

Melatonin (N-acetyl-5-methoxytryptamine) is an endogenous signaling molecule that can increase the resistance of plants to biotic and abiotic stresses. As an amphiphilic molecule, melatonin can freely cross the cell membrane and stabilize this membrane because of its ability to enhance the resistance of cells to free radicals. Therefore, the efficacy of exogenous melatonin application on alleviating CI and strengthening the chilling tolerance of plants has been tested [52].

In the case of banana, melatonin treatment entirely suppressed peel browning at 7 °C when fruit was stored for three days; in contrast, control fruit showed evident peel browning from the first day. In control fruits, after the peel pitting and brown patches began to appear, they became more severe as storage duration increased at low temperature. During storage from 0 to 9 days, the CI index increased gradually from 0 to 2.75 in melatonin-treated banana fruit, but it increased rapidly from 0 to 3.67 in control fruit.

The utilization of glycine betaine (GB) has been also reported to prevent CI in various horticultural products. In sweet pepper, antioxidant enzyme gene expression and activity were promoted by GB treatment and chilling tolerance was enhanced [32]. In the same case, GB was applied to peach fruits to reduce CI under refrigerated storage [53]. More recently, it was reported that GB improved CI symptoms in “Nanguo” pears by modulating phenylpropanoid, soluble sugar, antioxidant enzymes, and proline metabolism [54]. GB treatment also effectively alleviated CI in banana fruit through maintenance of high nonenzymatic and enzymatic antioxidant content, which was able to prevent ROS accumulation and thereby protect cells against oxidative damage [29].

Packaging material, such as blue transparent polyethylene, white opaque polyethylene, and bubble wrap composed of LDPE, can reduce CI symptoms in banana. Moreover, polypropylenes, such as white nonwoven fabric and white laminated nonwoven fabric, can be used for packaging. Polyethylene performs better as banana packaging because it has a higher CO₂ to O₂ permeability ratio than that of polypropylene as well as lower heat transfer [55]. Nguyen et al. [56] stated that using nonperforated polyethylene bags could reduce CI symptoms in bananas stored at 10 °C when the activities of phenylalanine ammonia-lyase and polyphenol oxidase were suppressed.

3.2.4 Current issues in storage

Losses of banana during storage are the main issue in the supply chain that contributes to postharvest losses. Obtaining appropriate storage conditions by controlling temperature and humidity is necessary to maintain the quality of bananas. The effect of packaging and temperature storage on banana have been investigated that quality and peel color development were generally achieved by the ripening of banana fruits at 20 °C and 25 °C with perforated plastic packaging [57]. Moreover, according to reported studies, CAS and MAP are accepted as preservation methods for extending the shelf-life of bananas [58]. However, the high capital cost of the gas control equipment that establishes an appropriate gas composition limits the widespread usage of CAS technology. For MAP, the occurrence of spoilage, which produces food toxicity, is a problem that still needs to be solved. Therefore, more appropriate packaging materials should be developed to avoid an anoxic package environment when the product is stored at the intended temperature. Maintaining proper storage conditions at 11 °C–15 °C and RH 95%–98% are important for preserving the quality of bananas, even though difficulties in applying the desired storage conditions exist at the retail level.

3.3 Transportation technology

In a supply chain, transportation is an unavoidable process required for the distribution of bananas to the market [59]. Bananas are susceptible to mechanical injury, such as bruising, cutting, breaking, impact wounding, and other forms of injury, during transportation [60]. Fruits can be damaged by mechanical stresses such as compression and impact forces during the storage, loading, and unloading of packages [61]. Typical mechanical damage to bananas presents as brown-black skin discoloration caused by the friction of fruits against either the corrugated carton walls or the sides of each banana finger.

The position of a banana package within a stacked pallet and the position of the pallet itself on the truck floor can lead to different levels of damage. Relatively higher levels of damage were observed in cartons stacked in the front position of the trailer compared to those stacked in the rear part. Furthermore, damage levels in the banana mechanical damage index were related to the level of input root-mean-squared vibration received by the pallets stacked in different positions on the truck [62].

Corrugated paperboard cartons are frequently used in food packaging due to the convenience in handling by growers and consumers. The effectiveness of different types of packaging for bananas, including one-piece corrugated cartons, two-piece corrugated cartons, and reusable plastic crates, has been investigated [63, 64]. The rigid sidewalls of reusable plastic crates caused a multifold increase in the bruising of bananas clustered in the top-tiers relative to the damage suffered in two-piece corrugated cartons; this emphasizes the need for nonrigid packaging materials in which soft fruits, such as bananas, can be packed. Based on their overall protective performance, the one-piece corrugated cartons provided the highest reduction in banana damage levels caused by in-transit vibrations.

