# Review of banana green life throughout the food chain: From auto-catalytic induction to the optimisation of shipping and storage conditions

Pierre Brat, C. Bugaud, C. Guillermet, F. Salmon

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Pierre Brat, C. Bugaud, C. Guillermet, F. Salmon. Review of banana green life throughout the food chain: From auto-catalytic induction to the optimisation of shipping and storage conditions. Scientia Horticulturae, 2020, 262, 10.1016/j.scienta.2019.109054. hal-03488740

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Review of banana green life throughout the food chain: from auto-catalytic induction to the optimisation of shipping and storage conditions

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P. Brat¹*, C. Bugaud¹, C. Guillermet², F. Salmon³

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*Corresponding author: Dr Pierre BRAT, CIRAD, Département PERSYST, UMR QualiSud, MUSE, 73 rue J.F. Breton, TA B-95/16, 34398 Montpellier cedex 5, France. Tel: +590 5 90 41 68 32. Fax: +590 5 90 86 80 77. Email address: pierre.brat@cirad.fr

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¹CIRAD, Département PERSYST, UMR QualiSud, MUSE. 73 rue J.F. Breton, TA B-95/16, 34398 Montpellier cedex 5, France.

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²CIRAD, Département PERSYST, UR GECO, Campus Agro environnemental Caraïbe, Quartier Petit Morne - BP 214, F-97285 Le Lamentin, Martinique, France.

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³CIRAD, Département BIOS, UMR AGAP, Station de Roujol, F-97170 Petit-Bourg, Guadeloupe, France.

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**Short version of title:** Banana green life Review

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ABSTRACT

Banana green life (GL) is the time between harvesting and the start of natural ripening. GL could be considered as a major quality criterion, as it defines whether or not a fruit is suitable for export and marketing. The ending of GL, when climacteric crisis occurs, is characterized by autocatalytic ethylene production. Ethylene synthesis and regulation are described. The main methods for determining Gl are based on detecting the CO2 peak or a decrease in green pigments, using a spectrometer (NDVI) for the latter. Temperatures during fruit growth and Black Sigatoka disease are the main pre-harvest factors affecting GL. The former can be managed by applying the thermal sums concept and the latter by adequate field practices. The effects of the main exogenous storage parameters (storage temperature, relative humidity, concentration of ethylene and O2/CO2 ratio in the atmosphere) could be modelled in some cases.

The most effective solutions for extending GL rely on either developing coatings using new preservative compounds, or designing packaging capable of controlling the temperature, CO2/O2 ratio and ethylene concentrations in the environment close to the fruits. It was demonstrated that 1-MCP is not relevant for increasing GL. A global and integrated approach involving the overall optimization of pre- and post-harvest factors needs to be applied to maintain green fruit until voluntary/artificial ripening is induced.

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Keywords: banana; green life; ethylene; shipping; storage

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42 **1. Introduction**

Banana is the fourth most consumed product in the world and approximately 50 billion tonnes of Cavendish bananas are produced globally every year (FAO, 2018). Bananas are not grown in Europe and thus need to be imported from the production sites, which are mainly located in Central and Latin America, and in West and Central Africa. Bananas are harvested green and ripened in market areas. Fruits that are allowed to ripen on the tree often split, and tend to be mealy. The pre-climacteric period after harvesting is vitally important for importers and ripeners, because bananas are transported before they are ripened.

During this period, mature green fruit have a low basal respiration rate and ethylene production is almost undetectable. This period is also called the “green life” (GL) and the longest practical “pre-climacteric period” is sought (Hailu et al., 2013b). The length of this period is influenced by several factors, such as temperature, relative humidity (RH) or the surrounding ethylene concentration. At the onset of fruit ripening in climacteric fruits, such as banana, a rapid rise in respiration and autocatalytic ethylene production is observed. Indeed, at this key point, a cascade of enzymes implicated in the ethylene synthesis pathway occurs, initiating the onset of fruit ripening, hence the end of the GL period. The first section of this review is therefore devoted to the fruit itself, with special focus on ethylene synthesis auto-induction mechanisms, the physiology of the fruit, and the correlative prediction of banana GL.

GL prediction is of great scientific, but also economic interest, so a dedicated section will focus on the different approaches and ways of evaluating this quality factor.

Field conditions largely affect ripening onset and numerous publications have studied the influence of agronomic conditions (temperature, fungal pressure, etc.). Indeed, fruit growing conditions, or disease control on plants in the orchard, will lessen or increase the GL period and therefore need to be closely considered.
Numerous post-harvest treatments and technologies, such as controlled or modified atmospheres, have been developed to maintain GL duration as much as possible, and thereby limit potential losses during the fruit shipping and delivery stages. A section will be dedicated to this and a special section will be developed on management at reefer level.

Lastly, post-harvest technology trends for the future will be proposed in order to raise
stakeholder awareness of the need to develop novel and safe ways of optimizing GL in the
coming decades.

2. Green life at fruit level

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2.1. Green life ending and occurrence of the auto-catalytic ethylene process

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As clearly depicted by Dominguez and Vendrell (1993), the ripening of climacteric fruits
such as banana shows a rapid rise in respiration and autocatalytic ethylene production. In these
fruits, ethylene is considered the trigger because exogenous ethylene induces ripening and
inhibition of its synthesis prevents it. They established that the regulation of ethylene
biosynthesis seems to depend on ACC (1-aminocyclopropane-1-carboxylic acid) availability,
but also on the capacity of the tissue to convert ACC to ethylene.

The pathway for ethylene biosynthesis was clearly depicted in apple, and other fruits such
as avocado, banana, and tomato by Kende (1993). At the onset of fruit ripening, the expression
of multiple ACC synthase genes is activated, resulting in increased production of ACC, which
is then oxidized to ethylene by ACC oxidase. In most cases, it is the ACC synthase activity that
determines the rate of ethylene biosynthesis. In banana fruit, ethylene biogenesis in the naturally ripening fruit is initiated by some factors involved in the regulation of the ageing process. To establish the succession of events occurring from the green to the ripening stage, and therefore to gain a better understanding of the banana GL ending process, the study by Areas *et al.* (1988) is worth looking at in detail. Ethylene synthesis was enhanced by using protein synthesis inhibitors meaning that these inhibitors affected the equilibrium mechanisms of ripening at tissue level. In fact, the capacity for ethylene production, and therefore the climacteric peak, is often associated with the integrity of the cell membrane integrity. Maintenance of the ethylene concentration in the fruit tissues under its threshold level during fruit GL might ultimately be a result of an intensive repair process that is highly dependent on protein synthesis (Areas et al., 1988). The biosynthesis of ethylene from methionine involved two key steps: i) the ATP (Adénosine-Triphosphate) break, which affects the conversion of methionine into S-adenosylmethionine (SAM) and ii) ACC oxidase activation, which affects the conversion of ACC into ethylene. After harvesting of the fruit, ACC and ethylene production are low in banana fruit during the pre-climacteric phase. About 0.20 $nl.g^{-1}.h^{-1}$ of ethylene and 0.25 $nmol.g^{-1}$ ACC were detected in immature banana during the pre-climacteric period (Li and Huang, 1988) and their levels remained nearly constant up to the onset of ripening. At the onset of ripening, meaning the end of GL, ethylene production and ACC levels increase markedly, with ethylene production reaching a peak before the respiratory climacteric (Li and Huang, 1988). Trace amounts of endogenous ethylene may be responsible for the gradual elevation of ACO (ACC-oxidase) activity during the pre-climacteric stage (Gao *et al.*, 1990).

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## 2.2. *Changes in biochemical compounds throughout fruit GL*

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There are only a few papers dealing with changes in fruit composition throughout the GL period. There are 7 stages in the banana fruit ripening process, with GL covering the first two stages with, for the Cavendish *cv.*, a sugar content close to 0%, a constantly high starch content (23 to 25%) and CO₂ respiration ~ 4 mg CO₂·kg⁻¹·h⁻¹ (Mees, 2017). The only visible parameter is chlorophyll declining gradually during the green life period and making the peel turn a deep green to green with a faint hint of yellow.

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There is no starch hydrolysis during GL and thus we could not relate the changes in fruit GL to histological changes such as starch hydrolysis (Jullien *et al.*, 2001). The water-soluble tannin content of banana pulp decreased and then increased during ripening (Chang and Hwang, 1990), while that of banana peel decreased continuously during storage, especially during ripening. A possible relationship between critically low levels of free indole-3-acetic acid (IAA), the onset of starch degradation and therefore banana GL, was suggested by Purgatto *et al.* (2002). In relation to starch degradation during banana ripening, IAA levels can be considered as one of the regulatory factors, or at least a compound that can efficiently delay the onset of the ripening process.

