For your delight and delectation

DESPITE their name, disposable nappies are notoriously difficult to dispose of. Studies of landfills suggest they may take centuries to rot away. But Alethia Vázquez-Morillas of the Autonomous Metropolitan University in Mexico City thinks she has found a method of speeding the process up.

As she and her colleagues describe in Waste Management, cultivating the right type of mushroom on soiled nappies can break down 90% of the material they are made of within two months. Within four, they are degraded completely. What is more, she says, despite their unsavoury diet the fungi in question, Pleurotus ostreatus (better known as oyster mushrooms), are safe to eat. To prove the point she has, indeed, eaten them.

The culinary use of oyster mushrooms was one reason why she picked them for the experiment. The species is frequently used in stir-fries and is often added to soups. The other reason was that Pleurotus ostreatus is widely used in what is known as mycoremediation—the deployment of fungi to clean up waste. It is, for example, already grown on agricultural materials such as wheat and barley straw, and industrial waste like coffee grounds and the leftovers from making tequila. Dr Vázquez-Morillas and her colleagues were trying to extend the oyster mushroom's own culinary range.

The reason nappies are difficult to break down has nothing to do with their use. Even a clean nappy would hang around for a long time in a dump. The main ingredient of a nappy is cellulose, an annoyingly persistent material. Pleurotus, however, grows on dead or dying trees in the wild and is thus well provided with enzymes that break cellulose down. And, since Mexicans alone throw away 5 billion nappies every year, there is plenty of material from this source for them to get their mycelia into.

The idea that the result might be sold and eaten may be controversial but it is not absurd. The nappies the researchers used were contaminated only with urine, not faeces. A healthy person's urine is sterile and Dr Vázquez-Morillas also treated the nappies with steam, to make sure. Such treatment would kill the nasty bugs in faeces, too, though, so mushrooms grown on treated nappies should, in theory, be safe to eat.

In practice, overcoming the yuck factor might be an insuperable barrier to marketing nappy-grown fungi, and the cost of the steam treatment could make the exercise futile. Mycoremediation of this sort does not, however, depend for its success on selling the results. Merely getting rid of what would otherwise hang around indefinitely is worthwhile. And of the fungi themselves, Dr Vázquez-Morillas observes, “they are cleaner than most of the vegetables you can find in the market, at least in Mexico.”