Regulation Of Banana Fruit Ripening With Exogenous Application Of Ethylene

Objectives

Ripening represents the maturation of fruits. Bananas are a good example of climacteric fruits, they only need to mature to ripen. Upon detaching from the plant, the fruits produce their own ethylene that initiates the ripening process. Similarly, the exogenously applied ethylene does not only induce ripening but also facilitates endogenous ethylene production. The study aims at understanding the process of fruit ripening, as well as studying the effects of ethylene application on banana ripening and its influence on the production of endogenous ethylene, respiration rate, and banana peel colour development.

The experiment involved green mature bananas which were fumigated with ethylene gas and later allowed to ripen at a temperature of 20±1⁰C. For the treated batches, the amount of endogenous ethylene produced increased with an increase in the exogenous ethylene, with the 100 µlL^-1 batch producing the most amount of endogenous ethylene. Control experiment reported a consistent green peel colour throughout the ripening period while the other trials had a consistent improvement in terms of colour. The experiment was designed to operate at a temperature of 20 ±1⁰C and from the TinyTag sensors, the temperature recorded shows no significant difference from the recommended temperature.

Regulation of Banana Fruit Ripening with Exogenous Application of Ethylene

Objectives

1. To understand the process of fruit ripening, it’s regulations and implications in addressing the requirements of the consumers and consequently facilitating marketing.
2. To study the effects of ethylene application on banana ripening and its influence on production of endogenous ethylene, respiration rate and banana colour development.

Introduction  

Fruits are classified as either climacteric or non-climacteric depending on how they respire as well as their reaction upon exposure to ethylene production. For the climacteric fruits, ethylene gas plays a very important role in their ripening, that is, exposure to ethylene will initiate the process of ripening. Ethylene gas is produced as a fruit ripens. Non-climacteric fruits, on the other hand, do not respond to ethylene but instead have to grow, mature and ripen while they are still attached to the mother plant failure to which they will not ripen. For climacteric fruits, ripening need not happen while attached to the plant but instead, as long as the fruits are ripe, ethylene gas can be dosed to the fruits initiating the ripening process.  Bananas are a good example of these fruits, they only need to mature to ripen. Upon detaching from the plant, the fruits produce their own ethylene that initiates the ripening process. Similarly, the exogenously applied ethylene induces ripening and the endogenous ethylene production (Marriott and Gravani, 2006).

As with bananas, they exhibit a sharp rise in ethylene gas followed by a sudden decrease in production, this allows for a quick pick at the onset of ripening. Ripening is associated with the increase in respiration and a change in the physiological and biochemical properties of the fruit which include; change in fruit color, fruit softening, conversion of starch into sugars, organic acid metabolism, and production of aroma and flavor (Bharat 2014). The endogenous ethylene production in a fruit is responsible for controlling these changes. In bananas, ethylene production is categorized into two; ethylene system I an ethylene system II. Ethylene system I trigger ethylene system II, which causes the autocatalytic climatic peak during fruit ripening (Plange et al., 2012).  During this process, the peel color changes from green to yellow, the texture, and softness of the fruits changes as well as the sweetness that results from the conversion of starch into sugars (Wills et al., 2007).

Climacteric and non-climacteric fruit ripening

Despite the fact that endogenous ethylene is most crucial in fruit ripening, exogenous application improves the ripening and the quality of the end product. Previous studies show that external ethylene can either be the manufactured type or artificial by the use of ethylene generating fruits. Fruits such as passion fruits, apples, and avocados are ranked as ethylene generators and therefore mixing bananas with these fruits doses ethylene into them, initiating ripening and its associated biochemical and physiological changes (Cornelius and James, 2007).

Ripening indicates the maturation of fruits and particularly occurs as a result of unique coordination between the developmental and biochemical pathways and as a result, peel colour, texture, aroma and nutritional quality are affected. Through this process, fruits become more palatable and how well this is achieved determines how well the fruits become commercially marketable. Exposure of bananas to ethylene gas advances the onset of an irreversible shoot in respiration and consequently a rapid ripening. It marks the end of growth of a banana fruit and the start of senescence, cellular delay of the tissues. It is an irreversible process that results from complex changes in the banana probably independently of each other. For commercial purposes, ripening plays a crucial role. This paper, therefore, aims at collecting relevant data related to banana ripening and demonstrate how they impact post-harvest physiological processes and the quality of harvested product (Saeed et al., 2006).