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## 2.3. Impact of exogenous factors on the auto-induction of ethylene synthesis

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### 2.3.1. Relative humidity storage effect.

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With storage at 20 °C, the impact of water loss on GL (d) could be estimated by linearization, the resulting equation being GL (d) = 22.15 – [5.3 x % Weight Loss per day] (p < .001) (Littmann, 1972). As a result, for a shipping time of ~10 d and a total fruit weight loss of 10% after shipping, GL was reduced from 22 to 16 d. Finger *et al.* (1995) also confirmed the
effects of water loss on banana fruit GL (*Musa accuminata* Colla), a water loss of 5% leading to a shortening of GL in pre-climacteric fruits. In fact, ethylene production in pre-climacteric fruits was stimulated by water stress, causing an increase in respiratory metabolism, such as observed in vegetative tissues. It is likely that the ethylene induced by water stress triggered autocatalytic ethylene production by the fruit and the climacteric rise of respiration. Indeed, it is commonly accepted that maintaining fruits at a low relative humidity (RH) accelerates water loss and also causes earlier changes in respiration and ethylene production (Burdon *et al.*, 1994). A gradual increase in the ACO activity of pulp and peel stored at a high RH was also observed.

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## 2.3.2. Storage temperature effect.

At room temperature, fruit respiration is maintained at the normal level, but if the fruit is stored at a lower temperature, fruit respiration will be reduced, resulting in a decreased of ATP, hence reduced ethylene biosynthesis. The purpose of storage temperature control is therefore to limit fruit respiration by suspending ethylene biosynthesis, thereby extending fruit post-harvest life, including green life. Fruit GL bears an inverse linear relationship to the temperature at which fruit are kept. An average regression equation established on the average of 4 experiments was determined (Wills *et al.*, 2001) as GL (day) = [-0.97 x θ] + 93 (θ storage temperature in °F). Thus, at 55 °F (12.8 °C) and 59 °F (15 °C), the expected GL was estimated at 40 and 36 days respectively, meaning that a 1 °F increases during storage reduces potential GL by one day. Blake and Peacock (1971) extended these former findings to higher temperatures (60 °F (15.5 °C) to 96 °F (35.5 °C)) and obtained the following regression equation GL (d) = 10 [(-0.02529 x T °F) + 2.898] meaning that at 15.5 °C (60 °F) the estimated GL of the fruit was 24 days. A 12 days difference in GL was therefore obtained applying the two
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regression equations mentioned above, highlighting the precaution that should be taken if using
it.
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The high sensitivity of banana fruit to the duration and intensity of low temperatures that
could provoke chilling injury (Jackman *et al.*, 1988) should be kept in mind at all times to
effectively match the constraints in the banana supply chain. Indeed, considering the shipping
duration (usually 10 to 20 days), hence the long cold-stress induced, the air-shipping
temperature of the reefer at 12.5-13 °C should really be considered as a critical value, hence a
limit for possibly extending GL. With a view to curtailing post-harvest losses and extending
fruit availability, Gonge *et al*. (2013) studied the combined effect of harvest maturity stages and
storage temperatures throughout the shipping of bananas (cv. Grande Naine). The greater the
physiological age is at harvest the lower is the GL (Table 1). Indeed, harvesting bunches at 90%
of the total maturity allows the GL to increase to 2 or 3 days, this behaviour being confirmed
whatever the storage temperature (14 or 16°C). Even if potentially interesting in terms of GL
extension, 12 °C implies chilling injury and is therefore not recommended. Joas (1987)
estimated the potential GL reduction at 7-8% of GL for a 1 °C higher storage temperature,
whatever the maturity at harvest, and therefore partially confirmed the data previously
presented (~ 4 days lost from 12 to 14 °C storage T °C). As a conclusion at this point, the faster
the fruit can be cooled after harvesting, then the longer would be the expected GL, the fruit
being less sensitive to any ethylene present in the post-harvest chain (Turner and Fortescue,
2012).
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Table 2 presents an overview of the regression equations established to estimate the
potential GL of banana depending on the limiting temperature and CO2 conditions. Averaging
the equations resulting from different studies, at 13°C the estimated GL is ~35 days and is
reduced by 3 days for a 1 °C increase in storage, which tallies well with the 7-8% GL time
reduction for 1 °C higher proposed by Joas (1987).
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### 2.3.3. Light effect.

### 2.3.4. Effect of O₂ and CO₂ concentration during storage.

Although not specifically demonstrated for banana, it has been reported for many years that light decreases the amount of ethylene produced (Sisler and Wood, 1988). The level of ethylene production could be stimulated above the dark level when both CO₂ and light were supplied. While maintaining the fruit at an ethylene concentration below 0.005 ppm, banana GL was reduced by ~50% when the fruit was exposed to light. The response did not seem to be dependent on the physiological age at harvest and was greatest when exposure started immediately after harvest (Peacock, 1972). Although this aspect is not relevant for bananas as they are of course shipped in boxes in the reefer, therefore in the dark, this tendency could help in understanding fruit green life behaviour in relation to mechanisms induced by ethylene.

The submitted hypothesis in view of the apparent role of CO₂ in facilitating the conversion of ACC into ethylene was that illuminated thylakoid membranes also convert ACC into ethylene, and that this conversion can be stimulated by added bicarbonate. However, since this reaction only occurs in the light, it is presumably free radical-mediated and non-enzymatic. The review by Sisler and Wood (1988) tried to clear up the controversial effect of CO₂, finding that low levels of carbon dioxide regulate ethylene synthesis and higher levels inhibit ethylene action. It seems, in fact, that CO₂ counteracts the effects of ethylene in many cases but it can also stimulate ethylene production by increasing the amount of ACC compared to ethylene, and by increasing the activity of that enzyme (Bufler, 1986).
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The importance of oxygen concentration in the storage atmosphere for controlling banana fruit ripening is very important. It is indeed possible to prevent any synthesis of ethylene, above the minute amounts found emanating from unripe fruit, by storing the fruit in oxygen atmospheres containing not more than 7.5% oxygen at the normal ripening temperature (18 °C). Under such conditions, the oxygen tension is sufficient to prevent either injury and will not impart taste or flavour to the fruit.

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Combined storage in reduced oxygen and increased carbon dioxide reduced respiratory activity during storage and thereby boosted banana GL. The lowest weight loss was established at 2% O₂ and 4 to 8% CO₂, with the weight loss for 14 storage days being reduced by more than 37% using this controlled atmosphere (Hailu *et al.*, 2013). We can confirm that this specific gas composition helps to optimize banana GL during storage. Indeed, Jedermann *et al.* (2014b) confirmed that elevated CO₂ (5%) and low O₂ (2%) concentration in storage vessels led to an increase in average green life of 4 d at 15 °C and 11 d at 18 °C.

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To obtain these required O₂ and CO₂ target values, use of the controlled or modified atmosphere is the only way of maintaining this gas composition throughout the shipping phase. Modified atmosphere (MA) packaging, created by the respiration of the fruit itself inside a bag having specific O₂/CO₂ permeability, is recognized as an easy and inexpensive method of extending banana GL compared to the controlled atmosphere. Chamara *et al.* (2000) used Low Density Polyethylene bags (LDPE) as a packaging system to extend storage life, thus creating a modified atmosphere able to extend the storage life of banana (cv. Kolikuttu). Stewart *et al.* (2005) improved this system by using silicone membrane and diffusion channel systems to provide suitable conditions for optimum storage of Giant Cavendish bananas. As a result, they achieved levels of 3.5% CO₂ and 3% O₂ in about 10 d using this membrane, with these gaseous conditions being rather close to optimum conditions for banana (2% O₂ and 4% CO₂, Hailu et al. (2013)).

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Table 3, extracted from the study by Jedermann et al. (2014b), can be considered as a good summing up of the relative humidity, temperature and modified atmosphere effect atreefer level. Using extreme conditions, namely 18 °C in dried air as opposed to 15 °C in a wet atmosphere at 5% CO2, reduced fruit GL to around one third of its original potential.

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Considering that the RH inside Low Density Polyethylene bags during the shipping of fruits with a conventional perforation density of 0.01 dm².m⁻² (4 holes in a conventional polybag), is higher than 99%, each additional degree during storage from 15 to 18 °C reduces GL by 5 d or more. The detrimental effect of a low RH is indeed again highlighted, since at 15 °C the measured banana GL will drop from 48 to 42 d in normal air at a 50% RH. Lastly, the positive effect of a high CO₂ concentration (5%) in terms of GL extension seems to be more intense at a high storage temperature (18 °C).

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2.3.5. Effect of ethylene concentration during storage.

