THE PROBLEM - ETHYLENE GAS
What is ethylene gas?
Ethylene gas is a naturally occuring gas and extremely small quantities of this gas activate and increase the ripening rate for plants. For our commercial customers, we have produced a technical article which may also be of interest to consumers. It is very important to understand that we are dealing with very small quantities of gas. This gas is measured in PPB and PPM-parts per billion and parts per million.
The destructive properties of ethylene gas ( C2H4, MW 28.05) has presented a problem to industries involved in the handling and storage of cut flowers, plants, fruits and vegetables. Ethylene gas is a naturally occurring gas which causes fruits and vegetables to ripen and has long been recognized as a significant factor in the storage behavior of fresh produce and products in the horticular industry. Presence of ethylene in amounts ranging from a few parts per billion (ppb) to a few parts per million (ppm) can reduce plant vigor, decrease life of various plant parts, and reduce stock quality. The sensitivity of plants to ethylene varies as listed here and shown in the table at the end of this document.
- 0.1 ppm is commonly considered the threshold level for ethylene action on plants.
- Bananas remain "green" when ethylene concentrations are below 0.1-0.2 ppm.
- Exposures of 0.01-0.08 ppm of ethylene can trigger carnation blooms.
- Douglas Fir and Western Hemlock seedings appear sensitive to ethylene concentrations as low as 0.05 ppm.
Not only is ethylene gas an ever-present pollutant resulting from exhaust fumes from internal combustion engines, improperly vented greenhouse heaters, industrial waste, etc., it is also produced by plants themselves.
Ripening fruit is well know for producing ethylene, however, other plant tissues also produce this gas and can lead to decay and wilting. Even when cut from the growing plant, fruits, vegetables, and flowers are still alive and respiring. Gaseous respiration products of fruits and vegetables, production by normal healthy flower and plant tissue, chopped or damaged leaf tissue, and diseased leaves and tissues can be significant sources of ethylene.
Mechanical injury also increases the production of ethylene, so the processing of raw fruits and vegetables causes an increase in ethylene production. The lifting of nursery seedings probably results in ethylene being produced in the roots of seedlings. Dormant nursery fruit stock is damaged if stored in an atmosphere containing ethylene.
Ethylene Reactions - The dihydroxylation of alkenes (of Which ethylene is one) by potassium permanganate (KMnO4) is a well know reaction and cleavage of the chain to produce aldehydes, ketones, or carboxylic acids frequently occurs.
THE SOLUTION - ETHYLENE ABSORBERS
The oxidation of ethylene ( CH2CH2) with potassium permanganate can be thought of as a two-step progress. 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. These reactions are shown in Eq. 1-3 and summarized in Eq. 4.
1. 3CH2CH2 + 2KmnO4 + H2O = 2MnO2 + 3CH3CHO + 2KOH
2. 3CH3CHO + 2KmnO4 + H2O = 3CH3COOH + 2MnO2 + 2KOH
3. 3CH3COOH + 8KMnO4 = 6CO2 + 8MnO2 + 8KOH + 2H2O
Combining Eq. 1-3, we get:
4. 3CH2CH2 + 12KMnO4 = 12MnO2 + 12KOH + 6CO2
Even if the reaction does not go all the way through to the carbon dioxide-producing step, many of the intermediate products formed either become irreversibly bound to the media or act as reactants themselves. Such is the case of the potassium hydroxide (KOH) formed in Eq. 1 and 2. The KOH will react with the acetic acid formed in Eq. 2 to produce the potassium acetate salt (KCOOCH3) through a simple acid-base neutralization reaction. This is shown below.
5. CH3COOH + KOH = KCOCH3 + H2O
Combining Eq. 1, 2, and 5 we get:
6. 3CH2CH2 + 4KMnO4 = 3KCOOCH3 + 4MnO2 + KOH + H2O
From the above and breakthrough challenge gas testing, it has determined that Purafil Select media has an ethylene removal capacity of 1.5-2.0 % by weight (0.012-0.016 g/cc). This can be used to estimate the amount of media required for a particular application.
ETHYLENE CONTROL - APPLICATIONS
Purafil media has been used for ethylene control almost since it was introduced to the market more that 25 years ago. The tree main applications we have been involved in are controlled atmosphere storage, fruit/vegetables ripening rooms, and the shipping of produce and hortical products. 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 :1) reduction of respiration rates during storage and 2) 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 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.
Most traditionally operated ripening rooms use ethylene levels greater than 1000ppm. 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 100ppm 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 be 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 recirculating air unit should be employed to reduce the ethylene concentration below threshold levels.
The area around the rooms should be vented and equipped with an ethylene scrubber as well. Produce may be inspected several times a day. When the ripening room doors are opened, ethylene is released into storage, production and service the 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 - As markets expand across the country and around the world, preserving freshness is a constant challenge. 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. A single propane - powered forklift can cause serious damage in highly ethylene gas - sensitive commodities. Uncontrolled, ethylene can destroy an entire shipment in a single day. Even trace levels can cause measurable damage in both fresh produce and floral products.
The effects of ethylene gas damage is common to us all: shrinkage of fresh produce and flower bulbs; spotting of leafy vegetables and eggplants; yellowing of cucumbers; broccoli and brussel sprouts; increase odor and sprouting of garlic and onions; wilting of vegetables and cut flowers; scald and loss of crunch in apples; and rind breakdown in citrus fruit. Controlling ethylene levels preserves freshness.
The use of epax e.. a.. packets during post-harvest handling of fruit, vegetables, and floral products, during storage and ripening, and during shipment by truck or sea container can effectively reduce ethylene concentrations and maintain product quality and shelf-life.
epax - Mother Natures helper
These packets are a natural way to keep fruit and vegetables FRESH mush longer.
How do these packets work?
How long are epax effective?
epax - Retail Pack
Contents: 1 Vacuum sealed pouch containing 3 individual ethylene absorbing packets - 1 pack of 3 - $2.60/pk
epax - Commercial Pack
Contents: 1 Vacuum sealed pouch containing 25 individual ethylene absorbing packets - 1 pack of 25 - $10.60/pk