Many of our favorite technologies to preserve food—pasteurization, canning, freezing—attempt a sort of suspended animation, where the food remains as unchanged as possible for as long as possible.
Fermentation, though, is different. Here, we deliberately invite microorganisms into our food that will not only preserve it, but also change it, producing wonderfully complex flavors and textures. While preservation was once the most important function of fermentation, now, in the age of refrigeration, we love those fermented flavors for their own sake. The longer fermented foods age, the more they change, and many only get better.
Kimchi is a perfect example of this. Lauryn Chun, founder of the California-based national brand Mother-in-Law’s Kimchi, writes that traditionally in Korea “old kimchi was never thrown out. A precious, valued ingredient, it was always treated with respect.”
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How Kimchi Is Made
A classic baechu kimchi (cabbage kimchi) is made by salting cabbage leaves to draw out their moisture, seasoning them, packing them in a clean container, and waiting. The presence of salt means that only certain microbes can survive, which helps to make the difference between fermentation and rotting. As soon as the salt starts to break open the cabbage’s cells, various species of lactic acid bacteria, which were already harmlessly living on the cabbage leaves before this all began, start to grow and ferment the cabbage.
Lactic acid bacteria are famous for converting sugars into lactic acid, and that’s what they do here. The production of acid rapidly lowers the pH of the cabbage, from a neutral starting pH around 7, down to 4.6 and beyond. At these acid levels, even fewer types of microbes can survive. But the ones that do survive, thrive. Other lactic acid bacteria species, which have the ability to create not just acid, but also carbon dioxide and alcohol, grow in the submerged portion of the kimchi, generating gas that gives the cabbage its delightful fizziness.
After one week, the cabbage retains much of its raw crunch, becomes tart, and tingles with carbonation. Its acidity level prevents it from spoiling, but keeping it cool at this point helps to slow down the fermentation and produce a wider variety of desirable flavor compounds. After three weeks, it’s more tart, because more lactic acid has been developed, and, beyond the tartness, it has additional flavor complexity. Meanwhile, its texture is limper, as the pectin that gives the leaves their internal structure is starting to be digested by the bacteria.
After three months, the transformations are even more profound, with the limp leaves turning soft, and the flavor increasingly savory as bacteria produce amino acids. Yeasts sometimes move in around this time, generating new families of taste, including vinegary flavors. By six months, the pectin breakdown may start to make the leaves mushy, but they’re still good to eat— especially cooked into dishes such as kimchi bokkeumbap.
Is It OK to Eat Old Kimchi?
So, does kimchi ever go bad? No! Mukeunji is a type of kimchi that’s typically aged for a year or more at cool temperatures so it doesn’t become wildly sour, just wonderfully flavorful and extremely tender. For long-term storage of kimchi, just keep the vegetables submerged in the brine, and watch out for visible fuzzy mold on top. So long as the surface of the kimchi isn’t allowed to dry out and grow mold, kimchi does not go bad. In fact, I've aged my own homemade kimchi for two years and it only got better and better.