The mysteries of fungi
Fungi belong to neither the plant nor animal kingdoms. They have their own kingdom (unsurprisingly called the fungi kingdom). Inhabitants include mushrooms, yeasts and moulds. A restaurant truffle lunch next week (it’s the height of the season here), prompts me to look at this most prized (and priced) fungus of them all.
The truffle belongs to the mycorrhizea family of fungi (myco fungi and rhiza root or rhizome) that live underground in partnership (symbiosis) with some tree species. The fungi infiltrate the roots, and the tree and fungus exchange nutrients to the benefit of both. This symbiosis is ancient, and thought to have been a first stage in the evolution of cellular land plants. The first plants (early Devonian — 400 million years ago) didn’t have roots, but rather formed a relationship with underground fungi for their nutrients.
The fungus that might produce a truffle starts as a spore. On germination, the spore produces thin threads (hyphae) that divide and grow, forming an intricate underground net (mycelium). Tree roots may send out chemical signals to trigger this process. If this mycelium encounters a net from another compatible spore, then a reproductive fungus forms and the mycelium net continues to grow. Otherwise, the fungus dies off. Ultimately, the net reaches a root and the hyphae begin to infiltrate it. The hyphae are very fine and the fungus can grow into the root between the root cells. To give some idea of how fine they are — a 1cm cube of soil could contain an incredible 2km of hyphae.
Meanwhile the mycelium expands out around the tree, giving the tree an extensive auxiliary root system to transport micronutrients back to it. The small diameter of the hyphae means that they infiltrate soil more effectively than tree-roots and, in addition, the hyphae secrete enzymes that break down plant and animal material and so increase the nutrients available to the tree. The fungus also kills off other plants and weeds competing for nutrients.
In turn, the tree feeds the fungus sugars that it sends to its roots to maintain and expand its own root network. While fungi don’t photosynthesise, this symbiosis gives the fungus the benefits of the tree’s photosynthesis while the fungus effectively feeds the tree (and vice versa).
When conditions are just right, the fungus starts to form part of its mycelium into a spore-containing growth. This will be the truffle (the fruiting part of the fungus). The truffle remains underground for protection as it matures. Other fungi (e.g. mushrooms) send this fruiting body above the soil to disperse their spores by wind or rain, and most have developed toxins and bright colours to deter predators (fortunately for us, not all).
The truffle has come up with a more ingenious dispersal method — stay hidden until maturity, then get eaten.
Truffle spores survive animal digestion, and the animals disperse the spores through their dung. The problem is that the truffle is out of sight, so it releases attracting odourants that yell “hey, over here animals” when the truffle is ready. The truffle is either at the interface between the leaf-layer and the soil, or just under the soil surface, so it is easily dug up and ingested. The truffle can attract animals of many kinds, from insects (beetles) to squirrels, mice, deer, bears and pigs.
Humans have come to admire truffles and increasingly there has been success with cultivation. Saplings can be inoculated with the fungus and grown in locations in which the soil and climate are conducive to both the tree and the fungus. The first success in the southern hemisphere was in New Zealand (1993) followed thereafter in Australia (Tasmania; 1999). Commercial grade truffles were harvested in the south-west of Australia (Manjimup) in 2004 and truffles are now farmed in the south-east of the continent (NSW, ACT and Victoria). An attraction of S. hemisphere cultivation is the sale of fresh, out of season, produce to the N. hemisphere market. Truffles sell for around AUD2–3,000/kg, an attractive industry if it can be established. Other growing regions are Chile, China and the US.
Truffle hogs (sows) are the traditional means to find them, although it is a challenge to stop a determined 100kg sow from devouring the truffle on the spot. Dogs are a more usual option. Many dog breeds can be trained to detect truffles, although only one breed (Lagotto Romagnolo) has been nurtured for this purpose.
Dogs need training, whereas sows have an innate instinct for the fungus. It is thought that this might be because one of the aroma molecules released by the truffle is the steroid androstenol, a boar pheromone (and related to androstenone in men’s perspiration). It is not that simple though — this steroid, when delivered alone, is ignored both by pigs and by trained dogs. Truffle odourants are a complex interplay of volatile aroma molecules that include alcohols, aldehydes, ketones, acids, esters, amines, aromatic ethers, hydrocarbons and sulphur compounds. It is not known whether the truffle itself creates these volatiles entirely, or whether there is a further interaction with other fungi (yeasts) and bacteria in the soil.
There are two varieties of truffle of main culinary interest, although there are many others. The black truffle (Tuber melanosporum) is found naturally around Northern Italy and Croatia, and the white truffle (Tuber magnatum) in the Périgord region of France. There is also a North African truffle of minor interest, a Chinese truffle that can be mistaken for a black truffle (sometimes fraudulently), and an Oregon truffle.
There is not much to say about cooking with truffles at home. Truffles are probably for restaurant dining more so than home cooking. The black truffle has the more complex flavour, and can withstand some cooking whereas the white cannot and is best served shaved over food at the last minute. Truffles loose their aroma fairly quickly, which can be taken up by other food items (e.g. eggs) stored with them.
Truffle volatiles are oil-soluble, however shop-bought truffle oil almost certainly contains no truffle. It is oil flavoured with 2,4-dithiapentane (a dimethyl sulphide), irrespective of whether there is a shaving of something black floating in it. This explains why it is always a disappointment. The odourant has been identified as one of the main black truffle volatiles. Look closely the label — if it says ‘truffle aroma’ rather than ‘truffle’, then it is oil flavoured with 2,4-dithiapentane. Even the most expensive are.
Unfortunately for some, not everyone tastes truffles the same way. A significant portion of the population (25%) cannot taste anything much at all. Another similar proportion can, but the taste is unpleasant (rotten wood/sweat/stale urine). That leaves only about 50% to appreciate them. It is down to a tiny genetic variation (polymorphism) in a single odour receptor. I will find out soon enough. It appears that constant exposure to the aroma can eventually result in appreciation by everybody, which might explain why all chefs seem to like truffles.
As an aside, not all mycorrhizea are symbiotic with trees, some are parasitic and deadly. They live off the tree, killing it. The largest known living thing is just such a fungus (a honey mushroom), located in Oregon. This single organism (genetically identical throughout) covers ~10 square kilometres, is between 2–8,000 years old and has killed the forest above it multiple times, each time building deeper layers of soil that support renewed growth.