MOLECULE OF THE MONTH / Birth of the smart tomato

Ethylene is what makes vegetables ripen and rot. Now food scientists are working on ways to get the better of it. John Emsley reports

John Emsley
Tuesday 21 March 1995 00:02

The Government has just approved for general sale in our shops and supermarkets a tomato concentrate made from genetically engineered plants. The new variety of tomato has been produced by the bioscience company Zeneca. It ripens more slowly, stays firmer longer, and can be transported over large distances. Its secret lies in the way it responds to the molecule ethylene.

The Zeneca tomato has taken 10 years to develop and came as a result of fundamental research into the walls of plant cells by Professors Don Grierson at Nottingham University and Wolfgang Schuch of Jealott's Hill research centre at Bracknell, Berkshire, which is part of Zeneca Seeds.

Dr Nigel Poole, the group manager, says: "The new tomatoes are the first genetically modified fruit to be approved for human consumption in both the UK and the US. They will be processed into tomato pure and are currently growing in California, but we hope soon to have permission to produce them in southern Europe."

The new tomatoes have been modified so that, unlike all other tomatoes, they respond to ethylene in a selective way: they ripen but they do not rot. Ethylene triggers both processes, and not only in tomatoes - it controls the ripening of bananas, avocados, nectarines and peaches as well. Ethylene, more correctly called ethene, has the formula C2H4 and is a gas at normal temperature; it condenses to a liquid at only -104C.

For 50 years, fruit importers have used ethylene to ensure a regular supply of ripe tropical fruits for the British market. These are picked unripe, and then ripened here by exposing them to ethylene gas. This is generated by passing alcohol vapour over an electrically heated catalyst, or it can be released by dissolving chloroethylphosphonic acid in water. Ripening of the fruit begins when ethylene in the air exceeds one part per million (1ppm).

In some countries ethylene is used to ensure uniform ripening of figs, mangoes and melons. The gas is also used to induce blossoming of pineapples, and to make olives easier to pick. In earlier times local people lit bonfires near fruit trees to accelerate ripening and harvesting, and it worked because wood smoke contains about 4ppm of ethylene. Ripe fruit itself gives off ethylene, which explains why one ripe tomato or banana will trigger the ripening of others, a trick that gardeners use when they have to pick their tomatoes green.

Ethylene does more than just ripen fruit: it also triggers the natural processes which tell flowers to shed their petals, and trees their leaves. Ethylene explains why our grandmothers never put cut flowers in a room with a gas fire. Coal gas had 10ppm of ethylene, and it was this which first sparked an interest in the gas's biological role.

In the last century it was not uncommon for some city trees prematurely to shed their leaves in summer. This curious phenomenon would often be linked to a nearby gas main. Research revealed that ethylene in the coal gas was the cause, and it was discovered to have other remarkable effects on plants. By the 1930s, biochemists had realised that ethylene was a plant hormone, produced during germination, growth, blooming and fruit ripening. Ethylene in plants is normally low, but can reach as much as 2,000ppm in ripe fruit.

In addition, ethylene can begin a ripe tomato's decay by activating fungal spores on its surface. These attach themselves during the growing period, then lie dormant until the fruit ripens, at which time they suddenly multiply. Last year, Moshe Flaishman and Pappachan Kolattudkudy, of Ohio State University, in the United States, discovered that the spores were triggered by ethylene.

Flaishman and Kolattudkudy demonstrated that ethylene in avocados and bananas also triggers fungi, but that a ripe orange - a fruit that does not use ethylene as a plant hormone - was unaffected. Other genetically engineered tomatoes, which were modified to produce no ethylene at all, were not affected either.

Ethylene is not only essential to plants; our modern way of life depends on it. This highly flammable gas is the source material for most products of the chemicals industry - last year 67 million tons. The gas is turned into hundreds of chemicals which make the thousands of products we use every day. Packaging, plastics, perfumes, paints and painkillers start out as ethylene.

The gas is manufactured by "cracking" crude oil or petroleum gas with steam at about 750C, a process that breaks these materials down into simpler molecules. A lot of ethylene ends up as polythene, the plastic that is simply made by stringing the molecules together in long chains.

While industry can use high temperatures to generate ethylene, plants have to make this molecule at ordinary temperatures, and they have evolved a complex sequence of chemical reactions for doing so.

Although we know how plants make ethylene gas, we are still unclear exactly why it acts as a chemical messenger. Dr Poole says: "There is still a lot of research to be done before we discover how it turns tomatoes red, but we know how it causes them to soften. Ethylene triggers the formation of polygalacturonase, the enzyme which breaks down pectin, the material which binds together the cell walls of plants. Without pectin, they quickly go soft."

It is possible to protect fruit and flowers against ethylene and thereby extend their lives. This can be done by storing fruit in sealed containers with silica impregnated with potassium permanganate, which mops up ethylene. In Australia, the Commonwealth Scientific and Industrial Research Organisation has developed a plastic wrapping film impregnated with a similar material which greatly extends the life of flowers wrapped in it.

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