POST HARVEST RIPENING AND PRESERVATION OF FRUITS

Post-Harvest Ripening of

Banana and Other Fruits

By Sunil P Bhat

Director

Advance Agro Ripe Pvt. Ltd., Pune (India )                     www.agroripe.com

Mr. Sunil Bhat is a hard cored Mechanical Engineer with over 25 years experience in the field of

Plant Engineering having  Domain knowledge in the field of post-harvest Preservation of

Fruits and Vegetables. He is member of Institution Of Engineers (India) and ISHRAE .He is a certified Engineer by “National Horticulture Board” (NHB) India for planning and designing cold chain infrastructure.

Introduction

Banana ranks third in cultivated area among fruits in India with 0.464 million hectare covering 12.46 per cent of the total area under fruit cultivation. However, it is first in total production (15.07 million tons), being nearly one- third (34.22%) of total fruit production. India occupies the first position in banana production globally. Among Indian states, Tamilnadu ranks first in area and production while productivity is the highest in Maharashtra. Most of the banana is produced on a small scale basis in different production systems.

There has been a phenomenal increase in banana production due to adoption of high density planting, use of tissue-cultured seedlings and drip irrigation, which have significantly improved productivity.

The Need to Promote Bananas

A banana has four times the proteins, twice the carbohydrates, three times the phosphorus, five times the vitamin A and iron, and twice other vitamins and minerals compared to an apple.  It is also rich in potassium and is one of the best value foods around. So maybe it is time to change the well-known phrase to: "A banana a day keeps the doctor away!"
There is no better snack than a banana as a quick fix for flagging energy levels. Containing three natural sugars – sucrose, fructose and glucose – combined with fiber, a banana gives an instant, sustained and substantial boost of energy. Research has proved that just two bananas provide enough energy for a strenuous 90-minute workout. No wonder the banana is the most favourite fruit of the world's leading athletes.    

Post-Harvest Losses

According to National Botanical Research Institute (NBRI), due to improper handling and ripening techniques, bananas worth Rs.150 crores (25% to 30%) are lost every year in Maharashtra alone. Losses occur at the following stages:

·                     Harvesting and pre-harvesting: Due to spoilage and trimming.

·                     Transport: Due to bruising, breakage and infection as a result of dust, heat, rain and humidity.

·                     Storage: Due to over ripening or under ripening.

·                     Processing and packing: Due to inefficiency and contamination.

·                     Marketing:  Due to loss of weight and quality during multi-level handling.

The problem is further complicated by the lack of storage facilities at the farm level, and farmers are forced to dispose off the entire produce immediately on harvesting. Thus, the margins of wholesalers and retailers are much higher than in the developed countries.

Scientific Ripening of Fruits – the Present Scenario

Large quantities of fruits like mango, papaya, sapote, etc. are ripened using calcium carbide which emits harmful substances like phosphor, arsenic and lead, which are health hazards. As per the old Prevention of Food Adulteration (PFA) Act (Section 44AA), use of carbide is strictly banned.  The only safe method accepted worldwide is the use of ethylene, which is a natural hormone for ripening under controlled temperature and relative humidity conditions.

After getting cleaned, packaged and quality checked, banana needs to be ripened before it arrives at retail outlets for purchase by the consumers. The wholesaler would need ripening facility under controlled conditions for fresh green bananas of appropriate physiological maturity brought from the farm or cold room to the ripening chamber. There is a need for the establishment of substantial ripening facilities in India, owing to the large production of banana in different parts of the country.

Customers have a preference for bananas with spotless yellow colour that make them more presentable, apart from their size. Thus a proper ripening facility ensures good price realization for the producer.

Ripening Chambers at Consumption Points

Ethylene gas is used for ripening most of the climacteric fruits like banana, mango and papaya under controlled condition of temperature, humidity and ethylene concentration in air tight, gas proof rooms. Ethylene, being a natural hormone, does not pose any health hazard for fruits; also, being a de-greening agent, it turns the peel from green to a perfect yellow and maintains the sweetness and aroma of the fruit. This adds value to the fruit. A ripening chamber does not require large investments and can be set up in the farm or at the trading point.

These chambers are multipurpose. During off-season, no ripening load period, they can be used for storing and preserving fresh vegetables and other fruits as well, ensuring maximum utilization of the cold chamber. This way of ripening not only increases the cosmetic value, sweetness and aroma of fruit but also increases its post ripening shelf life substantially. The same chamber and set-up can also be used for ripening mango, papaya and citrus fruits by minor adjustment in temperature and ethylene concentrations.

