Ethylene is a small unsaturated hydrocarbon consisting of two carbon atoms joined by double bond with the formula C2H4 . It is found naturally as a gaseous component in petroleum deposits, and the ability of ethylene to polymerize has been utilized by petrochemical industry to produce the now –ubiquitous plastics.
Ethylene is a natural plant hormone.
Ethylene, unlike the rest of the plant hormone compounds is a gaseous hormone .It affects the growth, development, ripening, and senescence (aging) of all plants. It is normally produced in small quantities by most fruits and vegetables. Many fruits produce larger quantities of ethylene and respond with uniform ripening when exposed to an external source of ethylene. Ethylene has been found not harmful or toxic to humans in the concentrations found in ripening rooms.
In fact, ethylene was used medically as an anesthetic in concentrations significantly greater than that found in a ripening room.
Ripening is a process in fruits that causes them to become more edible. In general, a fruit becomes sweeter, less green, and softer as it ripens.
Bananas are the most common fruit but other exotic fruit (eg kiwi fruit) are ripened by the same process.
Bananas are harvested and imported green .Then they are ripened in specially constructed rooms by exposure to controlled atmosphere containing ethylene gas at a concentration of typically 0.l% by volume in air. Some rooms are heated by gas or electricity. The ethylene is introduced from pressurized cylinders. If the introduction of ethylene is uncontrolled there is a risk that the ethylene may reach or exceed the Lower Explosive limit (LEL) and be ignited by unprotected electrical apparatus .The LEL for ethylene is 3.1 %.The use of ethylene 5%in nitrogen balance insure good balanced exposure of banana to ethylene and there is no possibility to reach the LEL.
a. Green bananas in cartons and at a fruit pulp temperature of around 11°C are loaded into the ripening room.
b. The room is closed, cooled or heated for 12 to 16 hours until the pulp temperature reaches 15 to 17°C. The heating is then switched off.
c. Ethylene is discharged into the room at a concentration of around 0.1%. The room is then kept closed for 24 hours. The ethylene acts as a catalyst initiating the hormonal process of ripening.
d. At the end of this time the room is ventilated to clear the ethylene.
e. The room is then closed again and the atmosphere controlled at a temperature of 15 to 17°C for three to four days. The fruit pulp may reach a temperature of 32°C during this process and gases, including carbon dioxide, are evolved in substantial quantities.
f. The room is finally ventilated and the ripe fruit removed. A common way of ventilating involves opening the doors for at least 5, usually 15 minutes before entry is made. Extractor fans may also be used.
NBThe amount of ethylene gas required for a ripening room is normally calculated on the free air space after the bananas have been loaded (i.e. if bananas take up to 35% of the room size. calculate the amount of ethylene required for remaining 65% free air space).
METHODS OF INTRODUCING ETHYLENE GAS
‘Ethy-Gen’ Catalytic Generators
A method by which a liquid concentrate ‘Ethy-Gen’ is decomposed in an electrically powered catalytic generator, to produce ethylene gas. The ‘Ethy-Gen’ concentrate is supplied in containers which produce about 12 ft3 (0.33 m3) of ethylene gas. The amount of liquid put in the generator depends on size of ripening room.
Each ethylene cartridge contains approximately 51g of pure ethylene and the ethylene concentration in the room may be controlled simply and accurately by using the appropriate number of cartridges. Ethylene is released by piercing the cartridge with a tool which is supplied.
Ethylene and Ethylene/Nitrogen Cylinders
The explosion risk from ethylene can be eliminated by the use of a mixture of gas consisting of 5% ethylene in nitrogen. Pure ethylene and the mixture can be obtained from British Oxygen Company Limited and Air Products Limited.
Pure Ethylene Cylinders
Pure ethylene can be obtained in cylinders.
