Bacteria make part of the bianchetto truffle’s smell

Which truffle, and what the bacteria do

The thiophene volatiles that characterise the fruiting bodies of Tuber borchii are made by the bacteria living inside them rather than by the fungus. The paper that showed it went online on 9 July 2014 and reached an issue of Environmental Microbiology only in August 2015, as volume 17, issue 8, pages 2647–2660. Six authors signed it, working at the Georg-August University of Göttingen, at Goethe University Frankfurt and at the French INRA unit for tree and micro-organism interactions at Champenoux; Richard Splivallo is the first author and Petr Karlovsky the last. The NLM record carries the full abstract.

One naming point before anything else, because it catches more than translators. The abstract calls the species “the white truffle Tuber borchii”, a phrase that collides head-on with Tuber magnatum, the Alba white truffle — a different species with a different smell. T. borchii is the bianchetto or whitish truffle, sold in Italy as tartufo marzuolo. Both reach the market as white truffles and both are routinely mistaken for each other. Everything below concerns T. borchii alone, and none of it transfers to Alba or to the Périgord black truffle, Tuber melanosporum.

What was actually dosed

The team characterised the bacterial community colonising the fruiting bodies and asked whether that community had anything to do with the sulphur-containing volatiles. It was dominated by α- and β-Proteobacteria, and the thiophene derivatives characteristic of this truffle turned out to be a biotransformation product: bacteria convert a non-volatile precursor into a volatile compound.

The decisive comparison was blunt. Fruiting bodies treated with antibacterial agents stopped producing thiophene volatiles entirely, while fungicides had no inhibitory effect — meaning they did not stop thiophene production, a narrower statement than saying they did nothing at all. From that the authors concluded that during the sexual stage of the truffle, which is the fruiting body itself, these volatiles are synthesised exclusively by bacteria and not by the truffle.

It is worth being clear about what kind of procedure this was. Harvested fruiting bodies were dosed on a laboratory bench; nobody watched a truffle growing in soil, and nobody demonstrated that the bacteria had been removed. What was demonstrated is that production of one class of compound was abolished. The second result is stranger and more suggestive: every bacterial phylum and class the team tested could make thiophene volatiles from T. borchii fruiting-body extract, whatever it had been isolated from, truffle or otherwise. The ability may therefore be widespread among bacteria and tied to ordinary primary metabolism rather than to any special adaptation for living inside a truffle.

Trace compound, or defining note

Establishing a bacterial origin says nothing about whether a human nose notices. A separate paper by Richard Splivallo and Susan Ebeler, online on 10 January 2015 and printed in the March issue of Applied Microbiology and Biotechnology (volume 99, issue 6, pages 2583–2592), answered that with gas chromatography-olfactometry, in which an assessor sniffs and describes each separated compound as it leaves the column. Thiophene derivatives proved to be major contributors to the aroma of T. borchii, and of the four detected, 3-methyl-4,5-dihydrothiophene contributed most to the overall smell. This is not a chemical footnote, then, but a defining part of what the species smells like.

The same record notes that the relative concentration of thiophene derivatives was unaffected by freezing, yet differed between samples collected in Italy and in New Zealand. Storage conditions and differences in the bacterial community of the fruiting bodies were offered as candidate explanations, with the authors stating outright that further work is needed to confirm them. These are measurements of the relative concentration of one group of compounds, not guidance about keeping or preparing truffles.

Where the finding stops

It does not license the sentence “truffle smell is made by bacteria”. It licenses a narrower one: in one species, one class of odorant has a bacterial origin. A minireview from 22 September 2015 sharpens that limit into something more interesting, describing thiophene derivatives as odorants unique to T. borchii and recording that they turn up in no more than one or two truffle species at all — so they are not an ingredient of the Alba or Périgord bouquet.

Nor is it known where the raw material comes from. The biosynthetic pathway remains elusive, and so does the origin of the precursor itself: it is present in fruiting-body extract, but nothing in this work shows the fungus making it. The involvement of yeasts or other bacteria cannot be excluded either, as the authors of the primary paper say in the last sentence of their abstract.

The fungal-versus-bacterial division is blurrier than it looks in any case. The commonest sulphur volatile in truffles is dimethyl sulphide, detected in 85 per cent of the species examined so far; the minireview is careful to say only that it might come from truffle mycelium and also from some of the α- and β-Proteobacteria that dominate the fruiting bodies. The eight-carbon volatiles octan-3-one and oct-1-en-3-ol — the smell most people would simply call mushroomy — were believed to be strictly fungal, yet the data assembled there suggest certain bacterial classes may produce them too.

A last distinction: none of this concerns flavoured truffle oils. Their smell rests on 2,4-dithiapentane, which does occur naturally in one or two truffle species but reaches the bottle by synthesis. That is a separate industrial story.

Something to ask the next menu

The next time a menu, a market catalogue or a product label promises truffle aroma, two questions are available to anyone: which species, and which compound. The answer is missing surprisingly often, and without it a sentence about smell carries no information at all. The wider point is worth keeping. A fruiting body is not a single organism exhaling its own scent; it is an inhabited structure, and part of its chemistry is done by somebody else.