Post-Harvest Processing of Bananas

Pre-cooling and Storage

The optimum transport and storage conditions for mature green bananas are 13-14°C and 90-95% relative humidity. Although pre-cooling is not generally carried out, it is advisable to cool down fruit exposed to temperatures above 30°C soon after harvesting to remove field heat. Failure to do so can irreversibly inhibit ripening and result in heat damage indicated by failure to de-green properly, excessive pulp softening of green fruit, boiled appearance of the pulp and incomplete starch-to-sugar conversion. In addition, insufficient field heat removal or failure to precool can result in failure to reach the desired storage temperature, heat accumulation in the cold room and thus reduced life.

Pre-cooling is done by forced-air or evaporative cooling. Different banana cultivars respond differently to Controlled Atmosphere (CA) conditions. Generally, storage in CA at 2-5% O₂, 2-5% CO₂, 90-95% RH and 12-15°C in the absence of ethylene can extend the post-harvest life of mature green bananas to 4-6 weeks. After storage, such fruit can still be ripened to good quality by treating it with ethylene. Excessively high CO₂ concentrations can be toxic to bananas and cause pulp softening of green fruit, internal browning and off flavor. The use of CA during ocean transport has made it possible to harvest bananas at full mature stage. Bananas also respond well to Modified Atmosphere Packaging (MAP), and green bananas can be stored in MAP at 13-14°C for more than 30 days. Ripe bananas can be stored in MAP at 13-14°C for up to 7 days.

Ethylene and Ripening

Bananas, mangos, papaya, sapote, etc. are climacteric fruit that exhibit typical climacteric patterns in both their respiration and ethylene production rates during ripening. Since exposure to ethylene accelerates ripening in these fruit, bananas must be kept away from other ethylene-producing fruit such as mangoes and melons. Bananas are harvested when mature but still green and ripened at the destination market by treatment with 100-150 μL L^-1 (ppm) ethylene at 15-20°C (depending on the required ripening rate) and 90-95% RH. Careful attention should be paid to temperature and CO₂ management during ethylene treatment and ripening. To avoid suppression of ethylene action, CO₂ levels should never be allowed to exceed 1%.

In CA, the low O₂ and high CO₂ levels suppress ethylene production by the fruit.

As soon as the product has reached the desired ripening temperature, ethylene is released into the ripening room from pressurized gas cylinders (Figure 1) or a generator that converts ethanol to ethylene. Ethylene stimulates fruit ripening at concentrations ranging from 0.1 to 1.0 ppm. However, ethylene concentration in the ripening room is set at around 100 ppm to ensure that all the fruit are constantly saturated with ethylene for the duration of the exposure period, and to make a provision for possible leakages from the room. After the product has been exposed to ethylene for 24 hours, the ripening room is ventilated to get rid of excess CO₂ in the atmosphere, since ethylene action is inhibited by high levels of CO₂. Levels higher than 1% inhibit the effect of ethylene in initiating ripening. After ethylene exposure, the room is ventilated continuously at a rate of 1 room volume every 2 to 6 hours to maintain the CO₂ levels below 1%. If the room is not equipped with a continuous ventilation system, ventilation can be achieved by opening the door for 10 to 20 minutes once or twice a day while the refrigeration fans are running. However, this practice can result in undesirable temperature fluctuations that can interfere with the precise control of the ripening process. Once the product has reached the desired colour or stage of ripening, the room temperature is lowered again to normal storage temperature before the product is removed to storage, transported to the market or processed.

Banana or other fruit ripening rooms must be cleaned and disinfected regularly to prevent infection of the fruit. Room surfaces must be scrubbed down with a suitable disinfectant.

General Ripening Procedure

·         Quickly load produce in the ripening room and gradually raise or reduce the flesh temperature to the ripening level.

·         Temperature monitoring is critical. Flesh temperature must be accurately maintained during ripening.

·         Treat with ethylene for 24 hours.

·         After this, ventilate continuously.

·         Lower the flesh temperature to storage temperature when the desired ripening stage is reached.

Ethylene (C₂H₄)

Fruit ripening is a genetically programmed process that is controlled by plant hormones and accelerated or retarded by certain environmental factors. Plant hormones control the expression or suppression of specific genes involved in these processes. Some plant hormones delay ripening, while others such as ethylene accelerate the process in climacteric commodities such as bananas, mango, papaya, sapota etc. Ethylene is a unique gaseous plant hormone. It stimulates respiration, accelerates fruit softening as a result of cell wall hydrolysis due to the stimulation of the transcription of cell wall degrading enzymes such as polygalacturonase, causes de-greening (yellowing) due to the stimulation of chlorophyll breakdown, causes de-compartmentation of the cell due an increase in membrane permeability, changes the metabolism of organic compounds such as carbohydrates, organic acids and proteins and stimulates the production of aroma volatiles.

