Ask Paul: What Is the Difference Between Fermentation and Rotting?

We like to think there’s no overlap, no chance of confusing fermentation and rotting, and I’d love to pull out a simple, clean distinction between the twobut unfortunately things are a little fuzzy. (Like mold.)

Both terms refer to the same realm of biological processes, in which microorganisms transform food. Interestingly, unlike most cooking procedures, the transformations happen whether or not we initiate them. Unless foodstuffs are specially preserved, bacteria, molds, and yeasts will spontaneously seize the opportunity to digest their sugars, fats, and proteins, creating a wild array of new compounds. When we like the results of the transformation, we call it fermentation; when we don’t, we call it rotting. 

Ripe fruit, which is jammed with sugar, is likely the first food we humans noticed undergoing this transformation. Yeast that naturally live on fruit skins convert the fruit’s sugar into alcohol, and at some point in deep prehistory, people discovered that the results weren’t so bad—kind of tangy and delicious—and sought them out. It wasn’t long until we started deliberately encouraging the conversion of sugar into alcohol; there is evidence of beverages made from fermented rice and fruit in 7,000 BCE. Today, the enormous world of fermented foods includes olives, sausages, cheese, and pickles; coffee and chocolate, hot sauce, and risen bread.

Because the distinction between fermentation and rotting is such a continuum, and because the rot end of the continuum is universally considered disgusting, products created by flavorful decomposition can be divisive: One person’s delicious fermentation is another person’s disgusting rot. 

As fermentation guru Sandor Ellix Katz eloquently writes, “Please do not take the idea that the boundary between fermented and rotten is blurry and slippery as a suggestion to start eating anything you would previously have rejected as rotten. Learning a sense of boundaries around what it is appropriate to eat is necessary for survival. But precisely where we lay those boundaries is highly subjective, and largely culturally determined.”

Take hákarl, the famously strong-smelling Icelandic delicacy often referred to as “rotten shark.” Over the course of several months, the pressed meat of the Greenland shark becomes rubbery in texture and fiercely redolent of fishy ammonia, but it’s actually quite pleasant to eat. If you’re not used to it, though, you definitely have to mentally suppress some of your body’s warnings that it may be harmful.

Meanwhile, certain European mountain-dwelling cultures allow bovine milk to coagulate and then rest unrefrigerated for months while bacteria create flavor compounds also found in human sweat, as well as gas pockets that can explode if unchecked. Just like hákarl, if you’re not used to cheese (for instance, if you grew up in much of East Asia), it can be repulsive, a point made well in a recent article by Jiayang Fan.

In order to keep the process of fermentation within the bounds of our taste, we tend to control it with a number of simple methods: salt, temperature, acidity. Put cucumbers in a jar of plain water and soon scummy pink and gray mold will find a happy new home. But mix up a 4- to 5-percent saltwater brine and you’re making pickles: Most microbes, including molds and yeasts, can’t survive in that salty environment, but the lactic acid bacteria that we want in our pickling jar like it just fine.