We humans are sweaty beasts. Amazingly enough, we can produce up to 2-4 litres of sweat per hour, and body parts that are clothed in socks and tightly laced up shoes are likely to get particularly moist.
Sweat itself is not to blame for the foul odour of stinky shoes. Believe it or not, sweat is completely odourless – it has no smell. However, it does contain minerals and nutrients that provide a tasty snack for bacteria, and it is these bacteria that produce “body odour”.
There are a handful of different bacteria involved in the production of the familiar cologne of smelly feet, but the main culprit is the catchily-named brevibacterium epidermis. This microorganism loves to gorge itself on dead skin cells; the warm, damp, sweaty environment of the foot, combined with the abundant dead skin we shed from our soles and toes, make the feet the perfect home for these brevibacteria.
While chomping away on our dead foot skin, Brevibacterium spews out a chemical called methanethiol, which has a distinctively pungent aroma, similar to sulphur. And to our noses it smells of cheese.
But why does the smell remind us of a well matured stilton? Cheese manufacture involves the “controlled spoilage” of milk – letting milk go off in a specific way. This is often achieved using bacteria, and one such bacteria is a close relative to our very own brevibacterium epidermis… brevibacterium linens. This bacteria is responsible for the ‘ripening’ of a number of tasty cheeses, including Limberger, Munster and Port du Salut. While chewing through milk proteins, ‘maturing’ the cheese, these brevibacterium flavour the cheese with the very same methanethiol that is a key component of that distinctive stinky feet smell.
In fact, some say that brevibacterium first got into Limberger cheese as a result of the Belgian monks of Limberg traditionally mixing the cheese curds with their bare feet.
Whether you believe this dairy legend (or think that it stinks) one thing we can be sure about is that it is no coincidence that our feet (and consequentially our socks) smell like cheese. The answer is simple – hungry brevibacteria pumping out stinky chemicals. Yum.
Answer by Nick Waszkowycz. Image credit: Skånska Matupplevelser on flickr.
Ferchichi, M., et al. “Production of methanethiol from methionine by Brevibacterium linens CNRZ 918.” Journal of general microbiology 131.4 (1985): 715-723.
Rattray, Fergal P., and Patrick F. Fox. “Aspects of Enzymology and Biochemical Properties of Brevibacterium linens Relevant to Cheese Ripening: A Review.” Journal of dairy science 82.5 (1999): 891-909.
Brevibacterium linens, MicrobeWiki
Don Voorhees Disgusting Things: A Miscellany (2008)