Cooking with Kimchi: Fresh, Ripe and Sour Kimchi Explained

How fermentation changes kimchi

After salting and seasoning, lactic acid bacteria in the cabbage start to turn its sugars into lactic acid, with a little carbon dioxide. That is why ripe kimchi tastes tangy and can taste faintly fizzy. As the acid builds, the chili, garlic and fish sauce mellow and blend, the cabbage softens, and the taste moves from salty-spicy toward sour.

The speed depends almost entirely on temperature. At room temperature kimchi ripens in one to two days. In a fridge at about 1–4 °C it takes one to three weeks to become nicely tangy, and it keeps souring slowly for months. Korean kimchi refrigerators hold kimchi just above freezing to slow this as much as possible.

Fresh, ripe and sour: which dish wants which

Fresh kimchi (geotjeori or saekimchi, from the day it is made to a few days old): crunchy, salty and spicy, with no sourness. Best eaten as is, with bossam (boiled pork), noodles, or a bowl of rice. It is not good for cooking, because it tastes flat when heated.

Ripe kimchi (about 1–3 weeks in the fridge): pleasantly tangy and well balanced. This is the stage most people mean by "kimchi". It is the best all-rounder: a side dish, and the right choice for kimchi fried rice, kimchi pancakes (kimchi-jeon), dumpling filling and stir-fried kimchi with pork or tofu (dubu-kimchi).

Sour kimchi (sin-kimchi, a month or more): clearly sour and softened. This is what you want for kimchi jjigae, budae-jjigae and kimchi soups. Simmering mellows the acid into a deep, savory broth, and the soft cabbage melts into the stew.

Mukeunji (aged kimchi, kept 6 months to a year or more, often made with extra salt for long keeping): very sour, deeply savory and soft. It is braised whole with pork or mackerel (mukeunji-jjim), rinsed and wrapped around rice, or used in long-simmered stews.

Rinsing, cutting and using the brine

Rinsing: for very sour or very salty kimchi, a quick rinse under cold water removes some salt, chili and acid. It is common for mukeunji used as a wrap, for kimchi given to children, and for mild dishes. Squeeze the kimchi after rinsing. For stews, do not rinse. The acid and seasoning are the point.

Cutting: cut whole cabbage kimchi with scissors or a knife on a dedicated board, because it stains wood and plastic. For stews and fried rice, bite-sized pieces of 2–3 cm work best. For pancakes, chop it finer so the batter holds together.

1. Kimchi brine (gukmul) is concentrated flavor, so keep it.
2. Add a few spoonfuls to kimchi jjigae, kimchi fried rice or kimchi pancake batter for more color and tang.
3. Use it to season cold noodle broth or a simple soup.
4. Pour it over a new batch of kimchi, or over sliced radish, to help it ferment.

Storing and buying kimchi

Keep kimchi in an airtight container in the coldest part of the fridge, pressed down so it stays under its brine. Air exposure causes off-flavors and white film yeast on the surface. Use clean utensils every time, and take out only what you need for the meal. A separate container for kimchi limits the smell in the rest of the fridge.

To slow ripening, keep it cold and closed. To speed it up, for example when a stew recipe needs sour kimchi, leave the jar at room temperature for a day or two and then refrigerate it.

Store-bought kimchi is usually sold fresh and is often pasteurized or ripened very little. Check the "made on" or "best before" date. A newer jar will taste fresh. Swelling or fizzing when the lid is opened is normal. It means the kimchi is alive and fermenting. Vegan kimchi without fish sauce or salted shrimp is widely available. It ripens a little more slowly and tastes slightly lighter.