This article was published in Scientific American’s former blog network and reflects the views of the author, not necessarily those of Scientific American
I'm fascinated by the biology of soil and the history of "dirtiness"--where dirt and bacteria are allowed to be and where we must clean them away. Mary Douglas defines dirt in her classic book Purity and Danger: An Analysis of Concepts of Pollution and Taboo as "matter out of place":
[Dirt] is a relative idea. Shoes are not dirty in themselves, but it is dirty to place them on the dining-table; food is not dirty in itself, but it is dirty to leave cooking utensils in the bedroom, or food bespattered on clothing; similarly, bathroom equipment in the drawing room; clothing lying on chairs; out-door things in-doors; upstairs things downstairs; under-clothing appearing where over-clothing should be, and so on.
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This is part of why the recent news of an expensive French restaurant in Tokyo serving a dirt-based menu is so surprising and wonderful. Fancy food and soil certainly don't belong together, but this is no ordinary dirt; the food-grade dirt is specially sourced and lab-tested to ensure cleanliness, heated, strained, and smoothed into chocolate-like sauces, spooned onto elaborate dishes to give an exquisite "earthiness" to the flavor. These symbols of fanciness and good taste, along with the expensive price tag give the dirt a new place at the table. For more about the dirty menu and the history of geophagy (eating dirt) check out this article from Smithsonian.