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Ethylene, which is auto-catalytically synthesized at levels as low as 0.01 µl.L⁻¹, triggers the ripening process (Prasanna *et al.*, 2007). It should be noted that while this observation is commonly accepted, the threshold of ethylene concentration likely to initiate the ripening process is debatable. Indeed, the ethylene concentration needed to initiate banana fruit ripening is highly dependent on the physiological age at harvest (sometimes called maturity at harvest) and on the storage temperature. In fact, it is unambiguous that there is no threshold of ethylene concentration for ripening, but instead that several factors such as ethylene contact time. The physiology of the fruit at harvest or the temperature background are the key factors delimiting fruit GL (Wills *et al.*, 1999). Indeed, for a long time, a lower ethylene limit that could initiate fruit ripening was discussed and the limit of 0.1 ppm (Inaba and Nakamura, 1988) was commonly accepted in the banana chain for many years. The study by Wills et al. (2001)

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254 confirmed banana to be one of the most ethylene-sensitive climacteric fruits. By fixing two
255 limits, a 10% loss in storage life that could be tolerable in most marketing situations, and a 30%
256 loss that could limit marketing opportunities, the authors established a regression equation at
257 20°C for banana GL (d) = [- 9.24 x Surrounding Ethylene Concentration] + 3.3. It appears
258 therefore that the marketing limit (30%) is reached with less than 0.03 ppm, while the tolerable
259 marketing limit (10% loss) will be at less than 0.01 ppm. Note that this value is not so far from
260 the limit of detection used in this paper, which was 0.005 ppm. Furthermore, the authors
261 established that 0.1 ppm, formerly accepted as a limit ethylene value, provoked a ~50%
262 reduction in GL under these conditions.

263 A reduction in surrounding ethylene concentration as a trigger to maintain or extend
264 banana GL was extensively studied by Wills et al. They investigated the impact of a wide range
265 of ethylene concentrations, ranging from rather high to ultra-low, during storage using different
266 banana cultivars from Australia (Wills et al., 1999). It appeared that, whatever the cultivar, GL
267 at 20 °C does not exceed 4 days at 1 ppm of ethylene, while rising to ~ 11 d at 0.1 ppm, ~24 d
268 at 0.01 ppm and, lastly, more than 33 d at 0.001 ppm. However, these extremely low ethylene
269 concentrations are not reachable in the food chain, even using specific ethylene absorbers in the
270 cardboard box, or at reefer level. Wills and Golding (2015); Wills et al. (2014) quantified the
271 relationship between ethylene concentration, temperature and GL of Australian-grown
272 Cavendish bananas. Fruits were stored at 15, 20 or 25 °C in air containing 1.0, 0.1, 0.01 or
273 0.001 ppm ethylene, and green life was taken as the number of days to reach the respiratory
274 climacteric. They found that banana green life decreased with increasing storage temperatures
275 and ethylene concentrations according to the relationship: GL (d) = 5.74 - (21.81 x X) - (0.82 x
276 X²) - (0.097 x Z) + (0.55 x Z), where X = log of ethylene concentration (ppm) and Z = storage
277 temperature (°C). These authors concluded that it would not seem unreasonable to expect that
keeping bananas at a low ethylene concentration would give a similar post-harvest life as at low
temperatures in current commercial situations, where ethylene levels can readily accumulate.

3. GL determination and forecasting

If we consider that GL is the time interval between the harvest date and the initiation of
the climacteric phase. This associated end-phase is therefore correlated both with an increase
in respiratory intensity with CO2 exhausted from 10-20 mg CO2.kg⁻¹.h⁻¹ to 60-120 mg CO2.kg⁻¹.h⁻¹, and with degreening of the peel associated with chlorophyll degradation. Any
determination methods are based on these physiological changes. Chillet *et al.* (2008) detected
the climacteric peak of a banana placed in a semi-permeable container, through an analysis of
O₂ and CO₂ levels resulting from fruit respiration. This method allowed accurate measurement
(± 1 d) of GL compared to methods based on fruit colour or decrease in texture. Anyway, the
successive phases: ethylene auto-induction and respiration peak, and green colour and texture
decreasing range no more than 2 d, this time day difference being far less the GL distribution
among the fruits of a single bunch.

New spectrometric methods have been developed to limit the two main limitations of the
methods described above. These spectral approaches are not destructive and can therefore be
applied directly on the fruit still attached to the bunch in the orchard; they give an immediate
response while the former method needs, by principle, to wait for the end of GL. Among criteria
for determining the end of green life, the parameter NDVI (normalized difference vegetation
index) was measured with a hand-held spectrometer (Pigment Analyser, CP Falkensee,
Germany) (Praeger *et al.*, 2013). NDVI indicates chlorophyll content in plant material and is
calculated from the reflected light intensity I in the following manner: NDVI = (I<sub>780 nm</sub> - I<sub>660 nm</sub>)
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/ (I<sub>780 nm</sub> + I<sub>660 nm</sub>) (Richardson *et al.*, 2002; Zude *et al.*, 2009). The NDVI values for green bananas at ripening stage 1, just after harvest, vary from 0.3 to 0.2, while at the end of GL NDVI falls between +0.05 and -0.05. This pigment analyser, which gives the NDVI value, could therefore be considered as a potential tool for directly and instantly giving a good estimation of the potential GL of the fruit in the orchard. It should however be validated on large fruit samples including different physiological ages at harvest, disease severity or shipping conditions.

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**4. GL optimization in the orchard**

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In general, any action that involves physiological stress on a fruit causes a reduction in the pre-climacteric period, which can be associated with increased ethylene production resulting from the stress. Extremes of temperature, fungal infection, exposure to light, dehydration and mechanical injury are all common causes of stress during normal commercial operations that reduce the pre-climacteric period of banana.

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**4.1. Pre-harvest factors affecting GL in the field**

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**4.1.1. Nutrients and agronomic conditions management.**

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Excess nitrogen produces crops with smaller fingers and bunches, and reduced their conformity. Bunches can be poorly filled and have a reduced GL if harvest occurs to late (Robinson, 1996). For instance, Srikul and Turner (1995) reported that fertilising with 450 kg N.ha⁻¹.year⁻¹ increased the relative fruit growth rate of cultivar “Williams” (Cavendish

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subgroup) bananas by 7% (P < 0.05) and hasten the relative loss of GL by 46% (P < 0.05).

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From 0 to 300 kg N.ha⁻¹ year⁻¹, N had no significant effects on fruit growth or GL. Robinson

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(1996) also mentioned that excess of manganese in leaves leads to a reduction in green shelf-

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life, thus contributing to the disorder known as "ripe mixture". Lastly, calcium, known to

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increase the shelf-life of some fruits, such as apples does not seem to be efficient on bananas

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(Tixier *et al.*, 2010). Lastly, calcium is known to strengthen cell tissue and thus can increase

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the shelf-life of some fruits, such as plum fruits (Sinha *et al.*, 2019). However, it does not seem

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to be the case with bananas as Tixier et al. (2010) showed that applications of calcium in the

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field, by fertilization or spraying, had no influence on the GL of Cavendish and CIRAD 920

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hybrid bananas.

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As revealed by Mahmoud and Elkashif (2003), plant spacing has a significant impact on

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fruit GL. The wider spacing resulted in significantly longer green life than the closer spacing in

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both the plant and ratoon crops. This was because plants grown at a wider spacing produced

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large, well developed bunches with large fingers which were more likely to have a longer green

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life than those produced at a closer spacing.

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### 4.1.2. Water stress.

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Water management is one of the first factors to consider in expressing the potential of

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banana. Excess water weakens fruit and may result in a short GL (Lassoudière, 2007).

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Conversely, a water deficit imbalances the fruit growth/development ratio, thus fruits will be

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often harvested at a higher physiological stage which means a lower GL. Srikul and Turner

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(1995) reported that a soil water deficit can reduce GL by 48% (P < 0.05) in a semi-arid

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environment. Indeed, exposure of the roots to drying soil changes the chemistry of the fruit and
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after flowering, the estimated fruit GL is 28 d at 13.5 °C and that a one-week delay in bunch
harvesting would reduce GL by approximately 3 d.

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On the basis of the harvest thermal-sum method (i.e. daily sum of average temperature at the orchard since inflorescence emergence with a zero basis at 14°C) (Umber et al., 2011), these former limitations could be avoided. An harvest point really representative of the outer temperature and of the minimum Cavendish growth temperature could be provided. To ensure a GL of not less than 25 d at 98% RH, the recommended thermal sum for Cavendish is 900 ± 50 dd (degree.day) (Ganry and Chillet, 2008). Jullien et al. (2001) confirmed that fruit GL correlates well with bunch age expressed in dd accumulated since inflorescence emergence. In actual production conditions, the harvest determination method used in the FWI to ensure EU banana export potential is a compromise between the grade measurement and thermal sum methods. Indeed, to ensure an optimized yield, but avoiding any risk of ripening in the reefer during sea-shipping, the thermal sum is only used as a limit not to be exceeded, but bunches could be earlier harvested if the grade of the fruit reaches the required value.

