Let's face it, truffles are a little bit mysterious. Far more people have heard food writers sing the praises of the knobby subterranean mushrooms than have actually tasted them. And not even all the people who've tasted them feel a rhapsody coming on afterwards. Truffles, as M F K  Fisher writes in Serve It Forth, “may or may not be as good as they are rare and dear”.

With the white variety, one of the more rare and dear, going for thousands of dollars a pound at auction, and their charms apparently fading quickly in the hours and days after harvest, is it any wonder that disappointment might rear its head?

But the reports from people who have managed to obtain a taste of a fresh, authentic truffle note a very particular flavour and scent. The words “musky,” “garlick-y,” “sulphurous,” and “funky” come up a lot. It's believed that some of the distinctive aroma comes from a molecule called androstenone, a hormone that is also produced by male pigs and whose presence in truffles is said to be the reason that pigs make fine truffle hunters.

Not all humans can smell adrostenone, thanks to naturally occurring genetic variety in smell receptors; those who can report it smelling like sandalwood, vanilla, or a bit like urine, perhaps adding to the potential for dissatisfaction with your tagliatelles aux truffes

So far, what we know about truffle scents comes from sampling the air around the mushrooms  

Some of the odour may come from a source that you're more intimately familiar with, though – microbes. Bacteria, yeast, and other microscopic creatures are behind a number of scents, including that of sweat, which is odourless until armpit-living bacteria digest a precursor molecule to turn it into one that stinks. And while the origins of truffle smell are still being uncovered, research investigating the microbes that live in and on them suggests that at least one set of truffle odour molecules is actually made by bacteria.

So far, what we know about truffle scents comes from sampling the air around the mushrooms, identifying the odour molecules present, and trying to deduce how they were made. The molecules have names like 3-methylbutanal and 2-methyl-4,5-dihydrothiophene, which smell like straw or toast and like onion, respectively. Different truffle species have different assortments of scent molecules at each stage in their lives, and a recent review article examines 35 truffle odorants whose smells range from meaty and dusty to buttery and creamy.

Laying out the classes of mushrooms, yeast, and bacteria known to produce these molecules, the researchers note some interesting things. Dimethyl sulphide, for instance, smells sulphurous and is emitted by 85% of truffle species. The mushrooms can generate it themselves, but it can also be produced by Alphaproteobacteria and Betaproteobacteria, two groups of bacteria that heavily colonize truffles. Other common truffle scents, like 3-methyl-1-butanol, which smells of chocolate and whiskey, and hexanal, which smells grassy, also could be coming from both the colonisers and the colonised.

Rarer truffle scents, though, are more likely to be linked to bacteria. In fact, last year researchers reported that the thiophene derivatives, a group of sulphur-containing molecules that contribute to the scent of white truffles, could not be made by the mushrooms themselves. Instead, the molecules were traced to bacteria, which digested odorless molecules to produce them.

The molecules may be a by-product of some important biological process

Treating the truffles with antibiotics eliminated the scent molecules, and the researchers reported that the ability to make them appears to be common to all the truffle bacteria they discovered, suggesting that the molecules may be a by-product of some important biological process.

The pungency of truffles isn't just for our entertainment – for the truffles themselves, it may be a matter of life or death. Scientists think that the fact that animals can smell the mushrooms underground and will dig them up suggests that being eaten is a way for the truffles to ensure that their spores are spread far and wide, in the faeces of their devourers.

Could it be that some of what animals smell is actually from the microbes, not from the truffles themselves? It's already been shown that fruit flies are drawn to fruit not because of the fruit's own smell, but rather because of scents made by yeast living on its surface. It's a fascinating idea.

The authors of the review end by saying that to really understand which molecules come from where, however, researchers will need to obtain truffles completely free of microbes. That is a tall order: No wild-harvested mushrooms will do, and culturing truffles in the lab is, sadly, still beyond us. 

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