SCIENCE: Fermentation and its Role in Kimchi

The hands that start the preservation process eventually pass the baton to lactic acid fermentation, which ushers the vegetables along a dynamic journey that makes the ferment safe to eat indefinitely, as well as spectacularly tangy and complex. Here is an in-depth look at how the fermentation process works to make kimchi.

Salting

Salt draws out water so the kimchi isn’t too liquidy, removes sulfurous flavor, collapses some of the leaves’ cells so it’s easier for good bacteria to get in and do their job, softens the leaves’ structure so they shrink and pack tightly in a container, and slows the action of pectin-digesting enzymes that would otherwise make the cabbage mushy. Meanwhile, it pulls water from bad microbes, diminishing their numbers so they can’t spoil the cabbage and clearing the way for beneficial microbes such as several species of lactic acid bacteria (LAB) to carry out the fermentation process by converting sugars into lactic acid.

Note: It’s important that the salt measure about 2 to 3 percent of the weight of the cabbage: enough to inhibit unwanted microbes but not so much as to make the kimchi too salty.

Seasoning

The cocktail of seasonings flavors the cabbage and also feeds the LAB so they can vigorously multiply and ferment the vegetables. The faintly fruity heat of gochugaru is standard, as are garlic, scallions, and ginger. Also standard is the element of sweetness with carrots, Asian pear or apple, and plum syrup, which doubles as food for the microbes. Fish sauce, dried large-eyed herring, salted shrimp, and dried large anchovies, pull it all together with briny, umami-rich funk.

Primary Fermentation

The LAB thrive as the mixture sits, consuming sugars and producing lactic acid. As that acid builds, it lowers the mixture’s pH and makes it increasingly inhospitable to unwanted bacteria so that the kimchi can be stored indefinitely, and it gives the kimchi its tang. The LAB also generate carbon dioxide, which forms bubbles in the juices that give the cabbage delightful effervescence.

Time and temperature are big factors. At room temperature (65 to 70 degrees) the mixture is tangy after two days but can be left up to five to build funk. Slightly cooler temperatures work, too; the reactions slow, so there’s even more flavor development. Note: avoid anything warmer, which speeds lactic acid production while secondary flavors lag behind, resulting in kimchi that’s tangy but less complex.

Secondary Fermentation

As soon as the kimchi hits your sweet spot for tanginess (3 days in the 65- to 68-degree test kitchen works for us), move it to the fridge for long-term storage indefinitely. Kimchi does not go bad; it simply matures as fermentation chugs along at fridge-cold temperatures. As long as the vegetables are submerged in the brine and there is no visible fuzzy mold at the surface, it’s not only good to eat, kimchi gets better with age. The flavor and textural changes it undergoes give the ferment tremendous range and utility over the course of its life cycle.