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4.3. *GL optimization at bunch level*

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Banana bunch covering using thin plastic (low density polyethylene) at flowering is a
physical protection method that improves the visual quality of fruit by promoting skin
coloration and reducing blemishes. It can also change the micro-environment for fruit
development, which can have several beneficial effects on internal fruit quality (Santosh et al.,
2017). Jullien et al. (2008) showed that a difference of 2 °C between the air temperature inside
and outside the bag was measured. They also showed that plastic bags induce a difference in
accumulated dd between older fruits at the top of a bunch and younger fruits at the bottom. The

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earlier the bagging date, the larger is the difference in dd, and therefore in GL. Research reports on fruit bagging have given contradictory information on the effect of bagging on both the physical and compositional quality of fruits. Kimani *et al.* (2017) mentioned that bunch covers did not significantly influence GL (p>0.05).

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The solution of 'sealed' bunch cover should be emphasized since this hermetic bag adds the effect of modified atmosphere (MA) (Johns and Scott, 1989a). Atmospheres of 1.5-3% O₂ and 5-10% CO₂ have been claimed to be optimum for modified atmosphere within the sealed bag storage of pre-climacteric bananas at 20 °C. However, the MA requirements of growing bunches could differ markedly from the MA storage requirements of harvested fruit (Johns and Scott, 1989a). Unfortunately, leakage is almost unavoidable, even under shed storage conditions and consequently, the selective permeability of the polyethylene will be offset by the non-selective nature of the leaks (Wade and Graham, 1987). The bunch/bagging effect on GL was improved by Johns and Scott (1989b), who established that the inclusion of KMnO₄ in sealed covers, creating a modified atmosphere at bunch level, increased GL by an average of 8 d.

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## 4.4. *Seasonal effect*

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According to Marin et al. (1996) and despite of similar grade and age, GL was different between fruit harvested at different times of the year. Fruit harvested at the hot and dry season had a longer green life than those harvested during “rainy” winter in Costa-Rica. Seasonal transition seems to have an impact on the reduction of the pre-climacteric period for fruit developing rapidly initially and maturing slowly (summer/winter transition). The effect of climatic factors was later studied by Bugaud *et al.* (2007), who did not find any correlation between climatic conditions and the GL of bananas harvested at a constant physiological age.
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These results well matched with those previously obtained by Bugaud *et al.* (2006); Chillet *et al.* (2006), who showed that climatic variations had no significant impact on the GL of bananas harvested at the same temperature sum. It seems, in fact, that the shortening of GL during the hot wet season was probably more due to the higher Sigatoka disease pressure during that season.

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### 4.5. Black Sigatoka disease management

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Sigatoka Disease (SD) and Black Leaf Streak Disease (BLSD), caused by the pathogenic fungi *Mycosphaerella musicola* and *Mycosphaerella fijiensis*, respectively, are the only biotic stress in the field known to have an impact on GL (Chillet et al., 2006). Both pathogens affect banana leaves, thus reducing the photosynthetic surface area, but also affect fruit physiology and reduce post-harvest GL, up to field-ripe fruit in case of higher infestations (Castelan *et al.*, 2012). A number of studies were conducted to understand this effect, but until recently most of them had been conducted on SD. As BLSD is more aggressive, and is becoming predominant in most tropical countries (De Lapeyre de Bellaire *et al.*, 2010), a few specific studies are available on its relation with GL.

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#### 4.5.1. A reduction related to disease damages.

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A reduction in GL is often related to an indirect effect of SD, through the reduction of leaf area. The fewer leaves present on the plant during the flowering-harvest period, the lower the GL is. A minimum of 3 to 4 leaves at harvest for minimum quality fruit and to ensure fruit shipping potential is commonly recommended. Ramsey *et al.* (1990) and Daniells *et al.* (1994) even

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stated that a minimum of 10 leaves at harvest was necessary to prevent premature ripening, with all bunches having fewer than 4 viable leaves at harvest were field-ripened. This share decreased logistically as the number of viable leaves at harvest increased up to 10 leaves. A reduction in the number of leaves below 6 leaves after flowering through severe leaf pruning was also linked to a reduction in GL by Robinson *et al.* (1992). As a consequence, the number of leaves at the harvesting stage seemed a practical criterion that is easy to control in the field for evaluating potential fruit GL, and for limiting the ripening risk during transit (Ramírez *et al.*, 2008). Until now, exporters have often determined a number of leaves as a threshold (usually 4 to 5 viable leaves at harvest) beyond which fruit should not be exported, or the harvest grade must be reduced, to supposedly avoid fruit ripening during transit. However, this criterion has proved to be highly empirical, and has often led to unsuitable decisions.

The qualitative descriptions of SD levels related to GL reduction was known for a long time. Premature ripening occurred at very low spotting levels, at least eight leaves without spotting should remain at harvest to ensure an optimized GL (Stover, 1974). Yet, this level is seldom reached under production conditions, and only in regularly sprayed fields and/or under dry climatic conditions. However, it should be noted that in most of these first studies, fruits were harvested following a calliper grade threshold and/or the number of weeks between flowering and harvest, without considering their physiological age. It is possible that, in some cases, fruits from infested plants took more time to reach harvest grade, and thus were considerably more mature than indicated by size and appearance at the time of harvest, with a lower potential GL.

Taking into account the physiological age of the fruit at harvest by the thermal sum method, Ganry and Chillet (2008) proved that quantitative evaluations of the disease level were more correct in predicting GL than the qualitative or indirect effects of the disease. Chillet *et al.* (2009) confirmed that SD had a direct effect on banana GL through the harvesting of fruits
at the same physiological age. Under these conditions, GL decreases as SD pressure increases, even though the fruits have an identical diameter, proving that the reduction in GL is not caused by a trophic effect, but rather by a direct effect of the disease. The best disease level indicators for GL are quantitative: the disease severity indexes at flowering and at harvest. Even though BLSD is more aggressive than SD, the same results were obtained with BLSD, with no greater reduction in GL than for SD (Castelan et al., 2012). A linear negative relationship between disease parameters and GL was further established for SD by Castelan *et al.* (2013): the SD effect is gradual and continuous, with no threshold effect. Such work has not yet been conducted for BLSD, but it can be assumed that the same observation would be made.

In addition to the severity of the disease, the duration of the disease is also critical for its impact on GL. Under southern tropical conditions in Brazil, with a longer flowering-harvest period (FHP), the reduction in GL was drastic in plots highly infested by both SD and BLSD, with only field-ripened fruits at the time the bunches reached 960 dd (Castelan et al., 2012). Thus, the longer the disease stays on the plant, the greater is the reduction in GL.

### 4.5.2. Potential of agronomic practices as a tool to limit this stress.

Some sanitary practices, such as deleafing (or leaf pruning), which consists in systematically eliminating all necrotic parts of the leaves by cutting them off, proved to be efficient in limiting the negative effect of SD and BLSD on GL. Necrotic leaf removal from SD-infested plants led to a large reduction in SD effects on fruit quality, preventing premature ripening (Chillet *et al.*, 2013). This technique allowed the harvesting of bunches with as few as 3 leaves at the time of harvest. The same observation was made by Guillermet *et al.* (2016) on BLSD under both humid and dry tropical conditions. Under these conditions, strict elimination of necrotic tissue, and adequate cultural practices including optimal plant growth and harvesting
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time, allowed the exporting of bunches with as few as no leaves at harvest, without ripening in transit.

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These results show that systematic removal of necrotic parts of leaves can be an efficient technique for limiting production losses. It furthermore reveals that the presence of necrosis from SD or BLSD is responsible for GL reduction, whatever the number of leaves at harvest. Thus, this criterion might not be the most appropriate for evaluating bunch suitability for export. It also highlights the existence of a potential “stress signal” emitted by necrosis, and directed to the bunch, responsible for the reduction in GL. The longer this signal is emitted, hence the longer the necrosis stays on the plant, the greater is the reduction in GL (Castelan et al., 2012). The timing of the signal also seems critical.

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### 4.5.3. Markers of GL reduction and hypothesis on the nature of the stress.

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For both SD and BLSD, GL reduction is associated with an altered maturation process in the fruit. Although the initial rate of ethylene production is identical in fruits from infected or uninfected plants, fruits from infested plots show higher emissions of CO₂ and C₂H₄ production at the time of the climacteric peak (Castelan et al., 2012). This elevation is higher for BLSD than for SD. BLSD is also involved in modifications to the starch metabolism during ripening, and the enzymatic activities linked to its degradation (Saraiva et al., 2013).