The hazards arising from this process fall into two main categories:
a. Introduction of pure ethylene from cartridges may result in localised and short-lived flammable gas/air mixtures.
b. Extensive flammable gas/air mixtures may result from the uncontrolled addition of ethylene from a large cylinder or from a multiple discharge of small cartridges or the use of the catalytic generator in too small a room.
c. Where gas-fired heating equipment is used, flame failure may result in quantities of unburnt gas entering the room and creating a flammable atmosphere.
a. Excess addition of ethylene/nitrogen mixture from cylinders;
b. The evolution of carbon dioxide during the ripening process;
c. The combustion of fuel gas toxic combustion products (chiefly CO) will be produced if there is not sufficient oxygen present at the burner for complete combustion.
Incidents to date show that the main hazard to arise from this process is the combustion and explosion of excess quantities of pure ethylene resulting from uncontrolled discharge from large capacity cylinders. There may be a further hazard from asphyxiating gases in the ripening room, eg carbon dioxide and nitrogen if entry is made before ventilation is complete.
Suggested Tomato Ripening Tips
Proper temperature, humidity, air circulation, ventilation, ethylene and mature tomatoes are required for ripening:
Temperature range for ripening: 64° to 70°F (18° to 21°C)
Humidity for ripening & storage: 85 – 95% RH (90% ideal)
Air Circulation: Sufficient to provide even pulp temperatures throughout the ripening room.
Ventilation: Use “flow through” ventilation or vent room 10-20 minutes every 12 hours (manually or by automatic fan). Our diagram of “Flow Through Ventilation,” designed to continuously vent Carbon Dioxide from the ripening room and introduce fresh air, resulting in more uniform ripening, can be seen at this link: “Suggested Flow-Through Diagram”. It also describes how to calculate the fan size for each room.
Ethylene: Maintain 100-150 ppm until a “breaker” or Stage #2+ is reached, usually 24-36 hours (depending on temperature and maturity).
Mature Tomatoes: Mature green tomatoes will ripen after harvesting in the same manner as they would on the plant, thanks to an external source of ethylene in the ripening room that triggers the fruit to release its own ethylene. However, immature tomatoes will erratically respond to external ethylene and possibly result in poor quality or delayed ripening. Harvest or receive only mature fruit.
Ripen tomatoes as soon as possible:
Whenever possible, avoid “holding” and delayed ripening. Tomatoes will respond their best and ripen evenly when external ethylene is applied soon after harvest. On average, fruit ripened at 64° to 70°F to a breaker stage can then be stored for more than two weeks at 55°F (12.5°C) until a full red (stage #6) color is reached.
Regularly take pulp temperature readings (at least twice per day) of each load of tomatoes and refer to these readings as you ripen.
Maintain pulp temperatures in the correct range at all times:
The greatest cause of tomatoes suffering a big loss in flavor and retail quality is cold. When tomato pulp temperatures are allowed to stray out of the proper temperature range, internal damage results in a mushy appearance of flavor decline. Therefore, it is crucial to ensure that pulp temperatures remain above 55°F at all times and at all stages of ripeness. When shipping tomatoes on mixed loads at air temperatures less than 55°F, use some form of protection such as insulated or plastic pallet covers to hold the fruit’s heat in.
Check equipment often and smoke test rooms for air leaks at least once per year.
Degreening also is called gassing, sweating or curing. Its purpose is to improve external skin color and export market acceptance. Improve external skin color and export market acceptance.
Timing of De-greening
Ethylene treatment should be given immediately after harvest.
Ethylene De-greening Methods
* Gas Treatment
* Liquid Treatment
The general procedure for de-greening citrus with ethylene gas involves exposing green-skinned fruit held at a high temperature and relative humidity to low concentrations of ethylene for several days. The source of ethylene can be a high-pressure gas cylinder or an ethylene generator.
The ethylene gas treatment procedure for effective de-greening should adhere to the following protocol:
Temperature: 28-29°C (82-85°F) for humid production areas
20-25°C (68-78°F) for arid production areas
Relative Humidity: 90-96%
Ethylene Concentration: 3-5 ppm
Duration: ≤ 2 days
Oranges: 24-36 hours
Tangerines: 6-12 hours
The source of ethylene can be a high-pressure gas cylinder or an ethylene generator.
High Pressure Cylinders
The flow of ethylene from the high pressure cylinder into the de-greening rooms is finely controlled by a two-stage regulator, needle valves and flowmeters.
