Despite its miniscule molecular weight, the volatile plant hormone ethylene, C2H4, a monounsaturated hydrocarbon, plays an exceedingly important role as a ripening agent in horticulture (1). Ethylene's involvement in fruit and vegetable ripening was first directly encountered in the early 20th century when orange and lemon growers changed the type of stove used for heating storage sheds to prevent the harvested citrus from freezing (2,3). Curiously, the citrus fruit in kerosene-heated sheds ripened more rapidly than when the sheds were heated with wood or electricity. The heating conundrum was eventually solved when ethylene — the only gaseous plant hormone — was identified as the causative ripening agent. Small amounts of ethylene are produced during combustion of kerosene.
In current harvesting practice, various fruits and vegetables such as oranges and tomatoes are picked "green" (as they are less sensitive to bruising during shipment), stored in enormous cold storage warehouses, and brought to ripeness by exposure to ethylene when market conditions are favorable. The ripening process is complex because it is not only accelerated by ethylene, but it is also autocatalytic, producing additional ethylene as it proceeds. Thus, if one wishes to store fruit for several weeks — plums are especially amenable to prolonged cold storage — it is essential to maintain virtually ethylene-free conditions. The sensitivity of fruits and vegetables to ethylene varies widely. Kiwis are so sensitive that the forklifts used to move the pallets of fruit are battery-powered because the small amounts of ethylene in the exhaust of propane-powered forklifts is enough to initiate premature ripening (4).
One enviably simple — and effective — method to help growers minimize ethylene-induced ripening problems is to add a single packet (Figure 2, left panel), a sachet (to quote the inventors), of ETHYLENE CONTROLTM to each box of plums (5). The consequence is to prolong by several weeks the shelf life of a box of plums in cold storage.
As a practicing organic chemist from Boston visiting my brother's tree fruit (plums, peaches, and nectarines) farm in the San Joaquin Valley of central California, I was curious to learn the chemical underpinnings of the sachet of ETHYLENE CONTROLTM that my brother added to each package of plums before sending it to the cold storage warehouse. I asked how the sachets work, but he did not know. Standard analytical tools were in short supply on his farm, but fortunately, where there is a will, there is often a way.
Opening one of the packets of ETHYLENE CONTROLTM revealed ca. 3mm diameter lumps of a nearly black material with a metallic sheen (Figure 2 above, right panel). The initial appearance of the material was largely uninformative, and given the minimal facilities available on my brother's farm, there were not many experiments to try. But addition of water to the sample to see if it was water-soluble precipitated an unexpected aha! moment (6), because the water quickly turned a brilliant purple (see Figure 1 in introductory paragraph). For one from the generations who learned chemistry when its descriptive aspect was emphasized — but not for most current students — the purple color was a dead giveaway: permanganate anion, MnO4-. To the author's knowledge, apart from recently created dyes and pigments, permanganate anion is the only species with such a striking purple color. And permanganate anion's presence in ETHYLENE CONTROLTM (7) makes total chemical sense: the use of potassium permanganate to convert alkenes to vicinal diols is a standard reaction covered in Introductory Organic Chemistry (8,9). But to the author at least, it is a clever leap of inspiration for a non-chemist to imagine using potassium permanganate, not to functionalize ethylene, but to scavenge it. The marketplace evidently concurs as the company Ethylene Control has been in business for thirty-five years (10).
The Classroom Demonstration. (Note: If teaching OChem, this demonstration is timely after presenting the dihydroxylation of alkenes. To view a video of the author performing this demonstration click here.) After explaining that ethylene is a gaseous plant hormone which stimulates ripening of fruit and at the same time is released during the ripening process, one introduces students to the concept of using cold storage warehouses to prolong the shelf life of freshly picked fruit — sort of like using refrigerators at home does, except that the refrigerators are full of forklifts. Projecting an image such as Figure 3 (note the yellow forklift) (11), or showing this video of a working cold storage warehouse (12) should convey the scale of operation.
At this point in the class I ask, "Who has been in a commercial cold storage warehouse?" A few hands usually go up and I ask what was being stored, which helps engage students. I explain that in order to maximize the storage time for the fruit, it is necessary to capture the volatile ethylene that is released during the ripening. The farmer accomplishes that capture by placing a sachet of ETHYLENE CONTROLTM in each box of plums. While explaining this, display Figure 2. Ask if anyone has any idea of what chemistry is going on (usually no one provides a "correct" answer). Then open a sachet [available for free from the author, or from the company Ethylene Control (in a pinch, simulated by substituting granular potassium permanganate)] and place the solid granules into a petri dish and display using a video camera. What is seen (Figure 2, right panel) is not very informative. Then, while still displaying, add some water and swirl. The bright purple color comes as a wonderful surprise (Figure 1). Most students have not encountered that striking color before. Nevertheless, ask if anyone knows or wants to speculate about the identity of the dark solid granules. Once the name of the species responsible for the color is given, ask if anyone can explain the purpose of the permanganate anion in the sachet.
Hazards: ETHYLENE CONTROLTM should not be eaten; otherwise, according to the manufacturer (13), it can be discarded in normal (household) trash since its use is approved by several organic produce grower's organizations.
In closing, most students, i.e. those who have not previously encountered agrarian issues, find this demonstration an illuminating perspective on a new lifestyle, and an example of a practical application of the chemistry they have been learning about.
References (Works Cited):
(1) For a leading reference, see Dao, H., Sun., X. Ma, B., Zhang, J.-S., Guo., H. Ethylene, Hormone Metabolism and Signaling in Plants. Academic Press: Cambridge, MA 2017, pp 203-241.
(2) McPhee, J. Oranges, 1st ed.; Farrar, Straus and Giroux: New York, NY,1967; pp 112-113.
(3) For a review see Saltveit, M. E., Yang, S.-F., Kim, W. T. History of the Discovery of Ethylene as a Plant Growth Substance. Discoveries in plant biology. 1998, 1, 47-70.
(4) Personal communication from J. Fitzgerald Kelly, farmer.
(5) https://ethylenecontrol.com/ (accessed Nov 2020).
(6) Jones, D., The Aha! Moment: A Scientist's Take on Creativity. The Johns Hopkins University Press: Baltimore, MD, 2011.
(7) Strictly speaking, the potassium permanganate is impregnated in inert zeolite: US Patent 5278112A to F. Klatte, 1994.
(8) Cf. Vollhardt, K. P. C., Schore, N. Organic Chemistry: Structure and Function, 8th ed.; W. H. Freeman: New York, NY 2018, p 557. Wade, L. G., Simek, J. W. Organic Chemistry, 9th ed.; Pearson: New York, NY 2017, pp. 398-399.
(9) For a review, see Álvarez-Hernández, M. H., Martínez-Hernández, G. B., Avalos-Belmontes, F., Castillo-Campohermoso, M. A., Contreras-Esquivel, J. C., Artés-Hernández, F., Potassium Permanganate-Based Ethylene Scavengers for Fresh Horticultural Produce as an Active Packaging, Food Engineering Reviews, 2019, 11, 159-183.
(10) https://www.perishablenews.com/produce/ethylene-control-celebrates-30-years/ (accessed Nov 2020)
(11) https://images.app.goo.gl/FzuqKzUaNqEinfTg8 Image from TJ Mullinax/Good Fruit Grower, copyright Good Fruit Grower (accessed Nov 2020)
(12) For a video of a cold storage warehouse in operation see:
(13) Personal communication from D. Biswell, founder and CEO Ethylene Control