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So far, only hypotheses can be put forward regarding the origin of this “stress signal”. The synthesis of metabolites by the fungus could have a direct impact on fruit physiology. Some toxins, such as juglone, are secreted by *M. fijiensis* and involved in the BLSD infection process (Chuc-Uc *et al.*, 2011; El Hadrami *et al.*, 2005). Ethylene is also produced by diseased tissues, but is unlikely to be transported very far in the plant, and is probably quickly lost in the air. Secondary plant metabolites involved in host-pathogen relationships could also impact fruit
ripening, as has been shown for methyl-jasmonate and salicylic acid on other pathosystems (Fan *et al.*, 1998). Lastly, changes in hormone balances, such as indole-3-acetic acid and gibberellic acid following pathogen infection, may also have an effect on fruit quality (Saraiva et al., 2013). It is probable that these factors are not exclusive, but act in synergy on processes leading to GL reduction.

To conclude, Sigatoka diseases are an essential factor to consider in most banana producing countries in order to manage fruit post-harvest quality. A better understanding of changes in the physiological metabolism induced by these diseases could enable adequate practices, in order to avoid the stress or interrupt its transmission to the fruit. More generally, additional work is needed on BLSD in order to quantify its impact on GL, and potentially adapt management practices.

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## 5. Post-harvest fruit treatment to optimize GL potential

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### 5.1. Fruit damages and post-harvest fungal diseases effect

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Peel abrasion was proved to be negatively correlated to fruit GL (Santana Llado and Marrero Dominguez, 1997). These authors varied mechanical injury by peeling the fruit surface at different degrees of severity (0, 1 and 4 cm²) and observed GL shortening, which was also reflected in the onset and peak production of ethylene. Again, the stress generated by the
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mechanical injuries could impact the oxidative status of the fruit, enhance cell membrane
degradation and, as a consequence, reduce the potential fruit GL.

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Several fungi at the post-harvest stage also have the potential to produce ethylene and therefore have a negative impact on fruit GL. The *in vitro* ethylene production capacity of *Colletotrichum musae*, the main agent causing banana anthracnose (De Lapeyre de Bellaire and Mourichon, 1997), was confirmed and explained by Daundasekera *et al.* (2003); Fukuda *et al.* (1993). In fact, it seems that the fungus has a dual effect: producing ethylene itself, but also acting as a stress factor and indirectly inducing ethylene biosynthesis in the plant (Bleecker and Kende, 2000). Daundasekera *et al.* (2003) suggests the ability of *C. musae* to use host banana tissue to provide substrates for ethylene production but, at this point, the relative contribution of pathogen-produced ethylene versus host system I and system II (autocatalytic) ethylene production in this pathogen-host relationship is not known and remains to be studied. The fungal and fruit peeling injury effect is worth discussing with regard to oxidative protein degradation and to the subsequent auto-catalytic fruit ripening process. Indeed, it can be suggested that any external peel aggression could be translated as stress factors, thereby triggering auto-catalytic ethylene synthesis and consequently resulting in reduced fruit GL potential.

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## 5.2. *Hormonal compounds and gas treatment*

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Auxins, kinins and gibberellins have been found to regulate aging processes in many plants, including fruits. Particularly, dipping whole banana fruit in gibberellic acid (GA3) solution was found to postpone the climacteric peak rise and therefore to extend GL (Archana and Sivachandiran, 2015; Osman and Abu-Goukh, 2008). A concentration of the dipping

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solution at 1.10⁻⁴ M was proved to increase GL from 13 (without GA₃) to 18 d at 20 °C and under saturating RH conditions. It should, however, be pointed out that the concentration of GA₃ actually fixed in the tissue is not known, with peel and mainly cuticular wax acting as a hydrophobic barrier. Huang and Jiang (2012), using another plant growth regulator, forchlorfenuron (CPPU), increased by 2 d the GL of banana stored at 25 °C and at 75-95% RH. The combined effects of CPPU (phenylurea type molecule) and GA₃ were also studied by Huang *et al.* (2014). The most beneficial effect was obtained by dipping the fruits in a mixture composed of 10 mg.L⁻¹ of CPPU and 50 mg.L⁻¹ of GA₃: The treatment delayed the occurrence of the peak of respiration but had a steady ethylene production rate at the green stage. The molecular mechanism was not known and is still to be proved.

Nitrous oxide (N₂O), although not tested on banana but on other climacteric fruits (tomato, avocado), also blocks or delays ACC and ACO activity, and could be worth considering (Gouble *et al.*, 1995). N₂O, which is an anaesthetic, is chemically neutral and nontoxic and therefore clearly differs from the very toxic but efficient ethylene oxide in extending GL (Williams *et al.*, 2003). As a consequence, autocatalytic ethylene evolution is inhibited and GL therefore extended. Again, although this treatment is promising, it should be considered with regard to food legislation, which does not accept this gas treatment for fruits and vegetables.

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### 5.3. 1-Methylcyclopropene (1-MCP) treatment

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An exciting new strategy for controlling ethylene production, and thus fruit ripening and senescence, especially of climacteric fruits, as well as the senescence of vegetative tissues, has emerged with the discovery and commercialization of the inhibitor of ethylene perception, 1-
methylcyclopropene (1-MCP) (Jiang *et al.*, 2004; Watkins, 2006). 1-Methylcyclopropene is a cyclopropene derivative initially used as a synthetic plant growth regulator. It is structurally related to the natural plant hormone ethylene and is used commercially to slow down the ripening of fruit and to help maintain the freshness of cut flowers. The use of cyclopropenes to inhibit ethylene action was patented by Sisler and Blankenship (1996). Macnish *et al*. (2000) established an extension of the GL of bananas stored at 20 °C when treated with 15 µL.L⁻¹ 1-MCP and subsequently exposed to ethylene. Jiang *et al*. (1999b) showed that exposure for 24 h at 0.5 or 1 µL.L⁻¹ 1-MCP at 20 °C extended GL from 16 to 31 d compared to control fruits. Jiang *et al*. (1999a) demonstrated that 1 µL.L⁻¹ 1-MCP for 1 h at 20 °C delayed the ripening effects induced by 100 µL.L⁻¹ ethylene. Lastly, Golding *et al*. (1998) showed that the effectiveness of 1-MCP on banana fruit was closely related to its colour stage. A commercial breakthrough in 1-MCP application technology resulted from the formulation of 1-MCP as a stable powder in which it is complexed with γ-cyclodextrin, so that 1-MCP is easily released as a gas when the powder is dissolved in water (Edgington *et al.*, 2013; Zhang *et al.*, 2013).

Despite good results for 1-MCP delaying fruit softening (Bagnato *et al.*, 2003), soluble solid accumulation and colour development, an uneven ripening of peel following treatment with 1-MCP has been reported (De Martino *et al.*, 2007; Harris *et al.*, 2000). This uneven ripening of the peel appears when 1-MCP is applied after ethylene treatment to bananas that have not completely lost their green peel colour, and results in a dull-grey appearance. Moreover, these symptoms are often combined with areas on the peel that remain green, similar to blotchy ripening of tomatoes. It has been observed on banana fruit treated with ethylene at ripening stage 2, before or after 1-MCP treatment, that respiration was completely inhibited by 1-MCP, whereas ethylene was slightly affected and this response affected the quality characteristics of bananas on a different level (DeMartino *et al.*, 2004). In particular, the occurrence of the yellow colour was completely delayed whenever the 1-MCP treatment was
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carried out. Similar results, but with a lower response, were obtained when treating bananas
with 1-MCP at a more advanced ripening stage.

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It therefore clearly appears that using 1-MCP in the banana food chain will present many drawbacks in terms of logistical vs. quality improvement:

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- Gassing fruit with 1-MCP at the green stage after harvesting at the production site causes some fruits, depending on their maturity on harvesting, to display severe visual defects (uneven green discoloration) when ripening is induced by ethylene after shipping, whatever the ethylene concentration and contact time (Feygenberg et al., 2010).

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- Gassing fruit with 1-MCP at colour stage 2 (light green) after ripening induction with ethylene is effective in greatly slowing down and even blocking fruit ripening. As the fruit cannot be consumed at this ripening stage, this option is not acceptable (DeMartino et al., 2004).

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- Gassing fruit with 1-MCP at colour stage 3-4 (green-yellow) after ripening induction with ethylene is not effective in slowing down ripening (DeMartino et al., 2004).

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As a conclusion, since the effectiveness of 1-MCP varies with fruit maturity, and in any commercial consignments there is a mixture of fruit maturity, 1-MCP has limited commercial potential for the storage of unripe Cavendish bananas (Harris et al., 2000). The study by Golding et al. (1998) confirmed that the application of 1-MCP significantly delayed and disrupted degreening, even when propylene was applied continuously, but that incomplete and uneven yellowing of the peel was a feature of fruit treated with 1-MCP fruit. Indeed, although yellowing is initiated by ethylene, completion of the process involves enzymes whose biosynthesis may be irreversibly disrupted by 1-MCP.

