It’s a squeamish moment, finding a hair that doesn’t belong to you snaking through your spaghetti—one that usually warrants a swift return of the plate to the restaurant kitchen, or profuse apologies from whichever family member’s ’do matches the length and color of the sample. In her seminal book, Purity and Danger, anthropologist Mary Douglas outlines the cultural codes that create this aversion. As “matter out of place,” that stray hair symbolizes a disruption to our established order. We are shocked to see the pedestrian become perverse.

Yet our aversion to a single strand does seem misplaced given that human hair is added to many commercially baked goods in the form of an amino acid called L-Cysteine. While this is not the only surprising ingredient that lurks in ou...