Bananas, mango, papaya, etc. should preferably be ripened in a forced air room to prevent heat build-up and facilitate even distribution of ethylene gas. The refrigeration equipment must be adequate to raise or lower the temperature between 14˚C and 18˚C in a few hours. Air circulating fans must be strong enough to provide an air flow rate of 0.02-0.06 m³ per minute per kg fruit in the room. Although fruits can be ripened in non-forced air store rooms, it is best to use forced air rooms for this purpose since they provide for more accurate temperature control and even distribution of ethylene in the room. When ordinary cold stores are used, boxes should be stacked in an open stacking pattern such as pigeonhole stacking where open spaces are left in the stack to improve air flow during ripening and storage . It is also important to leave adequate space between the pallets and the cold room walls to allow for unrestricted air circulation since cooling of the pallet is mostly by conduction.

Ripening Protocol

Upon arrival in the ripening room, boxes should be selected from the middle of each pallet and the pulp temperature of a fruit from each box checked. (It should be around 16˚C). The stage of maturity should be determined visually or with a pair of calipers. Individual fingers should be between light three-quarter and full three-quarter size. Over-sized fruit ripens rapidly and should be handled with great care because the peel can easily split during handling, while under-sized fruit will not ripen normally.

After determining the maturity, pallets are placed in the ripening room and the air circulation system turned on. The fruit is heated or cooled to the desired ripening temperature (14˚C-18˚C; do not exceed 20˚C pulp temperature during the ripening cycle). Temperature controls the rate of ripening and high temperatures will result in ‘green’ ripening, i.e. softening of the pulp without de-greening of the peel. As soon as the pulp has reached the set temperature, ethylene is introduced into the ripening room with an ethylene generator or bottled ethylene to maintain the levels at 100 ppm for a duration of 24 hours. After ethylene treatment, the room should be vented to get rid of excess ethylene and CO₂. Thereafter, the rooms should be vented at least twice per day for 20 minutes or continuously with exhaust fans to keep the CO₂ levels below 1%. CO₂ levels above 1% will inhibit the ripening process.

The fruit should be kept at the required temperature until it has reached the desired stage of ripeness (firmness). Pulp temperatures must be recorded throughout the room on a daily basis and the relative humidity should be kept at 90-95% throughout the ripening cycle. Once the fruit has reached the desired ripeness, it should be cooled down to 14˚C to slow down ripening, and placed in a cold store at 14˚C. Ripened fruit are less prone to chilling injury than unripe fruit. Further ripening after storage can be controlled by time and temperature. The higher the pulp temperature, the shorter is the time required to reach eat ripeness. The pulp temperature should never be allowed to rise above 20˚C during ripening. Please see Table 1.

Table 1: Fruit pulp temperature (⁰C)

	Days in the ripening room
Ripening Schedule	First	Second	Third	Fourth	Fifth	Sixth	Seventh	Eighth	Ninth
4 days	18 ethylene	18	16.5	15.5	Store at 14.5
5 days	16.5 ethylene	16.5	16.5	16.5	16.5	Store at 14.5
6 days	16.5 ethylene	16.5	15.5	15.5	15.5	14.5	Store at 14.5
7 days	15.5 ethylene	15.5	15.5	15.5	15.5	14.5	14.5	Store at 14.5
8 days	14.5 ethylene	14.5	14.5	14.5	14.5	14.5	14.5	14.5	14.5

Uneven Ripening

Uneven ripening in a box, pallet or load is a common problem encountered in fruit that are ripened after harvesting. The most common causes of uneven ripening are improper ripening techniques, insufficient ethylene levels, incorrect exposure time, incorrect ripening temperature, RH below 90%, temperatures above 21˚C during ripening, improper air circulation, excessive holding periods before the start of the ripening cycle, variable fruit age, variable fruit maturity, wide variations in pulp temperature upon arrival at the ripening room, exposure to temperatures below 12˚C prior to ripening and exposure to extreme high temperatures prior to ripening (heat damage).

Conclusion

Fruit ripening chambers are proliferating throughout the country as the fruit handler community has realized the importance of deploying a scientific method for ripening in a cold room, exercising control over temperature, RH and gas levels of ethylene, CO₂ and oxygen. A revolution has taken place in this sector during last the five years, as ripening chambers have reached even the smallest villages and farms. Refrigeration engineers play an important role in setting up these facilities.
This sector has a vast potential for rural employment generation. National Horticulture Board (NHB) India offers handsome incentives of 35% to 50 % for setting up ripening and cold preservation chambers for all fruits and vegetables and other agricultural produce.