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5.4. *Fruit coating and dipping*

Recently, increased efforts have been made to develop coatings using new preservative compounds that could generate a modified atmosphere by creating a semipermeable barrier against oxygen and carbon dioxide (Kudachikar *et al.*, 2011). Among them, Maqbool *et al.* (2011b) applied an edible coating composed of 10% gum Arabic plus 1% chitosan. Coated fruits had far fewer cracks, showed a smoother surface, and fruit GL was extended for up to 33 d. This aqueous dispersion of sucrose esters of fatty acids, and the sodium salt of carboxymethylcellulose, indeed modifies the internal atmosphere of the fruit and delays the subsequent climacteric phase. Lastly, coating with Arabic gum was improved using 0.4% cinnamon oil (Maqbool *et al.*, 2011a). Indeed, by controlling *Colletotrichum Musae*, the main fungus encountered after the harvest on banana, these authors indirectly preserved potential banana GL (cf. § 5.1.). Basil oil was also shown to maintain banana GL by controlling fungal attacks that could develop during shipping (Anthony *et al.*, 2003; Siriwardana *et al.*, 2017).

As salicylic acid has been shown to affect the biosynthesis and action of ethylene, the effects of salicylic acid were explored (Srivastava and Dwivedi, 2000). By dipping the fruits in 500 µM of salicylic acid for 6 h, this compound acted as an ethylene antagonist and prolonged fruit GL by up to 3 d at 25 °C.

Lastly, although heat treatments (thermotherapy) have been used for over a century to free plant materials from pathogens, this post-harvest treatment of fruit could also be used to modify fruit responses to other stresses and maintain fruit quality during storage (Paull and Chen, 2000). The detrimental effect on final fruit quality was, however, not really highlighted
655 in these studies; heat treatment at more than 55 °C definitely *degrades* the subcellular cuticular
656 wax layer and therefore increases peel browning at the ripening stage.

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658 **6. GL optimization throughout the banana cold chain**

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660 **6.1. *Reefer, pallet box and carton levels scale***

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662 Although palletized bananas have mainly been transported on dedicated vessels in the
663 past, transportation is now shifting to containers on third-party vessels. While such
664 transportation enables a more flexible reaction to fluctuating market volumes, shipping by third-
665 party vessels in rented refrigerated containers (reefers) can cause more quality-related
666 problems, unless adequate care is taken to maintain optimum transport conditions (Jedermann
667 et al., 2014b).

668 Refrigerated containers, or "reefers", as they are regularly known, are of particular interest
669 to shipping companies and similar organizations, because they permit relatively high freight
670 rates and margins that are not possible with dry containers. The temperature settings of the
671 reefer are prescribed in a range of 13 to 18 °C. In the case of goods with a high water content
672 and 2^nd^ order biotic activity (BA 2), such as fruit and vegetables in which respiration processes
673 predominate, ventilation has not only to ensure that temperature and humidity requirements are
674 met, but also gas exchanges. Ventilation ensures that the necessary oxygen is supplied and that
675 harmful gases (carbon dioxide, ethylene) are removed.
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The supply chain of the banana trade is a typical cold chain. The banana farm is the
starting point of the banana cold chain, and the final customer is the end point (Figure 1 from
Chen and Notteboom (2012)).

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As bananas are exported over long distances, requiring days of sea shipment to reach the ripening location and thereafter the consumer location, fruit physiology and exogenous gas sensitivity need to be addressed jointly with the industrial chain and logistical requirements. Indeed, it takes between 5 and 35 d for bananas to reach the final customers, depending on the area considered. If any bananas ripen during transportation, they not only cannot be sold, but also endanger the whole container of bananas. Jedermann *et al.* (2013); Jedermann *et al.* (2014a); Jedermann *et al.* (2014b); Praeger *et al.* (2013) evaluated some of the suggested innovations integrating logistical food chain constraints. Ethylene management should be particularly discussed regarding the structure of the cold chain and integration of the “neighborhood” of each entity (cluster, carton, pallet box). Indeed, during shipping, each banana cluster cannot be individually considered, as there are other clusters in the same polybag that very probably come from another bunch and therefore have a different physiology and fruit maturity at harvest. This consideration can be scaled up to the carton or pallet box levels: each one impacting on and being impacted by its neighbors in terms of the temperature and gas composition of the surrounding atmosphere. Inside the box, the bananas are usually protected by a polyethylene bag to prevent dehydration. Indeed, post-harvest packaging of fruits in cartons lined with polyethylene film resulted in a longer GL (Elkashif *et al*., 2010). The bag may either have some small holes to allow penetration of a controlled atmosphere (CA) into the box, provided by the ship, or may be completely sealed to generate a higher CO₂ concentration and lower O₂ concentration by respiration of the fruit.

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Controlling the temperature, CO2, O2 and ethylene concentrations at reefer, pallet box and carton levels is a real challenge and numerous studies and innovative technologies have been developed to achieve the required parameters to optimize banana GL.

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## 6.2. Modified Atmosphere Packaging (MAP)

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MAP is one of the most accepted methods for extending the shelf life of perishable and semi-perishable food products by altering the relative proportions of atmospheric gases surrounding the produce. However, the high capital cost of gas packaging machinery to control the atmosphere at the reefer scale has limited the use of this technology. This gives rise to the concept of active packaging (Sen et al., 2012). Besides being independent of package permeability and gas packaging equipment, this packaging system is more rapid and accurate than traditional MAP. The concept of active packaging is one of the emerging technologies in food packaging. It has been defined as a system in which the product, the package and the environment interact in a positive way to extend GL or to achieve some characteristics that cannot be obtained otherwise (Miltz et al., 1995). The active system can be an integral part of the package or be a separate component placed inside the package and different substances capable of either absorbing or releasing a specific gas control the atmosphere inside the package. Active packaging components can work as either an absorbing or releasing system.

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## 6.3. GL optimization at carton level

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Polyethylene bags are extensively proposed as a trigger, not only to protect fruits from external damage but also as a technology to delay ripening of bananas during storage and thereby increase GL. Commonly, potassium permanganate was added to these polyethylene bags as an ethylene absorbent to delay the potential GL even more. The most popular method at carton level is the oxidation of ethylene by potassium permanganate (KMnO₄), adsorbed on an inert carrier with a large surface area, such as silica gel, alumina, and activated carbon. The stripping chemical mechanism is rather simple, as described here: 3 C₂H₄ + 12 KMnO₄ → 12 MnO₂ + 12 KOH + 6 CO₂. Considering the low basal level of ethylene vs. the CO₂ (~ 1 ppm of ethylene / 4 % of CO₂) concentration, the release of 2 moles of CO₂ for 1 mole of ethylene does not have a great impact on the controlled atmosphere composition (Mangaraj and Goswami, 2009). Chamara et al. (2000) extended the GL of bananas cv. Kolikuttu' from 4 to 20 d at 25 °C and 85% RH by packing them in LDPE bags (75 µm) complemented with ethylene scavenger. Thus, GL could be greatly boosted using a modified atmosphere naturally obtained through the equilibrium between natural fruit respiration and PELD permeability and ethylene scavengers. Consequently, packing bananas as individual hands in LDPE (0.075 mm) bags with a 1:1 surface to weight ratio (cm².g⁻¹) containing 50 mL of saturated potassium permanganate absorbed onto suitable porous matrices could be recommended to increase GL.

Passive modified atmosphere (2% O₂ and 5% CO₂) packaging of bananas completed with an ethylene scrubber was found to increase fruit GL five-fold compared to fruits stored without wraps. Again, this gas composition is a result of the tryptic: fruit quantity/surface area of available film ratio, CO₂ exhausted at a given temperature, and gas permeability of the PEHD polybag. The combined effect of ethylene and CO₂ scrubber in the polyethylene bag during fruit shipping at 13 °C was tested by Chauhan *et al.* (2006). The increase in GL was only 2 d more using the two absorbers, while ethylene absorber alone led to an increase in GL from 8 to 13 days. It appears that the concentration of ethylene surrounding the fruit is really the more
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strategic parameter to be controlled during shipping until the initiation of the ripening stage, to
ensure an optimized GL.

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Another approach was using only material permeability to ensure the required atmosphere
during shipping to maintain the potential GL. These materials included Banavac®, also called
unvented polybag (Narayana *et al.*, 2002) which consists of 0.4 mm thick polyethylene bags in
which the carbon dioxide content is raised to 5%, the oxygen content is reduced to 2%
("modified atmosphere"). The ethylene that arises is absorbed by adding potassium
permanganate. However, Banavac® bags must be torn open before the ripening can be initiated,
the high CO₂ concentration accumulating during ripening making this action necessary. The
need to rip open the bags before gassing results in added labor costs.

