All You Ever Wanted to Know about Banana Ripening

BANANA (AND OTHER FRUIT) RIPENING ROOMS

BACKGROUND

Bananas and other unripe fruit (e.g. mango, papaya, avocado, tomato, stone fruit), are ripened in specially constructed rooms by exposure to a controlled atmosphere containing ethylene gas at a concentration of typically 100-200ppm (0.01% to 0.02% by volume in air). The ethylene is introduced from pressurized cylinders, or cartridges or a catalytic generator.

Ethylene obtained from cylinders is often at a higher level, up to 1000ppm (0.1%) since it is a one shot injection and more ethylene is introduced to compensate for room leakage. If ethylene generators are used, the maximum achievable concentration is in the order of 400ppm (0.04%) in a free space of 100m3.

The generators have the added advantage of producing ethylene gas continuously, which maintains gassing levels high enough to support ripening, even if there are air leaks from door gaskets and leaky room construction.

If the introduction of ethylene is uncontrolled there is a risk that the ethylene may reach or exceed the lower explosive limit (LEL) of 3.1% and be ignited by unprotected electrical apparatus or gas heating systems or other incendiary sources, resulting in a fire or explosion.

There are also potential acute risks from oxygen depletion and from the accumulation of other gases such as carbon dioxide; these health risks are exacerbated by the enclosed nature of the process.

Special sensors, for ethylene and carbon dioxide, are available, and better manage the process, for reduced costs, more consistent ripening results, and operator safety.

The main hazards arise from flammable or asphyxiating atmospheres being created by admitting excess ethylene gas into the room. Therefore, the use of cylinders of pure ethylene is strongly discouraged. Ethylene generators are the preferred option. If cylinders of pure ethylene are in use, they should be located outside in a safe well-ventilated position and be fitted with a pressure regulator and flow-meter, and be automatically timed so that the concentration of ethylene in the room does not exceed 25% LEL (7,000 ppm).

THE PROCESS

A typical ripening procedure for bananas is as follows:

1) Green bananas in cartons and at a fruit pulp temperature of around 14oC are loaded into the ripening room (lower temperatures can damage the fruit).

2) The room is closed and heated for 12-16 hours until the pulp temperature reaches 15-17oC.  This temperature is controlled and maintained by a thermostat. During summer conditions, a refrigeration system will cool the room to maintain the set temperature and  remove the heat of respiration, and the other cooling loads from fan motors and conduction.

3) Ethylene is supplied into the room at a concentration of around 100-400ppm (0.01 – 0.04%). The room is then kept closed for 24 hours. The ethylene acts as a catalyst initiating the hormonal process of ripening.

4) At the end of this time the room is ventilated to clear the ethylene gas and the carbon dioxide released during the initial ripening phase.

5) The room is then closed again and the atmosphere controlled at 17oC reducing to 15oC over three or four days. During this process gases, including carbon dioxide, are evolved in substantial quantities. Most of these gases would normally be vented off by ripening room operators to maintain the carbon dioxide level below 1%, as carbon dioxide levels exceeding 2% inhibit further ripening.

6) The room is finally ventilated and the ripe fruit removed. A common way of ventilating involves opening the doors for at least five, and usually fifteen, minutes before entry is made. Extractor fans may also be used.

METHODS OF INTRODUCING ETHYLENE GAS

Ethylene gas may be produced by one of the following three methods:

1) Ethylene generators which use a catalytic process to produce pure ethylene gas from a solution containing mainly ethyl alcohol (ethanol) but which may have trace quantities of other ingredients (e.g. methanol).

The solution (sometimes known as ‘ripening fluid’) is poured into a reservoir on the ethylene generator where it is slowly fed to an internal heater which vaporizes the ethanol which then passes to the catalytic converter. Each liter of solution produces about 0.33-0.4 m3 of ethylene gas. For the purposes of risk assessments the higher figure should be used.

2) Ethylene cartridges containing approximately 51g of pure ethylene (0.044 m3 at 20oC). Using the appropriate number of cartridges controls the ethylene concentration in the room simply and accurately. Ethylene is released by piercing the cartridge with the tool supplied.

3) Ethylene and ethylene/nitrogen cylinders. Bulk cylinders may be used containing either pure ethylene or a mixture of 5% ethylene in nitrogen (properly used, such a mixture can reduce the explosion risk).  It should be noted taht use of ethylene/nitrogen mixture, is the most expensive method.

5 Whichever method is used, the dutyholder should ensure that there are adequate means of dispersing the ethylene gases throughout the ripening room on its release.

HAZARDS

The hazards from this process fall into two main categories:

Fire/Explosion

1) Introduction of pure ethylene gas from cartridges may result in localized and short-lived flammable gas/air mixtures.

2) Extensive flammable gas/air mixtures may result from the uncontrolled addition of ethylene from a large cylinder or multiple discharges of cartridges.

3) Formation of flammable gas/air mixtures may result from spillage of ethanol-based solution used in ethylene generators.

4) Incidents to date show that the main acute hazard from this process is the combustion and explosion of excess quantities of pure ethylene resulting from uncontrolled discharges from large capacity cylinders, although other scenarios are possible.

Toxicity and asphyxiation

5) Oxygen deficiency in the room may result from excess addition of ethylene or ethylene-nitrogen mixture from cylinders.

6) The evolution of carbon dioxide during the fruit ripening process may result in a toxic or asphyxiating atmosphere. For example high levels of carbon dioxide may cause loss of consciousness and death from respiratory failure. Other effects such as headache and
exhaustion occur at much lower exposure levels.

7)  All exposures to carbon dioxide should not exceed 0.5% or 1.5% time weighted over an 8-hour or 15-minute period respectively.

8) Ventilation is a key factor both for controlling the process conditions and for ensuring a safe environment. The ripening room should be thoroughly ventilated between loads. As natural ventilation (i.e. open doors or vents) is unpredictable and unreliable, it can only be used with a safe system of work, which will involve the provision of permanent or portable oxygen meters to check the atmosphere before entry.

Mechanical ventilation is to be expected. Ventilation should be used to eliminate acute risks and adequately control other non-acute health risks e.g. exposure to excess levels of carbon dioxide.

Personal monitoring should be carried out as required to ensure that exposure is adequately controlled.

PREVENTION OF IGNITION SOURCES

No naked flames (e.g. gas fired heating systems) or other ignition sources should be allowed in the room after introduction of ethylene, until thorough ventilation has taken place, and it has been verified that the ethylene concentration is below 25% of the LEL.

STORAGE OF ETHYLENE CARTRIDGES AND ETHANOL BASED SOLUTION

Storage arrangements should ensure that:

1) quantities stored are as small as reasonably practicable;

2) a competent person is responsible;

3) no naked flames, smoking or other ignition sources are permitted near storage areas;

4) full and empty cartridges are stored in a safe, suitably ventilated area preferably in open air;

5) ethanol based solution for ethylene generators is stored in a suitable fire resisting bin or cabinet (quantities of up to 50 liters) which may be indoors. The storage of flammable liquids in containers’ gives further guidance, including arrangements for larger quantities.

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