Under practical conditions, a lack of cushioning leads to the mechanical injury of bananas. This damage often occurs due to vibrations and impact, especially during transportation from the orchard to the packing house in large plantations because of typically poor road conditions. During transportation, loading and unloading activities are also a cause of banana losses in the supply chain. The respondents when handling was given less emphasis, which indicates that traders have limited awareness about the impact of poor transportation, loading and unloading, and the storage system.

4. Future perspectives and challenges

Banana is a climacteric fruit that shows peak respiration and ethylene production during ripening. In postharvest handling of bananas, the physiological, biochemical, and physical characteristics of the fruit should be considered because these parameters closely relate to quality deterioration after harvesting. Since drastic changes occur in bananas during ripening, the pre-climacteric stage has been recognized as an optimum harvesting time for commercial purposes. Moreover, CI occurs in bananas that are stored at <11 °C because of their susceptibility to low temperature. In addition, given the high sensitivity of banana peel to physical stress, mechanical damage such as browning appears quickly during distribution to consumers. Controlling the harvesting time of bananas is one alternative method for reducing mechanical damage during transportation. Based on the above data, it can be concluded that, for distant markets, banana should be harvested at 75% maturity and stored at 12 °C. For nearby markets, fruits can be harvested at 90% maturity and stored at 14 °C. For local markets, it is recommended that fruits are harvested at 100% maturity and stored at 16 °C. In summary, the longer the distribution distance to the banana market, the earlier the bananas should be harvested.

Problems still exist on postharvest technology applied to bananas. Various postharvest technologies, such as artificial ripening treatment and cushioning packaging, have been developed and used practically. However, some technologies such as CAS and MAP are difficult to apply because of their high cost. Limited knowledge and insufficient infrastructure are the main reasons for banana waste, especially in developing countries. Such postharvest losses are caused by obstacles related to harvesting handling procedures, the failed artificial ripening process, poor storage infrastructure, and transportation. Furthermore, a lack of control over cleanliness, temperature, and humidity during storage are significant problems that affect the quality and quantity of products. People, especially in developing countries, are still unaware of the biological causes of these losses, including the respiration rate, ethylene production, and compositional changes (associated with color, texture, flavor, and nutritional value).

Postharvest technologies contribute to food security in multiple ways that are helping the world achieve sustainable development goals. The postharvest strategy should be better integrated into agricultural programs to provide technical advice and affordable solutions. To ensure high quality from the farm to consumers, the supply chain of bananas should be shorter and more efficient because the fruit is a perishable product. The best practices of postharvest handling must be applied as soon as possible to maintain the quality of bananas. A summary of the drawbacks of postharvest activities and recommendations for banana handling is presented in Table 2.

Table 2: Summary of the shortage and recommended postharvest handling of bananas

Point of activities	Real conditions	Recommendations
Harvesting and field transport	Rough and muddy field tracks Unsecured bunches during field transport	Optimizing the cushioning of banana to avoid peel injures
Ripening process	Improper stacking produced an imbalance ripening process	Avoid heterogeneous ripening induced by “ethylene pollution” Proceed to the ripening step without delay
Storage	Weakening and failure of the cartoon at high RH	Control the temperature at 13.5 ± 0.5 °C Maintain RH > 95%–98%
Transportation	Unsecured pallets produce mechanical injuries. High vibration transmissibility.	Use cushioning and the proper packaging during transportation

5. Conclusion

In this review, a comprehensive assessment of postharvest technology applied to bananas was provided. Postharvest physiology in banana has already been explored; consequently, the proper postharvest technology can be adopted for bananas according to their characteristics. As banana is a climacteric fruit, rapid deterioration in quality can occur; this should be prevented from the early postharvest stages. Artificial ripening requires the use of some agents for commercial purposes; however, the level of doses related to food safety must be considered. In addition, banana is sensitive to chilling temperature during storage; the optimum storage temperature is 11 °C–15 °C. Several technologies have been developed to prevent CI in bananas. Cushioning and adequate packaging of bananas remains important from the perspective of reducing mechanical damage to the fruit during transportation. Hopefully, this study can contribute to improving postharvest management on banana, which can reduce banana losses and maintain the quality of this popular fruit.

ACKNOWLEDGEMENT

The authors would like to express an appreciation to Unifrutti Japan Corporation (Tokyo, Japan) and T-NET JAPAN Co., Ltd. (Kagawa, Japan) for allowing us to include photos of their ripening room in this article and providing valuable information on it.

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