A commercially available electric ethylene
generator is available for de-greening citrus.
It converts ethanol, in the presence of heat and a catalyst, to ethylene. The generator is located inside the de-greening room.
Liquid Treatment (Ethephon)
The general procedure for de-greening citrus with liquid ethylene (ethephon) involves dipping green-skinned fruit in a tank of water at ambient temperature to low concentrations of ethylene for several minutes or less.
The liquid ethylene treatment procedure for effective de-greening should adhere to the following protocol:
Water Temperature: ambient (22-29°C) (72-85°F)
Water Sanitation: potable, 150 ppm hypochlorous acid, pH 6.5, fungicide
benomyl (500 ppm) or thiabendazole (1000 ppm)
Ethephon Concentration 500 ppm
Duration: 1-3 minutes
The source of ethylene is the chemical [(2chloroethyl) phosphonic acid], which is a liquid ethylene-releasing compound with the chemical name of ethephon.
*The water should be sanitized with 150 ppm hypochlorous acid (2.4 pints of household bleach in 100 gallons of water) and maintained at a pH of 6.5.
*The sanitizing strength of the wash water should be frequently tested with the appropriate meters and/or test strips, and adjusted when necessary.
* A fungicide is usually incorporated in the water (500 ppm benomyl or 1000ppm thiabendazole).
Degreening Chamber considerations:
CO2 level < 1% Air circulation(velocity) 1 room volume/min
Ventilation 1-2 air changes/hr
*Air circulation within the degreening room should produce about one change per minute. * Outside air ventilation should be adequate to maintain carbon dioxide level below one percent.
* Adequate insulation of the walls and ceiling of the de-greening chamber/room is necessary to avoid over-heating.
*The room should be designed so the internal air movement is horizontal.
* The treatment chamber should be well insulated in order to maintain the desired ethylene concentration.
Undesirable Effects of De-Greening
Postharvest decay may be accelerated due to excess ethylene exposure. The two principal diseases which can increase are stem-end rot (Diplodianatalensis) and anthracnose (Colletotrichumgloeosporioides). Both pathogens can be controlled by a postharvest application of benomyl (500 ppm) or thiabendazole (1000 ppm) after de-greening.
Diplodia Stem-end Rot
Stem-end rot fungus usually infects fruit from the button at the stem-end of the fruit. It proceeds through the core more quickly than the rind, leading a soft brown to black decay to appear at both ends of the fruit. All citrus types are susceptible.
Tangerines are particularly susceptible to anthracnose decay if degreening time exceeds 36 hours.
Stem-end Rind Breakdown
After degreening , a holding period is necessary where fresh, cool air is introduced into the room for 12 to 24 hours before packing. If not properly aired and cooled, rind stain may appear after running over the packing line.
Irregular, brown lesions develop on skin with an objectionable appearance.
Important degreening notes:
* Fruit do not have to be dry before treatment. However, grapefruit will not de-green well if the peel is moist
* Recommended degreeningconditions :
Temperature 82o to 85o F
Relative humidity 92 to 95%
Ethylene 1 to 5 ppm
* Warmer or cooler temperatures slow de-greening.
* A high RH is necessary during de-greening to maintain peel freshness and prevent dessication.
*Higher ethylene concentrations do not speed up the rate of degreening, but increase postharvest decay.
Notes on Ethylene Usage:
Ethylene acts on both:-
Apple, apricot, avocado, banana, feijoa, mango, papaya, passion fruit, peach, pear, plum, tomato and watermelon.
Blueberry, cherry, cucumber, fig, grape, grapefruit, lemon, melon, olive, orange, pineapple, strawberry and tamarillo.
Ethylene is known to affect the following plant processes.
• Stimulates the release of dormancy.
• Stimulates shoot and root growth and differentiation (triple response)
• May have a role in adventitious root formation.
• Stimulates leaf and fruit abscission.
• Stimulates Bromiliad flower induction.
• Induction of femaleness in dioecious flowers.
• Stimulates flower opening.
• Stimulates flower and leaf senescence.
• Stimulates fruit ripening.