Fruit treatment: The experiment involved green mature bananas, Musa sp. AAA group, Cavendish subgroup. The fruits which were harvested at the pre-climacteric stage were sourced from Woolworths limited. The different batches of bananas were placed in 60 liters container and fumigated with varying concentrations of ethylene gas, that is, 0, 10, 50 and 100 μlL^-1 for 24 hours. The plastic containers were then sealed. The ethylene gas was obtained from a cylinder containing ethylene in nitrogen from where it was injected into the sealed container through the injection port located on the lid of the container.30 grams of soda lime was also added to the container to prevent accumulation of CO₂ which impacts respiration and consequently the ripening process.

After injection of the ethylene, the containers were placed in ambient conditions and allowed to ripen at the temperature of 20±1⁰C. During the ripening process, changes in ethylene production, respiration rate, and colour development were recorded on every alternate day. A completely randomized block design with three replications was used, with each replication being five fingers.

Banana fruit ripening and ethylene production

Determination of ethylene production: The endogenous levels of ethylene in the treated and the control experiment was measured using a Sensor Sense B.V, Nijmegen, the Netherlands. Pranamornkith et al., (2012) procedure was used for this purpose. Each sample was run for 20 minutes at a flow rate of 4.0 L per hour and the average reading of last 15 minutes recorded as the endogenous ethylene from the fruit.

Determination or respiration rate: the rate of respiration of the fruits was estimated as the production of CO₂ using an infra-red gas analyser (Servomex Series 1400, Sussex, UK). The fruits were incubated for one hour in an airtight glass jar which contained rubber septum.  

Changes in peel colour: A colour chart was used to determine the banana skin colour. Measurement was recorded on every other day during the ripening process and was based on a scale of 1 to 8, with 1 representing very green bananas and 8 being speckled and over-ripe fruits.

Results

Maximum temperature recorded= 23⁰C

Minimum temperature recorded= 19⁰C

Required temperature of the indoor ripening=20±1⁰C

Mean Ethylene production (µmol C2H4  kg-1  h-1)
Treatments (µlL-1)	Ripening period
Day 1	Day 2	Day 3	Day 4	Day 5	Day 6
0	0.04167	0.04467	0.04033	0.04333	0.04667	0.05300
10	0.06633	0.07433	0.09800	0.09433	0.09067	0.08333
50	0.09000	0.08667	0.10467	0.09633	0.08833	0.07800
100	0.09800	0.09667	0.11467	0.10667	0.09833	0.08200

Table 1: Mean ethylene production by the ripening bananas

Mean Respiration Rate  CO2  mlkg-1.h-1
Treatments (µlL-1)	Ripening period
Day 1	Day 2	Day 3	Day 4	Day 5	Day 6
0	32.50218	23.26143	27.08286	26.90082	25.70370	23.97241
10	74.86527	76.61659	110.77210	106.36714	96.78297	92.82726
50	77.39939	99.20366	109.92063	66.63398	80.63655	84.90403
100	61.58895	110.19882	121.68880	117.47991	67.70774	87.77498

Table 2: Mean rate of respiration by the ripening bananas

Effects of ethylene treatments on changes in peel colour of banana fruit during ripening
Treatments (µlL-1)	Ripening period
Day 1	Day 2	Day 3	Day 4	Day 5	Day 6
0	1.0	1.0	1.0	1.0	1.0	1.0
10	1.0	1.0	1.2	2.3	3.0	4.4
50	1.0	1.2	2.0	4.2	5.0	5.1
100	1.0	1.6	3.1	4.9	5.4	5.9

Table 3: Rating of the peel colour based on the colour chart

Discussion

Effects of ethylene treatments on changes in ethylene production in banana fruit during ripening

From table 1 above the four treatments indicated a significance difference when it comes to endogenous ethylene production. The non-treated batch, the control, produced a consistent amount of ethylene gas throughout the ripening period. For the treated batches, the amount of endogenous ethylene produced increased with increase in the exogenous ethylene, with the 100 µlL^-1 producing the most amount. The difference between the batch treated with 10 µlL^-1 and 50 µlL^-1, is large as compared to the difference between that treated with 50 µlL^-1 and 100 µlL^-1. This is well illustrated in figure 1 below. Contrary to the control which reported a linear increase in production throughout the 6-day ripening period, the ethylene from the treated samples increases from day 1 to day 3 from which the amount reduces consistently.

