Ethylene. To trigger the fruit within a ripening room to ripen uniformly

• Ethylene. To trigger the fruit within a ripening room to ripen uniformly and predictably, an external source of ethylene is required for most fruits. Ethylene is a natural plant hormone that affects the growth, development, ripening, and senescence (aging) of all plants. Ethylene has been found not harmful or toxic to humans in the concentrations found in ripening rooms (typically less than 250 ppm). In fact, ethylene was used medically as a anesthetic in concentrations significantly greater than that found in a ripening room.  For more information on ethylene, please see our Resources page.

Ethylene is explosive at 27,000 ppm; however, all that is required to ripen fruit is 100 - 150 ppm.  There are several sources of ethylene application systems available; the safest form is ethylene generators and concentrate.  In fact, the United Kingdom's HSE has stated in a document that "the use of cylinders of pure ethylene should be vigorously discouraged."

Ripening rooms should have a smoke test performed to check for air leaks at least once per year. This will help show where ethylene will escape and cause premature ripening in other rooms or damage to other types of fruit.

What is the effect of ethylene on fruit ripening?

Ethylene can promote ripening in tomatoes, bananas, citrus, pineapples, pears, melons, mangoes, avocados, papayas and more - a clear indication that the action of ethylene is general and widespread amongst a number of fruits. It is clear that ethylene is a ripening hormone - a chemical substance produced by fruits with the specific biological phenomenon of accelerating the normal process of fruit maturation and senescence.*

Is ethylene a harmful compound or toxic to human health in any way?

No! In fact, ethylene was used historically as an important anesthetic until less flammable compounds were developed. It is a colorless gas with a sweet ether-like odor. As an anesthetic, it was used as a concentration of 85% with 15% oxygen. Ethylene is a hydrocarbon gas and quite flammable and explosive at concentrations above about 3%. Remember, a non-toxic anesthetic for humans at a concentration of 85% or higher, yet as a fruit ripening hormone, ethylene gas is effective at 0.1 to 1 ppm. One part of ethylene per million parts of air that's one cupful of ethylene gas in 62,000 gallons of air - is enough to promote the ripening process in fruits.*

What do you mean 'promote' the ripening process?

Using tomatoes as an example, the life of a tomato fruit begins with fertilization of the flower ovules. After fertilization, the young fruit goes through a short period of cell division which is then followed by a rapid period of growth as these cells enlarge. During the final stages of growth and development, the tomato fruit reaches its full size and is now mature. This period of growth and development, from fertilization to development of the mature fruit, requires about 45-55 days, depending on the cultivar and the season. During the growth and development period, there are many chemical and physical changes occurring that have an impact on fruit quality and ripening behavior after harvest. Ripening is the final stage of the maturation process when the fruit changes color, and develops the flavor, texture and aroma that makes up what we define as optimum eating quality. The biological agent that initiates this ripening process after the fruit is mature is naturally produced ethylene - this simple plant hormone described and understood over 40years ago. While there are other factors involved in this "triggering" of the ripening process by ethylene, it is essentially a universal ripening hormone. When this internal concentration of naturally produced ethylene increases to about 0.1 - 1.0 ppm, the ripening process is irreversibly initiated. The process may be slowed, but it cannot be reversed once it is truly under way. So, here is the key point: additional and externally applied ethylene, provided prior to the time that the naturally produced internal concentration reaches the required 0.1 - 1.0 ppm level, will trigger or initiate - "promote" if you will - this natural ripening process at an earlier time.*

Doesn’t this still amount to an 'artificial' process?

No! The additional externally applied ethylene (the "gassing" so frequently referred to in the popular press) merely accelerates the normal ripening process. Numerous studies have shown that there are no important biochemical, chemical, or physiological differences between fruit ripened where the naturally produced ethylene has been the triggering mechanism or where additionally externally applied ethylene has triggered the process in the mature but unripe fruit.*

Nevertheless, doesn't the use of ethylene still allow the trade to 'cheat' the consumer with an inferior product?

For example, tomato fruit are not and cannot be "artificially reddened" by ethylene. The normal tomato ripening process, which includes pigment changes - the loss of green chlorophyll and conversion of carotenoids into red lycopene pigments - can be accelerated and brought about earlier by externally applied ethylene, but this is a normal process. In fact, some of the components of nutritional quality, such as Vitamin C content, benefit because of the fact that the fruits will be consumed after a shorter time interval from harvest as a result of ethylene treatments and hence, the initial level will not have degraded as far as the longer, unaccelerated process. Ethylene is actually used commercially on only a few crops, including: (a) bananas, (b) for removing the green color from citrus fruits, (e) almost all honeydew melons, and (d) to a limited extent, with tomatoes.*

What are the factors that result in the poor quality tomatoes we often see on the market?

Although many factors could be listed, there are four which play the dominant role in determining the quality of tomato fruits presented to the customer in the retail store:

1. variety
2. maturity at time of harvest
3. storage temperature during shipping and handling (this is probably the most common cause; tomatoes are often shipped or stored at improper temperatures, which causes severe taste loss...never allow tomato pulp temperature to go below 55°F!)
4. physical damage*

Does Ethrel or Ethephon work in ripening bananas and other fruits?

Ethephon is a plant growth regulator used to promote fruit ripening, abscission, flower induction, and other responses by releasing ethylene gas. Ethephon is registered for use by the US-EPA on a number of food, feed and nonfood crops, greenhouse nursery stock, and outdoor residential ornamental plants, but is used primarily on cotton. Formulations include formulation intermediates and soluble concentrates/liquids. Ethephon is applied to plant foliage by either ground or aerial equipment. It is not registered for use on bananas nor is it designed for post-harvest use. For commercial fruit ripening, the most effective and efficient ethylene application method is an ethylene generator.

How effective is "Banana Gas 32™", BANARG™ and other ethylene gas mixtures?

Since ethylene is explosive, these products contain an inert, non-flammable gas so that when released into an enclosed space like a ripening room, no explosive level of ethylene can be reached.  The inert used is either Carbon Dioxide or Nitrogen.  These mixtures will ripen fruit, but it does require large doses to reach 100 ppm of ethylene.  Also, when applying ethylene in this manner, the fruit is also being exposed to large amounts of carbon dioxide or nitrogen; these are ripening inhibitors and will counteract the ethylene by causing the fruit to decrease respiration...or "sleep" and not receive the ethylene action.  Better results are typically achieved using pure ethylene from generators or cylinders!

What is the best way to measure ethylene ppm levels?

There are several methods, and each has their advantage.  What is best for one may not be for another, so the user would have to decide what is "best."

• Air Sampling System / Tubes. These pass a measured amount of air from the area sampled through a filament that reacts with ethylene.  After a few minutes, the filament changes color and the approximate ethylene level can be read on the tube. Accuracy is 25%. By far the most popular way to measure ethylene.
• Digital Ethylene Detectors.  Almost instant readings. Accuracy is approximately 2%, but unit must be in calibration!
• Gas Chromatograph.  By far the most accurate, into the part per billion (ppb) level, this method is also very time consuming and normally not performed on site.

*Source for these questions: California Fresh Market Advisory Board, Informational Bulletin No. 12, June 1, 1976.

 "Banana Gas 32™" is a trademark of Praxair Tecnology Inc.; BANARG™ is a trademark of Linde Gas.