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In recent years, laser micro-perforation of the polybags has been the most promising
technology to ensure a passive but steadily modified atmosphere. Indeed, these
micro-perforations allow for the transmission of oxygen, CO2 and ethylene gases into and out
of the polybags (Varriano-Marston, 2008). Given the ethylene permeability of this wrap, this
technology provides a package that does not have to be ripped open to initiate the ripening
cycle. This approach was further optimized by Balasubramanian *et al.* (2013b), who provided
a polymeric film, which comprised one or more copolymers of ethylene with a polar monomer,
in which the oxygen transmission rate of the said wrap was 8,000 to 16,000 cm³.h⁻¹. These
perforated plastic bags could display non-uniform perforation density with the density of the
lower zone being greater than that of the upper zone (Balasubramanian *et al.*, 2013a).

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6.4. *Temperature and RH control at reefer level*

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At the industrial level in the reefer, Jedermann et al. (2014b) found an exponential function for the relationship between storage temperature T and the average GL for the temperature range between T = 12 °C and T = 30 °C: GL (d) = 159.86 e⁻⁰.¹²⁴T in normal air within a polyethylene bag et 98% RH. At an optimum storage temperature of 13 °C, the green life lasted about 30 d and for each 10 °C temperature increase, the green life was reduced by a factor of Q₁₀ = 3.46. Green life decreased to one week at a storage temperature of 25 °C. The parameters of this equation should be modified depending on whether fruits are stored with or without CO₂ control. Thus, it becomes GL (d) = 122.14 e⁻⁰.⁰⁹T if the CO₂ level reaches 5% or more, meaning the estimated fruit GL at 15 °C rises from 25 d to 32 d at 5% CO₂ (Jedermann et al., 2013; Mees, 2017). Still at reefer level, Lang *et al.* (2011) proposed an interesting synthesis of the temperature, RH and CO₂ effect on banana GL during sea exports from Latin America to Europe (Table 3).

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While the high sensitivity of banana fruits to ethylene was shown at laboratory level (individual banana fingers or clusters), the management and optimization of GL at reefer level needs to consider all the key parameters as a whole. Indeed, and as an example, one 18 kg box of yellow bananas produces enough ethylene in 2.2 d to increase the concentration in the whole container with a free air volume of 47.4 m³ to a critical threshold of 0.1 ppm, if the air flaps are closed, and no ethylene is released into the atmosphere (Jedermann et al., 2014b).

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Controlling, optimizing and estimating the internal fruit temperature at reefer level is a real challenge. The speed of the cooling process depends largely on the quality and age of the cooling equipment, but surprisingly there is no preferred location for the hottest point inside the container (Jedermann et al., 2014b). As an example, after 8 d of shipping with a set-point fixed at 13.9 °C, the temperature difference could reach 18 °C in an old reefer while being only 14.5-15 °C in a new one. Cooling efficiency is therefore a key parameter that needs to be clearly defined. Furthermore, trucks or rail cars are frequently not refrigerated, so bananas only start

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<figure><img src="image_2.png" /></figure>

their cooling process once loaded into the vessel hatch. Cooling delays can range from 4 to 36
h. This cooling delay is frequently considered one of the causes of premature ripening in transit.

<figure><img src="image_3.png" /></figure>

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Delays in cooling can have a marked influence on banana ripening during subsequent
transit (Madrid, 1996). For fruit exposed to 12 h of cooling delay (30 °C instead of 14 °C, steady
RH at 95%), the GL duration dropped by 12%, while GL was reduced by ~20% for fruit exposed
to 24 h of cooling delay. Average shipment and delivery times from the tropics to the ripeners
are around 18 – 20 days, so that losses of 4 to 6 days of green life can have a significant impact
on the ‘ripe and turning’ incidence.

<figure><img src="image_10.png" /></figure>

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The Cargo Loss Prevention Information Handbook (Handbook, 2003) is very useful for
knowing the temperature rise during banana transport if the cooler system of the reefer is
damaged. As an example, 20 t of bananas that are temperature stabilized at 13 °C in a reefer
will have their temperature reach 16.5 °C after only 48h. Given the impact of temperature rises
on banana GL, this damage will undoubtedly provoke fruit ripening in the reefer, but also tend
to increase the average temperature of the ship. To avoid such uneven ripening initiation, Haass
et al. (2015) developed the so called “Intelligent Container”. This Intelligent Container is able
to calculate the green life of its cargo and this leads to the feasibility of quality driven
distribution. Thus, banana losses should be decreased and both transport costs and carbon
emissions should be reduced even further. The use of chlorophyll fluorescence in fruit and
vegetable storage (HarvestWatch™) could be a good solution for optimizing the O₂
concentration in a dynamic controlled-atmosphere (DCA) (Prange et al., 2012).

<figure><img src="image_24.png" /></figure>
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Temperature-related problems are mainly caused by an imbalance between generated and removed heat. Jedermann et al. (2013) identified the width of the gaps between pallets as the major influence on the spatial temperature profile. The best cooling was achieved with a 4 cm wide gap between pallets and a gap from the wall, therefore meaning that optimized stowage of pallets is needed, but also that the reefer's wall should not be flat but present instead some regular notches. Although developed on citrus fruit, the study by Defraeye et al. (2015a) is worth discussing. The existence of gaps between the pallets led to significant airflow short-circuiting, which reduced the cooling rate at all box heights on the pallet, but it improved cooling uniformity between the boxes to some extent. Simulations showed that low airflow rates typical for refrigerated containers do not only induce slower fruit cooling, compared to Forced-Air Precooling (FAC) airflow rates, but also the cooling heterogeneity between different layers of boxes (in height) and between individual fruits in a single box is greater. In addition, the existence of gaps between pallets invokes airflow short-circuiting, leading to greatly reduced fruit cooling rates.

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Optimized airflow circulation to reduce ethylene concentration inside and outside the box and thereby maintain banana GL is achieved with fan optimization at the polybag, box and pallet levels. Indeed, i) polyethylene bags should not have fewer than 4 pinholes (0.01 dm²·m⁻²) for 20 kg of banana fruit; ii) boxes are provided with perforations to ensure proper flow of cooling air (perforation not less than 8% of the total area) and these boxes should be perfectly aligned to ensure airflow passage; iii) pallet stowage should be optimized. To optimize ventilation using carton perforations, Sanabria et al. (2003) improved the ventilation pattern by developing a shade box with the usual plurality of ventilation apertures, but with those apertures being in communication when cartons were regularly stowed, thereby advantageously improving airflow through an entire palletized load of containers.
841

As maintaining market quality is of vital importance for the success of the banana industry, it is necessary not only to cool the product, but to cool it as quickly as possible after harvest. Since the rate of respiration is influenced by temperature, precooling to remove the field heat before storage will reduce the respiration rate, hence deterioration will decline accordingly (Brosnan and Sun, 2001). The process of precooling is the removal of field heat, which arrests the deterioration process, so as to maintain a high level of quality that ensures customer satisfaction (Brosnan and Sun, 2001). Forced air cooling was developed to accommodate products requiring relatively rapid removal of field heat immediately after harvest. Forced air or pressure cooling is a modification of room cooling and is accomplished by exposing packages of produce to a higher air pressure on one side than on the other. This technique involves particular stacking patterns and the baffling of stacks, so that the cooling air is forced through (rather than around) the individual containers. Consequently, a relatively small pressure difference between the two sides of the containers exists, resulting in good air movement and excellent heat transfer, hence faster cooling.

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The economic cost of this high-speed temperature reduction should however be pointed out. If it is essential that the heat load be removed quickly, whatever the cooling technique used.

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A poorly ventilated, or worse still, unventilated container of bananas with rapid respiration rates can quickly change the atmosphere, reversing the normal levels of oxygen and carbon dioxide in less than 24 hours. New reefer containers are bottom-air delivery units, which means that air is constantly supplied from the bottom of the container through the T-bar floor. Cool air circulates through and around the cargo as it absorbs cargo heat. In order to remove the gas generated by respiration, thereby reducing ethylene concentration surrounding the fruit and maintaining GL up to 28 days, it is recommended that fresh air be exchanged from the outside to the inside at a rate of 30 m³.h⁻¹ for banana.
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Although closely related to ventilation efficiency, dedicated reefers have been developed to specifically reduce exogenous ethylene. The main requirement during storage identified by the agrifood sector is ‘online’ monitoring of ethylene in the ppb range with low-cost devices in the storage chambers (Janssen et al., 2014). Indeed, if gas sensing systems for CO2 fruit monitoring in containers were already available, they would enable more reliable predictions of GL by measuring the ethylene concentration inside a container. The “intelligent” container developed by Lang et al. (2011) might be a solution to this challenge. The online control of ethylene, hence GL, seems to be such a challenge that Herdeman (1997) proposed bypassing this challenge by only reducing ethylene once the bananas were stabilized at the desired holding temperature.

