Disgusting, delicious durians

Abstract

On the stairway in a rather nice hotel where I stayed once in Thailand, a prominent plaque insisted: No durians. Bananas, fine; papaya, no problem; rambutan, knock yourself out. But the spiky, foot-long products of Durio spp (commonly Durio zibethinus)? Absolutely not! Yet durian flesh is widely regarded as exquisite (Figure 1). So why ban it? The renowned English naturalist Alfred Russel Wallace can answer that: “When brought into a house the smell is often so offensive that some persons can never bear to taste it” (The Malay Archipelago 1869; 1: 117, London: Macmillan & Co). Sadly, the above plaque offered no solution to the evolutionary conundrum of why a fruit, ostensibly seeking the dispersal of its seeds through its wonderful taste, should reek enough to ward potential helpers away.

That durians stink is uncontested. Writers have described them as smelling like everything from rotten onions to raw sewage, and the experience of eating the flesh as ranging from consuming carrion in custard to ingesting raspberry blancmange in a lavatory, and even to kissing a corpse (https://tinyurl.com/zu6r56uu). Getting beyond the stench is hard, but it brings its reward, as Wallace himself noted: “This was my own case when I first tried it in Malacca, but in Borneo I found a ripe fruit on the ground, and, eating it out of doors, I at once became a confirmed durian eater”.

The how part of the durian's funk has more recently been clarified. Analyses have revealed the fruit to produce over 40 odor-active compounds, many reminiscent of onions (raw, rotten, and roasted), along with others that conjure up the aromas of skunk, cabbage, and sulfur, tempered with soup-seasoning and caramel (J Agric Food Chem 2012; 60: 11253–62). And as durians get riper they get smellier, producing ever more ethionine, which enzymes then convert into the fruits’ signature “stink bomb”: ethanethiol (J Agric Food Chem 2020; 68: 10397–402). Even in minute quantities humans can detect its malodorous, garlicky-cabbage whiff (and given our paltry olfactory powers, that really does say something about ethanethiol!). But where is the evolutionary advantage in all this?

It's a tricky one. Some might argue that the colors, scents, sizes, tastes, and shapes of fruits have evolved to match the abilities of the animals that disperse them; clearly it's little help being too big for an intended bird's beak, or being red if a target primate can’t distinguish that color. But others might disagree, arguing that fruits are commonly eaten by many disperser species; just how could they match the needs (including the aromatic requisites) of all of them? So what about durians? Is their odor a useless, counterproductive byproduct as it might appear to be, or could it be a very useful signal that worked out because some potential dispersers, more inquisitive or more desperate for food, found, like Wallace, repayment for being so olfactorily bold? Indeed, hosts of durian-eaters, including humans, orangutans, elephants, rhinos, tapirs, and even tigers, would appear to have been thus compensated. Perhaps then, there is no more to this question than meets the nose. “It's important to remember that whether something smells good or bad is rather subjective”, says Omer Nevo (German Centre for Integrative Biodiversity Research; Leipzig, Germany). “Whether you find something attractive or repulsive is often not really about the actual smell, but your expectations, experience, and context. When fruits need to communicate with animal seed dispersers, they often use rather random scents – a mix of not-too-special plant volatile organic compounds that generate a unique bouquet that marks the fruit as ripe. So a durian's foul smell can in principle be just that: the plant needed to attract animals and ended up using a set of volatiles that these creatures learned to associate with a reward, and therefore find attractive. Durians ended up using chemicals that (for us at least) are quite smelly, but in the right context and with the right experience, not necessarily disgusting to us or other target animals.”

Durians, then, are the naturally occurring Camembert cheeses and Swedish surströmming (tinned, fermented herring that stinks to high heaven) of this world, balk-worthy to begin with but which sell very well because they are fantastic once you try them (well, at least the Camembert; I still can’t do surströmming despite all my Swedish friends’ encouragement). All durians had to do was get our attention once; passing on information to the next generation about what's good to eat did the rest.

So, if you are traveling with a durian in Southeast Asia, perhaps pondering the evolutionary questions surrounding its peculiar perfume, you may need to eat it before you roll up at your night's accommodation, especially if it's that rather nice place in Thailand. And if you will permit me, a word of caution to Swedish travelers: while the proprietor makes no explicit ban on the consumption of effervescing Baltic fish products on his premises, I probably wouldn’t push my luck.