The ripening process utilizes oxygen gas in the respiration process which breaks down starch to sugars. Ethylene gas is of utmost importance as it initiates and determines the rate of ripening. Carbon (iv) oxide is released in the process which slows down the ripening process. It is for this reason why soda lime was added into the plastic containers to prevent CO2 accumulation. Ethylene gas which is produced as the fruit matures plays a very crucial role in the ripening of fruits (Hailu, et al., 2012).

Exogenous application of ethylene

Effects of ethylene treatments on respiration rate in banana fruit during ripening

Form table 2, there is no defined pattern for the rate at which the fruits respire. This is best illustrated in figure 2 below. The rate of respiration is least for the control experiment as compared to the other trials. The batch treated with 100 µlL^-1 recorded the highest rates of respiration. In all the trials, the rate of respiration is high for the first 3 days from which it gradually decreases.

The study revealed that color is a very important parameter when it comes to banana quality. It’s is one of the aspects that any buyer will consider before they make any purchase decision. From table 3 above, the control experiment showed a consistent green colour throughout the ripening period, that is, a 1-day banana indicated no colour difference with a day 6 banana. Colour improved with increase in the applied exogenous and endogenous ethylene. Bananas treated with 100 µlL^-1 appeared more yellow compared to the other batches. There was a significant difference between the non-treated batch and the batch treated with 10 µlL-1, with the non-treated illustrating a very green banana, the 10 µlL^-1 portrayed a more yellow peel colour.

Treated bananas showed improve quality as far as colour is concerned. Colour development is more uniform throughout the ripening period than is the case with the control. This is attributed to the consistent diffusion/ production of endogenous ethylene from the fruits (Lelievre et al, 1997). The colour of the banana peels has been related to changes in the pigments contained within the skin of the banana. It has also been related quite well to the sugar content, flavor development as well as a change in the textural properties of the pulp (Salvador, Sanz and Fiszman, 2007).

Temperature

Banana ripening is done at a well-controlled temperature and relative humidity. Air circulation is also required so as to vigorously disperse ethylene gas and facilitate the removal of carbon dioxide and the heat produced as a result of respiration. Too high temperatures result in increased rate of respiration and irregular ripening of the fruits which results in poor keeping quality. Lower temperatures result in slow rates of ripening and may cause slight taste degradation. Very low temperatures cause chilling injury. The experiment was designed to operate at a temperature of 20 ±1⁰C. From the TinyTag sensors, the temperature recorded shows no significant difference from the recommended temperature.

Experimental procedure

A good place for ripening fruits should provide optimum conditions of 60-75% relative humidity as well as the reduced temperature of about 16-22⁰C which is optimum for banana ripening. Therefore, any ripening room requirements are; adequate size to avoid damaging of bananas through heaping and sufficiently ventilated to disperse carbon dioxide. Adequate cooling to prevent cellular heat damage on bananas, room temperature control and humidity control to prevent water loss from the banana peels (Mohapatra et al., 2010).

Temperature influences the ripening rate of any climacteric fruit. An increase in temperatures from 14-30⁰C enhances the rate of ripening and softening of fruits. Respiration and the rate of ethylene production also increase with an increase in temperature. High temperature causes damage to ripe fruits while the low temperature of below 14⁰ C will cause uneven ripening as a result of the chilling injury (Horton 2012).

Conclusion

Ethylene gas which is produced as the fruit matures plays a very crucial role in the ripening of fruits. Exposure of climacteric fruits to ethylene advances the onset of an irreversible rise in respiration and rapid ripening. Exogenous ethylene plays a major role in fruit ripening as well as in the production of the endogenous ethylene. Post-harvest losses occur from either unripe or overripe fruits as it affects the rate at which bananas are accepted by the consumer. There is, therefore, a need to improve and expand ripening methods and infrastructure to address market needs, where fruits acquire the right eating quality.

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