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Different methods of ethylene removal (ventilation, catalytic oxidation, photochemistry under UV-C light, and KMnO4) were evaluated (Lawton, 1991). As ambient levels in the sea transport environment rarely exceed 0.01 ppm, and as the best available method for the removal of ethylene has so far been ventilation with air, the costs and benefits of ethylene removal technologies are worth evaluating (Keller et al., 2013). Compared with the other ethylene oxidation methods, the photocatalytic oxidation process can be considered as a very promising and reliable technology, combining the efficiency of catalytic oxidation with both low energy consumption and low heat load resulting from UV irradiance. Using TiO2, photocatalytic processes chemically oxidize organic compounds into carbon dioxide and water at near-ambient temperatures to complete mineralization. It has proven to be an efficient method for removing ethylene from horticultural storage facilities in order to extend the postharvest storability of produce, and can further be used in combination with other means, such as cold storage, controlled atmospheres, or modified atmosphere packaging (Keller et al., 2013). Since photocatalytic material is not consumed during the oxidation process, there is no need for the
## 7. Conclusion
911 time, a continuum between producers, post-harvest methods and shipping arrangements will be
912 necessary to counteract how Black Sigatoka affects GL.

913 As technologies evolve, stakeholders will be able to pilot the post-harvest life of the fruit
914 using intelligent containers that would adapt the composition of the atmosphere and the
915 temperature to the cargo. In any event, research is required to try and estimate fruit GL after
916 harvest as accurately as possible, using spectral and non-invasive technologies for instance.
917 This will considerably change all our approaches to GL optimization and will considerably
918 reduce fruit losses after shipping.

919

920

921

922

923
924 References

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1160 Turner, D.W., Fortescue, J.A., 2012. Bananas (Musa spp.). Crop Post-Harvest: Science and Technology, Perishables 3, 24.

1162 Umber, M., Paget, B., Hubert, O., Salas, I., Salmon, F., Jenny, C., Chillet, M., Bugaud, C., 2011.

1163 Application of thermal sums concept to estimate the time to harvest new banana hybrids for export. Scientia Horticulturae 129, 52-57.

1165 Wade, N., Graham, D., 1987. A model to describe the modified atmospheres developed during the storage of fruit in plastic films. ASEAN Food Journal 3, 105-111.

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1187

1188

<caption>TABLE 1. Effect of maturity and storage temperature on banana green life (d) (cv. Grande naine) (taken from Gonge et al. (2013)).</caption>

1190

<table><thead><tr><th rowspan="2">Level of maturity</th><th colspan="3">Storage T °C</th><th rowspan="2">Mean green life (d)</th></tr><tr><th>12 °C</th><th>14 °C</th><th>16 °C</th></tr></thead><tbody><tr><td>75%</td><td>35</td><td>28</td><td>21</td><td>28</td></tr><tr><td>90%</td><td>29</td><td>24</td><td>16</td><td>23</td></tr><tr><td>100%</td><td>26</td><td>21</td><td>14</td><td>20</td></tr></tbody></table>

1191

1192

1193
1195 **TABLE 2.** Regression equations of estimated green life (GL) duration (d) according to the range of temperatures and

1196 the CO₂ concentration.

<table><thead><tr><th rowspan="2">Regression equations</th><th rowspan="2">Reference</th><th>Estimated</th><th>Estimated</th><th rowspan="2">Limiting conditions</th></tr><tr><th>GL at 13 °C</th><th>GL at 15 °C</th></tr></thead><tbody><tr><td>GL (d) = [- 0.97 x T °F] + 93</td><td>Peacock and Blake<br>(1970)</td><td>40</td><td>36</td><td>13 &lt; T °C &lt; 22<br>Normal air</td></tr><tr><td>GL (d) = 10 [(-0.02529 x T °F) + 2.898]</td><td>Blake and Peacock<br>(1971)</td><td>32</td><td>25</td><td>15.5 &lt; T °C &lt; 35.5<br>Normal air</td></tr><tr><td>GL (d) = 159.86 e<sup>-0.124 T °C</sup></td><td>Jedermann et al.<br>(2014b)</td><td>32</td><td>25</td><td>12 &lt; T °C &lt; 30<br>Normal air</td></tr><tr><td>GL (d) = 122.14 e<sup>-0.09 T °C</sup></td><td>Mees (2017)</td><td>38</td><td>32</td><td>12 &lt; T °C &lt; 30<br>CO₂ &gt; 5%</td></tr></tbody></table>
1198

1199

1200

1201

<table><caption>**TABLE 3.** Green lifeª of Cavendish bananas harvested in spring 2012 and stored under different relative humidity (RH), T °C, and CO₂ concentration conditions (taken from Jedermann et al. (2014b)).</caption><thead><tr><th rowspan="2">RH</th><th colspan="3">Effect of RH</th><th colspan="2">Effect of elevated CO<sub>2</sub></th></tr><tr><th>50-60%</th><th>80%</th><th>98%,<br/>&lt; 2% CO<sub>2</sub></th><th>98%,<br/>&lt; 2% CO<sub>2</sub></th><th>98%,<br/>5% CO<sub>2</sub></th></tr></thead><tbody><tr><td>normal air</td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>T 15 °C</td><td>42 ± 4</td><td>37 ± 5</td><td>48 ± 2</td><td>57 ± 1</td><td><strong>61 ± 2</strong></td></tr><tr><td>T 18 °C</td><td><strong>23 ± 7</strong></td><td>28 ± 4</td><td>32 ± 2</td><td>30 ± 4</td><td>49 ± 17</td></tr></tbody></table>

1202

1203

1204

1205

1206

1207

<footnote>ªonly hand-top position GL is presented.</footnote>
<table><caption>**TABLE 4.** Summary of recommendations and correlated banana chain good practices to ensure optimum green life at the orchard, packing-house and reefer levels of intervention.</caption><thead><tr><th>Levels of intervention</th><th>Recommendations(s)</th><th>Good practice(s) to set up</th><th>Key publication(s)</th></tr></thead><tbody><tr><td rowspan="3">At orchard level</td><td>Limit stress induced by fertilization and drought (when harvest at a specific fruit size/grade)</td><td>Low soil N supply and no soil water deficiency</td><td>Srikul and Turner (1995)</td></tr><tr><td>Control of Sigatoka diseases</td><td>Elimination of necrotic parts of the leaves</td><td>Chillet et al. (2013)</td></tr><tr><td>Optimisation of the harvest point: compromise yield and GL</td><td>Harvest stage at 900 ± 50 dd</td><td>Ganry and Chillet (2008)</td></tr><tr><td>At packing house level</td><td>Avoid peel abrasion after harvest</td><td>Do not injure the fruits during de-handling and cluster fruit preparation steps</td><td>Santana Llado and Marrero<br>Dominguez (1997)</td></tr></tbody></table>
<table><tr><td>Control fungal attack</td><td>Use natural or chemical treatments that prevent or limit <i>Colletotrichum Musae</i> during shipping</td><td>Daundasekera et al. (2003)</td></tr><tr><td rowspan="2">Limit ethylene exhausting as much as possible</td><td>If possible, pre-cool fruit immediately after harvest</td><td>Defraeye et al. (2015)</td></tr><tr><td>Limit cooling delay (&lt; 4 h)</td><td>Madrid (1996)<br/>Herdeman (1997)</td></tr><tr><td rowspan="2">Reduce ethylene accumulation +<br/>Control the temperature at 13.5 ± 0.5 °C</td><td>Optimise ventilation at pallet and reefer level</td><td>Jedermann et al. (2014b)<br/>Jedermann et al. (2013)</td></tr><tr><td>Shade storing of the reefers</td><td>Rodríguez-Bermejo et al. (2007)</td></tr><tr><td>Limit water loss</td><td>Maintain RH &gt; 95-98%</td><td>Jedermann et al. (2014b)</td></tr><tr><td>At ripening room level</td><td>Avoid heterogeneous ripening induced by “ethylene pollution”</td><td>Proceed to the ripening step without delay</td><td>Jedermann et al. (2014a)</td></tr></table>
1212 Itemized list of Figures:

1213

1214 FIGURE 1. Schematic representaion of the banana cold-chain (from Chen and Notteboom (2012)).

1215

1216 FIGURE 2. Theoretical pathway and controlling parameters of banana GL.

1217

1218
<figure><img src="image_25.png" /><figcaption>FIGURE 1.</figcaption></figure>
<figure><img src="image_26.png" /><figcaption>FIGURE 2</figcaption></figure>