Ethylene gas (C2H4 ) is an odorless, colorless gas that exists in nature and is also created by man-made sources. In nature, the largest producers are plant and plant products (ie. fruits, vegetables and floral products) which produce ethylene within their tissues and release it into the surrounding atmosphere. It is also a product of man-made processes, such as combustion.
Ethylene, also known as the ‘death or ripening hormone’ plays a regulatory role in many processes of plant growth, development and eventually death. Fruits, vegetables and flowers contain receptors, which serve as bonding sites to absorb free atmospheric ethylene molecules. The common practice of placing a tomato, avocado or banana in a paper bag to hasten ripening is an example of the action of ethylene on produce. Increased levels of ethylene contained within the bag, released by the produce itself , serves as a stimulant for re-absorption to initiate the production of more ethylene.
The overall effect is to hasten ripening, aging and eventually spoilage. A refrigerator acts in much the same way. Kept closed to retain the desired temperature, it also enables an increased concentration of ethylene to accumulate. Any closed environment, such as truck trailer, shipping container, warehouses and cold rooms, will have a similar effect.
Artificial ripening by using ethylene is generally resorted to at the latter part of post harvest storage stage, when the produce needs to reach the consumers (retail outlets) with a degree of ripeness, which brings out it’s best in terms of taste, color, texture and nutritional value. One of the most common example is the ‘forced’ ripening of bananas during high demand periods.
Ripening fruit is well known for producing ethylene, however, other plant tissues also produce this gas. Even when cut from the plant, fruits, vegetables and flowers are still alive and respiring. Ethylene can be produced from virtually any form of vegetation, may it be chopped, damaged, diseased leaves and tissues, or normal healthy flower or plant tissues or the gaseous respiration products of fruits and vegetables. Mechanical injury also increases the production of ethylene, so the processing of raw fruits and vegetables causes an increase in ethylene production.
Some bad effects of ethylene presence are:
- Undesired ripening and softening of fruits in storage eg. scald and loss of crunch in apples.
- Accelerated senescence and loss of green color in leafy and immature fruit & vegetables eg. yellowing of cucumbers & broccoli.
- Abscission of leaves eg. cauliflower, cabbage, foliage plants.
- Increased odor & sprouting (stimulation or retardation) in garlic & onion.
- Induction of phenolic synthesis, such as, bitterness in carrot roots, toxic enzymes in sweet potato roots, spotting on lettuce, lignification of asparagus.
- Physiological disorders of ornamental crops eg. ‘sleepiness’ of carnations (failure of bloom to open), flower and leaf abscission, inhibition of shoot and root elongation.
Presence of ethylene in amounts ranging from a few parts per billion (ppb) to a few parts per million (ppm) can reduce plant vigour, decrease life of various plant parts and reduce stock quality. 0.1 ppm is commonly considered the threshold level for ethylene action on plants.
The Solution - Enthylene Oxidation
Potassium permanganate is known to oxidize ethylene. Ethylene is initially oxidized to acetaldehyde (CH3CHO) which it turn is oxidized to acetic acid (CH3COOH). Acetic acid can be further oxidized to carbon dioxide (CO2), and water (H2O). To reach this final step, however, there must be sufficient potassium permanganate available for the reactions. Assuming this is the case, the oxidation of ethylene would proceed to the formation of carbon dioxide.
3CH2CH2 + 4KMnO4 = 3KCOOCH3 + 4MnO2 + KOH + H2O
Ethylene Control - Applications
Potassium permanganate impregnated alumina media is one of the favoured chemical media for removal of ethylene. The three main application areas are:
- controlled atmosphere storage
- fruit / vegetables ripening rooms
- shipping / transportation of the horticultural produce
These are described briefly in the following.
Controlled Atmosphere Storage - The whole concept of controlled atmosphere (CA) storage of horticultural products has been based on the control of two major factors affecting plant senescence:
- reduction of respiration rates during storage.
- delay of ethylene-triggered changes leading to senescence.
The practice of CA storage involves the use of increased levels of carbon dioxide (CO2) and decreased levels of oxygen (O2) in the atmosphere, low storage temperatures, and the prevention of the buildup of internally-generated ethylene to threshold levels which could trigger changes leading to senescence and death.
Controlling ethylene gas will maintain the quality and extend the life of horticultural products, allowing them to be stored for a much a longer period of time. While refrigeration and humidity control will slow ripening and decay, they will not halt ethylene control.
Ripening Rooms - Produce is purposely not picked at the "peak of perfection" because there would be precious little time left to sell, distribute, and get in to market. Fruits and vegetables are picked "green" and flowers are gathered before their buds open so there is ample time between harvesting and distribution to wholesale and retail outlets. It is here where much of the "green" produce is allowed to ripen - some naturally, but much is helped along by exposure to elevated levels of ethylene.
Traditionally operated ripening rooms use ethylene levels greater than 1000 ppm. A significant amount of the gas can leak into the area around the ripening room or escape as operators enter the ripening room. A well-sealed room can operate with ethylene levels of 100 ppm or less and still get effective ripening. By insuring that ripening rooms are gas-tight and using less ethylene can reduce this source of ethylene contamination by more than 90%. Using lower levels also reduces the amount of this potential air pollutant released to the environment.
Placing the ripening rooms in buildings separate from traditional cold storage buildings or CA storage facilities will minimize the potential for ethylene contamination and exposure. If the ripening room must be located in the storage area, several precautions should be taken. Ethylene should by vented from the ripening room to the outside after the exposure period is complete. Exhaust fans capable of moving six to eight room volumes per hour will allow the ethylene level in the room to be reduced to approximately 1% of the original level in relatively short period of time. Because of temperature and humidity concerns, makeup (dilution) air is typically drawn from the refrigerated area surrounding the ripening room. Even after venting, the ethylene levels may still be high enough to continue the ripening process and cause product to have to be sold at a lower price or even discarded. Thus a “re-circulating air scrubber” should be employed to reduce the ethylene concentration below threshold levels.
When the ripening room doors are opened, ethylene is released into storage, production and service areas. Even after the ethylene has been vented from the room, much of the ripened produce can continue to produce ethylene at levels high enough to be harmful to sensitive products.
Shipping of Produce - Preserving freshness of horticulture produce has been a constant challenge for the food industry, especially during long haul transportation and shipment.. While ethylene gas is used under controlled conditions as ripening agent, even small amounts of ethylene gas during shipping and storage causes fresh produce to deteriorate faster. Even trace levels can cause measurable damage in both fresh produce and floral products.
Controlling ethylene levels preserves freshness.