A rare amino acid called ethionine is present in the pulp of the durian (Durio zibethinus), a fruit called the ‘king of all fruits,’ and plays a key role in the formation of the characteristic durian odor, according to new research from the Leibniz-Institute for Food Systems Biology at the Technical University of Munich.
Durian fruits (Durio zibethinus). Image credit: Truthseeker08.
Durian is a Southeast Asian tropical plant known for its hefty, spine-covered fruit and sulfury and onion-like odor.
The edible part of the fruit, a yellowish pulp, consists of the arils encasing the seeds. The pulp exhibits a sweet taste and a pleasant custard-like consistency.
Its odor, which combines some fruitiness with a dominating sulfury note, however, is extremely strong and, from the point of view of Westerners not used to durian consumption, very extraordinary and rather repellent.
“We had already shown that the fruit’s stench is essentially due to the odorant ethanethiol and its derivatives,” said study authors Dr. Nadine Fischer and Dr. Martin Steinhaus.
“However, the biochemical pathway by which the plant produces ethanethiol remained unclear.”
The primary aim of the study was to screen durian for the presence of ethionine, the precursor of the foul-smelling substance.
After the researchers succeeded in detecting ethionine, they studied the influence of variety and ripening stage on its concentration and confirmed its role as ethanethiol precursor.
“Quantitation of ethionine in samples of different varieties (Monthong, Krathum, Chanee, and Kanyao) showed concentrations between 621 and 9,600 μg/kg,” they said.
During fruit ripening, the concentration of ethionine increased as well as the ethanethiol concentration.
“Our findings suggest that as the fruit ripens, a plant-specific enzyme releases the odorant from ethionine,” Dr. Fischer said.
“This is consistent with our observation that during fruit ripening not only the ethionine concentration in the pulp increases, but also at the same time that of the ethanethiol. The latter explains why a ripe durian emits an extremely strong smell.”
“Animal tests and cell culture studies have verified that the amino acid is not harmless,” Dr. Steinhaus added.
“Rats that incorporated high doses of the amino acid together with their food developed liver damage and cancer of the liver.”
“In order to consume a comparable dose of ethionine that had toxic effects in animal tests, a person weighing 70 kg would in one day have to eat 580 kg of fruit pulp of the Krathum variety which is especially rich in ethionine,” he said.
“However, low concentrations of ethionine may even have positive immunomodulatory effects,” he noted.
“This raises the question of whether eating the fruit entails health risks. Further studies certainly need to be conducted.”
The study was published in the Journal of Agricultural and Food Chemistry.
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Nadine S. Fischer & Martin Steinhaus. Identification of an Important Odorant Precursor in Durian: First Evidence of Ethionine in Plants. J. Agric. Food Chem, published online December 11, 2019; doi: 10.1021/acs.jafc.9b07065